Organometallic compounds, also from metallation with metals

The mercury may be replaced from mercury dimethyl by sodium, magnesium, zinc, or aluminium under suitable condition with formation of organometallic compounds of these metals with mercury diethyd, the reaction takes place with sodium, magnesium, cadmium, beryllium, zinc, aluminium, bismuth, and tellurium with mercury di-n-propyl, the metals sodium, beryllium, zinc and aluminium react, and sodium also reacts with mercury di-n-octyl. 

Thus, several mechanistic pathways based on polarization effects have been proposed to explain the catalysis of the alcohol-isocyanate reaction. These propositions appear to be often unsatisfactory and cannot explain even the majority of the experimental results reported in the literature. For an example, why is the polyurethane formation catalyzed by potassium acetate (JJ and not at all by MgC03 nor CsCl (14) The role played by the nature of the metal remains also unexplained. Robins reports the incremental temperature rise noted 1 minute after the mixing of reagents and catalyst (7, 16). This parameter is related to the catalytic activity and is an effective way to show the role played by the organometallic compound in its interaction with the alcohol. A similar conclusion can be drawn from Table I (15) where the Sn+ derivative is much more active than the Sn+2 oxidation state or Pb+. ... 

Regioselectivity of C—C double bond formation can also be achieved in the reductiv or oxidative elimination of two functional groups from adjacent carbon atoms. Well estab llshed methods in synthesis include the reductive cleavage of cyclic thionocarbonates derivec from glycols (E.J. Corey, 1968 C W. Hartmann, 1972), the reduction of epoxides with Zn/Nal or of dihalides with metals, organometallic compounds, or Nal/acetone (seep.lS6f), and the oxidative decarboxylation of 1,2-dicarboxylic acids (C.A. Grob, 1958 S. Masamune, 1966 R.A. Sheldon, 1972) or their r-butyl peresters (E.N. Cain, 1969). 

In organometallic compounds of the form RR R"C—M, pretty well the whole spectrum of bonding is known from the essentially covalent, via the polar-covalent, RR R"C, —M4+, to the essentially ionic, RR R"CeM . In their reactions, predominant retention, racemisation, and inversion of configuration have all been observed the outcome in a particular case depending not only on the alkyl residue, but also on the metal, and particularly on the solvent. Even with the most ionic examples it seems unlikely that we are dealing with a simple carbanion thus in the reaction of EtI with [PhCOCHMe]6 M , the relative rates under analogous conditions are found to differ over a range of 104 for M = Li, Na and K. 

The synthetic potential of transition metal atoms in organometallic chemistry was first demonstrated by the formation of dibenzenechrom-ium (127). Apart from chromium, Ti, V, Nb, Mo, W, Mn, and Fe atoms each form well-defined complexes with arenes on condensation at low temperatures. Interaction has also been observed between arenes and the vapors of Co, Ni, and some lanthanides. Most important, the synthesis of metal-arene complexes from metal vapors has been successful with a wide range of substituted benzenes, providing routes to many compounds inaccessible by conventional reductive preparations of metal-arene compounds. 

The first organometallic compound of the transition metals to be characterized (1827) was Zeise s salt, K[(C2H4)PtCl3]-H20 (Fig. 18.1). It forms when K2[PtCl4] in aqueous ethanol is exposed to ethylene (ethene) a dimeric Pt—C2H4 complex with Cl bridges is also formed. In both species, the ethylene is bonded sideways to the platinum(II) center so that the two carbon atoms are equidistant from the metal. This is called the dihapto-or T]2 mode. A ligand such as an allyl radical with three adjacent carbons directly bonded to a metal atom would be trihapto- or t 3, and so on. 

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