PhP=CPh₂ and Related Phosphaalkenes: A Solution Equilibrium between a Phosphaalkene and a 1,2‐Diphosphetane

Abstract

The synthesis of phosphaalkenes, ArP=C(R)Ph (1, a: Ar = Ph, R = Ph; b: Ar = o‐Tol, R = Ph; c: Ar = Mes, R = H), bearing sterically less hindered substituents is reported. Phosphaalkenes 1a–b were prepared by treating Ph2C=O with ArP(Li)SiMe3, whereas 1c was accessed from the AlCl3‐mediated reaction of ArP(SiMe3)2 and PhC(O)H. Both 1a and 1b dimerize to afford their respective 1,2‐diphosphetanes (2a and 2b). Compound 2a was characterized by X‐ray crystallography. Dissolution of pure 2a in THF resulted in a temperature dependent equilibrium with monomer 1a (ΔHo = –94.6 ± 14.6 kJ mol–1; ΔSo = –284 ± 48 J mol–1 K–1). Although monomer E/Z‐1c was identified in solution by its characteristic downfield 31P NMR chemical shift (δ = 247.1, 231.5), it was accompanied by the formation of what we believe are oligomers (δ = –25 to 10 ppm, br.). Attempts to trap phosphaalkenes 1a and 1b by treatment with W(CO)5(MeCN) afforded mixtures of complexes W(CO)5(1a–b) (4a–b) and W(CO)4(1a–b)2 (5a–b), the disubstituted species being subjected to X‐ray crystallographic characterization (5a).

Triphenylphosphaalkenes in Chemical Equilibria

“…In analogy to a report by Gates et al, three species were expected when benzophenone is added to an ethereal solution of PhP(Li)TMS (Scheme ). The 31 P NMR spectrum of the reaction mixture (Figure a) shows these three compounds, i.e.…”

From P‐Functional Thiazol‐2‐thione Derivatives to Phosphaalkenes

“…Immediately after addition, a new resonance was observed at 187.2 ppm along with the signal assigned to 5 (57.9 ppm). The downfield shift of the new signal is characteristic of phosphaalkenes having aryl or alkyl substituents at the alkylidene carbon atom, and was tentatively assigned to phosphaalkene 6 . Within 30 min, new signals between –37.7 and 54.4 ppm emerged (ca.…”

“…The resonance at 141.1 ppm was tentatively assigned to the end ‐on phosphaalkene tungsten complex 8 by comparison to data for known end ‐on complexes. Specifically, the coupling constants are similar to those of [W(CO) 5 (PhP=CPh 2 )] ( δ = 188.1 ppm, 1 J W,P = 270.8 Hz),[23a] and [W(CO) 5 (PhP=CMe 2 )] ( δ = 176.0 ppm, 1 J W,P = 261.0 Hz), with the chemical shift lying significantly upfield given the significant shielding effect exerted by the thiazol‐2‐thione substituent on the phosphorus nucleus, In order to gain additional support for this assignment of 8 , the solvent was removed in vacuo at –30 °C [shortly after having added DBU ( ≃ 5 min)]. The solid obtained was used for an EI‐MS experiment that showed m/z 651, clearly assignable to the molecular ion peak of phosphaalkene complex 8 , and the base peak at m/z 327 representing the fragment ion ([ 6 ] + ) after loss of the W(CO) 5 group.…”

Triarylalkenes from the site-selective reductive cross-coupling of benzophenones and aldehydes

“…Lithiated phenyl(trimethylsilyl)phosphane (PhP(TMS)Li) was identified as the most suitable coupling reagent as it can be prepared easily from PhP(TMS) 2 and is known to engage in phospha-Peterson reactions to form phosphaalkenes. 18 In a procedure that was slightly modified compared to literature precedence, 19 ethereal solutions of PhP(TMS)Li that were obtained by the lithium ethoxide -promoted selective removal 20 of one TMS group from PhP(TMS) 2 were treated with ketones A-G. A diagnostic colour change from light yellow to dark blue was observed in all cases and the reaction is completed within a few minutes. Phosphaalkene formation can be followed by the disappearance of the 31 P NMR signal of the starting lithium salt at À143 ppm and the appearance of the characteristic signals of phosphaalkenes 1A-E at downfield chemical shift (+234 ppm for 1A, see Fig.…”