Olefin transition-metal complexes olefinic protons

Although complexes with C—H—metal three-center, two-electron bonds were first observed several years ago (40-42), they have received increasing attention recently as model systems for C—H activation by transition metal complexes (43). A general route to such compounds involves the protonation of diene (35,44-51) or olefin complexes (52-56). The resulting 16-electron species are stabilized by the formation of C—H—metal bridges. Irradiation of the complexes [Cr(CO)s L] [L = CO, P(CH3)3, P(OCH 3)3 jin presence of conjugated dienes having certain substituents provides a photochemical route to electron-deficient >/4 CH-diene complexes. 

Scheme 1.24B shows the other possibility of catalytic hydrogenation of olefin driven by a transition metal monohydride. The monohydride can be sometimes obtained by cleavage of dinuclear transition metal complexes such as Co2(CO)g on treatment with dihydrogen or it can be generated by protonation of an electron rich metal complex [71]. Insertion of an olefin into the M-H bond to form an alkyl may proceed similarly to Scheme 1.24A. 

The catalytic hydrocarbonylation and hydrocarboxylation of olefins, alkynes, and other TT-bonded compounds are reactions of important industrial potential.Various transition metal complexes, such as palladium, rhodium, ruthenium, or nickel complexes, have widely been used in combination with phosphines and other types of ligands as catalysts in most carbonylation reactions. The reactions of alkenes, alkynes, and other related substrates with carbon monoxide in the presence of group VIII metals and a source of proton affords various carboxylic acids or carboxylic acid derivatives.f f f f f While many metals have successfully been employed as catalysts in these reactions, they often lead to mixtures of products under drastic experimental conditions.f i f f f In the last twenty years, palladium complexes are the most frequently and successfully used catalysts for regio-, stereo-, and enantioselective hydrocarbonylation and hydrocarboxylation reactions.f ... 

Reviews.—Recent reviews involving olefin chemistry include olefin reactions catalysed by transition-metal compounds, transition-metal complexes of olefins and acetylenes, transition-metal-catalysed homogeneous olefin disproportionation, rhodium(i)-catalysed isomerization of linear butenes, catalytic olefin disproportionation, the syn and anti steric course in bi-molecular olefin-forming eliminations, isotope-elfect studies of elimination reactions, chloro-olefinannelation, Friedel-Crafts acylation of alkenes, diene synthesis by boronate fragmentation, reaction of electron-rich olefins with proton-active compounds, stereoselectivity of carbene intermediates in cycloaddition to olefins, hydrocarbon separations using silver(i) systems, oxidation of olefins with mercuric salts, olefin oxidation and related reactions with Group VIII noble-metal compounds, epoxidation of olefins... 

A proton is the electrophile most commonly used to remove an organic ligand from a transition metal. These protonolyses typically occur by mechanisms that retain the stereochemistry of the organic ligand. For example, the deuterium in the product of the reaction in Equation 12.4 occupies the same position about the double bond as did the ruthenium in the initial vinyl complex. Olefin stereochemistry was also shown to be retained during cleavage of a vinyl-paUadium bond with Qeavage of a metal-alkyl bond has also been shown to occur with retention of stereochemistry (Equation 12.5). ... 

Many transition metal hydrides and low-valent complexes that can generate an M—H bond by protonation catalyze hydrogen migrations in olefins. Rhodium trichloride or rhodium(I) compounds plus HCl rapidly isomerize 1-butene to an equilibrium mixture of butenes in which trans-2-butene is the largest single component. Most of the complexes that catalyze olefin dimerization also catalyze isomerization. The isomerization mechanism postulated by Cramer is similar to his dimerization mechanism except that no insertion step is involved (185). 

Pitcher, Buckingham, and Stone 285) have discussed the anomalous chemical shift of fluorine atoms bonded to the a-carbon atom of perfluoro-alkyl-transition metal derivatives in terms of mixing of nonbonding electrons of the halogen with orbitals of the metal. Bennett, Pratt, and Wilkinson (27) have discussed the shielding of protons of olefins in the complexes of these ligands with transition metals. 

Inability to observe metal-proton coupling satellites in certain silver complexes (mentioned in the discussion in various places above) could be interpreted to denote rapid exchange of olefin between different metal atoms. In view of the small coupling constants for metals in the second transition series however [cf. small coupling constants for rhodium, 7r-(Cs)M discussion above], it may be that the satellites simply are not resolvable from the... 

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