Revisiting the Phospha-Wittig–Horner Reaction

Abstract

P,P-Dichlorophosphines 2a−c (RPCl 2 , R = Ph (a), t-Bu (b), 2,4,6-Me 3 Ph (c)) and P,P-dibromophosphines 4d,e (RPBr 2 , R = (i-Pr) 3 SiCC (d) and H 2 CCH (e)) react with triethylphosphite under Michaelis−Arbuzov conditions to give phosphinodiphosphonates 3a−e in quantitative yields. After complexation to W(CO) 5 and treatment with CH 3 ONa, phospha-Wittig−Horner reagents 9a,b are obtained on a multigram scale in good overall yield. Phospha-Wittig−Horner reagents with unsaturated substituents at III P (10d,e) can be prepared in analogous procedures; however, they prevail in an unusual ylide form that allows conjugation between the lone pair and the acetylene and vinyl π-systems, respectively. Phosphinophosphonate 9a has been characterized by X-ray crystallography and is shown to react smoothly with acetone within minutes. The resulting W(CO) 5 -coordinated phosphaalkene is shown to dimerize to a 1,2-diphosphitane or to undergo a 1,3-proton shift depending on the reaction conditions. In addition, a one-pot synthetic sequence starting from W(CO) 5 -coordinated phosphinodiphosphonates 5d,e has been developed to engage compounds with vinyl and acetylene substituents in phospha-Wittig−Horner reactions.

Cascade Reactions Forming Highly Substituted, Conjugated Phospholes and 1,2‐Oxaphospholes

“…Polyheteroaromatic structures based on the phosphole motif (I; Figure 1) have achieved a considerable degree of maturity and found application, for example, in organic light-emitting diodes (OLEDs); [4] phosphorus analogues of polyphenylvinylenes (II) [5] and oligoacetylenes (III) [6] have also been reported. Considering the reliability of the Wittig reactions for the conversion of aldehydes and ketones into alkenes, we were intrigued by the option to use the analogous phospha-Wittig-Horner (pWH) reaction [8][9][10] to prepare P-containing oligoacetylenes from appropriate acetylenic ketones. [7] In contrast to the plethora of synthetic procedures that are available for the preparation of pure hydrocarbons, the synthetic arsenal for the construction of I-III is limited.…”

“…The Li salt of 1 [9] is added to 1,5-bis(TES)-penta-1,4-diyne-3-one (2 a, TES = SiEt 3 ), [13,14] resulting in a red solution from which a bright red product 3 can be isolated in 57 % yield after silica gel chromatography. The Li salt of 1 [9] is added to 1,5-bis(TES)-penta-1,4-diyne-3-one (2 a, TES = SiEt 3 ), [13,14] resulting in a red solution from which a bright red product 3 can be isolated in 57 % yield after silica gel chromatography.…”

“…[9] Lithium salt A undergoes a formal [2+2] addition with one of the acetylene bonds of the starting ketones, presumably through nucleophilic attack of the low valent III P at the acetylenic ketone in a Michael fashion, followed by ring closure. [9] Lithium salt A undergoes a formal [2+2] addition with one of the acetylene bonds of the starting ketones, presumably through nucleophilic attack of the low valent III P at the acetylenic ketone in a Michael fashion, followed by ring closure.…”

“…[7] In contrast to the plethora of synthetic procedures that are available for the preparation of pure hydrocarbons, the synthetic arsenal for the construction of I-III is limited. Considering the reliability of the Wittig reactions for the conversion of aldehydes and ketones into alkenes, we were intrigued by the option to use the analogous phospha-Wittig-Horner (pWH) reaction [8][9][10] to prepare P-containing oligoacetylenes from appropriate acetylenic ketones. Whereas traditional phosphonium ylides react with diacetylenic ketones exclusively in a 1,2-addition, which makes Wittigtype reactions a reliable tool for the preparation of 1,1diethynylethenes, [11,12] herein we demonstrate that a pWH reagent 1 exhibits a completely different reactivity.…”

Redox Switching in Ethenyl‐Bridged Bisphospholes

“…Despite their seemingly complex molecular structure, bisphosphol-3-ols 1 a and 1 b are easily prepared in a one-pot cascade reaction from phosphinophosphonates [7] and diethynylketones in good overall yields. [8] Compounds 1 a,b are highly substituted and, as such, possess a rich chemistry.…”