A Cis-isomers

Figure 1.3 1,4-polyisoprene with R=CH3 (a) cis isomer natural rubber (b) trans... 

The reduction produces a mixture of four diasteomers in which a trans- (ACPC derivatives) or a cis- isomer (ACHC derivatives) is the major product. In the former case, treatment of the mixture with 4 N HCl in dioxane led to the precipitation of the trans isomer hydrochloride salt. One recrystallization from acetonitrile enriched the optical purity to >99% de [120, 122] In the latter case, epimerization... 

Scheme 6. Interplay of the C8- and C -production channels for the cyclo-oligomerization of 1,3-butadiene with zero valent PR3/P(OR)3-stabilized nickel complexes as the catalyst. Free energies (AG, AGJ in kcalmol-1) are given relative to the favorable rf-synrfiC A-cis isomer of 2a for catalysts bearing strong a-donor ligands namely I (L = PMe3), III (L = PPrj), VI (L = PBU3), and -acceptor ligands namely V (L = P(OMe)3), IV...

A typical photochemical isomerization of the azobenzene amphiphile was found in an ethanol solution. A trans isomer converted to a cis isomer with ultraviolet irradiation. Back reaction from cis to trans was accelerated when a weak n-n absorption band of the cis isomer at ca.450nm was excited (Figure 21a). An alternative irradiation of uv and visible light to the ethanol solution gave reversible changes of the ji-ji transition between 355nm and 325nm attributed to the trans and cis isomers, respectively. 

The hydroboration of alkynes gives predominantly cis isomer, as in the following example. Diborane adds as BH3 to alkynes in a cis isomer and forms vinyl boranes which react with acetic acid at low temperature to give cis-alkenes. 

Butadiene can form three repeat units as described in structure 5.47 1,2 cw-1,4 and trans-, A. Commercial polybutadiene is mainly composed of, A-cis isomer and known as butadiene rubber (BR). In general, butadiene is polymerized using stereoregulating catalysts. The composition of the resulting polybutadiene is quite dependent on the nature of the catalyst such that almost total trans-, A, cis-, A, or 1,2 units can be formed as well as almost any combination of these units. The most important single application of polybutadiene polymers is its use in automotive tires where over 10 t are used yearly in the U.S. manufacture of automobile tires. BR is usually blended with NR or SBR to improve tire tread performance, particularly wear resistance. 

The crystal structure of five members of the ferrioxamine family has been determined ferrioxamine Di , ferrioxamine E °, desferrioxamine E , the retro-isomer of fer-richrome E and ferrioxamine B . While all of the Fe(III)-ferrioxamine structures (Table 2) crystallize as racemic mixtures of A- and A-cis isomers , the configuration of the binary ferrioxamine-B in EhuD complex (2.0 A resolution) is A-C-trans,cis, indicating that the interaction between ferrioxamine-B and FhuD is enantioselective, and also exhibils geomelric seleclivily. 

Cycloalkenes Absorption of the internal double bond in the unstrained cyclohexene system is essentially the same as that of a cis isomer in an acyclic system. The C=C stretch vibration is coupled with the C—C stretching of the adjacent bonds. As the angle a... 

Other recent developments include crown ethers which contain potential switch mechanisms for complexation and transport. When photoresponsive chromophores are linked to crown ethers, ion binding can change on photoirradiation. The crown (120) is formed as its tram isomer with no alkali metal affinity, but photoirradiation gives a cis isomer capable of alkali metal complexation 500 this is an example of a reversible all or none ion-binding capability. A... 

The ju--propylidene(2) complex [ -C(CH3)2]( -CO)[(t)5-C5H5)-Ru(CO)]2 has been established by X-ray diffraction methods to exist as a cis isomer in the crystalline state. However, both cis and trans isomers were detected in solution by NMR techniques. The interconversion of these two compounds is concluded to involve bridge-terminal carbene... 

Formation of a cis isomer from an all-trans carotenoid results in a shift of the spectral bands... 

It is important to correctly identify the provitamin A peak(s) of interest in the chromatogram. A tentative identification can be made by a combination of retention time and spectral characteristics, using a photodiode array detector. Identification is aided by comparisons with authentic carotenoid standards in more than one chromatographic mode. Because of the ease of cis-trans isomerization when solutions of carotenoids are exposed to heat, light, oxygen, etc., it is difficult to ascertain whether a cis isomer occurs in nature or whether it is formed during its isolation. 

Salmonella typhimurium. Ferrichrome. Ferrichrome iron transport in enb7 proceeds by two concurrent mechanisms (Figure 12) (62). In one, the iron is snatched out rapidly with effective accumulation of the free ligand in solution. With added iron, the latter enters the cell at a rate identical to that of chromichrome. Again the A-cis isomer is active without dissociation, and reductive release is virtually certain. 

However, despite large differences in ligand molecular structure, all of the hydroxamate siderophores whose structures have been determined to date have been found to be cis complexes with a coordination geometry about the ferric ion which is substantially identical to the simple tris-(benzhydroxamato)-Fe(III) complex. Thus, while ferrioxamine E is racemic but with a cis geometry (13), x-ray structure analyses of ferrichrome A (14) and ferrichrysin (15) have shown both to be A-cis isomers. 

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