Alkynyl carboxylate esters

Alkynyl carboxylate esters, 75, are obtained in the reaction of lithium acetylides with bis(acyloxy)iodobenzene, 73 [Eq. (32)] [59], These reactions are likely to proceed through the intermediacy of the respective alkynyliodonium carboxylates (74), although no such salts have been isolated to date as they spontaneously decompose, via loss of iodobenzene, to the alkynyl carboxylates, 75. Only benzoate esters (75 R = CgHj) are sufficiently stable to isolate and store pure for longer periods. Simple alkylcarboxylates such as acetates are not stable although the hindered pivaloate ester (65 R = t-Bu, R = t-Bu) has been isolated in low yield [59]. Among the reasons for the instability of these esters is their sensitivity to moisture they both readily add water and undergo subsequent hydrolyses [60]. Because of the sensitivity to moisture, the isolated yield [41] of bisalkynyl benzoates, 76, from the bisalkynyliodonium triflates, 35, is only 6-15% [Eq. (33)]. 

Alkynyl carboxylates, RC=C02CR. A wide variety of these previously unknown esters can be prepared in a two-step procedure from 1, as formulated for the alkynyl benzoates (2).3... 

Silyl esters have also been transformed successfully [137] another example reports the O-acylation of serine with a /l-alkynyl carboxylic acid anhydride accompanied by an in situ isomerization of the alkyne to the allene [138],... 

The heteroatom-substituted carbene complexes most frequently used in organic synthesis are carbonyl complexes of the type (CO)5M=C(X)R (M Cr, Mo, W X OR, NR2 R H, alkyl, aryl, vinyl, alkynyl, etc.). To some extent such complexes behave as carboxylic esters or amides, the (CO)5M=C group having electronic properties similar to those of a carbonyl group (Figure 2.16). 

In 2006, Zeitler demonstrated the use of alkynyl aldehydes in redox esterification [114]. As in previous examples, the author proposes the formation of an activated carboxylate that acts as an acylating agent Eq. 19. A variety of a,P-unsaturated carboxylic esters 199 are formed in moderate yields with E-selectivity up to >95 5. 

Stable esters coming formally from alkynols and carboxylic, sulphonic or dialkyl phosphoric acids were prepared for the first time via alkynyliodonium salts with the corresponding anion. Several alkynyl carboxylates were obtained by anion exchange of alkynyl iodonium tosylates the initially formed salts, PhI+C=CR RCO2, were converted spontaneously into the esters on elution through a chromatographic column packed with an anion-exchange resin (Pol+ArCOj) ... 

A concise synthesis of highly substituted furans, pyrroles, butenolides, and 2-butene-4-lactam esters starts from alkynyl adducts of a Fischer carbene complex 21 (Scheme 27) < 1998JOC3164>. Incorporation of an aldehyde yields a reactive vinyl tungstencarbonyl complex 22 that can be oxidatively transformed to an ester group, furnishing the furan carboxylic ester 23. 

Acetylenic ethers and esters represent an important class of functionalized acetylene derivatives of the hypothetical alkynols, RC=COH. The chemistry of acetylenic ethers has been well developed since their first preparation about 100 years ago Several exhaustive reviews covering the literature on acetylenic ethers and their analogs up to 1985 have been published in the last 30 years. In contrast to acetylenic ethers, esters of alkynols were unknown until the mid-1980s when the first preparation of alkynyl tosylates was reported. In the following years a wide variety of alkynyl carboxylate, phosphate and sulfonate esters has been prepared from alkynyl iodonium salts. The chemistry of these novel derivatives of alkynols has been summarized in a recent review. In the last 10 years considerable interest and research activity has arisen toward alkynols themselves and such derivatives as alkynolate salts and silyl ynol ethers. The present chapter will cover the chemistry of acetylenic ethers and esters as well as related derivatives of ynols with emphasis on new developments in this subject during the last 5-10 years. 

More recently we have reported the preparation of alkynyl esters by the reaction of lithium alkynolates with the corresponding acid chlorides (equations 14 and 15, Section II.B.2). This method works well for the synthesis of alkynyl phosphates, however, it has only limited applicability in the preparation of alkynyl carboxylates. ... 

Pure alkynyl esters 94-96 are colorless or pale yellow liquids or solids with a low melting point. Most of them are relatively stable and can be stored in a refrigerator for extended periods. Of the three classes of alkynyl esters the sulfonates 94 are the most thermally stable some of them may even be distilled in vacuum at temperatures of 100-130 The carboxylate esters 96 are the least stable, in fact, so far only alkynyl benzoates (96, R = Ar) and pivalates (96, R = -Bu) have been isolated in pure form. Alkynyl esters of such acids as formic and acetic are too unstable to isolate k All three classes of alkynyl esters and, especially, carboxylates are water sensitive and rapidly decompose by hydrolysis in the presence of moisture. 

Treatment of 15 with sodium carboxylates (101) in aqueous THF gave keto esters (102) as products (equation 46). Carbonylation of 11 in alcohol solvents, catalyzed by Pd(OAc)2, results in alkynyl carboxylates (103) (equation 47). Coupling of alkynyl(phenyl)iodonium... 

The conversion of alkynyl(phenyl)iodonium ions to alkynyl esters with carboxylate ions via the MC mechanism (equation 45) has been proposed81. Evidence for the viability of this process is provided by the generation of 3,3-dimethyl-1-cyclopentenyl benzoate in addition to the expected alkynyl benzoate when the iodonium triflate shown in equation 90 is mixed with sodium benzoate in dichloromethane (yields not reported)3. 

Reaction with an amine, AIBN, CO and a tetraalkyltin catalyst also leads to an amide.Benzylic and allylic halides were converted to carboxylic acids electroca-talytically, with CO and a cobalt imine complex. Vinylic halides were similarly converted with CO and nickel cyanide, under phase-transfer conditions.Allylic (9-phosphates were converted to allylic amides with CO and ClTi=NTMS, in the presence of a palladium catalyst. Terminal alkynes were converted to the alkynyl ester using CO, PdBr2, CuBr2 in methanol and sodium bicarbonate. ... 

Alkynyl(phenyl)iodonium sulfonates are stable, microcrystalline substances that can be stored and used indefiniteiy with little or no decomposition. They have been employed in the formation of aryl (2-furyl)iodonium tosylates, alkynyl sulfonate, carboxylate and phosphate esters, tricoordinate vinyliodinane species,fO and alkylidenecarbene-iodonium ylides. <... 

The reaction of alkoxide ions with alkynyliodonium salts is unproductive, leading to only decomposition products rather than the desired alkoxyacetylenes. Similarly, reaction of R3SiO does not lead to any siloxyalkynes. In contrast the softer sulfonate, carboxylate, and phosphate nucleophiles all readily react with alkynyliodonium salts leading to the corresponding alkynyl sulfonate, carboxylate and phosphate esters [4]. 

Hydrolysis of alkynyl esters 116-118 under acidic conditions results in the formation of the carboxylic acids derived from the acetylenic moiety of the esters (equations 71-73). 

---