Substituent Steric Effects

Three different aryl iodides were employed (4-MeOC, H4l, 4-MeCOCgH4l, and 2-MeCgH4l) to evaluate electronic and steric contributions from the coupling partners. Steric and electronic factors on the aryl iodide had no effect on influencing the relative reactivity of the different silanes, regardless of the method of activation. 

The divergence of steric and electronic effects between the two promoters employed clearly suggests the operation of different mechanisms for the two systems. This issue has been addressed through kinetic studies discussed in the following section. 

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The reaction can be conveniently monitored by i.r. absorption spectroscopy by observing the intensity of the band at 1650 cm.-1 (C=N). The submitters observed almost complete disappearance of this band whereas the checkers found it still present in medium intensity in their product. In those instances where the a-carbon bears three alkyl substituents, steric effects retard the rate of addition, and in some cases (i.e., [Pg.15]

A mechanistic study by Deady and Korytsky, which showed that dealkylation is exactly the reverse of the alkylation reaction (79TL451), implies, according to the principle of microscopic reversibility, that the amount of strain in the TS for quaternization is two-thirds of that in the final quaternary salt, in close agreement with the early estimate by Brown. The ratio of substituent steric effects is 3/1 for disubstituted pyridines in methylation vs. demethylation. Because of smaller sensitivity to steric effects in thiazoles, steric acceleration in the reverse reaction is small (76AJC1745 79BSF(2)484). 

On the aryl ring of aryltrialkyltins can be introduced both electron-releasing and electron-withdrawing substituents. Steric effects can be important, however. The presence of an alkyl group ortho to tin often retards the cou-... 

Homoallylic nucleophilic substitution is known (equation 23), but its synthetic utility is hampered by competing direct substitution without opening of the cyclopropane ring. The product ratio is largely influenced by the nature of the attacking nucleophile and the cyclopropane substituents (steric effects). ... 

A substituent size descriptor defined as the volume V of the portion of the substituent within a sphere centred at the link atom [Meyer, 1986b], Tlie radius of the sphere was chosen equal to 0.3 nm to comprise the substituent portion responsible for the steric effect of the substituent. It was used, together with the - ovality index calculated on the substituent, to estimate substituent steric effects for substituents with equal volume V , much larger steric effects are observed for globular substituents. 

Steric substituent constants (or steric substituent parameters) are descriptors of substituent groups that measure the substituent steric effects on the reactivity centres of a molecule, based on differences in the rate and equilibrium constants of selected chemical reactions. 

Kier, L.B. (1987d). The Substituent Steric Effect Index Based on the Molecular Graph. Quant Struct.-ActRelat, 6,117-122. 

Kier, L.B. (1987d) The substituent steric effect index based on the molecular graph. Quant, Struct, -Act, Relat., 6, 117-122. 

When it comes to the actual stability of PX3-substituted carbonyl molecules, especially those with two or more substituents, steric effects become very important.68 For highly substituted molecules, such as Ni(PX3)4, steric effects become much more important than electronic effects in determining the position of exchange equilibria such as... 

Iron complexes containing bidentate alkyl and aryl phosphorus ligands cleave a variety of C-H bonds under mild conditions, Hydrido acetylide complexes were prepared by oxidative addition of primary acetylenes in the Fe(DPPE)2 and the Fe(DMPE)2 systems [DPPE = bis(diphenylphos-phino)ethane, DMPE = bis(dimethylphosphino)ethane]. The Fe(DMPE)2 system also cleaves C-H bonds of activated methyl groups, aromatic compounds, and certain other sp hybridized molecules. The C-H cleavage reactions are reversible, resulting in equilibrium mixtures of isomeric products in many cases. Studies of substituted benzenes show that while product stability is favored by electron withdrawing substituents, steric effects play a predominant role in the determination of product distribution. 

The factors that control the site of attack of nucleophiles on arene complexes are complex, and the position of attack is controlled by the electronic properties of the substituents. These properties dictate whether attack will occur meta or para to the substituent. Steric effects discourage attack ortho to the substituent. In many cases, the attack of nucleophiles on coordinated arenes is reversible. Thus, the final productive reaction may result from addition at a site of the arene that is not the kinetic site for attack. For example, nucleophilic attack onto the ortho, meta, or para positions of the phenyl chloride coordinated to Cr(CO)j in Equation 11.53 is reversible, and it is only attack at the ipso position that leads to productive substitution chemistry. - ... 

Mohring, P. C., CoviUe, N. J. Group 4 metallocene polymerisation catalysts Quantification of ring substituent steric effects. Coord. Chem. Rev., 250, 18-35... 

A number of carbon-carbon bond formation reactions have been studied in detail by Chambers (22) and Filler 28). Some of these transformations show that significant steric hindrance is imposed by the 2,4,6-tris(trifluoromethyl)phenyl substituent. Steric effects became especially apparent in the failure of several attempts to prepare the acetophenone derivative RfC(0)CH3 and the related vinyl ketone RpC(0)CH=CH2 (28), The elusive 2,4,6-tris(trifluoromethyl)acetophenone is... 

In practice it turns out that there are significant differences between the various polymerization heats of different monomers. This is caused by both the loss of coupling energy in the conversion of monomer molecules into the polymer molecule and the energy loss associated with the formation of stresses in the pol5mier chain as a result of mutual interaction of substituents (steric effect). The pol5mierization heats of selected monomers are listed in Table 8.1. 

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