Olefin structures hydroformylation

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

With Rh, the phenomena are strikingly different since the aromatic ring does not influence the selectivity which is always very high, no matter what olefinic structure (except for 1,1-diphenylpropene). Isomer selectivity depends on several factors, mainly the structure and the stereochemistry of the alkenylbenzene. With conjugated alkenylbenzenes, addition of the CHO group occurs preferentially on the a carbon when the a and / carbons are monosubstituted. When the ft carbon is disubsti-tuted, because of steric requirements, hydroformylation is less selective and occurs on both the a and y carbon ... 

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 ...

Structure (ATO) gives a product distribution that is dominated by adipic acid. This is thought to result because the narrower channels inhibit the release of cyclohexanol and cyclohexanone and the reaction proceeds further to the more mobile linear products, such as adipic acid. Selectivity is also observed in the aerial oxidation of linear alkanes. If the reaction is performed over large-pore solids, w-alkanes are oxidised preferentially at carbon atoms at C2 and C3 positions in the chain, in accordance with the C-H bond strengths at these positions. If a small-pore structure such as CoAPO-18 is used, however, the product selectivity favours Cl oxyfunctionalised products. The synthesis of terminally oxidised alkanes would be of use for many applications, because linear terminal alcohols could be prepared from alkane feedstocks, rather than from a-olefins (via hydroformylation). 

As mentioned above, hydroformylation reactions occur under atmospheric pressure at normal temperature with stoichiometric amounts of cobalt carbonyls. However, with catalytic amounts of cobalt catalysts a minimum CO partial pressure is necessary for reformation and stability of Co2(CO)8, or HCo(CO)4, as the case may be (see page 15). A small increase of the CO partial pressure above this value first results in an increase of the reaction velocity until a maximum is reached depending on temperature and olefin structure. However, further increase of the CO-partial pressure causes a decrease in the reaction velocity [38, 40, 120], (see also section on reaction mechanism). 

In principle, all olefins participate in the hydroformylation reaction however, their reactivities vary markedly. I. Wender et aL [199] made a systematic investigation of the reaction rate as a function of the olefin structure and found a variation of a factor of 50 (see table 7). 

In 2004 Caporali investigated the hydroformylation of 1-hexene and cyclohexene using HRh(CO)(PPh3)3 [61]. The collected data indicated that the rate-determining step in the hydroformylation cycle depends upon the structure of the olefin. With an alpha-olefin like 1-hexene, the slowest step seems to be the hydrogenolysis of the acyl rhodium complex. In the presence of cyclohexene as a model for an internal olefin, the rate-determining step is the reaction of the olefin with the rhodium hydride complex (intermediate II in Fig. 6). 

Table 1 Hydroformylation of structurally diverse olefins 2a-d using the cartridge catalysis system MeOPEG75o-PPh2 (l)/[Rh(acac)(C0)2]/scC02...

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]. 

With regard to the structure of the olefins, tetrasubstituted olefins do not undergo hydroformylation reaction under typical reaction conditions, and olefinic substrates containing functional groups sometimes give poor yields and unexpected products. If there is no plane of symmetry in the substrate across the double bond, at least two isomeric aldehydes are obtained. Although methods for shifting the... 

As a result of the recognized role of transition metal hydrides as l reactive intermediates or catalysts in a broad spectrum of chemical reactions such as hydroformylation, olefin isomerization, and hydrogenation, transition metal hydride chemistry has developed rapidly in the past decade (J). Despite the increased interest in this area, detailed structural information about the nature of hydrogen bonding to transition metals has been rather limited. This paucity of information primarily arises since, until recently, x-ray diffraction has been used mainly to determine hydrogen positions either indirectly from stereochemical considerations of the ligand disposition about the metals or directly from weak peaks of electron density in difference Fourier maps. The inherent limi-... 

The experiments in Tables I and II have dealt with the reactions of 1-hexene only. It is known that hydroformylation rates for various olefins are in the order, 1-hexene > 2-hexene > cyclohexene (15). Little dependence on structure would be expected in the aldehyde hydrogenation step, however. 

In view of the many differences noted above between the hydroformylation of olefins and epoxides, it is not surprising to find that changes in structure result in a different order of reactivity in each case. Thus for epoxides the reactivity to cobalt hydrocarbonyl is cyclohexene oxide > propylene oxide, whereas with olefins the order is terminal olefins > internal olefins > cyclic olefins (145). 

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