Cross-Couplings in the Presence of Bulky Phosphines

It may be risky to raise mechanistic conclusions on quatitative observations regarding rate accelerations upon changes on any reaction variable in complex catalytic processes such as cross-coupHng reactions. Nevertheless, some interesting hints can be obtained from work aimed at developing new conditions for the coupHng of the less reactive organic substrates such as aryl chlorides [99,100] and alkyl electrophiles [101]. 

Highly reactive palladium catalysts based on this type of bulky biphenylphos-phines can be readily generated from phenylethylamine-derived paUadacycles [122] or, even more conveniently, from biarylamine-derived precatalysts [123]. Complexes Pdl2 can also be conveniently prepared in situ by the reaction of Pd(r -l-PhC3H4)(r -C5H5) with tertiary phosphines L [124]. 

Coordinatively, unsaturated [(l,6-diene)PdL] (L = phosphine) complexes are also efficient catalysts for the Suzuki-Miyaura coupHng of aryl chlorides with phenyl-boronic acid [125, 126]. 

For the coupHng of primary alkyl tosylates, the bulkier phosphine P(tBu)2Me gave the best results [131]. As expected, the oxidative addition of an alkyl tosylate to Pd(0) results in predominant inversion of configuration, while the transmetallation occurs with retention [131]. The complex [Pd(P(tBu)2Me)2] is also a catalyst for the room-temperature coupHng of primary alkyl bromides that possess P-hydrogens with boronic acids [132]. Complex 11, the oxidative addition product of an alkyl bromide to [Pd(P(tBu)2Me)2], has been isolated and structurally characterized [132] 

Other related nickel-catalyzed Negishi, Hiyama, and Suzuki-Miyaura reactions of unactivated secondary alkyl halides, radical intermediates may be involved [140, 142-144], 

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