Anti-solvent reverse addition

Extensive rearrangements occur on reaction of ground state ( P) oxygen atoms with norbornene the major products can be accounted for by a fairly standard sequence of radical addition/reverse-addition steps. Electrolysis of the diester (531) in a mixed solvent (AcOH-Bu OH-Et3N) gave mainly the exo-anti-diester (532), a key... 

The formation of the ionic intermediates, long considered to be irreversible, has been shown to be reversible.229-231 The 1 1 complex (23) dominating in protic sol-vents may be converted to 24 in a solvent-assisted process. The ionic intermediate was first postulated as a cyclic bromonium cation by Roberts and Kimball.234 This was later supported by stereochemical results indicating nearexclusive anti addition stereochemistry,111 by kinetic studies,235 and more recently... 

The ratio of the three products depends on the reacting silane and alkyne, the catalyst, and the reaction conditions. Platinum catalysts afford the anti-Markovnikov adduct as the main product formed via syn addition.442- 146 Rhodium usually is a nonselective catalyst404 and generally forms products of anti addition.447 151 Minor amounts of the Markovnikov adduct may be detected. Complete reversal of stereoselectivity has been observed.452 [Rh(COD)Cl]2-catalyzed hydrosilylation with Et3SiH of 1-hexyne is highly selective for the formation of the Z-vinylsilane in EtOH or DMF (94-97%). In contrast, the E-vinylsilane is formed with similar selectivity in the presence of [Rh(COD)Cl]2-PPh3 in nitrile solvents. 

In order to form the activated complex required for the formation of product D, rotational changes of the less dipolar anti-form A to the more dipolar s jn-conformer B are necessary, to give an activated complex C with more parallel bond dipoles, which is thus more dipolar and better solvated than the reactant molecule. In agreement with this explanation is the observation that the reverse refro-Diels-Alder reaction exhibits no large solvent effect, since the activated complex C is quite similar to the reactant D [807], A very subtle solvent effect has been observed in the Diels-Alder addition of methyl acrylate to cyclopentadiene [124], The polarity of the solvent determines the ratio of endo to exo product in this kinetically controlled cycloaddition reaction, as shown in Eq. (5-43). The more polar solvents favour endo addition. 

There is no doubt that the solvent plays an important role in process (1). It seems to be accepted that activation energy for charge separation should be reduced with increasing polarity Various nucleophiles, e.g. SCN , I, MeO, amines, etc., do, in fact, react faster in polar than in non-polar solvents Eliminations, the microscopic reverse of additions, take the syn path more often in non-polar or associating solvents . All of this is supportive of the idea that polar solvents favour anti attack, other things being equal. 

Photoaddition of alkenes to the azathymine (227) affords the adducts (228) the structures of which were determined by -ray crystallography. Acetone-sensitized irradiation of the azauracil derivative (229) yields the cyclobutane (230). Irradiation of the uracil (231) in isotropic solvents is known to yield all four (2-1 2)-dimers in low yield especially at low concentrations. A study has shown that dimerization in smectic media affords a high yield (94X) of the trans-anti dimer (232). In frozen solutions the specificity is reversed and the CIS-anti dimer (233) is formed. The influence of a variety of media on this process was studied in detail. A theoretical treatment of the photochemical addition of alkenes to psoralens such as (234) has been published. ... 

The results for bicyclo[2.2.2]octene derive from the fast reversible formation of mercurium ions, which, if captured rapidly by the solvent, give products of anti addition, but, if attacked only slowly, give products derived from carbonium ions, i.e., rearranged products or syn adducts. 

Dichloroindium hydride, generated by transmetallation between tributyltin hydride and indium trichloride, predominantly reduces unsaturated ketones (enones) with 1,4 selectivity in the presence of aldehydes. Under anhydrous conditions, the successive aldol reaction between the resulting enolates and the remaining aldehydes proceeds with high anti-selectivity. The stereochemistry, however, is reversed to be syn-sclccti vc by the use of water and methanol as an additive and solvent, respectively [12 b]. 

There is a strong dependence of selectivity on the specific hydride used and the reaction solvent employed. The selectivity of 9 is reversed with the use of L-Selectride. Unlike most other reducing species that favor anti addition, L-Selectride favors syn addition. The effect of solvent on the reaction with L-Selectride is phenomenal the selectivity changes from 1 1.5 in THF (entry 6) to 1 8 in toluene (entry 7)... 

To the best of our knowledge, there is only one example in the literature of a photoaddition reaction influenced by CD and involving two different molecules, that is, the photocycloaddition of 5-X-adamantan-2-ones (X = F, Cl, Br, OH, phenyl, ferf-butyl) with fumaronitrile (Scheme 31) [319]. The reaction performed in p-CD aqueous solution led to a remarkable change in product distribution with respect to that found in isotropic solvent. The syn/anti ratio of the frans-oxetane formed was reversed (Table 23). The effect was maximum with the bulkier substituents and was not observed on addition of a- and y-CD. The rationale is that deep inclusion in the P-CD... 

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