Reactions of Cyanoboranes with a Palladium−PMe₃ Complex:  Mechanism for the Catalytic Cyanoboration of Alkynes

Abstract

Reversible oxidative addition of the B−CN bond to palladium was observed in a reaction of a 13C-labeled cyanoborane, having an N,N‘-dimethyl-o-phenylenediamido ligand on the boron atom, with a palladium−PMe3 complex, which was generated from (π-cyclopentadienyl)(π-allyl)palladium(II) with 2 equiv of PMe3. The same borylbis(trimethylphosphine)palladium(II) cyanide complex was formed in the reaction of borylbis(trimethylphosphine)palladium(II) chloride with trimethylsilyl cyanide. Reaction of the palladium−PMe3 complex with a cyanoborane bearing a tethered carbon−carbon triple bond gave a (2-borylalkenyl)palladium(II) cyanide complex, whose structure was determined by a single-crystal X-ray analysis, via insertion of the C−C triple bond into the B−Pd bond of a transiently formed borylpalladium(II) cyanide complex. The (2-borylalkenyl)palladium(II) cyanide complex underwent reductive elimination at 60 °C. Significant retardation of the reductive elimination step by addition of 2 equiv of trimethylphosphine was observed.

Catalytic carboborations

“…7). 27 This intermediate could be identified by a single‐crystal X‐ray analysis and underwent reductive elimination upon heating at 60°C. From these experiments, we elucidate that the catalytic cycle consists of oxidative addition of the B–CN bond, insertion of the triple bond into the B–Pd bond, and reductive elimination of the B–C bond (Fig.…”

“…A reaction mechanism was elucidated on the basis of stoichiometric experiments 27. A 1:1 reaction of 13 C‐labeled cyanoborane 6d* with a mixture of a palladium complex (1 equiv) and trimethylphosphine resulted in 20% conversion into an oxidative addition complex 9* (Fig.…”

Density functional study on the reaction mechanism of palladium‐catalyzed addition of cyanoboranes to alkynes

“…Recently, catalytic reactions, i.e., additions of cyanoboranes to alkynes, have been developed by Suginome et al20–23 These cyanoborations include intermolecular and intramolecular reactions. The intermolecular cyanoboration is the addition of BCN bond in cyclic cyanoborane derivatives to alkynes,20, 23 while the intramolecular cyanoboration is the addition of BCN bond to carbon–carbon triple bond within the homopropargylic cyanoboryl ether 21–23.…”

“…Recently, catalytic reactions, i.e., additions of cyanoboranes to alkynes, have been developed by Suginome et al20–23 These cyanoborations include intermolecular and intramolecular reactions. The intermolecular cyanoboration is the addition of BCN bond in cyclic cyanoborane derivatives to alkynes,20, 23 while the intramolecular cyanoboration is the addition of BCN bond to carbon–carbon triple bond within the homopropargylic cyanoboryl ether 21–23. For both intermolecular and intramolecular cyanoborations, the similar cis ‐fashion products were obtained, and a possible reaction mechanism of alkyne cyanoboration was suggested by Suginome et al, which involves alkyne coordination, oxidative addition, alkyne insertion, and reductive elimination steps 22, 23.…”

“…The intermolecular cyanoboration is the addition of BCN bond in cyclic cyanoborane derivatives to alkynes,20, 23 while the intramolecular cyanoboration is the addition of BCN bond to carbon–carbon triple bond within the homopropargylic cyanoboryl ether 21–23. For both intermolecular and intramolecular cyanoborations, the similar cis ‐fashion products were obtained, and a possible reaction mechanism of alkyne cyanoboration was suggested by Suginome et al, which involves alkyne coordination, oxidative addition, alkyne insertion, and reductive elimination steps 22, 23. To study the mechanism of intermolecular alkynes cyanoboration, the stoichiometric reactions of cyanoboranes with palladium complexes were performed 23.…”