Sigma bonds and bond rotation

Research suggests there are roles for pheromones in the lives of humans as well. For example, studies have shown that the phenomenon of menstrual synchronization among women who live or work with each other is likely caused by pheromones. Olfactory sensitivity to musk, which includes steroids such as androsterone, large cyclic ketones, and lactones (cyclic esters), also varies cyclically in women, differs between the sexes, and may influence our behavior. Some of these compounds are used in perfumes, including cive-tone, a natural product isolated from glands of the civet cat, and pentalide, a synthetic musk. 

Two groups bonded by only a single bond can undergo rotation about that bond with respect to each other. 

When we do conformational analysis, we will find that certain types of structural formulas are especially convenient to use. One of these types is called a Newman projection formula and another type is a sawhorse formula. Sawhorse formulas are much like dash-wedge three-dimensional formulas we have used so far. In conformational analyses, we will make substantial use of Newman projections. 

Learn to draw Newman projections and sawhorse formulas. Build handheld molecular models and compare them with your drawings. 

In Figs. 4.4a,b we show ball-and-stick models and a Newman projection formula for the staggered conformation of ethane. The sta ered conformation of a molecule is that conformation where the dihedral angle between the bonds at each of the carbon-carbon bonds is 180° and where atoms or groups bonded to carbons at each end of a carbon-carbon bond are as far apart as possible. The 180° dihedral angle in the staggered conformation of ethane is indicated in Fig. 4.41 . 

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In L2-dichloroethane, the t o C atoms are joined by a sigma bond. Rotation about a sigma bond does not destroy the bond, and the bond is therefore free (or relati ely fi ee) to rotate. TliuSj all angles are permitted and the molecule is nonpolar because the C-Cl bond moments cancel each other because of the averaging effect brouglit about by rotation. In c/5-dichloroetliylene tlie two C-Cl bonds are locked in position. Tlie... 

UNSUBSTITUTED BUTADIENE. Butadiene anchors were presented in Figures 1(3) and 13. The basic tetrahedral character of the conical intersection (as for H4) is expected to be maintained, when considering the re-pairing of four electrons. Flowever, the situation is more complicated (and the photochemistiy much richer), since here p electrons are involved rather than s electrons as in H4. It is therefore necessary to consider the consequences of the p-orbital rotation, en route to a new sigma bond. 

You may recall that we discussed the bonding in ethene in Chapter 7. The double bond in ethene and other alkenes consists of a sigma bond and a pi bond. The ethene molecule is planar. There is no rotation about the double bond, since that would require breaking the pi bond. The bond angle in ethene is 120°, corresponding to sp2 hybridization about each carbon atom. The geometries of ethene and the next member of the alkene series, QHg, are shown in Figure 22.6. 

Initially the excited electron does not change its spin.) The bonding effects of the two electrons cancel. There is now only a sigma bond between the bonded atoms, and rotation about the bond can occur. 

Sigma-bonded C s can rotate about the C—C bond and hence a chain of singly bonded C s can be arranged in any zigzag shape (conformation). Two such arrangements, for four consecutive C s, are shown in Fig. 4-1. Since these conformations cannot be isolated, they are not isomers. 

At this point we should reiterate that the relative positions of atoms in many structures are continuously changing. The term different conformations of a molecule is used if two different three-dimensional arrangements of the atoms in a molecule are rapidly interconvertible, as is the case if free rotations about sigma bonds are possible. If rotation is not possible, we speak of different configurations, which represent isomers that can be separated. Obviously, the conformation(s) with the lowest energy [the most stable form(s)] is(are) the one(s) in which a molecule will preferentially exist. In the case of six-membered rings such as cyclohexane, three stable conformations (i.e., the chair, twist, and boat form) exist (Fig. 2.9). 

