Borazines stability

A modification of the standard CNDO/2 MO calculation, in which pairs of atoms associated with the same or different molecules are differentiated, leads to reasonable results for 7r-7r-type molecular complexes.294 It was suggested that benzene-borazine (stabilization energy 2—5 kcal mol-1) and borazine-borazine (5—18 kcal mol ) can exist in the ground state, the molecules being arranged symmetrically in parallel planes. 

Borax stability Boraz anaphthalene Borazine [6569-51-3] Boraz ines Borazon... 

Poly[5-(alkylamino)borazines] 7-10 exhibited suitable viscoelastic and thermal stabilities to be extruded in the molten state through the monohole spinneret of a lab-scale melt-spinning apparatus. Thus, an extruded filament (diameter, 200 gm) was drawn with a windup unit, that is, a graphite spool. This provided green fibers with a wide range of diameters (16 =s (f> =s 50 gm Table 2), depending on polymer architecture. 

Waggoner, K. M. Hope, H. Power, P. P. Angew. Chem. Int. Ed. Engl. 1988, 27, 1699. Six-membered heterocycles [RMER ]3 have been subject to detailed preparative and theoretical studies since they are isoelectronic to borazine B3N3Hs. However, it was demonstrated that only B3P3-heterocycles exhibit a considerable delocalization of the 6 k-electrons, whereas the delocalization in other six-membered heterocycles of the desired type contributes very little to their stabilization. Power P. P. J. Organomet. Chem. 1990, 400, 49. 

As shown in Table 3.21, the stabilization corresponding to Fig. 3.46(a) (36.8 kcal mol-1) is nearly twice that of benzene (20.4 kcal mol-1), but the complementary interaction corresponding to Fig. 3.46(b) is essentially switched off (less than 1 kcal mol-1). Thus, borazine has a strong mono-directional cyclic pattern of conjugations that is quite unlike that of benzene. 

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. 

The gas-phase chemistry of borazine B3N3H6 (147) and the conjugate N-protonated acid B3N3H7+ indicates analogies with benzene,189 although the aromatic stabilization energy of neutral borazine is only 30% that of benzene, and the reactivities of benzene and borazine are not similar (Scheme 62). Comparable conclusions were reached when HOMA and Iq aromaticity indices were used.190a... 

The stabilization of iminoboranes can yield five different tj ies of products cyclodimers (1,3,2,4-diazadiboretidines, Di), cyclotrimers (borazines, Tr), bicyclotrimers (Dewar borazines, Tr ), cyclotetramers (octahydro-l,3,5,7-tetraza-2,4,6,8-tetraborocines, Te), and polymers (polyiminoboranes, Po) these substances are isoelectronic with cyclobutadienes, benzenes, Dewar benzenes, cyclooctatetraenes, and polyalkynes, respectively, which are all known to be products of the thermodynamic stabilization of alkynes. 

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. 

Polymers are formed, together with borazines, from iminoboranes RBNR with a-unbranched alkyl groups. By absolute control of the temperature, the stabilization would presumably be directed toward the borazine. Loss of thermal control will cause a loss of kinetic control, so that hot iminoborane molecules will trimerize or polymerize rather unspecifically. 

The most characteristic reaction of amine boranes is their conversion into aminoboranes and, subsequently, to borazine at higher temperatures.9,17 For complexes of low stability, the transfer of H from the boron to the donor is a characteristic process,11 40 as in the utilization of diborane as an electrophilic reducing agent. The neutral complexes of boranes are fairly stable towards hydrolysis. The key step of the hydrolysis was formerly assumed to involve displacement of BH3 by a proton, whereas in recent studies ionic intermediates, containing five-coordinated boron (R3N—BH4) are also taken into consideration.41,42 The hydrolytic... 

As with Dewai benzene, the increased stability of this structure over the planar borazine is thought to result from the extreme interactions of the alkyl... 

A detailed comparison of spectral data concluded that borazine has a delocalized 7T-electron system like that of benzene (112) other workers, however, have concluded from spin-coupled calculations that borazine has litde aromatic stabilization as compared to benzene (113). 

One of the major problems in the chemistry of three-coordinated boron-nitrogen compounds is their relative instability towards hydrolysis 79>8°). This feature reduces their possible applications in many areas such as in the medicinal field or utilization for polycondensation-products. However, steric hindrance by bulky substituents on the borazine ring leads to compounds with enhanced hydrolytic stability, particularly when no solvent is present in which they may dissolve. This observation has resulted in extensive research on sterically hindered boron-nitrogen ring compounds. 

B-2,6-Dimethylarylborazines have not been obtained in a direct borazine synthesis, though they can be prepared by Grignard arylation of l,3,5-trimethyl-2,4,6-trichloroborazine 01.92), These compounds show high hydrolytic stability, even in acid and basic medium, and they can be subjected to substitution reactions at the aromatic sites 91>. For example, free radical bromination 93> or Friedel-Crafts acetylation 94> has been accomplished. 

A borazine derivative has been obtained from the reaction of pyraza-bole with o-phenylenediamine [Eq. (27)]. The resultant product is characterized by high stability U2). 

By reactions of l,2,3(5-tetramethyl-4,6-di (alkylamino)borazines with several diamines and 4,4 -diisocyanatodiphenylmethane, it was shown that the thermal stability of the bis(alkylamino)borazine depends on the substituents at the exocyclic nitrogens. Reactions with diamines led to polycondensates such as the ones cited above. However, reaction with diisocyanates gave different results depending on the substitution pattern of the exocyclic amino groups 140,141),... 

B-Siloxyborazine derivatives are formed by reaction of B-mercapto-alkylborazines with NaOSi(CH3)3 144> and their thermal stability has been investigated 150>. It was found that borazines with electron-donating groups on boron show reduced thermal stability as compared to hexaorganoborazines. 

As the BN unit is isoelectronic with a C2 fragment, replacement of the latter by the former in various organic compounds leads to azaborane structural analogs. Some examples are shown below. Planar borazine (or borazole) B3N3H6 is stabilized by n -delocalization, but it is much more reactive than benzene in view of the partial positive and negative charges on the N and B atoms, respectively. 

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