Phosphorus-Involved Wagner–Meerwein Rearrangement of Phosphiranes: An Entry to Four-Membered Phosphacycles

Abstract

Phosphenium ions [R 2 P] + are important and highly reactive dicoordinate phosphorus species. Herein, we report a rearrangement of the carbocation into the phosphenium cation driven by ring strain. This phosphorus-involved Wagner−Meerwein rearrangement pathway converted the 1-acylphosphirane complex into phosphetane and 1,2-dihydrophosphete derivatives depending on the reaction temperature. The generation of the intermediate phosphenium cation was identified by the intramolecular reaction with ether, which also disclosed its strong Lewis acidity. This work expands the boundary of the phosphorus−carbon analogy.

Protonation of P-Stereogenic Phosphiranes: Phospholane Formation via Ring Opening and C–H Activation

“…Here, we have reported a new type of reaction for the phosphirane functional group, involving ring opening with P–C cleavage and formation of a new P–C bond by activation of a t -butyl C–H group under mild conditions. These observations emphasize the high reactivity of the strained phosphirane ring in 1 – 2 and in proposed intermediates P–H phosphiranium cations and phosphenium ion 13 ( 26 ) and the importance of the nucleophilicity/basicity of the anions in the chemistry of such cations. In comparison to stable N–H aziridinium cations ( Scheme 1 ), the increased P–H phosphiranium cation reactivity may be a consequence of weaker P–H and P–C bonds.…”

BF3-Promoted Ring Expansion of Iminylphosphiranes and Acylphosphiranes for Divergent Access to 1,2-Azaphospholidines and 1,2-Dihydrophosphetes

“…Since we have successfully trapped the phosphenium cation with an ortho-methoxyphenyl group, 15 the reaction was further explored with 1-iminylphosphirane complex 1i to support the reaction mechanism and identify the in-situ-generated phosphenium cation. Treatment of phosphirane 1i with 2.0 equiv of BF 3 •Et 2 O at 100 °C produced 1,2-azaphospholidines 2i and 2i′ in 41% total yield (Scheme 5).…”

“…It was known that the Wagner–Meerwein rearrangement of cyclopropylmethyl carbocation is a key step of ring expansion of cyclopropanes to cyclobutane derivatives . We recently reported a rearrangement of the carbocation into the phosphenium cation through a phosphorus-involved Wagner–Meerwein rearrangement . Phosphenium ions [R 2 P] + are highly reactive dicoordinate phosphorus species with six electrons in their valence shell containing a lone pair on phosphorus and a vacant p -orbital.…”

“…14 We recently reported a rearrangement of the carbocation into the phosphenium cation through a phosphorus-involved Wagner−Meerwein rearrangement. 15 Phosphenium ions [R 2 P] + are highly reactive dicoordinate phosphorus species with six electrons in their valence shell containing a lone pair on phosphorus and a vacant p-orbital. Based on the above information, a plausible mechanism for the presented ring expansion process is outlined in Scheme 4.…”