Diisopinocampheylborane reaction with alkenes

Kinetic resolution.l33 Since enantiomers react with chiral compounds at different rates, it is sometimes possible to effect a partial separation by stopping the reaction before completion. This method is very similar to the asymmetric syntheses discussed on p. 102. An important application of this method is the resolution of racemic alkenes by treatment with optically active diisopinocampheylborane,134 since alkenes do not easily lend themselves to conversion to diastereomers if no other functional groups are present. Another example is the resolution of allylic alcohols such as 45 with one enantiomer of a chiral epoxidizing agent (see 5-36).135 In the case of 45 the discrimination was extreme. One enantiomer was converted to the epoxide and the other was not, the rate ratio (hence the selectivity factor)... 

Monoisopinocampheylborane (method A) has a lower steric requirement than diisopinocampheylborane (method C). This feature makes it well suited for reactions with more hindered alkenes. Thus, the two reagents are complementary. The optical purities of the alcohols obtained from the hydroboration of disubstituted T-alkenes or trisubstituted alkenes with monoisopinocampheylborane are high (Table 2) and increase with an increase in the steric... 

Diisopinocampheylborane reacts with sterically less hindered (Z)-alkenes 34 at a considerably faster rate even at low temperatures (-25°C) to provide chiral alkyldiisopinocampheylboranes 36. Reaction of 36 with an aldehyde leads to the elimination of a-pinene and furnishes the chiral alkyl-boronates 37 in moderate-to-high ee (Scheme 28.11). The enantioselectivity can be further enhanced via recrystallization of the chelates 42 of alkylborinates 40 with chiral amino alcohols such as prolinol (Scheme 28.11). ... 

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. 

Diisopinocampheylborane is usually used for the hydroboration of relatively unhindered, disubstituted Z-alkenes. Optical purities of the product alcohols are high (Table 2) while the absolute configuration is consistently the same for similar structures. Models predicting the product configuration in the hydroboration of alkenes with mono- and diisopinocampheylborane have been proposed54-59. The preferred transition state structure for the reaction of (Z)-2-butene with ( — )-diisopinocampheylborane is shown in Section D.2.5.2.1.3. 

In Fig. 4, reaction A is a highly stereoselective reduction of 1-aryl alkanones with (-)-chlo-ro diisopinocampheylborane [21]. Upon co-ordination of the ketone oxygen with the Lewis acidic chirotopic and non-stereogenic [22] boron atom of the chiral reagent, two diastereo-isomeric complexes arise. The sterically less hindered one is preferentially formed and leads the major (,S)-enantiomer, which is isolated after a work-up that allows recovery of a-pinene, the chiral alkene from which the borane is prepared. 

As a final example of a process in which enantioselective synthesis is achieved using an optically active reagent, let us consider some of the results of hydroboration with optically active boranes. The boranes are normally prepared by reaction of diborane with one of several available terpene-derived alkenes. One such reagent is diisopinocampheylborane, which is prepared from diborane and a-pinene. ... 

Chemistry-based kinetic resolution methods, which make use of the preferential reaction of one enantiomer with a chiral reagent (e.g. hydroboration of racemic alkenes with diisopinocampheylborane) or an achiral reagent in the presence of an appropriate chiral catalyst (e.g. Sharpless epoxidation of racemic allylic alcohols with t-BuOOH in the presence of (2R,3R)- or (25,35)-diisopropyl tartrate and Ti(Oi-Pr)4) have not been exploited so far for the isolation of e.p. labeled substances. In contrast, biochemical methods have been widely used, particularly for the resolution of racemic a-[ " C]amino acids and various [ C]carboxylic acids. Such methods, including ... 

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