Iodide-induced elimination

Dehalogenation of vicinal dibromides by sodium iodide in acetone or alcoholic solvents, viz. 

For the reaction of sym-tetrabromoethane with iodide ion in methanol a secondary isotope effect (Ah/ d) of 1.28 at 80°C was observed . On theoretical grounds, an inverse isotope effect ( h/ d — 0.7) is predicted for a rate-determining displacement reaction and a normal isotope effect for an elimination reaction . Methyl and bromine substituents have similar spatial requirements, so it is thus not surprising that the direct elimination mechanism, which is observed for 2,3-dibromobutane, is followed by jyw-tetrabromoethane. 

The need for coplanarity of the eliminating halogens in the transition state is confirmed by studies with cyclohexyl derivatives. In the rigid cumbersome triterpenoid series, 11,12-diequatorial dibromides are inert but the diaxial isomers undergo a/iri-elimination with iodide ion - . In the cyclohexyl system, /ra 5-l, 2-dihalides (or halide-sulphonate esters) undergo rapid 

Although little is known concerning the identity of the transition state for these iodide induced reactions, the strict adherence to anti stereospecificity indicates the importance of rr-orbital overlap and a high degree of double bond character seems probable. The 6-fold rate difference (almost the maximum predicted from eclipsing of methyl substituents) between the rates of formation of Irani-and c/i-2-butenes from 2.3-dibromobutane supports this hypothesis -.  

An ElcB reaction or elimination from mono- or di-iodo intermediates are suggestions which have been proposed to account for the iodide-induced eliminations of toluenesulphonate esters of 1,2-diols in the carbohydrate, alicyclic and steroid fields. In the reaction of 1,2-ditosyloxy cyclohexanes with iodide ion, the rate of appearance of iodine is slower than that of sodium p-toluenesulphonate . This observation excludes a direct elimination reaction on the substrates and the following schemes were proposed (181). 

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The generation of an alkene by the reaction of a v/c-disulfonate ester with iodide (the Tipson-Cohen reaction) has been known since 1943 and in some cases it has proved useful where other methods have failed, as in the preparation of the spirocyclic triene (54 Scheme 22). The mechanism probably involves an initial nucleophilic displacement to give an iodohydrin sulfonate, which then undergoes iodide-induced elimination to the alkene. Methanesulfonates can be used as well as arenesulfonates. 

Thieno-o-quinodimethanes 46 and 48, generated in situ by iodide-induced 1,4-elimination from the respective 2,3-bis(chloromethyl)thiophene 45 and 2,3-bis(bromomethyl)benzo[b]thiophene 47 precursors, undergo Diels Alder... 

The regioselective addition of nitryl iodide to alkenes, followed by base-induced elimination, gives nitroalkenes. Nitryl iodide is generally prepared by the reaction of AgN02 and iodine. 

The reactive indolo-2,3-quinodimethanes are generated in situ generally from N-protected 2,3-disubstituted indoles (514,515). Generation of reactive indolo-2,3-quinodimethanes was achieved by fluoride-induced, 1,4-elimination of silylated indolyl ammonium salts, and was applied in the synthesis of substituted tetrahydrocarbazoles (516). Subsequently, the iodide-induced 1,4-elimination of ]V-benzoyl-2,3-bis(bromomethyl)indole (534) methodology was developed for the synthesis of reactive indolo-2,3-quinodimethanes and was applied for the first time in the synthesis of substituted carbazoles (e.g., 536) (517) (Scheme 5.14). 

These cycloadditions with o-quinodimethanes provide a broad variety of useful fullerene functionalizations, since o-quinodimethanes can be prepared using several routes and the resulting cycloadducts are thermally stable [42], There exist several alternatives to the iodide-induced bromine 1,4-elimination of 1,2-bis (bromomethyl)-benzenes [44-47]. o-Quinodimethanes have been prepared by thermolysis of 3-isochromanone (42) [43], benzocyclobutenes (43) [48-50], isobenzothiophene 2,2-dioxides (44) [42] and sultines [51,52] or by photolysis of o-alkylphenones such as 45 [53-55] and could be added to Cjq in good yields (Scheme 4.7). Indene, thermally rearranged to isoindene, also adds to Cjq in similar fashion to quinodimethanes [56]. 

In a related Sequence of reactions, cyclization of 549 with POCl3 and then aqueous workup produced the carbinolamine 552. N-Methylation of 552 with methyl iodide followed by treatment with strong base resulted in an intramolecular Cannizzaro process to deliver the intermediate 553, which was converted to ( )-tazettine (397) by tosylation and base-induced elimination (215). 

