Synthesis alkylboranes

The organoboranes have proven to be very useful intermediates in organic synthesis. In this section we discuss methods by which the boron atom can be replaced by hydroxy, carbonyl, amino, or halogen groups. There are also important processes that use alkylboranes in the formation of new carbon-carbon bonds. These reactions are discussed in Section 9.1. 

In Baldwin s formal total synthesis of haliclamines A and B, a Suzuki coupling of 3-bromopyridine was the central operation [52], Chemoselective hydroboration of diene 66 employing 9-BBN occurred at the less hindered terminal olefin. Suzuki coupling of the resulting alkylborane with 3-bromopyridine then furnished alkylpyridine 67 as a common intermediate for the synthesis of haliclamines A and B. 

DIMETHYL-3-PHENYLPRO-PIONALDEHYDE, 54, 46 Alkylboranes, oxidation, 52, 59 synthesis, 52, 59 Alkyl bromides, from alcohols, 54, 66... 

Alkylboranes formed in the hydroboration of alkenes and alkynes seldom are isolated for the most part they are used as reactive intermediates for the synthesis of other substances. In the reactions of alkylboranes, the B-C bond... 

The second reaction (Equation 11 -3) achieves the synthesis of a primary alcohol by the oxidation of the alkylborane with hydrogen peroxide in basic solution. Starting with a 1-alkene, one can prepare a primary alcohol in two steps ... 

Carbonylation of alkylboranes also can produce ketones. The conditions are similar to those in the aldehyde synthesis except that the hydride reducing... 

Three types of reaction systems have been designed and applied for the enantioposition-selective asymmetric cross-coupling reactions so far. First example is asymmetric induction of planar chirality on chromium-arene complexes [7,8]. T vo chloro-suhstituents in a tricarhonyl("n6-o-dichlorobenzene)chromium are prochiral with respect to the planar chirality of the 7t-arene-metal moiety, thus an enantioposition-selective substitution at one of the two chloro substituents takes place to give a planar chiral monosubstitution product with a minor amount of the disubstitution product. A similar methodology of monosuhstitution can be applicable to the synthesis of axially chiral biaryl molecules from an achiral ditriflate in which the two tri-fluoromethanesulfonyloxy groups are enantiotopic [9-11]. The last example is intramolecular alkylation of alkenyl triflate with one of the enantiotopic alkylboranes, which leads to a chiral cyclic system [12], The structures of the three representative substrates are illustrated in Figure 8F.1. 

After addition of the alkyne anion to the alkylborane, iodination facilitates alkyl group migration from boron to carbon in a transfer that resembles the one seen in the synthesis of (Z)-alkenes described in Section B4.1. Elimination to give the product alkyne occurs under the iodination reaction conditions (Figure B4.4). 

Intermolecular dimerization has also been effected by a comparable protocol.24-26 Treatment of triethylborane with silver nitrate and sodium hydroxide in water at 25°C led to the rapid evolution of M-butane (72%), ethylene (9%), and ethane (9%). Reaction of two different alkylboranes led to statistical mixtures of dimerized and cross-coupled products. Furthermore, this strategy has been used successfully in the synthesis of olefins from dihydroborated internal acetylenes,27 and in polymerizations of bifunctional organoboron compounds.28... 

Recently, the following synthesis of secondary alcohols by the reaction of tri-alkylboranes with bis(phenylthio)methyllithium has been found (Eq. 22) . The... 

Arylboranes, ArBH2 and Ar2BH, have been studied as an alternative to alkylboranes in order to avoid potential problems related to retrohydroboration reactions. Moreover, fiuther stabilization may be readily achieved by 2,6-disubstitution on the phenyl ring. Smith and Pel ter reported the synthesis and properties of mesitylborane (9) and tripylborane (10). Mesitylborane (9), which is obtained from Mes2BH (11) and BH3 THF, shows not only better stability in solution compared to thexylborane (2), but also exhibits very high regio- and chemoselectivity in hydroboration reactions. Polymer-supported versions of these arylboranes have been developed. ... 

Protonolysis of Alkylboranes with Carboxylic Acids Synthesis cf Alkanes 3.10.73 Protonolysis cf Alkenylboranes Synthesis ofAlkenes and Dienes... 

Protonolysis of Alkylboranes with Carboxylic Acids Synthesis of Alkanes... 

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. 

Multicarbon homologation of olefins.1 In a new general ketone synthesis, thexylborane is allowed to react with an equimolar quantity of an olefin to give a thexylmono-alkylborane (1). Addition of an w-alkenyl acetate yields a thexyldialkylborane (2). 

Although water, in some cases, has undergone addition across the C-C double bond in a vinylcyclopropane under acidic, - neutral, " and basic " - aqueous conditions, e.g. formation of 2, the reaction has more often been performed using a two-step procedure. Most frequently used was a one-pot synthesis consisting of hydroboration followed by oxidation with hydrogen peroxide/sodium hydroxide. The alkylboranes, formed in the first step, can be isolated. - The first step has generally been carried out using diborane in tetrahydrofuran 398,410,430,745,852,1025,1590,1609,1793,1833.1877-1884 9-borabi-... 

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