Pericyclic Reactions between Iminoboranes RB⋮NR‘ and Alkynes:  [4 + 2] vs [2 + 2] Transition States

Abstract

Computational comparisons of the [2 + 2] cyclization and [4 + 2] ene-type reactions between iminoboranes RBtNR′ and alkynes HCtC(R′′) show that the latter are strongly preferred. The preference arises from the [4 + 2] reactions exhibiting lower barriers and greater exothermicities than do the [2 + 2] reactions. This behavior mimics that of reactions between alkynes and supports viewing iminoboranes, particularly (F 3 C)BtNMe and higher fluorinated analogues, as acting like polar alkynes. The hypothetical 1-aza-2-boracyclohexyne BC(CF 3 ) 2 CF 2 CH 2 C(Me) 2 N shows remarkable potential reactivity, in that its [4 + 2] enetype reaction with propyne exhibits no activation barrier. This arises from the loss of ring strain energy in the azaboracyclohexyne as the propyne approaches and the B-C bond forms.

4π‐Electron B–N Monocycles: Stability and (Anti)aromaticity

“…Substituted 4 was also studied theoretically as a product of pericyclic reactions between iminoboranes and alkynes. The BNCC ring is nonplanar to avoid 4π antiaromaticity and has two double bonds (B=N and C=C, 1.48–1.49 and 1.37 Å, respectively) and two single bonds (B–C and C–N, 1.52 and 1.45 Å, respectively) . No data are available for the inherently charge‐separated 5…”

Unusual substituent effects on the bonding of iminoboranes

“…25 Based on the bonding descriptions above, it may be anticipated that disubstituted iminoboranes with two electron-withdrawing substituents will oligomerize very easily, since the oligomerization process is favored if the system presents a doubly-bonded nitrogen atom with a free lone pair and an electron deficient boron atom. 32 The unique bonding and electron distribution characteristics of disubstituted iminoboranes may suggest new synthetic routes to produce cyclic compounds containing BN motifs through Diels-Alder or pericyclic reactions. 31,32 The characteristics of ELF may also assist in understanding the bonding trends in the two series of monosubstituted derivatives.…”

“…32 The unique bonding and electron distribution characteristics of disubstituted iminoboranes may suggest new synthetic routes to produce cyclic compounds containing BN motifs through Diels-Alder or pericyclic reactions. 31,32 The characteristics of ELF may also assist in understanding the bonding trends in the two series of monosubstituted derivatives. As illustrated in Fig.…”

“…For this purpose we have considered the three series of derivatives: X-BRN-H, H-BRN-X, and X-BRN-X, for X = H, CH 3 , NH 2 , OH, and F. A systematic study of series of iminoboranes may lead to a better understanding of the dramatically different effects of substitution on B-N coupling constants for RBNR compared to C-C coupling constants for RCCR, 25 as well as the propensity of iminoboranes to oligomerize. 6,26,27 It may also provide clues for designing new synthetic routes to produce BN containing cycles, 6,[28][29][30][31][32] an intriguing possibility in light of the recently-synthesized boryl compounds in which boron behaves as a nucleophile. 33…”

3 Boron

“…121 Computational studies have been used to compare the [2 + 2] vs. [4 + 2] pericyclic reactions between R 2 BLNR 2 and alkenes, and indicate the preference for the [4 + 2] addition due to lower reaction barriers. 122,123 The dicationic dicobalt(III) arene-bridged boratabenzene complex [{(g 5 -C 5 H 5 )-Co(g 6 -C 5 H 5 B)} 2 (1,4-C 6 H 4 )] has been synthesised and structurally characterised. Variable temperature magnetic susceptibility measurements on the related neutral Co(II) complex have also been reported.…”