The dimerisation and competitive cycloaddition of 2-methoxycarbonylbuta-1,3-diene with electron-rich dienes has been investigated. Experimental results provide evidence that the enthalpy of the -system significantly influences the energy of the transition state of cycloadditions of this type. This has been corroborated by ab initio calculations. We propose an early reorganisation of the -electrons in such cycloadditions to explain the influence stated above.

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Supplementary Material Available: ORTEP diagrams and tables of fractional coordinates, thermal parameters, bond distances, and bond angles for 5 and 47 and full assignments of and 13C NMR spectra of the Diels-Alder adducts (17 pages). Ordering information is given on any current masthead page.

Reactions of the isodicyclopentadienide anion with TiC13 in tetrahydrofuran at 25 "C, with ZrC1, in dimethoxyethane at reflux for 70 h, and with HE14 in dimethoxyethane (reflux, 100 h) have been performed in order to assess the a-facial stereoselectivity of complexation to group 4 transition metals. In all three instances, a single complex was isolated. X-ray crystallographic analysis of the titanocene and zirconocene dichlorides demonstrate both to be the result of dual coordination to the exo surfaces. By l H NMR comparison, the hafnocene is shown to be closely similar and therefore of the same stereochemistry. The three-dimensional features of the complexes are discussed.