Alkynes alkoxycarbonylation

In 1959 Carboni and Lindsay first reported the cycloaddition reaction between 1,2,4,5-tetrazines and alkynes or alkenes (59JA4342) and this reaction type has become a useful synthetic approach to pyridazines. In general, the reaction proceeds between 1,2,4,5-tetrazines with strongly electrophilic substituents at positions 3 and 6 (alkoxycarbonyl, carboxamido, trifluoromethyl, aryl, heteroaryl, etc.) and a variety of alkenes and alkynes, enol ethers, ketene acetals, enol esters, enamines (78HC(33)1073) or even with aldehydes and ketones (79JOC629). With alkenes 1,4-dihydropyridazines (172) are first formed, which in most cases are not isolated but are oxidized further to pyridazines (173). These are obtained directly from alkynes which are, however, less reactive in these cycloaddition reactions. In general, the overall reaction which is presented in Scheme 96 is strongly... 

A synthesis of 2-acyl and 2-alkoxycarbonyl-indoles was carried out by Tamariz and coworkers via an intramolecular Friedel-Crafts heteroannulation of enaminone precursors <06SL749>. A lac type palladium-catalyzed intramolecular indolization of alkyne-tethered 2-chloroanilines has been reported by Lu and co-workers <06OL3573>. 

Intramolecular alkoxycarbonylation of alkynols is parallel to what has been described for alkenols except that functionalization of the triplebond produces a double bond. No lactone formation is observed in the Pd(II)-catalyzed oxidative cyclization-carbonylation of alkynes. Instead [(methoxycarbonyl)methylene]tetrahydrofurans are selectively formed [134, 135]. Moreover, starting from an enynol, furan-2-acetic ester is obtained resulting from a final aromatization step [136]. 

In contrast with olefins, alkoxy-alkoxycarbonylation of alkynes to give /J-alkoxyacrylic esters is not a common reaction, and has occasionally been observed as a side reaction of the main reaction pathway leading to maleates and fumarates [77,80]. However, with suitably functionalized alkynes bearing a coordinating group (such as o ,a -disubstituted propargylic acetates) it may become the major reaction course (Scheme 15) [88]. 

Kagan H, Namy JL (1999) Influence of Solvents or Additives on the Organic Chemistry Mediated by Diiodosamarium. 2 155-198 Kakiuchi F, Murai S (1999) Activation of C-H Bonds Catalytic Reactions. 3 47-79 Kakiuchi F, Chatani N (2004) Activation of C-H Inert Bonds. 11 45-79 Kalck P, Urrutigoi ty M, Dechy-Cabaret O (2006) Hydroxy- and Alkoxycarbonylations of Alkenes and Alkynes. 18 97-123 Kanno K, see Takahashi T (2005) 8 217-236 Keen SP, see Gibson SE (n6e Thomas) (1998) 1 155-181 Kendall C, see Wipf P (2005) 8 1-25... 

Highly selective transformation of terminal acetylenes to either linear or branched carboxylic acids or esters may be achieved by appropriately selected catalyst systems. Branched esters are formed with high selectivity when the acetylenes are reacted with 1-butanol by the catalyst system Pd(dba)2/PPh3/TsOH (dba = dibenzylideneacetone) or palladium complexes containing PPh3. Pd(acac)2 in combination with various N- and O-containing phosphines and methanesulfonic acid is also an efficient catalyst for the alkoxycarbonylation of 1-alkynes to yield the branched product with almost complete selectivity.307,308... 

Catalytic systems to afford linear esters selectively are scant.306,309 A report in 1995 disclosed that palladium complexes based on l,l -bis(diphenylphosphine)fer-rocene showed excellent regioselectivity for the formation of linear a,p-unsaturated esters.309 The results with phenylacetylene are remarkable because this compound is known to exhibit a distinct preference for the formation of the branched products on palladium-catalyzed carboxylations. Mechanistic studies indicate that the alkoxycarbonylation of alkynes proceeds via the protonation of a Pd(0)-alkyne species to give a Pd-vinyl complex, followed by CO insertion and alcoholysis.310... 

Cyclopenta[c]quinolizines (284) reacted with alkynic esters in boiling nitrobenzene to give cyclopenta[cflf][3.3.3]cyclazines (285), the alkoxycarbonyl groups of which could be readily removed by alkaline hydrolysis followed by vacuum pyrolysis of the resulting acids (69JCS(C)239>. 

As is to be expected, an alkynic ketone undergoes a Michael addition with a carbanion, leading eventually to a pyranone (50JA1022). Using malonic esters, a 3-alkoxycarbonyl derivative results, which is hydrolyzed to the 2-oxopyran-3-carboxylic acid under alkaline conditions, but to the pyranone by sulfuric acid. Rapid ester exchange is observed with the initial products, the alcohol used as solvent determining the nature of the alkyl group in the 3-carboxylic esters (Scheme 90). 

Carbamoyl or alkoxycarbonyl complexes 87 are obtained by the attack of amines or alkoxides to metal carbonyls. They are important intermediates of carbonylation reactions and undergo insertion of alkene and alkyne. 

---