Alkyne direct oxidation

This benzene oxide may look very dubious and unstable, but benzene oxides can be made in the laboratory by ordinary chemical reactions (though not usually by the direct oxidation of benzene). We can instead start with a Diels-Alder reaction between butadiene and an alkyne. Epoxidation with a nucleophilic reagent (HO-O- from H2O2 and NaOH) occurs chemoselectively on the more electrophilic double bond—the one that is conjugated to the electron-withdrawing carbonyl group. 

A variety of conjugated dienes can be synthesized stereospecifically via the hydroalumination of al-kynes. For example, the first-formed adduct from an alkyne can insert a second mole of alkyne and upon hydrolysis yield the ( , )-1,3-alkadiene (equation 23). Alternatively, the first-formed adduct can directly oxidatively dimerize to the ( , )-1,3-diene (equation 64). ... 

Copper Catalysts Direct oxidative functionalization of tertiary amines is of importance both enzymatically and synthetically. The combination of CuBr—TBHP has proved to be as an efficient system in the oxidative activation of sp3 C—H bonds adjacent to a nitrogen atom [10]. Various types of cross-dehydrogenative coupling (CDC) reactions have been developed, including compounds with activated methylene groups [11], indoles [12], and terminal alkynes (Scheme 11.2) [13]. Because 1,2,3,4-tetrahydroisoquinoline derivatives are important structure motifs of natural... 

The a-substituted alkenylmetals used in Pd-catalyzed cross-coupling have been mainly those containing Mg, Zn, B, and Sn, as shown in Table 12 as well as Schemes 54-57. Of these, a-substituted alkenylmetals containing Mg and Zn can readily be prepared by direct oxidative metaUation of 2-halo-l-alkenes that are easily accessible by Markovnikov addition of HX to 1-alkynes (Scheme 54). a-Substituted alkenyltin compounds have been prepared and used in the construction of bicyclic diene systems via intramolecular Stille coupling, as shown in Scheme 55. ... 

Owing to their tendency to dimerize to furoxans (1,2,5-oxadiazole 2-oxides), nitrile oxides 5 are usually generated in situ, i.e., in the presence of suitable dipolarophiles such as alkenes, alkynes, etc., from stable precursors such as aldoximes 12 (X = H) or from primary nitroalkanes 13 (Scheme 2) [5,57-67]. Generation of nitrile oxides 5 from aldoximes 12 (X = H) involves either direct oxidation or halogenation of aldoximes 12 (X = H) to hydroximoyl halides 12 (X = Cl or Br) followed by dehydrohalogenation [5,57-67,79,80]. Alternatively, nitrile oxides 5 are conveniently generated via dehydration of primary nitroalkanes 13 [ 17,38,39,65,66,81-95]. This review covers the literature in the last 10-15 years pertaining to the chemistry of isoxazoHnes synthesized from primary nitroalkanes 13. 

Preparation of Acetylenic Ketones by Direct Oxidation of Alkynes... 

The direct oxidative annulations with alkynes by Csp -H/Csp -H bond cleavage have not been developed yet only two examples were described in this area using nickel(O) by Hiyama [198] and rhodium(lll) by Wang [199]. With ruthenium (11) catalyst, Lam reported the first annulations with alkynes of 2-aryl-l,3-dicarbonyl compounds by Csp -H and Csp -H bond activation [200]. The reaction of 2-aryl-3-hydroxy-2-cyclohexenones and cyclic 2-aryl-l,3-dicarbonyl compounds with 2.5 mol% of [RuCl2(/ -cymene)]2 and 2.2 equiv. of Cu(OAc)2 was performed in 1,4-dioxane at 90°C, and led to spiroindene derivatives in 50-84%... 

Dlsubstltuted alkynes are oxidized to a-diketones by NaOCl (or NaI04) with a catalytic amount of Ru04. Symmetrically dlsubstltuted acyclic olefins or large ring allcyclic olefins are directly converted to a-dlketones by KMn04 Ac20 In the cold. 

S. Okumura, Y. Takeda, K. Kiyokawa, S. Minakata, Hypervalent iodine(lll)-induced oxidative [4+2] annulation of o-phenylenediamines and electron-deficient alkynes direct synthesis of quinoxalines from alkyne substrates under metal-free conditions, Chem. Commun. 49 (2013) 9266-9268. 

