Delocalized electrons stereochemistry

Nonclassical ions, a term first used by John Roberts (an outstanding Caltech chemist and pioneer in the field), were defined by Paul Bartlett of Harvard as containing too few electrons to allow a pair for each bond i.e., they must contain delocalized (T-electrons. This is where the question stood in the early 1960s. The structure of the intermediate 2-norbornyl ion could only be suggested indirectly from rate (kinetic) data and observation of stereochemistry no direct observation or structural study was possible at the time. 

Electrocyclic processes are also observed with ionic systems, both cationic and anionic. The decisive factor in the stereochemistry is the number of electrons in the delocalized polyenic system. Thus the mode of ring opening or ring closure depends on the n electrons in the open polyene. 

There are two other mechanistic possibilities, halogen atom abstraction (HAA) and halonium ion abstraction (EL), represented in Schemes 4.4 and 4.5, respectively, so as to display the stereochemistry of the reaction. Both reactions are expected to be faster than outer-sphere electron transfer, owing to stabilizing interactions in the transition state. They are also anticipated to both exhibit antiperiplanar preference, owing to partial delocalization over the C—C—Br framework of the unpaired electron in the HAA case or the electron pair in the EL case. Both mechanisms are compatible with the fact that the activation entropies are about the same as with outer-sphere electron donors (here, aromatic anion radicals). The bromine atom indeed bears three electron pairs located in two orthogonal 4p orbitals, perpendicular to the C—Br bond and in one s orbital. Bonded interactions in the transition... 

In the ground state, aminomethylenemalonates possess an essentially planar geometry, which maximizes the electron delocalization in the molecules. In the heteropolar transition state, the plane of the groups R3 and NR R2 and the plane of the two carbonyl groups occupy orthogonal positions. More details of the dynamic and static stereochemistry of push-pull ethylenes, as in compounds 1 and 2, are discussed in two excellent reviews (73TS295 83TS83). 

Photoinduced electron transfer promoted cyclization reactions of a-silyl-methyl amines have been described by two groups. The group of Pandey cyclized amines of type 135 obtaining pyrrolidines and piperidines 139 in high yields [148]. The cyclization of the a-silylated amine 140 leads to a 1 1 mixture of the isomers 141 and 142 [149]. The absence of diastereoselectivity in comparison to analogous 3-substituted-5-hexenyl radical carbocyclization stereochemistry [9] supports the notion that a reaction pathway via a free radical is unlikely in this photocyclization. The proposed mechanism involves delocalized a-silylmethyl amine radical cations as reactive intermediates. For stereochemical purposes, Pandey has investigated the cyclization reaction of 143, yielding... 

In the extreme class III behaviour,360-362 two types of structures were envisaged clusters and infinite lattices (Table 17). The latter, class IIIB behaviour, has been known for a number of years in the nonstoichiometric sulfides of copper (see ref. 10, p. 1142), and particularly in the double layer structure of K[Cu4S3],382 which exhibits the electrical conductivity and the reflectivity typical of a metal. The former, class IIIA behaviour, was looked for in the polynuclear clusters of copper(I) Cu gX, species, especially where X = sulfur, but no mixed valence copper(I)/(II) clusters with class IIIA behaviour have been identified to date. Mixed valence copper(I)/(II) complexes of class II behaviour (Table 17) have properties intermediate between those of class I and class III. The local copper(I)/(II) stereochemistry is well defined and the same for all Cu atoms present, and the single odd electron is associated with both Cu atoms, i.e. delocalized between them, but will have a normal spin-only magnetic moment. The complexes will be semiconductors and the d-d spectra of the odd electron will involve a near normal copper(II)-type spectrum (see Section 53.4.4.5), but in addition a unique band may be observed associated with an intervalence CuVCu11 charge transfer band (IVTC) (Table 19). While these requirements are fairly clear,360,362 their realization for specific systems is not so clearly established. 

Thus the changes of the stereochemistry always depend on electronic factors the more delocalized nucleophilic anions react mainly with inversion, the ones with more concentrated charge with retention. 

