ORGANIC MASS SPECTROMETRY, VOL. 22, 279-282 (1987)

Fast Atom Bombardment Induced Ring Rupture of Silatranes

Lin Yan, Wengang Chai,? Guanghui Wang, Guanli Wu, Kaijuan Lu and Yue Luo Institute of Chemistry, Chinese Academy of Sciences, PO Box 2709, Beijing, China

During a comparison study of the fast atom bombardment (FAB) and electron impact mass spectra of I-organyl- 2,9,10-trioxa-6-aza-l-silatricyclo[4.3.3.O'~6]dodecanes, an unusual ion peak at m / z 164 was noticed in the FAB spectra. Accurate mass measurement indicated that m / z 164 corresponds to protonated l-[N,N-bis(Z- hydroxyethyl)amino]- 1-propanol (BHAP), which is produced by a ring-rupture reaction. Further experiment showed that the intensity of the [BHAP+H]+ ion peak increased markedly as the 7 keV Aro bombardment proceeded, suggesting a FAB-induced reaction. The possible mechanism is also discussed.

INTRODUCTION

Since its first description in 1981, fast atom bombard- ment (FAB) mass spectrometry' has found wide applica- tion in the fields of biomedical and bio-organic chemistry, owing to its simplicity and its ability to pro- duce reliable spectra of non-volatile and/or thermally labile compounds. With this technique, the sample is dissolved in a viscous liquid matrix (usually glycerol), and then bombarded with a beam of energetic particles (such as Aro and Xeo of 5-8 keV). The exact mechanism still remains uncertain, and there is an entire chemistry to be learned before the process is fully understood.

It was well recognized in the early days that proton- ation, cationization and polymerization occurred during FAB mass spectral analysis.2 Recently, more interest has been attracted by FAB-induced reactions. By means of an extensive study of the effect of an Aro beam with an energy of 5 keV on glycerol, Field3 was able to show that new materials were formed during the course of bombardment. Kurlansik et aL4 presented evidence of recombination of molecular fragments. Different types of reductions have been noticed by several a ~ t h o r s . ~ - ~ Some adduct ions, such as [MH+12n]',' [M+2H]+ and [M + 3H]i,7 have also been thoroughly studied.

In this paper the authors describe a ring-rupture reaction, induced by fast Aro bombardment, that was observed during the mass spectral analysis of a new type of organosilicon compound, the silatranes 1-6 (1 -organyl-2,9,10-trioxa-6-aza- 1 -silatrkyclo- r4.3.3 .01,6]dodecanes). These are biologically active compounds and have aroused interest for both theoreti- cal and practical reasons.'

EXPERIMENTAL

The FAB experiment was carried out on a KYKY ZhP-5 double-focusing mass spectrometer (Scientific Instru-

t Author to whom correspondence should be addressed.

1: R=CH, 2: R=CH,CI 3: R=(CH,),CN 4: R=CH,SCN 5: R=C,H,

I 6: R = CH,NHC6H, R

ment Factory, Beijing, China) fitted with a standard KYKY fast atom gun (same supplier). The energy of the Aro beam was generally 7 keV with a gun monitor current of 1.2 mA. An accelerating voltage of 6 kV was used, and resolution was maintained at 1500. FAB spectra were recorded on UV sensitive chart paper. The m / z values and relative intensities were counted and measured, respectively, using manual methods.

In general, samples were loaded on to the copper tip, which has a sample area of 2mm2, by suspension in about lop3 cm3 of glycerol. After insertion of the probe into the ion source, the full spectra were taken in the range m / z 500-70 at a scan speed of 50s dec-'.

For the Aro beam bombardment comparison experi- ment, the sample was mixed with DMF/glycerol (1 : 1 by volume), and the slurry was then agitated by ultra- sound for 20 min (the final temperature of the mixture was about 40 "C). The clear saturated solution was separ- ated and lo-' cm3 loaded on to the probe tip. Immedi- ately after insertion of the probe, repetitive scans were taken, with a scan time of 30 s over the range m / z 350-80 and an interval of 30 s between scans. The accurate mass measurement of the ion at m / z 164 in compound 3 was carried out by using a peak-matching technique on the same instrument at a resolution of 3500. Glucose was mixed with 3 on the probe tip and the peak at m / z 163, [MH - H20]+ (163.0603), was used as a reference.

