Mechanisms allylic halide

Much of our understanding about conjugation effects in allylic systems comes from rate and product studies of nucleophilic substitution, especially those that take place by the SnI mechanism. Allylic halides react faster than their nonallylic counterparts in both S l and Sn2 reactions, but for different reasons. SnI reactions will be described first, followed later in this section by Sn2. 

Lithiation at C2 can also be the starting point for 2-arylatioii or vinylation. The lithiated indoles can be converted to stannanes or zinc reagents which can undergo Pd-catalysed coupling with aryl, vinyl, benzyl and allyl halides or sulfonates. The mechanism of the coupling reaction involves formation of a disubstituted palladium intermediate by a combination of ligand exchange and oxidative addition. Phosphine catalysts and salts are often important reaction components. 

These mechanisms ascribe in jortance to the Lewis acid-Lewis base interaction between the allyl halide and the organolithium reagent. When substitution is complete, the halide ion is incorporated into the lifliium cluster in place of one of the carbon ligands. 

Allylic bromination, 339-340 mechanism of, 339-340 Allylic carbocation, electrostatic potential map of, 377, 489 resonance in, 488-489 SN1 reaction and, 376-377 stability of, 488-489 Allylic halide, S l reaction and. 377 S j2 reaction and, 377-378 Allylic protons, ]H NMR spectroscopy and, 457-458... 

Alkylation by allylic halides is usually a satisfactory reaction, and in this case the reaction may proceed through a cyclic mechanism.81 For example, when l-14C-allyl chloride reacts with phenyllithium, about three-fourths of the product has the labeled carbon at the terminal methylene group. 

Allylic halides usually give both SN2 products and products of substitution with an allylic shift (SN2 products) although the mixed organocopper reagent RCu— BF3 is reported to give mainly the SN2 product.22 Allylic acetates undergo displacement with an allylic shift (SN2 mechanism).23 The allylic substitution process may involve initial... 

The conjugate addition reactions probably occur by a mechanism similar to that for substitution on allylic halides. There is probably an initial complex between the cuprate and enone.44 The key intermediate for formation of the new carbon-carbon bond is an adduct formed between the enone and the organocopper reagent. The adduct is formulated as a Cu(III) species which then undergoes reductive elimination. The lithium ion also plays a key role, presumably by Lewis acid coordination at the carbonyl oxygen.45 Solvent... 

Despite the lower reactivity of solvated perfluoroalkylzinc reagents, perfluoroalkyl iodides undergo synthetically useful zinc-mediated reactions under Barbier conditions which often employ ultrasound and co-catalysts. Under these conditions, the zinc reagents are not well characterized and radical intermediates and SET mechanisms are proposed in some cases. Representative examples are presented below and include the ultrasound-promoted, zinc-mediated perfluoroalkylation of various substrates as reported by Ishikawa and coworkers (Scheme 10)123 126. Yields of carbinols could be improved by use of Ti(II) co-catalyst. Ultrasound promoted the coupling of perfluoroalkyl iodides with vinyl and allyl halides in the presence of Pd co-catalysts. 

Halogenations may also occur by a free-radical mechanism.121,218 Besides taking place in the gas phase, halogenation may follow a free-radical pathway in the liquid phase in nonpolar solvents. Radical halogenation is initiated by the alkene and favored by high alkene concentrations. It is usually retarded by oxygen and yields substitution products, mainly allylic halides. 

Reactivities comparable to allylic halides are found in the nucleophilic displacement reactions of benzylic halides by SN1 and SN2 mechanisms (Table 14-6). The ability of the benzylic halides to undergo SK1 reactions clearly is related to the stability of the resulting benzylic cations, the electrons of which are extensively delocalized. Thus, for phenylmethyl chloride,... 

There have been many proposals for the occurrence of a so-called SN2 mechanism that would produce a concerted substitution with rearrangement for allylic halides. One possible example is... 

If an allyl halide is used as the quaternizing agent, an intramolecular 1,5-cyclization of the ylid may take place. In this way, various dihydro-indolizines, which are prone to oxidation and thus difficult to prepare, have been made.67 The probable mechanism is shown in Scheme 9. 

