Equilibration, cis/trans

Trans stereochemistry of the alkene product is established during the second reduction step when the less hindered trans vinylic anion is formed from the vinylic radical. Vinylic radicals undergo rapid cis-trans equilibration, but vinylic anions equilibrate much less rapidly. Thus, the more stable trans vinylic anion is formed rather than the less stable cis anion and is then protonated without equilibration. 

Azobenzene is oxidatively cyclized to benzo(c)cinnoline (CIX) when exposed to sunlight in strongly acidic solutions,172 or in an acetic acid and ferric chloride mixture.124 As irradiation of azobenzene in neutral solutions is known to result in cis-trans equilibration, it seems that the... 

In the absence of a base and in an aprotic neutral solvent such as acetonitrile, cis-trans equilibration occurs via the protonated lactone to furnish with high stereoselectivity the thermodynamically more stable trans- s,omev ... 

While the electrocyclic ring-opening of aziridines to azomethine ylides is well known, the reverse reaction has also been studied as a route to aziridines . This process is implicit in the cis-trans equilibration of substituted aziridines <67JA1753, 7UA1779), and may be involved in the reaction of diazoalkanes with imines <84T2569>. Convincing evidence for the electrocyclization of... 

Methanol and propan-2-ol add non-stereospecifically to cholest-4- and -5-ene as do ethylene and tetrafluoroethylene to 3 -acetoxypregna-5,16-dien-20-one although only 16a,17a-addition products were isolated from allene, acetylene, and dichloroethylene the derived 20-anti-oxime isomerises to the syn-isomer on irradiation in THF and cyclises internally in benzene solution. Irradiation of the trans-acid (561) in methanol resulted only in a cis-trans equilibration, although the trans-acid (562) produced the unsaturated lactone (563). 

In nearly all cases, (Z)-alkene selectivity is higher for tertiary than for unbranched aliphatic aldehydes. The eombination of a tertiary aldehyde and bulky phosphorus ligands in the ylide usually results in the highest Z E ratios, except for the cases already mentioned where oxaphosphetane intermediates undergo cis-trans equilibration according to control experiments. The kinetic oxaphosphetane ratios follow the general rule that cis selectivity is higher for tertiary than for unbranched aldehydes. If this rule is not reflected in the empirical alkene ratios, then equilibration of intermediates is a distinct possibility. 

Equations (5) and (6) presume that the cyclic Grignards 3 and 4 are intermediates in the formal 1,2-vinyl migrations. The subsequent finding (52) that the cyclopropylmethyl Grignard is preparable, though rapidly rearranging, supports this presumption. Further proof of its intermediacy is derived from experiments by Maercker (50, 50a) and by Hill (33), shown in Eqs. (7) and (8) respectively. In Eq. (7), cis-trans equilibration... 

The kinetics of the cis-trans equilibration of the [Cr(ox)a(OH2)J- ion have been measured the reaction proceeds by an acid-catalysed ring-opening mechanism. The accelerating effect of micelles on the rate of this reaction in benzene solution has been discussed previously (Section 3). ... 

Table 3 Thermodynamic parameters for bridge-terminal ligand interchange and cis trans equilibration in the complexes [(/j >-C5H5)ML2] from ref 11)...

The existence of a repulsive S/S or S/O gauche effect has been demonstrated by cis/trans equilibration studies with 1,3-dithians (158). Carbon-13 n.m.r. data have been reported for 2-X-substituted (X = Me, SiMej, GeMej, SnMej, or PbMe3) and a series of methyl-substituted 1,3-dithians. Incremental shifts of the carbon atoms of the ring have been used for assignments of the stereochemistry of S—O of anancomeric compounds (159). Other spectral studies have dealt with... 

The following catalysts may also be used for cis-trans equilibration ... 

In the first step of the mechanism of this reduction, the ir framework of the triple bond accepts one electron to give a radical anion. This anion is protonated by the ammonia solvent (step 2) to give an alkenyl radical, which is further reduced (step 3) by accepting another electron to give an alkenyl anion. This species is again protonated (step 4) to give the product alkene, which is stable to further reduction. The trans stereochemistry of the final alkene is set in the first two steps of the mechanism, which give rise preferentially to the less stericaUy hindered trans alkenyl radical. Under the reaction conditions (liquid NH3, -33°C), the second one-electron transfer takes place faster than cis-trans equilibration of the radical. This type of reduction typically provides >98% stereochemically pure trans alkene. 

Photocyclization of 5,5-dimethyl-3-hexen-2-one provides an aliphatic example, with the added twist that the a, 3-unsaturated ketone undergoes photoinduced cis/trans equilibration, which allows the cis-isomer to undergo 8-hydrogen abstraction." ... 

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