Hydroformylation olefin structure effects

Some Effects of the Olefinic Structure on the Orientation of the Hydroformylation Reaction... 

Marko (91) studied the effect of olefin structure on the ratio of hydroformylation to hydrogenation products, and concluded that the ratio declined with increasing branching of the olefin. This is not surprising in view of the known decrease in rate of hydroformylation with increased... 

Table 12. Effects of Olefin Structure on Hydroformylations Using AcacRh(C0)2 in 1M PPhs at 145 ...

Our study on the synthesis, structure and catalytic properties of rhodium and iridium dimeric and monomeric siloxide complexes has indicated that these complexes can be very useful as catalysts and precursors of catalysts of various reactions involving olefins, in particular hydrosilylation [9], silylative couphng [10], silyl carbonylation [11] and hydroformylation [12]. Especially, rhodium siloxide complexes appeared to be much more effective than the respective chloro complexes in the hydrosilylation of various olefins such as 1-hexene [9a], (poly)vinylsiloxanes [9b] and allyl alkyl ethers [9c]. 

A major interest for those practicing hydroformylation syntheses is the selectivity to the product desired. The factors which affect the yield of a specific aldehyde are (1) the structure of the olefinic substrate (a-olefin or internal olefin, branching, cyclic), (2) the isomers formed during the reaction (directly, with concomitant isomerization), (3) the effects of functional groups, and (4) the subsequent reactions of the product aldehyde. 

Reaction rate depends on the structure of the hydrocarbon olefin (see Table 1). Internal olefins are less reactive than terminal olefins. Branching in remote locations has little effect, whereas branching at one of the olefinic carbons reduces the reactivity by another order of magnitude. In addition, the product derives from isomerized olefin. Essentially no quaternary aldehydes are formed by hydroformylation. For example, the product from 2,3-dimethyl-2-butene is 3,4-dimethylvaleraldehyde . 

An alternative explanation of the experimental results is afforded by the assumption that the reaction involves the formation of an intermediate wherein the position of the double bond of the olefin is rendered indistinguishable. The observed rate of hydroformylation would then be the rate of formation of this intermediate. Owing to the great effect of steric factors on the rate of hydroformylation, one would expect that the rate of formation of this intermediate would be sterically dependent. Once formed, the intermediate reacts further to yield the aldehydes, the structures of which depend only on the intermediate and not on the reacting olefin. The exact nature of such an intermediate is unknown. 

In this review we summarized the results of the latest ab initio studies of the elementary reaction such as oxidative addition, metathesis, and olefin insertion into metal-ligand bonds, as well as the multistep full catalytic cycles such as metal-catalyzed hydroboration, hydroformylation, and sila-staimation. In general, it has been demonstrated that quantum chemical calculations can provide very useful information concerning the reaction mechanism that is difficult to obtain from, and often complementary to, experiments. Such information includes the structures and energies of unstable intermediates and transition states, as well as prediction of effects of changing ligands and metals on the reaction rate and mechanism. 

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