Configuration cisoid

Conformation. The exact conformation of the isoprene molecule is stiU in doubt. It is generally accepted that rotation is restricted around the central C—C single bond. Isoprene may be considered as an equiHbrium of two conformations, namely a cisoid s-cis) conformation in which both vinyl groups are located on the same side of the C—C bond, and a transoid s-trans) one with the vinyl groups located on the opposite sides of the bond. The predominance of the trans-planar or nonplanar configuration has been supported by experimental data (10—14). 

Alternatively, liquid phase polymerization (in bulk monomer at a temperature of 20° C) furnishes an isomer (II) characterized by a cis-transoid (or trans-cisoid) configuration of the main chain, with carboxyl groups located on both sides of it. These isomers will be shown later to differ in chemical and physicochemical properties. 

The use of UV-VIS spectra to analyse dienes and polyenes was historically the first method of choice. The spectra of isolated non-conjugated polyenes is actually the superposition of the spectrum of each one of the double bonds. For each double bond the spectrum depends on the various substituents and also on its location in the molecule. It also depends on the stereochemistry, since conjugated double bonds have either E or Z configuration around each jr-bond but also a cisoid and transoid conformer3 around the single bond marked as s-cis and s-trans4. 

The high enantioselectivity shown in the above reactions can be attributed to two important factors. First, coordination of the Lewis acid with the a-hydroxy ketone moiety of dienophile 17 or 19 leads to the formation of a rigid five-membered chelate 21. This chelate causes the differentiation of the two dia-stereotopic faces of the enone system. Second, arising from the established absolute configuration of 17 and 19, within 21, the Diels-Alder reaction proceeds with the enone fragment at its cisoid position (yyu-planar). 

The Cr(CO)6 and Cr(CO)3(arene) catalysts hydrogenate conjugated dienes by 1,4-addition of hydrogen. The diene coordinates in a cisoid configuration (equation 7)23. This... 

The dipole moment of 2,2 -bipyridine in benzene or carbon tetrachloride has been reported as less than 0.68, 0.91, 0.69, and 0.61 d. Because the conformation with the two nitrogen atoms transoid to each other should have a zero dipole moment and the cisoid configuration a value of 3.8 D, the consensus is that the molecule is in the transoid conformation and is approximately planar in solution with an angle of about 20° between... 

EIGURE 6. Dialkylmagnesium compounds form unusually strong adducts with sparteine. NMR data indicate that sparteine adopts cisoid configuration on co-ordination... 

The observed stereochemical outcome suggested that a cisoid configuration of the crotyl metal was kinetically favored in the cyclic transition state. Indeed, recent calculations supported that transition structures with cisoid crotylmetals are more stable whatever the olefinic configuration of the alkenyl metal142. 

Three-bond carbon-phosphorus couplings JCP depend on configuration, as expected, transoid couplings being larger than cisoid ones, as demonstrated for cyclohexyl- and 2-norbornylphosphonates in Table 4.52. The Karplus relation (4.13) can be derived from the data for 2-norbornyl derivatives [376]. 

However, the problem arises from our measurement of the hole size of 6.35. Certainly the hole size of the ligand in a planar configuration is too small for the nickel(n), but it is possible for the ligand to fold to adopt a cisoid configuration. It is in this form that the ligand is co-ordinated to the nickel the metal is octahedral, with the two remaining coordination sites occupied by bromide ions and with near optimal Ni-N and Ni-S distances (Fig. 6-37). 

Cycloadditions are easier to treat than unimolecular reactions they only require an evaluation of the best FO overlap (rule 4). Let us look at the cyclodimerization of butadiene. Woodward and Hoffmann suggested that the experimentally observed endo compound is due to secondary interactions (shown by the double arrows above), which increase the stabilizing the FO s interaction.23 Cisoid configurations are often adopted by the dienophile in Diels-Alder reactions,24 as first suggested by Dewar.20 For... 

O-Alkylation leads to derivatives in which rotation around Ar—Ar bonds is impossible, since the O-alkyl groups cannot pass each other. Hence, the single diphenol units become chiral and the macrocycle may be considered as resulting from R or S configured atropisomeric subunits. These compounds may also be regarded as inherently chiral since in a linear analogue the biphenyl unit can racemize not only via a cisoid transition state, where the O-Y groups have to pass each other (the only possibility in the macrocycle) but also via a transoid transition state. 

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