Borane Addition Compounds of Bases

Heitsch, Abstracts, 153d National Meeting, American Chemical Society, Miami Beach, Florida, 1967, p. L-109. 

Submitted by ROBERT C. MOORE, SIDNEY S. WHITE, JR., and HENRY C. KELLY Cheeked by DOUGLAS L. DENTONt and SHELDON G. SHOREt 

---

High-quality albuterol was obtained in good yield from this process. However, several environmental disadvantages were identified. The preparation of the keto aldehyde hydrate (KAH) generated dimethyl sulfide, methyl bromide, and trimethyl-sulfonium bromide (this compound sublimed in the condenser). In addition, reduction of the Schiff base with dimethylsulfide borane, although very attractive in simplifying... 

A further method for the synthesis of the title compounds with only hydrogen as byproduct is the base-catalyzed dehydrogenative coupling (index D) of ammonia and tris(hydridosilylethyl)boranes, B[C2H4Si(R)H2]3 (R = H, CH3). Initially, the strong base, e.g. n-butyl lithium, deprotonates ammonia. The highly nucleophilic amide replaces a silicon-bonded hydride to form a silylamine and lithium hydride, which then deprotonates ammonia, resuming the catalytic cycle. Under the conditions used, silylamines are not stable and by elimination of ammonia, polysilazane frameworks form. In addition, compounds B[C2l-L Si(R)H2]3 can be obtained from vinylsilanes, H2C=CHSi(R)H2 (R - H, CH3), and borane dimethylsulfide. 

Thus far we have seen that Schiff base ligands tend to promote six-coordination, but an unusual reaction has been found to occur between TiCl2(salen) and borohydride. No reduction at titanium is seen, but rather, addition of BtLr to the imino function of the ligand occurs to give a dimeric seven-coordinate titanium(IV) compound, (24), containing amine-boranes as... 

With this repertoire of bonding possibilities at our disposal, we car construct the molecular structures of various boron-hydrogen compounds, both neutral species and anions. The simplest is the tetrahydroborate126 or borohydride ion, BH. Although borane is unstable with respect to dimerization, the addition of a Lewis base, H , satisfies the fourth valency of boron and provides a stable entity. Other Lewis bases can coordinate as well. 

CP-96,345 (2) was synthesized by a route with literature precedent based on the 1,4-addition of aryl Grignard reagents to benzylidene quinuclidone derivatives such as 6.3 The hindered borane reagent 9-BBN afforded compound 7 with the desired cis-stereochemistry. Subsequent deblocking and reintroduction of the benzyl side chain furnished analogs depicted by 8 for exploring benzylamine side chain SAR. 

Hydroboration is the addition of borane, BH3, to an alkene to form a trialkyl-borane. Borane cannot be prepared as a pure compound because it reacts with itself (2BHj B2H0) to form diborane BgHe, a toxic gas that ignites spontaneously in air. However, BH3 forms a stable Lewis acid-base complex with ethers and is most commonly used as a commercially available solution of BH3 in tetrahydrofuran (THF). 

---