In 2008, our group disclosed a novel method that involved the one-pot synthesis of multisubstituted pyridines 92 by Rh-catalyzed oxime-assisted alkenyl C-H bond functionalization of a, -unsaturated oximes with alkynes [48]. The scope includes a variety of a, -unsaturated oximes and symmetrical alkynes. This report is one of a few examples of Rh(I)-catalyzed alkenyl C-H bond functionalization. The mechanism is thought to occur by oxime-directed oxidative insertion of the Rh catalyst into the alkenyl C-H bond to form the hydrometalacycle LI. Hydrorhodation onto the alkyne then occurs to give L2, followed by reductive elimination to provide L3. Intermediate L3 can undergo a 6. r-electrocyclization and then a dehydration reaction to form pyridine (Eq. (5.89)). Overall, this Rh(I)-catalyzed reaction is a redox neutral. 

Hydroxyl group-directed oxidative annulations with alkynes for the production of fluorescent pyrans were reported (Eq. (7.7)) [12]. Not only naphthols but also 4-hydroxycoumarin and 4-hydroxy-substituted quinolin-2-one underwent this ruthenium(II)-catalyzed C-H/O-H bond functionalization process in a highly chemo- and regioselective manner. Competition reactions showed that electron-deficient alkynes are more reactive. Deuterium experiments also revealed a reversible C-H bond ruthenation step via carboxylate assistance. 

Recently, Ackermann and coworkers reported aliphatic hydroxyl-directed oxidative annulation reactions of benzyl alcohols with alkynes to form isochromene derivatives (Eq. (7.11)) [16]. This C-H/O-H functionalization process performed smoothly by using [RuCl2(/ -cymene)]2 (5 mol%)/AgPFg (20 mol%) as catalyst and Cu(OAc)2 H2O (20 mol%) as oxidant under an atmosphere of air. Various tertiary benzylic alcohols, a,a-dimethylallyl alcohol, and diverse internal alkynes are appropriate substrates for this transformation. The reaction occurred with moderate to high regioselectivity for unsymmetrical alkylarylacetylene, and an irreversible C-H metalation step is involved in the catalytic cycle. 

General mechanism of H-Het addition to alkynes involves oxidative addition of H-Het to the metal center followed by multiple bond coordination and formation of the 71-complex (Scheme 2). The key point of the addition reaction is the direction of alkyne insertion insertion into the M-H or M-Het bonds and regioselectivity determine the structure of the final product - anti-Markovnikov (linear) or Markovnikov (branched). Reductive elimination of C-Het or C-H bonds is the final product releasing step in the catalytic cycle (Scheme 2). 

General Reaction Chemistry of Sulfonic Acids. Sulfonic acids may be used to produce sulfonic acid esters, which are derived from epoxides, olefins, alkynes, aHenes, and ketenes, as shown in Figure 1 (10). Sulfonic acids may be converted to sulfonamides via reaction with an amine in the presence of phosphoms oxychloride [10025-87-3] POCl (H)- Because sulfonic acids are generally not converted directiy to sulfonamides, the reaction most likely involves a sulfonyl chloride intermediate. Phosphoms pentachlotide [10026-13-8] and phosphoms pentabromide [7789-69-7] can be used to convert sulfonic acids to the corresponding sulfonyl haUdes (12,13). The conversion may also be accompHshed by continuous electrolysis of thiols or disulfides in the presence of aqueous HCl [7647-01-0] (14) or by direct sulfonation with chlorosulfuric acid. Sulfonyl fluorides are typically prepared by direct sulfonation with fluorosulfutic acid [7789-21-17, or by reaction of the sulfonic acid or sulfonate with fluorosulfutic acid. Halogenation of sulfonic acids, which avoids production of a sulfonyl haUde, can be achieved under oxidative halogenation conditions (15). 

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... 

Like alkenes (Sections 7.4 and 7.5), alkynes can be hydrated by either of two methods. Direct addition of water catalyzed by mercury(II) ion yields the Markovnikov product, and indirect addition of water by a hydroboration/ oxidation sequence yields the non-Markovnikov product. 

The hydroboration/oxidation sequence is complementary to the direct, mercury(ll)-catalyzed hydration reaction of a terminal alkyne because different products result. Direct hydration with aqueous acid and mercury(IJ) sulfate leads to a methyl ketone, whereas hydroboration/oxidation of the same terminal alkyne leads to an aldehyde. 

There are a number of procedures for coupling of terminal alkynes with halides and sulfonates, a reaction that is known as the Sonogashira reaction.161 A combination of Pd(PPh3)4 and Cu(I) effects coupling of terminal alkynes with vinyl or aryl halides.162 The reaction can be carried out directly with the alkyne, using amines for deprotonation. The alkyne is presumably converted to the copper acetylide, and the halide reacts with Pd(0) by oxidative addition. Transfer of the acetylide group to Pd results in reductive elimination and formation of the observed product. 

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