Alkene hydride complexes

Finally, the iridium carbene complex 580, obtained from the double C-H activation of 2-ethylphenol by Tp Ir(C6H5)2(N2) (211), illustrates the rare incidence of an equilibrium between alkylidene hydride and alkene hydride complexes. The alkylidene forms in admixture with ca. 5% of the alkene hydride isomer 581, illustrating a preference for the a- over hydrogen in the second activation step. However, in isolation 581 is observed to re-establish the same equilibrium mixture (i.e. 20 1 580 581, Scheme 61) a rare example of a metal-alkene converting to a metal-alkylidene, the reverse reaction being more typical. 

Hydroformylation regioselectivity can be established by either four-coordinate or five-coordinate intermediates. In one possible mechanism, the regioselectivity is controlled by the relative concentrations of four-coordinate intermediates, 3c, 3t, 10c and lOt, which undergo irreversible formation of alkene hydride complexes, 4. Accordingly, this mechanism requires that interconversion of isomeric pairs 4ee/4ae and 4e/4a be slow relative to irreversible migratory insertion of alkene. 

Alternatively, the regioselectivity can be determined during migratory insertion of alkene into the Rh-H bond in five-coordinate intermediates, 4. The concentrations of these five-coordinate alkene hydride complexes are controlled by the relative concentrations of PPh3 and CO. In addition, these stereoisomers (4ee/4ae, 4e/4a), which differ in the nature of the apical hgand, presumably undergo rapid interconversion via a pseudorotation mechanism, as has been observed for 2ee/2ae [8],... 

Contrary to CO, alkenes very easily insert into a metal-hydride bond to give a metal-alkyl complex, and the reaction is reversible. Indeed, depending on the conditions, the metal-alkyl complex can give rise to the extrusion reaction, also called (3-elimination, yielding the metal-alkene-hydride complex. This P-elimination reaction is the mechanism of decomposition of metal-alkyl complexes that have at least one (3 hydrogen and at most 16 valence electrons on the metal. 

As a final example in this section, a contribution by Grubbs et al. is discussed. The chloride-free ruthenium hydride complex [RuH2(H2)2(PCy3)2] (37) is believed to react, in the presence of alkenes, to form an unidentified ruthenium(O) species which undergoes oxidative additions with dihalo compounds, e.g., 38, to give the corresponding ruthenium carbene complex 9 (Eq. 4) [20]. 

The corresponding reactions of transient Co(OEP)H with alkyl halides and epoxides in DMF has been proposed to proceed by an ionic rather than a radical mechanism, with loss of from Co(OEP)H to give [Co(TAP), and products arising from nucleophilic attack on the substrates. " " Overall, a general kinetic model for the reaction of cobalt porphyrins with alkenes under free radical conditions has been developed." Cobalt porphyrin hydride complexes are also important as intermediates in the cobalt porphyrin-catalyzed chain transfer polymerization of alkenes (see below). 

Complex a is readily converted into a Fe-y-H agnostic complex b within an early picosecond timescale and then the 7i-allyl hydride complex c is generated by hydride abstraction. The energy level of the 2-alkene isomer d, which is calculated by DPT experiments, is similar to that of the 1-alkene complex b. In the next step, Fe (CO)3(t -l-alkene)(ri -2-alkene) f, which is generated via intramolecular isomerization of the coordinated 1-alkene to 2-alkene and the coordination of another 1-alkene, is a thermodynamically favored product rather than formation of a Fe(CO)3(ri -l-alkene)2 e. Subsequently, release of the 2-aIkene from f regenerates the active species b to complete the catalytic cycle. 

A similar involvement of palladium hydride, palladium alkyl, and palladium acyl complexes as intermediates in the catalytic cycle of the Pd-catalyzed hydroxycarbonylation of alkenes was reported for the aqueous-phase analogs. The cationic hydride PdH(TPPTS)3]+ was formed via the reduction of the Pd11 complex with CO and H20 to [Pd(TPPTS)3] and subsequent protonation in the acidic medium. The reaction of the hydride complex with ethene produced two new compounds, [Pd(Et)(TPPTS)3]+ and Pd(Et)(solvent)(TPPTS)2]+. The sample containing the mixture of palladium alkyl complexes reacted readily with CO to afford trans-[Pd(C(Q)Et)(TPPTS)2]+.665... 

We have demonstrated that a series of first row, Group 8 organometallic hydride complexes effect intermolecular hydride addition to coordinated n2-alkene, n2-vinyl ether, and a-alkoxyethy-lidene compounds (64). For example, one equivalent of CpFe(C0)PPh3(H) quantitatively reduces CpFe(CO)2(T12-CH2=CH2)+ to CpFe(C0)2CH2CH3 within one-half hour and leaves... 

Finally, whilst rhenium hydride complexes have not been reported to hydrogenate alkenes, there are several reports of the dehydrogenation of alkanes in the presence of tBuCH=CH2 as an hydrogen acceptor (Scheme 6.14) [136-142]. For example, cycloalkanes are transformed catalytically into the corresponding cyclic alkene, which shows that, in principle, a Re-based catalyst could be designed. 

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