FULL PAPER

DOI: 10.1002/ejoc.201100551

The First Benzo[1,2:4,5]dicyclobutenones and Their TricarbonylchromiumComplexes

Ismail Abdelshafy Abdelhamid,[a,b] Osama Mahmoud Ali Habib,[a,c] andHolger Butenschön*[a]

Keywords: Small ring systems / Chromium / Polycycles / Regioselectivity / Arynes / Fused-ring systems

Treatment of 1,4-dibromo-2,5-dimethoxybenzene (6) withNaNH2 followed by 1,1-dimethoxyethene (7) afforded the bi-and tricyclic acetals such as 8 and 9 as the products of [2+2]cycloadditions of the intermediate arynes. After chromato-graphic separation and acetal hydrolysis the title compounds15 and 16 were obtained in high yields as the first represen-

Introduction

Benzocyclobutene and its derivatives have been the ob-jects of study for more than 50 years and play an importantrole as precursors in the synthesis of natural products con-taining anellated six-membered rings, such as steroids, an-thracyclines, or alkaloids.[1–8] The striking feature of benzo-cyclobutenes is the potential for ring-opening reactionsleading to reactive ortho-quinodimethanes, which readilyenter into cycloaddition reactions. In order to eliminate theplanes of symmetry present in ortho-quinodimethanes,metal π complexes of benzocyclobutene and substituted de-rivatives have been prepared and investigated, both by our-selves and by Kündig’s group. This chemistry was reviewedsome time ago.[9]

Benzene rings containing more than one anellated cyclo-butane ring are much less common, although Suzuki re-cently reported some significant progress in this field.[10–15]

Remarkably, the complexes 1[16] and 2[17] are the only exam-ples of arene tricarbonylchromium complexes containingmore than one anellated cyclobutane ring. They do not bearfunctional groups, and their chemistry has not been investi-gated.

In an attempt to prepare arene tricarbonylchromiumcomplexes containing two functionalized anellated cyclobu-tane rings we focused on the linear fused systems derivedfrom the ligand of 1. The anti diketone 3 had been reportedby Liebeskind in 1989;[18] however, in 2001 Ibrahim-Oualiand Santelli found that the real constitution of the isolated

[a] Institut für Organische Chemie, Leibniz Universität HannoverSchneiderberg 1B, 30167 Hannover, GermanyE-mail: holger.butenschoen@mbox.oci.uni-hannover.de

[b] Chemistry Department, Faculty of Sciences, Cairo University12613 Giza, A. R. Egypt

[c] Chemistry Department, Faculty of Science, Assiut University71716 Assiut, A. R. Egypt

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tatives of their class. Complexation of the acetals withCr(CO)6 gave the corresponding tricarbonylchromium com-plexes in excellent yields. Subsequent deacetalization af-forded the diketone complexes 18 and rac-20, which under-went stereoselective nucleophilic diadditions at –78 °C.

compound was that of 5, as shown by an X-ray crystalstructure analysis of the precursor diacetal.[19] The syn dike-tone 4 is also unknown. Compounds derived from 3 or 4are of interest as precursors in the synthesis of anthraceneand higher acene-derived compounds, which are valuableboth in medicinal chemistry[20,21] and in materials sci-ence.[22–26] Here we report the synthesis of the first diket-ones derived from 3 and 4 and their tricarbonylchromiumcomplexes.

Results and Discussion

For the synthesis of the tricyclic systems we envisaged asequential elimination/addition/elimination/addition pro-cess based on the benzocyclobutenone synthesis describedby Stevens.[27] In view of recent results from Suzukiet al.,[10–15] we anticipated that the intermediacy of arynescontaining single anellated cyclobutane rings should bepossible. 1,4-Dibromo-2,5-dimethoxybenzene (6) wasviewed as a favorable starting material, because no bent sys-tems such as 5 need be expected. In addition, 6 – unlike 2-iodotosylarenes[15] – is commercially available. Treatment of

I. A. Abdelhamid, O. M. A. Habib, H. ButenschönFULL PAPER

6 with sodium amide (4 equiv.) in the presence of 1,1-di-methoxyethene (7, 5 equiv.) afforded the desired tricyclic di-acetals 8 and 9 as the main products in 55% and 25 %yields, respectively. A small amount of 10 (6%) was presum-ably the result of partial acetal hydrolysis of 8. In addition,the benzocyclobutenone derivatives 11 and 12 were isolatedas side products, each in 5% yield. These side products, ob-viously the results of incomplete reaction, were separatedby flash chromatography and characterized spectroscopi-cally. Compounds 8 and 9 were easily distinguished by in-spection of the numbers of their 1H and 13C NMR methoxysignals. The constitution of 11 as the syn isomer was estab-lished by a HMBC measurement indicating a 4J couplingbetween the aryl hydrogen atom 4-H and the CH2 carbonatom C-7, thereby excluding its anti regioisomer. Com-pound 12 was characterized accordingly. Although a syndiacetal similar to 8 has been obtained by Suzuki et al. in22% yield,[15] 9 is the first anti diacetal of its kind.

The reaction is thought to proceed by elimination/ad-dition sequences with arynes 13 and 14 as key intermedi-ates. Their constitutions raise the question of the regioselec-tivity of the [2+2] cycloadditions. The exclusive isolation ofthe syn regioisomers 11 and 12, but not of the correspond-ing anti isomers, suggests a regiochemical difference, pre-sumably of electronic origin, because the immediate vicini-ties of the triple bonds are sterically identical. PM3 calcula-tion[28,29] of the charge densities at the triple bonds in 13and 14 by use of the Spartan[30] program package indicatesthat the carbon atoms in the para positions relative to theBr substituent in 13 or to the methoxy groups in 14 areslightly more positively polarized than the adjacent ones inthe triple bonds, as indicated in the formulas 13 and 14.The charge distributions seem to be the result of the induc-tive electron withdrawal by the bromine substituent in 13and by the bis(methoxy) group in 14, which is more efficientat the carbon atoms para to these. With the assumption ofa cycloaddition mechanism starting with a nucleophilic at-tack by C-2 of 1,1-dimethoxyethene (7),[27] 11 and 12 arepreferred over their anti isomers, and 8 and 10 are formedin higher combined yield than 9. Alternatively, the anti iso-mer of 11, if formed, might be the more reactive one, thusreacting more rapidly to give 8 and 9 such that a smallamount of the syn isomer 11 remains.

