Migratory insertion of isocyanide into a ketenyl-tungsten bond as key step in cyclization reactions.

Treatment of the side-on tungsten alkyne complex of ethinylethyl ether [Tp*W(CO) 2 (η 2 - C , C '-HCCOCH 2 CH 3 )] + {Tp* = hydridotris(3,4,5-trimethylpyrazolyl)borate} (2a) with n -Bu 4 NI afforded the end-on ketenyl complex [Tp*W(CO) 2 (κ 1 -HCCO)] (4a). This formal 16 ve complex bearing the prototype of a ketenyl ligand is surprisingly stable and converts only under activation by UV light or heat to form a dinuclear complex [Tp* 2 W 2 (CO) 4 (μ-CCH 2 )] (6). The ketenyl ligand in complex 4a underwent a metal template controlled cyclization reaction upon addition of isocyanides. The oxametallacycles [Tp*W(CO) 2 {κ 2 - C , O -C(NHXy)C(H)C(Nu)O}] {Nu = OMe (7), OEt (8), N(i-Pr) 2 (9), OH (10), O 1/2 (11)} were formed by coordination of Xy-NC (Xy = 2,6-dimethylphenyl) at 4a and subsequent migratory insertion (MI) into the W-ketenyl bond. The resulting intermediate is susceptible to addition reactions with protic nucleophiles. Compounds 2a-PF 6 , 4a/b, and 7-11 were fully characterized including XRD analysis. The cyclization mechanism has been confirmed both experimentally and by DFT calculations. In cyclic voltammetry, complexes 7-9 are characterized by a reversible W(ii)/W(iii) redox process. The dinuclear complex 11 however shows two separated redox events. Based on cyclic voltammetry measurements with different conducting electrolytes and IR spectroelectrochemical (SEC) measurements the W(ii)/W(iii) mixed valent complex 11 + is assigned to class II in terms of the Robin-Day classification.