Palladium acetylenic alcohols

The intramolecular addition of the O-H bond to alkynes catalyzed by palladium complexes has been developed by K. Utimoto et al. (Eq. 6.59) [104]. An alkynyl alcohol can be converted to a cyclic alkenyl ether in the presence of a catalytic amount of [PdCl2(PhCN)2 or [PdCl2(MeCN)2] in ether or THE at room temperature. When the reaction was carried out in MeCN-H20 under reflux in the presence of a catalytic amount of PdCl2, hydration of the acetylenic alcohol occurred and the ketoalcohol was obtained in good yield instead. 

Acetylenic alcohols, usually of propargylic type, are frequently intermediates in the synthesis, and selective reduction of the triple bond to a double bond is desirable. This can be accomplished by carefully controlled catalytic hydrogenation over deactivated palladium [56, 364, 365, 366, 368, 370], by reduction with lithium aluminum hydride [555, 384], zinc [384] and chromous sulfate [795], Such partial reductions were carried out frequently in alcohols in which the triple bonds were conjugated with one or more double bonds [56, 368, 384] and even aromatic rings [795]. 

Metal- and alloy-containing membranes are currently applied mainly in ultrapure hydrogen production. Pilot plants with palladium alloy tubular membrane catalyst were used in Moscow for hydrogenation of acetylenic alcohols into ethylenic ones. In the Topchiev Institute of Petrochemical Synthesis, a laboratory-scale reactor of the same type was tested... 

The palladium-polyvinyl alcohol catalyst has proved useful in the reduction of acetylenes to ethylenes (15). Thus, 3-methyl-butyn-l-ol-3 has been reduced to 3-methyl-buten-l-ol-3 in excellent yield. Furthermore it was also advantageously utilized in the hydrogenation of cystine, in which case only 10 mg. of palladium were required (15a), and in the catalytic hydrogenation of apozymase (15b). 

Depending on the catalytic species, palladium-catalyzed mono- and dicarbonyl-ation of alkynes may be achieved. Monocarbonylation of acetylenic alcohols in the presence of thiourea is an elegant route to a-methylene-7-butyrolactone 202, the structure of which is widely distributed in certain natural products98,99). The synthesis of a vernolepine derivative (203) has been attempted by this method 100l Pro-... 

Silanes can also be added to acetylenes, alkylacetylenes, acetylenic alcohols and their derivatives, etc., under conditions very similar to those effective with olefins, i.e.9 at high temperatures,367 with catalysis by peroxides,339,368 or in the presence of platinum361,369 or hydrogen hexachloroplatinate 370 palladium has proved a particularly effective catalyst for this reaction with acetylenes.371 The corresponding vinylsilicon compounds are formed, e.g. ... 

Following the reaction between the acetylenic alcohol 428 and (EtO)2PCl-pyridine, distillation yielded the cyclic products 430 and 431 via 429 together with a subsequent series of hydride shifts (Scheme 53) prolonged heating of the mixture in the presence of palladium-charcoal under nitrobenzene afforded only diethyl (3,5-dimethylphenyl)phosphonate. ... 

The palladium-catalyzed cyclization and cross-coupling of acetylenic alcohols with 2-iodo-thiophene to generate enol ethers should probably be included here (Equation (61)) <92JOC22l3>. Similarly, (trimethylsilyl)ethynylmagnesium bromide can be coupled with 2-iodothiophene under Pd catalysis to produce (595 R = SiMes). Alkynylzinc chlorides also react with 2-bromo or 2-iodothiophene to produce (595). Phenylethynylcopper reacts with 2-iodothiophene to form (595 R = Ph). 

An interesting synthesis of J -butenolides (28) via carbonylation of vinyl-mercurials (27) has been reported. This palladium-catalysed conversion goes in virtually quantitative yield, but its synthetic utility is limited by the availability of the /ra/i5-chlorovinylmercurials (27). These compounds were synthesized from the corresponding acetylenic alcohol by reaction with mercuric chloride-sodium chloride, but yields were generally low (ca. 30%) and the reaction appears to be limited to primary and tertiary acetylenic alcohols. 

The stereochemistry of electrochemical reduction of acetylenes is highly dependent upon the experimental conditions under which the electrolysis is carried out. Campbell and Young found many years ago that reduction of acetylenes in alcoholic sulfuric acid at a spongy nickel cathode produces cis-olefins in good yields 126>. It is very likely that this reduction involves a mechanism akin to catalytic hydrogenation, since the reduction does not take place at all at cathode substances, such as mercury, which are known to be poor hydrogenation catalysts. The reduction also probably involves the adsorbed acetylene as an intermediate, since olefins are not reduced at all under these conditions and since hydrogen evolution does not occur at the cathode until reduction of the acetylene is complete. Acetylenes may also be reduced to cis olefins in acidic media at a silver-palladium alloy cathode, 27>. 

A significant part of the examples of transition metal catalyzed formation of five membered heterocycles utilizes a carbon-heteroatom bond forming reaction as the concluding step. The palladium or copper promoted addition of amines or alcohols onto unsaturated bonds (acetylene, olefin, allene or allyl moieties) is a prime example. This chapter summarises all those catalytic transformations, where the five membered ring is formed in the intramolecular connection of a carbon atom and a heteroatom, except for annulation reactions, involving the formation of a carbon-heteroatom bond, which are discussed in Chapter 3.4. 

Many such activated acyl derivatives have been developed, and the field has been reviewed [7-9]. The most commonly used irreversible acyl donors are various types of vinyl esters. During the acylation of the enzyme, vinyl alcohols are liberated, which rapidly tautomerize to non-nucleophilic carbonyl compounds (Scheme 4.5). The acyl-enzyme then reacts with the racemic nucleophile (e.g., an alcohol or amine). Many vinyl esters and isopropenyl acetate are commercially available, and others can be made from vinyl and isopropenyl acetate by Lewis acid- or palladium-catalyzed reactions with acids [10-12] or from transition metal-catalyzed additions to acetylenes [13-15]. If ethoxyacetylene is used in such reactions, R1 in the resulting acyl donor will be OEt (Scheme 4.5), and hence the end product from the acyl donor leaving group will be the innocuous ethyl acetate [16]. Other frequently used acylation agents that act as more or less irreversible acyl donors are the easily prepared 2,2,2-trifluoro- and 2,2,2-trichloro-ethyl esters [17-23]. Less frequently used are oxime esters and cyanomethyl ester [7]. S-ethyl thioesters such as the thiooctanoate has also been used, and here the ethanethiol formed is allowed to evaporate to displace the equilibrium [24, 25]. Some anhydrides can also serve as irreversible acyl donors. 

Other unsaturated substrates arylated by various diaryl iodonium salts included butenone, acrylic acid, methyl acrylate and acrylonitrile [46]. Allyl alcohols with diaryliodonium bromides and palladium catalysis were arylated with concomitant oxidation for example, from oc-methylallyl alcohol, aldehydes of the general formula ArCH2CH(Me)CHO were formed [47]. Copper acetylide [48] and phenyl-acetylene [49] were also arylated, with palladium catalysis. 

---