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Hydrolysis of the diacetals 8 and 9 with 50 % HCl/THF(1:1) at reflux for 5 h afforded unprotected 2,7-dimeth-oxytricyclo[6.2.0.03,6]deca-1,3(6),7-triene-4,10-dione (15)and 2,7-dimethoxytricyclo[6.2.0.03,6]deca-1,3(6),7-triene-4,9-dione (16), the first diketones derived from 3 and 4, in88% and 89 % yields, respectively.

The new diketones were characterized by inspection oftheir spectroscopic data, which reflect their symmetries. Asa result of the presence of a center of inversion in 16 butnot in 15 the number of IR absorptions is significantlysmaller for the anti isomer 16. In addition, comparison ofthe mass spectrometric fragmentation patterns of 15 and 16is instructive: the syn isomer 15 shows a molecular ion atm/z = 218 (68%). The fragment at m/z = 203 (100%), corre-sponding to the loss of a CH3 group, is the base peak, fol-lowed by signals of very low intensity at m/z = 189 (�5 %)and m/z = 188 (�5%) as the result of a subsequent methyl-ene or methyl fragmentation. In contrast, for the anti iso-mer 16 the molecular ion at m/z = 218 (100%) is the basepeak, followed by signals at m/z = 203 (21 %) and m/z =189 (12%). Obviously, the loss of a methyl group is the firstfragmentation in 16 as well. However, whereas the subse-

Benzo[1,2:4,5]dicyclobutenones and Their Tricarbonylchromium Complexes

Scheme 1. Resonance formulas of radical cations with m/z = 203 obtained by mass spectrometric demethylation of 2,7-dimethoxytricy-clo[6.2.0.03,6]deca-1,3(6),7-triene-4,10-dione (15). (a) Demethylation at MeO-C-2; (b) demethylation at MeO-C-7.

quent methylene fragmentation is possible for the anti iso-mer 16 with comparable ease, this is not the case for thesyn isomer 15.

This observation is consistent with the correspondingspin delocalization possibilities, as indicated by the reso-nance formulas of the different fragment radical ions of 15.Scheme 1 shows the resonance formulas resulting eitherfrom demethylation of 15 at the methoxy group at C-2 orfrom demethylation of the methoxy group at C-7. In thefirst case (Scheme 1a) a higher degree of delocalization, in-cluding the carbonyl groups, is possible; in the opposite caseit is not (Scheme 1b). In accord with this line of thoughtwe assume that the methoxy group at C-2 is the first to bedemethylated.

Unlike in the case of 15, the sites of the first demethyl-ation in 16 cannot be distinguished because of the center ofsymmetry present in this molecule (Scheme 2).

Scheme 2. Resonance formulas of radical cations with m/z = 203obtained by mass spectrometric demethylation of 2,7-dimeth-oxytricyclo[6.2.0.03,6]deca-1,3(6),7-triene-4,9-dione (16).

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Whereas the mass spectrum of 15 does not show signifi-cant peaks between m/z (%) = 203 (100) [M – CH3]+ andm/z (%) = 160 (16) [M – CH3 – H2CCO – H]+, that of 16also shows fragmentation peaks at m/z (%) = 189 (12) [M –CO – H]+ and m/z (%) = 175 (13) [M – H2CCO – H]+, aswell as at m/z (%) = 161 (15) [M – CH3 – H2CCO]+, indicat-ing a larger variety of fragmentation pathways.

In order to obtain tricarbonylchromium complexes of di-ones 15 and 16 the corresponding diacetals 8 and 9 weretreated with hexacarbonylchromium in dibutyl ether/THF(10:1) at reflux for 20 h. The corresponding chromium com-plexes 17 and rac-19 were obtained in excellent 89% and85% yields, respectively. Subsequent treatment of 17 withformic acid at 25 °C afforded the unprotected complex 18in 89% yield, whereas treatment with HCl (50%) gave 18(65 %) together with the uncomplexed dione 15 (20%). Ace-tal hydrolysis of rac-19 with formic acid gave a 94% yieldof rac-20, whereas hydrolysis with HCl (50 %) gave rac-20(80%) together with the dione 16 (10 %). Compounds 18and rac-20 are the first benzo[1,2:4,5]dicyclobutenone tri-carbonylchromium complexes to be reported.

As a result of the electron-withdrawing properties of thetricarbonylchromium group and the rigidity of the anellatedcyclobutanone ring, the reduction of (benzocyclobutenone)-tricarbonylchromium with lithium aluminum hydride takesplace at –78 °C as an immediate reaction with almost quan-titative yield and diastereoselectivity.[31,32] Because the issueof diastereoselectivity is even more relevant for the doublereductions of the dione complexes 18 and rac-20 we chosethe less reactive, usually more selective, diisobutylaluminumhydride (DIBAL-H) as the reducing agent. Treatment at–78 °C followed by acidic workup afforded the syn diols 21and rac-22 in 90% and 96% yield, respectively. The relative

I. A. Abdelhamid, O. M. A. Habib, H. ButenschönFULL PAPER

configurations were assigned with regard to that of the re-duction of (benzocyclobutenone)tricarbonylchromium[31,32]

and the stereochemical outcomes of nucleophilic attack atvarious (arene)tricarbonylchromium complexes with anel-lated cyclopentanone or cyclohexanone moieties[33] and re-lated substrates.[34] The higher reactivity of lithium alumi-num hydride as the reducing agent results in decreasedselectivity, with formation of unseparated diastereoisomericmixtures of 21 (69 %) and other diols (20%) from 18 andof rac-22 (68%) and other diols (24 %) from rac-20 as indi-cated by the NMR spectra, which, however, showed sub-stantial signal overlap.