The electron impact (El) mass spectra were obtained with an AEI MS 50 mass spectrometer (Kratos) and DS-30 data system. The electron energy was 70 eV and the accelerating voltage 8 kV. The ion source was kept at 200 "C and the inlet probe temperature ranged from 50 to 25OoC, depending on the volatility of each com- pound.

The silatranes 1-6 were synthesized for a different purpose."

Received 8 September 1986 Accepted 24 November 1986

0030-493X/87/050279-04$05.00 @ 1987 by John Wiley & Sons, Ltd.

280 LIN YAN ETAL.

Table 1. EI mass spectral data for compounds 1-6

m j z (relative intensity. %) ton 1 2 3 4 5 6

203 (38.8) 237 (2.9) 256 (6.6) 260 (0) 265 (0.5) 294 (4.5) [M-R]+ 188 (73.3) 188 (1 00) 188 (100) 188(100) 188(100) 188(100) [M - R - CZH,O]+ lM(17.3) 144 (13.2) 144 (1 1.9) 144 (13.2) 144 (6.7) 144 (9.6) [M - R - C3H6OIf 130 (12) 130 (3.8) 130 (4.9) 130 (3.9) 130 (1.4) 130 (2.3)

[MI+'

[M - CzH30]' lsa(100) 194 (5.6) 213 (11.4) 217 (0.3) 222 (0.3) - [M - R - CHZO]' 158(19.2) 158 (1.8) 158 (3.4) 158 (1.2) - - [M - R - 2CZH,O]+ 100 (14.5) 100 (4.0) 100 (5.1) 100 (6.5) 100 (2.4) 100 (2.1)

1CSH120NI+ 102 (9) 102 (2.6) 102 (4.9) 102 (4.6) 102(1.9) 102 (2.1)

[C6H140ZN1+ 132 (23.6) 132 (5.7) 132 (19.1) 132 (12.1) 132 (23.9) 132 (8) [CSH1202NI+ 118 (1 1.3) 11 8 (1.4) 118(5.5) 118 (4) 118(7.1) 118(2.7)

[C4HioON 1 + 88 (20.5) 88 (1 5.4) 88 (50.9) 88 (33.2) 88 (63.7) 88 (1 4.3) Other ions 1 16 (25.5) 116 (5.8) l lS(6.1) llS(6.7) 116(2.5) 116(3.4)

56 (16.2) 5 6 ( l l ) 56 (22.3) 56 (16.2) 56 (13.9) 56 (4.4)

42 (24.1) 42 (1 7.2) 42 (27.7) 42 (24.1) 42 (1 3.4) 42 (4.6) 32 (93.9) - 32 (93.2) 32 (30.4) 32 (21.8) 32 (5.3)

74 (1 0.2) - - - 74 (1 0.2) 74 (7)

44 (20.2) 44(10.1) 44 (24.1) 44 (20.2) 44(11.3) 44 (3.5)

at m/ z 150 appeared. The latter corresponds to proton- ated triethanolamine. RESULTS AND DISCUSSION

The EI mass spectral data of compounds 1-6 are given in Table 1. The relative intensities of the molecular ion peaks varied from 0 to 38.8'/0, depending mostly upon the electronic effects of the substituent R." Most of the major fragments observed were even-electron ions, among which [M - R]+ ions formed by the cleavage of the Si-R bond were the most abundant (except in 1). Owing to the stability of the silatrane ring structure towards electron impact, ions produced by ring rupture possess much lower abundances.

The striking feature of the FAB spectra of compounds 1-6 (Table 2) is the very intense peak at m / z 164, which did not appear in the EI spectra at all. Accurate mass measurement indicated that the ion at m / z 164 has the elemental composition C7HI8O3N, which suggested a protonated 1 -[ jV,N-bis(2-hydroxyl)amino]-l-propanol (BHAP), (HOCH2CH2),NCH2CH2CH20H + H', for- med during FAB. The ring rupture was further indicated by the FAB spectra of the symmetrical silatranes 7 and 8,12 where the ion peak at m / z 164 was absent and one

R

Careful comparison was then made to determine the effect of energetic Aro beam bombardment. Compound 3 was bombarded for 18 min until the total ion current decreased sharply. In the course of the irradiation, spectra were taken every minute. Three typical spectra are shown in Fig. 1. In the first several scans, the intensity of the m / z 164 peak was much weaker than that of m / z 188. As the irradiation time was prolonged, the former increased and the latter decreased. Finally, the intensity ratio of m/ z 164 to m / z 188 was reversed. The behaviour of compound 5 under Aro bombardment is depicted in Fig. 2, where the intensity ratio is plotted against scan number and time. As shown in this figure, the intensity

