Halide-assisted conditions

In addition, thioglycosylations using halide-assisted conditions (O Scheme 8) [75] and Koenigs-Knorr conditions (O Scheme 9) [76], the latter both with stannyl sulfides and free thiols, have been described. 

An alternative view of these addition reactions is that the rate-determining step is halide-assisted proton transfer, followed by capture of the carbocation, with or without rearrangement Bromide ion accelerates addition of HBr to 1-, 2-, and 4-octene in 20% trifluoroacetic acid in CH2CI2. In the same system, 3,3-dimethyl-1-butene shows substantial rearrangement Even 1- and 2-octene show some evidence of rearrangement, as detected by hydride shifts. These results can all be accoimted for by a halide-assisted protonation. The key intermediate in this mechanism is an ion sandwich. An estimation of the fate of the 2-octyl cation under these conditions has been made ... 

Dialkyl peroxides can be obtained by the alkylation of alkyl hydroperoxides with alkyl halides. Mild conditions can be achieved by assisting the departure of the halide ion with the aid of a suitable silver salt (Eq. 19)3S). 

Pyrazinoquinodimethanes were found to undergo 4+2 cycloaddition to [60]fullerene under thermal- or microwave irradiation-assisted conditions <97JOC3705>. The base-mediated alkylation of quinoxalin-2(l/f)-one with co-phenylalkyl halides affords a mixture of N- and O-functionalized products that are separable by chromatography <97H357>. 

Recently, the reaction has been found to proceed using on-water microwave-assisted conditions [52], Barluenga et al. have also reported a three-component variation of the methodology, in which an initial palladium-catalyzed alkenyl halide amination was used to generate the imine component in situ [53]. 

Palladium-catalyzed aminations of aryl halides is now a well-documented process [86-88], Heo et al. showed that amino-substituted 2-pyridones 54 and 55 can be prepared in a two-step procedure via a microwave-assisted Buchwald-Hartwig amination reaction of 5- or 6-bromo-2-benzyloxypyri-dines 50 and 51 followed by a hydrogenolysis of the benzyl ether 52 and 53, as outlined in Fig. 9 [89]. The actual microwave-assisted Buchwald-Hartwig coupling was not performed directly at the 2-pyridone scaffold, but instead at the intermediate pyridine. Initially, the reaction was performed at 150 °C for 10 min with Pd2(dba)3 as the palladium source, which provided both the desired amino-pyridines (65% yield) as well as the debrominated pyridine. After improving the conditions, the best temperature and time to use proved... 

Use of mild conditions was crucial and the development of diimide reduction of singlet oxygenates, silver-salt-assisted displacement of halide by peroxide nucleophiles, peroxymercuration and demercuration, peroxide transfer from organotin to alkyl triflates, and oxygen trapping of azoalkane-derived diradicals have all played a part in providing the rich harvest of new bicyclic peroxides described herein. 

Direct high-temperature microwave-assisted aminations of a variety of aryl halides [257] or aryl triflates [258] under transition metal-free conditions have also been reported, probably involving a benzyne mechanism. 

Selective N-alkylation of 6-amino-2-thiouracil with different halides has been performed efficiently by use of MW-assisted methods in the presence of small amounts of DMF to improve energy transfer (Eq. 34 and Tab. 5.12). No reaction was observed under the same conditions in a thermoregulated oil bath. 

The earliest developments in transition metal-assisted formation of aromatic C-P linkages were the result of the efforts of Tavs28 that were concerned with the use of Ni(II) halide salts for the reaction of aryl halides with trialkyl phosphites. These reactions involved conditions reminiscent of the Michaelis-Arbuzov reaction (heating at an elevated temperature) and produced arylphosphonate products in reasonable yield (Figure 6.8). 

Silver(I)-ion- assisted Me2SO oxidations of organic halides have been observed for primary and secondary bromides and iodides (174). The reactions occur under ambient conditions yielding the corresponding carbonyl, plus traces of alcohol. The fate of the silver ion is unreported. 

The addition of a hydrogen-donor solvent to the sub-bituminous coal at these mild conditions, appears to assist in melting the coal to permit access of the molten halide catalyst. 

Under these conditions, silver-assisted electrocyclic ring opening provided the haloallyl cation, which was subsequently trapped by isocyanate anion. Interception of the cationic species with isocyanate was successful since bromide was removed from the reaction mixture as a precipitate (AgBr). Finally, treatment of intermediate 9 with methanol furnished the desired carbamate in 96% yield. This example demonstrates the usefulness of the silver(I)-mediated process. Removal of free halide from the reaction mixture affords a long-lived cationic species that can be captured by a different nucleophile, such as solvent, the silver(I) counteranion, or an intramolecular nucleophile. This reactivity has been exploited in many different ways throughout the years and is examined in greater detail later in this chapter. 

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