Organoboron compounds palladium complexes

The application of in situ-generated (alkoxy)palladium(II) species (Scheme 14.23) can be extended to reactions of a-carbonates with organoboron compounds. Crosscouplings of allenes 108 with aryl (or alkenyl) boron acids or their esters catalyzed by a palladium(O) complex afforded the 2-aryl(alkenyl)-l,3-butadienes 109 in excellent yields (Scheme 14.24) [53], The coupling reactions of 9-BBN-derived intermediates such as ester 111 can be accelerated by applying K3P04 as additive (Eq. 14.15). 

G. Cross-coupling Reactions of Organoboron Compounds in the Presence of Palladium Complexes... 

Oxidative addition [1, 38] of 1-alkenyl, i-alkynyl, allyl, benzyl, and aiyl halides to a palladium(O) complex affords a stable rra .s-<7-palladium(II) complex (11). The reaction proceeds with complete retention of configuration for alkenyl halides and with inversion for allylic and benzylic halides. Alkyl halides having /3-hydrogens are rarely useful because the oxidative addition step is very slow and may compete with /3-hydride elimination from the a-organopalladium(II) species. However, it has been recently shown that iodoalkanes undergo the cross-coupling reaction with organoboron compounds (Section 2.4.5). 

An alternative transmetallation route found during our investigations is based on the fact that organoboron compounds readily transfer their organic groups to (alkoxo)palladium(II) complexes under neutral conditions (Scheme. 2-20). 

Miyaura, N. Cross-coupling reaction of organoboron compounds via base-assisted transmetalation to palladium " complexes. J. 

A palladium-phosphine complex supported on polystyrene-PPh2 resin catalyzes the cross-coupling reaction of various organoboron compounds with organic halides or triflates under conditions comparable to that of the homogeneous catalysts, such as Pd(PPh3)4 (Eq. 32). The catalyst is... 

It should be noted that cross-coupling in the presence of phosphine complexes of palladium usually requires high amounts of catalyst, with initial loadings of 25-30 mol% not being uncommon. An entirely new approach to the Suzuki reaction is phosphine-free palladium catalysis. The use of palladium catalysts without the addition of phosphine ligands for cross-coupling with organoboron compounds in aqueous media opened a new chapter in the story of this powerful synthetic method. This approach allows catalyst efficiency to be dramatically increased, and the reaction to be performed under milder conditions. 

---