Synthesis and Evolution of Phosphanylcarbene-Borane Adducts
Résumé
Photolysis of [ bis(dicyclohexylamino)phosphanyl](trimethylsilyl)diazomethane leads to the corresponding stable carbene 1 b, which reacts with triethylborane affording a borane-carbene adduct 2b, characterized by spectroscopy in solution before rearrangement and fragmentation into f-dicyclohexylamino-C-(ethyl)(trimethylsilyl)phosphaalkene 3b; trimethylsilyltrifluoromethanesulfonate adds to l b giving the methylenephosphonium 4b. The formation of transient adducts of electrophilic carbenes with Lewis bases is well documented,' but there are only two reports of main group 3 adducts of carbene,2 including a stable alane-carbene complex.2a We have shown that the stable [bis(diisopropylamino)phosphanyl] (trimethylsily1)carbene la3 presents both carbene and phosphorus-carbon multiple bond reactivity, while calculations4 concluded that l a was best formulated as a phosphorus vinyl ylide. Since classical phosphorus ylides are known to react at carbon with Lewis acids giving stable adducts,S it was tempting to try to prepare a phosphanylcarbene-borane complex. Monitoring the reaction of triethylborane with [bis(diisopropylamino)phosphanyl](trimethylsilyl)carbene l a in toluene solution at-80°C by 31P NMR spectroscopy, two new signals were observed at 8 +116.0 (2a) and 249.4 (3a). After a few minutes, the high-field signal disappeared, and after work-up, phosphaalkene 3at and (diethyl)(diisopropylt Selected spectroscopic data for: lb, 2b, 3a, 3b and 4b. lb: 31P NMR (C6D6) 6-31.4; 29si NMR (c6D6) 6-19.7 (d, Jpsi 59.3HZ); l3C NMR (c6D6) 6 3.9 (d, J p c 10.