Phenyl, trans effect

Two types of three-coordinate selenium(II) complexes, isolated as the triselenocyanate and the triselenourea salts, and which have essentially similar structures, have been identified.66 Several three-coordinate tellurium(II) complexes have also been obtained67 when one of the ligands has a very strong trans effect, for example a phenyl group, as in phenylbis(thiourea)tellurium(II) chloride.68... 

Another TS model was proposed by Schlosser and Schaub (66) as illustrated by structure 143. This model explicitly recognizes the importance of phosphorus substituents, but it invokes a dominant P-phenyl propeller effect that requires kinetic cis selectivity for all Ph3P=CHR as well as PhjP CHX reactions and kinetic trans selectivity for reactions of ylides that do not contain the propeller . The formation of trans alkenes from ylides Ph3P= CHX would have to be due to equilibration. None of these generalizations is consistent with Table 22 or with the control experiments that demonstrate kinetically controlled decomposition of a variety of oxaphosphetanes. 

Solvent exchange at the [M(PR3)2(solv)2H2] cations, with M = Rh or Ir, R = phenyl or cyclohexyl, solv = acetone or acetonitrile, have been followed by proton nmr spectroscopy, with the establishment of rate constants and Arrhenius parameters. The most marked feature is the enormous acceleration induced by the trans effect of the hydride ligand, taking these reactions from the normal very slow rates characteristic of rhodium(III) and iridium(III) into the nmr time scale. Rates are considerably faster for... 

The effect of substrate structure on product profile is further illustrated by the reactions of cis- and trons-stilbene oxides 79 and 83 with lithium diethylamide (Scheme 5.17) [32]. Lithiated cis-stilbene oxide 80 rearranges to enolate 81, which gives ketone 82 after protic workup, whereas with lithiated trans-stilbene oxide 84, phenyl group migration results in enolate 85 and hence aldehyde 86 on workup. Triphenylethylene oxide 87 underwent efficient isomerization to ketone 90 [32]. 

The cyclopropane cyclizations by elimination of triflinic acid (CF3S02H) are readily effected by basic treatment of triflones (trifluoromethyl alkyl sulfones) with activated /-protons (equations 46 and 47)39. The cyclopropane diesters 45 are formed on treatment of 44 with potassium hydride in DMSO or sodium methoxide in methanol (equation 48). In contrast, the monoester 46 failed to give the desired cyclopropane40. Addition of carbanions derived from /f, y-unsaturated phenyl sulfones to a, /i-unsaturated carboxylic esters and subsequent elimination of benzenesulfinate ion give cyclopropanes possessing the unsaturated side chain and the ester function in trans positions (equation 49)41. 

The magnitude of the electrical effect is comparable to that of the trans-heterovinylene sets. The difference in magnitude between the a values for the syn and anti phenyl ketoximes is significant and suggests that the inductive effect alone cannot account for the observed substituent effect. If the inductive effect were operating by itself, the a values for syn and anti sets would be the same. 

The rates of radical-forming thermal decomposition of four families of free radical initiators can be predicted from a sum of transition state and reactant state effects. The four families of initiators are trarw-symmetric bisalkyl diazenes,trans-phenyl, alkyl diazenes, peresters and hydrocarbons (carbon-carbon bond homolysis). Transition state effects are calculated by the HMD pi- delocalization energies of the alkyl radicals formed in the reactions. Reactant state effects are estimated from standard steric parameters. For each family of initiators, linear energy relationships have been created for calculating the rates at which members of the family decompose at given temperatures. These numerical relationships should be useful for predicting rates of decomposition for potential new initiators for the free radical polymerization of vinyl monomers under extraordinary conditions. 

It is concluded from these results that with this kind of non-C2 symmetric ligand (that led necessarily to poor enantioselectivities in homogeneous phase), it is possible to exploit support effects to change the trans/cis selectivity and to improve the enantioselectivity. This is demonstrated for the trans-cyclopropanes obtained with ligand 10a in styrene. Due to the relative disposition of the ester and phenyl groups in the transition state, support ef-... 

Table 3 summarizes the scope and limitation of substrates for this hydrogenation. Complex 5 acts as a highly effective catalyst for functionalized olefins with unprotected amines (the order of activity tertiary > secondary primary), ethers, esters, fluorinated aryl groups, and others [27, 30]. However, in contrast to the reduction of a,p-unsaturated esters decomposition of 5 was observed when a,p-unsaturated ketones (e.g., trans-chalcone, trans-4-hexen-3-one, tra s-4-phenyl-3-buten-2-one, 2-cyclohexanone, carvone) were used (Fig. 3) [30],... 

Similar results were obtained from a study of 2-phenyl-5-t-butyl-l,3 2-dioxaphosphorinane (111) in that the cw-isomer was thermodynamically more stable than the trans. However, in this case even the trans-isomer adopts a conformation (112) with the P-phenyl group and, perforce, the t-butyl group axial. A similar situation has already been noted in the phosphite (108), and it may be that the special case of a phenyl group produces some type of pseudo anomeric effect. 

Baba, A., Yamamoto, T., Yamamoto, H., Suzuki, T., and Moroji, T., Effects of the major metabolite of phencyclidine, the trans isomer of 4-phenyl-4-(l-piperidinyl) cyclohexanol, on [3H]N-(l-[2-thie-nyl]cyclohexyl)-3,4-piperidine([3H TPC) binding and [3H] dopamine uptake in the rat brain, Neurosci. Lett., 182, 119, 1994. 

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