Propene, rotation

The conformation of simple alkenes can be considered by beginning with propene. There are two families of conformations available to terminal alkenes eclipsed and bisected conformations, as shown below for propene. The eclipsed conformation is preferred by about 2 kcal/mol and represents a barrier to rotation of the methyl group. A simple way to relate the propene rotational barrier to that of ethane is to regard the tt bond as a banana bond (see p. 7). The bisected conformation of propene is then seen to correspond to the eclipsed conformation of ethane, while the more stable eclipsed conformation corresponds to the staggered conformation of ethane. ... 

Scheme 3 Potential free energy profile (kJ/mol) for propene rotation in HCo(CO)3( j -C3H6)...

Methyl rotors pose relatively simple, fundamental questions about the nature of noncovalent interactions within molecules. The discovery in the late 1930s1 of the 1025 cm-1 potential energy barrier to internal rotation in ethane was surprising, since no covalent chemical bonds are formed or broken as methyl rotates. By now it is clear that the methyl torsional potential depends sensitively on the local chemical environment. The barrier is 690 cm-1 in propene,2 comparable to ethane,... 

Propene and cis-butene-2 possess the molecular symmetries C2 and C2 v (28, 29), respectively. (Fig. 4 the geometry parameters given are derived from force field calculations (19, 30) and agree well with experiment.) The barriers // observed for the rotation of the methyl groups amount to 1.98 kcal mole-1 for propene (28), and 0.75 kcal mole-1 for cis-butene-2 (29). The torsional force constants K for the methyl groups... 

So far we have discussed conformations of a molecule obtained by rotation along sp -sp3 bond i.e., between two tetrahedral carbon atoms. But there are many compounds in which one carbon is in a state of sp2 hybridisation. Examples are substituted alkenes where one carbon atom is tetrahedral and the other trigonal, for example propene ... 

As a consequence of the energy decrease in both u and 7r, only the —rr interaction need be considered when dealing with the rotational barrier in CH3CH=0. As in the propene case, the eclipsed conformer will be favored. However, as can be seen from the diagrams below, the eclipsed form will be favored to a lesser extent in acetaldehyde relative to propene. 

Experimentally, acetaldehyde is known to exist in the eclipsed conformation. It has a methyl rotational barrier of 1.16 kcal/mol94-96 as contrasted with a barrier of 2.00 kcal/mol in the case of propene. 

Experimentally, it is known that the cis isomer in 1-substituted propenes is more stable and has a lower rotational barrier. Some pertinent data are shown in Tables 13—14. In most cases, the experimental results agree with our predictions. An interesting trend obtains in the alkyl vinyl ether series. Specifically, two types of nonbonded attraction can obtain in these molecules ... 

Table 2. Propene insertion for the benzoxantphos system, energies and geometrical parameters of transition states. Energies in kJ.mol"1, distances in A and angles in degrees, (a) Dihedral angle Hhvdndc-Rh-Cali.mc-C llktr . for defining alkene rotation.

Bercaw and Yoder obtained similar results using doubly labelled CH2=CD- CH3 in polymerisations with 84 and 85 at 50-75 °C and low propene concentrations. There was P-D elimination and olefin rotation, leading to migration of the label from the methyl substituent into the main chain, but no evidence for this process involving a Ti-allyl intermediate [131]. 

Delocalization of the odd electron into extended n systems results in considerable radical stabilization. The C—H BDE at C3 of propene is reduced by 13 kcal/mol relative to that of ethane. That the stabilization effect in the allyl radical is due primarily to delocalization in the n system is shown by the fact that the rotational barrier for allyl is 9 kcal/mol greater than that for ethyl. Extending the conjugated system has a nearly additive effect, and the C—H BDE at C3 of 1,4-pentadiene is 10 kcal/mol smaller than that of propene. Delocalization of the odd electron in the benzyl radical results in about one-half of the electron density residing at the benzylic carbon, and the C—H BDE of the methyl group in toluene is the same as that in propene. 

Alkenes which have no symmetry planes perpendicular to the plane of the double bond such as Pmr-butene-2 or propene can coordinate to platinum in two enantiomorphous ways (77) and (78). If an optically active ligand is also bound to platinum(H), then two diastereoisomers are found which can be separated by fractional crystallization657,658 or by HPLC.659 Both cis and trans isomers of complexes PtCl(N—0)(alkene) have beenprepared, where N—O is an anion derived from an amino add (equations 235a and 235b).660-664 Epimerization cannot occur by simple rotation of the alkene about its bond axis, but only by a mechanism involving cleavage of the platinum(II)-alkene bond. 

Kinetic studies by Doering and his collaborators at Harvard150-154 based on five sets of chiral 1,2-disubstituted cyclopropanes, with 1-cyano, 2-(phenyl or propen-2 -yl or -(E)-propenyl or phenylethynyl) (3) and 1 -phenyl-2-(propen-2 -yl) (4) substitution, established the ralative rotational propensities of these substituents and tested the proposition that they might be related to substituent moments of inertia. In all of these cases, the balance between one-center and two-center epimerizations from a trans isomer, reflected in (kt + k2) kl2, was fairly constant, ranging from 1.4 1 to 2.1 1. The kinetic advantages for one-center epimerizations at cyano-substituted carbons for the four cases studied were modest and not especially system-dependent the k, k2 ratios were 2.5,2.2,2.4 and 1.8, thus establishing that rotational propensities are not dictated by some simple function of the moments of inertia of substituents. 

In order to establish this supposition of a restricted rotation around the coordination bond, it should be assured that the contribution of the dissociation mechanism is negligibly small in the hydrogen-exchange reaction. For this purpose, a 1 1 mixture of (ZJ-propene-l-rf, (composition d0, 3.6% Z-l-dlt 86.1 % E-l-, 7.5% Z-l, 2-d2,1.4% E-l,2-d2,0% and 1, l-d2,1.4%) and propene-d6 (purity, propene-powder catalyst at room temperature, and HD gas (98 %) was added as a cocatalyst. In this experiment, the amounts of Mo—H and Mo—D sites are approximately equal during reaction. The primarily exchanged products formed by associative and dissociative mechanisms are described respectively in Scheme 11. 

Scheme 12. Rotational motions required for the hydrogen exchange of (Z)-propene-l-di by the associative mechanism.

Figure 1.6.1 Various group contribution types for propene nitrile. The SMILES rotation is used to indicate the groups. The Csp hybridize carbon atom is in bold.

Absolute configuration of the chiral centers in the isotactic copolymer main chain can be determined by comparing the CD spectrum of the copolymer that is a polyketone with that of (S)-3-methylpentan-2-one [126]. A recent model study has confirmed this assignment, showing that the copolymer with ( -configuration in the main chain exhibits plus optical rotation in (CF3)2CHOH and minus in CHC13 [128-130], The study has also revealed that the enantiose-lectivity for the propene insertion is at least 95% ee. 

Ab initio MRCI calculations showed that the barrier from trimethylene to propene is 7.9kcalmol 1 higher than that from trimethylene to cyclopropane.11 Thus, cyclopropane stereomutation may occur through trimethylene as an intermediate (Chart 3). Trimethylene biradical may cyclize by double rotation of the two C C bonds in conrotatory or disrotatory fashion or successive single rotation. The calculations showed that the PES at the... 

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