In other nucleophilic diadditions the dione complexes 18and rac-20 were treated at –78 °C with vinylmagnesium bro-

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mide (5 equiv.). In each case the double addition took placewith high diastereoselectivity: the adducts 23 and rac-24were obtained exclusively, in 85% and 88% yields, respec-tively. The configurations were assigned with regard tothe corresponding reactions of (benzocyclobutenone)tri-carbonylchromium and related compounds.[31,32,35]

Conclusions

In conclusion, here we report the first synthesis ofbenzo[1,2:4,5]dicyclobutenone derivatives, including theirtricarbonylchromium complexes. Remarkably, the two re-gioisomers 15 and 16 differ significantly in their mass spec-trometric fragmentation patterns. Nucleophilic addition re-actions at the keto functionalities of the complexes 18 andrac-20 afford the corresponding diols with high diastereo-selectivity. Attempts directed towards oxyanion-driven ring-expansion reactions with these substrates, as well astowards the synthesis of tetraketones corresponding to 15and 18, are underway in our laboratory.

Experimental Section

General: All operations were performed under argon by usingSchlenk techniques. Reaction vessels were heated at reduced pres-sure with a heat gun and flushed with argon or nitrogen. This pro-cedure was repeated three times. Solvents were dried and distilledbefore use. Diethyl ether and THF were distilled from sodium wire/benzophenone under nitrogen; petroleum ether (PE), tert-butylmethyl ether (TBME) and ethyl acetate were dried with calciumchloride. Hexane, dibutyl ether, dichloromethane and acetonitrilewere dried with calcium hydride. Column chromatography was car-ried out by using flash chromatography.[36] Silica gel (J. T. Baker,40 µm) was degassed three times by heating it at reduced pressurefollowed by argonization. IR: Perkin-Elmer FT 1710, Golden GateATR. s = strong, m = medium, w = weak, br. = broad. MS: Finni-gan AM 400 (ionization potential 70 eV). FAB-MS: VG-Autospec.LC-MS (ESI): Micromass LCT with Lock-Spray unit (ESI), Loop-Mode, HPLC-Alliance 2695 column (Waters). HRMS: VG-Au-tospec or Micromass LCT spectrometer with Lock-Spray unit(ESI). 1H NMR: Bruker WP 200 (200.1 MHz), AVS 400(400.1 MHz). Chemical shifts δ refer to δTMS = 0.00 ppm or toresidual solvent signals. 13C NMR: Bruker AVS 200 (50.3 MHz)and AVS 400 (100.6 MHz). Chemical shifts δ refer to δTMS =0.00 ppm or to solvent signals. The multiplicity of the signals wasdetermined by ATP or DEPT measurements. Signals with negativephase for CH or CH3 are labeled with “–”, those with positivephase for C or CH2 are labeled with “+”. Melting points (m.p.)were measured with a Büchi apparatus without any correction. Ele-mental analyses were carried out for CHN with an Element VarioEL instrument, with acetanilide as the standard.

Benzocyclobutene Derivatives 8–12: 1,1-Dimethoxyethene (7, 22.0 g,250.0 mmol) and then 1,4-dibromo-2,5-dimethoxybenzene (6,14.8 g, 50.0 mmol) were added dropwise to a suspension of sodiumamide (7.8 g, 200.0 mmol) in THF (150 mL). The mixture wasstirred at reflux for 24 h. After consumption of 6 (GC monitoring)

Benzo[1,2:4,5]dicyclobutenones and Their Tricarbonylchromium Complexes

the black brown suspension was allowed to cool to 25 °C and fil-tered through a 5 mm thick layer of silica gel with elution withTHF (50 mL). After solvent removal the resulting product mixturewas separated by column chromatography (SiO2, deactivated withtriethylamine, 400� 30 mm, petroleum ether/TBME (tert-butylmethyl ether) 7:1, then 5:1).

3-Bromo-2,5,8,8-tetramethoxybicyclo[4.2.0]octa-1,3,4-triene (11):Colorless solid (0.76 g, 2.5 mmol, 5%). M.p. 70–72 °C. 1H NMR(400 MHz, CDCl3): δ = 3.37 (s, 2 H, 7-H), 3.45 (s, 6 H, 2 OCH3),3.82 (s, 3 H, OCH3), 3.97 (s, 3 H, OCH3), 7.03 (s, 1 H, 4-H) ppm.13C NMR (100 MHz, CDCl3): δ = 39.8 (+, C-7), 51.9 (–, OCH3),56.4 (–, OCH3), 60.5 (–, OCH3), 104.9 (+, C-8), 112.9 (+, C-3),120.4 (–, C-4), 125.4 (+, C-6), 133.7 (+, C-1), 145.2 (+, C-2), 150.2(+, C-5) ppm. IR (ATR): ν̃ = 2963 (w), 2940 (w), 2836 (w, OCH3),1578 (w), 1474 (s), 1420 (s), 1327 (s), 1256 (s), 1206 (m), 1146 (s,OCH3), 1074 (s, OCH3), 1059 (m), 1046 (s), 1030 (s), 1019 (s), 1007(s) cm–1. MS (70 eV): m/z (%) = 302 (50) [M]+, 286 (100) [M –CH3]+, 272 (80) [M – OCH3]+, 258 (36) [M – OCH3 – CH3]+, 243(22) [M – 2OCH3]+. HRMS (ESI): calcd. for C12H15

79BrO4

302.0154; found 302.0154. C12H15BrO4 (303.15): calcd. C 47.54, H4.99; found C 47.46, H 4.93.

