S-trans configuration

Several forms are imaginable for the [Ni°(butadiene)2L] and [Ni°(butadiene)J active catalysts, depending on the monodentate (p2) or the bidentate (p4) coordination mode of butadiene from either its s-cis or its s-trans configuration. The two butadienes can be coordinated in bis(p2), p4, p2, and bis(p4) modes for the PR3/P(OR)3-stabilized catalyst complex, giving rise to formal 16e, 18e, and 20e species. On the other hand, bis(p4)- and p4,p2-butadiene species and also tris(p2)- and p4,bis(p2)-butadiene compounds are possible species for the [Ni°(butadiene)2] and [Ni°(butadiene)3] forms for the [Ni°(butadiene)J active catalyst. In general, for butadiene to coordinate in a bidentate fashion, the p4-cis mode is thermodynamically favorable relative to the p4-trans mode, while the p2-trans mode prevails for monodentate coordination. 

Cyclic dipeptides, especially when N-alkylated, undergo extremely fast epimerization (79JA1885). For example, cyclo(L-Pro-L-Phe) is rapidly converted to its diastereomer, cyclo(D-Pro-L-Phe) (80% conversion), by treatment with 0.5 N NaOH at 25°C for 15 min. This diastereomer is the one in which the proline residue has epimerized and not the more activated phenylalanine. CNDO/2 calculations seem to provide a rationale for this. It is not yet completely clear why such base-catalyzed epimerizations of piperazinediones are so easy the conformation of the molecule may play a role in this (79MI1). It is also worth noting that even in linear peptides, rm-amides of N-alkyl-amino acids, which consist of s-trans and s-cis rotamers of almost equal energy, are more prone to racemization than the sec-amides, which exist only in the s-trans configuration. Of course, the amide functions of piperazine-2,5-diones are obliged to assume the s-cis conformation. 

The planar molecule 1,3-butadiene might have either the s-cis or the s trans configuration in which the CH2 groups lie either on the same side or on opposite sides of the carbon-carbon single bond, respectively, (a) It... 

Some thermal cyclization reactions of allene derivatives in their own crystals are described. Intramolecular cyclization of diallene derivatives within their crystals proceeds stereoselectively according to the Woodward-Hoffmann rule. For example, heating of colorless crystals of meso- 19 and rac-diallene 23 at 150 °C for 1 h gave the in,out 22 and a 1 1 mixture of in,in 26 and out,out 27 cyclization products, respectively, in quantitative yields (Scheme 2.2.1) [14]. Since no liquid state was observed during the cyclization reactions, the reaction in the crystal is a real crystal-to-crystal reaction. An X-ray analysis showed that 19 and 23 have an s-trans configuration in the crystal [15], In order to cyclize to 22, and to 26 and 27, 19 and 23 should first isomerize to their s-cis-isomers 20 and 24, respectively, in the crystal. The conformational change, from the s-trans form to the s-cis form, requires... 

Fig. 3. The plane of shielding in the s-cis and s-trans configuration of a,li-un-saturated ketones.

Geraniol would not be expected to convert as fast as nerol into a-terpineol in that the conversion of geraniol would involve inversion of configuration about the double bond due to geraniol s trans configuration. 

As indicated in these equations, the main factor determining the stereochemistry of enchainment is the mode of coordination of the transition metal center with the monomer to form either a syn or anti r-allyl type of intermediate. In general, the coordination with two double bonds of the 1,3-diene in an s-trans configuration (see (b) Eq. (2.89)) is less common than the coordination in an s-cis configuration shown in Eq. (2.88) (Zohuri et al., 2012). This interpretation is complicated by the fact that the syn and anti 7r-allyl complexes are in equilibrium. These simple... 

Recently low-temperature conditions and comparison with calculated spectra were used to prove the X -cis-Xl-s-trans configuration assumed for rhodopin-20-al (5) (1). This provided additional information about the out-of-plane twisting of the polyene chain of (5) (54). 

According to solution phase NMR analysis and solid state X-ray analysis, the structures possess s auche configuration of the central C—C bond in contrast to the s-trans configuration reported for the majority of known 1,3-dienes. This is the first... 

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