Mass Spectral Fragmentation of Ethene and Propene Dithioketals of Cyclobutanone

Mohammad Mohammadit and James D. VYillett Department of Chemistry, University of Idaho, Moscow, Idaho 83843, USA

A detailed analysis of the electron impact induced fragmentation of ethene and propene dithioketals of cyclobutanone is presented. The tetradeuterio analog of the cyclic dithioketals was prepared as an aid in analyzing the fragmentation patterns. Cleavage of the 1,3-dithiolane ring system is favored at 15 eV, whereas cyclobutane ring cleavage is favored at 70eV. Thus, the primary fragmentation pathway seems to be - -

dependent upon the ionization energy.

INTRODUCTION

In general, it is assumed that stereochemistry (confor- mational effects) plays a small role in directing the electron impact induced fragmentation pathways ob- served in organic molecules.' In the cyclic dithioketals, indications are that this assumption is invalid. Our preliminary examinations2a-2c of the mass spectra of the series of dithioketals has indicated many compet- ing paths, most of which seem to be governed by stabilization of charge by sulfur and pronounced varia- tions in the fragmentation pathways with changes in ring size (ground state geometry). For example, the 1,2-dithiolane cation radical and 1,2-dithiacyclobutane cation radical are formed when the distance between the two sulfur atoms is relatively short.

In addition, when mass spectra of mesocyclic dithio ethers were r e p ~ r t e d , ~ the presence of 1,2-dithiolane and 1,2-dithiane cation radicals were dominant fea- tures of a fragmentation pattern. Because of this un- usual behavior, we decided to investigate the mass spectra of 5,8-dithiaspiro[3.4]octane (1) and 5,9-di- thiaspiro[3S]nonane (2) to see whether there is an interaction between the two sulfur atoms prior to any bond breaking process.

DISCUSSION AND RESULTS

5,S-Dithiaspiro[3.5]octane (1,l') (Figs. 1 and 2)

The parent ion for this compound is of very low intensity. This is unusual for cyclic dithioketals.2

The major fragment occurs at m/z 118 (100%). In the tetradeuterio analog l', the correspond- ing fragmentation results in fragments with two differ- ent m/z values, at m/z 120 (100%) and m/z 122 (97%). Thus, at 70 eV, both pathways occur with approxi- mately equal probability. This confirms that the loss of

'F Author to whom correspondence should be addressed; Depart- ment of Chemistry, University of Tennessee, Knoxville, Tennessee 37916, USA.

I

40

40 t 40 60 80 100 120 140

m/z

Figure 1. Mass spectra of 1 at (a) 70eV and (b) 15eV.

ethene from the parent ion, to produce the major fragment ion, occurs via two pathways, A and B (Scheme 1):

mlz 118 m/z 146

1 mlz 118

Scheme 1

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M. MOHAMMADI AND J. D. WILLETT

40 t

m/.?

Figure 2. Mass spectra of 1’ at (a) 70eV and (b) 15eV.

In the case of the 15eV spectrum, the ion at rnlz 118 is the only observable fragment. The relative intensity of the parent ion was 5%, identical to the value observed in the 70eV spectrum. In the (low ionization energy) spectrum of the deuterium analog, the major ion now appears at mlz 122 (loo%), with the ion at mlz 120 having a relative intensity of 39%. This would indicate that at the lower ionization energy, fragmentation of the sulfur ring is markedly favored over that of the cyclobutane ring. It is worth remembering that the ‘normal’ cleavage of cyc- lobutanes involves loss of e ther~e .~ It appears that in this particular molecule, the loss of ethene from the 1,3-dithiolane ring is exceptionally favorable.

The 70eV spectrum contains only two other frag- ments with relative intensities greater than 10%. These appear at mlz 90 (21%) and mlz 58 (26%). Both are shifte.d by 2 u in the case of the deuterium compounds. The fragment ion at mlz 90 contains two sulfur atoms, whereas the ion at mlz 58 contains only one. The pathways shown in Scheme 2 are in accord with these data.

m/z 118 rnlz 90

f-7

H x+- H

+*

8 H A H

mlz 118 rnlz 58 Scheme 2

262 ORGANIC MASS SPECTROMETRY, VOL. 17, NO. 6, 1982

There is a small fragment (metastable peak) in the 70eV spectrum which appears to arise via loss of a sulfhydryl radical from the major fragment at m/z 118. The fragment at mlz 85 is shifted to m/z 87 (11%) in the deuterated compound, and thus must contain one of the a-carbons of the cyclobutane ring. As the ion contains only two deuterium atoms, it must arise via loss of -SH from mlz 118 produced via pathway B. The hydrogen lost must originate from the 1,3-dithiolane ring (Scheme 3):

mlz 118

H h mlz 85

Scheme 3

We are still not certain as to the exact nature of the fragment ion at m/z 118. It is quite likely that it is best represented by the cyclic dithioester, rather than the 3-membered cyclic disulfide.

