Synthesis boranes

Brown, H. C. 1975, Organic Synthesis via Boranes, Wiley New York London... 

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... 

Similai selectivity is observed in the synthesis of allylsilanes where X = CF3SO3 and Y = Si(CH3) 3 (304). Alkenyl- and alkynylborates containing a leaving group in the y-position rearrange to aUyhc and aUenic boranes, respectively (305). 

Fig. 7. Preparation of Alpine-borane (93) and use in the synthesis of homochiral butyrolactones and arylhydroxytetronic acids. Rg and denote small and...

Borane complexes are the most widely used commercial boron compounds, after sodium borobydride. Examples used in organic synthesis are amine borane complexes and borane complexes of tetrahydrofuran and dimethyl sulfide. 

Synthesis. An efficient, convenient synthesis for the preparation of ammonia borane [13774-81 -7] the inorganic analogue of ethane, is shown in equation 1 where THF is tetrahydrofuran (6). 

A synthesis using Hquid ammonia and NaBH has also been described (7). Both methylamine borane [1722-33-4] (CH3)H2N BH, and ethylamine borane... 

The synthesis of a number of other amine—borane complexes from THF BH [14044-65-6] have also been described (10). 

Using a procedure similar to the synthesis of amine boranes, a series of amine cyanoboranes where the amine = (CH3)3N, (CH3)2NH, (CH3)NH2, C3H3N, or (CgH3)NH2 have been prepared as shown in equation 4 (11). 

Compounds of the type LBH2X can be prepared by the reaction of the appropriate amine borane and hydrogen haHdes or halogens. The synthesis of the trimethyl amine iodoborane [25741-81-5] adduct (eq. 8) yields a precursor for the preparation of the trimethyl amine isocyanoborane [60045-36-5] adduct as shown in equation 9 (15). 

Synthesis. One of the more common routes for the synthesis of aminoboranes involves the aminolysis of the appropriate boron hahde. Trisaminoboranes are most convenientiy prepared by adding BCI3 to an excess of amine in an inert solvent at low temperatures (42). For example for tris(dimetby1amino)borane [4375-83-1]. ... 

Boracyclohexa-2,5-diene, 1-alkyl-synthesis, 1, 642 Borane, amino-bonds, 1, 630... 

In the early work on the synthesis of prostaglandins, zinc borohydride was used for the reduction of the 15-ketone function and a 1 1 mixture of epimeric 15(S)- and 15(/ )-alcohols was generally obtained. Subsequent studies led to reaction conditions for highly selective reduction to the desired 15(S)-alcohol. Some of the results are summarized in the following table. The most practical method is E which utilizes borane as the stoichiometric reductant and a chiral, enzyme-like catalyst which is shown. 

A more convenient high-yield synthesis of Bi2Hi2 is by the direct reaction of amine-boranes with BiqHu in the absence of solvents ... 

The synthesis of cZos o-borane dianions B H (1-7) relies principally on thermolysis reactions of boranes in the presence of either BH4 or amino-borane adducts. " The yields... 

Borane complexes of P-heterocycles as versatile precursors for the synthesis of chiral phosphine ligands used for asymmetric catalysis 98S1391. 

Olefins with 9-borabicyclo[3.3.1]nonane residues in the synthesis of borane-containing polyolefins 99JOM(581)176. 

Scheme 4 outlines the synthesis of key intermediate 7 in its correct absolute stereochemical form from readily available (S)-(-)-malic acid (15). Simultaneous protection of the contiguous carboxyl and secondary hydroxyl groups in the form of an acetonide proceeds smoothly with 2,2 -dimethoxypropane and para-toluene-sulfonic acid and provides intermediate 26 as a crystalline solid in 75-85 % yield. Chemoselective reduction of the terminal carboxyl group in 26 with borane-tetrahydrofuran complex (B H3 THF) affords a primary hydroxyl group that attacks the proximal carbonyl group, upon acidification, to give a hydroxybutyrolactone. Treat-... 

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