Dimerization of Diboradiazacyclobutadienes To Form Tetraboratetraazacyclooctatetraenes:  Computational Study of Boron−Nitrogen Ring Formation and Ring Opening

Abstract

Computational studies of the dimerization of the diboradiazacyclobutadienes (HBNH) 2 and (MeBNMe) 2 to form the tetraboratetraazacyclooctatetraenes (RBNR) 4 suggest that the preferred pathway involves "face-to-face" dimerization of the four-membered ring, followed by asynchronous, one-step scission of the two transannular BN bonds to form the eightmembered monocyclic product. Detection of the proposed intermediates in either system is unlikely, as they lie in shallow potential energy wells. That in certain situations the fourand eight-membered rings establish equilibria is consistent with the comparatively small energy difference between (MeBNMe) 2 and (MeBNMe) 4 .Iminoboranes RBtNR′ prefer to oligomerize rather than react with other molecules. 1-3 Paetzold 4 and coworkers, among others, 5 have examined in detail the relationship between the size of R/R′ and the degree of oligomerization, finding in broad terms an inverse correlation, as one would expect. However, inexplicable subtleties exist, and these are readily seen in the behavior of the iminoborane tetramers, 6 here termed tetraboratetraazacyclooctatetraenes. 7 For example, NMR studies show that equilibria between the iminoborane dimers (diboradiazacyclobutadienes) and the tetramers can occur at or somewhat above room temperature for R ) Me, R′ ) tBu 8 and for R ) R′ ) iPr 9 but not for R ) nPr, R′ ) iPr, R ) R′ ) iBu, or several other combinations. 10 The mechanism by which the equilibria are established remains lightly studied. Paetzold proposed the mechanism shown in Scheme 1. 4 The diboradiazacyclobutadiene 1 dimerizes to the tricyclic "Dewar cyclooctatetraene" analogue 2. This undergoes sequential breakage of the internal transannular BN bonds to form first fused bicyclic 3 and then the product tetraboratetraazacyclooctatetraene 4. All steps proceed reversibly, so that equilibria between the four species can occur. Experimentally, however, only the end points 1 and 4 were observed, suggesting that 2 and 3, if they are intermediates, lie in very shallow potential energy wells.Close inspection of the mechanism suggests several issues meriting computational examination. First, does the diboradiazacyclobutadiene dimerization occur "edgeto-edge", to give a staircase-like tricycle, or "face-to-face", to give a tub-like tricycle? Can these tricycles equilibrate, possibly through decomposition back to the fourmembered rings and then recombination? Second, need the internal bond breaking occur stepwise, or can it occur synchronously (or nearly so)? Third, given that rather strong bonds (ca. 88-90 kcal mol -1 ) 11,12 must break to convert the tricyclic intermediate to the bicyclic intermediate and monocyclic product, why are the proposed intermediates 2 and 3 so difficult to observe? Are the wells they lie in really so shallow?We report here an examination of these questions through calculations of the structures and energetics of the parent perhydrogen and permethyl homologues represented in Scheme 1 and the transition states connecting them. We find that t...

A Unified Orbital Model of Delocalised and Localised Currents in Monocycles, from Annulenes to Azabora‐heterocycles

“…The nonplanarity of B 2 N 2 H 4 has been noted in previous ab initio calculations, [26] which gave a BN distance of 1.457 , and although the parent molecule itself has not been synthesised, X-ray structures of five substituted molecules [14b] are known with mean BN distances varying between 1.430 and 1.486 , and all are planar apart from the severely hindered tetra-tert-butyl derivative. Borazine has been the subject of many ab initio calculations.…”

“…No experimental data are available for the parent B 4 N 4 H 4 , but experimental and theoretical structural data on its derivatives have been reviewed by Gilbert and Gailbreath. [26] Calculations at different levels give a planar (B3LYP/6-31G+*) or near-planar (MP2/6-31G+*) structure for B 4 N 4 H 8 with a BN length of 1.436 , intermediate between single and double bonds. Equivalent calculations on the permethylated mono-cycle show it to adopt a tub structure.…”

“…Equivalent calculations on the permethylated mono-cycle show it to adopt a tub structure. [26] X-ray structures for derivatives with bulky substituents are tub-like with alternating BN bonds as in, for example, (tBuN) 4 (MeB) 4 . [29] Solution 1 H NMR spectra of B 4 N 4 (CH 2 R) 4 (Me) 4 show a nonplanar eight-membered ring.…”

BN‐Phenanthryne: Cyclotetramerization of an 1,2‐Azaborine Derivative

“…Tetramer 8 is possibly formed by dimerization of 10, a known process in the chemistry of acyclic iminoboranes (Scheme 4). [7,18,19] The dimerization of 10 is exothermic by À83.9 kcal mol À1 , and hence the cyclotetramerization (4 6!8) is an extremely exothermic process (À315.3 kcal mol À1 ) which indicates the high energy nature of 6. The trimer 7 of BNaryne 6 could be detected in the reaction mixture by mass spectrometry, but was formed in such low amounts that it could not be isolated in addition to 8.…”

BN‐Phenanthrin: Cyclotetramerisierung eines 1,2‐Azaborinderivats

“…Das Tetramer wird vermutlich durch die Dimerisierung von 10 gebildet, ein aus der Chemie der acyclischen Iminoborane bekannter Prozess (Schema 4). [7,18,19] Die Dimerisierung von 10 ist mit À83.9 kcal mol À1 exotherm, weshalb die Cyclotetramerisierung (4 6!8) ein sehr stark exothermer Prozess (À315.3 kcal mol À1 ) ist, der den hohen Energieinhalt von 6 widerspiegelt. Das Trimer 7 des BN-Arins 6 konnte zwar mittels Massenspektrometrie in der Reaktionsmischung nachgewiesen werden, wird aber in so geringen Mengen gebildet, dass es neben 8 nicht isoliert werden konnte.…”