6063-64-5Relevant articles and documents
Synthesis, properties, and structural investigations of 1,3,2-diazaborolidines and 2,3-dihydro-1H-1,3,2-diazaboroles
Schmid, Günter,Polk, Michael,Boese, Roland
, p. 4421 - 4429 (2008/10/08)
A series of variously substituted 1,3,2-diazaborolidines have been prepared by different methods. 1,3-Diisopropyl-2-methyl-1,3,2-diazaborolidine (1a), 1,3-diethyl-2-methyl-1,3,2-diazaborolidine (2a), 1-ethyl-2,3-dimethyl-1,3,2-diazaborolidine (3a), and 1,2,3-trimethyl-1,3,2-diazaborolidine (4a) are formed from the corresponding lithiated ethylenediamines and CH3BBr2 in diethyl ether (method C). 2-Methyl-1-(trimethylsilyl)-1,3,2-diazaborolidine (5a), 1-tert-butyl-2-methyl-1,3,2-diazaborolidine (6a), and 1-isopropyl-2-methyl-1,3,2-diazaborolidine (7a) can be prepared either by method C, by method A, using the ethylenediamines and H3CB[N(CH3)2]2 to eliminate HN(CH3)2, or by method B, starting with CH3BBr2, NR3, and the corresponding ethylenediamines. The unsaturated 2,3-dihydro-1H-1,3,2-diazaboroles 1b-7b are synthesized by catalytic dehydrogenation in either liquid (1b-3b) or gaseous (4b-7b) state. Diazaboroles can act as 6-π-electron donors in Cr(CO)3 complexes. 1b-4b react with (CH3CN)3Cr(CO)3 under various conditions to form the corresponding complexes 1c-4c. The monosubstituted rings 5b-7b are not suited to form stable Cr(CO)3 complexes. One of the two rings in 8 can be combined with a Cr(CO)3 fragment to give 9. The yellow 1H-1,3,2-diazaborole-tricarbonylchromium complexes 1c-4c decompose slowly at room temperature. 2,3-Dihydro-2-methyl-1,3-bis(trimethylsilyl)-1H-1,3,2-diazaborole (10) can be metalated at one N atom by NaNH2 and K(O-t-Bu) to give the salts 11a and 11b. These alkali-metal derivatives can easily be protonated by HCl or CH3OH to form the N-H derivative 5b. X-ray structure analyses have been performed on the diazaborolidines 2a and 4a and on the diazaboroles 1b, 2b, 4b, and 8. The structures of 2a and 4b have been determined at two different temperatures. 1b, 2b, and 2a crystallize in the monoclinic space groups P21/n, P21/c, and Cc, respectively. 4a crystallizes hexagonally in the space group P32; 4b, tetragonally in the space group P42. X-X-Difference electron densities of 4a, 2a, and 4b show that the B-N bonds in the saturated compounds 4a and 2a possess remarkable double-bond character. The electron distribution in the 1,3,2-diazaborole 4b corresponds with that in 6-π-electron systems.
