Stereoisomerism cis and trans

In order to answer these questions, Stang and Dueber (186) investigated the solvolysis of the stereoisomeric cis and trans 1,2-dimethyl-2-phenylvinyl triflates 232a and 232b, Rj = R2 = CH3. In 60% aqueous ethanol buffered with... 

Cyclooctatetraene gave stereoisomeric cis and trans) bicyclo[4,2,0]octane-7,8-diols and their diacetates in water and acetic acid, respectively [298,299], and the anodic acetoxylation of anthrancene resulted in the predominant formation of trans-9, lO-diacetoxy-9,10-dihydroanthracene [300]. 

Bicyclic cyclopropyl compounds are anodically dimethoxylated to give stereoisomeric cis and trans) cyclopropane ring-opened products, as in Eq. (48) [322]. Shono and coworkers [323] reported a different type of ring-opening reaction of a cyclobutyl compound through anodic methoxylation. They also found that anodic oxidation of borneol and isoborneol in methanol resulted in a rearrangement to provide methoxylated stereoisomeric products with the same endo-exo ratio, as in Eq. (49) [324] ... 

A detailed study of the synthesis and conformation of stereoisomeric cis- and trans-tetramethylene- and -pentamethylene-dihydro-l,3-oxazines has been undertaken, e.g. (50). 3-Phenyl-4-hydroxy-2i/-l,4-benzoxazines, which may have some interest as antifungal agents, can be prepared as outlined in Scheme 109 4 jjjg hydroxylamines are rather unstable in air. 

Configurations around any double bond give rise to cis and trans stereoisomerism. 

The thermal polymerization of S has a long history.310 The process was first reported in 1839, though the involvement of radicals was only proved in the 1930s. Carefully purified S undergoes spontaneous polymerization at a rate of ca 0.1% per hour at 60 C and 2% per hour at 100 °C. At 180 aC, 80% conversion of monomer to polymer occurs in approximately 40 minutes. Polymer production is accompanied by the formation of S dimers and trimers which comprise ca 2% by weight of total products. The dimer fraction consists largely of cis- and trans-1,2-diphenylcyclobutanes (90 and 91) while the stereoisomeric tetrahydronaphthalenes (92 a nd 93) are the main constituents of the trinier fraction.313... 

Since vinyl anions generally retain configuration 39> while isomeric vinyl radicals rapidly interconvert 40) these results constitute evidence that reductions of alkyl iodides do proceed via radical intermediates. Isomerization of stereoisomeric vinyl anions is ruled out by the lack of effect of phenol on the stereochemistry of the products (Scheme III). Since cis and trans-3-hexene are formed in differing proportions from the two halides, it may be concluded that the stereoisomeric vinyl radicals are being intercepted by electron trans-... 

A third type of configurational interdependence exists if two elements are so interrelated that a change in the configuration of one automatically alters that of the other. This characterization applies to the two centers of 1,4-cyclohexanediol of the type Cg+g hi (5,51). Consequently only two isomers exist and a single pair of descriptors suffices for their distinction. We can remove the mutual dependence of the two elements by waiving the requirement that a line of stereoisomerism be occupied by bonds. The H and OH ligands have different distributions in the isomers about the line between C(l) and C(4), and the usual terms cis and trans express this relationship. Undoubtedly this is the most convenient description and the only one now available, but should we go further and say that the proper element of stereoisomerism in this case is this achiral line of torsion, and that its further factorization into two graphochiral centers is unwarranted ... 

A polymer such as -[-CH=CH-CH(CH3)-CH2-hr which has two main-chain sites of stereoisomerism, may be atactic with respect to the double bond only, with respect to the chiral atom only or with respect to both centres of stereoisomerism. If there is a random distribution of equal numbers of units in which the double bond is cis and trans, the polymer is atactic with respect to the double bond, and if there is a random distribution of equal numbers of units containing the chiral atom in the two possible configurations, the polymer is atactic with respect to the chiral atom. The polymer is completely atactic when it contains, in a random distribution, equal numbers of the four possible configurational base units which have defined stereochemistry at both sites of stereoisomerism. 

Stereoisomerism in 1,4-poly-1,3-dienes results in significant differences in the properties of the cis and trans polymers. The trans isomer crystallizes to a greater extent as a result of higher molecular symmetry and has higher Tm and Tg values (Table 8-1). These trends are... 

