Dewar borazines

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. 

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. 

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 steric bulk of the substituents on B and N in an iminoborane RB=NR has a major influence on the outcome of the oligomerisation process. Small substituents favour the formation of borazines. For example, only the borazine Me3B3 N3Me3 is formed for the combination R = R = Me. An increase in the steric bulk of R and/or R may lead to the formation of a Dewar borazine C, a four-mem-bered ring D or an eight-membered ring E. Speciflc examples of each of these ring systems and the mechanisms of their interconversions are discussed below. 

Scheme 9.4 Formation of a Dewar borazine from iminoboranes.

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. 

The iminoborane, i-PrB=N-/-Bu, trimerizes to (i-PrBN-/-Bu)j which has a Dewar-borazine type structure.174 Compare this structure to that of borazine. 

Dewar borazine derivatives 12.29 can be stabilized by the introduction of sterically demanding substituents. Figure 12.20b shows the structure of )V,iV, iV"- Bu3- ,5, 5 -Ph3B3N3 the open-book conformation of the B3N3 framework mimics that of the Cg-unit in Dewar benzene. By... 

With intermediate steric requirements, specifically if R = t-Bu and R = i-Pr or sec-Bu, thermal oligomerization forms trimers that are not typical borazines. These trimers are bicyclic rings with structures analogous to Dewar benzene . They are commonly referred to as Dewar borazines. 

A wide variety of dimers (RNBR )2 react quantitatively with additional monomer to expand into borazines , and (t-BuNBPr-i)2 reacts with t-BuNBPr-i to form the Dewar borazine as expected ( 15.2.5.1.1). 

Owing to the similar steric requirements for dimers and for Dewar borazines, the bicyclo (i-PrNBBu-t)3 transforms into dimer at 200°C . 

Dewar borazine derivatives 13.29 can be stabilized by the introduction of sterically demanding substituents. 

The iminoborane. r-PrB=N-r-Bu. trimerizes lo U-PrBN i-Bu)]i whbh has a Dewar > borazine type structure.Compare this structure to that of borazine. 

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