Metal-mediated [6 + 2] cycloadditions of alkynes to cycloheptatriene and N-carbethoxyazepine

Article abstract of DOI:10.1021/ja00112a005

UV photolysis of [(η⁶-C₇H₇R)Cr(CO)₃] (1, R = H, Ph, CH{CO₂Me}₂) and R?C=CR? (R? = Ph, Tol, SiMe₃, Et) in toluene or n-hexane gives the [6 + 2] cycloadducts [(η^2:4-C₉H₇{R}R?2)Cr(CO)₃] (2). Heating 2 in toluene releases the 7,8-disubstituted bicyclo[4.2.1]nona-2,4,7-trienes 3 and [(η⁶-C₆H₅Me)Cr(CO)₃]. Irradiation of solutions of 1 and dimethyl acetylenedicarboxylate at O °C yields the bicyclotriene as the major product but no organometallic complex. The diphenyl derivative 2a undergoes a reversible intramolecular metal migration to the η⁶-aryl species 4. Irradiation of the azepine complex [(η⁶-C₆H₆N{CO₂Et})Cr(CO) ₃] (5) and R?C≡CR? (R? = Ph, SiMe₃) in toluene also forms [6 + 2] cycloadducts [(η^2:4-C₈H₆N{CO₂Et}R? ₂)Cr(CO)₃] (6). The heterobicyclotrienes 7 are isolated from 6 by treatment with iodine (R? = SiMe₃) or by heating in toluene (R? = Ph). X-ray crystal structures are reported for 6b (R? = SiMe₃) [orthorhombic, P2₁2₁2₁, a = 8.871(1), b = 10.148(1), and c = 26.017(4) A, R = 0.041 for 2500 independent reflections] and a byproduct (8) that results from the endo [6 + 4] cycloaddition of N-carbethoxyazepine to 5; monoclinic, P2₁/c, a = 10.665(9), b = 14.976(7), and c = 13.004(7) A?,β= 100.05(7)°, R = 0.047 for 1044 independent reflections. Heating cycloheptatriene and alkynes R?≡CR? (R? = SiMe₃, Me, Et) in dibutyl ether with a catalytic amount of Cr(CO)₆ or Fe₂(CO)₉ also gives good yields of the [6 + 2] cycloadducts 3, whereas the thermal reaction with dimethyl acetylenedicarboxylate in the presence of Cr(CO)₆ gives the known norcaradiene [4 + 2] adduct 9.