Alkenes conformational analysis

The configuration of the product in diastereoselective hydrogenation -whether 1,2-syn or 1,2-anti - is related to the substitution pattern of the starting alkene. The allyl alcohol with a 1,1-disubstituted olefin unit affords the antiproduct, while the syn-product is formed from the allyl alcohol with a trisubsti-tuted olefmic bond (Table 21.8, entries 6-9). The complementarity in diastereoselective hydrogenation of di- and tri-substituted olefins may be rationalized based on the conformation analysis of the intermediary complex (Scheme 21.1)... 

The formation of frau.v-alkene was interpreted by conformational analysis ... 

The computational study of the osmium dihydroxylation of aliphatic al-kenes is much more complicated than the case of aromatic alkenes due to the large number of conformations that the former could adopt. To overcome this issue, we considered the system to be composed of two different parts the catalyst and the olefin. For the catalyst, the conformation considered is that from the X-ray structure. As already shown in the study of styrene [95], and in some experimental works [98], the catalyst is a fairly rigid molecule. For the aliphatic alkenes under study, there is a large number of possible conformations in addition, the stability of an olefin conformation is also affected by the interactions between the olefin substituent and the catalyst. Therefore, the catalyst must be included in the conformational search. The conformational analysis was done using a scheme based on the systematic search approach [99]. The strategy consisted of two parts first we developed a method to identify all of the possible conformations afterwards, we screened all of the possible conformations at MM level to select the most stable. Finally, we only carried out the relatively expensive QM/MM calculations on these selected conformations. 

Protic solvents shift the alkene E)j Z) ratio in the direction of the (E)-form. The alkene [E)I Z) ratio of salt-free Wittig reactions is thus influenced not only by the electronic character of R, but also by the solvent and the stereochemistry of the formation of the 1,2-oxaphosphetane in the first rate-determining step. According to Eq. (5-48), the thermodynamically less stable (Z)-l,2-oxaphosphetane is formed in the first activation step. A conformational analysis of the activated complex leading to the 1,2-oxaphosphetane intermediate provides a reasonable explanation for this unexpected cis-selectivity [143, 556]. 

The influence of methyl groups in the 3-position on the epoxidation of Z or E double bonds in highly flexible macrocycles has been discussed in terms of local conformers 138,172. This concept has been shown to be useful for the rough estimation of diastereoselectivities, where complete conformational analysis is laborious however, attempts to localize relative energies of transition states by MM2 calculations of the corresponding alkenes and epoxides strongly underestimate torsional interactions, the parameters utilized for epoxides have moreover, meanwhile been revised114. 

How To Name Alkenes and Cycloalkenes 156 How To Name Alkynes 158 4.8A Newman Projections and How To Draw Them 162 4.8B How To Do a Conformational Analysis 163... 

Oxirans. - The synthesis of l,2-anhydro-3,4-di-0-benzyl-6-deoxy-a-D-glucopyranose and its conformational analysis have been reported. A range of epoxides have been prepared by base treatment of bromohydrins, which were made by reaction of hydrogen bromide with aldonolactones. A one-pot conversion of vicinal diols into epoxides employs halohydrin ester intermediates generated from cyclic orthoacetates and either acetyl bromide or trimethylsilyl chloride. Levoglucosenone has been transformed into l,6 3,4-dianhydro-p-D-talopyranose by way of a trn/w-iodo-acetoxylation of the alkene moiety... 

The beta silicon effect , referring to the strong stabilization of carbocations -substituted with silicon, is another well-known manifestation of hyperconjugation. The C-Si empty orbital on a cationic center, very significant delocalization is observed. Finally, an important feature in the conformational analysis of alkenes is the tendency for allylic sp centers to prefer to eclipse the C=C double bond (Scheme I) (see Conformational Analysis I Conformational Analysis 2 and Conformational Analysis 3). This preference, typically on the order of 2 kcal moI , is ascribed to improved hypercon-jugative interactions of the non-eclipsing C-H bonds with the jr orbital of the alkene for this conformation. 

An analysis of polymer end groups provided insight into the mechanism of stereo-control in such catalysts. The first polymerisation step, where propene inserts into a Zr-Me bond, is in fact not stereoselective, while the insertion into a Zr-iso-butyl bond proceeds with high enantioselectivity. Ligand stereo-control operates therefore by an indirect mechanism the ligand determines the conformation of the polymery] chain, and this in turn influences the preferred orientation of the incoming alkene [127], as illustrated in structure 89 for a syndiospecific case. 

Analysis of the far IR-spectra of 3,4-dihydro-2//- pyran (13) (72JCP(57)2572> and 5,6-dihydro-2/f- pyran (14) (81JST(71)97> indicates that for both molecules the most stable conformation is a half-chair form. The barrier to planarity is greater for the former compound. These preferred structures are in accord with the half-chair conformation established for cyclohexene and its derivatives. The conformational mobility of cyclohexene is greater than that of the 3,4-dihydropyran. The increased stabilization of the pyran has been attributed to delocalization of the v- electrons of the alkenic carbon atoms and the oxygen lone-pairs (69TL4713). 

The absolute stereochemistry of the Al(6)-alkene alkaloids cocculine (56) and coccutrine (52) has also been established by X-ray analysis. It was found that the cyclohexene ring A exists preferentially in an approximate half-chair conformation in the free base, but this was altered to an envelope conformation on protonation of the nitrogen atom (49). 

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