Boranes Iminoboranes

The chemistry of iminoborane compounds containing the X>N—B< moiety has developed only within the last decade. The first representatives of this type of compounds were obtained by hydroboration of nitriles with sterically hindered boranes 10> or tetraalkyldiboranes 17> the resultant compounds appeared to be unique intermediates (stabilized by steric or reactivity effects) in the course of reactions that normally lead to borazines. The intermediates illustrated in Eq. (1) are mostly unstable at room temperature and, in general, cannot be isolated. 

However, the successful isolation of an iminoborane derivative in the reaction of trichloroacetonitrile with diborane indicated that the stability of imino-boranes is not only a function of the nature of the borane but also of that of the imine 16>. 

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 ... 

Iminoboranes derived from borine or alkylboranes and related aldimino-boranes are listed in Table 3. 

Fluorocyanide was found to produce similar monomers 26). The iminoborane dimers obtained from chlorocyanide or bromocyanide with dimethylbromo-borane transform to monomers upon vacuum distillation. 

Transformations of dimeric iminoboranes into the corresponding nitrile-borane adducts, due to temperature changes during sublimation or distillation, have been observed for the reaction products of pentafluorobenzonitrile and tribromoborane or organodibromoboranes 24) ... 

This cited reaction illustrates that the C=N double bond of iminoboranes is quite stable. Indeed, the C=N bond in these compounds tends to increase its bond order, forming corresponding nitriles, rather than to undergo further 1,2-additions leading to aminoboranes. This suggestion is confirmed by several reported transformations of iminoboranes to nitrile-borane adducts (Eq. (20)) M). Addition across the C=N double bond of iminoboranes is virtually unknown. This event is also true for related imines (e.g., dichloromethylenealkyl-amines) which yield imine-trihaloborane adducts with trihaloboranes rather than to undergo a 1,2-addition (c.f. Sect. VII). 

For a compilation of iminoboranes derived from haloboranes and organohaloboranes see Table 4. However, aldiminoboranes are listed in Table 3 and sulfur-containing compounds in Table 7. Bis- and tris(imino)boranes are listed in Table 6. 

A number of fully organosubstituted iminoboranes has been obtained originating from diphenylketimine or its derivatives and triorganoboranes or di-organohaloboranes. In particular, monomeric (diarylmethyleneamino)diorgano-boranes have been prepared in this manner. Four reaction routes have been used which correspond, in part, to procedures which have also been useful for the synthesis of compounds cited in Sects. Ill and IV 47). 

The reaction of tris(organothio)boranes with nitriles leads to B—S substituted iminoboranes. 1,2-addition of tris(methylthio)borane or tris(phenylthio) borane to trichloroacetonitrile yields the monomeric products (XIV) or (XV)32). 

Dimeric iminoboranes, on the other hand, are formed from fluoroacetonitrile and tris(organothio)boranes ... 

However, thermal decomposition of isopropylthiocyanate-trichloroborane does not yield an iminoborane, rather, tris(thiocyanato)-borane and isopropyl-chloride are produced. Methylisothiocyanate-trichloroborane does not rearrange under comparable conditions 33). 

The thioesterimide-haloboranes, upon thermal dehydrobromation, yield C-S substituted iminoboranes, whereas treatment with trie thy lamine leads to the rather unstable thioesterimides and triethylamine-borane adducts. 

Thioesterimide-haloborane adducts (c.f. Sect. VI.2, Eqs. (42-44)) can also be classified as imine-boranes. They are intermediated of low stability in the synthesis of monomeric sulfur substituted iminoboranes 27K... 

Infrared spectroscopy is an excellent tool in iminoborane chemistry, which readily permits, to distinguish between iminoboranes and nitrile-borane adducts and to identify monomeric and dimeric forms of iminoboranes. This event is due to the fact that the i>CN of CN multiple bonds absorbs outside the fingerprint region and can be considered to be a valuable group frequency even when mixed with other vibrational modes. In some cases other vibrations like NH, BH, B-halogen or B-S stretching modes are helpful for determining the structure of iminoboranes. 

