B-Alkyl-9-BBN compounds

The rate of reduction of aldehydes with B-alkyl-9-BBN compounds is greatly dependent on the structure of the alkyl group on boron [9]. In general, rate of reduction is very fast when alkyl group has tertiary ( -hydride, while with a primary (5-hydride provides a very slow rate. For example, B-3-pinanyl-9-BBN reacts rapidly and exothermally at room temperature and the VPC and NMR techniques have proven futile, since the reaction is so fast. Consequently, the... 

Scheme 4.3. Reaction of B-alkyl-9-BBN compounds with benzaldehyde to form olefin [5] (fj is the time for 50% completion of reaction at reflux THF (65 °C)).

The use of B-alkyl-9-BBN compounds, especially (15), as mild and very chemoselective reducing agents for aldehydes (Scheme 8), even in the presence of ketones, has been explored and the variation in reducing ability with different S-alkyl groups examined. The cyclic transition state (16) is consistent with an increase in rate both with increase in substitution /3- to boron and when the B—C C—H unit can adopt a planar arrangement. This system has also been... 

The B-alkyl-9-BBN undergoes an interesting reverse reaction to afford the parent alkene when treated with benzaldehyde. Consequently, the reaction is uniquely employed for the synthesis of exocyclic olefins (Chart 24.3). The hy-droboration of cyclic olefins with an internal double bond, followed by homologation with carbon monoxide in the presence of lithium trimethoxyaluminum hydride afford B-(cycloalkylmethyl)-9-BBN. This intermediate on treatment with benzaldehyde leads to an exocyclic methylene compound (Chart 24.3) [16]. Since the synthesis proceeds from the cycloalkene, thus it provides a valuable alternative to the customary methylenation of carbonyl compounds by Wittig and related procedures. The method also provides a clean synthesis of deuterium-labeled compounds (Eq. 24.10) [16], without positional scrambling or loss of label. Consequently, methylmethylene-d -cyclopentane in 52% isolated yield is obtained. 

Suzuki and Miyaura first demonstrated the utility of B-alkyl-9-BBN derivatives for coupling with vinyl halides (Figure 28.4). ° The reaction conditions are mild enough to tolerate many functional groups such as esters, nitriles, amides, amines, and sulfides and a wide variety of compounds (e.g., 119a-e) have been synthesized using this protocol (Figure 28.5). 

Organoboranes typically used in these reactions are Et3 B, i-PrsB, and alkylated 9-BBN derivatives. Interestingly, multiple organoboration steps may take place with di-, tri-, and tetra-1-alkynyl element compounds. Intermediates such as the... 

Cross-coupling between 1-alkenyl- and arylboron compounds with organic electrophiles have found wide application (see Sect. 1.5.1.1) in organic synthesis [125, 132, 269-271]. The value of this methodology is further realized with the use of alkylboranes, such as B-alkyl-9-borabicyclo[3.3.1]nonanes (B-R-9-BBN), which can be conveniently prepared Ijy hydroboration of olefins [272]. The hydroboration proceeds with high stereoselectivity and chemoselectivity. The choice of phosphines in a catalytic system sometimes affects the chemo- and regioselectivity of the hydroboration. Hydroboration of l-(ethylthio)-l-propyne with catecholborane can be satisfactorily carried out with PdClj (dppf) but the regioselectivity is best for the dppe and dppp complexes of Ni [273]. Notably, PPhj complexes perform poorly. 

B-l-alkynyl-9-BBN compounds are exceptionally mild reagents and are essentially inert at 25 °C toward alkyl halides, acid chlorides, amides, anhydrides, esters, nitriles, acetals, and ketals. 

Homologation via a-Haloenolates. Organoboranes can also be used to construct carbon-carbon bonds by several other types of reactions that involve migration of a boron substituent to carbon. One such reaction involves a-halocarbonyl compounds.19 For example, ethyl bromoacetate reacts with trialkylboranes in the presence of base to give alkylated acetic acid derivatives in excellent yield. The reaction is most efficiently carried out with a 9-BBN derivative. These reactions can also be effected with B-alkenyl derivatives of 9-BBN to give /i,y-unsaturatcd esters.20... 

Niedenzu initiated studies of B-substituted analogs of (47) and (48) and various X = chloro, fluoro, alkyl, aryl, alkoxy, and amido derivatives of (49). These compounds should be very useful for further synthetic transformations. Koster and coworkers have examined the reactions of sterically congested pyrazoles with (9H-9-BBN)2 and unique neutral, monomeric products with tricoordinate boron centers (50) are obtained. The compounds are stable toward dimerization. When bulky pyrazoles are allowed to react with activated trialkylboranes, for example, BEts at 170 °C,... 

B-Me- andB-MeO-9-BBN through this radical bromination under hydrolytic conditions where the facile 1,2-alkyl migration of a ring B-C bond occurs (eq 25). This latter compound serves as a convenient source of 9-oxabicyclo[3.3.1]nonane through base-induced iodination via an SE2-type inversion. ... 

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