Iminoboranes, reactions

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 ... 

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). 

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 ... 

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... 

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>. 

Alkylideneamino)-butylboranes, the first known dimeric iminoboranes obtained from hydroboration reactions of nitriles 10 were prepared according to Eq. (9). 

A variety of substituted dimeric iminoboranes were obtained from reaction (10) as a mixture of geometrical isomers however, only dimeric ethylideneaminodimethylborane could be separated (by vacuum distillation) into the cis- and trans-isomers (IV) and (V). 

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 ... 

Several arylsubstituted dimeric iminoboranes have been prepared by the reaction of (phenylmethyleneamino)trimethylsilane with diorganohaloboranes or organodihaloboranes 47> according to Eq. (12). 

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)-... 

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) ... 

Addition of B—F bonds across C=N groups has never been achieved. Therefore, the only B-fluoro substituted iminoborane so far known was obtained by the reaction of diphenylketimine-lithium with trifluoroborane 13> according to the general Equation (25). 

An analogous reaction with phenyldichloroborane leads to dimeric (diphenylmethlene-amino)phenylchloroborane47). A similar iminoborane was refered to in Sect. Ill (Eq. (12)). 

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). 

Halogen bonded to boron or carbon of the CNB grouping of the molecule seems to be of similar activity. Reactions of monomeric iminoboranes with organothiols lead to C-S substituted iminoboranes which will be discussed later (c.f. Sect. VI). 

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). 

Compound (XIII) exists as a mixture of monomeric and dimeric species at room temperature. The same situation is true for the reaction products from methyl- and isopropyl-thiocyanate, respectively, with triisopropylborane, whereas the iminoborane obtained from reacting methyl-thiocyanate with tri-n-butylborane is dimeric 3 . With the exception of (XIII) these compounds have not been characterized by analysis. 

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). 

Substitution Reactions on Halosubstituted Iminoboranes with Organothiols... 

The reaction of alkylthiols with halosubstituted dimeric iminoboranes is another route for the preparation of some derivatives discribed in Sect. V. 1., but also leads to dimeric iminoboranes with only one organothio substituent 33). 

Like iminoboranes, imine-adducts of boron compounds have only recently been prepared and investigated. The adduct obtained from the reaction of diphenylketimine and diborane readily loses hydrogen, even at 20°, to form 1,3,5-diphenylmethylborazine 41) ... 

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... 

The first iminoborane that could be handled at — 30°C in a liquid 1 1 mixture with Me3SiCl was (FjCglBNfBu (9). It was synthesized by the general elimination reaction in Eq. (1) (Hal = halide) from the corresponding aminoborane. A hot tube procedure must be employed for... 

Production of iminoboranes by a hot tube procedure is obviously restricted to those reactants that can be transported into the gas phase without decomposition. Owing to this restriction the reactions according to Eqs. (3) and (4) cannot be applied to reactants with alkyl groups larger than C4H9 or with aryl groups. 

A striking difference between alkynes and iminoboranes appears to be their kinetic stability. As was pointed out in Section II, iminoboranes are metastable, in general, at temperatures far below room temperature. Alkynes are also metastable, but their stabilization requires either high temperature or effective catalysts. We assume the polarity of the B—N bond to be a chief reason for these differences. This idea is supported by the observation that strongly polar alkynes (e.g., FC=CH, FC=CfBu) do oligomerize or polymerize at room temperature quite rapidly (25). Polar additions will generally be the predominant reaction for iminoboranes (Sections V,VI). 

Apart from a real interconversion, cyclodimers can be transformed into borazines, however, by addition of iminoboranes [Eq. (25)]. In order to carry out such a reaction, a solution of the iminoborane in a dropping funnel, kept at — 80°C, is slowly dropped into a solution of the cyclodimer at 50°C. The yield of borazines is quantitative. The procedure can be applied to components with the sEime set of ligands, but different sets may also be applied, permitting the synthesis of borazines with an unsymmetric arrangement of more than two different ligands (13.19). 

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. 

Polar reagents AY (Section V,B-D) generally attack both triply bonded atoms of iminoboranes to yield aminoboranes [Eq. (27a)]. In special cases, the cationic fragment A of AY is added to the nitrogen atom and Y remains a separate anion [Eq. (27b)] such a reaction path seems to be governed by steric factors, but seems also to be restricted to aminoiminoboranes (70). 

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. 

Apparently, Eq. (29) represents a polar nonradical addition. If a two-step mechanism is conceived, intermediates of the type [XB=NRH] will be reasonable, though such cations proved to be rather unstable as isolated species (unless X represents a x-electron donating group) (33). Intermediates of the type HY—B(X)=NR would explain the fast reaction with protic bases of vanishing Bronsted acidity. The results, however, mentioned in Sections V, A, and V, C, favor to some extent the picture of iminoboranes as preferring electrophilic to nucleophilic attack. The high activity of amines can also be rationalized in terms of a concerted process, with a transition state of type VI. 

Azidoborations of iminoboranes are smooth and facile reactions [Eq. (33a)] (14, 19). For X = alkyl, the azidoboration products cannot easily be distinguished from hypothetical alkyloboration products... 

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