The Conjugative Ability of the Trimethylsilyl Group

The original Hammett equation, Δ = ρσ, is transformed in a constrained tetralinear relationship where each straight line with variable intercept term correlates one of the following four groups or subsets of dipolar substituents: normal and special substituents (depending on the absence or the presence of a lone electron pair in their atom next to the aromatic ring) and, in each of these classes, separating meta and para derivatives. There are a total of four fitting parameters in the resulting plurilinear Hammettian transformation (PHT) from which the statistically corrected parameters λ and γ are derived; λ and γ are the asymptotic values in a hyperbolic model for the representation of Δ4 vs Δ4/Δ3. This meta–para interrelationship is assumed to hold in the absence of through‐resonance effects which, in turn, are allowed for by the use of alternative sigma scales of substituent constants. By applying the PHT to a large number of selected literature data, parameters λ and γ are determined for the ionization equilibria of 3‐ and 4‐monosubstituted benzoic acids, anilinium ions, phenols and pyridinium ions. In these reactions series, parameter λ, which measures the para/meta ratio of field/inductive effects, is lower than unity and shows a marked dependence on the basic molecular framework. It is best modelled in terms of a through‐space field effect approach. The ratio γ/γ0, where γ0 is referred to the unified sigma‐zero scale, is shown to correspond to the original Hammett's reaction constant ρ. It is concluded that the PHT constitutes an improved Hammett equation for the analysis of substituent effects in benzene derivatives taking into account statistical errors and making allowance for different transmission coefficients for the field/inductive effect from meta and para positions in different reaction series.

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“…Dropping the upperscript th, the relevant equations for the theoretical plurilinear transformation are A 3 n = ~3 n d n (37)…”

Section: Theoretical Casementioning

confidence: 99%

Plurilinear improvement of the Hammett equation

Reis

Segurado

Oliveira

1995

J of Physical Organic Chem

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“…Although there is divergence of opinion on the electronegativities of silicon and germanium,l7 there is no doubt that silicon is more electropositive than carbon; consequently the Me3Si and Me3Ge groups should act as electron releasing groups in the absence of n-bonding effects. Consistent with this view the dissociation constants 18 of anilines, dimethylanilines, phenols and benzoic acids with the Me3Si group in the meta and para positions show an electron release effect except when it is in a position para to a group capable of conjugation with the benzene ring. Dipole moment measurements suggest that the Me3Si group has an electron donating M u e n a but can act as an electron donor or acceptor when it is attached to a benzene ring, according to the nature of the para substituent.19 Aromatic groups attached to silicon cause substantial decreases in the activation energy for 36Cl-exchange in R3SiCl systems and although such effects are not easy to separate from steric effects it is probable that increased n-bonding in the (bipyramidal) transition state relative to the (tetrahedral) initial state is at least partly responsible for this effect9…”

Section: Chemical Aspects : Comparison With Existing Evidencementioning

confidence: 53%

The electron spin resonance spectra of the phenyltrimethylsilane and -germane anions: ?-bonding between silicon and germanium and the aromatic ring

Bedford¹,

Bolton²,

Carrington³

et al. 1963

Trans. Faraday Soc.

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“…Hence, the assumption of a single Interaction mechanism is not valid* Hammett (18) recognized this discrepancy and attributed it to inductive and resonance effects (4). It was argued that o m represents the inductive effect and the difference (o p -a m ) the resonance (i.e., mesomeric effect) (19,20,21). Further improvements in the separation into inductive and mesomeric effects are discussed below.…”

Section: However the Same Linear Relationship Can Also Hold For Casementioning

confidence: 99%

“…We define the terms on the right side of equation 21 (23) where 0 and W represent the octanol and aqueous phases respectively and P x and P H the equilibrium constants or the partition coefficients for the substituted and unsubstituted molecules. Here we use the usual assumption of additivity of terms, representing the parts of the molecule, R and X, and the interaction between them, Ij^.…”

mentioning

confidence: 99%

Parameters and Methods in Quantitative Structure-Activity Relationships

Osman

Weinstein

Green

1979

ACS Symposium Series

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In studies of quantitative structure activity relationships (QSAR), the relative potencies of a series of drugs are subjected to analysis with the hope that biological potency will be described by a mathematical equation. QSAR is an actuarial or statistical method in which only objective data are used with no intrusion of models or mechanistic hypotheses. The equation that is obtained not only accounts for the relative potencies of the compounds, but from it are deduced predictions of the potencies of untested compounds; if the equation is valid, the predictions are ineluctable. The method thus has the capacity of yielding new (structurally related) drugs with desired potency, perhaps drugs with enhanced selectivity or fewer side effects.Further, in the method that is most widely used, the equation contains a parameter(s) that reflects a property related to the structural characteristics of the molecules. Therefore, the equation has often been assumed to provide insight into mechanisms that determine the action of drugs. In fact, the parameters that account for the biological potency are usually not truly empirical, observed entities, but rather abstractions. These parameters are frequently reified, burdened with a concrete meaning they were not designed to bear, and in this process the true molecular mechanisms may become obfuscated. Further, to infer a mechanism from a correlation is to assume that the correlation implies a causal relationship between the dependent and independent variables, when in fact the two may be conjoint but not causally related. Causality can be inferred only if a testable mechanistic hypothesis is offered that relates the correlation parameters to the observed biological activity.The attempt to correlate biological activity with chemical structure in quantitative terms assumes that a functional dependence exists between the observed biological response and certain physicochemical properties of molecules. Without a mechanistic model one obtains it by an empirical correlation. A rational way to define such empirical relationships is within the extrathermodynamic approach (1). Although extrathermodynamic relationships 0-8412-0521-3/79/47-112-021$13.55/0

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The Conjugative Ability of the Trimethylsilyl Group

Cited by 67 publications

References 6 publications

Plurilinear improvement of the Hammett equation

Plurilinear improvement of the Hammett equation

The electron spin resonance spectra of the phenyltrimethylsilane and -germane anions: ?-bonding between silicon and germanium and the aromatic ring

Parameters and Methods in Quantitative Structure-Activity Relationships

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