Hydroboration synthesis of alkylboranes

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. 

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... 

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. 

Protonolysis of alkenylboranes generates an alkene and this reaction is faster than the analogous protonolysis of alkylboranes.57 Once again, protonolysis proceeds stereospecifically with retention of configuration. In Negishi s synthesis of the sex pheromone of Lobesia botrana (65), hydroboration of 64 was followed by protonolysis with acetic acid at 0°C to give 65, for a net conversion of an alkyne to an alkene.58 As with alkylboranes, protonolysis of vinylboranes with deuteroacetic acid allows the synthesis of deuterated alkenes. 

Numerous other reactions include the synthesis of trialkylcarbinols from trialkylboranes (Brown, H. C. Acc. Chem. Res. 1969, 2, 65) and hydroboration-protonolysis leading to alkanes (Brown, H. C. Murray, K. /. Am. Chem. Soc. 1959,81,4108). Isomerization and disproportionation of the alkylboranes was also seen (reference 230b). 

Specify the alkene needed for synthesis of each of the following alkylboranes by hydroboration ... 

Although clearly of use in the synthesis of certain alkylboron compounds, particularly tri-n-alkylboranes and alkylboron hydrides, the hydroboration reaction (developed mainly by H. C. Brown) is of considerable significance in synthetic organic chemistry, as the resulting organoboron compound can be used (without being isolated) as an intermediate in the conversion of olefins or acetylenes into other hydrocarbons (parafi or olefins), alcohols, aldehydes, ketones, carboxylic acids or amines. 

Hydroboration (addition of B-H to C=C) is a well-known reaction which takes place under mild con4itions almost quantitatively, for the preparation of various alkylborane compounds in organic synthesis. However, the direct use of this reaction in polymer synthesis has been very limited. 

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. ... 

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-... 

The usefulness of the hydroboration reaction in synthesis arises from the fact that the alkylboranes formed can be converted by further reaction into a variety of other products. On hydrolysis (protonolysis), for example, the boron atom is replaced by a hydrogen atom. Particularly important is the oxidahon to alcohols. Some of the transformations are described below. 

The hydroboration of olefins is a classic reaction in organic synthesis. - Dialkylbo-ranes add rapidly to alkenes in the absence of catalyst. However, dialkoxyboranes, such as catecholborane and pinacolborane, add more slowly to olefins and alkynes. Thus, transition metal complexes could catalyze the addition of dialkoxyboranes to olefins and alkynes without interference from the background reaction. The potential to alter chemoselectivity, regioselectivity, enantioselectivity, and diastereoselectivity has led a munber of groups to develop metal-catalyzed versions of hydroboration. " Enantioselective hydroboration would alleviate the need to use boranes containing stoichiometric amounts of chiral substituents to generate optically active alkylboranes. 

Palladium-catalyzed hydroboration, haloboration, and similar reactions involving addition of organoboron compounds to alkynes have been reviewed.Alkylborane addition to alkynes followed by intra- or intermolecular cross-coupling with a vinyl halide or triflates according to the Suzuki-Miyaura protocol constitutes a very powerful synthetic route for the generation of two carbon-carbon bonds in cascade. This subject and its applications in synthesis have been reviewed. ... 

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