Iminoboranes cyclodimers

The stabilization of iminoboranes can yield five different tj ies of products cyclodimers (1,3,2,4-diazadiboretidines, Di), cyclotrimers (borazines, Tr), bicyclotrimers (Dewar borazines, Tr ), cyclotetramers (octahydro-l,3,5,7-tetraza-2,4,6,8-tetraborocines, Te), and polymers (polyiminoboranes, Po) these substances are isoelectronic with cyclobutadienes, benzenes, Dewar benzenes, cyclooctatetraenes, and polyalkynes, respectively, which are all known to be products of the thermodynamic stabilization of alkynes. 

Apart from a real interconversion, cyclodimers can be transformed into borazines, however, by addition of iminoboranes [Eq. (25)]. In order to carry out such a reaction, a solution of the iminoborane in a dropping funnel, kept at — 80°C, is slowly dropped into a solution of the cyclodimer at 50°C. The yield of borazines is quantitative. The procedure can be applied to components with the sEime set of ligands, but different sets may also be applied, permitting the synthesis of borazines with an unsymmetric arrangement of more than two different ligands (13.19). 

Equation (25) may shed light on the general path of the iminoborane oligomerization. I propose the formation of cyclodimers to be the first stage of such oligomerizations. If a cyclodimer is stable to an excess of iminoborane, it will be isolated (Table III). Otherwise the cyclodimer is attacked by the excess iminoborane according to Eq. (25), and the borazine is formed via the Dewar borazine. In special cases, the Dewar borazine will be the final product. The first step determines the rate of such a sequence of reactions. If the cyclodimerization step becomes relatively fast, so that the first and the second step are comparable in rate, both the cyclodimer and the cyclotrimer will be found this is true for the thermal stabilization of sBuBNsBu. Catalysts for the cyclodimerization make the first step more rapid than the second one. 

A plausible alternative mechanism involves as a first step the formation of a linear dimer, XB=NR—BX=NR, according to the left part of Eq. (21). This linear dimer will be more stable in entropy but less stable in energy than the corresponding cyclodimer. The second step would be addition of iminoborane to the open-chain dimer giving the borazine in either a concerted or a two-step mechanism. The intramolecular cyclization of the open-chain dimer, according to the right-hand side of Eq. (21), would compete addition of an iminoborane. We cannot definitely exclude a mechanism via open-chain dimers. 

Thermal or catalytic cyclodimerization of iminoboranes is obviously a [2 -I- 2]-cycloaddition (Section IV). A mixture of two different iminoboranes may be stabilized by formation of three different cyclodimers. If the relative stability of the two iminoboranes, however, differs distinctly, the mixed cyclodimer will be formed preferentially by... 

The nitrone PhCH=N(Me)—O was successfully applied as a 1,3-dipole to MesB=NMes and to Me3Si-(iBu)N—B=N[Pg.164]

Diels-Alder reactions with alkynes as dienophiles have been known for a long time. Iminoboranes, however, will more readily cyclodimerize than react with dienes, and even the cyclodimers are generally superior to dienes in the competition for excess iminoborane. Among many attempts, therefore, only two reactions with iminoboranes as dienophiles have been successful, and in both of them the diene is cyclo-pentadiene [Eq. (55)] (9, 14). 

The formation of Dewar borazines from iminoboranes and their cyclodimers is also a [4 + 2]-cycloaddition, whether or not the Dewar borazines are the final products or are rearranged to normal borazines (Section IV,E). 

The same iminoborane is thermally stabilized by cyclotrimerization, but may be cyclodimerized by the catalytic aid of /BuN=C (Section IB). The cyclodimer (BuBN[Pg.166]

As indicated in Scheme 9.4, the formation of Dewar borazines involves the dimerisation of an iminoborane followed by the [4 + 2] cycloaddition of the cyclodimer thus formed with an iminoborane. For smaller R/R groups, the isomerisation of the bicyclic Dewar borazine to a monocyclic borazine occurs readily. 

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