Electrophiles carbonyls, relative reactivity

Cyclopropane formation occurs from reactions between diazo compounds and alkenes, catalyzed by a wide variety of transition-metal compounds [7-9], that involve the addition of a carbene entity to a C-C double bond. This transformation is stereospecific and generally occurs with electron-rich alkenes, including substituted olefins, dienes, and vinyl ethers, but not a,(J-unsaturated carbonyl compounds or nitriles [23,24], Relative reactivities portray a highly electrophilic intermediate and an early transition state for cyclopropanation reactions [15,25], accounting in part for the relative difficulty in controlling selectivity. For intermolecular reactions, the formation of geometrical isomers, regioisomers from reactions with dienes, and enantiomers must all be taken into account. 

Although borane appears superficially similar to borohydride, it is not an ion and that makes all the difference to its reactivity. Whereas borohydride reacts best with the most electrophilic carbonyl groups, borane s reactivity is dominated by its desire to accept an electron pair into its empty p orbital. In the context of carbonyl group reductions, this means that it reduces electron-rich carbonyl groups fastest. The carbonyl groups of acyl chlorides and esters are relatively electron-poor (Cl and OR are very electronegative) borane will not touch acyl chlorides and reduces esters only slowly. But it will reduce amides. 

The oxygen-substituted 1,3-azoles exist in their carbonyl tautomeric forms. The bromination of thiazol-2-one, at C-5, is a nice demonstration of relative reactivity here the double bond carries both sulfur and nitrogen, and it is the latter, i.e. the enamide rather than the enethiol ester character, that dictates the site of electrophilic attack. ... 

There are some anomalous cases in which a specific atom shows both high electrophilicity and nucleophilicity due to the limitation of various basis set-dependent charge calculation procedures, and hence it is more appropriate to rationahze this concept of relative electrophilicity/nucleophUicity. Relative nucleophilicity is the nucleophilicity of a site relative to its own electrophilicity, and vice versa for relative electrophilicity. The idea of relative nucleophilicity/electrophilic-ity was first proposed by Roy et al. [81] to predict intramolecular reactivity sequences of carbonyl compounds. We have used a similar ratio for the first time to find the best di-octahedral smectite for nitrogen heterocyclics adsorption in terms of intermolecular interaction [82] and as well for the adsorption property of para and meta substituted nitrobenzene [83]. 

AT-heterocyclic carbenes show a pure donor nature. Comparing them to other monodentate ligands such as phosphines and amines on several metal-carbonyl complexes showed the significantly increased donor capacity relative to phosphines, even to trialkylphosphines, while the 7r-acceptor capability of the NHCs is in the order of those of nitriles and pyridine [29]. This was used to synthesize the metathesis catalysts discussed in the next section. Experimental evidence comes from the fact that it has been shown for several metals that an exchange of phosphines versus NHCs proceeds rapidly and without the need of an excess quantity of the NHC. X-ray structures of the NHC complexes show exceptionally long metal-carbon bonds indicating a different type of bond compared to the Schrock-type carbene double bond. As a result, the reactivity of these NHC complexes is also unique. They are relatively resistant towards an attack by nucleophiles and electrophiles at the divalent carbon atom. 

The reactivity of these metal hydride-metal carbonyl reactions can be correlated with the nature of the reactants in a manner consistent with the proposed mechanism nucleophilic attack by hydride on coordinated CO. Thus reactions involving the highly nucleophilic group IV hydride, Cp gZrHg, are much faster than those of group V metal hydrides. On the other hand, the relatively electrophilic neutral binary metal carbonyls all react with Cp2NbH under mild conditions (20-50° C), whereas more electron-rich complexes such as cyclopentadienylmetal carbonyls (Cp2NbH(C0), CpV(CO) ) or anionic carbonyls (V(CO)g ) show no reaction under these conditions. 

The different reactivity of the two double bonds of 20 has therefore been related to the relative energies of the transition states leading to the intermediates 24a and 24b. The preferential addition of an electrophile at the y—S bond has been attributed to its more nucleophilic character and to the fact that conjugation with the ester carbonyl is not disrupted. Furthermore, with the assumption that a later transition state should favor attack at the a—ft bond, since a more stable (delocalized) intermediate can be formed, and taking into account that product distribution data show that the lower-energy transition state leading to addition to the y—S bond is favored with these electrophiles (chlorine and bromine), it has been concluded that the chlorine reaction has an earlier transition state than the bromine one in accordance with the relative product distribution data. 

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