Organic syntheses via transition metal complexes .89. 5-aza-1-metalla-1,3,5,7-octatetraenes and 2H-pyrrole and 2H-dihydroazete complexes from (1-alkynyl)carbene complexes (M=Cr, W) and alkenyl N-H imidates

Addition of alkenyl N-H imidates RCH=(CRC)-C-1(OEt)=NH (8a-d; R, R-1 = Ph, Me, H) to (1-alkynyl)carbene complexes (CO)(5)M=C(OEt)C=CPh (6a,b; M = Cr, W) affords novel 5-aza-1-metalla-1,3,5,7-octatetraenes (CO)(5)M=C(OEt)CH=C(Ph)N=C(OEt)CR1=CHR ((Z)-9a-h; by Michael addition) together with 2,5-diethoxy-2H-pyrrole complexes 10a-h (by 3 + 2 cycloaddition) as well as 2,4-diethoxy-2H-dihydroazete complexes 11e-h (by 2 + 2 cycloaddition) in good overall yields. The product ratio 9:10:11 is strongly influenced by the nature of the metal (chromium or tungsten). Thus, formation of 5-aza-1-metalla-1,3,5,7-octatetraenes 9 is favored in the case of M = W, while 2H-dihydroazete complexes 11 are major products in the case of M = Cr. Compounds 9a-d undergo smooth cyclization at 80-90 degrees C to give 2,3-diethoxy-2H-pyrrole complexes 12a-d (which are regioisomers of 10a-d) in 86-93% yields, from which (metal-free) W-pyrroles 13a-d are generated on thermolysis at 110 degrees C. X-ray structure analyses are reported for the 5-aza-1-metalla-1,3,5,7-octatetraene 9b, the (regioisomeric) 2H-pyrrole complexes 10b and 12b, and the 2H-dihydroazete complex 11e.