J . Org. Chem. 1986, 51, 5078-5082
5078
Gas-Phase Wittig Rearrangement of Carbanions Derived from Benzyl Ethers Peter C. H. Eichinger, John H. Bowie,* and Thomas Blumenthal Department of Organic Chemistry, University of Adelaide, South Australia, 5001 Received M a y 5, 1986
The CA mass spectra of ions PhC-HOR and Ph(R)CH-0- (R = alkyl and phenyl) are very similar, suggesting that the Wittig rearrangement PhC-HOEt Ph(Et)CH-0- occurs in the gas phase. Major fragmentations can be interpreted in terms of 1,2-elimination of groups from the Wittig product ion. For example, 2 H labeling of C,H, and (C6H4)-CH0 PhC-HOEt and Ph(Et)CH-O- show that the major processes produce Me-CH=CH-O+ C2Hs,with the latter process involving prior scrambling of phenyl hydrogens. An analogous process, Ph2CH-0-* (C6H4)-CH0+ C6H6 gives the major peak in the spectrum of "PhC-HOPh". -+
+
Introduction In recent years there have been significant advances in the analytical applications of negative ion mass spectrometry, particularly in the areas of negative ion chemical ionization and fast atom b0mbardment.l These techniques often yield (M-H+)- ions, but since the ions are generally of low energy, they fragment little, thus providing sparse structural information. Collision-induced dissociations of (M-H+)- ions can often provide this structural information together with fundamental information concerning ion One of the better known carbanion rearrangements in the condensed phase is the Wittig rearrangement.4 The rearrangement could, in principle, involve either of the intermediates shown in Scheme I, and to support such mechanistic proposals, it has been shown that aldehydes are often byproducts of the reacti0n.j The migratory aptitude of substituents R' is allyl = benzyl > methyl > ethyl > phenyl in solution reactions! Since the migratory aptitudes are in the order of free radical stabilities,6 the radical pair mechanism for the reaction is f a ~ o r e d . Re~ cent work6-l0 in this area includes analogous rearrangements of thioethersQ and silyl ethers.'O In this paper we address the following question: Can the CA mass spectra of carbanions from benzyl ethers be interpreted in terms of Wittig rearrangements (Scheme I) or are the fragmentations typical solely of the precursor benzyl ether (M-H+)- ions?
Results and Discussion Full experimental details are provided in the Experimental Section. The compounds used for this study together with their spectra are listed in Tables I-IV. Two examples are illustrated in Figures 1 and 2 . (1)Bowie, J. H. Mass Spectrom Rev. 1984, 3, 161.
(2) Bowie, J. H.; Blumenthal, T.; Laffer, M. H.; Janposri, S.; Gream, G. E. Aust. J. Chem. 1984,37,1447. Bowie, J. H.; Stringer, M. B.; Duus, F.; Lawesson. S.-0.: Larsson. F. C. V.: Madsen. J. 0. Aust. J . Chem. 1984, 37, 1619. (3) Stringer, M. B.; Bowie, J. H.; Holmes, d. L. J . Am. C h e m . Soc. 1986, 108, 3338. (4) Wittig, G. Angew. Chem. 1954, 66, 10. Zimmerman, H. E. In Molecular Rearrangements; de Mayo, P., Ed.; Interscience: 1963; Vol. 1,p 345. Baldwin, J. E.; DeBernardis, J.; Patrick, J. E. Tetrahedron Lett. 1970, 353. (5) Hauser, C. R.; Kantor, S. W. J . Am. Chem. Soc. 1951, 73, 1437; Cast, d.; Stevens, T. S.; Holmes, J. J . Chem. Soc. 1960, 3521. (6) Lansbury, P. T.; Pattison, V. A.; Sidler, J. D.; Bierber, J. B. J . Am. Chem. Soc. 1966,88, 78. Schafer, H.; Schollkopf, U.; Walter, D. Tetrahedron Lett. 1968, 2809. (7) Schollkopf, U. Angew, Chem., Int. Ed. Engl. 1970, 9, 763. (8) D'Orchymont, H.: Goeldner, M. P.; Biellmann. 