Research on Chemical Intermediates, 13 (1990) 1-5 1 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

L O W - T E M P E R A T U R E P H O T O L Y S I S O F D I A Z O K E T O N E S : T H E C O N F O R M A T I O N A L I S O M E R I S M

OF B E N Z O Y L P H E N Y L M E T H Y L E N E A N D T H E I R S P E C T R U M OF A N T H R O N Y L I D E N E

M. T O R R E S , A. C L E M E N T A N D O . P . S T R A U S Z * Department of Chemistry

University of Alberta Edmonton, Alberta, Canada T6G 2G2

(Received 9 August 1989; accepted 12 October i989}

ABSTRACT

Low-temperature photolysis of benzoylphenyldiazomethane, 1, and diazoanthrone, 2, in the polycrystal pha~e resulted in the high-intensity ESR spectra of benzoylphenylmethylene l a and anthronylidene 2a, re- spectively. Photolyses of argon matrix-isolated 1 and 2, coupled with IR spectroscopy, have now been carried out. Photolysis of I yielded the IR spectrum of diphenylketene as the only IR-observable product and indicated the presence in the madriz of the two s-E, s-Z, isomers of 1. On the other hand, photolysis of 2 yielded the IR spectrum of carbene 2a ~ the only observable product.

Direct spectroscopic detection of carbenes was initially confined to ESR and only for ground triplet state species. The high sensitivity of ESR combined with the virtual absence of interferences made the pioneering work of Yager, Wasserman and Trozzolo a great success [1].

Other kinds of spectroscopic methods such as UV, of variable sen- sitivity, and IR, of much lower sensitivity, both subject to interferences and requiring higher concentrations of the intermediate carbenes, have become more readily available with the improvements of low-temperature argon matrix isolation (LTMI) techniques [2]. Initially, LTMI was applied to substituted carbenes with singlet or triplet ground states having high reactivity but being incapable of intramolecular rearrangement [3].

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It has been well established that c~-ketocarbenes undergo Wolff re- arrangement (WR) to the ketene [4] and although the ESR spectra of several ketocarbenes have now been reported [5] their fast WR prevented the buildup of sufficiently high concentrations to be detectable by other means. Chapman et al. [6] have recently circumvented the problem by the use of highly-strained polycyclic c~-diazoketones in which the ring strain slows down, if not prevents, their WR and they reported the first IR and UV spectra of a-ketocarbenes.

We have previously studied the low-temperature photolysis of ben- zoylphenyldiazomethane 1 [7] and diazoanthrone 2 [8] in the polycrystal phase and found that it resulted, not only in the appearance of the ESR spectra of the ketocarbenes, benzoylphenylmethylene, l a , and anthronyli- dene, 2a, respectively, but also in the appearance of quintet state ESR spectra. The latter were assigned to the radical pair formed between two neighbouring triplet ketocarbenes, l a or 2a. The high intensities of the ESR signals of l a and 2a and also the relatively high intensities of the corresponding quintet radical pair signals suggest the possibility of IR de- tection of l a and 2a. To this end we have carried out a LTMI photolysis study of 1 and 2 and report the results here.

The IR spectrum of argon matrix-isolated 1 shows splitting in the CN2 (2095(m), 2070(s) cm -1) and CO (1645(m), 1630(m) cm -1) absorp- tion bands. Upon photolysis with either A = 365 or 435 nm (isolated from the radiation of a medium pressure mercury lamp with appropriate interference filters) all the bands of the initial deposit suffer an inten- sity reduction with the concomitant appearance of new bands at 2100(s), 1600(m), 1500(m), 760(s) and 690(s) cm -1. These new b a n d s can be as- signed to diphenylketene by comparison with its published spectrum [9]. However, not all the bands of the initial deposit diminish at the same rate; bands at 2095(s), 1630(s), 1290(m), 1270(s), 915(w), 785(w) and 660(w) cm -1 either disappear or diminish much more rapidly than the rest of the spectrum. Extended photolysis at A -- 365 nm resulted in a slow but steady decrease of the band intensities of the rest of the initial spectrum and a steady increase in the intensities of the diphenylketene bands. No new absorptions that could be assigned to l a were observed.

These results suggest the presence of two rotational isomers of 1 in the matrix. The presence of an isomeric oxadiazole [10] (for which no diazo or carbonyl absorptions are expected) or diazirene (for which a carbonyl but no diazo absorption is expected) can be excluded on the basis of the strong diazo and carbonyl absorptions in the spectrum and their concomitant disappearance upon photolysis.

A similar (2088(m), 2071(s) cm -1) split in the CN2 band of 1 in CCI4 solution has been reported by Paliani et al. [11] who assigned these bands to the s-E and 8-Z conformers of 1, respectively.

