Hydroboration reagents for

Unsymmetrical functional tetraorganotins are generally prepared by tin hydride addition (hydrostaimation) to functional unsaturated organic compounds (88) (see Hydroboration). The realization that organotin hydrides readily add to atiphatic carbon—carbon double and triple bonds forming tin—carbon bonds led to a synthetic method which does not rely on reactive organometatiic reagents for tin—carbon bond formation and, thus, allows the synthesis of... 

Since borane BH3 reacts with only one or two equivalents of a sterically hindered alkene, it is possible to prepare less reactive and more selective borane reagents R2BH and RBH2 respectively. In addition to disiamylborane 8 and thexylbo-rane 10, the 9-borabicyclo[3.3.1]nonane (9-BBN) 14 is an important reagent for hydroboration, since it is stable to air it is prepared by addition of borane 2 to cycloocta-1,5-diene 13 ... 

This reagent can be used for the enantioselective hydroboration of Z-alkenes with enantiomeric excess of up to 98%. Other chiral hydroboration reagents have been developed. ... 

Several 2-butenylboron reagents have been synthesized by the hydroboration of allenes9 36. Diisopinoeampheyl(3-methyl-2-butenyl)borane is an excellent reagent for the asymmetric iso-prenylation of aldehydes (typically 89 95% ee), and is conveniently synthesized by the hydroboration of 3-methyl-l,2-butadiene using diisopinocampheylborane9. 

Catecholborane and pinacolborane, in which the boron has two oxygen substituents, are much less reactive hydroborating reagents than alkyl or haloboranes because the boron electron deficiency is attenuated by the oxygen atoms. Nevertheless, they are useful reagents for certain applications.161 The reactivity of catecholborane has been found to be substantially enhanced by addition of 10-20% of N,N-dimethylacetamide to CH2C12.162... 

Alkynes are reactive toward hydroboration reagents. The most useful procedures involve addition of a disubstituted borane to the alkyne, which avoids complications that occur with borane and lead to polymeric structures. Catechol borane is a particularly useful reagent for hydroboration of alkynes.212 Protonolysis of the adduct with acetic acid results in reduction of the alkyne to the corresponding cw-alkene. Oxidative workup with hydrogen peroxide gives ketones via enol intermediates. 

Boronic esters have been used in a wide range of transformations. These useful reagents have been transformed into numerous functional groups and are essential reagents for several C-C bond-forming reactions. Transition metal-catalyzed hydroboration of olefins often leads to mixtures of branched and linear products. Several groups have reported asymmetric reductions of vinyl boronic esters [50-52] with chiral rhodium P,P complexes however, the first iridium-catalyzed reduction was reported by Paptchikhine et al (Scheme 10) [53]. 

Alkenylcatecholborane 11 is a good reagent for the conjugate addition and is easily obtained by the hydroboration of an alkyne with catecholborane. One-pot asymmetric synthesis of the conjugate addition product, /9-alkenyl ketone, is possible starting from an alkyne and catecholborane without isolation of the alkenylcatecholborane [12]. 

Sulfonylhydroxylamines and hydroxylamine O-sulfonic acid have found wide apph-cation in synthesis of amines from achiral or chiral organoboranes and boronate esters and the hydroboration-amination methodology is successfully used for direct amination of alkenes. 0-Sulfonyloximes were also found to be good reagents for synthesis of amines from organomagnesium, -copper and -zinc reagents. 

An interesting new method for the preparation of the methoxycarbonyl (and related) compound (35) involves the hydroboration-carbon monoxide insertion in bis-alkenic amines (34). The best reagent for this process is thexylborane, followed by cyanidation (82JOC1494). The yields are not very good and seven-membered ring compounds (36) can also be formed, but the procedure is short and simple. 

WitlUiindered alkenes, it is more difficult to add three a.kenes to borane. This becomes the basis for unique, borane derivatives. Sw Hydroboration Reagents. 

Notes 1. Use as other hydroborating reagents. Its value is in the increased reagent stability and solvent solubility. Like other hydroborating agents, it is stable to an array of functional groups. It is useful for the reduction of ozonides. 

A useful reagent for asymmetric hydroboration. See Brown s Hvdroboration Reaction. 

Hydroxy-6-methyl-5,6-dihydropyran-2-one (597) is brominated by NBS at C-3 (78JHC1153). More than one product is often obtained in this type of reaction, for example from 2,3-dihydro-4//-pyran (593), but in acetic acid this reaction gives 2-acetoxy-3-bromotetrahydropyran (599) (58JOC1128). 2,3-Dihydro-4H-pyran reacts normally with hydroboration reagents to give tetrahydropyran-3-ol in 80% yield (70JOC2282). 

These two alkylboranes are important reagents for the hydroboration of other alkenes, since they frequently show in this further reaction a regioselectivity which is greater than that of borane itself. 

Hydroboration.1 The usual hydroboration reagents, BH3THF and BH3-S(CH3)2, are sensitive to oxygen and moisture and require special handling. 1 he complexes of BH3 and phosphorus compounds are generally stable, but much less reactive. The complex of BH3 and triphenylphosphine, m.p. 189°, can be used for hydroboration if activated by addition of methyl iodide (to form a phosphonium iodide) or sulfur (to form a triphenylphosphine sulfoxide). The complex of borane and triphenyl phosphite does not require activation and hydroborates alkenes in a reasonable time in refluxing DME or THF. Trialkyl phosphite complexes are not useful. 

Asymmetric hydroboration.2 In a review of asymmetric hydroboration, Brown el al. conclude that this is the preferred reagent for asymmetric hydroboration of unhindered n j-alkenes. Thus, (R)-(—)-2-butanol can be prepared from cis-2-butene with ( —)-l in 98% ee and (S)-( + )-2-butanol is obtained using (+)-l in 95% ee. The alcohols obtained in this way have the same absolute configuration. 

Borane transforms a wide range of alkenes into trialkylboranes under mild conditions but the trifunctional nature of borane and its trialkylborane products imposes some limitations on its use. Many of the synthetically useful reactions of the trialkylboranes (see Chapters B.2 and B.3) use all three alkyl substituents, but some reactions only utilize either two or even one of the alkyl substituents. This sets a maximum yield (based on the alkene starting material) for these latter transformations of 66% and 33% respectively which is clearly undesirable especially if the alkene involved is the product of a multi-step synthetic sequence. To overcome this problem, and others such as the production of intractable polymers on addition of borane to dienes and alkynes, monoalkylborane and dialkylborane hydroborating reagents were introduced. Some commonly used reagents are depicted in Figure B 1.2 and two are described in more detail below. 

The two transition states for the addition of a homochiral hydroborating reagent to the two faces of a prochiral alkene are diastereoisomeric and of different energy. 

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