Frustrated Lewis Pair Chemistry Derived from Bulky Allenyl and Propargyl Phosphanes

The dimesitylpropargylphosphanes mes 2 P−CH 2 −C≡C−R 6 a (R=H), 6 b (R=CH 3 ), 6 c (R=SiMe 3 ) and the allene mes 2 P−C(CH 3 )=C=CH 2 ( 8 ) were reacted with Piers’ borane, HB(C 6 F 5 ) 2 . Compound 6 a gave mes 2 PCH 2 CH=CH(B(C 6 F 5 ) 2 ] ( 9 a ). In contrast, addition of HB(C 6 F 5 ) 2 to 6 b and 6 c gave mixtures of 9 b (R=CH 3 ) and 9 c (R=SiMe 3 ) with the regioisomers mes 2 P−CH 2 −C[B(C 6 F 5 ) 2 ]=CRH 2 b (R=CH 3 ) and 2 c (R=SiMe 3 ), respectively. Compounds 2 b , c underwent rapid phosphane/borane (P/B) frustrated Lewis pair (FLP) reactions under mild conditions. Compound 2 c reacted with nitric oxide (NO) to give the persistent FLP NO radical 11 . The systems 2 b , c cleaved dihydrogen at room temperature to give the respective phosphonium/hydridoborate products 13 b , c . Compound 13 c transferred the H + /H − pair to a small series of enamines. Compound 13 c was also a metal‐free catalyst (5 mol %) for the hydrogenation of the enamines. The allene 8 reacted with B(C 6 F 5 ) 3 to give the zwitterionic phosphonium/borate 17 . The ‐PPh 2 ‐substituted mes 2 P‐propargyl system 6 d underwent a typical 1,2‐P/B‐addition reaction to the C≡C triple bond to form the phosphetium/borate zwitterion 20 . Several products were characterized by X‐ray diffraction.