Halide ions and methyl

Table 5 The experimental and theoretical secondary a-deuterium KIEs and the components of the vibrational contribution to these KIEs for three SN2 reactions between halide ion and methyl halides at 300 K."...

Fig. 10 The secondary a-deuterium KIE for the identity SN2 reactions between halide ions and methyl halides versus the elongation of the C—X bond on going from the reactant to the transition state. Data from Glad and Jensen (1997), modified, with...

These methylene-bridged complexes III are extremely robust, air-stable substances. We have sought without success to effect insertions into the Pd-C bonds. The complexes are unreactive toward carbon monoxide (at 5 atm at 30°C) or sulfur dioxide. Reaction with methyl isocyanide or pyridine results in displacement of the terminal halide ions and produces cations that have been isolated as hexa-fluorophosphate salts [Pd2(dpm)2( -CH2)(CNCH3)2][PF6]2( (CN) = 2217 cm-1) and [Pd2(dpm)2(/Lt-CH2)(py)2][PF6]2. Treatment of III with fluoroboric or trifluoroacetic acid slowly results in the protonation of the methylene group which is converted into a terminal methyl group. The resulting brown complex, which has been isolated as its tetra-fluoroborate salt has been shown by H-l and P-31 NMR spectroscopy and X-ray crystallography to have Structure IV. 

Other theoretical studies discussed above include investigations of the potential energy profiles of 18 gas-phase identity S 2 reactions of methyl substrates using G2 quantum-chemical calculations," the transition structures, and secondary a-deuterium and solvent KIEs for the S 2 reaction between microsolvated fluoride ion and methyl halides,66 the S 2 reaction between ethylene oxide and guanine,37 the complexes formed between BF3 and MeOH, HOAc, dimethyl ether, diethyl ether, and ethylene oxide,38 the testing of a new nucleophilicity scale,98 the potential energy surfaces for the Sn2 reactions at carbon, silicon, and phosphorus,74 and a natural bond orbital-based CI/MP through-space/bond interaction analysis of the S 2 reaction between allyl bromide and ammonia.17... 

SN2 reactions between methyl p-nitrobenzenesulfonate and halide ions and amines in several different ionic liquids have been investigated.76 The rates of reaction can be understood using the Hughes-Ingold rules77 if one considers the ion-ion interactions and ion-dipole interactions that occur with the ionic liquid solvent. Changing the ionic liquid solvent can reverse the relative reactivity of Cl-, Br-, and I- nucleophiles. 

When an enolate ion is treated with an alkyl halide it results in a reaction called alkylation (Fig.E). The overall reaction involves the replacement of an a-proton with an alkyl group. The nucleophilic and electrophilic centres of the enolate ion and methyl iodide are shown (Fig.F). The enolate ion has its negative charge shared between the oxygen atom and the carbon atom because of resonance and so both of these atoms are nucleophilic centres. Iodomethane has a polar C—I bond where the iodine is a weak nucleophilic centre and the carbon is a good electrophilic centre. 

The choice of system deserves a few comments. Hydroxide ion and methyl halides were chosen for the present study. Nucleophilic displacement reactions have already been studied extensively in the gas phase (for reviews, see references 1 and 9-13) and current techniques are limited to reactions where only one of the two reactants is charged—substrate or nucleophile. Reactions with a negatively charged nucleophile offer an attractive choice because they have been investigated so extensively in solution. Methyl halides are the substrates of choice because elimination is not a possible pathway (4). Hydroxyl is a convenient nucleophile because its large hydration energies (14) minimize decomposition of the hydrated ions during their preparation and reaction. 

Since sodium methyl acetylide is the salt of the extremely weak acid, methyl acetylene, the acetylide ion is a stronger base, thus this reaction involves substitution of acetylide ion for halide ion. From this it can readily be seen that the metal ion, sodium, bonds to the released halide ion and the acetylide ion bonds to the alkyl group yielding a higher alkyne, plus a metal halide as the byproduct. 

Aikyiation of Acetyiide Anions (Section 7.5A) Acetylide anions are nucleophiles and will displace halide ion from methyl and 1° haloalkanes. 

Multi-redox-mediated electrosynthesis may significantly expand the scope of the electrosynthesis. Various combinations of different kinds of mediators have been smdied intensively so far. Nicotinamide adenine dinucleotide (NAD)/methyl viologen double mediatory systems are utilized successfully for various enzymatic reductions [14]. The combinations of halide ions and 2,2,6,6-tetramethylpiperidine nitroxyl (TMPO)... 

