Mechanisms of Stereomutation and Thermolysis of Spiro-1,2-oxaphosphetanes: New Insights into the Second Step of the Wittig Reaction

Abstract

The experimentally observed stereomutation of spiro-1,2-oxaphosphetanes is shown by DFT calculations to proceed through successive M(B2) or M(B4) and M(B3) mechanisms involving two, four, and three Berry pseudorotations at phosphorus, respectively. Oxaphosphetane decomposition takes place in a single step via a polar transition state. The calculated activation parameters for this reaction are in good agreement with those determined experimentally.

Synthesis of 2,3-Dihydro-1-phenylbenzo[b]phosphole (1-Phenylphosphindane) and Its Use as a Mechanistic Test in the Asymmetric Appel Reaction: Decisive Evidence against Involvement of Pseudorotation in the Stereoselecting Step

“…3 Such intermediates can undergo a fast intramolecular ligand exchange process, termed Berry pseudorotation,4,5 that may have a significant effect on the outcome of the reaction. 6,7 However, in the case of the Appel reaction conditions, 8 while the involvement of chloro-and alkoxy-phosphonium species is well established, 9,10,11 the extent of participation of pentaco-ordinate phosphorane intermediates is less clear. 8,9 Our interest in this question arose through our development 12 of an asymmetric version of the Appel conditions (Scheme 1), whereby racemic arylmethylphenyl phosphines are treated with hexachloroacetone (HCA) and a chiral non-racemic alcohol (R*OH) to give high yields of enantioenriched phosphine oxides with enantiomeric excess (ee) up to 82%.…”

Synthesis, Properties and Stereochemistry of 2-Halo-1,2λ5-oxaphosphetanes

“…One of the most interesting and intriguing classes of organophosphorus compounds are the 1,2-oxaphosphetanes—four-membered heterocycles containing pentacoordinated phosphorus [1,2,3,4,5,6,7]. Since 1,2-oxaphosphetanes are well-known intermediates in the Wittig reaction, a number of efforts have been made for their structural characterization both in solution and the solid state [8,9,10,11,12,13].…”

“…Among the stable oxaphosphetanes [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22], 2-halo-1,2λ 5 -oxaphosphetanes 14 , which possess relatively high stability and diverse reactivity, attract particular interest [23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38] (Scheme 7). These oxaphosphetanes containing fluorine, chlorine or bromine atoms bonded to phosphorus are an interesting class of pentacoordinated phosphorus heterocycles possessing peculiar properties.…”

Synthesis and Properties of Four-Membered Phosphorus Heterocycles-2-Fluoro-1,2λ5-Oxaphosphetanes

“…[1][2][3] The reaction of phosphorus ylides with carbonyl compounds, leading to the formation of olefins, is one of the major reaction in synthetic organic chemistry. 4 Extensive experimental 5,6 and theoretical work 7,8 has been devoted to clarify the reaction mechanism. A variety of different species, betaines, 8 diradicals, and 1,2-oxaphosphetanes, 1-3 were proposed as possible reactive intermediates.…”