Hydroboration borane sulfides

HYDROBORATION Borane-Dimethyl sulfide. Borane-Tetrahydrofurane. Borane-Triphenylphosphate. Disiamylborane. Diphenylamine-Borane. Thexylborane. 

Diborane [19287-45-7] the first hydroborating agent studied, reacts sluggishly with olefins in the gas phase (14,15). In the presence of weak Lewis bases, eg, ethers and sulfides, it undergoes rapid reaction at room temperature or even below 0°C (16—18). The catalytic effect of these compounds on the hydroboration reaction is attributed to the formation of monomeric borane complexes from the borane dimer, eg, borane-tetrahydrofuran [14044-65-6] (1) or borane—dimethyl sulfide [13292-87-0] (2) (19—21). Stronger complexes formed by amines react with olefins at elevated temperatures (22—24). 

Borane—dimethyl sulfide complex (BMS) (2) is free of these inconveniences. The complex is a pure 1 1 adduct, ca 10 Af in BH, stable indefinitely at room temperature and soluble in ethers, dichioromethane, benzene, and other solvents (56,57). Its disadvantage is the unpleasant smell of dimethyl sulfide, which is volatile and water insoluble. Borane—1,4-thioxane complex (3), which is also a pure 1 1 adduct, ca 8 Af in BH, shows solubiUty characteristics similar to BMS (58). 1,4-Thioxane [15980-15-1] is slightly soluble in water and can be separated from the hydroboration products by extraction into water. 

The products are Hquids, soluble in various solvents and stable over prolonged periods. Monochloroborane is an equiUbtium mixture containing small amounts of borane and dichloroborane complexes with dimethyl sulfide (81). Monobromoborane—dimethyl sulfide complex shows high purity (82,83). Solutions of monochloroborane in tetrahydrofuran and diethyl ether can also be prepared. Strong complexation renders hydroboration with monochloroborane in tetrahydrofuran sluggish and inconvenient. Monochloroborane solutions in less complexing diethyl ether, an equiUbtium with small amounts of borane and dichloroborane, show excellent reactivity (88,89). Monochloroborane—diethyl etherate [36594-41-9] (10) may be represented as H2BCI O... 

Dibromoborane—dimethyl sulfide is a more convenient reagent. It reacts directly with alkenes and alkynes to give the corresponding alkyl- and alkenyldibromoboranes (120—123). Dibromoborane differentiates between alkenes and alkynes hydroborating internal alkynes preferentially to terminal double and triple bonds (123). Unlike other substituted boranes it is more reactive toward 1,1-disubstituted than monosubstituted alkenes (124). 

Among chiral dialkylboranes, diisopinocampheylborane (8) is the most important and best-studied asymmetric hydroborating agent. It is obtained in both enantiomeric forms from naturally occurring a-pinene. Several procedures for its synthesis have been developed (151—153). The most convenient one, providing product of essentially 100% ee, involves the hydroboration of a-pinene with borane—dimethyl sulfide in tetrahydrofuran (154). Other chiral dialkylboranes derived from terpenes, eg, 2- and 3-carene (155), limonene (156), and longifolene (157,158), can also be prepared by controlled hydroboration. A more tedious approach to chiral dialkylboranes is based on the resolution of racemates. /n j -2,5-Dimethylborolane, which shows excellent enantioselectivity in the hydroboration of all principal classes of prochiral alkenes except 1,1-disubstituted terminal double bonds, has been... 

Certain base adducts of borane, such as triethylamine borane [1722-26-5] (C2H )2N BH, dimethyl sulfide borane [13292-87-OJ, (CH2)2S BH, and tetrahydrofuran borane [14044-65-6] C HgO BH, are more easily and safely handled than B2H and are commercially available. These compounds find wide use as reducing agents and in hydroboration reactions (57). A wide variety of borane reducing agents and hydroborating agents is available from Aldrich Chemical Co., Milwaukee, Wisconsin. Base displacement reactions can be used to convert one adduct to another. The relative stabiUties of BH adducts as a function of Group 15 and 16 donor atoms are P > N and S > O. This order has sparked controversy because the trend opposes the normal order estabUshed by BF. In the case of anionic nucleophiles, base displacement leads to ionic hydroborate adducts (eqs. 20,21). 