The split in the diazo band of c~-diazoketones, as an indication of geometric isomerism, has drawn strong criticism based on the fact that a similar split is also present in some cr-diazoketones with an s-Z fixed conformation [12]. Moreover, only the s-E conformer of 1 could be de- tected in solution with lSC NMR [13] and in the crystalline phase, X-ray diffraction also showed the presence of only the s-E conformer [14]. The splitting of the IR spectrum observed here, however, proves the presence of both conformational isomers in the matrix deposit of 1. Deposition of 1 was carried out by sublimation of the crystals onto the cold plate of the cryostat and therefore the equilibrium distribution of the conformers in the deposit is expected to represent the gas-phase conformational equilib- rium. Also, it is reasonable to expect a similar equilibrium distribution in the solution phase using non-polar solvents, in line with the IR spec- troscopic observation [11], but the reason why the equilibrium was not observed in the 13C NMR study remains unexplained.

c~-Diazoketones are more stable in their s-Z conformation in the ab- sence of steric hindrance [15] and they will mimic the behaviour of vinyl ketones in which the stronger CO, CH2 coupling in the s-Z conforma- tion results in a higher CO and lower C=CH2 absorption wavenumber for the s-Z than for the s-E conformer [16]. Therefore, we assign the 2095 and 1630 cm -1 absorptions to the s-Z, and the 2070 and 1645 cm -1 ab- sorptions to the s-E conformers of 1, respectively, which is the reverse of Paliani et al.'s [11] assignment. Thus we conclude that in an argon matrix s-Z 1 undergoes photolysis at a faster rate than s-E 1, in agreement with the solution photolysis of cr-diazoketones, in which the preferred WR for the s-Z diazoketenes and/or the s-E ketocarbenes is well documented [17].

Photolysis of diazoanthrone 2 represents a different case, in that its solution-phase photolysis results in the formation of azine, bianthronyl and abstraction-recombination products of triplet 2a [18]. Photolysis of argon matrix-isolated 2 at A = 365 nm resulted in the disappearance of the initial diazoanthrone spectrum and in the appearance of a new spectrum with absorption at 1625(m), 1580(s), 1290(m), 1145(w), 1025(w), 925(m), 755(s) and 660(m) cm - I , assigned to anthronylidene 2a, vide infra. Sub- sequent irradiation at ~ > 210 nm did not cause any further change in the spectrum of 2a, indicating its resistance to photoisomerization.

Assignment of this new spectrum to anthronylidene 2a is based on the expected reactions [6] in reactive matrices according to the scheme:

N2

-N 2

O O

0 II C

3

0

4 (Mw ~ 224)

O

O S (MW = 208)

Thus, photolysis of 2 in an argon matrix containing 10% CO resulted, in addition to the spectrum of 2a, in a ketene absorption at 2110 cm -1. The absorption of ketene 3 grew after annealing the photolyzate at 25K. Likewise, when the photolysis of 2 was carried out in an argon matrix containing 20~0 O2, in addition to 2a, the spectra of two new species were observed, one with a characteristic CO absorption at 1750 cm -1, assigned to lactone 4, and another one assigned to anthraquinone 5, by comparison with the argon-matrix IR spectrum of an authentic sample. Annealing the photolyzate at 25K caused a decrease in the 2a spectrum intensity and an increase in the spectral intensities of 4 and 5. Mass spectral analysis of the photolyzate after warming up to room temperature indicated the presence of two compounds with the proper molecular masses of 208 and 224.

A C K N O W L E D G E M E N T

We thank the Natural Sciences and Engineering Council of Canada for continuing financial support.

R E F E R E N C E S

i. A.M. Trozzolo, Acc. Chem. Res., 1(1968)329 and references therein. 2. B. Meyer, Low Temperature Spectroscopy, Elsevier, Amsterdam, 1971.

3. L. Andrews, in H.E. Hallam (F_xL), Vibrational Spectroscopy of Trapped Species, John Wiley and Sons, London, 1978, p. 192.

4. W. Kirmse, Carbene Chemistry, 2nd ed., Academic Press, New York, 1971; R.A. Moss and M. Jones (F_.ds.), Carbenes, Wiley, New York, 1973; H. Meier and K.P. Zeeler, Angew. Chem. Int. Ed., 14(1975)32; M. Torres, E.M. Lown, H.E. Gunning, and O.P. Strausz, Pure Appl. Chem., 52(1980)1623.

5. For a review see M. Torres, I. Safarik, H. Mural, and O.P. Strauss, Rev. Chem. Int., 7(1986)243.

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