Propene, rotational isomers

Figure 4.11. A depiction of the rotational isomer of propene, CH3-CH=CH2,with a hydrogen of the methyl group (-CH3) lying in the plane defined by the three carbon atoms.

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 3. Propene insertion for the homoxantphos system, energies and geometrical parameters of transition states. Energies in kJ.mol 1, distances in A and angles in degrees, (a) The CW B in isomer could not be localized (b) Dihedral angle Hhydnde-Rh-Caikenc-Caiiiene for defining alkene rotation.

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. 

Like a-melhyislyrene (2-phenyl-1-propene) the isomeric alkenes a-ethylstyrene (2-phenyl-1-butene), (Z)- and ( )-2-phenyl-2-bulene are hydrocarboxylated under similar conditions19. However, different optical rotations are observed for methyl 3-phenylbutanoate, the common reaction product (values between 8.1 to 10.5). This supports the assumption of individual induction steps for each isomer and excludes the isomerized substrate being released from the complex. 

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