Transition states hypercoordinated

Many more cyclic and polycyclic equilibrating carbocations have been reported. Some representative examples, namely, the bisadamantyl (499),859 2-norbornyl (500),40 7-perhydropentalenyl (501),188 9-decalyl (502),188 and pentacylopropylethyl (503)860 cations, are given in Scheme 3.19. All these systems again involve hypercoordinate high-lying intermediates or transition states. 

The much more highly charged silicon atom can interact far more readily with nucleophiles. Silyl cations may even be complexed simultaneously and symmetrically by two electron pair donors (hypercoordination), in contrast to carbocations. With ammonia, the methyl cation gives the very stable protonated methyl amine, H3C-NH3 a second ammonia molecule is only weakly bound to this complex. If both NH3 groups are forced to be equidistant from carbon, a Sn2 transition state results, 20 kcal mol" higher in energy than the minimum. 

To show how the study of hypercarbon compounds helps us to understand the mechanisms of many organic reactions, reactions in which carbon atoms become temporarily hypercoordinated in intermediates or transition states even though the reagents and products contain only normally coordinated carbon atoms. 

Hie involvement of hypercoordinated carbon species in Sn2 reactions was commented on in Section 1.4 (Fig. 1.10). Compounds have been synthesized that keep the displaced and displacing atoms close to the carbon atom undergoing nucleophilic substitution, in order that the relative energies of the classically bonded reagent or product and the hypercoordinated transition state can be both more readily assessed and modihed (see Chapter 6). 

For many years, a lively controversy centered over the actual existence of nonclassical carbocalions. " The focus of argument was whether nonclassical cations, such as the norbornyl cation, are bona fide delocalized bridged intermediates or merely transition states of rapidly equilibrating carbenium ions. Considerable experimental and theoretical effort has been directed toward resolving this problem. Finally, unequivocal experimental evidence, notably from solution and solid-state C NMR spectroscopy and electron spectroscopy for chemical analysis (ESCA), and even X-ray crystallography, has been obtained supporting the nonclassical carbocation structures that are now recognized as hypercoordinate ions. In the context of hypercarbon compounds, these ions will be reviewed. 

The cyclopentyl cation (34) shows a single peak in the ll NMR spectrum of 5 H 4.75 even at -150°C. In the NMR spectrum/ a 10-line multiplet centered around 95,4ppm with. /< h = 28.5Hz was observed.This is in excellent agreement with values calculated for simple alkyl cations and cyclopentane and supports the complete hydrogen equilibration by rapid 1,2-shifts (through the 35 hypercoordinate intermediate or transition state) [Eq. (6.24)]. 

Computational studies of the Rh-P,N system have shown that the C-C activation product is the most stable (A < -12kcalmol relative to the C-H activation product) and its formation is fast and irreversible. C-H activation is fast and reversible. When choosing structure 132 as the entry channel, structure 133 is the common intermediate for both C-H and C-C activations and structure 134 is a possible transition state, all having hypercoordinate carbon atoms. 

Shea and coworkers have recently reported the reaction of 1 -boraadamantane with dimethylsulfoxonium methylidc to give a polymethylene pro uct. Methylene incorporation takes place through migratory insertion invo vg hypercoordinate transition state 152 (Scheme 6.39). Repetitive homo lead to macrocyclic organoboranes, which, upon oxidation, give a three a star polymethylene polymer. 

Nucleophilic substitution on silicon—stable hypercoordinated species Another demonstration of the role of ionic structures is the nucleophilic substitution on Si, which proceeds via pentacoordinated intermediates [81,82], in contrast to the situation in carbon where the pentacoordinated species is a transition state. Recently, Lauvergnat et al. [83], Shurki et al. [84], Sini et al. [85], and Shaik et al. [86] have performed BOVB/6-31G (and a few other basis sets) calculations for a C-X and Si-X bonds (X = H, F, Cl) and made an interesting observation that the minimum of the ionic curve... 

Using the above basic ideas it is possible to discuss reactivity patterns and transition state structure in a variety of organic reactions, and to understand the variation of isoelectronic species from transition states to stable hypercoordinated clusters of metallic and nonmetallic elements. 

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