Iminoboranes formation

Iminoboianes have been suggested as intermediates in the formation of compounds derived from the pyrolysis of azidoboranes (77). The intermediate is presumed to be a boryl-substituted nitrene, RR BN, which then rearranges to the amino iminoborane, neither of which has been isolated (78). Another approach to the synthesis of amino iminoboranes involves the dehydrohalogenation of mono- and bis(amino)halobotanes as shown in equation 21. Bulky alkah-metal amides, MNR, have been utilized successfully as the strong base,, in such a reaction scheme. Use of hthium-/i /f-butyl(ttimethylsilyl)amide yields an amine, DH, which is relatively volatile (76,79). 

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... 

Although dimeric (t-butylmethyleneamino)dibutylborane can be prepared by the reaction of tri-t-butylborane with pivalonitrile at temperature above 150 °C by elimination of butene. preparation at lower temperatures involving triethylamine-borane according to Eq. (11), illustrates that the formation of this iminoborane proceeds via di-n-butylborane as an intermediate 6 ... 

Reaction (15) was originally performed more than a century ago but as that time the product was formulated as the ehlorocyanide-trichloroborane adduct 18). Analogous 1,2-ad-ditions of tribromoborane to chlorocyanide and of trichloroborane to bromocyanide result in the formation of substituted dimeric iminoboranes 23)-... 

Mixtures of dimeric iminoboranes with iodoacetonitrile-trihaloboranes were obtained from iodoacetonitrile and trichloro- or tribromoborane tri-iodoborane formed only the adduct 31). Trichloroborane has been shown to give 1,2-addition to an oxyalkylnitrile after intermediate formation of an adduct ... 

In contrast, reaction of mdo-decaborane(14) with iminoboranes RB=N R did not result in the formation of doso-azadodecaboranes (RN)(RB)BioHio because the iminoborane dimerized more rapidly and, therefore, does not insert into the Bio cluster [68]. On the other hand, the aminoiminoborane tmp-B=N-Bul reacted readily with Bi0Hi4 to give 6-(tmpH)(But)B-Bi0Hi3. Most likely, the fist step in this... 

This article describes what is known about the formation, structure, and reactivity of iminoboranes. The chemistry of iminoboranes is in its beginnings, and so we cannot paint a complete picture. A comparison between iminoboranes and the corresponding alkynes will serve as a background throughout this novel field of boron chemistry. 

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 same type of product was isolated from the reaction of the iminoborane alkyl azides R N3 (R = Me, Et, Pr, Bu, iBu, sBu, ra-CsHu, cyclo-CsHg, cyclo-CgHn, Ph(3H2) (19). The azidosilane Me3SiN3 may also behave as a 1,3-dipole [Eq. (40b)], but addition of the SiN bond to iminoboranes [Eq. (40a)] is usually the preferred reaction (Section V,C,8). This is not so when Me3SiN3 is present during the formation of diaryliminoboranes, ArB NAr, as intermediates Both reaction pathways [Eqs. (40a) and (40b)]... 

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. 

It is known that hydroboration reaction of cyano groups gives iminoborane species which dimerize to form B-N four-membered ring (cyclodiborazane) in the case of appropriate borane used [16]. When this reaction is used for the bifunctional monomers, formation of polymeric materials consisting of cyclodiborazane units can be expected. Concerning the preparation of poly(cycIodiborazane)s, we have recently reported two new methods, i.e., allylboration polymerization of dicyano compounds with triallylborane [17], and polycondensation between bis(silylimine)s and chlorodialkylboranes [18]. 

It is known that hydroboration reactions of cyano groups give iminoborane species which dimerize to form boron-nitrogen four-membered rings (cyclodiborazane) (26). When this reaction is used with bifunctional monomers, formation of polymeric materials consisting of cyclodiborazane units can be expected. 

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