Cis-stereoisomers

The CIS and trans forms of 1 2 dimethylcyclopropane are stereoisomers Stereoisomers are isomers that have their atoms bonded m the same order—that is they have the same constitution but they differ m the arrangement of atoms m space Stereoiso mers of the cis-trans type are sometimes referred to as geometric isomers You learned m Section 2 18 that constitutional isomers could differ m stability What about stereoisomers We can measure the energy difference between as and trans 1 2 dimethylcyclo propane by comparing their heats of combustion As illustrated m Figure 3 20 the two compounds are isomers and so the difference m their heats of combustion is a direct measure of the difference m their energies Because the heat of combustion of trans 1 2 dimethylcyclopropane is 5 kJ/mol (12 kcal/mol) less than that of its cis stereoisomer it follows that trans 1 2 dimethylcyclopropane is 5 kJ/mol (12 kcal/mol) more stable than as 1 2 dimethylcyclopropane... 

Their heats of combustion (Table 3 2) reveal that trans 1 4 dimethylcyclohexane is 7 kJ/mol (17 kcal/mol) more stable than the cis stereoisomer It is unrealistic to believe that van der Waals strain between cis substituents is responsible because the methyl groups are too far away from each other To understand why trans 1 4 dimethylcyclo hexane is more stable than cis 1 4 dimethylcyclohexane we need to examine each stereoisomer m its most stable conformation... 

As m the 1 4 dimethylcyclohexanes the 6 kJ/mol (1 5 kcal/mol) energy difference between the more stable (trans) and the less stable (cis) stereoisomer is attributed to the strain associated with the presence of an axial methyl group m the cis isomer... 

The difference m stability between stereoisomeric alkenes is even more pronounced with larger alkyl groups on the double bond A particularly striking example compares as and trans 22 5 5 tetramethyl 3 hexene m which the heat of combustion of the cis stereoisomer is 44 kJ/mol (10 5 kcal/mol) higher than that of the trans The cis isomer IS destabilized by the large van der Waals strain between the bulky tert butyl groups on the same side of the double bond... 

Cycloalkenes that have trans double bonds m rings smaller than 12 mem bers are less stable than their cis stereoisomers trans Cyclooctene can be isolated and stored at room temperature but trans cycloheptene is not stable above -30°C... 

The relative amounts of the two products however are not equal more as 1 2 dimethyl cyclohexane is formed than trans The reason for this is that it is the less hindered face of the double bond that approaches the catalyst surface and is the face to which hydro gen IS transferred Hydrogenation of 2 methyl(methylene)cyclohexane occurs preferen tially at the side of the double bond opposite that of the methyl group and leads to a faster rate of formation of the cis stereoisomer of the product... 

Since the presence of a plane of symmetry in a molecule ensures that it will be achiral, one a q)ro h to classification of stereoisomers as chiral or achiral is to examine the molecule for symmetry elements. There are other elements of symmetry in addition to planes of symmetry that ensure that a molecule will be superimposable on its mirror image. The trans,cis,cis and tmns,trans,cis stereoisomers of l,3-dibromo-rranj-2,4-dimethylcyclobutaijte are illustrative. This molecule does not possess a plane of symmetry, but the mirror images are superimposable, as illustrated below. This molecule possesses a center of symmetry. A center of symmetry is a point from which any line drawn through the molecule encouniters an identical environment in either direction fiom the center of ixnimetry. 

Vazquez-Caicedo AL, Sruamsiri P, Carle R, Neidhart S. 2005. Accumulation of all-trans- 5-carotene and its 9-cis and 13-cis stereoisomers during postharvest ripening of nine Thai mango cultivars. J Agric Food Chem 53 4827-4835. 

Optimal yields were obtained by slow addition of the alkene substrates to a solution of the ruthenium vinylalkylidene and this allowed just two equivalents of the acyclic alkene to be used without significant formation of polymeric products. Unlike the acyclic cross-metathesis reactions, which generally favour the formation of tram products, the above ring-opening metathesis reactions yielded products in which the cis stereoisomer is predominant. Particularly noteworthy was the absence of significant amounts of products of type 31, formed from metathesis of one cyclic and two acyclic alkenes. In fact, considering the number of possible ring-opened products that could have been formed, these reactions showed remarkable selectivity (GC yields > 80%). 