Allyl complexes of the type ri1-allyl-Fp are prepared by reaction of [Cp(CO)2Fe] Na+ with allyl halides or, alternatively, by deprotonation of [Ti2-alkene-Fp]+ complexes. The most important reaction of V-allyl-Fp complexes is the [3+ 2]-cycloaddition with electron-deficient alkenes [33]. The reaction proceeds via a non-concerted mechanism, to afford Fp-substituted cydopentanes (Scheme 1.10). 

The reaction of allylic halides with nickel tetracarbonyl to form coupled products has been known for over two decades (9), but it is only in recent years that an insight into the mechanism has been obtained. Isolation of the intermediate 7r-allylnickel complexes and the discovery that these react with activated olefins and organic halides in general have led to a considerable increase in the scope of the reaction. 

Benzyl halides have been reported to react with nickel carbonyl to give both coupling and carbonylation (59). Carbonylation is the principal reaction in polar nonaromatic solvents, giving ethyl phenylacetate in ethanol, and bibenzyl ketone in DMF. The reaction course is probably similar to that of allylic halides. Pentafluorophenyl iodide gives a mixture of coupled product and decafluorobenzophenone. A radical mechanism has been proposed (60). Aromatic iodides are readily carbonylated by nickel carbonyl to give esters in alcoholic solvents or diketones in ethereal solvent (57). Mixtures of carbon monoxide and acetylene react less readily with iodobenzene, and it is only at 320° C and 30 atm pressure that a high yield of benzoyl propionate can be obtained (61). Under the reaction conditions used, the... 

In a thorough study it was determined that the Barbier coupling of allylic halides 69a to aldehydes, ketones, or a,p-unsaturated carbonyl compounds 68 catalyzed by Cp2TiCl2 47a in the presence of manganese as the terminal reducing agent proceeds most likely via a radical mechanism (Fig. 21) [149,150], The active titanocene(III)... 

Retention in the reactions of 15 is established both from presumed retention in the Sn-Li exchange step of a stannylation-destannylation sequence and by evidence that the s-BuLi-(-)-sparteine complex used to make the organolithium reliably removes the pro-/ proton adjacent to a carbamate (see below for crystallographic evidence involving a similar compound).11 The stereochemistry of the products 16, almost all formed essentially in enantiomerically pure form, was proved for the C02 adduct and the Mel adduct by comparison with known compounds. The only electrophiles for which incomplete retention of stereochemistry has been observed are the benzylic or allylic halides. These probably react in part by single electron transfer SE1 mechanisms, rather than by partial SE2inv.15 For example, 15 reacts with allyl bromide to give 16 (E = allyl) with only 42% ee. 

Evidence based on product mixtures now suggests, at least in the cases of a-halocarbonyl and perhaloalkyl starting marterials, that these reactions are in fact atom transfer radical cyclizations (equation 166)324,325. In them, the palladium catalyst is proposed to have roles both as the radical initiator and as a trap for iodine, similar to the more commonly used hexabutylditin. Intramolecular allyl halide-alkyne cyclizations proceed with trans-addition to the triple bond this is evidence that a still different mechanism may be operating in these cases (equation 167)1,326. 

XT Then certain allyl halides react with a suitable inorganic salt in the presence of a heterocyclic monomer, polymerization ensues (J). If the halide is a polymeric halide, graft copolymer results. The objectives of the present work are (1) to synthesize and characterize such graft copolymers and (2) to test some of their mechanical properties and relate them to the content of branch which has been grafted from the polymer. 

Allylic halides and tosylates show enhanced reactivity toward nucleophilic displacement reactions by the Sn2 mechanism. For example, allyl bromide reacts with nucleophiles by the SN2 mechanism about 40 times faster than n-propyl bromide. 

Alternatively, allyl halides are fluorinated with metal fluorides in general, yields are poor except for cyclic systems (Table 7), and for fluorinated halogenated propanes, e.g. 2, in which the fluoriiiation proceeds through an S>,2 mechanism. ... 

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