Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Regioselectivity solvent effects

Further studies on regioselectivity, solvent effects, temperature effects, and catalyst loadings were also conducted [27]. A series of 2-ethyl-3-methylquinolines was obtained and substituents were added at the 5-, 6-, 7- and 8 positions. As seen in Table 6.2, introduction of an electron-donating group favored product formation, whereas electron-withdrawing groups inhibited product formation. [Pg.145]

The solvent has no influence on the stereoselectivity of bromine addition to alkenes (Rolston and Yates, 1969b), but it could have some effect on the regioselectivity, since this latter depends not only on polar but also on steric effects. Obviously, it modified the chemoselectivity. For example, in acetic acid Rolston and Yates find that 2-butenes give 98% dibromides and 2% solvent-incorporated products whereas, in methanol with 0.2 m NaBr, dibromide is only about 40% and methoxybromide 60%. There are no extensive data, however, on the solvent effects on the regio- and chemoselectivity which would allow reliable predictions. [Pg.237]

In summary, the interactions responsible for the typical solvent effects on Diels-Alder reactions are comparable to those of Lewis acids. The rate acceleration, the increase of regioselectivity and the higher endo/exo selectivity on changing the solvent may be explained by the FMO theory. [Pg.1053]

Another focus of this chapter is the alkynol cycloisomerization mediated by Group 6 metal complexes. Experimental and theoretical studies showed that both exo- and endo- cycloisomerization are feasible. The cycloisomerization involves not only alkyne-to-vinylidene tautomerization but alo proton transfer steps. Therefore, the theoretical studies demonstrated that the solvent effect played a crucial role in determining the regioselectivity of cycloisomerization products. [2 + 2] cycloaddition of the metal vinylidene C=C bond in a ruthenium complex with the C=C bond of a vinyl group, together with the implication in metathesis reactions, was discussed. In addition, [2 + 2] cycloaddition of titanocene vinylidene with different unsaturated molecules was also briefly discussed. [Pg.153]

Many mechanisms had been proposed in the past to rationalize this selectivity (tri-oxanes, perepoxide, exciplex, dipolar or biradical intermediates) however, it is now generally accepted that the reaction proceeds through an intermediate exciplex which has the structural requirements of a perepoxide. This assumption is supported by (a) the lack of stereoselectivity in the reactions with chiral oxazolines and tiglic acid esters (b) the comparison of the diastereoselectivity of dialkyl substituted acrylic esters with structurally similar non-functionalized aUtenes (c) the intermolecular isotope effects in the photooxygenation of methyl tiglate and (d) the solvent effects on regioselectivity. ... [Pg.853]

An alternative two-step mechanism involving a spin-paired diradical intermediate has also been considered for 1,3-cycloadditions.18,68,69 However, ab initio calculations70-72 on a wide variety of 1,3-dipoles and dipolarophiles are found to coincide essentially with a synchronous 1,3-cycloaddition mechanism.15,17 On the other hand, a two-step mechanism passing through two transition states separated by an intermediate has been derived using the MINDO/3 method, and found to be compatible with substituent and solvent effects as well as stereospecificity observed in 1,3-cycloadditions.73 However, several factors beyond FMO interactions, such as closed shell repulsions, geometrical distortions, polarization, and secondary orbital interactions, all influence mechanisms, rates, and regioselectivities in cycloaddition reactions.74... [Pg.224]

A study of the intramolecular alkoxymercuration of ( )-5-arylpent-4-en-l-ols (70) has indicated that the regioselectivity is closely related to the Hammett constants of the para-substituents on the benzene ring. Large solvent effects on the regioselectivity were also observed (Scheme 17 and Tables 2 and 3). By contrast, the related oxymercuration of -methylstyrene is 100% a-sclcctivc. This comparison shows that the regioselectivity of the intermolecular reaction is controlled by electronic factors, whereas the cyclization is governed by a delicate balance of steric and electronic effects.65... [Pg.406]

Various mechanisms for the insertion reaction are conceivable (a) ionic stepwise, (b) radical (chain or nonchain), and (c) concerted. Generally, ionic or radical mechanisms give a mixture of products with cis and trans stereochemistry. In some special cases of the ionic reaction, however, exclusive formation of a trans product has been observed (62, 66). Therefore, stereoselectivity does not necessarily imply a concerted mechanism other evidence, e.g., regioselectivity, kinetic data, solvent effects, and substituent effects, must be sought out. [Pg.253]

It should be noted that the stereoselectivity is also completely different from that associated with triplet 1,4-biradicals. Thus, a highly exo-selective formation ofbicyclic oxetanes was observed during PET-promoted PB reactions, whereas a highly endo-selective formation ofbicyclic oxetanes was reported for PB reactions that proceeded via triplet 1,4-biradicals (see Scheme 7.25). The competitive reaction pathway for electron-rich alkenes explained a notable solvent effect on the regioselectivity and stereoselectivity of the PB reaction of dihydrofuran (Scheme 7.15). Thus, an endo-selective formation of 3-alkoxyoxetane was observed when using benzene, whereas the exo-isomer of 2-alkoxyoxetane was detected as a product of the PB reaction in acetonitrile (Scheme 7.15). [Pg.230]

Hintz, S. Frolich, R. and Mattay, J. (1996) PET-oxidative cyclization of unsaturated silyl enol ethers. Regioselective control by solvent effects. Tetrahedron Letters, 37, 7349-7352. [Pg.314]


See other pages where Regioselectivity solvent effects is mentioned: [Pg.107]    [Pg.107]    [Pg.10]    [Pg.25]    [Pg.6]    [Pg.109]    [Pg.58]    [Pg.67]    [Pg.1052]    [Pg.145]    [Pg.831]    [Pg.854]    [Pg.675]    [Pg.373]    [Pg.831]    [Pg.854]    [Pg.204]    [Pg.373]    [Pg.139]    [Pg.510]    [Pg.432]    [Pg.432]    [Pg.192]    [Pg.143]    [Pg.279]    [Pg.261]    [Pg.137]    [Pg.571]    [Pg.102]    [Pg.545]    [Pg.587]    [Pg.859]    [Pg.42]   


SEARCH



Solvent effects on regioselectivity

Solvents regioselectivity

© 2024 chempedia.info