Table 2. FAB mass spectral data for compounds 1-6

Ion 1 m f z (relative intensity. %)

2 3 4 5 6

204 (30.5) 238 (4.4) 257 (1 5.7) 261 (15.8) 266 (1 7.9) - - 330 (2.3) 349 (0.9) 353 (1.9) 358 (2.2) -

P H I +

[MH+BHAP]+ 367(4.2) - 420 (1.9) - 429 (2.2) - 202 (2.1) 202 (5) - 202 (6.2) - -

- 178 (loo) 178 (2.9) 178 (39.7) - -

[MH+G]+

[M-R]' 188 (14.9) 188 (15) 188 (32.9) 188 (26.9) 188 (100) 188 (2.2)

[BHAP+H]+ 164(100) 164 (44.6) 164(100) 164 (1 00) 164 (52.5) 164 (1 00) 146(12.2)

144(12.7) 144 (4.4) lU(6 .1) 144 (3.6) 144 (3.4) 144 (8.3) 132 (1 6.7) 132 (33.8) 132 (26) 132 (1 9.9) 132 (26.9) 132 (9.0)

118 (21.1 ) i i a (30) 118(17.1) 118 (1 1.5) 118 (6.8) 118 (19.4)

102 (12.7) 102 (16) 102 (1 1.6) 102 (9) 102 (4) 102 (1 2.8)

Other ions 146 (1 6.9) 146 (5.9) 146 (9.2) 146(10.9) -

130 (4.5) 130 (5.6) 130 (4.4) 130 (5) - -

106 (5.5) 106 (26.6) 106 (5) - 106 (3.2) -

FAB INDUCED RING RUPTURE 281

M+Hf 4 0.. 20.

ZOO 250 300 50 100 150

201 ,_ ,_ ,_,_ & 0 -

50 100 150

_1_1 200 250 300

M+H+ 20 t 50 100 150 200 250 300

m/z

Figure 1. Typical FAB spectra of compound 3 for prolonged irradi- ation times: (a) 2 min; (b) 9 min; (c) 18 min.

ratio increases dramatically, reflecting a change in the concentration of the m / z 164 and 188 ions in the target mixture.

For comparison, the same sample solution was left in the ion source for the same period of time without Aro bombardment. Two scans were taken at the initial and the final minute. The atom gun was turned on only when the scans were in progress, and so the total irradiation time was about 1 min. Spectra thus obtained were essen- tially identical, and this indicates that the ring rupture is not purely a result of glycerol solution chemistry.

The above results strongly suggested that the ring- rupture reaction was induced by the fast Aro bombard- ment. The exact mechanism is not very clear, but it is unlikely to be a direct outcome of the localized high temperature, of the order of 104K,13 produced by the irradiation.

The kinetic study14 of acid catalysed solvolysis of silatranes in alcoholic solution demonstrated that pro- tonation at the nitrogen site results in the cleavage of the S i c N coordinate bond, followed by extensive sol- volysis of the ring structure. The present authors tend to believe that the same process occurred in the FAB experiment, where protonation was induced by irradi-

16

14

12

OD 10

4 -. F I

- . 8

U c

9 2 6 F 0 -

I c 1 2 3 4 5 6 7 8 9 10 1 1 12

Scan no. Time (rnin)

Figure 2. The intensity ratio of m / z 164 to m / z 188 in the FAB spectrum of compound 5 as a function of scan number and time.

ation and played an important role in the ring-rupture reaction. The reversible reaction for the preparation of silatranes is shown in Scheme 1, where (a) is the alkali- catalysed ring formation and (b) the acid-catalysed ring rupture. In the preparation (Scheme l(a)), R’ is R and R is C2H5, while in the glycerol-assisted FAB experi- ment (Scheme l(b)), R could be either R or H and R could be either CH2CH(OH)CH20H or H. At this stage, it is not clear whether glycerolysis (R”= CH2CH(OH)CH20H) or hydrolysis (R” = H) occurs, since water exists in glycerol in small amounts.

RSi(OR”), + HOCH,CH2CH2N(CH,CH20H)2

E V ‘OHf R’Si(OCH2CHZ),N + 3ROH

I / OCH2CH2CH2

Scheme 1.

Acknowledgements

The authors wish to thank M.S. Xu for his technical assistance in obtaining the EI spectra and L. Jia for useful discussions during this work.

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