Halides, alkyl, reaction with Subject

The reaction of benzene with a carbocation leads to an arene in what is known as Friedel-Crafts alkylation. The reaction of an alkyl halide with a strong Lewis acid gives a carbocation, which is subject to rearrangement. Friedel-Crafts alkylation reactions are subject to poly alkylation because the arene is more reactive than benzene. 

Only activated aryl halides (e.g. 2,4-dinitrochlorobenzene and picryl chloride) react with pyridine, and the facility of these reactions is subject to much the same steric and electronic controls as alkylation. Picryl chloride forms characteristic yellow TV-picryl derivatives with pyridines. These have been used to help separate, purify and characterize particular liquid pyridines. Pyridine also undergoes quaternization with diphenyliodonium fluoroborate (equation 19). Treatment of pyridine 1-oxides with arenediazonium salts yields aryloxy quaternary salts (equation 20). 

Homofamesyl iodide 7 was prepared by the reaction sequence shown in the margin. Of interest here is the two-step transformation of an alkyl halide into a Crextended alkyl halide.9 Compound 30 is first subjected to a nucleophilic substitution by an urganolithium species with formation of a homoallylic phenyl thioether This is then methylated in a second step to an intermediate sulfonium salt. The final SN2 reaction with an iodide ion releases thioanisol as a stable leaving group to give compound 7. 

Although compounds of composition P(OR)3 are well known and are useful as ligands (Section 10-17) and in other ways, they are subject to the Arbuzov reaction with alkyl halides, which converts them to diesters of phosphonic acids ... 

Very recently, the preparation of zinc /3-diiminate derivatives became the subject of interest due to their catalytic activity in the copolymerization of CO2 with epoxidesd They can be prepared in high yields by direct reaction of the corresponding /3-ditmines with diaUcylzinc compounds or by treatment of the /3-diimine-lithium salts with zinc halide followed by an alkylation reaction (Scheme 9). 

The indirect reduction of many organic substrates, in particular alkyl and aryl halides, by means of radical anions of aromatic and heteroaromatic compounds has been the subject of numerous papers over the last 25 years [98-121]. Many issues have been addressed, ranging from the exploration of synthetic aspects to quantitative descriptions of the kinetics involved. Saveant et al. coined the expression redox catalysis for an indirect reduction, in which the homogeneous reaction is a pure electron-transfer reaction with no chemical modification of the mediator (i.e., no ligand transfer, hydrogen abstraction, or hydride shift reactions). In the following we will consider such reactions and derive the relevant kinetic equations to show the kind of kinetic information that can be extracted. 

The ability to open the furan ring by oxidation has also been utilized for synthesis of trialkylacetic acids A 2-furoic ester is first subjected to a Friedel-Crafts reaction with a tertiary alkyl halide, and the 5-alkyl-2-furoic acid obtained from the product by hydrolysis is then oxidized by alkaline permanganate to the trialkylacetic acid 154... 

The alkylation, with the more reactive of alkyl halides, of the sodium salts of monoes-terified phosphonothioic acids (equation 22) (see also Scheme 11) or of the disodium salts 107 results in preferential S-alkylation, and the same situation obtains for the salts of phosphinothioic acid " methylation can also be carried out with dimethyl sulphate. Alkylations may also be performed under phase-transfer conditions. From both practical and theoretical perspectives, the subject is more complex, since the course of alkylation reactions depends on the nature of the alkylating agent, on the polarity of solvent and whether this is protic or non-protic and on the concentrations of reactants a study of these features has been the subject of two reports In non-polar or weakly polar aprotic media, or in EtOH, alkylation occurs almost exclusively on sulphur, but in dipolar aprotic solvents, O-alkylation also takes place. The relative yields of sulphur- and oxygen-substituted derivatives, [Qs/Qol depends, for a given solvent, on the nature of substituents on phosphorus, i.e. essentially, whether the substrate is a thiophosphoric, thiophosphonic or thiophosphinic acid. With alkyl tosylates as alkylating agents at 0.02 m in hmpt, the alkylation of sodium 0,0-dialkyl or diphenyl phosphorothioates results in 100% overall conversions with [Qs/Qo] 5 the overall yields for sodium diphenyl- or diisopropyl-phosphinothioates are lower (50-100%) with [Qs/Qo] 1 ... 

Alkyl halides react with Lewis acids to form tertiary, secondary, or primary carbocations. The tertiary cation (160, r1=r2=r3= alkyl) is sufficiently stable that good yields of alkylated products are obtained upon reaction with benzene. Formation of the primary cation (160, R1=r2=H, R = alkyl) is energetically more difficult and is, of course, subject to rearrangement primary alkyl derivatives are rarely observed under these conditions. Secondary cations (160, R =H, r2=R = alkyl) are readily formed but are also subject to rearrangement. The extent of rearrangement is dependent on the reaction conditions and the nature of the... 

The most common route to sulphenes, the elimination of hydrogen halide from a sulphonyl halide, has been the subject of a recent mechanistic and kinetic study which led to the conclusion that this reaction most probably proceeds by an El mechanism. Tsuge and Iwanami used this reaction for the generation of benzoylsulphene (130) from benzoylmethane-sulphonyl chloride. The reaction of (130) with a series of anils (131) was found to be somewhat dependent on the presence or absence of triethylamine and very dependent on the nature of R in (131). For R = alkyl, the (4 -I- 2) cycloadduct (132) is the predominant or only product, whereas for R = aryl the (2 + 2) cycloadduct (133) is the reaction product. Similar results were obtained for the reaction of (130) with carbodi-imides. The reaction of (130) with aryl cinnamylideneamines gave Diels-Alder adducts (134). ... 

The first problem with Friedel-Crafts alkylation is therefore that alkyl cations are subject to rearrangement. Assume that primary cations obtained from primary halides will always rearrange before they react with benzene. Secondary halides lead to secondary cations, which may react directly or rearrange, depending of the substituent pattern. The reaction of 2-bromopropane... 

A great deal of work has been done on the use of cobalt-cyanide complexes as catalysts for homogeneous hydrogenation, as is evinced by the citation of 144 references in a recent review of this subject. Basic to the behaviour of [Co(CN)5] as a hydrogenation catalyst is its reaction with molecular hydrogen. The latest contribution to the study of the mechanism of this reaction concludes that it is concerted and is not, as are the reactions with hydrogen peroxide and alkyl halides, a stepwise process with free-radical intermediates. Whether this concerted mechanism involves a termolecular reaction according to the stoicheiometry ... 

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