2,4,4,7,10,10-Hexamethoxytricyclo[6.2.0.03,6]deca-1(8),2,6-triene(8): Colorless solid (8.53 g, 27.5 mmol, 55%). M.p. 54–56 °C. 1HNMR (400 MHz, CDCl3): δ = 3.36 [s, 4 H, 5(9)-H], 3.46 (s, 12 H,4 OCH3), 3.89 (s, 3 H, OCH3), 3.98 (s, 3 H, OCH3) ppm. 13C NMR(100 MHz, CDCl3): δ = 40.1 [+, C-5(9)], 52.0 (–, 4 OCH3), 57.3(–, OCH3), 59.5 (–, OCH3), 104.9 [+, C-4(10)], 128.6 [+, C-1(3)],131.5 [+, C-6(8)], 142.5 (+, C-2), 145.0 (+, C-7) ppm. IR (ATR): ν̃= 2954 (w), 2829 (w, OCH3), 1490 (s), 1422 (s), 1287 (s), 1233 (s),1208 (m), 1142 (s, OCH3), 1128 (s, OCH3), 1071 (s, OCH3), 1044(s, OCH3), 1017 (s, OCH3) cm–1. MS (70 eV): m/z (%) = 310 (42)[M]+, 295 (100) [M – CH3]+, 279 (35) [M – OCH3]+, 265 (23) [M –OCH3 – CH3]+, 249 (46) [M – 2OCH3]+, 219 (13) [M – 3OCH3].HRMS (ESI): calcd. for C16H22O6 310.1416 [M + H]+; found310.1417. C16H22O6 (310.34): calcd. C 61.92, H 7.15; found C61.79, H 6.99.

2,4,4,7,9,9-Hexamethoxytricyclo[6.2.0.03,6]deca-1(8),2,6-triene (9):Colorless solid (3.88 g, 12.5 mmol, 25 %). M.p. 140–142 °C. 1HNMR (400 MHz, CDCl3): δ = 3.45 [s, 4 H, 5(10)-H]+, 3.49 (s, 12H, 4 OCH3), 3.90 (s, 6 H, 2 OCH3) ppm. 13C NMR (100 MHz,CDCl3): δ = 42.9 [+, C-5(10)], 52.1 (–, OCH3), 57.5 (–, OCH3),104.9 [+, C-4(9)], 124.7 [+, C-3(8)], 133.9 [+, C-1(6)], 142.9 [+, C-2(7)] ppm. IR (ATR): ν̃ = 2940 (w), 2834 (w, OCH3), 2080 (w),1490 (s), 1460 (s), 1416 (m), 1279 (s), 1204 (s, OCH3), 1135 (s,OCH3), 1068 (s, OCH3), 1055 (s, OCH3), 1034 (m, OCH3) cm–1.MS (70 eV): m/z (%) = 310 (26) [M]+, 295 (100) [M – CH3]+, 279(32) [M – OCH3]+, 265 (29) [M – OCH3 – CH3]+, 249 (72) [M –2OCH3]+, 219 (20) [M – 3OCH3]+. HRMS (ESI): calcd. forC16H22O6 310.1416 [M + H]+; found 310.1418. C16H22O6 (310.34):calcd. C 61.92, H 7.15; found C 61.89, H 6.79.

3-Bromo-2,5-dimethoxybicyclo[4.2.0]octa-1,3,5-trien-8-one (12):Colorless solid (0.65 g, 2.5 mmol, 5%). M.p. 110–112 °C. 1H NMR(400 MHz, CDCl3): δ = 3.88 (s, 3 H, OCH3), 4.00 (s, 2 H, 7-H), 4.20(s, 3 H, OCH3), 7.24 (s, 4-H) ppm. 13C NMR (100 MHz, CDCl3): δ= 50.1 (+, C-7), 56.7 (–, OCH3), 60.8 (–, OCH3), 110.9 (+, C-3),126.7 (–, C-4) 132.3 (+, C-6), 133.4 (+, C-1), 144.6 (+, C-2), 148.5(+, C-7), 182.9 (+, C-8) ppm. IR (ATR): ν̃ = 3083 (w), 2362 (w),1763 (s, CO), 1492 (s), 1431 (s), 1409 (m), 1359 (m), 1247 (s), 1207(w), 1189 (w), 1070 (w), 1048 (s), 984 (s) cm–1. MS (70 eV): m/z (%)= 257 (97) [M]+, 255 (100) [M – 2H]+, 242 (38) [M – CH3]+, 226(20) [M – OCH3]+, 212 (36) [M – OCH3 – CH3]+. HRMS (ESI):calcd. for C10H9

81BrO3 257.9735; found 255.9724.

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2,7,10,10-Tetramethoxytricyclo[6.2.0.03,6]deca-1,3(6),7-trien-4-one(10): Colorless solid (0.79 g, 3.0 mmol, 6%). M.p. 152–154 °C. 1HNMR (400 MHz, CDCl3): δ = 3.32 (s, 2 H, 9-H), 3.47 (s, 6 H, 2OCH3), 3.91 (s, 3 H, OCH3), 3.92 (s, 2 H, 5-H), 4.14 (s, 3 H,OCH3) ppm. 13C NMR (100 MHz, CDCl3): δ = 41.7 (+, C-9), 49.3(+, C-5), 52.2 (–, OCH3), 57.5 (–, OCH3), 60.7 (–, OCH3), 104.2(+, C-10), 132.9 (+, C-6), 133.0 (+, C-3), 135.7 (+, C-1), 137.9 (+,C-8), 141.4 (+, C-7), 144.9 (+, C-2), 184.4 (+, C-4) ppm. IR (ATR):ν̃ = 2938 (w), 1758 (s, CO), 1570 (m), 1490 (s), 1427 (m), 1415 (w),1381 (w), 1283 (m), 1265 (m), 1231 (m), 1136 (m), 1114 (s, OCH3),1063 (s, OCH3), 1045 (s, OCH3), 1014 (m) cm–1. MS (70 eV): m/z(%) = 264 (56) [M]+, 249 (100) [M – CH3]+, 233 (64) [M –OCH3]+, 219 (38) [M – OCH3 – CH3]+, 203 (34) [M – 2OCH3]+,219 (13) [M – 2OCH3 – 2CH3]+. HRMS (ESI): calcd. for C14H16O5

264.0998 [M + H]+; found 264.0999. C14H16O5 (264.27): calcd. C63.63, H 6.10; found C 63.14, H 6.13.

