Hypercoordinate intermediates

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)]. 

Recent examples of experimental and computational studies showing the involvement of hypercoordinate intermediates are cyclometallation of benzylamine with palladium acetate (91) and cyclopalladation of aryl imino-phosphoranes (92). ... 

The reaction was first described for pinacol (2,3-dimethyl-2,3-butanediol), a ditertiary 1,2-diol and catalyzed by sulfuric acid (73). Now it is known that the pinacol rearrangement is characteristic of all types of 1,2-diols, and most electrophilic catalysts are capable of promoting the process. Two possible mechanisms may account for the experimental observations. The stepwise mechanism involves the )8-hydroxy carbocationic intermediate 6 and the pentacoordinate carbocation 7 (Scheme 1). According to the concerted mechanism, the product carbonyl compoimd is formed through the hypercoordinate intermediate 8. 

It has been well recognized that the hydrolysis of alkoxysilanes and chlorosilanes is effectively catalyzed when fluoride anions are present due to formation of hypercoordinated silicon intermediates.803 More in-depth studies by Bassindale et al. showed that the reaction of PhSi(OEt)3 with stoichiometric amounts of Bu4NF surprisingly yields an encapsulation complex, namely tetrabutylammonium octaphenyloctasilsesquioxane fluoride 830, in which the fluorine atom is situated inside the cubic siloxane cage (Scheme 114). The Si--F distance of average 2.65 A is shorter than the sum of van der Waals radii (3.57 A), which renders the coordination number of the silicon atoms at [4+1]. 

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. 

All these experiments illustrate the great reactivity of hypervalent species. They confirm the possibility of pentacoordinated intermediates in the nucleophilic activation. This possibility cannot be ruled out only on the basis of the argument of a more crowded and less electrophilic species than tetracoordinated silicon. Furthermore, after these results, it becomes interesting to understand why these hypercoordinated species react faster than the tetracoordinated species. Two possible explanations are the increase in the length of Si —X bonds which corresponds to a higher lability and the increase of the electrophilicity of the central silicon atom. 

Keywords silicon complexes, reactive intermediates, hypercoordination... 

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. 

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. 

Clear evidence for a C-C protonated C4H1C ion (55) (which would resemble 52) has been obtained by Siskin/ while studying the I II TaFs catalyzed ethylation of excess ethane with ethylene in a flow system [Eq. (5.12)]. n-Butane was obtained as the only product no isobutane was detected. This remarkable result can be explained by C-H bond ethylation of ethane by the ethyl cation, thus producing the hypercoordinate 55 carbocation intermediate, which, subsequently, by proton elimination, yields n-butane (56). The use of a flow system that limits the contact of the product n-butane (56) with the acid catalyst is essential. Prolonged contact causes isomerization of n-butane to isobutane to occur (see Chapter 6). 

The trishomocyclopropenium ion (CeH/, 199) was first proposed by Winstein and coworkers as an intermediate in the solvolysis of czs-bicyclo[3.1.0] hexyl tosylate and extensive efforts were directed toward its generation under stable ion conditions. The persistent cation 199 was first prepared by Masamune et al. by the ionization of czx-bicyclo[3.1.0]hexane in superacid media and it has since been generated from the corresponding alcohol [Eq. (5.29)]. The NMR spectra of structure 199 are consistent with an ion of Csv symmetry. The three equivalent C-H groups are found at high field in the C NMR spectrum (8 C 4.9,7c h = 195.4Hz) in accordance with their hypercoordinate environments. 

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. 

First, we survey the major types of compounds that contain hypercarbon. The relationships that link these apparently disparate species are demonstrated by showing how the bonding problems they pose can be solved by the use of three- or multicenter electron-pair bond descriptions or simple MO treatments. We also show the role played by hypercoordinated carbon intermediates in many familiar reactions (carbocationic or otherwise). Our aim here is to demonstrate that carbon atoms in general can increase their coordination numbers in a whole range in reactions. 

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... 

---