Tertiary phosphine complexes of low-valent zirconium and their properties

Abstract

1053protonated carbonyl compounds can relax to their ground states. In this context, the photoreactions of 3 and 4 described here can perhaps be better compared to those of 1,3-dienes rather than conjugated enones.'Secondly, it is of interest to consider the mechanism of the thermal isomerization about the C -0 bonds. Several possibilities can be eliminated from consideration. Thus, the minimal effect of introducing a methyl group a t C3 on the rate of isomerization about the C-0 bond renders a direct rotation process unlikely as this involves a large change in the charge distribution in the system. The absence of any appreciable medium effect eliminates any base addition route, while the substantially different activation energies for isomerism about the C-0 and C2-C3 bonds rules out the intervention of a common 0-methyloxetium ion in these two rearrangements.Rather, it appears that the most likely process occurring is an inversion or "lateral shift" mechanism8 as has also been found with the isoelectronic aldimines and k e t i m i n e~.~ Acknowledgment.

Butadiene Complexes of Titanium(II) and Titanium(0): Synthesis, Butadiene Dimerization Catalysis, and Crystal Structures of TiMe2(η4-1,4-C4H4Ph2)(dmpe) and Ti(η4-C4H6)2(dmpe)

“…It is not entirely clear why the titanium(II) complex TiMe 2 (dmpe) 2 is able to cyclodimerize butadiene while other homogeneous early transition metal catalysts such as Cp 2 M(diene) afford linear dimers. − The different products reflect the fact that the vinyltitanacycloheptene intermediate generated from TiMe 2 (dmpe) 2 readily undergoes reductive elimination with formation of a C−C bond, whereas similar intermediates with other ancillary ligands do not. Linear butadiene dimers can only be formed if a C−H bond is broken; evidently, this C−H bond breaking step (or some subsequent step) is slow relative to C−C reductive elimination for the metallacyclic intermediates generated from TiMe 2 (dmpe) 2 but fast in other systems.…”

“…The dimerization of butadiene to linear C 8 alkenes can also be achieved by using low-valent titanium catalysts generated in situ by treatment of CpTiCl 3 , CpTiCl 2 , or Cp 2 TiCl 2 with “Mg(isoprene)” or Grignard reagents , Butadiene polymers are also produced in these reactions. The allyl complex Zr(η 8 -C 8 H 8 )(η 3 -C 3 H 5 ) 2 catalyzes the dimerization of butadiene to 1,3,6-octatriene …”

Zirconium and Hafnium: Introduction

“…Thus cyclohexadiene (the 1,3-isomer more readily than the 1 , 4 ) is converted into benzene and cyclohexene at 60 °C. 56 Other hydrogen transfer reactions include (i) the formation of the same products from cyclohexa-l,3-diene and hex-1-ene and (ii) the conversion of but-1-ene into but-2-ene. 57 Homogeneous hydrogenation using an organozirconium catalyst was first noted in 1972, using…”

1,3-Diene complexes of zirconium and hafnium prepared by the reaction of enediylmagnesium with MCl2Cp2. A remarkable difference between the zirconium and hafnium analogs as revealed by proton NMR and electronic spectra

“…The interaction of 4-6 with magnesium salts seems to be weaker than that of the Zr derivatives 1-3. (1) M = Zr = Hf Preparation and Characterization of Cp2M( 1,3-diene) Complexes A zirconium complex of isoprene was prepared by the 1:1 reaction of ZrCl2Cp2 with [Mg(C5H8)]" in THF at 20 °C for 2-3 h. After evaporation of the reaction mixture to dryness, the Cp2Zr(isoprene) complex was extracted into hot benzene-hexane (1:2) and was isolated in the form of…”