Synthesis and examination of amine-cyanocarboxyboranes, the boron analogues of alpha-cyanocarboxylic acids: X-ray structural study of the first lactam containing a boron atom in the lactam ring

摘要

The first representatives of chiral boron atom-containing amine-cyanomethoxycarbonyl boranes (A . BH(CN)COOMe) have been synthesized either from the corresponding amine-bromocyanomethoxycarbonylborane complexes with [BU4N]CN or from Me3N.BH(CN)COOMe and an amine in a base-exchange reaction. Acid hydrolyses of methyl esters generated the free acids (A.BH(CN)COOH), which are isoelectronic to the alpha-cyano carboxylic acids. Their pK(a) values and hydrolysis half-lives in acidic medium (that is rate of proton reduction) have been determined. Similarly to the alpha cyano carboxylic acids, the cyano group attached to the boron (in alpha position to the COOH group) increased the acid strength of carboxy boranes with 2.0-2.5 orders of magnitude. Independently from the type of the amine, pK(a) values of the amine-cyanocarboxyhoranes (6.34-5.82) decrease consistently with the increase of pK(b) values of the amines. Hydrolytic decomposition rate of the alkylamine complexes increases with increasing pKb values of the amines while the opposite was found for pyridine base complexes. When considering both types of the amines, hydrolysis half-lives of the complexes range over several orders of magnitude from 0.005 to 400 h. Based on these observations protonation of the amine nitrogen atom appears to be the rate determining step in the hydrolysis process. With loss of methanol, 2-NH2-py.BH(CN)COOMe transformed into a five membered lactam derivative. X-ray diffraction revealed that the pyridine ring is coplanar with the five membered lactam ring. In the crystal two molecules are connected in a head to tail arrangement by strong intermolecular H-bonds between N(2)-H and the carbonyl oxygen (01) with a donor and acceptor distance of 2.867(3) A. Three new cyanomethoxycarbonylborates having the composition of K[BHn(CN)(3-n)COOMe] (n = 1, 2) and K[B(OH)(CN)(2-) COOMe] have also been synthesized and their properties examined. (C) 2004 Elsevier B.V. All rights reserved.