Complexes with Borane

Complexes with borane can be obtained due to the presence of the tricoordinated phosphorus atom [Eq. (103)] (90IZV1120). When heated... 

Fluorophosphates, 16 183 Fluorophosphine complexes with borane, 13 439-444 coordination, 13 41CM14 bonding, in, 13 410-414 with nontransition metal halides, 13 445-447... 

Nonbonded complexes. The equilibrium constants, enthalpies, and entropies for the weak complexation of pyrazine with dichloromethane, chloroform, or carbon tetrachloride have been determined from changes in the n — 77 absorptions of solutions at various concentrations (in cyclohexane) and temperatures 568 similar data for pyrazine-aromatic hydrocarbon complexes were obtained from variations in the H NMR chemical shift values.1037 The spectral effects of complexation with borane have been studied in the pyrazine diborane adduct and its methyl derivatives.254... 

This naked selenolate is more reactive than the one complexed with borane 1 [1]. For example, in the presence of HMPA, 4 undergoes an SN2-type ester cleavage to produce the corresponding acids and alkyl phenyl selenides (Sect. 3.3) [6 a]. Uncomplexed selenolate 4 can also be prepared by the reduction of benzeneselenol (PhSeH) with sodium hydride (NaH) (Scheme 4b) [6aj. 

Activation of aziridines. Lithiation of aziridines with x-BuLi becomes routine after complexation with borane. 

Treatment of the amino alcohol with borane provides the oxazaborolidine catalyst, which presumably complexes with borane to provide the reducing agent. 

One of the more widely used solutions to this challenge is the chiral borohydride analogue invented by Itsuno in Japan and developed by Corey, Bakshi, and Shibata. It is based on a stable boron heterocycie made from an amino alcohol derived from proline (see the box below for the synthesis), and is known as the CBS catalyst after its developers. The active reducing agent is generated when the heterocycie forms a complex with borane. Only catalytic amounts (usually about 10%) of the boron heterocycie are needed because borane is sufficiently reactive to reduce ketones only when complexed with the nitrogen atom. The rest of the borane just waits until a molecule of catalyst becomes free. 

As might be expected from the ability of THF to complex with borane (vide supra), reaction of ethers with electron-deficient species is common and,indeed, the reactions of ethers with electron-deficient species defines most of their chemistry. 

Addition to Aldehydes. The Vedejs oxazole metalation has often been applied in the addition of oxazoles to aldehydes. In order to avoid ring opening to the isonitrile, the oxazole is first complexed with borane, then reacted with BuLi to generate the 2-metalo nucleophile (eq 15). This has been applied in the synthesis of a-ketooxazole inhibitors, and angiotensin II (AT2)... 

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