Tetraphenyl substituent effects

The di-7r-methane rearrangement has been studied in a sufficient number of cases to develop some of the patterns regarding substituent effects. When the central sf carbon is unsubstituted, the di-7i-methane mechanism becomes less favorable. The case of 1,1,5,5-tetraphenyl-l,4-pentadiene is illustrative. Although one of the products has the expected structure for a product of the di-7t-methane rearrangement, labeling with deuterium proves that an alternative mechanism operates ... 

Steric studies of benzo[c]furan derivatives are scant. There is an important collision between the peri phenyl substituents in the 1,3,4,7-tetraphenyl derivative such that these substituents rotate out of the main plane and face each other with marked effects upon the UV spectra and important fluorescence characteristics of the system (80AHC(27)3l). 

Obviously, in 2,4,6,8-substituted bispidine molecules there is a large steric hindrance for the rotation of the substituents. Cooperative effects in the restricted rotation of the aryl groups in 2,4,6,8-tetraphenyl-3,7-diazabicy-clo[3.3.1]nonanes have been described (165). The rotation of the 2,4-phenyl substituents seems to be correlated with that of the 6,8-phenyl groups. This emerges from large negative activation entropies for the rotation. 

On addition of a second pair of phenyl substituents at C4 and C6, the interallylic distance further lengthens so that the contribution of structure C to the TS wave function is further enhanced. The ability of the second pair of phenyl substituents to increase the optimal value of R from 7 = 2.218 A to 7 = 2.649 A allows each of the phenyl substituents in l,3,4,6-tetraphenyl-l,5-hexadiene to provide more stabilization for the Cope TS than the pair of phenyl substituents in 1,3-diphenyl-1,5-hexadiene. This cooperative effect of the four phenyl groups in 1,3,4,6-tetraphenyl-1,5-hexadiene lowers AT/ for Cope rearrangement by, not twice, but by four times as much as the pair of phenyl substituents in 1,3-diphenyl-1,5-hexadiene. 

The reaction thus always leads to a six-membered ring, by 1,4-addition. Of the three double bonds concerned in the reaction, only one is retained in the product, and that at a different position. Rearrangements rarely occur in these reactions. Moreover, they are stereospecific on addition of maleic acid to a diene the adduct produced is a cw-dicarboxylic acid, and on addition of fumaric acid it is a frms-dicarboxylic acid. Substituents on the diene or philodiene are not wholly without effect on the course of the addition for example, 1,2,3,4-tetramethylbutadiene (3,4-dimethyl-2,4-hexadiene) reacts smoothly,33 but the tetraphenyl analog shows no tendency to add a philodiene. Bulky substituents at the 2,3-positions can completely suppress the addition, presumably because they prevent free rotation and thus disfavor formation of the con-... 

Little meaningful physical data on these complexes has appeared yet. The study of their reactivity is complicated by the effects of the substituents on the cyclobutadiene ring and the other ligands present. Until more information is available on the properties of the recently prepared unsubstituted cyclobutadieneiron tricarbonyl (XVIII) 38) and similar molecules, it is hard to be certain which properties are due to the presence of a cyclobutadiene group. Thus while cyclobutadieneiron tricarbonyl (XVIII) is easily oxidized by ferric chloride in ethanol (as are other diene-iron tri-carbonyl complexes, albeit to different types of product), tetraphenyl-cyclobutadieneiron tricarbonyl (XIII) is very resistant to this reagent, and indeed to most others, presumably mainly due to the steric hindrance of the phenyls. 

The SERS spectral data obtained on colloidal Ag as well as Ag island films indicate a weak macrocycle interaction and small structural modifications of the naphthalocyanine complex on the surface. An energy transfer mechanism contribution to the observed enhancement was proposed. The whole spectral data point that the naphthalocyanine complex is oriented with the naphthalocyanine plane face-on to the surface. In both vanadylnaphthalocyanine and vanadylpor-phine tetraphenyl substituted complexes the vanadyl group is perpendicular to the coordination site and opposed to the surface. In both complexes the phenyl substituents, oriented perpendicular to the macrocycle plane, are responsible of the weak adsorbate-substrate interaction. Several different experimental conditions such as scanning speed, laser power and excitation lines wavelengths, concentration and metal surfaces were intended to obtain SERS of the porphine complex without success this was interpreted in terms that the complex has no interaction with the surface. Thus, it was concluded that the extension of the whole r-system plays a significant role in the mechanism involving the spectral enhancement by surface effect. 

The effect of carboxylic anchoring group position with respect to the meso-tetraphenyl-substituted A3B-type ZnPs on PCE of the DSSC was evaluated in the above-mentioned research (Schemes 13 and 19) [28]. Moreover, the effect of the Zn atom introduced into free-base porphyrin and the steric effect of peripheral substituents on PCE were evaluated as well. 

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