2,7-Dimethoxytricyclo[6.2.0.03,6]deca-1(8),2,6-triene-4,10-dione(15): Hydrochloric acid (3 n, 20 mL) was added to 8 (3.10 g,10.0 mmol) in THF (20 mL). After the mixture has been stirred for5 h at reflux it was allowed to cool to 25 °C and extracted twicewith CH2Cl2 (100 mL each), and the collected organic layers werewashed with water (100 mL) and dried with MgSO4. After solventremoval at reduced pressure, 15 (1.94 g, 8.9 mmol, 89%) was ob-tained as a buff solid. M.p. 192–194 °C. 1H NMR (400 MHz,CDCl3): δ = 3.96 [s, 4 H, 5(9)-H], 4.00 (s, 3 H, OCH3), 4.21 (s, 3H, OCH3) ppm. 13C NMR (100 MHz, CDCl3): δ = 49.4 [+, C-5(9)],57.8 (–, OCH3), 61.4 (–, OCH3), 118.0 [+, C-1(3)], 134.9 [C-6(9)],140.1 (+, C-7), 144.1 (+, C-2), 183.3 [+, C-4(10)] ppm. IR (ATR):ν̃ = 2922 (w), 1751 (s, C=O), 1561 (m), 1491 (s), 1444 (m), 1297(s), 1054 (s, OCH3) cm–1. MS (70 eV): m/z (%) = 218 (67) [M]+, 203(100) [M – CH3]+, 189 (3) [M – 2CH3]+, 160 (15) [M – 2CO]+.HRMS (ESI): calcd. for C12H10O4 218.0579; found 218.0578.C12H10O4 (218.21): calcd. C 66.05, H 4.62; found C 65.56,H 4.60.

2,7-Dimethoxytricyclo[6.2.0.03,6]deca-1(8),2,6-triene-4,9-dione (16):Hydrochloric acid (3 n, 20 mL) was added to 9 (3.10 g, 10.0 mmol)in THF (20 mL). After the mixture had been stirred for 5 h at re-flux it was allowed to cool to 25 °C and extracted twice withCH2Cl2 (100 mL each), and the collected organic layers werewashed with water (100 mL) and dried with MgSO4. After solventremoval at reduced pressure, 16 (1.92 g, 8.8 mmol, 88%) was ob-tained as a yellow solid. M.p. 236–238 °C. 1H NMR (400 MHz,CDCl3): δ = 3.87 [s, 4 H, 5(10)-H], 4.15 (s, 6 H, 2 OCH3) ppm. 13CNMR (100 MHz, CDCl3): δ = 48.3 [+, C-5(10)], 60.3 (–, OCH3),136.9 [+, C-1(6)], 140.3 [+, C-3(8)], 142.6 [+, C-2(7)], 184.9 [+, C-4(9)] ppm. IR (ATR): ν̃ = 2951 (w), 1752 (s, CO), 1490 (s), 1407(s), 1364 (m), 1257 (s), 1168 (m), 1096 (w), 1049 (s, OCH3) cm–1.MS (70 eV): m/z (%) = 218 (100) [M]+, 203 (20) [M – CH3]+, 189(11) [M – 2CH3]+, 160 (12) [M – 2CO]+. HRMS (ESI): calcd. forC12H10O4 218.0579 [M + H]+; found 218.0579. C12H10O4 (218.21):calcd. C 66.05, H 4.62; found C 65.60, H 4.60.

Tricarbonyl{2,4,4,7,10,10-hexamethoxytricyclo[6.2.0.03,6]deca-1(8),2,6-triene}chromium(0) (17): Compound 8 (3.1 g, 10.0 mmol)and hexacarbonylchromium (3.3 g, 15.0 mmol) in dibutyl ether(100 mL) and THF (10 mL) were heated at reflux for 20 h. Afterhaving cooled to 25 °C the mixture was filtered through a frit cov-ered with a layer (2 cm) of silica gel. The solvents were removedat reduced pressure, and the crude product was purified by flashchromatography (200 � 20 mm, TBME/PE 1:5, then TBME) togive 17 (3.97 g, 8.9 mmol, 89%) as a yellow, moderately air-stablesolid. M.p. 108–110 °C. 1H NMR (400 MHz, CDCl3): δ = 3.39 [s,

I. A. Abdelhamid, O. M. A. Habib, H. ButenschönFULL PAPER4 H, 5(9)-H], 3.49 (s, 12 H, 4 OCH3), 3.76 (s, 3 H, OCH3), 4.0 (s,3 H, OCH3) ppm. 13C NMR (100 MHz, CDCl3): δ = 40.5 [+, C-5(9)], 51.2 (–, OCH3), 58.9 (–, OCH3), 61.4 (–, OCH3), 100.0 [+,C-4(10)], 100.5 [+, C-1(3)], 104.8 [+, C-6(8)], 127.3 (+, C-2), 132.3(+, C-7), 233.7 (CrCO) ppm. IR (ATR): ν̃ = 2944 (w), 2835 (w),1938 (s, CrCO), 1872 (s, CrCO), 1845 (s, CrCO), 1551 (w), 1483(m), 1402 (m), 1285 (m), 1233 (m), 1132 (m, OCH3), 1067 (m,OCH3), 1050 (s, OCH3), 860 (w), 806 (w) cm–1. MS (70 eV): m/z(%) = 446 (15) [M]+, 415 (8) [M – OCH3]+, 362 (43) [M –3CO]+,330 (41) [M –3CO – OCH3 – H]+, 279 (19) [M – Cr(CO)3 –OCH3]+, 249 (19) [M – Cr(CO)3 – 2OCH3]+, 233 (100) [M –Cr(CO)3 – 2OCH3 – CH3]+, 217 (6) [M – Cr(CO)3 – 2(CH3)2-O]+, 203 (24), 52 (29) [52Cr]+. HRMS (ESI): calcd. for C19H22O9Cr446.0669; found 446.0671. C19H22CrO9 (446.36): calcd. C 51.12, H4.94; found C 50.90, H 5.16.

