B-3-pinanyl-9-BBN

Reduction of a,p-ynones. a,/J-Acetylenic ketones are reduced by 1 to (S> propargyl alcohols in 86-96% enantioselective purity. It is thus comparable to B-3-pinanyl-9-BBN in enantiomeric yields, although the reductions require 1-2 days and an excess of neat reagent for complete reaction. 

Midland, M. M., Lee, P. E. Efficient asymmetric reduction of acyl cyanides with B-3-pinanyl 9-BBN (Alpine-borane). J. Org. Chem. 1985, 50, 3237-3239. 

A simple conversion of (-I- )-a- to (-I- )-P-pinene has been effected through the addition product with 9-BBN (9-borabicyclo[3.3.1]nonane). B-3-Pinanyl-9-BBN (806) undergoes exceptionally rapid olefin-alkyl group exchange after 30 hours reflux in diglyme it has reached equilibrium, the major component being the myrtenyl derivative 807, from which (-f-)-P-pinene [(-t-)-142] is liberated with benzaldehyde. From (-I- )-a-pinene of 92% optical purity, (+ )-p-pinene of... 

Hydroxy-esters. - Once again, most contributions to this area involve the synthesis of chiral hydroxy-esters. Almost complete enantioselectivity is achieved in the reduction of a-keto-esters to a-hydroxy-esters using Alpine-Borane (B-(3-pinanyl)-9-BBN) derived from either (+)- or (-)-a-pinene when the reactions are carried out at relatively high concentrations (>2M). Many other types of prochiral ketones are also reduced with excellent asymmetric inductions although 3-keto-esters may not be particularly suitable substrates as ethyl acetoacetate is reduced to ethyl 3-hydroxybutanoate with an enantiomeric excess of only... 

The rate of reduction of aldehydes with B-alkyl-9-BBN compounds is greatly dependent on the structure of the alkyl group on boron [9]. In general, rate of reduction is very fast when alkyl group has tertiary ( -hydride, while with a primary (5-hydride provides a very slow rate. For example, B-3-pinanyl-9-BBN reacts rapidly and exothermally at room temperature and the VPC and NMR techniques have proven futile, since the reaction is so fast. Consequently, the... 

The relative rate constants for the reduction at 25 °C are given in Table 4.20 [12]. As the reductions with B-3-pinanyl-9-BBN are too fast, the relative rates are obtained by competition experiments. 

The rate of reduction with B- -octyl-9-BBN ofp-substituted benzaldehydes correlates with (p + 1.03) [12]. Relative rates ofp-substituted benzaldehydes with B-3-pinanyl-9-BBN gave a of + 0.49. Electron-withdrawing groups increase the rate of reduction of substituted benzaldehydes. This effect is presumably also important for alkynyl ketones [7], where the small steric size of the acetylene and its negative inductive effect can combine to make the reduction possible. 

The systematic studies conducted by Midland and coworkers [8] have shown that among the various B-alkyl-9-BBN obtained from (+)-a-pinene, (-)-(3-pi-nene, (-)- camphene, and (+) 3-carene (Chart 26.2), B-3-pinanyl-9-BBN (Alpine-Borane) shows remarkable enantioselectivity as illustrated in Table 26.1 [8]. 

Therefore, this avoids the necessity of prior preparation [6] of 1-deuterated aldehydes. The absolute configuration of the major products are consistently K)- when deuterated Alpine-Borane (from (+)-a-pinene) is used for the reduction. The product of (S)-configuration is readily obtained by using deuterated B-3-pinanyl-9-BBN prepared from (-)-a-pinene and 9-BBN-9-D. On the other hand, reduction of deuterated aldehyde with Alpine-Borane from (-h)-a-pinene affords (S)-alcohols and (f )-alcohols are obtained when (-)-a-pinene is used (Chart 26.3). 

Table 26.2 Reduction of aldehydes with deuterated B-3-pinanyl-9-BBN [8]...

Consequently, a deuterated B-3-pinanyl-9-BBN finds applications in the reduction of variety of aromatic, aliphatic, and a,p-unsaturated aldehydes to the corresponding chiral primary 1-deuterio alcohols either with (R)- or (S)-con-figuration. 

Addition of alane and borane to alkenes affords a host of alkylated alanes and boranes with various reducing properties (and sometimes bizarre names) diisobutylalane (Dibal-H ) [104], 9-borabicyclo[3.3.1]nonane (9-BBN) (prepared from borane and 1,5-cyclooctadiene) [705], mono- [106,107] and diiso-pinocampheylborane (B-di-3-pinanylborane) (both prepared from borane and optically active a-pinene) [108], isopinocampheyl-9-borabicyclo[3.3.1 Jnonane alias B-3-pinanyl-9-borabicyclo[3.3.1]nonane (3-pinanyl-9-BBN) (prepared from 9-borabicyclo [3.3.1]nonane and a-pinene) [709], NB-Enanthrane prepared from 9-borabicyclo[3.3.1]nonane and nopol benzyl ether) [770] and others. ... 

Two other purely chemical routes to chiral acetic acid have been pursued in our laboratory. In one of these, outlined in Scheme 4, stereospecifically o-deuterated or -tritiated 3,5-dimethoxybenzyl alcohol is prepared by reduction of the aldehyde with Midland s reagent (B-3-pinanyl-9-borabicyclo [3.3.1 ] -nonane, cr-pinanyl-9-BBN) (35), followed by conversion to the tosylate and reductive displacement with lithium aluminum hydride or superhydride (lithium... 

Alpine-Borane (Midland) Alpine-Borane is a trademark of Aldrich Chemical Company for B-3-pinanyl-9-borabicyclo[3.3.1]nonane. It is prepared by hydroboration of a-pinene with 9-borabicyclo[3.3.1]nonane (9-BBN). Synthesis of (1 )-Alpine-Borane is prepared from (17 )-(+)-a-pinene, and since (+)- and (-)-a-pinenes are commercially available, both enantiomers of the reagent can easily be prepared. The reagent is only suitable for very reactive substrates, such as labeled aldehydes and acetylenic ketones (Scheme 2.136) [64],... 

Midland, M. M., Petre, J. E., Zderic, S. A., Kazubski, A. Thermal reactions of B-alkyl-9-borabicyclo[3.3.1]nonane (9-BBN). Evidence for unusually facile dehydroboration with B-pinanyl-9-BBN. J. Am. Chem. Soc. 1982,104, 528-531. 

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