It was observed that 4,4-dimethyl-2-pentene was more readily oxidized than the saturated hydrocarbon—2,2-dimethylpentane—although the latter gave a significantly larger ion yield. In the case of this olefin, which was used as the stereoisomeric mixture, the cis and trans isomers were equally attacked in terms of hydrocarbon oxidized. 

Some heteropolycycloalkanes, e.g. indolizidine, quinolizidine, tropane, and decahydro-quinoline, are parent skeletons of alkaloids. Carbon-13 shifts of these and other parent polycycloalkanes [408] (Tables 4.64 and 4.65) are predominantly determined by heteroatom electronegativity, ring size, stereoisomerism, and dynamic effects. Aza- and thiade-calins are representative examples (Table 4.64). As described for cis- and trans-decalin (Section 4.1.3), carbon nuclei in the cis isomers of aza- and thiadecalins are shielded... 

The C—N—N—C atoms arc coplanar in each form and the intcrconvcrsion of 3 and 4 requires a relatively high amount of energy. On the other hand, hydrogen peroxide (H2O2), which may be thought of as existing in similar cis and trans stereochemical arrangements (5 and 6). does not exhibit stereoisomerism... 

From this point of view, a brief comparison of the acyloxylation of cis or trans stil-benes under electrochemical and chemical conditions is also relevant. Anodic oxidation (Pt) of cis or trans stilbenes in the presence of acetic or benzoic acid gives predominantly mesa diacylates of hydroxybenzoin or, if some water is present, threo monoacylate. None of stereoisomeric erythro monoacylate and rac diacylate was obtained in either case. There was no evidence of isomerization of cis-to-trans stilbene under the electrolytic conditions employed (Mango Bonner 1964 Koyama et al. 1969). The sequence of reaction steps in Scheme 2-23 was proposed. Adsorption-controlled one-electron oxidation of the substrate takes place. Then an adsorption-controlled rotation proceeds of cis stilbene monoadduct into thermodynamically more stable trans benzoxonium ion. The trans benzoxonium ion is the common intermediate for conversions of both cis and trans stilbenes and, of course, for all the final products (Scheme 2-24). Hence, oxidized molecules of stilbene are, at the electrode, involved in a reaction with acylate ions. There is no passing into the solution volume, with the following electron being exchanged there with unoxidized molecules of stilbene. 

Stereochemical integrity may also be lost when the reaction of a stereoisomer occurs via an intermediate which retains a stereogenic element, but whose bonding permits interconversion of stereoisomers faster than its conversion of stereoisomeric products. [2 + 21-Cycloaddition of TCNE and cis-propenyl methyl ether [30] yields cis- and trans-adducts, 22 in Scheme 9.13, in ratios which depend on the solvent (84 16 in favour of the cis-adduct in acetonitrile). The dipolar 23 was proposed as an intermediate. The initial bonding destroys the double bond character between Cl and C2 of the enol ether reactant, and the much... 

Figure 6.3 Stereoisomerism of poly(l-vinylene-3-cyclopentylene). Isotactic and syndiotactic cis and trans polymers...

Cis and trans isomers are only one type of stereoisomerism. The study of the structure and chemistry of stereoisomers is called stereochemistry. We will encounter stereochemistry throughout our study of organic chemistry, and Chapter 5 is devoted entirely to this field. 

Stereoisomerism. The first trials to use the XPS valence band spectra to distinguish between two stereoisomers were unsuccessful. Cis- and trans- poly(isoprene) - with short branched chain and small substitution effects-, as well as cis- and trans-poly(l,4dichloro-2,3epoxybutene) - with longer branched chain and more intense substituent effects- did not show in our first measurements significant differences in their valence band spectra that could be attributed to the searched effect. Before... 

The possibility of stereoisomerism at the acetal carbon atom in a-rings is of considerable practical importance when they are prepared from aldehydes other than formaldehyde. For the cis and trans isomers XXXI and XXXII it is impossible to predict whether, in any given case, one will be... 

In a stereospecific reaction, a given substrate isomer leads to one product while another stereoisomer leads to the opposite stereoisomeric product. Thus, the starting material specifies the stereochemical outcome of the reaction (as in Sf.,2 reactions and epoxidation of cis- and tran -alkenes). All stereospecific reactions are necessarily stereoselective. 

---