Tricarbonyl{2,7-dimethoxytricyclo[6.2.0.03,6]deca-1(8),2,6-triene-4,10-dione}chromium(0) (18): Formic acid (150 mL) was added to11 (4.0 g, 9.0 mmol) at 0 °C in the dark. The mixture was stirredfor 2 h at 25 °C, becoming orange. After completion of the reaction(TLC), water (200 mL) was added. The mixture was extracted twicewith CH2Cl2 (100 mL each) and the collected organic layers werewashed with water (100 mL) and dried with MgSO4. After the col-lected organic layers had been dried over magnesium sulfate, themixture was filtered through a short column (SiO2, 5 � 2 cm,TBME, then THF), and the solvent was removed at reduced pres-sure to give 18 (2.91 g, 8.0 mmol, 89%) as an orange solid. M.p.147–149 °C. 1H NMR (400 MHz, [D6]acetone): δ = 3.96 (s, 3 H,OCH3), 4.12 (s, 3 H, OCH3), 4.14 (d, 2Jexo-2,endo-2 = 16.8 Hz, 2 H),4.48 (d, 2Jexo-2,endo-2 = –16.8 Hz, 2 H, endo-2-H or exo-2-H, endo-2-H or exo-2-H) ppm. 13C NMR (100 MHz, [D6]acetone): δ = 50.3[+, C-5(9)], 59.0 (–, OCH3), 61.7 (–, OCH3), 93.8 [+, C-1(3)], 105.2[+, C-6(8)], 128.9 (+, C-7), 132.4 (+, C-2), 181.2 [+, C-4(10)], 230.9(CrCO) ppm. IR (ATR): ν̃ = 2947 (m), 2360 (w), 1963 (s, CrCO),1875 (s, CrCO), 1753 (s, CO), 1561 (m), 1491 (m), 1389 (m), 1257(s), 1049 (s, OCH3), 981 (m), 912 (s), 845 (w) cm–1. MS (70 eV):m/z (%) = 354 (6) [M]+, 270 (22) [M – 3CO]+, 255 (9) [M – 3CO –CH3]+, 218 (85) [M – Cr(CO)3]+, 203 (100) [M – Cr(CO)3 –CH3]+, 189 (10) [M – Cr(CO)3 – 2CH3]+, 52 (28) [52Cr]+. HRMS(ESI): calcd. for C15H10O7Cr 353.9832; found 353.9834.

Tricarbonyl{2,4,4,7,9,9-hexamethoxytricyclo[6.2.0.03,6]deca-1(8),2,6-triene}chromium(0) (19): Compound 9 (3.1 g, 10.0 mmol)and hexacarbonylchromium (3.3 g, 15.0 mmol) were heated at re-flux in dibutyl ether (100 mL) and THF (10 mL) for 20 h. Afterhaving cooled to 25 °C the mixture was filtered through a frit cov-ered with a layer (2 cm) of silica gel. The solvents were removedat reduced pressure, and the crude product was purified by flashchromatography (200 � 20 mm, TBME/PE 1:5, then TBME) togive 19 (3.79 g, 8.5 mmol, 85%) as a yellow, moderately air-stablesolid. M.p. 160–162 °C. 1H NMR (400 MHz, CDCl3): δ = 3.43 (s,12 H, 4 OCH3), 3.54 [s, 4 H, 5(10)-H], 3.88 (s, 6 H, 2 OCH3) ppm.13C NMR (100 MHz, CDCl3): δ = 43.0 [+, C-5(10)], 52.0 (–,OCH3), 57.5 (–, OCH3), 93.5 [+, C-4(9)], 104.9 [+, C-3(8)], 105.7[+, C-1(6)], 124.7 [+, C-2(7)], 233.7 (CrCO) ppm. IR (ATR): ν̃ =2939 (m), 2835 (w), 1952 (s, CrCO), 1862 (s, CrCO), 1551 (w), 1490(m), 1277 (m), 1219 (m), 1133 (s, OCH3), 1068 (s, OCH3), 857 (m),776 (m) cm–1. MS (70 eV): m/z (%) = 446 (7) [M]+, 415 (2) [M –OCH3]+, 362 (7) [M –3CO]+, 295 (100) [M – Cr(CO)3 – CH3]+, 279(43) [M – Cr(CO)3 – OCH3]+, 249 (95) [M – Cr(CO)3 – 2OCH3]+,233 (65) [M – Cr(CO)3 – 2OCH3 – CH3]+, 217 (3) [M – Cr(CO)3 –2OCH3 – 2CH3]+, 203 (8), 52 (14) [52Cr]+. HRMS (ESI): calcd.for C19H22O9Cr 446.0669; found 446.0667. C19H22CrO9 (446.36):calcd. C 51.12, H 4.94; found C 50.82, H 4.95.

www.eurjoc.org © 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Eur. J. Org. Chem. 2011, 4877–48844882

Tricarbonyl{2,7-dimethoxytricyclo[6.2.0.03,6]deca-1(8),2,6-triene-4,9-dione}chromium(0) (20): Formic acid (150 mL) was added to 19(4.0 g, 9.0 mmol) at 0 °C in the dark. The mixture was stirred for2 h at 25 °C, becoming brown. After completion of the reaction(TLC), water (200 mL) was added. The mixture was extracted twicewith CH2Cl2 (100 mL each), and the collected organic layers werewashed with water (100 mL) and dried with MgSO4. After the col-lected organic layers had been dried over magnesium sulfate, themixture was filtered through a short column (SiO2, 5 � 2 cm,TBME, then THF), and the solvent was removed at reduced pres-sure to give 20 (2.97 g, 8.4 mmol, 94%) as a brown solid (m.p.152–154 °C). 1H NMR (400 MHz, [D6]acetone): δ = 3.91 [d,2Jexo-5(10),endo-5(10) = –16.6 Hz, 2 H, endo-5(10)-H or exo-5(10)-H],4.07 (s, 6 H, 2 OCH3), 4.26 [d, 2 H, endo-5(10)-H or exo-5(10)-H] ppm. 13C NMR (100 MHz, [D6]acetone): δ = 49.4 [+, C-5(10)],60.6 (–, OCH3), 99.1 [+, C-1(6)], 99.3 [+, C-3(8)], 131.1 [+, C-2(7)],182.8 [+, C-4(9)], 231.9 (+, CrCO) ppm. IR (ATR): ν̃ = 2946 (w),1973 (s, CrCO), 1925 (s, CrCO), 1898 (s, CrCO), 1755 (s, CO), 1549(m), 1493 (m), 1391 (s), 1269 (s), 1174 (m), 1051 (s, OCH3), 978(s), 918 (s) cm–1. MS (70 eV): m/z (%) = 354 (23) [M]+, 270 (95) [M–3CO]+, 255 (25) [M –3CO – CH3]+, 218 (10) [M – Cr(CO)3]+, 203(2) [M – Cr(CO)3 – CH3]+, 189 (2) [M – Cr(CO)3 – 2CH3]+, 52(100) [52Cr]+. HRMS (ESI): calcd. for C15H10O7Cr 353.9832; found353.9833.