Borane dissolved in THF or dimethyl sulfide undergoes addition reactions rapidly with most alkenes. This reaction, which is known as hydroboration, has been extensively studied and a variety of useful synthetic processes have been developed, largely through the work of H. C. Brown and his associates. 

Hydroboration. This complex of borane is more reactive than borane-dimethyl sulfide (4, 124, 191 5, 47) and has the added advantage that after hydroboration, 1,4-oxathiane can be removed easily as the water-soluble sulfoxide. 

An alternative method of hydroboration is to use diisopinocampheylborane (12) (Scheme 4). This reaction is particularly useful for sterically hindered alkenes. Diisopinocampheylborane (12) is prepared from borane-dimethyl sulfide and (+)-pinene.[23-24] Treatment of 4-meth-ylenecyclohexanone ethylene ketal with diisopinocampheylborane (12) gives the borane 13.[25] Further treatment with 2 equivalents of an aldehyde results in the elimination of pinene and the formation of a new dialkyl boronate, e.g. treatment of 13 with acetaldehyde gives the diethyl cyclohexylmethylboronate 14J261 The dialkyl boronates thus produced can be transesterified with pinanediol to give 15[26] or with other cyclic diols. 

Al complexes prepared in situ from Al[OCH(CH3)2]3 and two equivalents of (K)-BINAPHTHOL (9) and (i )-H8-BINAPHTHOL (10) promoted the enanti-oselective reduction of propiophenone with borane-dimethyl sulfide and gave the S alcohol in 83% and 90% ee, respectively (Scheme 7) [47]. The reaction was much slower and afforded a racemic product in the absence of Al[OCH(CH3)2]3 under otherwise identical conditions. The addition of a catalytic amount of Al(OC2H5)3 increased both the rate and enantioselectivity in the hydroboration of ketones with a chiral amino alcohol [48]. 

Commercially available solutions of H3B-THF in THF (Aldrich-Boranes, Inc., 2371 N. 30th St., Milwaukee, WI 53210) which are used in this synthesis, contain approximately 5% of sodium tetrahydroboratefl —) as a stabilizer. Tri(sec-butyl)borane prepared from these solutions should be distilled. An alternative commercial (Aldrich-Boranes, Inc.) hydroborating agent free of sodium tetrahydroborate(l —) is borane-methyl sulfide. 

Diisopinocampheylborane (Ipc2BH) (1), 1, 262-263 4, 161-162 6, 202 8, 174 11, 188. Two methods have been reported for preparation of either ( + )- or (-)-Ipc2BH of high optical purity from commercially available (-)- or (+ )-a-pinene of lower optical purity. Essentially pure material (99% ee) selectively crystallizes from the reaction of a-pinene with borane-dimethyl sulfide in THF-diethyl ether after storage at 0° for several hours. Somewhat purer reagent (99.9% ee) can be prepared by hydroboration of ( + )-oi-pinene (84 or 91.6% ee) with optically pure (- )-monoisocampheylborane (8, 267).1... 

Thexylchloroborane-Dimethyl sulfide (1). This borane can be prepared by treating thexylborane-dimethyl sulfide with hydrogen chloride or by hydroboration of 2,3-di-methyl-2-butene with monochloroborane-dimethyl sulfide. 

Preparative Methods although now commercially available, the reagent is readily prepared by hydroboration of (+)-longifolene with Borane-Dimethyl Sulfide (BH3 -SMe2) in a 2 1 ratio (eq 1). 

Diborane is not stable at room temperature and is usually generated in situ. It is available from Alfa Inorganics but only in a very dilute solution in THF. Adams et al. report that dimethyl sulfide -borane provides a useful substitute for diborane. It is less reactive than diborane, but can be used for most hydroboration and reduction applications. 

