Alkenes, reactions with metal atoms

Transition-metal atoms have been shown to deoxygenate epoxides to alkenes (36). Chromium and titanium atoms emerged as the most effective species in this regard, abstracting over two equivalents of oxygen. By studying the reaction of a wide range of epoxides with chromium atoms, the reaction... 

This forms individual hydrogen atoms adsorbed to the surface of the metal. These hydrogen atoms are now available for addition across the alkene. The addition reaction begins when the alkene coordinates with the metal surface ... 

Transition metal atoms react much more readily with alkenes than with alkanes because the initial interaction between the metal atom and an alkene is much less repulsive than for M+alkanes. To insert into a C-H bond of an alkane, the metal atom has to break a C-H bond and form an M-C and an M-H bond. The first step in a reaction with an alkene, however, is formation of a 7r-complex in which the C=C bond is merely weakened, not broken.119 The availability of the DCD bonding scheme (Sec. 1.1) leads... 

The 1,3-dipolar cycloaddition of nitrones to vinyl ethers is accelerated by Ti(IV) species. The efficiency of the catalyst depends on its complexation capacity. The use of Ti( PrO)2Cl2 favors the formation of trans cycloadducts, presumably, via an endo bidentate complex, in which the metal atom is simultaneously coordinated to the vinyl ether and to the cyclic nitrone or to the Z-isomer of the acyclic nitrones (800a). Highly diastereo- and enantioselective 1,3-dipolar cycloaddition reactions of nitrones with alkenes, catalyzed by chiral polybi-naphtyl Lewis acids, have been developed. Isoxazolidines with up to 99% ee were obtained. The chiral polymer ligand influences the stereoselectivity to the same extent as its monomeric version, but has the advantage of easy recovery and reuse (800b). 

Intermediates corresponding to the coordination step are considered as sufficiently close to transition states of the insertion reaction, and hence as suitable preinsertion intermediates, only if the insertion can occur through a motion of the nuclei that is near to the least—principle of least nuclear motion.13,30,31 For instance, for alkene polymerizations preinsertion intermediates correspond to geometries with (a) a double bond of the olefin nearly parallel to the metal growing chain bond and (b) the first C-C bond of the chain nearly perpendicular to the plane defined by the double bond of the monomer and by the metal atom (50° < Gi < 130°, rather than 0i 180° see below). 

Many pyridine-indole compounds are biologically active. A growing number of methods for the preparation of indolylstannanes have been developed. 2-Trialkylstannylindoles, for example, have been synthesized via directed metalation followed by reaction with tin chloride [91-93]. The latest indolylstannane syntheses include Fukuyama s free radical approach to 2-trialkylstannylindoles from novel isonitrile-alkenes [94], and its extension to an isonitrile-alkyne cascade [95]. Assisted by the chelating effect of the SEM group oxygen atom, direct metalation of 1-SEM-indole and transmetalation with BujSnCl afforded 2-(tributylstannyl)-l//-indole 108, which was then coupled with 2,6-dibromopyridine to give adduct 109. 

RMn(CO)5.3 Alkylmanganese pentacarbonyl complexes are obtained by reaction of alkyl halides with NaMn(CO),. Under pressures of 2-10 kbar, these complexes undergo an insertion reaction with alkenes to give usually a single manganacycle in which the electron-withdrawing group of the alkene is attached to the same carbon atom as the metal. Demetallation is accomplished by photolysis... 

The interaction of metal atoms with monoalkenes has been investigated on both a spectroscopic and preparative scale. It appears that the primary interaction between a metal atom and an alkene at low temperature is the formation of a ir-complex. This may subsequently lead to a thermally stable 7r-alkene complex or to rearrangement products by hydrogen abstraction or reaction with another alkene moiety, depending on the electronic requirements of the metal and the particular alkene considered. 

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