A new dipolarophile bearing a chirality-controlling heterocyclic auxiliary at the p-position is readily accessible from (5)-A -benzylvalinol and methyl ( )-4-oxo-2-propenoate. However, the dipolarophile is available only as an 86 14 equilibrium mixture of trans and cis stereoisomers (Scheme 11.20) (84). When this is used without separation in the reaction with the Al-hthiated azomethine ylide derived from methyl (benzylideneamino)acetate in THE at 78 °C for 3.5 h, a mixture of two diastereomeric cycloadducts (75 25) was obtained in 82% yield. These two cycloadducts are derived from the trans and cis isomers of acceptor, indicating that both cycloadditions were highly diastereoselective. 

In conclusion, 4-Amino-benzoicacid can be hydrogenated to 4-aminocyclohexane carboxylic acid on 5wt.% Ru- and Rh/A Os catalysts with 100% yield in 2 wt. % Na0H-H20 at 80-100 °C, 10 MPa H2, for 5h. However, the ratio of trans/cis stereoisomers of the product does not exceed a value of 1/1. Raw reaction mixture... 

Like the 1,4-dimethyl derivatives, trans-1,2-dimethylcyclohexane has a lower heat of combustion (see Table 3.2) and is more stable than cis-1,2-dimethylcyclohexane. The cis stereoisomer has two chair conformations of equal energy, each containing one axial and one equatorial methyl group. 

The ir ir transition in the UV spectrum of trans-stilbene (frans-C6H5CH=CHC6H5) appears at 295 nm compared with 283 nm for the cis stereoisomer. The extinction coefficient emax is approximately twice as great for trans-stilbene as for c/.s-stilbene. Both facts are normally interpreted in terms of more effective conjugation of the it electron system in trans-stilbene. Construct a molecular model of each stereoisomer, and identify the reason for the decreased effectiveness of conjugation in cw-stilbene. 

The axial methyl group in the cis stereoisomer is involved in unfavorable repulsions with the C-4 and C-6 axial hydrogens indicated in the drawing. 

The cis stereoisomer is more stable than the trans. It exists in a conformation with only one axial methyl group, while the trans stereoisomer has two axial methyl groups in close contact with each other. The trans stereoisomer is destabilized by van der Waals strain. 

Only when the methyl groups are cis in the starting alkene will the cis stereoisomer be the sole product following hydrogenation. Hydrogenation of a>3,6-dimcthylcyclohexane will yield exclusively cis-1,4-dimethy lcyclohexane. 

The observed product mixture (68% cis-1,2-dimethylcyclohexanc 32% trans-1,2-dimcthylcyclo-hexane) contains more of the less stable cis stereoisomer than the trans. The relative stabilities of the products therefore play no role in determining the stereoselectivity of this reaction. 

Because caronic acid is optically active, its carboxyl groups must be trans to each other. (The cis stereoisomer is an optically inactive meso form.) The structure of (+)-chrysanthemic acid must therefore be either the following or its mirror image. 

Products J 0 and J l are meaty in aroma and flavor. 1 possesses a characteristic meaty note reminiscent of 1iver-sausage whilst has a roasted meat odor and a burnt meat taste. In addition, the cis stereoisomer has a carrot-like and mushroom-like aroma and a herbaceous and tropical fruit note. The flavor thresholds in water of JJD and 1 1 are below 100 ppb. 

The C4V type stereoisomer exists for coordination polyhedra [158] of M(C204)(HC204) 3H2O, where M = Dy, Yb, Y or Er and there seems to be no example of Ci stereoisomer of a capped square antiprism at present. 

More satisfactorily, the regioisomeric selenoester 17, incorporating the tetrahydropyridine moiety at the 3-position of the indole ring, upon exposure to the above standard reductive conditions underwent a regioselective 6-endo cyclization to give the pyrido[4,3- >]carbazole 18 as a 2 1 mixture of trans-cis stereoisomers in 75% yield. No 5-exo cyclization product was detected <06JOC1746>. 

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