99%, from MeCD,OH, a commercial product]. Compounds I11 and XI1 were prepared from PhCD,Br (,H2 > 99%, from methyl benzoate and LiA1D,22 followed by bromination with PBr,) by the procedure outlined above. The labeled compounds V and VI were prepared from [2H5]and [2,4,6bromobenzene (,H5 > 99%, from [2H6]ben~ene23) respec2H3]iodobenzene (2H, = 98%, from [ 2,4,6-2H3]aniline23), tively, by the following method (cf. ref 24): Lithium (62.5 mg) was added to [2H5]bromobenzene(728.2 mg) in anhydrous diethyl ether (4 mL) under nitrogen and allowed to stir a t 25 "C for 30 min. This solution was added to a precooled (0 "C) suspension of Cu'Br (321 mg) in anhydrous diethyl ether (3.3 mL), the mixture allowed to stir a t 0 "C for 30 min, and then iodomethyl ethyl ether (0.86 g) was added dropwise over a 10-min period. The reaction mixture was allowed to stir a t 0 "C for 2 h, poured into water (10 mL), and extracted with diethyl ether (5 x 5 mL). The ethereal extract was washed with aqueous sodium chloride (saturated, 2 X 5 mL), aqueous sodium metabisulfite (5%, 2 X 5 mL), and water (5 mL) and dried (Na2S04). Removal of the solvent followed by vacuum distillation gave [ 2,3,4,5,6-2H5]benzyl phenyl ether (497 mg, 78%; 2H5> 99%), bp 78-79 "C (18 mmHg). The phenyl methanols XV-XVII were made from the appropriate alkyl bromide [XV (methyl bromide), XVI (ethyl bromide), XVII (MeCD,Br)] by a standard Grignard reaction between the alkylmagnesium bromide and benzaldehyde (yields 70-80% 1. Compounds XVIII and X I X were made from [2H6]bromobenzene and [2,4,6-2H3]iodobenzeneby a standard Grignard reaction between the appropriate phenylmagnesium halide and propional (XVIII, ,H, > 99%, yield 81%; XIX, 'H, = 98%, yield 54%). The diphenylmethanol derivatives XX-XXIII were available from a previous
Acknowledgment. This p r o j e c t was s u p p o r t e d w i t h the a i d of a g r a n t f r o m t h e A u s t r a l i a n R e s e a r c h G r a n t s Scheme. Registry No. I (carbanion), 104977-34-6; I1 104977-35-7; I11 (carbanion), 104977-36-8; IV 104977-37-9; V (carbanion), 104977-38-0; VI 104977-39-1; VI1 (carbanion), 104977-40-4; VI11 104977-41-5; I X (carbanion), 104977-42-6; X 104977-43-7; X I (carbanion), 104977-44-8; XI1 104977-45-9; XI11 (carbanion), 104977-46-0; XIV 104977-47-1.
(carbanion), (carbanion), (carbanion), (carbanion), (carbanion), (carbanion), (carbanion),
(22) Kice, J. L.; Parham, F. M. J. Am. Chem. SOC.1960, 82, 6168. (23) Best, A. P., Wilson, C. L. J. Chem. Soc. 1946, 239. Vogel, A. I. A Textbook of Practical Organic Chemistry, 3rd ed.; Longmans, Green and Co.; New York, 1956; pp 598-599. (24) Whitesides, G. M.; Fischer, W. E.; San Filippo, J.; Bashe, R. W.; 1969, 91, 4871. House, H. 0. J. Am. SOC. (25) Bowie, J. H.; White, P. Y.; Bradshaw, T. K.; J. Chem. SOC.,Perkin Trans. 2 1972, 1966.
