Ring Contraction in Arylcarbenes and Arylnitrenes; Rearrangements of 1- and 3-Isoquinolylcarbenes and 2-Naphthylnitrene to Cyanoindenes

Abstract

Flash vacuum pyrolysis (FVP) of 1-(5-(13)C-5-tetrazolyl)isoquinoline 18 generates 1-((13)C-diazomethyl)isoquinoline 19 and 1-isoquinolyl-((13)C-carbene) 22, which undergoes carbene-nitrene rearrangement to 2-naphthylnitrene 23. The thermally generated nitrene 23 is observed directly by matrix-isolation ESR spectroscopy, but undergoes ring contraction to a mixture of 3- and 2-cyanoindenes 26 and 27 under the FVP conditions. The (13)C label distribution in the cyanoindenes was determined by (13)C NMR spectroscopy and indicates the occurrence of two parallel paths of ring contraction starting from 1-isoquinolylcarbene; path a via ring expansion to 3-aza-benzo[c]cyclohepta-1,2,4,6-tetraene 32 bifurcating to 2-naphthylnitrene 23 and 2-aza-benzobicyclo[3.2.0]heptatriene 39 (paths a1 and a2); and path b via ring closure of the carbene onto the ring nitrogen, yielding 1-aza-benzo[d]bicyclo[4.1.0]hepta-2,4,6-triene 34 and 3-aza-benzo[d]cyclohepta-2,3,5,7-tetraene 35. Product studies demand that the major path is route a1 via 2-naphthylnitrene 23, which then undergoes direct ring contraction to 1-cyanoindene; but the (13)C label distribution requires that the non-nitrene route b contributes significantly. The two reaction paths are modeled at the B3LYP/6-31G* level. The initially formed carbene 22 is estimated to carry chemical activation of some 40 kcal/mol. This allows both reaction channels to proceed simultaneously under low-pressure FVP conditions. FVP of 3-(5-tetrazolyl)isoquinoline 28 similarly generates 3-diazomethylisoquinoline 29 and 3-isoquinolylcarbene 30, which rearranges to 3- and 2-cyanoindenes 26 and 27.

Rearrangements of 4-Quinolylcarbene, 3-Quinolylcarbene, and 2-Quinolylcarbene to 1-Naphthylnitrene and Cyanoindenes by Falling Solid Flash Vacuum Pyrolysis

“…A correlation between observed D values and calculated nitrene spin densities for several nitrenes, including 1-naphthylnitrene 35, is reported in the accompanying publication. 7 2-Quinolylcarbene 25 was generated previously 9 by conventional FVP of either the 2-(5-tetrazolyl)quinoline 22 or 1,2,3triazolo[1,5-a]quinoline 24T and yielded virtually the same results as described for 14 and 18, viz. a ca.…”

“…Such barriers are very easily accessed under FVP conditions. The open-shell singlet 1naphthylnitrene 35 formed in this way can now undergo ring contraction to 1-cyanoindene 36 in a highly exothermic reaction with a calculated barrier of 38 kcal/mol, and 36 interconverts with 3-cyanoindene 12 and 2-cyanoindene 13 in a series of 1,5-shifts of H and CN as described previously 7,9 (Scheme 4 and Figure 2).…”

“…5,7 The aza-benzocycloheptatetraene 30 could undergo a similar rearrangement to tricyclic compound 43 and then to the spiroazirine 44, the biradical 45 and 1cyanoindene 36 as a mechanism for forming the cyanoindenes 12 and 13. The energies of the intermediates 44, 45, and 36 have been calculated previously 7 and are repeated in Scheme 8. However, the first step in Scheme 8 requires a high activation energy of 54 kcal/mol relative to nitrene 35 S 1 (or 32 kcal/mol relative to 30), which puts it considerably above all the barriers in Figures 1 and 2.…”

“…An analogous situation pertains to 9phenanthrylnitrene, which undergoes "normal" ring expansion to a dibenzoazacycloheptatetraene (analogous to 33) on FVP but to a cyclic nitrile ylide (analogous to 48) on photolysis. 17 (6) The rearrangement of 1-isoquinolylcarbene 9 to 2naphthylnitrene and to 3-and 2-cyanoindenes (Scheme 2) 7 is analogous to that of 2-quinolylcarbene to 1-naphthylnitrene and the cyanoindenes. The two energy surfaces are formally linked through 1-cyanoindene 36, but otherwise there is no known connection between them.…”

“…2 The rearrangement of 4-pyridylcarbene 1 to 3-pyridylcarbene 3 to 2pyridylcarbene 6 and finally to phenylnitrene 8 has been investigated in great detail (Scheme 1). 3−6 Recent 13 C labeling studies have allowed the mechanisms of the ring contraction of phenylnitrene to cyanocyclopentadiene, 5 and of 1-isoquinolylcarbene 9 to 3-and 2-cyanoindenes 12 and 13 (Scheme 2) 7 to be elucidated.…”