Cyclic iminoboranes: Analogues of cycloalkynes

1-Azido- and 1-[trimethylsilyl(trimethylsilyloxy)]boracycloalkanes, (-Y-BX-) [X = N-3 (2), N(OSiMe3)SiMe3 (3), Y = alkanediyl], were synthesized from the corresponding chloroboranes [X = Cl (1)]. The following alkanediyl ring fragments were considered: Y = -CH2-CHMe-CH2-CH2- (a), 1,5-cyclooctanediyl (b), -(CH2)(6)- (c), -(CH2)(7)- (d), -CMe2-(CH2)(6)- (e), -C(-CMe2-CH2-)-(CH2)(6)- (f). The thermal elimination of N-2 (from 2) or (SiMe3)(2)O (from 3) is accompanied by the migration of one of the two alkanediyl ends from boron to nitrogen under ring expansion to give the cyclic iminoboranes 4. Formed in solution, the iminoboranes react immediately with undecomposed starting material 2 or 3 under formation of the products 5 or 6 by azidoboration or aminoboration, respectively, of the BN multiple bond. The temperature for the decomposition of 2 depends on the ring size: the five-membered ring compound 2a and the bicyclic six-membered ring compound 2b decompose beneath 0 degreesC, the seven- and eight-membered ring compounds 2c and 2d in boiling hexane and toluene, respectively, whereas the eight-membered ring compounds 2e and 2f are decomposed in solution above 100 degreesC so slowly that only unidentified polymers are found. The boranes 3a and 3b, stable at room temperature, afford a temperature of 80 degreesC for decomposition in solution. The azidoboranes 2c-f can be transported into the gas-phase without decomposition and can be thermolyzed there at 270-285 degreesC. The cyclic iminoboranes 4c-e, formed in the hot tube, were condensed at -196 degreesC, but thereafter not characterized, because they either cyclotrimerized (4c,d) or cyclodimerized (4e,e'; two isomers depending on which end of the 1,1-dimethylheptamethylene unit migrates) beneath -60 degreesC under formation of the corresponding borazines 10c,d or of the diazadiboretidine isomer mixture 9e,e', respectively; the spirocyclic borane 2f gives a mixture of unidentified products on gas-phase thermolysis. The iminoboranes 4e,e' can be trapped by ethyloboration with BEt3 giving the products 8e,e'. The acyclic azidoborane R(Me)BN3 (2g; R = 1-methylcycloheptyl), formed after the ring-contracting rearrangement of a boracyclooctane derivative, gives the isolated and characterized mixture of the acyclic iminoboranes MeBequivalent toNR (4g) and RBequivalent toNMe (4g') upon gas-phase thermolysis; the stabilization of 4g,g' gives the Dewar borazines 11g and 11g'.