5,9-Dithiaspiro[3.5]nonane (2,2’) (Figs. 3 and 4)

The parent ion occurs at mlz 160 with a relative intensity of 31%. The fragment at mlz 132 (100%) contains two sulfur atoms. This fragment shifts to mlz 134 in the tetradeuterio analog, indicating loss of

13

n 60 %s 401 20 -

40 60 80

13,,

) 120 140 m/z

Figure 3. Mass spectra of 2 at (a) 70eV and (b) 15eV.

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MASS SPECTRAL FRAGMENTATION OF ETHENE AND PROPENE DITHIOKETALS OF CYCLOBUTANONE

I ‘- 100

6ot P

134

40t I I ‘Y 20

40 60 80 100 120 140 160 m h

Figure 4. Mass spectra of 2’ at (a) 15eV and (b) 70eV.

ethene from the C4 ring (Scheme 4):

mlz 160 rnlz 132 Scheme 4

The remainder of the spectrum is quite complex and consists of a series of fragments all having relative intensities under 20%, with the exception of the two ions at mlz 45 and mlz 58 (54% and 53% rel. int., respectively). The fragment ion at mlz 45 remains unchanged in the deuterated analog, whereas the ion at mlz 58 is shifted to mlz 60 (Scheme 5).

a+ 74(21%)

2 mlz 132 m/z 90 mlz 58 Scheme 5

The fragment at rnlz 58 could arise via any one of several different routes. We are certain, however, that the methylene carbon is derived from one of the a-carbons of the cyclobutane ring. An ion at mlz 74 is seen and is not changed on deuteration. This favors loss of thietane from mlz 132 as a source of the ion at mlz 58.

Two fragments of interest occur at mlz 113 (22%) and 118 (10%). Both retain all deuteriums in the

spectrum of the labeled material, being shifted to mlz 117 and mlz 122, respectively. From isotopic abun- dance measurements, it was determined that the ion at mlz 118 contains two sulfur atoms, while the ion at mlz 113 contains one sulfur. These two ions seem to result from cleavage of the 1,3-dithiane ring, and the pathways are substantiated by the presence of the appropriate metastable peaks.

The ion at mlz 118 is thought to arise via loss of C3H6 from the hetero-ring (Scheme 6): n

2 rnlz 118 (122 in deuterio analog)

s-s+ [ x H H

mlz 90 (92 in deuterio analog)

Scheme 6

This pathway is substantiated by the spectrum of the deuterated compound in which the mlz of this frag- ment is 122. It may be that the ion at mlz 90 (found at rnlz 92 in the deuterio analog) arises from the frag- ment at rnlz 118 via loss of ethene.

The ion at m/z 113 (22%) is more difficult to explain. There is a metastable peak indicating its direct formation from the parent ion via loss of 47 u. As this ion contains only one sulfur atom, it is appar- ent that this must represent the loss of a methyl thiol radical. In the spectrum of the deuterated compound, this ion does not lose deuterium (path B); however, the ion at mlz 117 (in the deuterio analog) has a relative intensity of only 15% and a new peak is present at mlz 114 (in the deuterio analog) (11Y0). This may indicate that there are two pathways opera- tive. One results in loss of no deuterium from the cyclobutane ring, and one results in the loss of three deuteriums from the cyclobutane ring. We are thus faced with two distinct pathways both giving rise to ions at rnlz 113 in the nondeuterated molecule, (A and B) (Scheme 7).

Thus, we have competition between cleavage of the cyclobutane ring and the 1,3-dithiane ring. The latter predominates at 70 eV. This phenomenon is even more pronounced in the case of the 15 eV spectra, for here both ions occur with a much greater relative intensity.

