Halides, aryl, also iodides

Alkylation reactions are subject to the same constraints that affect all Sn2 reactions (Section 11.3). Thus, the leaving group X in the alkylating agent R—X can be chloride, bromide, iodide, or tosylate. The alkyl group R should be primary or methyl, and preferably should be allylic or benzylic. Secondary halides react poorly, and tertiary halides don t react at all because a competing E2 elimination of HX occurs instead. Vinylic and aryl halides are also unreactive because backside approach is sterically prevented. 

Palladium complexes also catalyze the carbonylation of halides. Aryl (see 13-13), vinylic, benzylic, and allylic halides (especially iodides) can be converted to carboxylic esters with CO, an alcohol or alkoxide, and a palladium complex. Similar reactivity was reported with vinyl triflates. Use of an amine instead of the alcohol or alkoxide leads to an amide. Reaction with an amine, AJBN, CO, and a tetraalkyltin catalyst also leads to an amide. Similar reaction with an alcohol, under Xe irradiation, leads to the ester. Benzylic and allylic halides were converted to carboxylic acids electrocatalytically, with CO and a cobalt imine complex. Vinylic halides were similarly converted with CO and nickel cyanide, under phase-transfer conditions. ... 

Tributylstannyl)-3-cyclobutene-1,2-diones and 4-methyl-3-(tributylstan-nyl)-3-cyclobutene-l,2-dione 2-ethylene acetals undergo the palladium/copper-catalyzed cross coupling with acyl halides, and palladium-catalyzed carbon-ylative cross coupling with aryl/heteroaryl iodides [45]. The coupling reaction of alkenyl (phenyl )iodonium triflates is also performed by a palladium/copper catalyst [46],... 

This reaction involves the two reactants carbon monoxide and alcohol and produces esters, or lactones. The starting material, which will be considered here, is an alkene or an alkyne but it is also possible to start from activated halides (aryl- or allyl- iodides and bromides) to produce the same kind of organic products. 

In a few instances, it is also possible to generate unsymmetrical biaryls using Ullmann s arylcopper compound generated in situ. In these cases, one employs a mixture of an aryl iodide and another aryl halide (not an iodide ) and the other aryl halide must exhibit a higher propensity than the aryl iodide to couple to the arylcopper intermediate (example Figure 16.5, bottom). This is referred to as a crossed Ullmann coupling. 

Acceleration by KI in the substitution reaction of aryl halides with potassium diethyl phosphite or with the 2-naphthoxide ion has also been explained on the basis of an ET through the exciplex of the charge-transfer complex formed between the aryl halide and the iodide ions34a. It has also been reported that iodide ions catalysed the photostimulated reaction of bromoarenes with diethyl phosphite ion34b. 

Intramolecular aryl halide animations were also conducted with the original catalysts [99]. For example, the reactions in Eq. (6) proceeded with yields in excess of 80 %. In this case, the halide could be iodide or bromide, and [Pd(PPh3)4] proved to be a more effective catalyst than Pd[P(o-C6H4Me)3]2Cl2. ... 

The Ullmann reaction (Figure 13.4) represents another synthesis of substituted biphenyls. In this process an aryl iodide or—as in the present case—an aryl iodide/aryl chloride mixture is heated with Cu powder. It is presumed that under standard conditions the aryl iodide reacts in situ with Cu to form the aryl copper compound. Usually, the latter couples with the remaining aryl iodide and a symmetric biphenyl is formed. In a few instances it is also possible to generate asymmetric biaryls via a crossed Ullmann reaction. In these cases one employs a mixture of an aryl iodide and another aryl halide (not an iodide ) the other aryl halide must exhibit a higher propensity than the aryl iodide to couple to the arylcopper intermediate. It is presumed that the mechanism of the Ullmann reaction parallels the mechanism of the Cadiot-Chodkiewicz coupling, which we will discuss in Section 13.4. 

When Me3SiSnBu3 is treated with CsF, the fluoride anion should coordinate to the silyl group and not the stannyl group to produce a hypervalent silicate, and as a result, a stannyl anion would be generated.282 The stannyl anion reacts with vinyl iodide to produce a vinyl anion via a halogen-metal exchange and it reacts with the carbonyl group intramolecularly (Equation (113)). Aryl halides or allyl halides are also used in similar cyclizations.283,284... 

This new multicomponent cascade reaction can also be performed with a large variety of aryl halides, most favorably iodides (Scheme 7). The yields in this coupling-cycloaddition sequence are particularly high with the parent... 

Castro et al. 44) found that the order of reactivity of various halides in coupling reactions with ethynylcopper compounds in DMF is ArSX, ArX > RCOX ArCHaX, RCH=CHCH2X, ArCOCHaX > RCH= CHX, Alkyl-X. A somewhat different order is observed for the reactivity of some organic halides towards perhaloarylcopper reagents in THF (97, 147) Allyl Aryl > Alkyl and perfluoroaryl, perfluoro-vinyl > aryl > perfluoroalkyl. Acid halides were also far more reactive than aryl iodides in ethereal solvents 146). The ease of halide displacement is I > Br > Cl. 

Direct trifluoromethylsulfuration of aryl halides can also be achieved by employing methyl difluoro(fluorosulfonyl)acetate in the presence of sulfur and copper iodide (see Table 5). ... 

Highly selective synthetic transformations can be performed readily by taking advantage of the che-moselectivity of SmI . It has been pointed out that there is a tremendous reactivity differential in the Bar-bier-type reaction between primary organic iodides or tosylates on the one hand, and organic chlorides on the other. As expected, selective alkylation of ketones can be accomplished by utilizing appropriately functionalized dihalides or chlorosulfonates (equation 24). Alkenyl halides and, presumably, aryl halides can also be tolerated under these reaction conditions. 

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