Carbenes selectivity index

To further characterize the selectivity of carbene CXY, we can compare its discrimination between the olefins of the standard set with that of CCI2, which we designate the standard carbene. We plot the logarithms of the relative reactivities for CXY against analogous data for CCI2 (with the relative reactivities all adjusted to the standard alkene, isobutene, = 1.00), and then determine the slope of the correlation line, Wcxy- We call m xy the carbene selectivity index, defined as the least-squares slope of log (V o)cxy s. log (kJk QQX. [8,9,16,17] The m xv values for the carbenes of Table 1, determined in this manner, are m p = 1.48 mcci, 1-00 (by definition), and m Br, 0-65. 

The correlation between the carbcnc selectivity and its substitution has been disclosed In Moss s classical work (ll- He defined a selectivity index mexY of a carbene as the slope of the linear correlations obtained by plotting [log(fc/Ao)] c fK versus [log(/fMo)lcc/2 corresponds to the cyclo-... 

The relative reactivity of alkenes toward various carbenes and carbenoids has been compiled Recently, a selectivity index, mcxv which can be estimated from the dual... 

Dibromocarbene is one of the most reactive (the least selective) carbenes (Houben-Weyl, Vol. E19b, p 1598). Its selectivity index 0.65, if generated from bromoform and potassium... 

Benzyloxy(cyano)carbene would be expected to be electrophilic by virtue of the calculated selectivity index, Wp -HjOccN 111- This has not yet been experimentally proved as 3-bcnzyloxy-3-cyano-3//-diazirine is rather labile and cannot be isolated if it is synthesized by the substitution of chlorine by a cyano group in 3-benzyloxy-3-chloro-3if-diazirine consequently, this substitution reaction is carried out in the presence of an alkene, hence, preparing 1-benzyloxy-cyclopropane-l-carbonitrile derivatives. However, the cyanide ion present in the system induces the polymerization of electrophilic alkenes, such as acrylonitrile or methyl acrylate. 

The results with dimethoxyearbene highlight an inherent deficiency of Eq. 4 it is an empirical correlation of parameters normalized to the electrophilic car-bene, CCI2. Its electrophilic heritage means that although the equation can predict values for highly resonance stabilized, nucleophilic carbenes such as (MeO)2C or Me2NCOMe, these are virtual selectivity indexes. The nucleophilic carbenes simply do not add to the aUcenes of the standard set. However, the equation helps us define the Wcxy regions in which electrophilic and nucleophilic carbenes reside. The electrophilic species, which react appropriately with the standard alkenes of Table 1, exhibit w xy values between 0.29 (BrCCOOEt) and... 

Schoeller and Brinker applied single electron perturbation theory to carbene/ alkene additions and elaborated a construct that was almost indentical to our FMO analysis. [74] Schoeller also focused on estimating LUMO(carbene)/ HOMO(alkene) and LUMO(alkene)/HOMO(carbene) differential energies, and derived a selectivity index , S, which roughly paralleled and predicted the electrophilicity of the dihalocarbenes and the nucelophilicity of (e.g.), C(NH2)2, C(0H)2, and C(SMe)2. [74]... 

Flowever, PhCOMe has m icalcd) = 1.34. According to our empirical analysis of carbenic selectivity (Section 1.2), it should be an electrophile because its WcxY selectivity index lies between 0.29 and 1.48. What s wrong FMO... 

The electrophilicity index also accounts for the electrophilic activation/deactivation effects promoted by EW and electron-releasing substituents even beyond the case of cycloaddition processes. These effects are assessed as responses at the active site of the molecules. The empirical Hammett-like relationships found between the global and local electrophilicity indexes and the reaction rate coefficients correctly account for the substrate selectivity in Friedel-Crafts reactions, the reactivity of carbenium ions, the hydrolysis of esters, the reactivity at the carbon-carbon double bonds in conjugated Michael additions, the philicity pattern of carbenes and the superelectrophilicity of nitronium, oxonium and carboxonium ions. This last application is a very promising area of application. The enhanced electrophilicity pattern in these series results from... 

Our empirical analysis of selectivity in the carbene/alkene cyclopropanation reaction has now furnished both a quantitative index of selectivity and examples... 

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