Cyclic Iminoboranes: Analogues of Cycloalkynes

1‐Azido‐ and 1‐[trimethylsilyl(trimethylsilyloxy)]boracycloalkanes, (‐Y‐BX‐) [X = N₃ ( 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 N₂ (from 2 ) or (SiMe₃)₂O (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 °C, 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 °C so slowly that only unidentified polymers are found. The boranes 3a and 3b , stable at room temperature, afford a temperature of 80 °C 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 °C. The cyclic iminoboranes 4c ‐ e , formed in the hot tube, were condensed at ‐196 °C, 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 °C 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 MeB≡NR ( 4g ) and RB≡NMe ( 4g′ ) upon gas‐phase thermolysis; the stabilization of 4g , g′ gives the Dewar borazines 11g and 11g′ .