Styrene Substrate

Para-substituent of styrene substrate %ee Product and absolute configuration... 

To examine the nature of the enatioselectivity of the catalysis we have examined the catalytic cycle with two substrates, styrene which leads to predominantly the R form of the product (64% ee) and 4-(dimethylamino)styrene which gives predominantly the S form of the product (67% ee). Our simulations suggest that the t 3-allylic coordination of the styrene substrate plays an important role in defining the enatioselectivity of the hydrosilylation. As a first step, this theoretical study constitutes a valid contribution in rationalizing the enantioselective determining factors and possibly in designing a new catalyst with improved enantioselective properties. We are currently examining nature of the enantioselectivity in more detail as well as the dependence of the enantioselectivity on the electronic nature of the substrates [58]. 

The phenyl ring of styrene substrates directs a syn selectivity in their ene reactions with singlet oxygen. This effect was demonstrated by the photooxygenation of / ,/ -dimethyl styrene (20). This substrate, apart from the ene product, produces the 1,2-dioxetane and two diastereomeric diendoperoxides as shown in Scheme 9. 

Mononuclear oxazolines are among the most effective ligands for enantioselective hydrogenation of nonfunctionalized alkenes." " The styrene substrate 597 is one of the most studied nonfunctionahzed alkenes used to evaluate the efficiency of new chiral ligands (Scheme 8.185). Selected examples of enatioselective hydrogenation of 597 using iridium catalysts are shown in Table g jg 359,425,426,457-459... 

The data in Table 9 also allows one to reach conclusions regarding the elec-trophilicity vs nucleophilicity of the partially-fluorinated radicals since the three styrene substrates have a considerable range in IP values. /1-Fluorine, and to a lesser extent y-fluorine, substitution would appear to have a small impact on electrophilicity, whereas a single a-fluorine substituent seems to impart slightly nucleophilic properties. In a recent study, Takeuchi et al. have examined both computationally and experimentally radicals which bear both an a-fluorine substituent and an electron withdrawing ester function. They found that the a-fluorine substituent makes such radicals more electrophilic, but that they still add more readily to styrene than they do to acrylonitrile [127],... 

Heteroaromatic-substituted vinyl compounds were successfully subjected to the AA reaction, too. In contrast to styrene substrates, 2-vinylfuran was convert-... 

One can also characterize the reactivity and selectivity of carbenes using the venerable Hammett ap linear free energy methodology. [5,39] Additions of CXY could be examined with a series of ring-substituted styrenes, ArCH=CH2. Relative reactivities could be obtained, with styrene itself as the standard olefin, and then log could be plotted against the appropriate a constant for each substituted styrene substrate. The slope of the resulting correlation is the Hammett p value for the carbene addition. 

A negative value of p indicates that the reaction is accelerated by electron-donating substituents that is, positive charge develops on the olefinic carbon atoms in the addition reaction s transition state (see 2), which is then mitigated (and the energy of the transition state lowered) by electron release Irom the aromatic ring and its substituent. That means that CF2 behaves as an electrophile in its addition to these styrene substrates, the same conclusion that we reached for the olefins on the basis of above and Table 1). 

In addition to the extensive studies of hydrovinylations using styrene substrates, 1,3-dienes are also an especially useful substrate class. With phosphoramidite ligands, enantioselective diene hydrovinylation with ethylene provides a practical solution to the difficulties associated with the installation of exocyclic side chain stereochemistry, as the representative procedure below illustrates. ... 

Y. W. Leong, U. S. Ishiaku, M. Kotaki and H. Hamada, Interfacial characteristics of film insert molded polycarbonate filmypolycarbonate-acrylonitrile-butadiene-styrene substrate. Part 1 influence of substrate molecular weight and film thickness. Polymer Engineering Science, 46, 1674—1683 (2006). 

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