Nucleophilic substitution hypervalent

Very recent extensions of the formalism of Thiel and Voityuk to AMI have been reported by multiple groups. Voityuk and Rosch (2000) first described an AMl/d parameter set for Mo, and, using the same name for the method, Lopez and York (2003) reported a parameter set for P designed specifically to facilitate the study of nucleophilic substitutions of biological phosphates. Winget et al. (2003) described an alternative model, named AMI, that adds d orbitals to P, S, and Cl. As with MNDO/d, the primary improvement of this model is in its general ability to describe hypervalent molecules more accurately. Subtle differences in the various individual formalisms will not be further delved into here. 

On the other hand, evidence against an intermediate hypervalent tetracoordinated phosphorus anion in nucleophilic substitutions at phosphorus in tertiary phosphines was put forward by Kyba the fact that the substitution reaction 3 occurs with complete inversion of configuration at phosphorus was interpreted to mean that it proceeds without even one pseudorotation of 9, which makes the passage through such an intermediate unlikely ( ). 

The increasing interest in extra-coordinate derivatives of group 14 elements is caused by their structural peculiarities, their high reactivity and also by the possibility of using them for studying dynamic processes and modeling the pathway of nucleophilic substitution reactions at the central atoms. However, only a few publications deal with a model of hypervalency. 

As already mentioned, hypervalent iodine reagents are useful for the oxidatively assisted nucleophilic substitution of iodine in iodoalkanes (Section 5.4.2). In this way HTI was used for the preparation of some tosylates. A rearrangement occurred with neopentyl iodide, Me3CCH2I, which was converted into the ester Me2C(OTs)CH2Me (85%) [53]. The most successful application of such reactions for preparative purposes was the functionalization of some iodocubanes and iodohomocubanes, and especially the preparation of their triflates and mesylates from iodosylbenzene-trimethylsilyl triflate and [hydroxy(mesyloxy)iodo]benzene, respectively [54,55]. 

The kinetics of nucleophilic substitution at the silicon atom assisted by uncharged nucleophiles have been studied by Corriu et at. (248-251). Hydrolysis of triorganochlorosilanes induced with HMPA, DMSO, and DMF was used as the model. The reaction proceeded according to the third-order kinetic law, first order with respect to the nucleophile, the silane, and the silylation substrate. Very low values of activation enthalpy and high negative entropy of activation were observed (Table VI). These results were taken as evidence for the intermediacy of silicon hypervalent species (249,251) however, they are also perfectly consistent with... 

Silicon compounds with coordination number larger than four are the object of many studies first with respect to their application as catalysts in organic and inorganic syntheses and second as starting materials for the preparation of a broad variety of organosilicon compounds [1]. Additionally, hypervalent silicon hydride compounds can successfully be used as model compounds to study, for instance, the mechanism of nucleophilic substitution reactions, which is of great interest since the silicon atom is able to easily extend its coordination number [1]. Moreover, hypervalent silanes are suitable as starting materials for the synthesis and stabilization of low-valent silanediyl transition metal complexes [2-5]. 

In hypervalent sulfur compounds like (37) and (38), the apical bonds formed by the /r-orbitals are longer and more polar than the equatorial bonds. The special stability of many polycoordinated sulfur compounds is still concluded to be associated with the added orbital interaction with the energetically accessible 3J-orbitals. The discovery of sulfuranes has provided valuable insight into many nucleophilic substitution reactions on polycoordinate sulfur atoms and into ligand-coupling reactions which occur via a-sulfurane intermediates. 

Phosphoranide anion 1 is a stable lO-P-4 (7) hypervalent anionic species that could be considered as a transition state for a nucleophilic substitution reaction at a three coordinate phosphine center (2). The stability of 1 (5) can be attributed to several factors. 

In organic sulfur chemistry, these decet species have long been considered to be transition states or unstable intermediates in the nucleophilic substitution reaction (SN2), and not to exist as real molecules except in a few cases [13]. However, recently numerous heteroatom compounds containing hypervalent structures have proved to be of considerable interest to many organic chemists. 

Another important example of hypervalent bonding is the transition state in the Sn2 nucleophilic substitution reaction, which has the same orbital configuration and electron count as a 10-electron hypervalent system. For nucleophilic substitution, the hypervalent species is a transition state rather than a stable species, as shown schematically in Figure 5. Experimental and computational studies on transition states are generally difficult. Stable hypervalent systems can serve as more tractable benchmarks to test the accuracy of computational techniques used on 10-electron systems. 

Figure 5. Qualitative potential energy surfaces comparing two ten-electron systems, (a) An unstable nucleophilic substitution (Sn2) intermediate, (b) A stable hypervalent molecule.

The new methods (Schemes 23-25) valuable for the synthesis of complex pyrroloiminoquinone alkaloids are based on the hypervalent iodine-induced nucleophilic substitution of p-substituted phenol ethers via reactive cation radical intermediates. Thus, we found a novel hypervalent iodine induced nucleophilic substitution of p-substituted phenol ethers in the presence of a variety of nucleophiles, such as TMSN3, TMSOAc, and (3-diketones, etc., in 1994. For this reaction the reaction solvent was quite important, and CF3CH2OH and (CF3)2CHOH worked very well (Scheme 23) [76]. 

Scheme 23 Hypervalent iodine-induced nucleophilic substitution of p-substituted phenol ethers...

Kita Y, Tohma H, Hatanaka K, Takada T, Fujita S, Mitoh S, Sakurai H, Oka S (1994) Hypervalent iodine-induced nucleophilic substitution of para-substituted phenol ethers. Generation of cation radicals as reactive intermediates. J Am Chem Soc 116 3684-3691... 

This is a synthetically valuable process, as illustrated by the hypervalent iodine-mediated oxidative nucleophilic substitution of 269 with the silyl enol ether 271, leading to the highly functionalized naphthoid cyclohexa-2,4-dienone 272 (Scheme 3.113), which is an important intermediate product in the synthesis of aquayamycin-type angucyclinones [343,344],... 

The direct nucleophilic substitution of electron-rich phenol ethers using hypervalent iodine oxidants in the presence of Lewis acid or fluorinated alcohols and involving aromatic cation-radical intermediates was originally developed by Kita and coworkers in 1994 [362], Since then this procedure with some variations has been extensively applied by Kita and other researchers for various oxidative transformations. In the intermolecular mode, this reaction (Scheme 3.122) has been utilized for the preparation of the products 298 from N3, AcO , ArS, SCN , 3-dicarbony 1 compounds and other external nucleophiles [320]. The oxidative coupling reaction in the intramolecular mode provides a powerful synthetic tool for the preparation of various... 

Silicon-based Polymeric Materials Mechanistic Organosilicon Chemistry (a) Gas Phase and Photochemical Reactions (b) Hypervalent Silicon, Nucleophilic Substitution, and Biotransformations Structural Organosilicon Chemistry and New Organosilicon Compounds Organic Synthesis using Siiicon. 

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