Tricarbonyl{2,7-dimethoxytricyclo[6.2.0.03,6]deca-1(8),2,6-triene-endo-4,endo-10-diol}chromium(0) (21): Compound 18 (1.0 g,2.8 mmol) in THF (30 mL) and diisobutylaluminum hydride (DI-BAL-H) (5.4 mL, 1.6 m in toluene, 8.5 mmol) were mixed at –78 °Cand stirred for 30 min at this temperature. After addition of hydro-chloric acid (1 m, 5 mL) at –78 °C the mixture was extracted withdiethyl ether (4�20 mL) and dried with MgSO4. The mixture wasfiltered through a short column (SiO2, 5�2 cm, TBME, then THF)and, after solvent removal at reduced pressure, gave 21 (0.9 g,2.5 mmol, 90%) as a yellow solid. M.p. 146 °C (dec.). 1H NMR(400 MHz, [D6]acetone): δ = 3.16 [dd, 2Jendo -5(9) ,exo -5 (9 ) =–13.4, 3Jendo-5(9),exo-4(10) = 2.1 Hz, 2 H, endo-5(9)-H], 3.62 [dd,3Jexo-5(9),exo-4(10) = 5.6 Hz, 2 H, exo-5(9)-H], 3.77 (s, 3 H, OCH3),3.96 (s, 3 H, OCH3), 4.72 (s, 2 H, 2 OH), 4.82 [dd, 2 H, exo-4(10)-H] ppm. 13C NMR (100 MHz, [D6]acetone): δ = 40.3 [+, C-5(9)],58.1 (–, OCH3), 58.7 (–, OCH3), 66.8 [–, C-4(10)], 101.6 [+, C1(3)],103.0 [+, C-6(8)], 132.8 (+, C-2), 134.2 (C-7), 235.5 (CrCO) ppm.IR (ATR): ν̃ = 2922 (m, OH), 2362 (w), 1941 (s, CrCO), 1842 (s,CrCO), 1480 (s), 1405 (m), 1265 (m), 1039 (s, OCH3) cm–1. MS(70 eV): m/z (%) = 358 (28) [M]+, 302 (12) [M – 2CO]+, 274 (69)[M – 3CO]+, 256 (76) [M – 3CO – OH]+, 241 (37) [M –3CO – OH –CH3]+, 222 (40) [M – Cr(CO)3]+, 203 (62) [M – Cr(CO)3 – OH –2H]+, 179 (45), 52 (34) [5 2Cr]+. HRMS (ESI) : calcd . forC15H14O7Cr 358.0144; found 358.0142.

rac-Tricarbonyl{2,7-dimethoxytricyclo[6.2.0.03,6]deca-1(8),2,6-triene-endo-4,endo-9-diol}chromium(0) (rac-22): Compound rac-20(1.0 g, 2.8 mmol) in THF (30 mL) and diisobutylaluminum hydride(DIBAL-H, 5.4 mL, 1.6 m in toluene, 8.5 mmol) were mixed at–78 °C and stirred for 30 min at this temperature. After additionof hydrochloric acid (1 m, 5 mL) at –78 °C the mixture was ex-tracted with diethyl ether (4�20 mL) and dried with MgSO4. Themixture was filtered through a short column (SiO2, 5 � 2 cm,TBME, then THF) and, after solvent removal at reduced pressure,gave rac-22 (1.0 g, 2.7 mmol, 96%) as a yellow solid. M.p. 138–140 °C (dec.). 1H NMR (400 MHz, [D6]acetone): δ = 3.02 [dd,2Jendo-5(10),exo-5(10) = –13.4, 3Jendo-5(10),4(9) = 2.1 Hz, 2 H, endo-5(10)-H], 3.46 [dd, 3Jexo-5(10),4(9) = 5.6 Hz, 2 H, exo-5(10)-H], 3.93 (s, 6H, 2 OCH3), 5.10 (s, 2 H, 2 OH), 5.42 [dd, 2 H, 4(9)-H] ppm. 13CNMR (100 MHz, [D6]acetone): δ = 39.4 [+, C-5(10)], 58.6 (–,

Benzo[1,2:4,5]dicyclobutenones and Their Tricarbonylchromium Complexes

OCH3), 67.3 [–, C-4(9)], 96.9 (+, C1 or C6), 107.7 [+, C-3(8)], 130.8[+, C-2(7)], 235.3 (+, CrCO) ppm. IR (ATR): ν̃ = 3660 (m, OH),2951 (w), 1946 (s, CrCO), 1854 (s, CrCO), 1481 (s), 1403 (m), 1264(m), 1054 (s, OCH3), 985 (m) cm–1. MS (70 eV): m/z (%) = 358 (12)[M]+, 302 (3) [M – 2CO]+, 274 (25) [M – 3CO]+, 256 (27) [M –3CO – OH]+, 241 (3) [M – 3CO – OH – CH3]+, 222 (100) [M –Cr(CO)3]+, 207 (86) [M – Cr(CO)3 – OH]+, 179 (31), 52 (33)[52Cr]+. HRMS (ESI): calcd. For C15H14O7Cr 358.0144; found358.0142 [M + H]+.

