ChemInform Abstract: NACHBARGRUPPENBETEILIGUNG BEI DER UMSETZUNG EINIGER TRIARYLPHOSPHINE MIT BENZYLCHLORID

Abstract

Die Geschwindigkeit der Bildung der Triaryl‐benzylphosphoniumchloride (III) aus den Triarylphosphinen (I) und Benzylchlorid (II) in Benzol/Methanol wird durch ortho‐ständige Methoxyreste in den Phenylkernen (If), (Ih) und (Ik) deutlich erhöht, während ortho‐ständige Methylgruppen (Ib) und (Id) eine merkliche Verringerung der Reaktionsgeschwindigkeit verursachen.

Unequivocal Experimental Evidence for a Unified Lithium Salt-Free Wittig Reaction Mechanism for All Phosphonium Ylide Types: Reactions with β-Heteroatom-Substituted Aldehydes Are Consistently Selective for cis-Oxaphosphetane-Derived Products

“…This arose some years ago, when we reported on a curious phenomenon in the Wittig reactions of triphenylphosphonium benzylides with ortho -substituted benzaldehydes. Strong ortho -effects from substituents on phosphorus were already well known through the work of McEwen and co-workers, , and these had been extended to the Wittig reaction, although with conflicting results. , It was also known that Z -selectivity in stilbene synthesis could be induced by ortho -substituents with heteroatom lone pairs on the aldehyde. , However, remarkably, we found that this latter Z -selectivity could be substantially augmented by an additional ortho -substituent on the benzylide, despite the fact that such a substituent would ordinarily lead to E -selectivity. This counter-intuitive cooperative effect was strong enough to be preparatively useful ( Z / E up to 95:5), and the resulting Z -2,2′-disubstituted stilbenes have been used to good effect in synthesis by others. , At the time, we rationalized the results within the cycloaddition mechanism by proposing that the increased Z -selectivity arose from the induction by the ortho -heteroatom of a lower energy TS in the kinetically controlled cycloaddition step leading to a lower energy cis -OPA.…”

“…This results in a three-center, four-electron (3c4e) bond, with the acceptor orbital being one of the P–C bond σ* orbitals, analogous to the (orthogonal) interaction forming the P–O bond. The proposal of such a through-space interaction has long-time precedent in organophosphorus chemistry, having been used most notably by McEwen and co-workers in simple rationalizations of the rates of quaternization of ortho -substituted arylphosphines and ω- N,N -dimethylaminoalkylphosphines and hydrolysis of ortho -substituted arylphosphonium salts. , More recently, similar through-space interactions have been proposed to explain O–Se peri -interactions in 1,8-substituted naphthalenes , and anthroquinones and large long-range PP coupling constants in 1,8-diphosphanaphthalenes, − a phosphorus-containing carborane, tetraphosphine ferrocenyl complexes, biaryl bisphosphines, and calix[4]­arene bisphosphites …”

Role of through space 2p-3d overlap in the alkylation of (.omega.-N,N-dimethylaminoalkyl)diphenylphosphines and in the alkaline decomposition of related quaternary phosphonium salts

“…Three lines of support for this concept have been offered. (1) An X-ray diffraction study of benzyl(2-methoxyphenyl)diphenylphosphonium bromide has revealed that the P-0 distance is substantially shorter than the sum of the van der Waals radii of phosphorus and oxygen. Furthermore, the bond angles indicate that the o-methoxyphenyl group is actually leaning toward the phosphorus in order to facilitate a P-0 bonding interaction, and energy minimization calculations, in which the total steric energy is made up primarily of bond stretching energies and van der Waals nonbonded interactions, also indicate that a weak P-O bonding interaction exists.…”