Iminoboranes bonding

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 ... 

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. ... 

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). 

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... 

A B-Cl bond of pentafluorophenyldichloroborane adds quantitatively across the C=N group of trichloroacetonitrile and yields an equilibrium mixture of the monomeric and dimeric iminoborane derivative 26>. In contrast, benzon-itrile does not react at all with trichloroacetonitrile under comparable conditions 26 ... 

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). 

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). 

Replacement of halogen in dimeric iminoboranes containing tetracoordin-ated boron is not so readily accomplished. For example, bis[(dichloromethyl-eneamino)dichloroborane]reacts with C4H9MgCl in nonetheric solvents to yield dimeric (dichloromethyleneamino)butylchloroborane, whereas C4H9Ii replaces both boron-bonded chlorine atoms by butyl groups 25). 

The first examples of sulfur-containing iminoboranes have been obtained by the 1,2-addition of the B—S bond of (alkylthio)diorganoboranes to acetonitrile 3S>. 

Analogous monomeric iminoboranes which are fully alkylated at the B-atoms have also been prepared 27). Treatment of bromosubstituted iminoboranes with phenylthiol, however, also causes the B-bonded halogen to be replaced by thiophenyl groups ir> (Eq. 44). 

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. 

Monomeric iminoboranes exhibit a B-N bond order higher than unity due to p - p bonding between nitrogen and three-coordinate boron. This event results in an allene-type structure as shown in (I) exhibiting its antisymmetric stretching vibration around 1800 cm-1. This should have a predominant i>(CN) character, whereas in the symmetric mode of lower wavenumber the B-N charac-... 

B=N-CMe3 double bond. This generates a diazatetraborane derivative [24] (Figure 2.1-13). In contrast, B3H7THF reacts with the iminoborane Ehd-B N-Bu to give a /eamino-diborane in which the Bu groups stand trans to each other [25]. 

Iminoboranes may be identified as a class of molecules with an imino group NR and a varying group X (e.g., F, RO, R2N, R3C) bonded to boron. Iminoboranes (XBNR) belong to the family of neutral two-coordinated boron species which may be arranged systematically in the following way ... 

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... 

Apparently there is a substantial contribution to the B—N bond of HBNH from one a- and two orthogonal 7i-bonds. Expressed in simple terms, there is a B=N triple bond in iminoboranes. Concerning a structural formula for HBNH, the real situation is represented best by H—B=N—H. 

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). 

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.]... 

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). 

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). 

Alternative reaction pathways, corresponding to Eq. (46), are also observed in the reaction of iminoboranes with iminoalkanes the ligand R bonded to the iminoalkane nitrogen atom, seems to govern the reaction path [Eq. (47)] 9, 19). Offering the two C=N double bonds of... 

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)]... 

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