Mechanism SN2 type

In the case of electrophiles like Br", which can form cyclic intermediates, both 1,2-and 1,4-addition products can be rationalized as stemming from an intermediate like 17. Direct nucleophilic attack by W would give the 1,2 product, while the 1,4 product could be formed by attack at the 4 position, by an Sn2 type mechanism (see p. 422). Intermediates like 18 have been postulated but ruled out for Br and Cl by the observation that chlorination or bromination of butadiene gives trans 1,4... 

The small solvent isotope effect shows clearly that a proton transfer is not part of the rate-determining step of the hydrolysis. Christensen (1966, 1967) favors an SN2-type mechanism (164) for the hydrolysis, with nucleophilic attack of water on a sulfonyl group synchronous with the departure of ArSO. The alternative formulation (165), however, where a pentacovalent inter-... 

The H-site is formed of clusters ofnon-polar amino acid side chains which provide a highly hydrophobic surface, which, in the absence of a drug substrate is open to bulk solvent Binding of substrates to this site has been shown to relate to increased lipophilicity for substrates (4-hydroxyalkenes). The actual conjugation reaction, with the thiolate anion acting as a nucleophile proceeds via an SN2-type mechanism yielding the deactivated product... 

The relatively large value of 1.67 for s seems to fit an SN2 -Si or SN2 -Si mechanism better rather than an SN2-Si type mechanism. In addition, SN2-type mechanisms generally give a small negative p value. For example, solvolysis of primary alkyl p-toluenesulfonaies in ethanol has a p value of —0.746 [53]. The large positive p for base catalyzed hydrolysis is inconsistent with a simple SN2-type mechanism. 

Thus, to summarize the hydrolysis data, the alkyl substitutents on silicon were found to have a significant effect on the rates of aqueous hydrolysis of alkyl-trialkoxysilanes under basic and acidic conditions. The base- and acid-catalyzed rate constants were found to correlate well with modified Taft equations. The Taft coefficients, p and s, suggest a change in mechanism as the aqueous solution is varied from basic to acidic pH. Large positive. v and p under basic conditions suggest a two-step mechanism with a pentacoordinate intermediate. The smaller, positive p and s under acidic condition are consistent with an SN2-type mechanism. 

Complementary to the conjugate substitution reaction in which the nucleophile is transferred directly from the tetraalkyl ferrate to the allylic ligand, preformed low-valent Fe complexes can form reactive allyl-iron complexes via an SN2 -type mechanism (path C, Equations (7.8) and (7.9), Scheme 7.16], These complexes react with incoming nucleophiles and electrophiles in a substitution reaction. Depending on the nature ofthe iron complex employed in the reaction, either o- or Jt-allyl complexes are generated. 

Oxidation of sulfoxides with oxo(salen)Mn(V) complexes follows second-order kinetics. Sodium hypochlorite is used as a terminal oxidant. The oxidation of substituted sulfoxides yielded a reaction constant p = -2.57. The reduction of substituted Mn(V) complexes showed a reaction constant of 0.50. A valid reactivity-selectivity principle is operative in this system. An SN2- type mechanism has been proposed.44... 

An NMR study35 using the labelled epoxide (28) has shown that both the ringopening reaction by a rhodium tetraphenylporphyrin (27) and re-formation of the epoxide under basic conditions occur with inversion of configuration at the least substituted carbon (Scheme 13). This has been taken as evidence that both reactions occur by an SN2-type mechanism. 

Bachrach etal. investigated the mechanism for the reaction of thiolate (HS ) with 1,2-dithietane 17 (Equation 2) by applying B3LYP/ang-ee-pVDZ and MP2/6-31+G calculations. Both these methods concur on the SN2-type mechanism for a nucleophilic substitution at the sulfur atom in 1,2-dithietane <2002JOC8983>. 

The base catalyzed hydrolysis (figure 1.6) is a two-step process, with formation of a pentacoordinate intermediate. Acid catalyzed hydrolysis proceeds by an SN2-type mechanism. The leaving alkoxy group is rapidly protonated and a water molecule performs a nucleophilic attack at the central silicon atom. 

Unfortunately, the prevailing view expressed in the literature is not in agreement with the above conclusion the view that no SN 2 reactions are known which show isotope effects larger than about 1-04 (Humski et al., 1974) conflicts with the observations of Raaen et al. and Westaway (discussed above) which suggest to us that some Sn 2 reactions show a-deuterium isotope effects of at least 1-10. Observation of an a-deuterium isotope effect of 1-114 0-003 led Vitullo and Wilgis (19 75) to discount the classical SN2-type mechanism for the intramolecular substitution of the chloride [36] to give [37]. 

The addition of organic halides to electrophilic metal centers can follow several different mechanisms. For a concerted m-addition mechanism, a 3-center transition state of low polarity (21-XI) may be expected. Other possibilities include an SN2 type mechanism, involving either inversion (structure 21-XIIa) or retention (structure 21-XIIb) at the C atom, followed by dissociation of X" to give an ionic intermediate ... 

In accord with an SN2-type mechanism for intramolecular capture of the episulfonium ion, the phenylsulfenylation occurs with trans selectivity. The generality of this method is demonstrated by the selected examples presented in Table 2. 

In order to achieve high levels of diastereoselectivity, the attack of the nucleophile should proceed via an Sn2 type mechanism. This is the case when the acyloxy group at C2 forms a dioxolanium ion with the oxocarbenium ion. 

Existing evidence indicates that C-alkylations of metal enolates with common electrophiles proceeds by an SN2-type mechanism that is, the highest occupied molecular orbital (HOMO) of the enolate attacks the lowest unoccupied molecular orbital (LUMO) of the alkylating agent. Scheme 16 illustrates the principle of stereoelectronic control, which states that the electrophile should approach in a plane perpendicular to the enolate to allow maintenance of maximum orbital overlap in the transition state (24) between the developing C—C bond and the ir-orbital of the carbonyl group. 

Most of the enzyme modification reactions, and hence of the coupling reactions, are nucleophilic reactions, in particular bimolecular nucleophilic substitution reactions following an SN2-type mechanism. Therefore, the chemical reactivity is basically a function of nucleophilicity of the amino acid side chain. Following the overall nucleophilic order of Edwards and Pearson [22], the sulfhydryl group of cysteine is the most potent nucleophile in the protein, especially in its thiolate form. 

The detailed mechanism of oxidative addition is not known but it seems reasonable to assume that this is a simple one-step reaction. The above-mentioned stereochemical results allow one to reject a Sn2 type mechanism as was proposed for CH3I oxidative addition reactions (2S6). Moreover, a nucleophilic at-... 

Mechanistic studies with stereoisomeric 2,5-hexanediols showed that an intramolecular Sn2 type mechanism is operative high stereoselectivity with configurational inversion at one stereocenter has been reported [90,91,93]. meso-l.S-Hex-anediol (7), consequently, undergoes dehydration to yield tran5-2,5-dimethyloxo-lane (Scheme 6), whereas the racemic compound is converted to dy-2,5-dimethy-loxolane. 

The mechanism of the second-order term could not be established with any degree of certainty. One possibility discussed by the authors is an SN2-type mechanism although this would require an unusual seven coordinate metal complex at the transition state. Alternatively, the phosphine could first attack the carbene carbon to form 19 followed by loss of CO and migration of R3P to the metal as shown in equations (12). This latter mechanism found support in a subsequent study by... 

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