Hydroborations 9-borabicyclo nonane dimer

Wang, K. K., Scouten, C. G., Brown, H. C. Hydroboration kinetics. 3. Kinetics and mechanism of the hydroboration of alkynes with 9-borabicyclo[3.3.1]nonane dimer. Effect of structure on the reactivity of representative alkynes. J. Am. Chem. Soc. 1982,104, 531-536. 

Among the various hydroborating reagents, 9-borabicyclo [3.3.1] nonane dimer (9-BBN)2 has found the most extensive use because of its unique properties, convenient preparation, commercial availability and enormous synthetic applications. 

TMS-alkynes are oxidized at the terminal carbon to carboxylic acids by hydroboration/oxidation (dicyclohexylborane/NaOH, H2O2). This does not work with TIPS-alkynes. Instead, TIPS-alkynes are cleanly monohydroborated at the internal carbon by 9-borabicyclo[3.3.1]nonane dimer to give (Z)- -borylvinyl-silanes. These can be oxidized in high yields to a-silyl ketones, or cross coupled with a bromide R Br (R = aryl, benzyl, dimethyl-vinyl) in the presence of NaOH and tetrakis(triphenylphos-phine)palladium(0) to give /3,/3-disubstituted vinylsilanes (Suzuki reaction eq 14). The same nucleophilic substituted vinylsilane can be added to an aromatic aldehyde to provide access to ( )-3-silyl allyl alcohols. ... 

Brown and co-workers have continued their extensive studies of the mechanisms of hydroboration. Some of this work has recently been reviewed. Kinetic studies have been reported of the reaction of 9-borabicyclo[3.3.1]nonane dimer (3) with alkenes, alkynes, aldehydes, and ketones, in various solvents and with various Lewis bases and Br0nsted acids. Earlier studies suggested that the reaction of (3) with alkenes in a variety of solvents proceeded via the initial dissociation as in equation (4) to the monomer (4) followed by the hydro-... 

Transfers of hydride from boron or lithium to carbon usually occur in the context of addition of the complete M—H moiety to polar or non-polar unsaturation. Additions of boranes to alkenes have been extensively reviewed (Brown et al., 1983a), but the experimental characterization of the hydroboration transition state remains problematic. Dialkylboranes, including 9-borabicyclo[3.3.1]nonane (Wang and Brown, 1980), borinane (Brown et al., 1984), and disiamylborane (Chandrasekharan and Brown, 1985) have now been shown to be dimeric in hydrocarbon and ethereal solvents. With unreactive alkenes, their additions are first order in alkene and half order in the dimer. With reactive terminal alkenes, the reactions are first order only in dimer, with intermediate behaviour between these extremes. A reaction scheme (10) involving reaction of monomeric borane with the alkene satisfies the data, with the observed order depending on the ratio k i/k2. 

Borabicyclo[3.3.1.]nonane (9-BBN) is available by hydroboration of 1,5-cyclooctadiene with one equivalent of BH3," or commercially either as a crystalline dimer or as a THF solution. The reagent is thermally more stable than dicyclohexylborane. It is frequently used as an anchor group in organoborane reactions, allowing an efficient utilization of valuable alkenes. 

Borabicyclo[3.3.1]nonane (9-BBN) has been prepared by the thermal redistribution of 9-n-propyl-9-BBN, and the hydroboration of 1,5-cyclooctadlene with borane-tetrahydrofuran complex followed by thermal isomerization of the mixture of dialkylboranes at BS C. Solutions of 9-BBN have been prepared from the hydroboration of 1,5-cyclooctad ene with borane-methyl sulfide in solvents other than THF.6 The present procedure involves the cyclic hydroboration of 1,5-cyclooctadiene with borane-methyl sulfide in 1,2-dimethoxyethane.7 Distillative removal of the dimethyl sulfide in this special solvent system provides a medium that gives high purity, large needles of crystalline 9-BBN dimer in excellent yield. The material can be handled in air for brief periods without measurable decomposition. 

These results for 1,3-butadiene, 1,4-pentadiene and 1,5-hexadiene allow the courses of thermal isomerization in other cases to be predicted. Hydroboration of 1,5-cycloocta-diene yields quantitatively a 72 28 mixture of 9-borabicyclo[3.3.1]nonane (9-BBN) (IX) and 9-borabicyclo[4.2.1]nonane (X), both of which exist as dimers. In refluxing THF, the latter can be isomerized to 9-BBN within 1 h. ... 

Borabicyclo[3.3.1]nonane (9-BBN) has found use in the selective hydro-boration of alkenes in the presence of other reducible functional groups and its reaction with alkynylstannates has been studied. o-Stannyl- and a-silyl-substituted crotyl-9-BBN show promise as reagents for the stereo-regulated synthesis of acyclic systems. A series of papers covers the question of olefin-alkyl exchange in. 5-alkyl-9-BBN s, " the kinetics of reduction of substituted benzaldehydes with 9-BBN, and the kinetics and mechanism of hydroboration of alkynes with 9-BBN dimers. Selective dehalogenation of tertiary alkyl, benzyl, and allyl halides in the presence of secondary or primary alkyl or aryl halides is possible with (165). The... 

A variety of useful materials, hindered at boron and yet capable of dehvery of hydride, can be prepared in this way. Although their utility will become clearer as additional functional groups with which they can interact are discussed, both disia-mylborane [(CH3)2CHCH(CH3)]2BH) (Scheme 6.26) and 9-borabicyclo[3.3.1] nonane (9-BBN) (Equation 6.32) form dimers that readily dissociate so that they behave as monomeric species in the presence of reactive alkenes. Experimental determination of the kinetics vide infra) of hydroboration are much simplified in this way. 

It is of interest to examine the kinetics of hydroboration of the same alkene as shown in Scheme 6.30. It has generally not proved possible to use diborane itself for kinetic measurements. However, as shown in Scheme 6.31, the dimer of 9-borabicyclo[3.3.1]nonane (9-BBN), (i.e., [(9-BBN)2]) can be used. As predicted, the reaction is half-order in (9-BBN)2 and first order in alkene. 

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