Alkyne From acid

Three highly useful synthetic transformations are presented in this section the synthesis of isoflavones from chalcones, the synthesis of a-arylalkanones from arylalkenes, and the synthesis of a-arylalkanoic acids from aryl ketones. Two others are potentially useful methods, but are not as yet widely used the preparation of a-branched carboxylic acids from alkynes, and the ring expansion and ring contraction of cyclic alkenes and ketones. 

SCHEME 6.13 Synthesis of butenoic acids from alkynes and C02. 

Using stoichiometric amounts of an alkylzinc reagent and catalytic amounts of Ni(0)/DBU, Mori was able to prepare a number of p-aUcylated or arylated a,-P-unsaturated carboxylic acids from alkynes and CO2 (Scheme 5.12) [56, 57]. Through a frans-metalation reaction with the oxanickelacyclopentenone intermediate, the alkyl zinc reagent effected the reductive elimination of the product from the nickel center with concomitant regeneration of the active zerovalent catalyst. 

Alkynes, like alkenes, can be cleaved by reaction with powerful oxidizing agents such as ozone or KMnC, although the reaction is of little value and we mention it only for completeness. A triple bond is generally less reactive than a double bond and yields of cleavage products are sometimes low. The products obtained from cleavage of an internal alkyne are carboxylic acids from a terminal alkyne, CO2 is formed as one product. 

Examples of alkylation, dealkylation, homologation, isomerization, and transposition are found in Sections 1,17,33, and so on, lying close to a diagonal of the index. These sections correspond to such topics as the preparation of alkynes from alkynes carboxylic acids from carboxylic acids and... 

The use of stronger acid conditions provides somewhat better synthetic yields of alkanes from alkynes. A useful method consists of treatment of the substrate with a combination of triethylsilane, aluminum chloride, and excess hydrogen chloride in dichloromethane.146 Thus, treatment of phenylacetylene with 5 equivalents of triethylsilane and 0.2 equivalents of aluminum chloride in this way at room temperature yields 50% of ethylbenzene after 1.5 hours. Diphenylacetylene gives a 50% yield of bibenzyl when treated with 97 equivalents of triethylsilane and 2.7 equivalents of aluminum chloride after 2.8 hours. Even 1-hexyne gives a mixture of 44% n -hexane and 7% methylpentane of undisclosed structure when treated with 10 equivalents of triethylsilane and 0.5 equivalent of aluminum chloride for 0.5 hour.146... 

In addition to transition metals, recent work has demonstrated that strong Lewis acids will catalyze the addition of silanes to alkynes in both an intra- and an intermolecular fashion.14,14a-14c The formation of vinylsilanes from alkynes is possible by other means as well, such as the synthetically important and useful silylcupration15,15a of alkynes followed by cuprate protonation to afford vinylsilanes. These reactions provide products which can be complementary in nature to direct hydrometallation. Alternatively, modern metathesis catalysts have made possible direct vinylsilane synthesis from terminal olefins.16,16a... 

If an alkyne (or activated alkene) is present in the solution the sulfenic acid, from (57), or the alkanethiosulfoxylic acid (R SSOH), from (58), can be trapped, as shown in (59) and (60), respectively, and the course of the decomposition is relatively straightforward. [The thiocarbonyl compound formed in (57) normally forms polymer.]... 

Solvent-free conditions were used by Tanaka et al. in their assays to obtain intermolecular reactions from alkynes and anilines. The chosen catalyst was [AuMe(PPh3)] with an acidic promoter [92]. Reaction, whose effectiveness was greater in the case of aromatic amines, proceeded via Markovnikov by amine electrophilic attack of the alkyne in a similar way to the methanol addition proposed by Teles (see Section 2.1.3.2) and provided high yields and TONs. 

HF-pyridine reagent, however, can also be used for the hy-drofluorination of alkenes,7 alkynes,7 cyclopropanes,7 and diazo compounds,8 the halofluonnation of alkenes,9 the preparation of fluoroformates from carbamates,10 the preparation of a-fluorocarboxylic acids from a-amino acids,11 and as a deprotecting reagent in peptide chemistry.12 Examples of the hydrofluonnation of alkenes with HF-pyridine are given in Table II. 

Alkenes and Alkynes. A series of metal carbonyl cations, such as [Au(CO)n]+,287,288 [c-Pd( i-CO)2]2+,289 [Rh(CO)4]+,290 and [(Pt(CO)3)2]2+,291 was found to induce the formation of carboxylic acids from alkenes and CO in the presence of H2SO4 under mild conditions. A novel water-soluble Pd catalyst292 and Pd complexes of calixarene-based phosphine ligands293 showed high activity in the regioselective carboxylation of vinyl arenes to yield 2-arylpropionic acids or... 

Flash thermolysis of 4-substituted isoxazol-5(4//)-ones can be used to generate alkynes, isocyanides, aminoisocyanides and ketenimines (77C258, 76HCA2615). Decomposition of the oxime (538) at an oven temperature of 450 °C produces C02, benzonitrile and fulminic acid (539 equation 5) (79AG(E)467). This method thus offers a safe alternative to the synthesis of fulminic acid from the explosive metal fulminates. 

A series of 3-hydroxybut-4-ynoic esters (319) has been prepared from alkynic ketones and bromoacid esters in the presence of zinc (61AP234). Ring closure to the pyran-2-one occurs in a mixture of acetic and sulfuric acids. The preparation of 4,6- and 3,4,6-substituted pyranones is possible by this route, the substituents being different or identical, and as such is complementary to the alkynic route to the 3,6-disubstituted compound (57JA2602). 

Water is too weak a base to accept protons from alkynes consequently no measurable concentration of H30 is expected from the ionization of alkynes in dilute aqueous solutions. Therefore we have no quantitative measure of 1-alkyne acidity in aqueous solution other than that it probably is about 1010 times less acidic than water, as judged from measurements in other solvents to be discussed shortly. In the gas phase, however, the situation is reversed, and ethyne is a stronger acid than water ... 

Tradition reserves the use of the name acid for substances that transfer protons measurably to water. Thus the carboxylic acids stand out from alkynes, halides, alcohols, and simple aldehydes and ketones in giving water solutions that are acidic to indicator papers or pH meters as the result of proton transfers from the carboxyl groups ... 

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