In the case of certain diolefins, the palladium-carbon sigma-bonded complexes can be isolated and the stereochemistry of the addition with a variety of nucleophiles is trans (4, 5, 6). The stereochemistry of the addition-elimination reactions in the case of the monoolefins, because of the instability of the intermediate sigma-bonded complex, is not clear. It has been argued (7, 8, 9) that the chelating diolefins are atypical, and the stereochemical results cannot be extended to monoolefins since approach of an external nucleophile from the cis side presents steric problems. The trans stereochemistry has also been attributed either to the inability of the chelating diolefins to rotate 90° from the position perpendicular to the square plane of the metal complex to a position which would favor cis addition by metal and a ligand attached to it (10), or to the fact that methanol (nucleophile) does not coordinate to the metal prior to addition (11). In the Wacker Process, the kinetics of oxidation of olefins suggest, but do not require, the cis hydroxypalladation of olefins (12,13,14). The acetoxypalladation of a simple monoolefin, cyclohexene, proceeds by trans addition (15, 16). 

The following scheme summarizes the types of isomers that we have encountered. (Because the rotations about sigma bonds that interconvert conformations occur rapidly at room temperature, conformations cannot be separated and are not considered to be isomers.)... 

The other type of bond that can form is a pi (it) bond. Pi bonds are the type of bonds that make up multiple bonds and are formed when p orbitals on neighboring atoms align with one another in a parallel fashion. The electrons in the p orbitals distribute themselves above and below the axis (where the s bond has occurred). Pi bonds are weaker than sigma bonds. Atoms that have only single available p orbitals can form a single tt bond, where atoms with two available p orbitals can form two tt bonds. The formation of pi bonds prevents molecules from rotating around the internuclear axis in Figure 7.23 ... 

The monomer coordinates at a vacant coordination site on the cobalt atom. It assumes the trans form since the second coordination attachment on the other side of the cobalt is blocked by an amine molecule. The organic group in the bridge is favorably located for attack either on the C2 or C4 positions of the complexed monomer molecule to give 1 2 or trans 1 4 addition. A sigma bond is formed between the Co and Ci atom of the monomer, and after bond rotation into the bridge, the coordination site is then once more available for monomer insertion. 

I2 has a low bond dissociation energy (151 kJ/mol) and forms 21- on heating. I- adds to the C=C to form a carbon radical which rotates about its sigma bond and assumes a different conformation. However, the C—I bond is also weak (235 kJ/mol) and the radical loses I- under these conditions. The double bond is reformed and the two conformations produce a mixture of cis and trans isomers. 

Because of rapid rotations about sigma bonds and molecular symmetry, the three methyl hydrogens are all equivalent to each other. For the same reasons the two methylene protons are also equivalent to each other. In summary,... 

All the C-C and C-H bonds in the alkanes are sigma single bonds and free rotation about such bonds is possible (as shown by using a model). If there is free rotation, then some of the arrangements might look different but they are the same molecule. Remember that in actual practice the molecule will try to take up the most favourable positions in space, and will not take up some of the distorted shapes that can be made, as shown using a model kit. 

Conformational isomers (conformers) are stereoiso-meric forms characterized by different relative spatial arrangements of atoms that result from rotation about sigma bonds. Thus, unlike configurational isomers, conformers are interconverting stereochemical forms of a single compound. The nature of conformational and configurational stereoisomerism, as well as the role of stereoisomerism in drug activity is the subject of this article. 

As shown in Scheme 69, Kim and coworkers have reported highly stereoselective routes to ci5-1,2-dialkylcycloalkanecarboxylates by intramolecular alkylations of tosyloxy esters such as (142). The reaction appears to proceed via the eclipsed conformations of the enolate (143), rather than the bisected one that would result from a 180 rotation about the a, 3-sigma bond. 

Stereochemical labels on the cyclobutene confirm that this 4-electron electrocyclic ring opening is conrotatory. Another way to view the reaction is from the closure of the diene. The ends of the HOMO of the diene must rotate con to create a bonding overlap for the new sigma bond. Because of microscopic reversibility, opening and closure follow the same route if the opening is conrotatory, then the closure will likewise be con. 

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