Double bonds chiral carbon atoms

Cholesterol is derived from the steroid skeleton by adding a hydroxyl group at C3, methyl groups at CIO and C13, an eight-carbon alkyl group at C17, and introducing a double bond between carbon atoms 5 and 6.1 have taken some pains to indicate the stereochemistry at each of the eight chiral centers in this molecule its shape matters. 

Double bonds present along a polymer chain are stereoisomeric centers, which may have a cis or trans configuration. Polymers of 1,3-dienes with 1,4 additions of the monomeric units contain double bonds along the chains and may contain up to two stereoisomeric tetrahedral centers. Stereoregular polymers can be cis or trans tactic, isotactic or syndiotactic, and diisotactic or disyndio-tactic if two stereoisomeric tetrahedral centers are present. In the latter case erythro and threo structures are defined depending on the relative configurations of two chiral carbon atoms.1... 

Lithium and alkyllithiums in aliphatic hydrocarbon solvents are also used to initiate anionic polymerization of 1,3-butadiene and isoprene.120,183-187 As 1,3-butadiene has conjugated double bonds, homopolymerization of this compound can lead to several polymer structures. 1,4 Addition can produce cis-1,4- or tram-1,4-polybutadiene (19, 20). 1,2 Addition results in a polymer backbone with vinyl groups attached to chiral carbon atoms (21). All three spatial arrangements (isotactic, syndiotactic, atactic) discussed for polypropylene (see Section 13.2.4) are possible when polymerization to 1,2-polybutadiene takes place. Besides producing these structures, isoprene can react via 3,4 addition (22) to yield polymers with the three possible tacticites ... 

As in stereoselectivity, the degree of stereoelection is reduced as the chiral centre is positioned further away from the C = C bond only racemic a-olefins with a chiral carbon atom in the a- or -position to the double bond have been stereoselectively polymerised. Note that, in general, stereoelectivity is lower than stereoselectivity. 

Priority depends on the atomic number of atoms directly attached to the chiral carbon atom. If two or more directly attached atoms are identical, one proceeds along the groups until differences are found. In double and triple bonds the groups are considered to be duplicated or triplicated. 

The arrangement of bonds around a chiral carbon atom or a double bond is called configuration. When two structures differ by rotation around one or more single bonds, they are called conformers or rotamers, and usually they are more easily interconvertible than stereoisomers rotamers can, however, be stable compounds when considerable steric hindrance is present." In ethane viewed straight-on with superimposed carbon atoms, when the configurations of the two carbon atoms overlap, the confor-... 

As was illustrated in Scheme 2.1, there are many molecules which are chiral but which do not contain chiral carbon atoms. The optical activity of such substances frequently depends upon the barrier to conformation changes which effect racemization. For example, the lowest energy path for racemization of fran -cycloalkenes is a rotation of the plane of the double bond through an angle of 180°. This rotation is most easily seen by working with molecular models. It is represented below for the case of trans-cyclooctene, in which the rotation of the plane is about the C(8)-C(l) and the C(2)-C(3) bonds. 

C1/C2 chevrons, ferroelectric devices 644 cadmium selenium TFT address 233 Cano wedge, chiral nematics 347 f, 351 Canon technology, ferroelectric devices 648 capillaiy flow, shear viscosity 143 carbocyclic compounds, charge transfer systems 958 carbocyclic rings, smectogens 412 carbon atoms, intercalated smectics 808 carbon-carbon bonds, dimers 823 carbon-carbon double bonds, chiral smectics 498 carbonaceous phases 693 carbonyl connectors, antiferroelectrics 687 carbonyl groups... 

Next, consider the positions in the middle of the alkyl chain that have methyl group branches. The methyl group on the left is bonded to a double-bonded carbon atom, which does not have four groups bonded to it therefore, it is not chiral. The next two methyl groups are located on chiral centers. Because there are two chiral carbon atoms, 2 = 4 stereoisomers are possible. These are the two carbons by the right hand of the parenthesis. 

In the hydrogenation of an alkene using a transition metal catalyst, the planar molecule binds to the surface of the metal. If the alkene is achiral, the side presented to the surface of the metal is not important. The alkene can be hydrogenated from the top or bottom to give the hydrogenated product. If the alkene contains a chiral carbon atom near the double bond, however, two products are possible. Consider the catalytic hydrogenation of (i )-2-methyhnethylenecyclo-hexane. Two stereoisomers, S,2R and R,2R, form, but in unequal amounts. Approximately 70% of the product is the cis isomer (15,2./ ). 

A structural formula for cholesterol is shown below. How many chiral carbon atoms are there in the cholesterol molecule What is the configuration, R or S, of the carbon atom bonded to the —OH group What is the configuration, E or Z, of the double bond ... 

Besides compounds with double bonds, which were described in the preceding paragraph one should also mention compounds that contain such bonds and, in addition, possess chiral quaternary carbon atoms. In such cases, the molecular fragment containing a double bond acts as a substituent at the chiral carbon atom or another heteroatom. 

Rule 2. When atoms attached directly to a double-bonded carbon have the same priority, the second atoms are considered and so on, if necessary, working outward once again from the double bond or chiral center. For example, in l-chloro-2-methylbutene, in CH3 the second atoms are H, H, H and in CH2CH3 they are C, H, H. Since carbon has a higher atomic number than hydrogen, the ethyl group has the next highest priority after the chlorine atom. 

Cahn-Ingold-Prelog sequence rules (Sections 6.5, 9.5) A series of rules for assigning relative priorities to substituent groups on a double-bond carbon atom or on a chirality center. 

The chiral sites which are able to rationalize the isospecific polymerization of 1-alkenes are also able, in the framework of the mechanism of the chiral orientation of the growing polymer chain, to account for the stereoselective behavior observed for chiral alkenes in the presence of isospecific heterogeneous catalysts.104 In particular, the model proved able to explain the experimental results relative to the first insertion of a chiral alkene into an initial Ti-methyl bond,105 that is, the absence of discrimination between si and re monomer enantiofaces and the presence of diastereoselectivity [preference for S(R) enantiomer upon si (re) insertion]. Upon si (re) coordination of the two enantiomers of 3-methyl-l-pentene to the octahedral model site, it was calculated that low-energy minima only occur when the conformation relative to the single C-C bond adjacent to the double bond, referred to the hydrogen atom bonded to the tertiary carbon atom, is nearly anticlinal minus, A- (anticlinal plus, A+). Thus one can postulate the reactivity only of the A- conformations upon si coordination and of the A+ conformations upon re coordination (Figure 1.16). In other words, upon si coordination, only the synperiplanar methyl conformation would be accessible to the S enantiomer and only the (less populated) synperiplanar ethyl conformation to the R enantiomer this would favor the si attack of the S enantiomer with respect to the same attack of the R enantiomer, independent of the chirality of the catalytic site. This result is in agreement with a previous hypothesis of Zambelli and co-workers based only on the experimental reactivity ratios of the different faces of C-3-branched 1-alkenes.105... 

---