Iminoboranes stabilization products

A correlation between the iminoboranes and their thermal stabilization products is given in Table III. Thermal, in this context, means at room temperature. Mixtures of two products can be separated either by extraction of the soluble component (Tr/Po) or by distillation... 

The most widely used method for the synthesis of iminoboranes involves the 1,2-addition of boron-element bonds such as boron-hydrogen, boron-halogen, boron-carbon, or boron-sulfur bonds across the C=N bond of nitriles thereby producing variously substituted iminoboranes (Eq. (2)). The formation of iminoboranes as well as the stability of the products depends on the substituent on the nitrile group, the nature of the boron-element bond to be cleaved during the 1,2-addition across the C N bond, and to a lesser extent on the non-reacting boron substituents 26T... 

Finally, three examples are reported in which iminoboranes as intermediates do not react with trapping agents but stabilize themselves intramolecularly in the gas phase during a hot tube procedure [Eqs. (17)-(19)]. A prerequisite is the steric availability of side groups with respect to the BN bonds (9, 21). The products are well established either by solvolytic degradation (9) or by X-ray analysis (21). Note that... 

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. 

Reaction of aldehydes and ketones with iminoboranes has been widely investigated. Conditions for the [2 + 2]-cycloaddition between XBNR and R R"CO are relatively good stability of the iminoborane and lack of enolic protons in the oxo compound [Eq. (46)] 14, 19). Relatively less stable iminoboranes, but in some cases the stable ones too, may react with 0X0 compounds by a total opening of the B=N triple bond [Eq. (43)], presumably via a [2 + 2]-cycloaddition [Eq. (42)] (Section V,D). A relatively stable iminoborane and a ketone containing enolic protons may yield an open-chain product, probably through a six-membered cyclic transition state [Eq. (46b)] 19). 

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]

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