Synthesis of the First Stable Palladium Allenylidene Complexes

Oxidative addition of BrCCC(O)NR2 to [Pd(PPh3)4] affords the trans-alkynylbromopalladium complexes trans-[Br(PPh3)2Pd−CCC(O)NR2] (NR2 = NMe2 (2a), N(CH2)4) (2b)). Subsequent reaction of 2a,b with PiPr3 in excess gives trans-[Br(PiPr3)2Pd−CCC(O)NR2] (5a,b). The analogous reaction of 2b with P(C6H4OMe-4)3 gives trans-[Br(P{C6H4OMe-4}3)2Pd−CCC(O)NR2] (7b), and that of 2a with trifluoroacetate gives trans-[(F3CCOO)(PPh3)2Pd−CCC(O)NMe2] (9a). Methylation of 2a,b, 7b, and 9a with either MeOTf or [Me3O]BF4 and ethylation of 2a,b with [Et3O]BF4 yield the first cationic allenylidene complexes of palladium, trans-[R*(PR′3)2Pd−CCC(OMe)NR2]+X− (R* = Br, CF3COO; R′ = Ph, C6H4OMe-4, iPr; X = OTf, BF4).