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)2Pd–CCC­(O)­O-menthyl]
(2a). Subsequent reaction of 2a with trifluoroacetate
gives trans-[(F3CCOO)­(PPh3)2Pd–CCC­(O)­O-menthyl] (3a). Reaction of trans-[Br­(PPh3)2Pd–CCC­(O)­NC4H8] (2b) with 10 equiv of NaI gives trans-[I­(PPh3)2Pd–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)2Pd­(−CCC­{O}­NR2)2] (NR2 = N­(CH2)4 (9b), NMe2 (9c), N­(CH2)4O (9d), N­(CH2)5 (9e)), trans-[(PiPr3)2Pd­(−CCC­{O}­NMe2)2] (11c), and trans-[(PPh3)2Pd­(−CCC­{O}­NR2)2] (NR2 = N­(CH2)4 (13b), NMe2 (13c), N­(CH2)4O (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(PiPr3)2], or [PdCl2(PPh3)2], respectively. Methylation of complexes 9b–e with MeOTf yields dicationic bis­(allenylidene)
complexes of palladium, trans-[(PEt3)2Pd­{CCC­(OMe)­NR2}2]2+(OTf)–2 (NR2 = N­(CH2)4 (10b-OTf), NMe2 (10c-OTf), N­(CH2)4O (10d-OTf), N­(CH2)5 (10e-OTf). Alkylation of complex 9e with [Me3O]­BF4 or [Et3O]­BF4 gave trans-[(PEt3)2Pd­{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.