Carboxylic acid halides vinyl substitutions

Vinyl Substitutions with Carboxylic Acid Halides, Aryl Sulflnates and Arylsulfonyl Chlorides 856... 

The range of applicability of the Wittig reaction has in recent years become extremely extensive and there have been reports of its use for the preparation of alkyl- and aryl-substituted alkenes, unsaturated aldehydes, ketones, and carboxylic acid derivatives, vinyl halides, and vinyl ethers. In the preparation of these compounds there is often the possibility of cis-trans isomerism Bestmann and Kratzer1003 state that the trans-olefin is always obtained when an alkylidenetricyclohexylphosphorane is used, and Schlosser and Christ-... 

There are a few efficient methods for the stereoselective synthesis of vinyl halides, and this transformation remains a synthetic challenge. Research by S. Roy showed that the Hunsdiecker reaction can be made metal free and catalytic catalytic Hunsdiecker reaction) and can be used to prepare ( )-vinyl halides from aromatic a,p-unsaturated carboxylic acids. The unsaturated aromatic acids were mixed with catalytic amounts of TBATFA and the A/-halo-succinimide was added in portions over time at ambient temperature. The yields are good to excellent even for activated aromatic rings which do not undergo the classical Hunsdiecker reaction. The fastest halodecarboxylation occurs with NBS, but NCS and NIS are considerably slower. The nature of the applied solvents is absolutely critical, and DCE proved to be the best. This strategy was extended and applied in the form of a one-pot tandem Hunsdiecker reaction-Heck coupling to prepare aryl substituted (2 ,4 )-dienoic acids, esters, and amides. 

The ate complexes that result may be converted to vinyl halides or a,p-unsaturated carboxylic acids by procedures analogous to those employed in the previous examples. Organoaluminum reagents possess the characteristic of versatility, in that an alkyne may be converted stereospecifically to a cis- or trans-substituted alkene by appropriate choice of reagents. In the case of a,)8-unsaturated acids, the flexibility is even more pronounced, in that it is simply the order of addition of reagents that determines the stereochemistry of the product. 

In order to evaluate what caused the limitation to ort/io-substimted carboxylic acids, we investigated the decarboxylation step separately firom die cross-coupling step. In the presence of only the copper(I) phenanthroline/quinoline decarboxylation catalyst (15 mol%), a wide range of aromatic, heteroaromatic, and vinylic carboxylates protodecarboxylated to the corresponding hydrocarbons rai heating to 160°C (Scheme 12). However, added halide ions, as would inevitably be released in a decarboxylative cross-coupling (Scheme 6), prevented the decarboxylation for aU non-orf/io-substituted or not otherwise activated aromatic carboxylic acids. 

A v ety of reactions are catalyzed by electrochemically generated Ni(0) (62). Electrochemical reduction of Ni(bipy)3Br2 affords a reagent that couples acid chlorides and alkyl or aryl halides to form unsymmetrical ketones (63). Symmetrical ketones are formed from alkyl halides and carbon dioxide (64). Reductive electrochemical carboxylation of terminal alkynes, enynes and diynes can be accomplished with 10% Ni(bipy)3(Bp4)2 in DMF (65-68). Terminal allies lead selectively to a-substituted acrylic acids. Electrocatalytic hydrogenation on hydrogen-active electrodes has been reviewed (69). Radical cyclizations of vinyl, alkyl and aryl radicals can be carried out by indirect electrochemical reduction with a Ni(II) complex as a mediator (70). 

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