B-N Triple Bonds

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. ... 

One common feature does exist between the photolysis of borazine and of benzene in the absence of a second reagent that is the observation of a ring rupture in the matrix isolated species For benzene, the product is hexatriene diradical, vidiile in the case of borazine, the product has not been identified completely. IR data show it to involve a ring-opening with some loss of hydrogen, leaving B-N triple bonds and a terminal NB rnoiety ... 

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. 

Though adoption of a typical B=N bond length may be questionable, we suggest 141 pm as a possible value, referring to noncyclic aminoboranes. The most questionable course is to generalize the one experimentally established B=N triple bond distance of 126 pm. On the other hand, the steric situation at least will not be too different, if the substituents R and X in XB=NR, distant from each other, are altered. Thus we take 126 pm as a typical B=N triple bond distance. The preceding discussion is summarized in Table II. 

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.]... 

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). 

A B=N triple bond is used to add to a cyclopentadiene, bridging two of its carbon atoms. 

Also, it is worth mentioning (a) the process of N=N triple bond cleavage by Mo(III) complex [6], and (b) the reactivity of coordinated dinitrogen with electrophiles, including the protons and organic free radicals [7], and coordinated [8] and free [9] dihydrogen. 

Figure 7.12 outlines how hydantoin is produced during the synthesis of methionine depicted in Figure 7.11. Initially, the aminonitrile B is formed. With its amino group it adds to carbon dioxide, which—under the weakly basic reaction conditions—is followed by the formation of the carbamate ion D. In a fully analogous fashion, ammonia reacts with carbon dioxide to give ammonium carbamate (cf. Section 8.2). The intermediate D in Figure 7.12 permits the intramolecular and thus kinetically favored—i.e., fast—addition of the oxyanion to the C=N triple bond. The addition product is the negatively charged heterocycle F, which in...

The Beckmann rearrangement of cyclic oximes results in lactams. This is exemplified in Figure 11.38 with the generation of e-caprolactam, the monomer of nylon-6. The nitrilium ion intermediate cannot adopt the preferred linear structure because it is embedded in a seven-membered ring. Therefore, in this case the intermediate might better be described as the resonance hybride of the resonance forms A (C=N+ triple bond) and B (C+=N double bond). The C,N multiple bond in this intermediate resembles the bond between the two C atoms in benzyne that do not carry H atoms. 

Cycloaddition of 1,2,4,5-tetrazines with C-N double or triple bonds yields dihydro-1,2,4-tri-azines 13 or 1,2,4-triazines 14 (see Section B.2.2.1.3.). Various systems with C—N double bonds have been used imines, including dihydropyrroles and dihydroisoquinolines 341 344 hydra-zones, including dihydropyrazoles,343 348 oxime ethers,343 imidates, including dihydrooxa-zoles 345 357 thioiniidates, including dihydrothiazoles 345,349,350 357 358 amidrazones 357,358 amidines 359 and isocyanates or isothiocyanates.351 Nitriles are the examples of systems with a C-N triple bond.352 -356... 

Using structure (1) for N2O, with an N-N triple bond and an N-0 double bond, we would be let to predict that the bond-lengths are similar to the 1.10 A and 1.21 A for N2 and CH3N=0. The experimental lengths of 1.13 A and 1.19 A confirm this expectation (Section 2-3 (b)), and on this basis structure (1) is a suitable valence-bond structure for N2O. But the Lewis theory, with electron-pair bonds, does not permit a valence of five for first-row atoms, provided that only the 2s and three 2p orbitals of these atoms are valence orbitals for bonding. Therefore, in the Lewis theoiy, the quinquevalent structures are replaced by octet stractures such as structures (4)-(6) for N2O, and (7) and (8) (together with equivalent resonance forms) for MesNO and N2O4. 

Of the eight nonmetals listed in Table 21.1, nitrogen is by far the least reactive. Its inertness is due to the strength of the triple bond holding the N2 molecule together (B.E. N=N = 941 kj/mol). This same factor explains why virtually all chemical explosives are compounds of nitrogen (e.g., nitroglycerin, trinitrotoluene, ammonium nitrate,... 

Thiazyl halide monomers undergo a variety of reactions that can be classified under the general headings (a) reactions involving the 7i-system of the N = S triple bond, (b) nucleophilic substitution, (c) halide abstraction, and (d) halide addition. The cycloaddition of NSF with hexafluoro-1,3-butadiene provides an example of a type (a) reaction. 

---