Recent studies have demonstrated that the incorporation of an ansa bridge may have a profound influence on the chemistry of a metallocene system. 1 We are particularly interested in delineating the origin of these effects by studying well-defined permethylated 2 ansa metallocene complexes, [A(C 5 Me 4 ) 2 ]MX n . For example, we recently demonstrated that the [Me 2 Si] ansa bridge in [Me 2 Si(C 5 Me 4 ) 2 ]ZrX 2 derivatives creates more electrophilic metal centers than those in Cp* 2 ZrX 2 counterparts. 3 In this paper, we report that incorporation of the [Me 2 Si] ansa bridge into the permethylmolybdenocene system promotes intermolecular C-H and C-C bond activation reactions.Access to the [Me 2 Si] ansa-bridged permethylmolybdenocene system is conveniently provided by the synthesis (1) For leading reports describing differences in the chemistry of ansa and non-ansa systems, see: (a) Conway, S. L. J.; Dijkstra, T.; Doerrer, L. H.; Green, J. C.; Green, M. L. H.; Stephens, M. L. H. Cook, J.; Green, M. L. H.; Labella, L.; Simpson, S. J.; Souter, J.; Stephens, A. H. H. J. Chem. Soc., Dalton Trans. 1997, 3225-3243 and references therein. (d) Wochner, F.; Brintzinger, H. H. J. Organomet. Chem. 1986, 309, 65-75. (e) Smith, J. A.; Brintzinger, H. H. J. Organomet. Chem. 1981, 218, 159-167. (f) Dorer, B.; Diebold, J.; Weyand, O.; Brintzinger, H. H. J. Organomet. Chem. 1992, 427, 245-255. (g) Fendrick, C. M.; Schertz, L. D.; Day, V. W.; Marks, T. J. Organometallics 1988, 7, 1828-1838 and references therein.(2) Our reason for studying permethylated ansa-metallocenes is associated with the fact that such substitution frequently allows isolation of complexes that have no counterparts in the corresponding unsubstituted cyclopentadienyl system. For reviews of heavily substituted cyclopentadienyl ligands, see: (a) Janiak, C.; Schumann, H. Adv. Organomet. Chem. 1991, 33, 291-393. (b) Okuda, J. Top. Curr. Chem. 1991 (3) Lee, H.; Desrosiers, P. J.; Guzei, I.; Rheingold, A. L.; Parkin, G. J. Am. Chem. Soc. 1998, 120, 3255-3256. (4) The diiodide [Me2Si(C5Me4)2]MoI2 may also be obtained by treatment of [Me2Si(C5Me4)2]MoH2 with (CH2I)2. (5) The molecular structures of [Me2Si(C5Me4)2]MoCl2, [Me2Si(C5-Me4)2]MoI2, [Me2Si(C5Me4)2]MoMe2, [Me2Si(C5Me4)2]MoH2, [Me2Si(C5-Me4)2]Mo(Ph)H, [Me2Si(C5Me4)2]Mo(η 2 -C2H4), [Me2Si(C5Me4)2]Mo(Me)-CN,[Me2Si(C5Me4)2]Mo(CO),[Me2Si(C5Me4)2]Mo(η 2 -Bu t CN),and[Me2Si(C5Me4)2]-Mo(η 2 -Bu t CP) have been determined by X-ray diffraction. See the Supporting Information.

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A detailed account regarding a formal [3 + 3] cycloaddition method using 4-hydroxy-2-pyrones and 1,3-diketones is described here. This formal cycloaddition reaction or annulation reaction is synthetically useful for constructing 2H-pyranyl heterocycles. The usage of alpha,beta-unsaturated iminium salts is significant in controlling competing reaction pathways to give exclusively 2H-pyrans. Most significantly, experimental evidence is provided to support the mechanism of this reaction that involves a sequential Knoevenagel condensation and a reversible 6pi-electron electrocyclic ring-closure of 1-oxatrienes.