Wit - J . A m. Chem. SOC.1 992,114, 813-821 8 13 A...

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J. Am. Chem. SOC. 1992,114, 813-821 813 A Theoretical Study of the Wittig Olefination Reaction: MNDO-PM3 Treatment of the Wittig Half-Reaction of Unstabilized Ylides with Aldehydes’ Frank Mari, Paul M. Lahti,* and William E. McEwen* Contribution from the Department Chemistry, University Massachusetts, Amherst, Massachusetts 01003. Received March 18, 1991 Abstract: As with our previous report (Man, F.; Lahti, P. M.; McEwen, W. E. Heteroatom Chem. 1991,2,265) on the mythical Wittig half-reaction (H3P-CH2 + CHzO - the oxaphosphetane), use of the MNDO-PM3 molecular orbital method on the reaction of (C6HS)$=CHCH3 and the mythical analogue H3P-CHCH3 with aldehydes (acetaldehyde, propargylaldehyde, and glyoxylonitrile) to give the respective Z- and E-diastereomeric oxaphosphetanes has provided evidence that the mechanisms are best described as very asynchronous cycloadditions (borderline two-step mechanisms). The geometries of the transition states of these model reaction are such that the P, C, C, and 0 atoms that interact in the bond-breaking-forming process lie in the same plane. The C-C distance in the transition state varies from 1.96 8, to 2.14 A, whereas the P-O distance varies from 2.48 to 2.94 8,. Analysis of the evolution of the bond orders of these reactions shows that each C-C bond is about 3040% formed in the respective transition states, whereas no P-O bond has been formed to any significant degree. An analysis of localized molecular orbitals througout each reaction leads to the same conclusions. For the formation of the Z oxaphosphetanes, the enthalpies of activation AH* were calculated to be 1.5 to 3.5 kcal/mol higher than the AH* for the formation of the E oxaphosphetanes. The enthalpies of reaction, AHr, were calculated to be kcal/mol less negative for the formation of the Z oxaphosphetanes when compared to the formation of E oxaphosphetanes. In addition, values of AH’ are about 10 kcal/mol less and the values of AHr 5-8 kcal/mol less negative for reactions with H3P=CHCH3 than for the respective ones of (C6H5)3wHCH3 with the same aldehydes. Thus, the presence of the phenyl groups bonded to phosphorus makes an important contribution to the steric strain of formation of each oxaphosphetane and to the transition state leading to it. The calculated decreases as the electrophilicity of the aldehydes is increased, which is in good agreement with experimental evidence. The nature of the transition states suggests that along the reaction coordinate for real Wittig reactions carried out in polar solvents or in the presence of metal ions, the reactants do not have to be restricted to the ‘syn” approach. An ‘anti” approach of the reactants is considered as a possibility to explain Z stereoselectivity of some Wittig reactions. Introduction The unusual 2 stereoselectivity of alkene formation is perhaps the most striking mechanistic feature of the salt-free Wittig re- actions of certain phosphorus ylides2 (Scheme I).
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This note was uploaded on 03/07/2010 for the course CHEM 12A taught by Professor Alston during the Spring '10 term at Saddleback.

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Wit - J . A m. Chem. SOC.1 992,114, 813-821 8 13 A...

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