The Phospha-Bora-Wittig Reaction

Abstract

We report the phospha-bora-Wittig reaction for the direct preparation of phosphaalkenes from aldehydes, ketones, esters, or amides. The transient phosphaborene Mes*P=B-NR2 reacts with carbonyl compounds to form 1,2,3-phosphaboraoxetanes, analogues of oxaphosphetane intermediates in the classical Wittig reaction. 1,2,3-phosphaboraoxetanes undergo thermal or Lewis acid-promoted cycloreversion, yielding phosphaalkenes. Experimental and density functional theory studies reveal far-reaching similarities between classical and phospha-bora-Wittig reactions.

[2 + 2] Cycloaddition of phosphaalkenes as a key step for the reductive coupling of diaryl ketones to tetraaryl olefins

“…We believe that this reactivity is not exclusive to phosphaalkenes, as has recently been demonstrated by the first example of a related phospha-bora-Wittig reaction. 36 Investigations along those lines using heavy alkenes are currently on-going in our labs.…”

A Free Phosphaborene Stable at Room Temperature

“…This species underwent a facile [2+2] cycloaddition with p-methyl benzaldehyde at room temperature to afford 6 in 90% yield (Figure 6a), analogous to Cowley's transient phosphaborene Mes*P≡Btmp. 10 Treatment of 5 with pfluoroacetophenone gave rise to the formation of 7. The 31 P NMR spectrum of 7 displayed a doublet at -233.4 ppm ( 1 JPH = 212.3 Hz), which collapsed into a singlet upon proton decoupling, indicating the presence of a PH unit.…”

A Free Phosphaborene Stable at Room Temperature

“…Therefore, in targeting 5, we chose the bulky, Dipp-substituted NHB-phosphane 1 and NHN-boron dibromide 3 as precursors (Scheme 1). Species 1 was obtained in 91% yield ( 31 P NMR: -246.8 ppm, 11 B NMR: 35.2 ppm) through a salt metathesis reaction of the corresponding bromodiazaborolidine and sodium phosphide. 29 Successive reactions of 1 with benzyl potassium, trimethylsilyl chloride (TMSCl) and t BuLi eventually yielded 2 ( 31 P NMR: -286.4 ppm, 11 B NMR: 38.3 ppm) as a white solid in 67% yield.…”

“…Species 1 was obtained in 91% yield ( 31 P NMR: -246.8 ppm, 11 B NMR: 35.2 ppm) through a salt metathesis reaction of the corresponding bromodiazaborolidine and sodium phosphide. 29 Successive reactions of 1 with benzyl potassium, trimethylsilyl chloride (TMSCl) and t BuLi eventually yielded 2 ( 31 P NMR: -286.4 ppm, 11 B NMR: 38.3 ppm) as a white solid in 67% yield. In the meanwhile, the boron precursor 3 ( 11 B NMR: 12.7 ppm) was readily accessible via a cross-coupling reaction of the corresponding N-heterocyclic imine and boron tribromide.…”

“…These imply the presence of the delocalization of π electrons over the BPBNC chain. The B(1)−P(1)−B(2), P(1)−B(2)−N(1) and B(2)−N(1)−C(1) angles are 106.00(6) o , 168.47 (11) o and 135.6(1) o , respectively. These bond angles suggest the high nonbonded character of the inplane electron pairs at P(1) and N(1), in line with the electron localization function (ELF) 30 calculations (Figure S4-4).…”