Borane derivatives

Mono- and diaLkylboranes obtained by controlled hydroboration of hindered olefins and by other methods can serve as valuable hydroborating agents for more reactive olefins. Heterosubstituted boranes are also available and used for this purpose. These borane derivatives show differences in reactivity and selectivity. 

A. tris(Alkylamino)boranes-Derived Poly Z -(alkylamino)borazines]... 

Palladium-catalyzed coupling of the borane derived from A with 1-bromo-1-pentyne to give B as well as the coupling of iodoalkene C with alkyne D were the two key-steps. 

C. 4-(3-Cyclohexenyl)-2-phenylthio-1-butene. To the above solution of the borane derivative, 0.809 g (0.700 mmol) of tetrakis(triphenylphosphine)palladium(0) (Note 9), 1.47 g (5.60 mmol) of triphenylphosphine (Note 10), 35 mL of 3 M potassium phosphate in water (Note 11), and finally 15.1 g (70.0 mmol) of 1-bromo-1-phenylthioethene are added and the resulting mixture is heated at reflux for 3 hr with stirring. The light brown solution is cooled to room temperature and treated with 6.4 g... 

Advances in the use of anionic stereogenic phosphorus have been interesting. Acylation of lithium o-anisylphenylphosphide with chloro-formates bearing chiral alkyl groups provided a diastereomeric mixture that could be induced to undergo an inversion at phosphorus (at relatively low temperature) to form the more favorable diastereoisomer in a crystalline lattice.186 Subsequent conversion to the quaternary phos-phonium species was followed by removal of the acyl group and isolation of the chiral tertiary phosphine as the borane derivative (Equation 3.10). 

The boron-11 n.m.r. spectrum of the compound [(CH3)3CCH=N-B(C4H9)2]n exhibits a strong signal at -7,4 ppm and a weak one at -38,8 ppm. At 120 °C the signal intensities reverse thus indicating that the dimer reverts to the monomer with increasing temperature 6>. The chemical shifts of imine-borane derivatives (c.f. Eq. (5)) are observed near +1,0 ppm 7>. 

Boranes have opened the door to asymmetric reduction of carbonyl compounds. The first attempt at modifying borane with a chiral ligand was reported by Fiaud and Kagan,75 who used amphetamine borane and desoxyephedrine borane to reduce acetophenone. The ee of the 1-phenyl ethanol obtained was quite low (<5%). A more successful borane-derived reagent, oxazaborolidine, was introduced by Hirao et al.76 in 1981 and was further improved by Itsuno and Corey.77 Today, this system can provide high stereoselectivity in the asymmetric reduction of carbonyl compounds, including alkyl ketones. 

Even though qualitative bonding descriptions of metal atom clusters up to six or seven atoms can be derived and in some cases correlated with structural detail, it is clear that most structures observed for higher clusters cannot be treated thus. Nor do the structures observed correlate with those observed for borane derivatives with the same number of vertices. Much of borane chemistry is dominated by the tendency to form structures derived from the icosahedron found in elemental boron. However, elemental transition metals possess either a close-packed or body-centered cubic arrangement. In this connection, one can find the vast majority of metal polyhedra in carbonyl cluster compounds within close-packed geometries, particularly hexagonal close-packing. 

The reduction of an unsymmetrical ketone creates a new stereo center. Because of the importance of hydroxy groups both in synthesis and in relation to the properties of molecules, including biological activity, there has been a great deal of effort directed toward enantioselective reduction of ketones. One approach is to use chiral borohydride reagents.92 Boranes derived from chiral alkenes can be converted to borohydrides, and there has been much study of the enantioselectivity of these reagents. Several of the reagents are commercially available. 

Diborane and other lower boranes pyrolyze when treated with borohydrides and other metallo borane derivatives at elevated temperatures undergoing polyhedral expansion ... 

Fig. 1 The most spherical (closo) polyhedra found in borane derivatives such as B H In Figs. 1 and 2, the degree 6, 4, and 3 vertices are indicated by an asterisk, a square, and a triangle, respectively, and degree 5 vertices are unmarked...

A 100-mL flask protected with a positive pressure of nitrogen is charged with 11 mmol of a 0.25 M pentane solution of iodo(diisopinocampheyl)borane [derived from ( +)-a-pinene]. The solution is cooled to — 100 C and treated dropwise with a solution of 980 mg (10 mmol) of 1,2-epoxycyclohexane in 2 mL of pentane. After stirring the solution for 0.5 h at —100 CC, the mixture is quenched by the addition of 1.8 mL (excess) of butanal. The mixture is allowed to warm gradually to r.t. and stirred for an additional 1 h. By that time "B NMR indicates clean formation of boronate [R OB(OEt)Ipc, d = 31], The mixture is diluted with 50 mL of pentane and treated with a 4 M THF solution of 12 mmol of 2.2 -diethanolamine. After the solution is stirred for 0.5 h, the precipitated boronate 2,2 -diethanolamine complex is filtered off and the filtrate is washed with water and brine and dried with anhyd Na2S04. The solution is concentrated under reduced pressure and the residue is purified by column chromatography followed by crystallization from pentane to provide the title compound in 89% yield mp 44 - 45°C [a] 3 —31.6 (c = 5, CHClj). 

HBR 2 may be BH3 (B3H6 = diborane) or other borane derivatives (see below). 

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

The hypho (Gr.. net ) series of boranes, with electronic formula 2n + 8, has beer suggested to augment the closo, nido, and arachno series. Although no neutral boranes fit this scheme, some borane derivatives do. It is also passible to construct units consisting of more than one of the above types. These are called cmjuncio (Latin, joined subunits ) structures. 

This product is then treated with a borane derivative (equation 40). 

Alternative routes to boron nitride have been examined that do not require those challenging reaction conditions. These fall into three categories, which are (1) the pyrolysis of borazines (2) pyrolysis of organic or inorganic polymers that bear borazine groups as side units and (3) pyrolysis of polyhedral borane derivatives. 

---