Alkylborane reagents

Various aryl, alkenyl and even alkylborane reagents of different reactivity can be used for coupling with aryl, alkenyl, alkynyl and some alkyl halides, offering very useful synthetic methods. The cross-coupling of aryl and heteroarylboronic acids with aryl and heterocyclic halides and triflates provide useful synthetic routes to various aromatic and heteroaromatic derivatives. Sometimes, the reaction proceeds in the... 

Part 1 (Formation and activation of B-alkyl-(9-BBN) reagents) To a suspension of 9-BBN dimer (4.6 g, 18.8 mmol, 1.0 equiv) in i-Pr20 (10 ml) was added allylbenzene (4.4 g, 5.0 ml, 1.0 equiv) in a N2-filled glovebox. The reaction mixture was then heated at 60 °C for 1.5 h outside of the glovebox, cooled down to room temperature, and diluted with i-Pr20 to produce a 1.5 M solution (total volume 25 ml) of the alkylborane reagent. 

To activate the alkylborane reagent, a solution of the alkylborane reagent (1.5 M, 670 pi, 1.0 mmol, 2.0 equiv relative to the section below) was added to a slurry of KOt-Bu (78.5 mg, 0.70 mmol, 1.4 equiv relative to the section below) and hexanol (92.0 mg, 113 pi, 0.90 mmol, 1.8 equiv relative to the section below), which was vigorously stirred for 45 min at room temperature before being immediately used in the cross-coupling reactions. 

Common catalyst compositions contain oxides or ionic forms of platinum, nickel, copper, cobalt, or palladium which are often present as mixtures of more than one metal. Metal hydrides, such as lithium aluminum hydride [16853-85-3] or sodium borohydride [16940-66-2] can also be used to reduce aldehydes. Depending on additional functionahties that may be present in the aldehyde molecule, specialized reducing reagents such as trimethoxyalurninum hydride or alkylboranes (less reactive and more selective) may be used. Other less industrially significant reduction procedures such as the Clemmensen reduction or the modified Wolff-Kishner reduction exist as well. 

Reactions with Acyl Garbanion Equivalents. Alkyl substituted carbanions CRXY with potential leaving groups X, Y, and acyl carbanion equivalents or CHRX (342) react with alkylboranes, providing products with mixed alkyl groups derived from both reagents. 

Enantiomerically pure alkylboranes arc known to be excellent reagents for asymmetric reduction but they can also be used to generate enantiomerically pure /V-borylimines by partial reduction of nitriles. Addition of organolithium and Grignard reagents to these compounds affords secondary carbinamines in moderate to good yield but low enantioselectivity13,14. The best results reported so far are shown below. 

To immobilize such anions as borate, organoboron polymers were reacted with aryllithium reagents.31,32 The reaction of alkylborane polymers with n-BuLi was examined first however, the ionic conductivity of the resulting material was very low. Moreover, complicated peaks were observed in the H-NMR spectrum. Conversely, selective lithium borate formation was observed in the nB-NMR spectrum when PhLi was employed (scheme 6). An ionic conductivity of 9.45 X 10 7Scm 1 was observed at 50°C. The observed ionic conductivity was relatively low because of the decreased number of carrier ions compared with dissolved salt systems. However, the lithium transference number of this polymer was markedly high (0.82 at 30°C). 

Alkylmercury reagents can also be prepared from alkylboranes. 

It is clear from these representative results that regioselectivity in hydroboration is controlled by steric effects. As a result, nonsymmetric internal olefins usually yield a mixture of regioisomeric alkylboranes when they react with borane. Several hindered mono- and diakylboranes with sterically demanding alkyl groups, however, have been developed for use in selective hydroboration. Disiamylborane [bis(3-methyl-2-butyl)borane, 34], thexylborane (2,3-dimethyl-2-butylborane, 35), and 9-BBN (9-borabicyclo[3.3.1]nonane) are the most frequently used reagents. Improvements of regioselectivities in hydroboration of both... 

An example of the use of an X—Y reagent is conversion of alkylboranes to primary amines with hydroxylaminesulfonic acid, H2NOSOsH (Equation 11-4). The key steps are attack of the nucleophilic nitrogen at boron,... 

These two alkylboranes are important reagents for the hydroboration of other alkenes, since they frequently show in this further reaction a regioselectivity which is greater than that of borane itself. 

Trialkylboranes, including 9-alkyl-9-BBN, underwent cross-coupling with 1-alkenyl or aryl halides or triflates.750,751 The reaction was limitedly used for primary alkylboranes thus, hydroboration of terminal alkenes with 9-BBN was the most convenient to furnish the desired boron reagents in the presence of a base and PdCl2(dppf)750,751 or PdCl2(dppf)/2Ph3As (Equation (200)).752... 

The BH3 THF reagent is the form of borane commonly used in organic reactions. BH3 adds to the double bond of an alkene to give an alkylborane. Basic hydrogen peroxide oxidizes the alkylborane to an alcohol. In effect, hydroboration-oxidation converts alkenes to alcohols by adding water across the double bond, with anti-Markovnikov orientation. 

---