Organoboron additions

Organoboron reagents ate pariictdarly well suited for 1,4-additions of aryl and vinyl groups to enones. Hayasbi et al. developed a highly enantioselective RliQ)/ BlNAP-catalyzed 1,4-addilion of pbenylbotonic add lo cyclic and acyclic enones [24] fSclieme 7.5) and 1-alkenylpbospbonales [25]. 

In addition to hydroboration, haloboration and phenylboration have also been used in the synthesis of 7r-conjugated organoboron polymers (9)29 (Fig. 9) and polymers containing B—N bonds.30... 

Addition of the elements of water to a double bond can be achieved through hydroboration, followed by oxidation and hydorlysis of the organoboron... 

The regioselectivity of palladium-catalyzed additions of organoboronic acids to unsymmetrical alkynes is strongly dependent on steric and electronic effects (Scheme 12).62 Multi-component reaction has been reported for the synthesis of tetrasubstituted alkenes.58 The aryl group from an aryl iodide is generally added to the less hindered... 

The first example of asymmetric rhodium-catalyzed 1,4-addition of organoboron reagents to enones was described in 1998 by Hayashi and Miyaura. Significant progress has been made in the past few years. This asymmetric addition reaction can be carried out in aqueous solvent for a broad range of substrates, such as a,/ -unsaturated ketones, esters, amides, phosphonates, nitroalkenes. The enantioselectivity is always very high (in most cases over 90% ee). This asymmetric transformation provides the best method for the enantioselective introduction of aryl and alkenyl groups to the / -position of these electron-deficient olefins. 

Nitroalkenes are good candidates for the rhodium-catalyzed asymmetric 1,4-addition of organoboronic acids. Hayashi et al. reported that the reaction of 1-nitrocyclohexene with phenylboronic acid in the presence of rhodium/ -BINAP catalyst gave 99% ee of 2-phenyl-1-nitrocyclohexane (Scheme 38).117... 

In addition, Wipf and co-workers104 have used silver(i)-catalyzed addition of zirconocenes to 3,4,6-tri-O-benzyl-D-glucal epoxide 93 for the stereoselective synthesis of a-C-glucosyl compounds 95 and 96 following a similar mechanism as in the reaction with organoaluminium and organoboron reagents (Scheme 32). 

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). 

The catalytic H—B addition of organoboron compounds to unsaturated molecules is synthetically interesting in main-group chemistry, as a wide range of functional groups can be transformed from the selective new C—B bond [1]. Although considerable energy has been expended to use rhodium] I) complexes to catalyze the... 

Figure 3.15. Scope of Rh/(S)-binap-catalyzed asymmetric 1,4-addition of organoboronic acids to a,P-enones.

Not only organoboron reagents but also organosilicon reagents are suitable nucleophiles in the rhodium-catalyzed asymmetric 1,4-additions to a,p-enones in... 

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