Alkene exchange

Similarly, Al, H2 and Mc2C=CH2 react at 100° and 200 atm to give AlBu 3 in a single-stage process, provided a small amount of this compound is present at the start this is required because Al does not react directly with H2 to form AIH3 prior to alkylation under these conditions. Alkene exchange reactions can be used to transform AlBuj into numerous other trialkyls. AIBU3 reduced by potassium... 

Evans suggests that the catalyst resting state in this reaction is a 55c Cu alkene complex 58, Scheme 4 (35). Variable temperature NMR studies indicate that the catalyst complexes one equivalent of styrene which, in the presence of excess alkene, undergoes ready alkene exchange at ambient temperature but forms only a mono alkene-copper complex at -53°C. Addition of diazoester fails to provide an observable complex. These workers invoke the metallacyclobutane intermediate 60 via a formal [2 + 2] cycloaddition from copper carbenoid alkene complex 59. Formation of 60 is the stereochemistry-determining event in this reaction. The square-planar S Cu(III) intermediate 60 then undergoes a reductive elimination forming the cyclopropane product and Complex 55c-Cu, which binds another alkene molecule. 

Olefin metathesis refers to a process in which two alkenes exchange their alkylidene fragments. This reaction has recently been reviewed [743-747]. An exciting account on the discovery of olefin metathesis has been written by Eleuterio [748]. 

The reaction in which two alkenes exchange their alkylidene fragments is called alkene metathesis. The synthesis of libraries in liquid- and solid-phase employing this kind of C-C bond formation has been reported. There are three types of me-... 

Fig. 5 Illustration of the oxidative trajectory for alkene exchange and oxygenation of Pd -alkene complexes...

The reaction with ethylene is visualized as an alkene exchange to give the unstable parent /3-sultone, which undergoes ring opening and concomitant proton shift to give ethylenesul-fonic acid. The latter reacts with the fluorinated /3-sultone with ultimate formation of ethylenebis-sulfonic acid (Scheme 53) (70BAU574). 

The trend of structural selectivity can be summarized as degenerate metathesis of terminal alkenes (exchange of methylene groups) > cross-metathesis of terminal and internal alkenes > metathesis of internal alkenes > productive metathesis of terminal alkenes (formation of internal alkene and ethylene).87 Since different catalyst systems exhibit different selectivities, a simple general picture accounting for all stereochemical phenomena of metathesis is not feasible. 

Though hexamethylsilirane is far too reactive to provide a convenient source of dimethyl-silylene, it is a useful precursor to other siliranes formed through alkene exchange. The insertion of styrenes into the silirane ring suggests that its thermal decomposition proceeds through a diradical intermediate (Scheme 11) (76JOM(ll7)C51). 

One of the most curious catalytic reactions of alkenes ever discovered is alkene metathesis or alkene dismutation, in which two alkenes exchange alkyli-dene groups, usually over a tungsten catalyst. The essence of the reaction is illustrated by a commercial process for converting excess propene to a mixture of ethene and butenes ... 

When ethene reacts with deuterium on a metal catalyst, the products typically consist of (i) deuterated ethenes containing from one to four deuterium atoms, formed by alkene exchange, (ii) deuterated ethanes containing from zero to six deuterium atoms formed by addition and (iii) hydrogen... 

Reaction of the Tebbe reagent with 3,3-dimethylcyclopropeneand4-(dimethylamino)pyridine (DMAP) affords the titanacycle 1 in 33% isolated yield. Alternatively, 1 can be prepared by alkene exchange with other titanacyclobutanes. Acidolysis of 1 with hydrochloric acid gives a 89% yield of 1,1,2-trimethylcyclopropane. ... 

There is evidence with nickel and platinum that molecular addition is unimportant virtually the same alkane profile is given by non-equilibrated and equilibrated mixtures of hydrogen and deuterium. Unfortunately the measurements with platinum were made with propene where direct addition as shown by semi-logarithmic plots is absent (it appears with ethene but not propene, Figure 7.9). In addition to the evidence provided by the high values of q and s for ruthenium and osmium, which incidentally overcome the extensive alkene exchange to make ethane-rf2 the major product, the orders in deuterium are often somewhat... 

Section 14.15 The doubly bonded carbons of two alkenes exchange partners on treatment with transition-metal carbene complexes, especially those derived from ruthenium and tungsten. 

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