Dichloroborane and monochloroborane etherates or their methyl sulfide complexes have been prepared by the reaction of borane and boron trichloride [44]. The hydroboration of alkenes with these borane reagents is, however, usually very slow because of the slow dissociation of the complex. Dichloroborane prepared in pentane from BCI3 and trimethylsilane is unusually highly reactive with alkenes and alkynes hydroboration is instantaneous at -78 °C (Eq. 20) [45]. 

Borane-dimethyl sulfide is a particularly useful reagent. The complex is more stable than borane-THF and can be prepared in neat form. It is soluble in a range of organic solvents and retains sufficient looseness to allow it to react readily with alkenes. It carries out most of the reactions of borane-THF, perhaps somewhat more slowly, and gives the same selectivity almost irrespective of solvent. It has been used to hydroborate alkynylsilanes at the a-position. Borane-thioxane is an alternative to borane-dimethyl sulfide. °... 

Monochloborane-dimethyl sulfide coexists with small amounts of the borane and dichloroborane complexes, but the bromoborane-dimethyl sulfide complex appears to be almost pure. - These complexes react readily with alkenes at 25 °C and can be used for hydroborations in a variety of solvents. Dialkylha-loboranes are obtained in high yield as their dimethyl sulfide complexes (equation 24), but dimethyl sulfide is readily removed under reduced pressure if required. - The reagents are also useful for cyclic hydroborations of dienes such as cyclooctadiene (equation 25). An alternative approach to dialkylbro-moboranes involves the reaction of dialkyl(methylthio)boranes with bromine. ... 

Unsymmetrical, nonconjugated dienes are generally easier to monohydroborate because of intrinsic differences between the two double bonds. Both 9-BBN-H and disiamylborane favor attachment to a terminal double bond rather than an internal double bond (e.g. equation 32). On the other hand, 2-methyl-1, S-hexadiene reacts predominantly at the 1-position with 9-BBN-H and almost exclusively at the 6-position with disiamylborane. The products of dihydroboration of a,o>-dienes with 9-BBN-H can be redistributed with borane-dimethyl sulfide to give boracyclanes. In this way, some of the problems sometimes associated with direct hydroboration of dienes with borane (see Section 3.10.2.1) may be overcome. 

Alkyldibromoboranes isomerize only slowly and dibromoborane-dimethyl sulfide may have advantages over borane-THF for hydroboration of problematical alkenes such as 1-methylcyclooctene" (compare equation 30, Section 3.10.4.2). 

If alkyldichloroboranes are specifically required, dichloroborane-dimethyl sulfide is the reagent of choice. It is more stable and more convenient than the dichloroborane-diethyl ether complex, but its hydroborating properties are very similar." Dichloroborane complexes ethers even more strongly than monochloroborane, and its reactions with alkenes in this solvent are slow and lead to mixtures. Therefore, it is generally used in pentane and trichloroborane is added to liberate uncomplexed dichloroborane. Under these conditions it readily gives alkyldichloroboranes on reaction with alkenes or alkenyldichloro-boranes on reaction with alkynes. " " The latter reaction has been applied to alkynylsilanes (equation 44)." ... 

Dilongifolylborane is simply prepared by admixture of borane-dimethyl sulfide and 2 equiv. (+)-lon-gifolene in THF, whereupon the product crystallizes out of solution as the dimer, and can readily be separated from the solvent. It is used as a suspension for hydroboration reactions. Diisopinocampheylborane can be prepared in a similar way from (+)- or (-)-a-pinene, and early work was carried out with reagent so prepared. However, ot-pinene is often available only in purities up to ca. 95%, so that Ipc2BH produced by direct hydroboration can also be somewhat impure. Fortunately, equilibration of the reagent (dimethyl sulfide must first be removed if borane-dimethyl sulfide is used for the hydroboration) with cn-pinene, at 0 C over several days, results in preferential incorporation of the major enantiomer of a-pinene into the Ipc2BH. This then becomes available in 98-99% enantiomeric purity. 

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