Aryl halides trimethylsilylation

A number of approaches have been tried for modified halo-de-diazoniations using l-aryl-3,3-dialkyltriazenes, which form diazonium ions in an acid-catalyzed hydrolysis (see Sec. 13.4). Treatment of such triazenes with trimethylsilyl halides in acetonitrile at 60 °C resulted in the rapid evolution of nitrogen and in the formation of aryl halides (Ku and Barrio, 1981) without an electron transfer reagent or another catalyst. Yields with silyl bromide and with silyl iodide were 60-95%. The authors explain the reaction as shown in (Scheme 10-30). The formation of the intermediate is indicated by higher yields if electron-withdrawing substituents (X = CN, COCH3) are present. In the opinion of the present author, it is likely that the dissociation of this intermediate is not a concerted reaction, but that the dissociation of the A-aryl bond to form an aryl cation is followed by the addition of the halide. The reaction is therefore mechanistically not related to the homolytic halo-de-diazoniations. 

Tunney, S.E. and Stille, J.K., Palladium-catalyzed coupling of aryl halides with (trimethylstannyl)diphenylphosphine and (trimethylsilyl)diphenylphos-phine, /. Org. Chem., 52, 748, 1987. 

Halopyridines, like simple carbocyclic aryl halides, are viable substrates for Pd-catalyzed crosscoupling reactions with terminal acetylenes in the presence of Pd/Cu catalyst. The Sonogashira reaction of 2,6-dibromopyridine with trimethylsilylacetylene afforded 2,6-bis(trimethylsilyl-ethynyl)pyridine (130), which was subsequently hydrolyzed with dilute alkali to provide an efficient access to 2,6-diethynylpyridine (131) [106]. Extensions of the reactions to 2-chloropyridine, 2-bromopyridine, and 3-bromopyridine were also successful albeit at elevated temperatures [107]. 

S)-(-)-CITRONELLOL from geraniol. An asymmetrically catalyzed Diels-Alder reaction is used to prepare (1 R)-1,3,4-TRIMETHYL-3-C YCLOHEXENE-1 -CARBOXALDEHYDE with an (acyloxy)borane complex derived from L-(+)-tartaric acid as the catalyst. A high-yield procedure for the rearrangement of epoxides to carbonyl compounds catalyzed by METHYLALUMINUM BIS(4-BROMO-2,6-DI-tert-BUTYLPHENOXIDE) is demonstrated with a preparation of DIPHENYL-ACETALDEHYDE from stilbene oxide. A palladium/copper catalyst system is used to prepare (Z)-2-BROMO-5-(TRIMETHYLSILYL)-2-PENTEN-4-YNOIC ACID ETHYL ESTER. The coupling of vinyl and aryl halides with acetylenes is a powerful carbon-carbon bond-forming reaction, particularly valuable for the construction of such enyne systems. 

The mixed triarylphosphine 787 can be prepared by the reaction of (trimethylsilyl)diphenylphosphine (786) with aryl halides[647]. Ph3P is converted into the alkenylphosphonium salt 788 by the reaction of alkenyl tri-flates[648]. 

Trimethylsilyl groups also may be lost from vinylic or allylic positions in the arylation reaction.70 Halide ion facilitates die desilylation as well as the loss of alkoxy and acetoxy groups. Inclusion of soluble silver salts in reactions where trimethylsilyl groups are lost prevents this reaction.70 The reaction in the absence of silver ion is useful for preparing styrene derivatives. However, they also may be prepared directly from aryl halides and ethylene in fair to good yields.71... 

As already shown by Moore and co-workers9 and Tour and coworkers,10 addition of methyl iodide to a triazene resin at elevated temperature (110°) gives rise to aryliodides 17-1 (Nu = I) in excellent yields. We have shown that aryl halides 17-X (X = Cl, Br, I) are readily available by the action of lithium halides in the presence of an acidic ion-exchange resin or with the corresponding trimethylsilyl halide at room temperature.26 A mixture of acetic anhydride and acetic acid produces phenol acetates 17-OAc.26... 

Trimethylsilyl chloride has been employed in some synthetically useful reductions of aryl halides. The electrochemical reductive trimethylsilylation of aryl chlorides is a good... 

Reduction of an aryl halide at a cadmium-modified nickel cathode in DMF containing TBABF4 leads to a formylation reaction between aryl carbanions and the solvent [186]. Two papers [187,188] have appeared in which reduction of aryl halides gives an aryl carbanion, which, by acting as a base to deprotonate a suitable nitrile, can cause coupling of the nitrile with esters, aldehydes, and ketones. Electrochemical trimethylsilylation of aryl halides can be effected at a stainless-steel or carbon-cloth cathode in THF-HMPA containing TEABF4 and trimethylchlorosilane [189]. 

Tunney and Stille88 described the coupling of diphenyl(trimethylsilyl)phosphine or (trimethylstan-nyl)diphenylphosphine with various aryl halides, in the presence of a palladium catalyst, to give aryldiphenylphosphines, e.g., (56), according to Equation (17). Unfortunately the reaction conditions are not compatible with functional groups such as N02, CHO, NH2, or OH. 

Site-specific tritium-labeled arenes. Taylor reports that tritium-labeled aromatics can be obtained by wetting dried ether with tritiated water, adding an aryl halide, cooling to -70°, and then adding n-butyllithium. Trimethylsilyl derivatives of aromatics can be prepared using trimethylchlorosilane in the same way. When the organometaUic intermediate is formed first and then treated with tritiated water, only the unlabeled hydrocarbon is obtained in some cases. Apparently, the cross-metalation reaction is faster than the reaction of -butyl-lithium with either water or trimethylchlorosilane. 

Representative examples of Arbuzov reactions with alkyl phosphinites and phos-phonites are the preparation of some bisphosphinates (equation 122) functionalized trialkylphosphine oxides (equation 123) and phosphorylated sarcosine analogues (equation 124). Examples involving aryl halides (equation 125) ", a vinyl halide (equation 126) , an alkynyl halide (equation 127) and an acyl halide (equation 128) illustrate the diversity of the reaction. Arbuzov reactions that involve trimethylsilyl phosphinites or phosphonites occur under very mild conditions, usually at or below room temperature, e.g. equation 129 ... 

The preparation of quinolizinium salt from ketone has been extended to oxime, nitrile, and trimethylsilyl groups in addition, the quinolizinium salt has also been prepared directly from the reaction between pyridine and aryl halide under heating or photoirradiation. 

The terminal alkyne can also be generated in situ via copper-free Sonogashira cross-coupling of trimethylsilyl acetylene (5a) and aryl halides 6 and subsequent cleavage with TBAF, caibonylation, and cyclization (Scheme 26) (20090L(11)3210). This microwave-assisted sequence allows the rapid synthesis of the title compounds in moderate to good yields. Each compound can be easily varied with exception to trimethylsilyl acetylene. The application of electron-rich aryl bromides and iodides expectedly decreases the yields. 

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