Phenyl substituent role

The nature of the counterion has had a profound impact on catalysis, as will be seen. Structurally, it was of considerable interest to delineate the factors that influence selectivity and to examine whether the counterion plays a role in the solid-state geometry of these catalysts. While the hexafluoroantimonate copper complexes of bis(oxazoline) 55c are completely dissociated in the solid state, analogous triflate complexes exhibit weak bonding to one counterion in the apical position (2.62 A from the metal), Fig. 23. Association of the triflates in the solid state was also noted for Complex 266d. The water molecules are distorted toward the phenyl substituents, similar to the SbF6 complex 265d. 

The length of the oligomer chain plays a role in the stereospecificity, the presence of a few units in the initiating chain leads to an increase in diastereoselectivity, as was also found for growing propene oligomers. Thus, the hindrance between chain and metal catalyst contributes to the orientation of the phenyl substituent of the incoming styrene molecule. 

When R is primary alkyl, the second-order rate constant k2 is obtained by taking the slope of kobs vs. concentration of the nucleophile. The plot passes through the origin, indicating a pure SN2 mechanism without SN1 participation. The reference pyridinium ion is the 2,4,6-triphenyl derivative (because pyrylium precursors with phenyl substituents are more easily prepared) (82AHC(Suppl 2)1) but numerous other substituents have been introduced into the ring. Rate constant values reported in Table XIX, where release of steric strain has a major influence, are in agreement with the role of structural factors discussed in Section IV,A. 

Recognizing that the conducting polypyrrole films can be chemically modified (.1,2), the phenyl substituent assumes a particularly important role because it provides a means of introducing a wide selection of functional groups into the polymer. With this objective in mind, we have prepared a series of N-arylpyrrole polymers and find the thin poly-N-(p-nitrophenyl)pyrrole films of particular interest because they combine the electroactive properties of nitrobenzene and polypyrrole. With this combination, the polymer can be switched electrochemically between the cationic, neutral, and anionic form. 

Again, this result would probably also have been obtained through the performance of very expensive pure QM calculations on the full system. A pure QM calculation on the real system would nevertheless only prove the decisive role of the phenyl substituents of phosphine in the distortion but could not decide on whether their effect was electronic or steric. The fact that this distortion appears in the IMOMM calculation, where the QM effects of phenyls are neglected, in itself constitutes a direct proof that the origin of the distortion is purely steric. 

Pyridine derivatives will undergo aza-annulation at elevated temperatures. For example, reaction of ketone 416 with the diester 47 at 150-190 °C led to formation of quinolizone 417 (eq. 85).101 The ketone substituent R played an important role on the outcome of the reaction, in which substrates with an alkyl substituent gave much greater yields than those with a phenyl substituent. 

Therefore, the shielding field of the peripheral porphyrins influences even the central part of the template porphyrin. H NMR studies also show that the phenyl substituents are fixed in space they are perpendicular both to the Ru(II) porphyrin mean planes and the central, assembling, free-base porphyrin, forming the rims of a molecular box. In fact, the pentamer really plays the role of a molecular box when the central porphyrin is complexed with Zn(II) and is able to selectively recognize S-bonded Ru(II) complexes of Me.SO. 

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 aryl-isocyanide [3] or -cyanide [5,9] complexes of our study are invariably harder to oxidize than their alkyl homologues and, within any aryl series (with a variable phenyl substituent), a gross linear correlation is observed between EyO and the Hammett s op or Op constant, the former increasing with the latter, in accord with the stabilization of the HOMO as a result of an increase of the electron-acceptor character of a ligand moreover, the HOMO can be conjugated to the aryl substituent vs. op correlation) [3], again reflecting the important role of x-effects. 

The active catalyst is presumably a cationic complex with a 1 1 phosphine/ rhodium ratio. The phenyl substituent of the chiral phosphoramidite ligand Lll seems to play an important role for stereocontrol. Therefore, it has been postulated that t[ -coordination of the phenyl group takes place during the catalytic cycle and helps to create a well defined chiral environment (Fig. 10.40). 

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