The only remaining ions of importance in the 70 eV spectra occur at mlz 99 and mlz 100 (both 20%). These are shifted to mlz 102 and 103 in the case of the deuterated compound. The importance of these fragments is made obvious on examination of the 15 eV spectra where only the ion at mlz 100 (87%) is seen, this now being the most abundant fragment in the spectrum. It was assumed initially that the ions at mlz 99 and mlz 100 resulted from simple expulsion of thiirane (episulfide) from the parent ion followed by loss of a hydrogen atom. However, if this were the

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2 mlz 113 Scheme 7

case, we would expect a shift of 4 u on deuteration. We find instead a shift of only 3u, indicating that there is a loss of one a-deuterium during the forma- tion of this ion. It is also noteworthy that there is a metastable peak present for the formation of the ion at m/z 100 from the parent ion. The following is our rationalization of these data (Scheme 8):

‘I/ !S

0 2

Scheme 8

In the deuterated material the shift of mlz 60 to m/z 61 cannot readily be seen due to interference by the isotope peak of the ion at mlz 60.

C ~ N C ~ ~ ~ I ~ N S

In comparing the spectra of 5,8-dithiaspiro[3,4]octane (1) and (5,9-dithiaspiro[3.5]nonane (2), the first obvi- ous difference is in the intensities of their respective molecular ions (1 = 5% and 2 = 31%). The next rather striking difference relates to the ease of elimination of ethene and its origin. In 1 there is extremely facile loss of ethene which occurs equally from the 4-membered ring and the dithiolane ring in the 70eV spectrum. However, at ZSeV, loss of ethene from the 1,3- dithiolane moiety predominates (100 : 40) which is bet-

ter than 2: 1. Loss of ethene is also seen in compound 2, but such loss occurs exclusively from the cyc- lobutane ring system. The loss of cyclopropane from the sulfur ring does occur, but even in the case of the 15 eV spectrum where it is most intense, it represents a minor pathway (12*/0).

Again, however, if one considers the degree of cyclobutane ring cleavage versus cleavage of the heteroatom-containing ring, it is evident that both processes occur with about equal facility. This is par- ticularly evident from examination of the 15 eV spectra.

It is intriguing that there is such a facile loss of 60 u in compound 2 particularly in view of the evidence that this ion does not involve a simple loss of episulfide from the parent ion. The nature of this fragmentation process deserves further inquiry.

If one examines the 15eV spectra, one can derive the following assessment of cyclobutane versus hetero- ring cleavage pathways: in 1, cleavage of the cyc- lobutane ring and cleavage of the 1,3-dithiolane ring occur with equal frequency at 70 eV, but at 15 eV, hetero-ring cleavage predominates, i.e. 7 1% versus 29%. In 2, the cleavage of cyclobutane versus 1,3- dithiane is 58% to 42%, cyclobutane ring cleavage predominating.

EXPERIMENTAL

The 5,8-dithiaspiro[3.4]octane (1) and its 1,1,3,3-d4- analog (l’)5a-5c and 5,9-dithiaspiro[3.5]nonane (2)6 and its 1,1,3,3-d4-analog (2‘) were prepared by the literature methods. All compounds for mass spectral analysis were purified and checked for putity by gas chro~atography. The mass spectra were obtained with an Hitachi Perkin-Elmer RMU-6E instrument at 70 and 15eV.

REFERENCES

1. K. Eieman, Mass Spectrometer, Organic Chemical Appfica- tions, pp. 144-153. McGraw-Hjll, New York (1962).

2, (a) M. ~ohammadi, The Electron-tnduced-Fra~mentation of Some Cyclic Dithioketais, University of Idaho, MS Thesis (1971); (b) J. D. Willett, The Photochemistry of Some Cyclic Dithioketals, MIT. PhD Thesis 119653: (c) J. D. Willett. J. R.

4. J. Laune, Ind. Chim. Beige. 27, 245 11962). 5. (a} J. Seibt and T. Gaumann, Helv. Chim. Acta 46, 2857

(1963); (b) D. Djerassi, ~ o ~ a t s h . Chem. 95,166 f1964); (c) M. Fuhrer, Hs. H. Gunthard, Helv. Chim. Acia. 45 2036 (19623.

6. D. Seebach, N. R. Jones and E. J. Corey, J. Org. Chem. 33, 300 119681.

Grunwell and G. A. Berchtold, J. Org. Chem. 33,2297 (1968). 3. W. K. Musker, B. V. Gorewit, P. B. Roush and T. L. Wolford,

J. Org. Chem. 43, 3235 (1978).

Received 27 July 1981; accepted 15

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