Stable Iso‐osmabenzenes from a Formal [3+3] Cycloaddition Reaction of Metal Vinylidene with Alkynols

Abstract

The investigation on the synthesis and reactivity of metal vinylidene complexes has always been an active area, especially given their applications as well-known catalysts for metathesis and carbon–carbon and carbon–heteroatom coupling reactions. 2] Compared with other reactions of metal vinylidenes, the cyclization reactions are underdeveloped. Previous studies showed that the vinylidene M=Ca bond participated in cycloadditions (see a) as key steps in carbon– carbon coupling reactions, and some cycloaddition products have also been isolated. Interestingly, similar reactions of the metal vinylidene Ca=Cb bond (see b) have been realized as well. [5] However, cyclizations of the skeleton of metal vinylidene (M=Ca=Cb) as a unit (see c) remain scarce. In fact, the reported cyclization is a formal [3+2] cycloaddition reaction. To the best of our knowledge, the [3+3] cyclization reactions of metal vinylidene to construct six-membered rings have not been reported. Moreover, since the first isolation of metallabenzene was achieved by Roper and co-workers, considerable research interest has been attracted in this field. Nevertheless, the metallabenzene analogue isometallabenzene, which can be seen as a metal-containing 1,2,4-cyclohexatriene, has not been thoroughly explored. Until now, only the 16e isometallabenzene [Os{=C=C(Ph) CH(Ph) CH=C(CH2Ph)}Cl(PiPr3)2], prepared by double coupling reactions, was reported by Esteruelas and co-workers in 2004. Herein, we report an unprecedented formal [3+3] cycloaddition reaction between the hydride vinylidene complex [OsHCl2{=C=CH(PPh3)}(PPh3)2] (2) and alkynols under mild conditions to give stable 18e iso-osmabenzenes. The origin of the unexpected stability of these iso-osmabenzene complexes and their isomerization into h-cyclopentadienyl complexes through metalated cyclopentadiene intermediates is also described. Stirring of the osmium hydride alkenylcarbyne complex [OsHCl2( C C(PPh3)=CHPh)(PPh3)2]BF4 (1) in a mixture of H2O/CH3OH (3:2 v/v) at reflux produced the osmium hydride vinylidene complex 2, which was isolated as a lightyellow solid in 60% yield (Scheme 1). The other product, PhCHO, was detected by H NMR (d= 9.3 ppm; PhCHO) and GC analysis of the reaction mixture (see Figure S1 in the Supporting Information). Complex 2 was characterized by NMR spectroscopy and elemental analysis, and the structure was additionally confirmed by single-crystal X-ray diffraction (Figure S2).