Conformation transoid

Theoretical work by the groups directed by Sustmann and, very recently, Mattay attributes the preference for the formation of endo cycloadduct in solution to the polarity of the solvent Their calculations indicate that in the gas phase the exo transition state has a lower energy than the endo counterpart and it is only upon introduction of the solvent that this situation reverses, due to the difference in polarity of both transition states (Figure 1.2). Mattay" stresses the importance of the dienophile transoid-dsoid conformational equilibrium in determining the endo-exo selectivity. The transoid conformation is favoured in solution and is shown to lead to endo product, whereas the cisoid conformation, that is favoured in the gas phase, produces the exo adduct This view is in conflict with ab initio calculations by Houk, indicating an enhanced secondary orbital interaction in the cisoid endo transition state . 

For a discussion of the mechanistic course of the reaction, many aspects have to be taken into account. The cisoid conformation of the diene 1, which is in equilibrium with the thermodynamically more favored transoid conformation, is a prerequisite for the cycloaddition step. Favored by a fixed cisoid geometry are those substrates where the diene is fitted into a ring, e.g. cyclopentadiene 5. This particular compound is so reactive that it dimerizes easily at room temperature by undergoing a Diels-Alder reaction ... 

In case of a Z,Z-configurated diene 6, the transoid conformation is favored, because of unfavorable steric interactions of substituents at C-1 and C-4 in the... 

Solvent polarity is also important in directing the reaction bath and the composition and orientation of the products. For example, the polymerization of butadiene with lithium in tetrahydrofuran (a polar solvent) gives a high 1,2 addition polymer. Polymerization of either butadiene or isoprene using lithium compounds in nonpolar solvent such as n-pentane produces a high cis-1,4 addition product. However, a higher cis-l,4-poly-isoprene isomer was obtained than when butadiene was used. This occurs because butadiene exists mainly in a transoid conformation at room temperature (a higher cisoid conformation is anticipated for isoprene) ... 

The diene must be in the cisoid conformation. If it is frozen into the transoid conformation, as in 84, the reaction does not take place. The diene either must be frozen into the cisoid conformation or must be able to achieve it during the reaction. 

D. Restricted Rotation.—A study on solvent and stereochemical effects on the restricted rotation of the stabilized ylides (35) has shown that although the cisoid (Z) conformation (35a) is generally predominant there is an increase in the amount of the transoid ( ) conformation (35b) as the size... 

Cyclic dienes which are locked in the cisoid conformation, e.g. (82), are found to react very much faster than acyclic dienes in which the required conformation has to be attained by rotation about the single bond (the transoid conformation is normally the more stable of the two). Thus cyclopentadiene (82) is sufficiently reactive to add to itself to form a tricyclic dimer, whose formation—like most Diels-Alder reactions—is reversible. 

Another interesting chiral chain end effect is exhibited by the helical polymer block co-polymer, poly(l,l-dimethyl-2,2-di-/z-hexylsilylene)- -poly(triphenylmethyl methacrylate), reported by Sanji and Sakurai (see Scheme 7) and prepared by the anionic polymerization of a masked disilene.333 The helical poly(triphenylmethyl methacrylate) block (PTrMA) is reported to induce a PSS of the same sign in the poly(di- -propylsilylene) block in THF below — 20 °C, and also in the solid state, by helicity transfer, as evidenced by the positive Cotton effect at 340 nm, coincident with a fairly narrow polysilane backbone UV absorption characteristic of an all-transoid-conformation. This phenomenon was termed helical programming. Above 20°C, the polysilane block loses its optical activity and the UV absorption shifts to 310 nm in a reversible, temperature-dependent effect, due to the disordering of the chain, as shown in Figure 45. 

But irrespective of the mechanism (concerted or stepwise) the reaction is only possible from cisoid forms. In the frozen transoid conformation, the reaction does not occur. This is why the following does not react. 

That in which the two methyl groups are as far apart as possible. These are called antiox transoid conformation. 

The interaction diagrams for the above conformations are identical with that of methyl vinyl ether (Fig. 30) except that the oxygen lone pair AO is replaced by an unoccupied carbon 2p AO. With this in mind we conclude that the transoid conformation of the cation, Ts, will be more stable than the cisoid conformation, Cs, since the <(>j—Pz two electron stabilizing interaction is greater for the Ts conformation. 

If the vinylallene possesses a substituent at the vinylic terminal position, an endo adduct is preferentially obtained owing to the secondary orbital overlap. Only the E-isomer of propenylallene underwent the regio- and stereoselective cycloaddition with methyl vinyl ketone to afford the endo-isomer as the major product. The Z-iso-mer was unreactive because it preferred the transoid conformation [165]. 

Fig. II. (a) Schematic representation of hydrocarbons adsorbed on the [111] plane of platinum. Intersections of the lines of triangular net denote positions of the centers of platinum atoms. (1) Cyclohexane (2) all-cis conformation of cij-l,3,5-hexatriene (3) transoid conformations of cis- and trans-1,3,5-hexatriene (S4). (b) Adsorption configurations of acetylene and ethylene found most probable according to LEED studies 141).

INTERNAL TRANSMITTANCE TRANSMISSION DENSITY TOTAL TRANSMITTANCE BEER-IAMBERT LAW ABSORPTION SPECTROSCOPY Transoid conformation,... 

Lactams are another interesting class of compounds that have been thoroughly explored by Zhang [25]. Methods that are typically used in the synthesis of nonracemic lactams involve C-N bond formation of substrates with preset chiral centers. The Alder-ene carbon-carbon bond forming strategy allows for chiral centers to be set without the use of other pre-existing chirality in the substrate. Initially, enynes with secondary amide tethers were studied but no cyclization was observed. Zhang reasoned that the favored transoid conformation of the secondary amide prevents the necessary conformation required for cyclization (Scheme 8.2). 

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... 

The proton NMR spectrum of 2,2 -bipyridine has been obtained and analyzed in a variety of solvents by several authors. ° The full interpretation of the spectra is in accord with the transoid conformation I in solution. The behavior of the chemical shifts of protons at positions 3 and 3 indicates the existence of a strong deshielding effect exerted by the nitrogen atoms of the adjacent rings. Interestingly, the proton NMR spectra of 2,2 -bipyridine taken in various solvents indicate self-association and stacking of the molecules in some cases. The spectra of some substituted 2,2 -bipyridines, - - 2,3 -bipyridine, ° 2,4 -bipyridine, ° 3,3 -bipyri-dine, and 4,4 -bipyridine - - ° have been investigated in detail. It was... 

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