Organometallic compounds double bonds

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). 

The Peterson olefination can be viewed as a silicon variant of the Wittig reaction, the well-known method for the formation of carbon-carbon double bonds. A ketone or aldehyde 1 can react with an a-silyl organometallic compound 2—e.g. with M = Li or Mg—to yield an alkene 3. 

Because most carbenes are so reactive, it is often difficult to prove that they are actually present in a given reaction. The lifetime of formylcarbene was measured by transient absorption and transient grating spectroscopy to be 0.15-0.73 ns in dichloromethane. In many instances where a carbene is apparently produced by an a elimination or by disintegration of a double-bond compound, there is evidence that no free carbene is actually involved. The neutral term carbenoid is used where it is known that a free carbene is not present or in cases where there is doubt. a-Halo organometallic compounds (R2CXM) are often called carbenoids because they readily give a elimination reactions (e.g., see 12-37). ° ... 

The structure of the organometallic tin(II) compound 14, which was the first stable bivalent tin compound in non-polar organic solvents84, is changed when passing from the solution to the solid state 21 108). In the crystal discrete dimers are present (Fig. 5). Since the tin atom is pyramidal and the Sn—Sn distance quite large (276 pm), no normal ct, Jt-double bond can be responsible for this geometry. 

Only a few stable organometallic compounds containing double-bonded tin are known. Stannaimines are very reactive and can be scavenged in various ways. The sterically crowded groups of compound 82 confer to it some stability. Compound 82 decomposes slowly in solution below 0 °C, according to reaction 37. See also Table 8 and reaction 44241. 

Some organometallic compounds and Lewis bases can also act as initiators. In such cases initiation occurs by a direct attack of these compounds on the double bond of the monomer molecule. 

Organometallic compounds containing polymerizable carbon-carbon double bonds have been synthesized and their polymerization studied [Archer, 2001 Pittman et al., 1987]. Among the organometallic monomers studied are vinylferrrocene and trialkyltin methacrylate. Much of the interest in these polymerizations has been to obtain polymers with... 

Undoubtedly, the most notable feature of these new dendrimeric organometallic molecules is their ability to act successfully as effective homogeneous catalysts for the Kharasch addition reaction of polyhalogenoalkanes to olefinic C=C double bonds. Indeed, they show catalytic activity and clean regiospecific formation of 1 1 addition products in a similar way to that observed in the mononuclear compounds. Likewise, the nanoscopic size of these first examples of soluble dendritic catalysts allows the separation of such macromolecules from the solution of the products by ultrafiltration methods. 

Addition of Organometallic Compounds to Activated Double Bonds Hydro-alkyl-addition... 

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