Isomers solvent effect

A soln. of 2-carbomethoxy-2-cyclohexenone in chloroform added during 25 min to a mixture of startg. p-ketoester and 0.47 eqs. CS2CO3 in the same solvent at room temp., 2.5 h later CS2CO3 removed, and the crude product refluxed with / -TsOH in benzene product. Y 70% (99.5% cis-cis isomer). Solvent effects and reaction mechanisms s. P. Deslongchamps, J.-F. Lavallee, Tetrahedron Letters 29, 5117-8 (1988) 14a-hydroxysteroids by sequential Michael addition-intramolecular aldol condensation s. ibid. 6033-6. 

Startg. m. added dropwise at 0-5° during 10 min. to a stirred mixture of coned. H2SO4 and hexane, stirring continued 0.5 hr. at the same temp. 5,5,9-tri-methyl-2-decalone. Y 65% 93% trans- and 7% cw-isomer. Solvent effect on stereospecificity s. G. Ohloff et al., Helv. 56, 1414 (1973). 

Hi) The fact that the signal of the substituent in position 5 is more sensitive to solvent effects than that of the 3-substituent. This is true for protons and methyl groups, the best solvents for these studies being benzene, CDCI3, DMSO and HMPT. A beautiful illustration of this method is provided by the establishment of the structure of the six l,l -dimethyl-bipyrazolyl isomers (72JHC1373). 

Table 9.4 Solvent effect on enol triflate isomer ratio.

Taskinen and Nummelin (op. cit.) reported many other isomer equilibria in their paper. Most of these used cyclohexane as the solvent and I2 as the catalyst and so are not confounded by solvent effects. However, these authors noted that hydrogen atom transfer induced disproportionation (to form the aromatic benzene) dominates this reaction for the case of 49/50 isomerization and so they needed alternative reaction conditions. 

Further mechanistic work has revealed interesting solvent effects on diastereo-selectivity103. A method for enriching isomer ratios in favor of the trans or exo... 

The thermal isomerization of tert. -butyl-diisobutylborane to triisobutylborane and the stepwise isomerization of triisopropyl and tri-sec.-butyl boranes to the corresponding straight-chain isomers have been studied over a range of temperature, both neat and using either the end product trialkylborane or diglyme as solvent117 . In all cases the reactions were first order and showed no solvent effect. 

The solvent effects on the relative stabilities of 4-nitroimidazole and 5-nitroimidazole exhibit interesting patterns81. In the excited state the 5-nitro isomer is more stable than the 4-nitro isomer in aprotic solvents, while the stability order is reversed in the ground state. 

The photochemical isomerization 41B — 41A was observed in nonpolar solvents (83AJC1603). Equilibrium 1) was attained in CD3OD solution after several days, but following concentration and cooling of the solution, only the cyclic isomer 41B, solvated by methanol, was isolated in the solid state. Alcoholic solvents effectively quench the photochemical isomerization. 

Different stereoselectivities caused by solvent effects are demonstrated in the reduction of dihydroisophorone (3,3,5-trimethylcyclohexanone) with sodium borohydride which gave less stable tranj-3,3,5-trimethylcyclohexanol (with axial hydroxyl) by reduction in anhydrous isopropyl alcohol (55-56%), in anhydrous tert-butyl alcohol (55%), in 65% aqeuous isopropyl alcohol (59.5%), in anhydrous ethanol (67%), and in 71% aqueous methanol (73%) (the balance to 100% being the more stable cis isomer with equatorial hydroxyl) [849]. 

TABLE 6. Experimental (7max [ in cm mmol" ]) and calculated (B3LYP/6-311G(3df,2p)) electronic transitions of the C21, (anti/anti) isomer (including solvent effects, methanol)... 

To verify this mechanistic possibility, the solvent dependence of the ene products derived from the photooxygenation of the isomeric a,-unsaturated esters 93-E and 93-Z was examined (Scheme 23). For 93-E, the two ene products are formed from two different perepoxides. When the oxygen atom of the perepoxide intermediate is placed syn to the ester group, 93b is produced, whereas 93a is formed from the opposite case. For isomer 93-E, the expected solvent effect was found (93a/93b = 85/15 in CCI4 or benzene, and 70/30 in DMSO). On the other hand, for the isomer 93-Z, both products are formed from the same intermediate (the perepoxide oxygen is placed anti to the ester functionality), and no solvent dependence on the ene products was found (93a/93b = 95/5 in CCI4 and 93/7 in DMSO). 

Assuming there are no solvent effects, which isomer is likely to predominate in an equilibrium between the conformers A and B ... 

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