The culture of Pavlova pinguis (CCAP 940/2) permitted the study of a methyltransferase transforming the halide ions Cl, Br and I in monohalogenomethanes, the methyl group being derived exclusively from S-adenosylmethio-nine. The activity of this enzyme is optimal at pH 8.0, which corresponds almost exactly to that of seawater moreover, the enzyme is not inhibited by other halide ions, and activity decreases in the order Cl > Br< ) > I< ). However, the production of monohalogenomethanes in vivo was not detected in the culture media (Ohsawa et al., 2001). 

Electrophilic attack on the sulfur atom of thiiranes by alkyl halides does not give thiiranium salts but rather products derived from attack of the halide ion on the intermediate cyclic salt (B-81MI50602). Treatment of a s-2,3-dimethylthiirane with methyl iodide yields cis-2-butene by two possible mechanisms (Scheme 31). A stereoselective isomerization of alkenes is accomplished by conversion to a thiirane of opposite stereochemistry followed by desulfurization by methyl iodide (75TL2709). Treatment of thiiranes with alkyl chlorides and bromides gives 2-chloro- or 2-bromo-ethyl sulfides (Scheme 32). Intramolecular alkylation of the sulfur atom of a thiirane may occur if the geometry is favorable the intermediate sulfonium ions are unstable to nucleophilic attack and rearrangement may occur (Scheme 33). 

Halide ions may attack 5-substituted thiiranium ions at three sites the sulfur atom (Section 5.06.3.4.5), a ring carbon atom or an 5-alkyl carbon atom. In the highly sterically hindered salt (46) attack occurs only on sulfur (Scheme 62) or the S-methyl group (Scheme 89). The demethylation of (46) by bromide and chloride ion is the only example of attack on the carbon atom of the sulfur substituent in any thiiranium salt (78CC630). Iodide and fluoride ion (the latter in the presence of a crown ether) prefer to attack the sulfur atom of (46). cis-l-Methyl-2,3-di-t-butylthiiranium fluorosulfonate, despite being somewhat hindered, nevertheless is attacked at a ring carbon atom by chloride and bromide ions. The trans isomer could not be prepared its behavior to nucleophiles is therefore unknown (74JA3146). 

Among the cases in which this type of kinetics have been observed are the addition of hydrogen chloride to 2-methyl-1-butene, 2-methyl-2-butene, 1-mefliylcyclopentene, and cyclohexene. The addition of hydrogen bromide to cyclopentene also follows a third-order rate expression. The transition state associated with the third-order rate expression involves proton transfer to the alkene from one hydrogen halide molecule and capture of the halide ion from the second ... 

Nucleophilic displacement reactions One of the most common reactions in organic synthesis is the nucleophilic displacement reaction. The first attempt at a nucleophilic substitution reaction in a molten salt was carried out by Ford and co-workers [47, 48, 49]. FFere, the rates of reaction between halide ion (in the form of its tri-ethylammonium salt) and methyl tosylate in the molten salt triethylhexylammoni-um triethylhexylborate were studied (Scheme 5.1-20) and compared with similar reactions in dimethylformamide (DMF) and methanol. The reaction rates in the molten salt appeared to be intermediate in rate between methanol and DMF (a dipolar aprotic solvent loiown to accelerate Sn2 substitution reactions). 

Alpha hydrogen atoms of carbonyl compounds are weakly acidic and can be removed by strong bases, such as lithium diisopropylamide (LDA), to yield nucleophilic enolate ions. The most important reaction of enolate ions is their Sn2 alkylation with alkyl halides. The malonic ester synthesis converts an alkyl halide into a carboxylic acid with the addition of two carbon atoms. Similarly, the acetoacetic ester synthesis converts an alkyl halide into a methyl ketone. In addition, many carbonyl compounds, including ketones, esters, and nitriles, can be directly alkylated by treatment with LDA and an alkyl halide. 

In view of the chemical nature of alkylaluminums and methyl halides, complexation is most likely to be rapid and complete, i. e. K is large. Indeed Me3 Al and a variety of Lewis bases were found to complex rapidly2. Initiation, i.e., f-butyl cation attack on monomer, is also rapid since it is an ion molecule reaction which requires very little activation energy. Thus, it appears that Rj t. and hence initiator reactivity are determined by the rate of displacement Ri and ionization R2. 

Other carbanionic groups, such as acetylide ions, and ions derived from a-methylpyridines have also been used as nucleophiles. A particularly useful nucleophile is the methylsulfinyl carbanion (CH3SOCHJ), the conjugate base of DMSO, since the P-keto sulfoxide produced can easily be reduced to a methyl ketone (p. 549). The methylsulfonyl carbanion (CH3SO2CH2 ), the conjugate base of dimethyl sulfone, behaves similarly, and the product can be similarly reduced. Certain carboxylic esters, acyl halides, and DMF acylate 1,3-dithianes (see 10-10. )2008 Qxj(jatjye hydrolysis with NBS or NCS, a-keto aldehydes or a-... 

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