Iminoboranes

Reviews covering the Hterature through 1970 for all areas except iminoboranes (3) and through 1984 (4) are available. 

A new class of boron—nitrogen compounds was reported in 1975 when the first iminoborane having two-coordinate boron,... 

Synthesis. Iminoboranes, thermodynamically unstable with respect to oligomerization can be isolated under laboratory conditions by making the oligomerization kineticaHy unfavorable. This is faciUtated by bulky substituents, high dilution, and low temperatures. The vacuum gas-phase pyrolysis of (trimethylsilylarnino)(aLkyl)haloboranes has been utilized as an effective method of generating iminoboranes RB=NR as shown in equation 19 for X = F,... 

The products are trapped at Hquid nitrogen temperature. This strategy can also be employed in the synthesis of amino iminoboranes (76), where X = F, Cl. 

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). 

Properties and Reactions. The stmcture of (aLkyl)imiQoboranes RB=NR is characterized by ahnear C—B—N—C geometry and a B—N bond order approaching three. Amino iminoboranes can be described usiag three resonance stmctures ... 

NR qq g relative stability of iminoboranes and amino iminoboranes shows strong dependence on the stetic bulk offered by R and R7 This is illustrated by the stabiUty of at 0°C whereas the permethyl analogue, (CH3) S[(CH3), decomposes above —110°C (72). The... 

The chemistry of these compounds reflects the unsaturated nature of the B—N triple bond. Polar compounds add to iminoboranes, provided the addition proceeds more rapidly than oligomerization of R SIR ( ). For example, for R = R = CH(CH3)2 or /-C H. ... 

In an analogous fashion to the hydroboration reaction, a variety of boron-containing substrates react with iminoboranes. Addition of X2B—Cl, X2B—N3, X2B—SR, X2B—NR2, and X2B—R to the unsaturated B—N system is called chloro-, azido-, thio-, amino-, and alkyloboration, respectively. The azidoboration and chloroboration of two iminoboranes are shown ia equations 23 and 24 (72). 

A general type of stabilization for iminoboranes is a cyclodimerization, which yields diazadiboretidines (RBNR )2 that are isoelectronic with cyclobutadienes. 

Iminoboranes, R-N=B-R, are isoelectronic with alkynes and contain 2-coordinate boron their chemistry has recently been review-... 

The finding of preparatively available iminoboranes RB = NR some years ago opened exciting new possibilities not only in B—N chemistry, but also in coordination chemistry. The first examples of iminoborane-transition-metal complexes have now been published. The structurally completely characterized t-BuB = NBu-t adds, like its alkyne analog, to the 03(00)5 fragment as a bridging ligand. When Co2(CO)g and t-BuB = NBu-t are dissolved in pentane at 0°C, warming to RT and evaporation of unreacted iminoborane yields (t-BuBNBu-t)Co2(CO)5 (86%) as a black solid, which can be recrystallized from ether-nitromethane (1 3) ... 

Niobium and zirconium are also appropriate transition metals to coordinate iminoboranes side on (T) -Cp)2NbH3 and RB = NBu-t [R = t-Bu, Me3Si(t-Bu)N] react with the loss of two hydrogen atoms. The remaining H atom probably interacts additionally with the boron atom ... 

The complexes are formed with 68% (R = t-Bu) and 73% [R = Me2Si(t-Bu)N] yield in warm (85°C) toluene in the course of 90 min. Surprisingly, the B—N distance is extended only a little ( 0.03 A), compared with the free iminoborane. In contrast to this side-on coordinated iminoborane, the zirconium complex (rj -Cp)2ZrCl(H)... 

Some iminoboranes dimerize during reaction with a transition-metal complex to diazadiboretidines. These four-membered rings then act as four-electron donors in different complexes ... 

Hydrozirconation of iminoboranes -B=N-R leads to a four-membered ring with a Zr-H-B hydride bridge [209]. 

A synthesis of comblike organoboron polymer/boron stabilized imidoanion hybrids was examined via reactions of poly(organoboron halides) with 1-hexylamine and oligo(ethylene oxide) monomethyl ether and subsequent neutralization with lithium hydride (scheme 8). The obtained polymers (10) were amorphous soft solids soluble in common organic solvents such as methanol, THF, and chloroform. In the nB-NMR spectra (Fig. 11), neutralization of the iminoborane unit with lithium hydride... 

Borylphosphinoethene derivatives were synthesized by varying the dipolar reagents. Thus, (176) appeared to interact with /-butyl isocyanide as trialkylborane, yielding unstable iminoborane 178. The latter forms 2-phosphinoethenyl-l-bora-2,4-diazacyclopentane (179) with ben-zalaniline and 1 -hydroxy-1 -phosphinoethyl-1 -borata-2-ammonia-4-aza-cyclopentane-2 (180) with acetonitrile [Eq. (134)] (90IZV2147). 

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>. 

It is now recognized that iminoboranes are not merely unique intermediates but rather are members of an independent class of stable compounds which has its own chemistry such as the aminoboranes, borazines, or other examples of boron-nitrogen compounds. 

To date, monomeric (I) and dimeric (II) iminoboranes have been identified. These show a very different behaviour due to their difference in hybridization at the boron atoms. 

Dimeric iminoboranes are generally solids at room temperature and many of them are quite stable towards hydrolysis due to the tetra-coordination about... 

Most of the monomeric compounds are highly reactive liquids and are rapidly hydrolysed by atmospheric moisture. In the monomeric iminoboranes the N-B bond order is greater than unity and these compounds represent an allene-type system with cumulated multiple bonds as indicated by structure (I) 3S). 

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