3076 A zyxwvu Stereochemical Approach toward a More Detailed Understanding of Electron Impact Induced Elimination Reactions''' Mark M. Green, Richard J. Contribution from the Department zyxwv of Chemistry, University of Michigan, Ann Arbor, Michigan 48104. Received September 19, zyxw 1969 John M . S ~ h w a b , ~ ~ and R. B. Abstract: Electron impact induced elimination reactions involving the loss of small molecules, such as water and hydrogen chloride from alcohols and chlorides, possess considerable potential for the application of mass spectrometry to stereochemical problems. In the present work the elimination of water and hydrogen chloride has been found to obey identical steric driving forces in acyclic systems following the observation of equivalent propensity for the abstraction of the pro-R over the zyxwvut pro3 hydrogen at C-4 in (9-2-pentyl alcohol and chloride. This similarity is not extended entirely in cyclohexanol and cyclohexyl chloride. Diastereotopicdeuterium labeling has revealed that zyxwvutsr 1,4 elimination in both compounds is stereospecific while the 1,3 process is only stereospecific in the chloride. Ring opening has been proposed as a prerequisite to 1,3 elimination in cyclohexanol and this hy- pothesis allows correlation of the apparently dissimilar behavior of cyclic and acyclic alcohols and as well explains such anomalous results as a zyxwvutsr 90/1 difference for loss of water from the stereoisomers of 4-t-butylcyclohexano1 while the derived chlorides exhibit stereoinsensitiveloss of hydrogen chloride. Deuterium scrambling is found not to in- tervene in both low and high voltage measurement in these systems and thus offers a counterpoint to recent exam- ples in ketones. A general theory is proposed connecting site selectivehydrogen abstracting elimination reactions to the size of the heteroatom involved and findings on the model compounds studied here suggest that ground-state interatomic distances may be extrapolated to the ionized states in the mass spectrometer and thereby used to predict the positions of hydrogen abstraction. he mass spectra of aliphatic alcohols and chlorides T appear straightforward. Elimination of the func- tional group as HX is a highly favored fragmentation4 as might be assumed, resting on analogy with thermal pr0cesses.j This seeming analogy breaks down on detailed examination. Deuterium labeling studies6-* have demonstrated, in contrast to pyr~lysis,~ that loss of water and hydrogen chloride do not occur by a 1,2 elimination. Even more striking is the fact that the hydrogen involved in the loss of water from acyclic alcohols comes unexpectedly and with high specificity from C-4 while loss of hydrogen chloride from acyclic chlorides8 occurs predominantly by hydrogen abstrac- tion from C-3 (Figure 1). Additional interest in these eliminations arises from the behavior of the diastereo- mers of 4-t-butylcyclohexano1 and its derived chloride. That is, while the alcohols demonstrate high stereo- specificity for water loss, the chloride isomers (Figure 2)9 exhibit little difference for elimination of hydrogen chloride. (1) This work was supported by grants from the Petroleum Research Fund of the American Chemical Society, Research Corporation and the National Institute of General Medical Sciences. (2) Portions of this work have been reported: M. M. Green and J. Schwab. Tetrahedron Lett.. 2955 (1968): M. M. Green. J. Amer. Chem. Soc.., 90, 3872 (1968); .M. M.'Green and R. J. Cook, ibid., 91, 2129 (1969). ~ \ - - , (3) (a) National Science Foundation Summer Fellow, 1968; (b) visiting professor on leave from B. S. College, Dinapore, Magadh University, Bihar, India. (4) For early studies discussing these fragmentations, see F. W. Mc- Lafferty, Anal. Chem., 34, 2 (1962); R. A. Friedel, J. L. Schultz, and A. G. Sharbey, Jr., ibid., 28, 926 (1956). (5) A. Maccoll, Chem. Rev., 69,33 (1969). (6) W. Benz and I<. Biemann, J. Amer. Chem. Soc., 86, 2375 (1964). (7) S. Meyerson and L. C. Leitch, ibid., 86, 2555 (1964). (8) A. M. Duffield, S. D. Sample, and C. Djerassi, Chem. Commun., 193 (1966). (9) The alcohols have been reported by (a) C. E. Brion and L. D. Hall, J. Amer. Chem. Soc., 88, 3661 (1966); (b) L. Dolejg and V. Hanus', COIL Czech. Chcm. Commun., 33, 332 (1968); (c) M. M. Green, R. J. Cook, W. Rayle, E. Walton, and M. F. Grostic, Chem. Commun., 81 (1969). We have measured the spectra of the chlorides in the present work. Following the expectation that bond-forming re- actions (i.e., rearrangements) will best allow the ap- plication of mass spectrometry to stereochemical problerns,l0*l1 and noting the common occurrence of these simple eliminations, we were prompted to investi- gate these processes in more detail. Results Pentyl Alcohol and Pentyl Chloride. The reason for the difference in site selectivity for loss of hydrogen chloride and water is not readily apparent. Both fragmentations are not a function of source pressure, and thus are unimolecular, and as well likely take place in one step from the respective molecular ions, since corresponding metastable peaks are observed. l3 Study of the elimination of water from the secondary alcohol 2-pentanol allows a new kind of insight into the mechanism. By virtue of the chiral carbon in 2-pentanol, each methylene group now presents to the hydroxyl function stereochemically distinguishable (diastereotopic) hydrogens for abstraction and forma- tion of water., These arguments apply identically to the 2-pentyl chlorides. If one could substitute deu- terium in turn for the diastereotopic hydrogens on the methylene group of interest (e.g., C-4), the details of the approach of the X group might be revealed by observing the relative propensity for abstraction of the diastereotopic deuteriums. This goal may be reached (10) S. Meyerson and A. W. Weitkamp, Org. Muss. Specfrosc., 1, 659 (1968). (11) R . B. Roy and M. M. Green, 158th National Meeting of the American Chemical Society, New York, N. zyxw Y., 1969, Abstract 0-64. (12) There is no significant change in the relative intensity of the M - HX peaks as a function of pressure in the source on either the MS-902 or the CEC-103C. In addition the relative intensity of meta- stable peaks for loss of water from cyclohexanol is independent of the pressure in both the source and the analyzer region of the MS-902 mass spectrometer. (13) See F. W. McLafferty and R . B. Fairweather, J. Amer. Chem. Soc., 90, 5915 (1968), for a discussion of metastable abundances includ- ing the systems of interest here. Journal ojthe American Chemical Society-/ 92:lO May 20, 1970