Boranes adducts with amines

Borane-amines. 13,42 18,58 19,43—44 2 Borane adducts with tertiary amines in (e.g., 1 and 2 ) have been prepared for hydrt... 

Whittell, G.R., Balmond, E.L, Robertson, A.P.M., Patra, S.K., Haddow, M.F. and Manners, L, Reactions of amine- and phosphane-borane adducts with frustrated Lewis pair combinations of group 14 triflates and sterically hindered nitrogen bases. Ear. J. Inorg. Chem. 2010 (25), 3967-3975 (2010). 

The reaction between a Lewis acid R3M and a Lewis base ER3 is of fundamental interest in main group chemistry. Synthetic and computational chemists have investigated the influence of both the Lewis acid and the base on the solid state structure and the thermodynamic stability of the corresponding adduct, that is usually expressed in terms of the dissociation enthalpy De. This led to a sophisticated understanding of the nature of dative bonding interactions. In particular, reactions of boranes, alanes and gallanes MR3 with amines and phosphines ER3, typically leading to adducts of the type R3M <— ER3, have been studied.10... 

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). 

Octaborane(12) is a strong Lewis acid forming 1 1 adducts with ethyl ether, trimethyl amine and acetonitrile 223>. With sodium hydride in ether, approximately two thirds of a mole of hydrogen per mole of octaborane(12) is produced but the product is said to be apparently identical with the product of the reaction of sodium amalgam with octa-borane(12) which produces no hydrogen 223>. 

Preparation of B-fluoroborazines cannot be effected from trifluoro-borane and amines by a Brown-Laubengayer type of reaction. However, direct synthesis of B-fluoroborazine rings has been reported recently and involves the dehydrofluorination of primary amine-trifluoroborane adducts with the aid of the adducts of sterically hindered tertiary amines with trifluoroborane. 2,4,6-Trifluoroborazines and ammoniumtetra-fluoroborates sl) are obtained in excellent yield. 

The reactivity of BX3 fragments toward bifunctional Tewis bases (e.g. R2ENR2, RE(NR2)2, and E(NR2)3 E=P, As) are also of fundamental interest. It is clear that a combination of substituent inductive effects, E- N p r-d r electronic effects, as well as steric influences are responsible for the BX3 coordination site selectivity and selected bondbreaking chemistry found in these complexes. The mechanism for hydrolytic decomposition of amine-borane complexes also continues to be of interest. A comparison of the hydrolysis of benzylamine-borane with the hydrolysis of various arylamine-boranes has been reported, and the results are important for the selection of borane adducts as practical hydridic reagents in aqueous phase reductions. 

Reactions of Enamines.—Hydroboronation of the 2-enamine (397) gave a product (amine-borane adduct ) from which the 3a- and 3 -pyrrolidino-derivatives (398 3a 3)5 = 82 18) were liberated by refluxing methanol. The stereochemistry of this reduction is quite unusual, and not fully explained. Reaction of the 3,5-dienamine (399) with dichlorocarbene led to ring expansion, giving the A-homo-4-chloro-dienone (400). " Condensations of the enamine... 

Borane reacts with electron pair donors to form Lewis acid-base complexes. The most common forms for use in synthesis are the THF and dimethyl sulfide complexes. Stronger bases, particularly amines, form less reactive adducts. 

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