Synthesis, properties, and structural investigations of 1,3,2-diazaborolidines and 2,3-dihydro-1H-1,3,2-diazaboroles.
A series of variously substituted 1,3,2-diazaborolidines have been prepd. by different methods. 1,3-Diisopropyl-2-methyl-1,3,2-diazaborolidine, 1,3-diethyl-2-methyl- (I) 1,3,2-diazaborolidine, 1-ethyl-2,3-dimethyl- and 1,2,3-trimethyl-1,3,2-diazaborolidine (II) are formed from the corresponding lithiated ethylenediamines and CH3BBr2 in di-Et ether (method C). 2-Methyl-1-(trimethylsilyl)-1,3,2-diazaborolidine, 1-tert-butyl-2-methyl-1,3,2-diazaborolidine, and 1-isopropyl-2-methyl-1,3,2-diazaborolidine can be prepd. either by method C, by using the ethylenediamines and H3CB[N(CH3)2]2 to eliminate HN(CH3)2, or by starting with CH3BBr2, NR3, and the corresponding ethylenediamines. The unsatd. 2,3-dihydro-1H-1,3,2-diazaboroles (III) are synthesized by catalytic dehydrogenation in either liq. or gaseous state. Diazaboroles can act as 6-p-electron donors in Cr(CO)3 complexes. III (R, R' = Me, Et, CHMe2) react with (CH3CN)3Cr(CO)3 under various conditions to form the corresponding complexes IV. The monosubstituted rings III (R' = H) are not suited to form stable Cr(CO)3 complexes. One of the two rings in dimer V can be combined with a Cr(CO)3. The yellow 1H-1,3,2-diazaboroletricarbonylchromium complexes IV decomp. slowly at room temp. 2,3-Dihydro-2-methyl-1,3-bis(trimethylsilyl)-1h-1,3,2-diazaborole can be metalated at one N atom by NaNH2 and K(O-t-Bu) to give the salts VI (M = Na, K). These alkali-metal derivs. can easily be protonated by HCl or CH3OH to form the NH deriv. X-ray structure analyses have been performed on the diazaborolidines I and II and on the diazaboroles III and V. The structure of III have been detd. at two different temps. The electron distribution in the 1,3,2-diazaborole III corresponds with that in 6-p-electron systems.
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