Organoboron reagent

Although the C-Zn bond is rather unreactive toward electrophiles such as aldehydes, ketones, esters, and nitriles, it undergoes transmetalations with many transition metal complexes to furnish new organometallics capable of reacting with a variety of electrophilic substrates (see Section 7.10). 

There is a vast array of carbon-carbon-bond-forming reactions involving organoboron compounds. This section covers transfer reactions of carbon moieties from boron to an adjacent carbon. Examples include the transfer of CO, CN, carbanions derived from dichloromethyl methyl ether and dichloromethane, and allyl groups in reactions of allylic boranes with aldehydes. 

If the carbonylation reaction is done in the presence of a small amount of water at 100 °C, a second alkyl group migrates from B to the adjacent carbon to furnish, after oxidative workup, the corresponding ketone. 

The use of thexylborane (1,1,2-trimethylpropylborane) as the hydroborating agent permits (a) the synthesis of mixed trialkyboranes, and (b) cyclic hydroboration of dienes. When followed by carbonylation, these hydroboration-carbonylation sequences generate a variety of unsymmetrically substituted ketones and cyclic ketones, respectively. Since the thexyl moiety exhibits a low migratory aptitude in carbonylation reactions, it serves as an anchor group. 

Carbonylation of trialkylboranes in the presence of ethylene glycol results in migration of a second and a third alkyl group to give, after oxidation, the corresponding tert-alcohols.  

Compounds containing reactive C-H bonds have been shown to undergo cross coupling in the presence of catalyst and base witliout initial foimation and isolation of a main-group derivative. One classic version of this coupling is the reaction of an aryl halide with 

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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]. 

Figure 25 Synthesis of naturally occuring phenylpropenoid (3-D-glucopyranosides. (a) allyl alcohol/immobilized (3-glucosidase with ENTP-4000, (b) Ac20/4-dimethylaminopyridine/pyridine, (c) organoboron reagents/Pd(OAcyCu(OAc)2/LiOAc/DMF, (d) K2C03/Me0H.

Scheme 8.14. Palladium-Catalyzed Cross Coupling of Organoboron Reagents...

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. 

Since the C—B bond is almost completely covalent, transmetalation of an organoboron reagent to transfer the organic group will not occur without coordination of a negatively charged base or a F to the boron atom. As a consequence, the Suzuki reaction normally needs to be carried out in a basic solution, which poses some limitations to base-sensitive substrates. 

Scheme 29 One-pot synthesis of a-C-glycosyl compounds from glycals using organo-aluminium and organoboron reagents.

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). 

Zapf, A. Coupling of Aryl and Alkyl Halides With Organoboron Reagents (Suzuki Reaction). In Transition Metals for Organic Synthesis (2nd edn), Beller, M. Bolm, C. eds., 2004, 1, 211-229. Wiley-VCH Weinheim, Germany. (Review). 

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

Senda, T. Ogasawara, M. Hayashi, T. Rhodium-Catalyzed Asymmetric 1,4-Addition of Organoboron Reagents to 5,6-Dihydro-2(lH)-pyridinones. Asymmetric Synthesis of 4-Aryl-2-piperidinones. ]. Org. Chem. 2001, 66, 6852-6856. 

Addition of Organoboron Reagents to Other Eiectron-Deficient Oiefins... 

Scheme 3.8 Asymmetric conjugate addition of organoboron reagents to a,jS-unsaturated esters [20].

Reactions with Organoboron Reagents The Suzuki-Miyaura Reaction... 

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