Hetero Diels–Alder Reactions of Masked Dienes Containing Heavy Group 15 Elements

Treatment of N,C,N-chelated organopnictogen(I) compounds ArE (1–3) (Ar=2,6-(RN=CH)₂C₆H₃, E/R=As/Dmp (1), Sb/tBu (2), and Bi/tBu (3), where Dmp=2,6-Me₂C₆H₃) with various electron-deficient alkynes RC≡CR′ (R/R′=CO₂Me (DMAD), R/R′=H/CO₂Me, or R/R′=NC₅F₄) affords different types of heterocyclic compounds. In these reactions, 1–3 act as hidden dienes and participate in hetero-Diels–Alder (DA) reactions, a feature that has been only rarely reported for heavier pnictogen compounds. In this way, reactions of 1 furnished the set of 1-arsanaphthalenes 4–6. The most likely mechanism involves two steps, that is, an arsa-DA reaction giving a 1-arsa-1,4-dihydro-iminonaphthalene, followed by CH→NH proton migration. In contrast, this reaction sequence terminates at the first step in the case of the antimony analogue 2, thus giving the 1-stiba-1,4-dihydro-iminonaphthalenes 7 and 8. Reactions of the bismuth congener 3 gave one of two products depending on the alkyne used. Reaction of 3 with DMAD gave 1-bisma-1,4-dihydro-iminonaphthalene 9, whilst its reaction with two equivalents of HC≡C(CO₂Me) gave bismacyclohexadiene (10). All compounds have been characterized by NMR, IR, and Raman spectroscopies and X-ray diffraction analysis. The experimental data were complemented by a computational study, including a description of the reaction profiles of the hetero-DA reactions and an assessment of the aromaticities of the heterocyclic naphthalene derivatives.