Nitrile-borane adducts

Equilibria involving both forms 6.24,31,35) as Well as equlibria between the dimeric form and the nitrile-borane adducts, (III)24 31), are known to exist. 

Transformations of dimeric iminoboranes into the corresponding nitrile-borane adducts, due to temperature changes during sublimation or distillation, have been observed for the reaction products of pentafluorobenzonitrile and tribromoborane or organodibromoboranes 24) ... 

This cited reaction illustrates that the C=N double bond of iminoboranes is quite stable. Indeed, the C=N bond in these compounds tends to increase its bond order, forming corresponding nitriles, rather than to undergo further 1,2-additions leading to aminoboranes. This suggestion is confirmed by several reported transformations of iminoboranes to nitrile-borane adducts (Eq. (20)) M). Addition across the C=N double bond of iminoboranes is virtually unknown. This event is also true for related imines (e.g., dichloromethylenealkyl-amines) which yield imine-trihaloborane adducts with trihaloboranes rather than to undergo a 1,2-addition (c.f. Sect. VII). 

Infrared spectroscopy is an excellent tool in iminoborane chemistry, which readily permits, to distinguish between iminoboranes and nitrile-borane adducts and to identify monomeric and dimeric forms of iminoboranes. This event is due to the fact that the i>CN of CN multiple bonds absorbs outside the fingerprint region and can be considered to be a valuable group frequency even when mixed with other vibrational modes. In some cases other vibrations like NH, BH, B-halogen or B-S stretching modes are helpful for determining the structure of iminoboranes. 

In the spectra of those nitrile-boranes having a CN bond order of about 3 the absorption due to the CN triple bond is observed near 2300 cm-1, which is higher than the CN frequency of the free nitriles 19>. This event has been attributed to strict sp-hybridisation and to the nonexistence of canonical forms involving C=N double bonds in these adducts 9>. 

Treatment of the bis(propynyl)zirconocene 759 with B(C6Fs)3 results in a linear G-G coupling of the alkynyl ligands to form the zwitterionic complex 76 0582,583 (Scheme 186). Complex 760 reacts with nitriles R CN to form initially the 1 1 adduct 761 that concurrently equilibrates with 760 and the metallacyclocumulene 762 (and the nitrile-borane adduct) subsequently, irreversible reaction in the presence of excess nitrile yields the methylene-cyclopropene derivative 763.584,585 Calculations have shown that the conversion 760 —> 763 is probably triggered by nitrile addition to the metal with formation of a planar-tetracoordinate carbon intermediate that features coordination of the three-membered carbocycle through one of its G-G cr-bonds. 

Scheme 11.14 Catalytic reduction of nitrile-borane adducts by RjP/B(C6F5)3/H2. (Mes = C6H2Me3).

Two new syntheses of the juvenile hormone, juvabione (57a) have been eompleted. In the first synthesis, i -(-f )-limonene was treated with disiamyl-borane and the adduct oxidised to yield the two alcohols (58a) and (58b) which were separated by fractional crystallisation of their diastereoisomeric 3,5-dinitrobenzoates and subsequent hydrolysis. Eaeh alcohol was separately converted to the corresponding nitrile and these on treatment with isobutyl-lithium gave the optically pure ketones (59a) and (59b). Oxidative modification yielded... 

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