Theoretical Investigation of Stevens Rearrangement of P and As Ylides. Migration of H, CH₃, CHCH₂, SiH₃, and GeH₃Groups on P and As Atoms

Abstract

Ylides are considered as intermediates of the well-known Stevens rearrangement. To investigate the mechanism of this rearrangement, theoretical calculations for the rearrangements of monosubstituted ylides, ZH 2 MCH 2 f H 2 MCH 2 Z (Z ) H, CH 3 , CHdCH 2 , SiH 3 , and GeH 3 ; M ) P and As), were performed with the Møller-Plesset perturbation theory up to the fourth order. The IRC calculations at the RMP2 level show that the reaction mechanism is strongly dependent on the migrating group. While the methyl and vinyl migrations are antarafacial with large activation energies of 37-47 kcal/mol, the IRC of the SiH 3 migration along with that of GeH 3 migration displays that it is a suprafacial process. While the migration of the groups with the heavier elements of Si and Ge utilizes their hypervalency to have a small activation energy of 13 kcal/mol, the transition states for the migrations of the methyl and vinyl groups as well as the hydrogen atom are higher in energy. The RMP2 transition state for the methyl migration is less stable than the radical dissociation limit, H 2 MCH 2 + Z, the radical dissociation-recombination path being more favorable, and it may not exist as real. On the other hand, the transition state for the vinyl migration as well as that for the hydrogen migration is slightly more stable than the dissociation limit. If the entropy effect is properly taken into account, the radical dissociation-recombination path would be more favorable than the concerted mechanism for these migrations as well.

Carbene Complexes of Nonmetals

“…[44] In the oxonium ylide series, the detection of radical-derived products (e.g. RϪR) and a migratory aptitude of R that correlates with radical stability are indicative of the homolysis/ radical recombination mechanism as well.…”

One pot synthesis of a chiral N-phosphine substituted iminophosphorane: X-ray structure and in situ NMR study

“…1 H and 31 P{ 1 H} were recorded for all temperatures, while 13 C{ 1 H} APT, DEPT, 2 D 1 H, 13 C gHMQC and gHMBC were measured for selected temperatures. A 1 H, 15 N gHMQC spectrum acquired at Ϫ20 ЊC showed only the 15 N signal corresponding to the NaNH 2 (δ Ϫ387 ppm). ** The ylidic carbon of 4 could not be identified by APT or HMBC NMR experiments.…”

“…14 However, the conjugation of the P᎐ ᎐ C linkage in 8 with the iminophosphorane moiety may allow the rearrangement to occur at ambient temperature. 15 Unambiguous characterisation of 11 was achieved via a single crystal X-ray structure. ¶ The molecular geometry of 11 is unremarkable: the P-N distances and the P-N-P angle are similar to those found in other substituted iminophosphoranes, 4-7 and the conformations of the cyclopropyl groups are usual.…”

On the Aromaticity and Meisenheimer Rearrangement of Strained Heterocyclic Amine, Phosphine, and Arsine Oxides

“…The MR is related to that of Stevens (SR), originally discovered in nitrogen ylides, and it has also been shown to take place in sulfur ylides. − This reaction occurs thermally to give amines and sulfides derivatives. The mechanism of SR, which has long been a topic of discussion for years, , has been understood considering such experimental facts as stereospecifity , and intramolecularity. , Very recently, theoretical studies on SR have been reported for amine, phosphine, and arsine ylides , and related rearrangements of imino phosphoranes and iminoarsoranes . Moreover, a comparison has been performed between a concerted migration and a radical dissociation−recombination path.…”

“…The mechanism of SR, which has long been a topic of discussion for years, 36,37 has been understood considering such experimental facts as stereospecifity 38,39 and intra-molecularity. 40,41 Very recently, theoretical studies on SR have been reported for amine, phosphine, and arsine ylides 42,43 and related rearrangements of imino phosphoranes and iminoarsoranes. 44 Moreover, a comparison has been performed between a concerted migration and a radical dissociation-recombination path.…”