Metal-catalyzed isomerization complex compounds

In the presence of transition-metal complexes, organic compounds that are unsaturated or strained often rearrange themselves. One synthetically useful transition-metal catalyzed isomerization is the olefin migration reaction. Two general mechanisms have been proposed for olefin migrations, depending on the type of catalyst employed (A and B) (Scheme 3.8).137... 

Under this heading we shall note, first, that the hydrocarbon moiety in a hydrocarbon-metal complex can be oxidized and reduced by chemical means to provide species which may correspond to the half-hydrogenated states that are postulated in catalytic reaction mechanisms secondly, that organometallic compounds exhibit displacement reactions, and thirdly that organometallic compounds catalyze isomerization, hydrogenation, polymerization, and oxidation processes. 

The formation of reactive intermediates provides possible opportunities for new reaction design. An attractive highly reactive intermediate, carbenes, which demonstrate numerous useful synthetic pathways, most notably by addition to alkenes and alkynes and also insertion into X-H bonds, where X is both carbon and heteroatoms, suffers from problems associated with their accessibility. Undoubtedly, the most useful class of precursor is the diazo compounds, whose safety problems restrict their use. For the specific case of vinylidenes, an attractive possibility is a terminal alkyne which is isomeric with a vinylidene. Although the thermolysis appears to effect this transformation (Equation 1.1, path a), the extraordinarily high temperatures required make the prospect of a transition metal-catalyzed version (Equation 1.1, path b) attractive. The early studies of Werner [6] using Rh and Bruce and co-workers [7] using Ru proved the facility with which such species would form however, the studies focused on the formation and isolation of the vinylidene-metal complexes and their stoichiometric reactions. 

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). 

Some synthetically useful isomerization reactions of alkenes, other than nitrogen- or oxygen-substituted allylic compounds, were reported by the use of a catalytic amount of transition metal complexes. The palladium complex, /ra r-Pd(C6HsCN)2Gl2, effectively catalyzed the stereoselective isomerization of /3,7-unsaturated esters to a,/3-unsaturated esters (Equation (26)). 

Although various transition-metal complexes have reportedly been active catalysts for the migration of inner double bonds to terminal ones in functionalized allylic systems (Eq. 3.2) [5], prochiral allylic compounds with a multisubstituted olefin (Rl, R2 H in eq 2) are not always susceptible to catalysis or they show only a low reactivity [Id]. Choosing allylamines 1 and 2 as the substrates for enantioselective isomerization has its merits (1) optically pure citronellal, which is an important starting material for optically active terpenoids such as (-)-menthol, cannot be obtained directly from natural sources [6], and (2) both ( )-allylamine 1 and (Z)-allylamine 2 can be prepared in reasonable yields from myrcene or isoprene, respectively, The ( )-allylamine 1 is obtained from the reaction of myrcene and diethylamine in the presence of lithium diethylamide under Ar in an almost quantitative yield (Eq. 3.3) [7], The (Z)-allylamine 2 can also be prepared with high selectivity (-90%) by Li-catalyzed telomerization of isoprene using diethylamine as a telomer (Eq. 3.4) [8], Thus, natural or petroleum resources can be selected. 

It has been found quite recently that the isomerization to carbonyl compounds of oxiranes containing a rr-electron system is catalyzed by certain metal complexes. The experimental data acquired so far suggest that only the penta-cyanocobalt complexes are active towards the isomerization of aliphatic and alicyclic oxiranes. ... 

The role of transition-metal carbonyls and particularly those of the Group 6 metals in homogeneous photocatalytic and catalytic processes is a matter of considerable interest [1]. UV irradiation especially provides a simple and convenient method for generation of thermally active co-ordinately unsaturated catalyst for alkenes or alkynes transformation. By using tungsten and molybdenum carbonyl compounds as catalysts, alkenes and alkynes can be metathesized, isomerised and polymerized. Photocatalytic isomerization of alkenes in the presence of molybdenum hexacarbonyl was observed by Wringhton thirty years ago [2]. Carbonyl complexes of molybdenum catalyze not only... 

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