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Mass Spectrometry of Organic Compounds. IX.'" McLafferty Rearrangements in Some Bicyclic Ketones J. D. Henionlb and David G . I. Kingston*lc Contribution from the Department of Chemistry, State University of New York at Albany, Albany, New York 12222, and the Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061. Received June 23, 1973 Abstract: An examination of the mass spectra of isotopically labeled bicyclo[4.2.l]nonan-9-one(l),bicyclo[5.2.1]decan-10-one (Z), bicyclo[4.3.lldecan-lO-one(3), and bicyclo[5.3.l]undecan-ll-one(4) has shown that the ions of m/e 84 and 98 in the spectra of 2 and 4 most probably arise by McLafferty rearrangement. This conclusion is noteworthy in view of the large angle that the y hydrogens must make with the plane of the carbonyl group in these compounds.
A
mong the many and varied itinerations undergone by hydrogen atoms on electron impact of organic molecules,2 none has received as much attention as the rearrangement that has become known as the McLafferty rearrangement (eq 1). A recent review3 lists , O D R
----t
1 : fR +
(1)
over 250 references dealing with the mechanistic aspects of this and related rearrangements, and the rearrangement has been used times without number for purposes of structural elucidation. It has become important for diagnostic purposes precisely because it is so well documented and well understood. Thus, it is known3 that rearrangement involves the specific migration of a y-hydrogen atom to the carbonyl oxygen atom, followed by cleavage of the bond between the carbon atoms a: and p to the carbonyl group, to yield (usually) an ionized enol fragment and an olefin. Structural effects on the rearrangement are well documented; formation of stable product ions provides a substantial driving force for the reaction, while the greater tendency for secondary hydrogens to migrate compared with primary hydrogens, or hydrogen atoms compared with deuterium atoms, is well known. Steric effects on the rearrangement are less well understood, however, and the study of one aspect of these effects forms the subject of this paper. Previous studies of the effect of the stereochemistry of the substrate on the McLafferty rearrangement have been limited almost exclusively to an investigation of the maximum value possible for the H,-0 interatomic distance. In a definitive series of papers4 Djerassi and his coworkers showed from their investigation of the rearrangement of various steroids that rearrangement (1) (a) Part VIII: D. G. I. Kingston, H. P. Tannenbaum, and H. G. Kuivila, Org. Muss Spectrom., 9, 31 (1974); (b) abstracted from the Ph.D. Thesis of J. D. Henion. State University of New York at Albany; (c) Virginia Polytechnic Institute and State University. (2) J. T. Bursey, M. M. Bursey, and D. G. I. Kingston, Chem. Reo., 73, 191 (1973). (3) D. G. I. Kingston, M. M . Bursey, and J. T. Bursey, Chem. Reo., 74, 215 (1974). (4) D. H. Williams, J. M. Wilson, H. Budzikiewicz, and C. Djerassi, J . Amer. Chem. Soc., 85, 2091 (1963); D. H. Williams and C. Djerassi, Steroids, 3, 259 (1964); C. Djerassi, G. von Mutzenbecher, J. Fajkos, D. H. Williams, and H. Budzikiewicz, J. Amer. Chem. Soc., 87, 817 (1965); C. Djerassi and L. Tokes, ibid., 88, 536 (1966); L. Tokes, R. T. LaLonde, and C. Djerassi, J . Org. Chem., 32,1020 (1967).
did not occur if the interatomic distance between the y hxdrogen and the receptor oxygen was greater than 1.8 A. Similar conclusions were drawn from a study of the rearrangement of the exo and endo isomers of 2-acetylnorbornane. The other major steric factor which might influence the rearrangement is the angle T between the itinerant hydrogen atom and the plane of the carbonyl group (Figure 1). In normal acyclic aldehydes and ketones this angle can approach 0", and it has been stated6 that "the observed specificity for rearrangement of the y hydrogen is in accord with steric requirements for overlap of the highly directional orbital of the unpaired electron on oxygen." The point of interest in the current work concerns the effect of increasing the angle T from 0" to its maximum value of 90". Will McLafferty rearrangement still occur or will it become inoperative at some value of T between 0 and 90"? If the latter statement is correct, at what angle does the rearrangement become essentially inoperative ? The only work that bears directly on this question is the theoretical study of the McLafferty rearrangement by Boer, Shannon, and McLafferty.' These investigators used nonempirical molecular orbital calculations to estimate the energy of the transition state for hydrogen transfer and showed that transition states in which T is appreciably greater than 0" have significantly higher energies than those in which T is close to 0". They concluded that the planar transition state is the favored one. While this conclusion is undoubtedly valid for the simple system for which the calculations were made, it does not solve the problem of what happens when a molecule is constrained to approach rearrangement through a nonplanar transition state. Some insight into this question may be obtained from studies on various monocyclic ketones.* In these studies it has been shown that McLafferty rearrangement can take place, albeit with reduced ion abundance, in ketones as small as cyclononanone. A study of a
Journal of the American Chemical Sociefy 1 96:8 1 April 17, 1974
( 5 ) A. F. Thomas and B. Willhalm, Helu. Chim. Acta, 50,826 (1967).
( 6 ) H. Budzikiewicz, C. Djerassi, and D. H. Williams, "Mass Spectrometry of Organic Compounds," Holden-Day, San Francisco, Calif., 1967, p 155. (7) F. P. Boer, T. W. Shannon, and F. W. McLafferty, J . Amer. Chem. Soc.. 90,7239 (1968). (8) (a) D. H. Williams, H. Budzikiewicz, Z . Pelah, and C. Djerassi, Monarsh. Chem., 95, 166 (1964); (b) I
