Kinetic (not equilibrium) factors are dominant in Wittig reactions of conjugated ylides

Abstract

the Harvard University MRL. The rotating-anode facility is part of the MRL. The synchrotron measurements were carried out at the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory. Research at the NSLS is supported by the Office of Basic Energy Sciences, U.S. Department of Energy, under contract DE-AC02-76CH00016. Ian Tidswell was the recipient of a NATO studentship. We are grateful to Dr. David Osterman and Dr. Thomas Rabedeau for assistance in these experiments, to Dr. Abraham Ulman of Eastman Kodak for providing preprints of articles, and to Dr. Ralph Nuzzo of AT&T Bell Laboratories for helpful discussions.

The modern interpretation of the Wittig reaction mechanism

“…Rebuttal and alternative explanation: Another explanation ([2 + 2] cycloaddition) exists that is consistent with all of the observations that support the betaine mechanism, but which also accounts for several other facts that the betaine mechanism 60 cannot explain. This is discussed in sections 3.8 and 4.…”

“…61 At the time of the development c,62,63 of this mechanism 61 by 60 Wittig and co-workers, there was a significant amount of experimental evidence strongly suggesting the involvement of betaine intermediates in the Wittig reaction. There was also an absence of evidence to the contrary and the alternative explanations for the observed phenomena that have since been 65 amassed had not yet been recognised.…”

“…As a result, these ylides are referred to as stabilised ylides. This name 60 has also come to refer to all ylides in which R 2 is a conjugating group, even though certain ylides derived from phosphines other than triphenylphosphine are somewhat sensitive to hydrolysis in air. b M ost ylides undergo Wittig reactions with carbonyl 65 compounds very rapidly, 3 although quantitative rate data on this subject is sparse.…”

“…Thus allylidene or benzylidene triphenylphosphoranes (Chart 1 (e)) generally react to give approximately equal proportions of E and 15 Z alkene, while semi-stabilised alkyldiphenylphosphine-derived ylides (Chart 1 (f)) show moderate and in some cases very high E-selectivity. 4 Semi-stabilised ylides with two or more of the Pphenyl groups replaced by alkyl groups (Chart 1 (g)) 4 and also those derived from methyldibenzophosphole (Chart 1 (h)) 60 show 20 exceptionally high E-selectivity. As with non-stabilised ylides, the general observation applies that for a given semi-stabilised ylide Z-selectivity is highest for reactions with tertiary aldehydes, and E-selectivity is highest for reactions with primary aldehydes.…”

“…OPAs are also necessarily produced in each of the independent generations of betaine, and they can be detected by NM R in solution in such experiments, and in Wittig reactions. 59,60,68,69,70 Therefore experiments that necessarily involve betaine and that ultimately 20 give alkene and phosphine oxide do not mean that they must be involved en route from ylide and aldehyde to alkene and phosphine oxide in the Wittig reaction. alkylidenetriphenylphosphoranes was rationalised as being a consequence of addition of the ylide to the carbonyl in such a way as to give anti-betaine (with a P-C-C-O dihedral angle of 180°, as shown in Fig.…”