Palladium-catalyzed decarboxylative couplings

The palladium-catalyzed decarboxylative coupling of allyl 2-(benzo[c(jthiazol-2-yl)acetates 118 provides a facile approach to 2-(but-3-enyl)benzo[c(jthiazoles 122 <07JA4138>. The reaction is initiated by nucleophilic attack of Pd(0) on the allyl ester to give Pd-7t-allyl complex 119, which undergoes nucleophilic attack at the less substituted allylic carbon from the benzothiazole nitrogen to produce 120. Decarboxylative dearomatization leads to intermediate 121, and a subsequent aza-Cope rearrangement driven by rearomatization affords the final product 122 and accounts for the unusual regioselectivity. This appears to be the first report of a tandem allylation/aza-Cope reaction driven by decarboxylative dearomatization/ rearomatization. 

Voutchkova, A., Coplin, A., Leadbeater, N. E., Crabtree, R. H. (2008). Palladium-catalyzed decarboxylative coupling of aromatic acids with aryl halides or unactivated arenes using microwave heating. Chemical... 

Very recently, Tan and coworkers [20] reported the synthesis of unsymmetrical biaryls via palladium-catalyzed decarboxylative coupling between aryl carboxylic acids and phenylboronic acids. Under the optimized conditions, where the choice of solvent was found to be crucial for achieving a straightforward and simple coupling reaction, a variety of substituted benzoic acids were found to undergo decarboxylative coupling with several phenylboronic acids (Scheme 3.8). Despite poor substrate scope, this method could be useful and complementary to the Pd-catalyzed... 

The palladium-catalyzed decarboxylative coupling of potassium pentafluoroben-zoate with aryl chlorides, bromides and triflates was shown to be a useful method for the synthesis of polyfluorobiaryls from readily accessible polyfluorobenzoate salts. For instance, 2- and 3-chlorothiophens 218, 219 reacted with potassium pentafluorobenzoate to produce pentafluorophenyl derivatives 210 and 220. The reaction proceeded in refluxed diglyme in the presence palladium acetate(ll) [111],... 

A conjugated oligomer is synthesized through the palladium-catalyzed decarboxylative coupling of pyrrole-2,5-dicarboxylic acid derivatives with dibromoarenes (Scheme 4.45) [50]. 

Recently, the Chou group reported on the palladium-catalyzed decarboxylative coupling of substituted 2,5-cyclohexadiene-l-carboxylic acids. The products obtained are 5-arylcyclohexa-1,3-dienes. ... 

Several examples of transition metal catalysis for the synthesis of piperidines appeared this year. Palladium catalyzed intramolecular urethane cyclization onto an unactivated allylic alcohol was described as the key step in the stereoselective synthesis of the azasugar 1-deoxymannojirimycin . A new synthetic entry into the 2-azabicyclo[3.3.1]nonane framework was accomplished through a palladium mediated intramolecular coupling of amine tethered vinyl halides and ketone enolates in moderate yields . A palladium catalyzed decarboxylative carbonylation of 5-vinyl... 

On the basis of this palladium-mediated Michael addition cyclization process, a novel two-step synthetic entry into functionalized furan derivatives 67 has also been devised (Scheme 28). Substitution of benzylidene (or alkyli-dene) malonates for their ethoxymethylene analog (65) as activating olefins gave rise to the formation of the corresponding 2-ethoxy-4-arylidene tetrahy-drofurans 66. An in situ addition of potassium ferf-buloxidc induced a decar-boxylative elimination reaction which was followed by an isomerization of the exocyclic double bond. The entire process successively involved a conjugate addition, a palladium-catalyzed cyclization-coupling reaction, a base-induced eliminative decarboxylation, and finally, a double bond isomerization [73]. 

The copper-catalyzed decarboxylative coupling of a-keto carboxylic acids with indoles can proceed through C-H bond cleavage at their C3-position (Scheme 4.76) [78]. In contrast, the coupling with Ar-(pyrimidin-2-yl)indoles occurs at their C2-position under palladium catalysis [79]. 

The intramolecular version of palladium-catalyzed decarboxylative cross-coupling reaction of aromatic carboxylic acids with organic halides also worked well. For example, 2-bromophenyl-tethered benzoic acids 116 underwent cyclization in the presence of Pd(OAc)2/PPh3 and K2CO3 in NMP to give 117 in high yields (Scheme 22.27) [51]. A mechanism that involved a sequence of oxidative addition, cyclization of carboxylate 118, decarboxylation of 119, and reductive elimination was proposed. Indole-2-carboxylic acid 121 was also applicable to this type of cyclization. 

Palladium-catalyzed coupling of allylic acetate 113 (Scheme 23) with diethyl malonate, or with acetylated diethyl tartronate, yielded the Boc-protected allylic amines 114 in 99 1 E stereoselectivity.1 "1 Saponification of 114 (R1 = H) and decarboxylation gave the Tyr- Ala alkene isostere 116 in quantitative yield, whereas 114 (R = OAc) was converted into the Tyr-Gly alkene isostere 115 in a two-step procedure in 62% overall yield. Considering the number of steps and the overall yield, this procedure is among the most efficient to prepare Xaatp, CH=CH]Gly dipeptide isosteres. 

GooBen LJ, Zimmermann B, Knauber T (2008) Palladium/copper-catalyzed decarboxylative cross-coupling of aryl chlorides with potassium carboxylates. Angew Chem Int Ed 47 7103-7106... 

Transition metal-catalyzed cross-coupling reactions of halogenopyrazines is an efficient synthetic method for alkyl-, alkenyl-, and alkynylpyrazines (Section 6.03.5.4.2). Palladium and nickel complexes are particularly effective as catalysts in this reaction. The Wittig reaction of halogenomethyl-pyrazines likewise leads to the formation of alkenylpyrazines (Section 6.03.8.1). Dehalogenation of halogeno pyrazines is a very practical synthetic method for alkyl- or aryl-substituted pyrazines. For example, phenylpyrazine has been prepared by catalytic hydrogenolysis of the 2-chloro-3-phenyl compound in the presence of triethylamine. This product is also obtained by decarboxylation of... 

Pd-Catalyzed Reactions in the Presence of Ag Salts A bimetallic system containing a Pd catalyst has been used in some cases to achieve Ar-Ar bond formation. Palladium-mediated decarboxylative cross-coupling of aromatic acids and aryl iodides takes place efficiently under microwave heating (MW), affording the corresponding biaryls, as shown in Scheme 22.40 (path a) [55]. 

The palladium-catalyzed enyne synthesis is achievable via the decarboxylative coupling of propiolic acids with alkenyl bromides (Scheme 4.62) [63]. Propiolic acids also couple with benzyl halides through decarboxylation and subsequent sp C-sp C bond formation (Scheme 4.63) [64]. 

In 2005, Trauner and coworkers reported the synthesis of rhazinilam using a palladium-catalyzed intramolecular C-H arylation of a pyrrole unit with an iodoarene (Scheme 16.12a) [24]. Treatment of compound 57 with a palladium catalyst resulted in the formation of coupling product 59 through intermediate 58. After removal of the MOM group and decarboxylation, the synthesis of... 

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