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Palladium Alkynyl and Allenylidene Complexes: Very Efficient Precatalysts for C-C Coupling Reactions

Oxidative addition of BrC CC(=O)O-menthyl (1a) to [Pd(PPh3)(4)] affords the alkynyl complex trans-[Br(PPh3)(2)Pd-C CC(=O)O-menthyl] (2a). Subsequent reaction of 2a with trifluoroacetate gives trans-[(F3CCOO)(PPh3)(2)Pd-C CC(=O)O-menthyl] (3a). Reaction of trans-[Br(PPh3)(2)Pd-C CC(=O)NC4H8] (2b) with 10 equiv of NaI gives trans-[I(PPh3)(2)Pd-C CC(=O)NC4H8] (4b). The efficiency of the new palladium complexes 3a and 4b as precatalysts is compared with that of recently published cationic palladium allenylidene complexes in C-C coupling reactions. Further trans-bis(alkynyl)palladium complexes, trans-[(PEt3)(2)Pd(-C CC{=O}NR2)(2)] (NR2 = N(CH2)(4) (9b), NMe2 (9c), N(CH2)(4)O (9d), N(CH2)(5) (9e)), trans-[((PPr3)-Pr-i)(2)Pd(-C CC{=O}NMe2)(2)] (11c), and trans-[(PPh3)(2)Pd(-C CC{=O}NR2)(2)] (NR2 = N(CH2)(4) (13b), NMe2 (13c), N(CH2)(4)O (13d)), were synthesized by treating HC CC(=O)NR2 with AgNO3 followed by transmetalation of the alkynyl ligand from silver to [PdCl2(PEt3)(2)], [PdCl2((PPr3)-Pr-i)(2)], or [PdCl2(PPh3)(2)], respectively. Methylation of complexes 9b-e with MeOTf yields dicationic bis(allenylidene) complexes of palladium, trans-{(PEt3)(2)Pd{=C=C=C(OMe)NR2}(2)](2+)(OTf)(2)(-) (NR2 = N(CH2)(4) (10b-OTf), NMe2 (10c-OTf), N(CH2)(4)O (10d-OTf), N(CH2)(5) (10e-OTf). Allcylation of complex 9e with [Me3O]BF4 or [Et3O)BF4 gave trans-[(PEt3)(2)Pd{=C=C=C(OR)N(CH2)(5)}(2)](2+)(BF4)(2)(-) (R = Me (10e-BF4), Et (10e'-BF4)). Complexes 11c and 13b-d were methylated accordingly to obtain the corresponding bis(allenylidene) complexes 12c-OTf, 12c-BF4, 14b-OTf, 14c-OTf, and 14d-OTf The catalytic activities of the new bis(alkynyl) and bis(allenylidene) complexes were investigated and compared with those of the mono(alkynyl) and mono(allenylidene) complexes. The palladium alkynyl and allenylidene complexes are both found to be very efficient precatalysts for Heck, Suzuki, and Sonogashira reactions. Even aryl chlorides were efficiently coupled in Heck and Suzuki reactions when complex 9e was used as precatalyst. Powder diffraction, TEM, and DLS measurements indicate that palladium nanoparticles are not formed during catalysis in the Heck reaction.

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