A stereoselective synthesis of the (Z)-isomer of a,3-unsaturated esters (Scheme 9) from a-phenylthio-3-keto esters results from an iodide-induced anti elimination of the pyridinium salt (22) An analogous reaction based on a 3-phenylseleno alcohol derivative (Scheme 10) was exploited in a synthesis of the... 

A(, -Dimethyl(methylene)iminium salts have been the most widely used class of preformed iminium salts, mainly due to their applications in the synthesis of a,p-unsaturated carbonyl compounds, normally accomplished by subjecting the /V, -dimethyl Mannich base to quatemarization followed by base-induced elimination. Table 3 outlines various counterion forms of /V, -dimethyl(methylene)iminium salts that have been used in Mannich reactions as well as their synthetic precursors. The crystalline iodide (30), known also as Eschenmoser s salt , has seen the most widespread use and is prepared by thermal fragmentation of (iodomethyl)trimethylammonium iodide or, more conveniently, by a variant of the... 

Scheme 21 Xenon difluoride-induced aryl-iodide reductive elimination from Pd complexes. The reaction is very general with regard to the aryl group involved...

The NO2 radicals obtained from nitrite ions in acidic solution add to olefins regioselectively. The resulting radical is oxidized in situ by CAN to provide nitroolefins, even labile ones, in high yield and short reaction times.32 The second reaction in Fig. 5 shows another preparation by the addition of nitryl iodide, from potassium nitrite and iodine, followed by a base-induced elimination of hydrogen iodide. A radical mechanism is probably not involved. This mild one-pot method, a valuable alternative to other published techniques, is accomplished in a non-aqueous medium and gives good yields and regioselectivities even with sterically hindered substrates. 

One of the more fundamental methods for generation of the o-xylylene intermediate is 1,4-elimination from o-xylene derivatives. o-Xylene di-, tri-, or tetrabromides undergo intramolecular 1,4-dehalogenation by means of sodium iodide [102], lithium [103], zinc [104], copper [105], iron [106], and chromium [107]. Hoffman degradation of (o-methylbenzyl)trimethylammonium hydroxides [108], as well as the more recently developed fluoride ion-induced elimination from [o-[a-(trimethylsilyl)alkyl]benzyl]trimethylammonium halides [109], have also been effectively utilized in the generation of 2. 

C-C bonds can be formed by reaction with alkyl iodides or more usefully by reaction with metal carbonyls to give aldehydes and ketones e.g. Ni(CO)4 reacts with LiR to form an unstable acyl nickel carbonyl complex which can be attacked by electrophiles such as H+ or R Br to give aldehydes or ketones by solvent-induced reductive elimination ... 

These compounds contain a furan ring fused to a benzene moiety in the 2,3-position. This synthesis was also described by Flynn et al. [73] and is shown in Scheme 25 involved the coupling of 2-iodo-5-methoxyphenol 104, 4-methoxyphenylethyne 105 to form the intermediate o-alkynylphenolate 106. Aryl iodide 107 was added to the phenolate in DMSO with heat. Oxidative addition, palladium(II)-induced cyclization and reductive elimination resulted in the product 108 with an 88% yield. 

The o-quinodimethane (40) may also be generated by fluoride-induced 1/4/elimination of trimethyl(2-[(trimethylsilyl)methyl]-benzyl) ammonium iodide (43). When this elimination is carried out at low temperature, the reaction conditions are ecjually compatible with the presence of 27, and this provides an additional route to 42. ... 

Scheme 6.33 illustrates an example of some zinc-induced three-component coupling reactions of alkyl iodides, electron-deficient alkenes, and carbonyl compounds [51]. In this instance, the isopropyl radical is generated by a one-electron reduction of isopropyl iodide followed by elimination of iodide ion. The resulting radical then adds to acrylonitrile to form an a-cyano alkyl radical, which is con-... 

Addition of DBU to a solution of 149 in THF induced an elimination reaction accompanied by loss of a molecule of CO2 and provided the unstable amine 150, which was converted in situ into isocyanate 151 by reaction with phosgene and triethylamine. After filtration to remove hydrochloride salts, the solution of 151 was treated with samarium (II) iodide in the presence of lithium chloride. These conditions, which had been previously determined to be optimal for spirooxindole generation on a model system, provided compound 152 as an inseparable 7 1 mixture of diastereoisomers [43]. The major component of this mixture was determined by NOE analysis to have the required configuration, which is consistent with bond formation from the less hindered, convex face of 151 (Scheme 35). 

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