Tricarbonyl{2,7-dimethoxy-4,10-divinyltricyclo[6.2.0.03,6]deca-1(8),2,6-triene-4,10-diol}chromium(0) (23): Compound 18 (0.50 g,1.4 mmol) in THF (30 mL) was added dropwise at –78 °C to vinyl-magnesium bromide (4.4 mL, 1.6 m in diethyl ether, 7.0 mmol) inTHF (30 mL). After stirring at –78 °C for 16 h the mixture washydrolyzed by addition of HCl (1 m, 10 mL). After having warmedto 25 °C the mixture was extracted with portions of CH2Cl2

(30 mL) until the aqueous layer remained colorless. The collectedorganic layers were washed with water (2 � 50 mL), dried withMgSO4, and filtered through a P4 frit covered with a layer of silicagel (2 cm). After solvent removal at reduced pressure the crudeproduct was purified by flash chromatography (SiO2, 30�2 cm,TBME) to give 23 (0.49 g, 1.2 mmol, 85%) as a yellow solid. M.p.98–99 °C (dec.). 1H NMR (400 MHz, CDCl3): δ = 2.56 (br. s, 2 H,OH), 3.47 [br. s, 4 H, endo-5(9)-H, exo-5(9)-H], 3.77 (s, 3 H,OCH3), 3.89 (s, 3 H, OCH3), 5.29 (d, 3J = 10.7 Hz, 2 H, trans-CCH=CHH), 5.49 (d, 3J = 17.1 Hz, 2 H, cis-CCH=CHH), 6.18(dd, 3J = 10.7, 3J = 17.1 Hz, 2 H, CCH=CH2) ppm. 13C NMR(100 MHz, CDCl3): δ = 47.0 [+, C5(9)], 58.9 (–, OCH3), 59.9 (–,OCH3), 101.5 [+, C4(10)], 105.1 (+,=CH2), 115.7 [+, C1(3)], 119.8[+, C6(8)], 126.9 (+, C2), 134.6 (+, C7), 137.8 (–, –CH=CH2), 233.9(CrCO) ppm. IR (ATR): ν̃ = 3416 (m, OH), 2944 (w), 1939 (s,CrCO), 1842 (s, CrCO), 1555 (w), 1485 (s), 1398 (s), 1262 (s), 1060(m, OCH3), 1019 (m, OCH3), 912 (m) cm–1. MS (70 eV): m/z (%)= 410 (13) [M]+, 354 (8) [M – 2CO]+, 326 (76) [M – 3CO]+, 308(100) [M – 3CO – CH3]+, 294 (32) [M – 3CO – 2CH3]+, 274 (3)[M – Cr(CO)3]+, 240 (69) [M – Cr(CO)3 – 2OH]+, 225 (41). HRMS(ESI): calcd. for C19H1 8O7Cr 410.0458; found 410.0460.C19H18CrO7 (410.34): calcd. C 55.61, H 4.42; found C 55.89, H4.48.

Tricarbonyl{2,7-dimethoxy-4,9-divinyltricyclo[6.2.0.03,6]deca-1(8),2,6-triene-4,9-diol}chromium(0) (rac-24): Compound rac-20(0.50 g, 1.4 mmol) in THF (30 mL) was added dropwise at–78 °C to vinylmagnesium bromide (4.4 mL, 1.6 m in diethyl ether,7.0 mmol) in THF (30 mL). After stirring at –78 °C for 16 h themixture was hydrolyzed by addition of HCl (1 m, 10 mL). Afterhaving warmed to 25 °C the mixture was extracted with portionsof CH2Cl2 (30 mL) until the aqueous layer remained colorless. Thecollected organic layers were washed with water (2�50 mL), driedwith MgSO4, and filtered through a P4 frit covered with a layer ofsilica gel (2 cm). After solvent removal at reduced pressure thecrude product was purified by flash chromatography (SiO2,30�2 cm, TBME) to give rac-24 (0.51 g, 1.2 mmol, 88%) as a yel-low oil. 1H NMR (400 MHz, CDCl3): δ = 3.01 (br. s, 2 H, OH),3.32 (br. s, 4 H, CH2), 3.86 (s, 6 H, 2 OCH3), 5.30 (d, 3J = 10.7 Hz,2 H, trans-CCHCHH), 5.52 (d, 3J = 17.2 Hz, 2 H, cis-CCHCHH),6.16 (dd, 3J = 10.7, 3J = 17.2 Hz, 2 H, vinyl-H, CCH=CH2) ppm.13C NMR (100 MHz, CDCl3): δ = 46.6 [+, C5(10)], 59.7 (–, OCH3),96.5 [+, C4(9)], 109.8 (–, =CH2), 113.6 [+, C1(6)], 115.5 [+, C3(8)],130.7 [+, C2(7)], 138.7 (–, CH=CH2), 234.5 (CrCO) ppm. IR(ATR): ν̃ = 3401 (m, OH), 2944 (w), 1943 (s, CrCO), 1840 (s,CrCO), 1639 (w), 1477 (s), 1395 (s), 1261 (s), 1028 (m, OCH3), 988(m), 931 (m) cm–1. MS (70 eV): m/z (%) = 410 (8) [M]+, 326 (12)[M – 3CO]+, 308 (8) [M – 3CO – CH3]+, 294 (3) [M – 3CO –

Eur. J. Org. Chem. 2011, 4877–4884 © 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.eurjoc.org 4883

2CH3]+, 240 (58) [M – Cr(CO)3 – 2OH]+, 218 (100) [M – Cr(CO)3 –2CH=CH2 – 2H]+. HRMS (ESI): calcd. for C19H18O7Cr 410.0457;found 410.0460.

Acknowledgments

I. A. A. is indebted to the Alexander von Humboldt Foundationfor a postdoctoral research fellowship. O. M. A. H. is indebted tothe government of the Arab Republic of Egypt for a graduate fel-lowship. We thank Dr. Jörg Fohrer for assistance with Spartan cal-culations.

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Received: April 19, 2011Published Online: July 8, 2011