Dialkylboranes

Mono-, di-, and trialkylboranes may be obtained from olefins and the trifunctional borane molecule. Simple unhindered alkenes yield trialkylboranes and it is not possible to halt the reaction at the mono- or dialkylborane stage. With more hindered and trisubstituted alkenes the reaction can be controlled to stop at the dialkylborane stage. 

Mono- and dialkylboranes usually exist as bridged dimers, yy -dialkyldiboranes and yy -tetraalkyldiboranes. Only very hindered alkylboranes, eg, bis(2,3-dimethyl-2-butyl) borane (39), are monomeric. However, for convenience of presentation monomers are shown in the equations. 

Retardation of the reaction rate by the addition of dimethyl sulfide is in accord with this mechanism. Borane—amine complexes and the dibromoborane—dimethyl sulfide complex react similarly (43). Dimeric diaLkylboranes initially dissociate (at rate to the monomers subsequentiy reacting with an olefin at rate (44). For highly reactive olefins > k - (recombination) and the reaction is first-order in the dimer. For slowly reacting olefins k - > and the reaction shows 0.5 order in the dimer. 

Mono- and diaLkylboranes obtained by controlled hydroboration of hindered olefins and by other methods can serve as valuable hydroborating agents for more reactive olefins. Heterosubstituted boranes are also available and used for this purpose. These borane derivatives show differences in reactivity and selectivity. 

A number of less hindered monoalkylboranes is available by indirect methods, eg, by treatment of a thexylborane—amine complex with an olefin (69), the reduction of monohalogenoboranes or esters of boronic acids with metal hydrides (70—72), the redistribution of dialkylboranes with borane (64) or the displacement of an alkene from a dialkylborane by the addition of a tertiary amine (73). To avoid redistribution, monoalkylboranes are best used /V situ or freshly prepared. However, they can be stored as monoalkylborohydrides or complexes with tertiary amines. The free monoalkylboranes can be hberated from these derivatives when required (69,74—76). Methylborane, a remarkably unhindered monoalkylborane, exhibits extraordinary hydroboration characteristics. It hydroborates hindered and even unhindered olefins to give sequentially alkylmethyl- and dialkylmethylboranes (77—80). 

Primary dialkylboranes react readily with most alkenes at ambient temperatures and dihydroborate terminal acetylenes. However, these unhindered dialkylboranes exist in equiUbtium with mono- and ttialkylboranes and cannot be prepared in a state of high purity by the reaction of two equivalents of an alkene with borane (35—38). Nevertheless, such mixtures can be used for hydroboration if the products are acceptable for further transformations or can be separated (90). When pure primary dialkylboranes are required they are best prepared by the reduction of dialkylhalogenoboranes with metal hydrides (91—93). To avoid redistribution they must be used immediately or be stabilized as amine complexes or converted into dialkylborohydtides. 

Borabicyclo [3.3.1] nonane [280-64-8], 9-BBN (13) is the most versatile hydroborating agent among dialkylboranes. It is commercially available or can be conveniendy prepared by the hydroboration of 1,5-cyclooctadiene with borane, followed by thermal isomerization of the mixture of isomeric bicychc boranes initially formed (57,109). 

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... 

Table 4.3 provides some data on the regioselectivity of addition of diborane and several of its derivatives to representative alkenes. Table 4.3 includes data for some mono- and dialkylboranes that show even higher regioselectivity than diborane itself. These derivatives are widely used in synthesis and are frequently referred to by the shortened names shown with the structures. 

As oxidation also converts the original chiral terpene-derived group to an alcohol, it is not directly reusable as a chiral auxiliary. Although this is not a problem with inexpensive materials, the overall efficiency of generation of enantiomerically pure product is improved by procedures that can regenerate the original terpene. This can be done by heating the dialkylborane intermediate with acetaldehyde. The a-pinene is released and a diethoxyborane is produced.204... 

Organoboranes can also be made using organocopper reagents. One route to methyl and aryl derivatives is by reaction of a dialkylborane, such as 9-BBN, with a cuprate reagent.4... 

Thexylchloroborane can be alkylated and then converted to a dialkylborane by a reducing agent such as KBH[OCH(CH3)2]3, an approach that is preferred for terminal alkenes. 

The dialkylboranes can be prepared from thexylchloroborane. The thexyl group does not normally migrate. 

If an internal olefin is treated similarly and the resulting branched trialkylborane heated at about 160° in diethylene glycol dimethyl ether, the molecule is rearranged and finally the terminal trialkylborane is produced. The isomerization of the organoborane proceeds through a series of eliminations to regenerate olefin and dialkylborane, followed by... 

Another example of great synthetic interest, involves the hydroboration reaction of alkenes [62], In general, the addition of borane to alkenes proceeds stepwise, the final product being the trialkylborane. However, hindered alkenes react slowly, especially when the dialkylborane precipitates from the medium. It was found that trialkyl bor-anes could be obtained rapidly under sonication, even with highly hindered substrates (Eq. 3.5). Applications of this useful modification were published, among which were the reduction-hydroxylation of vinyl groups by 9-BBN [63,64]. 

Since the olefin is hydroborated to the dialkylborane stage (RgBH), a large amount of hydrogen is evolved on hydrolysis. Consequently, the addition of water should be carried out slowly (dropwise) and adequate ventilation is recommended. 

Chiral Dialkylboranes. Several allylic boranes have been developed as chiral auxiliary reagents (Fig. 5). The introduction of terpene-based reagents such as 12 and 64-68 has been pioneered by H.C. Brown, and the most popular class remains the bis(isopinocampheyl) derivatives (structures 12, 64-66). A wide variety of substituted analogs have been reported, including the popular crotylboranes but also a number of other reagents bearing heteroatom-... 

Other methods are also available for generation of boron enolates. Dialkylboranes react with acyclic enones to give Z-enolates by a 1,4-reduction.19 The preferred Z-stereochemistry is attributed to a cyclic mechanism for hydride transfer ... 

Alkenylboranes are less sensitive to substituents on the diene than are carbonyl-activated dienophiles.33 Relatively hindered dialkylboranes, such as S-vinyl-9-BBN, show steric effects which lead to a preference for the meta regioisomer and reduced endo exo ratios.34... 

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