Now we are ready to start the derivation of the intermediate scheme bridging quantum and classical descriptions of molecular PES. The basic idea underlying the whole derivation is that the experimental fact that the numerous MM models of molecular PES and the VSEPR model of stereochemistry are that successful, as reported in the literature, must have a theoretical explanation [21], The only way to obtain such an explanation is to perform a derivation departing from a certain form of the trial wave function of electrons in a molecule. QM methods employing the trial wave function of the self consistent field (or equivalently Hartree-Fock-Roothaan) approximation can hardly be used to base such a derivation upon, as these methods result in an inherently delocalized and therefore nontransferable description of the molecular electronic structure in terms of canonical MOs. Subsequent a posteriori localization... 

The dicyano complexes [Fe(bipy)2(CN)2]X and [Fe(phen)2(CN)2]X (X = C104, N03) are also low-spin with magnetic properties63 and Mossbauer spectra59 (Table 1) comparable with those of the tris complexes. Little or no tlg electron delocalization was apparent in the magnetic results. The stereochemistry of these mixed ligand complexes is presumably cis. 

The investigations carried out in this area were done primarily to determine the magnitudes of steric and electronic effects on the solvolytic rates and products of reaction in the cyclopropylcarbinyl cation system. The goal of most of these studies was to learn more about the nature of the charge delocalization in the cyclopropylcarbinyl system and of the stereochemistry of the cyclopropylcarbinyl-cyclobutyl and cyclopropyl-carbinyl-allylcarbinyl cation rearrangements. Key papers in these studies were those in 1966 by Schleyer and Van Dine, in 1971 by Majerski and Schleyer and in 1974 by Poulter and Spillner which demonstrated that in the simple cyclopropylcarbinyl system... 

It is thus clear that orbital localization permits justification of the classical concepts of localized binding and lone electron pairs, as opposed to delocalized n systems, as well as the notion of directed valency, a foundation of stereochemistry. Nevertheless, it must be noted that even if localization were perfect, the notion of bond electron pairs would be conventional In fact, if the descriptions in terms of the initial and the localized orbitals are fuUy equivalent in the sense that the total wave function is the same, quantities associated with a single pair of electrons are not invariant under the transformation. It has no absolute... 

The hardness of Mg(II) and Mn(II) ions (relative to Cu, Zn, Fe, Co, or Ni) prevents extensive donation of rr-electrons from ligands to metal within the metal complexes. Thus it is unlikely that the earlier proposed extensive delocalization of electrons could be the basis of catalysis in metal-dependent proteases. This conclusion has been verified by more recent spectroscopic and magnetic data, along with quantum mechanical calculations related to stereochemistry, reactivity, d-d transitions, and charge transfer complex formation . 

Radicals substituted a to the amide linkage, 24, have been used in several studies to control stereochemistry in radical transformations, while radicals substituted a to esters, 25, and ethers, 26, have been used on a few occasions. Resonance structures for each of these radicals (A and B) can be written as shown in 24-26, with stabilization resulting from delocalization of the odd electron into the adjacent functional group. This resonance delocalization also restricts the geometry of these radicals, maximum delocalization being obtained when overlap between the radical and adjacent group is highest. 

The TT electrons of carbon-carbon double bonds can also become involved in nucleophilic substitution reactions. This participation can facilitate the ionization step if it leads to a carbocation having special stability. Solvolysis reactions of the syn and anti isomers of 7-norbornenyl tosylates provide some dramatic examples of the influence of participation by double bonds on reaction rates and stereochemistry. The anfi-tosylate is more reactive by a factor of about 10 than the saturated analog toward acetolysis. The reaction product, anft -7-acetoxynorbornene, is the product of retention of configuration. These results can be explained by participation of the tt electrons of the double bond to give the ion 3, which is stabilized by delocalization of the positive charge. ... 

Five of the next seven chapters cover the reactions of hydrocarbons—compounds that contain only carbon and hydrogen. The other two chapters treat topics that are so important to the study of organic reactions that each deserves its own chapter. The first of these is stereochemistry and the second is electron delocalization and resonance. 

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