A listening of iminoboranes and imine-boranes is complied in Tables 3 - 8 the particular arrangement is somewhat arbitrary. In Table 3 all aldiminoboranes... 

Table 3. Iminoboranes derived from monoborine and organoboranes (aldimino-boranes)... 

Table 4. Iminoboranes derived from halogeno- and organohalogeno-boranes...

In particular, iminoboranes (XBNR) are isoelectronic with alkynes (XCCR). Well-known comparable pairs of isoelectronic species are aminoboranes (X2BNR2) and alkenes (X2CCR2), amine-boranes (X3BNR3) and alkanes (X3CCR3), borazines [(XBNR)3] and benzenes [(XCCR)3], etc. The structure of aminoboranes, amine-boranes, and borazines is well known from many examples. It has turned out that these BN species are not only isoelectronic, but also have structures comparable with the corresponding CC species. In the case of borazines, the aromatic character was widely discussed on the basis of theoretical and experimental arguments. The structural and physical properties of... 

Thermal decomposition of azidoboranes [Eq. (3)], and of [trimethyl-silyl(trimethylsilyloxy)amino]boranes [Eq. (4)], permits a simple synthesis of symmetric iminoboranes RBNR (17-19). A hot tube procedure at about 300°C and 10 Torr turned out to be useful. The iminoborane iPrBNiPr, for example, was prepared at a rate of 10 g/hour by the azidoborane method. No separation problems are met with this method. Handling the liquid reactants of Eqs. (3) and (4) is hazardous. 

The chloroborane reactants of Eqs. (13) and (14) can be aminated to isolable [silyl(silyloxy)amino]boranes, which decompose at 120 and 70°C, respectively, to give a mixture of products, three of which were identified in both cases [Eqs. (15) and (16)] (19). Again, the iminoboranes... 

It can be concluded from the discussions in Sections IIIA-D that B=N double bonds in aminoboranes and B N triple bonds in iminoboranes represent a realistic picture. It is here recommended, therefore, to indicate these bonds in structural formulas as usual, but to omit erroneous formal charges, e.g., amine-borane X3B—NR3 aminoborane X2B=NR2i iminoborane XB=NR. [Note that R3N-BX3 is recommended as the correct molecular formula for amine-boranes (46), but one is not bound to rules in constructing structural formulas, e.g., X3B—NR3.]... 

Each of six protic agents was added to both of two representative iminoboranes, iPrB=NiPr and BuB=NtBu the expected 12 amino-boranes were isolated, chiefly in good yield [Eq. (29)] (71). The yield of distilled pure products may be smaller, but primarily the addition of protic agents is a quantitative reaction, fast even far below 0°C. This means a distinct difference to the slow addition of the same protic agents to alkynes which affords catalytic support at temperatures above 0°C. 

The same type of reaction was achieved with the hydroxylamino-boranes R2B=N(SiMe3)OSiMe3 and the iminoboranes EtBNEt (R = Pr), iPrBNiPr (R = Bu), BuBNiBu (R = Et) (18, 19). Following the addition of [ B]Bu2B=N(SiMe3)—OSiMe3 to iPrBNiPr, B-NMR spectra excluded alkyloboration instead of aminoboration (19). 

Hydroxylamino groups differ from normal amino groups by their smaller 7i-donating power. Hydroxylaminoboranes, therefore, are stronger Lewis acids than aminoboranes, having a vacant boron p-orbital more easily available. The lack of reactivity of aminoboranes indicates that the Lewis acidity of the boranes plays an important role in the boration of iminoboranes. Again, iminoboranes seem to favor electrophilic attack. 

Upon treatment of indoles with allylic borane, [l,3]hydrogen shift from the nitrogen atom to C-3 proceeds, producing the imine complex 8. Further allylboration of the C=N bond formed occurs with rearrangement through the chair-like six-membered transition state 9 allylic group being added mainly or quantitatively in /ram-fashion relative to the 3-R. Hydrolysis of iminoborane 10 formed leads to the 2-allylated indoline 11. 

Boranes with Two Boron-Bonded Nitrogen Atoms 4.2.2.1 Derivatives of Amino-iminoborane, HNBNH2... 

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