Organic syntheses via transition-metal complexes. 99.1 cyclopentadiene annelation to enolizable cyclic ketones via (1-alkynyl)carbene complexes (M = Cr, W)

Article abstract of DOI:10.1021/om9810394

The procedure for a metal-mediated cyclopentadiene annelation to enolizable cycloalkanones is described. Its key step is based on the cyclization of a 1-metalla-1,3,5-hexatriene intermediate, which is generated from the 1-metalla-1,5-hexadien-3-yne (CO)₅M=C(OEt)C≡ CC(～)=CH(～) (3; M = Cr, W) precursor. Addition of secondary amines 4a,b to [2-(1-cyclopentenyl)ethynyl]carbene complexes 3a,b (M = W, Cr) affords the 4-amino-1-metalla-1,3,5-hexatrienes (3E)-5a-d, which cyclize spontaneously to the η(C,M)-cyclopentadiene complexes 6a-d. The ring closure is highly stereoselective and involves an anti addition (!) of the M=C bond to the terminal C=C bond of the 1-metalla-1,3,5-hexatriene unit. Ligand displacement from compound 6a is achieved by protonation to give an iminium salt 7a. Addition of aniline to compounds 3a,b yields 4-(NH-amino)-1-metalla-1,3,5-hexatrienes (3Z)-10a,b, which subsequently cyclize to cyclopentenimine(N,M) complexes syn-11a,b and anti-11a,b and finally afford the cyclopentenimine 12 by oxidative ligand disengagement. Addition of secondary phosphanes 13a,b to compound 3a produces a mixture of the isomeric cyclopentadiene(P,W)-tungsten complexes anti-14a,b, anti-15a,b, and anti-16b. X-ray structure analyses are reported of compounds (3E)-5a, 6c, anti-11a, anti-16b, and anti-17a.