Alkenes carbonyl-catalyzed isomerization

The Pd- or Ni-catalyzed isomerization reaction of carbonyl alkenes shown in Scheme 3 was recently reported. This is another thermodynamically favorable process. The results obtained with methyl 4-pentenoate appear to represent a case of an incomplete reaction. Further development of the reaction is desirable. 

A variety of metal carbonyls upon sonication will catalyze the isomerization of 1-alkenes to the internal alkenes (J 8),(27). Initial turnover rates are as high as 100 mol alkene isonierized/mol of precatalyst/h, and represent rate enhancements of 1(P over thermal controls. The relative sonocatalytic and photocatalytic activities of these carbonyls are in general accord. A variety of terminal alkenes can be sonocatalytically isomerized. Increasing steric hindrance, however, significantly diminishes the observed rates. Alkenes without 6-hydrogens will not serve as substrates. 

A survey of Wacker-type etherification reactions reveals many reports on the formation of five- and six-membered oxacycles using various internal oxygen nucleophiles. For example, phenols401,402 and aliphatic alcohols401,403-406 have been shown to be competent nucleophiles in Pd-catalyzed 6- TZ /fl-cyclization reactions that afford chromenes (Equation (109)) and dihydropyranones (Equation (110)). Also effective is the carbonyl oxygen or enol of a 1,3-diketone (Equation (111)).407 In this case, the initially formed exo-alkene is isomerized to a furan product. A similar 5-m -cyclization has been reported using an Ru(n) catalyst derived in situ from the oxidative addition of Ru3(CO)i2... 

When either an alcohol or an amine function is present in the alkene, the possibility for lactone or lactam formation exists. Cobalt or rhodium catalysts convert 2,2-dimethyl-3-buten-l-ol to 2,3,3-trimethyl- y-butyrolactone, with minor amounts of the 8-lactone being formed (equation 51).2 In this case, isomerization of the double bond is not possible. The reaction of allyl alcohols catalyzed by cobalt or rhodium is carried out under reaction conditions that are severe, so isomerization to propanal occurs rapidly. Running the reaction in acetonitrile provides a 60% yield of lactone, while a rhodium carbonyl catalyst in the presence of an amine gives butane-1,4-diol in 60-70% (equation 52).8 A mild method of converting allyl and homoallyl alcohols to lactones utilizes the palladium chloride/copper chloride catalyst system (Table 6).79,82 83... 

The ketenimine (93) reacts only at the C=N bond to give pyrroles and pyrrolines with methylene migration (equations 102,103). Carbon (hoxide codimerizes smoothly with methylenecyclopropanes to yield lactones with alkene isomerization (equations 104, lOS). The Pd -catalyzed rearrangement of y-butyrolactone (94) to butenolide (95) indicates that this CO2 insertion reaction is reversible. Hiis is fur-ter supported by the observation that lactone (94) can serve as a TMM precursor in the codimerization with aldehydes and electron-deficient alkenes (equations 106, 107). No cycloaddition to ketenes or simple ketone carbonyls has ever been reported. 

An interesting reaction discovered by Wakamatsu involves the cobalt-catalyzed carbonylation of aldehydes in the presence of primary amides to give A-acylamino acids in high yield (equation 26). By combining this reaction with known catalytic routes to aldehydes, for example isomerization of allyl alcohols or hydroformylation of alkenes, it is possible to achieve the direct synthesis of A-acylamino acids from precursors other than aldehydes. ... 

Wilkinson s catalyst catalyzes the isomerization of an alkene to its thermodynamically most stable isomer. Isomerization of allyl ethers in this manner gives enol ethers, which can be hydrolyzed to give the free alcohol and a carbonyl compound. 

Numerous metal hydrides, such as [HCo(CO)4] and [HRh(CO)(PPh3)3], or combinations of a metal complex and a hydride source (e.g., [Co2(CO)8]/H2, [Ni P(0Et3) 4]/H2S04 catalyze the isomerization of 1-alkenes to 2-alkenes. In the industrial carbonylation of ot-olefins, this double-bond isomerization is undesirable since the linear end products are of greater industrial importance. 

Carbonylation proceeds in the presence of chalcogen compounds without poisoning Pd catalysts. Pd-catalyzed stereo- and regioselective carbonylative double thiolation of 1-octyne with diphenyl disulfide (3) afforded the (Z)-j6-(phenylthio)--unsaturated thioester 4 [2], The thioester 4 can be converted to 3-(phenylthio)-2-alkenal 5 by Pd-catalyzed reduction with HSnBu3 under mild conditions [3]. When propargyl alcohol was subjected to the carbonylation in the presence of either diphenyl diselenide (6) or disulfide 3, 3-phenylselenobutenolide 7 or 3-phenylthiobutenolide was obtained. The transformation involves isomerization of the acylpalladium intermediate 8 to 9 [4]. 

The role of transition-metal carbonyls and particularly those of the Group 6 metals in homogeneous photocatalytic and catalytic processes is a matter of considerable interest [1]. UV irradiation especially provides a simple and convenient method for generation of thermally active co-ordinately unsaturated catalyst for alkenes or alkynes transformation. By using tungsten and molybdenum carbonyl compounds as catalysts, alkenes and alkynes can be metathesized, isomerised and polymerized. Photocatalytic isomerization of alkenes in the presence of molybdenum hexacarbonyl was observed by Wringhton thirty years ago [2]. Carbonyl complexes of molybdenum catalyze not only... 

Diisocyano-Rh dimers photocatalytically decompose water [13] and diisocyano complexes catalyze hydrogenation and isomerisation of alkenes and alkynes (although they are far less active than the Wilkinsons catalyst [14]). Alkene hydrogenation is a probe reaction for such reaction centres, especially since the hydrogenation of alk-l-enes over Wilkinson s catalyst [hydrido-carbonyl tris(triphenylphosphine).Rh ] in benzene is quite selective (i.e. was 45 times faster than the cis-aIk-2-ene [15]). Surprisingly, the active centres in such catalysts are still not entirely understood, despite extensive analysis [16]. Rh complexed with 4,4 -diisocyanobiphenyl and 1,4-diisocyanobenzene is active in hydrogenation and isomerization of 1-hexene [14], while Rh complexed with aliphatic amines is active in catalysis of hydrogenation of alkenes and cycloalkenes [16]. 

Another photocatalytic reaction of some interest is the cyclic dimerization of alkenes, which has been reported for a number of different metal carbonyls including Fe(CO)s, Cr(CO)6, Ni(CO)4 and several substituted analogs [83-85]. Although such dimerizations are catalyzed thermally by some of the same species, the stereochemistries of the products differ depending both upon which complex is utilized and whether the reaction is carried out thermally or photochemically. The dimerization of butadiene to 4-vinylcyclohexene photocatalyzed by the iron dicarbonyl Fe(CO>2(NO)2 has been studied by examining the intermediates formed when the photolysis was carried out in liq. Xe [85]. The key species was found to be the disubstituted species Fe(NO)2(Ti -butadiene)2 which was proposed to undergo isomerization to Fe(NO)2( n -butadiene)(ri2-butadiene) which can undergo an internal Diels-Alder cyclization to the vinyl cyclohexene. 

This one-step transformation of an alkene to an allylic acetate compares well with other methods of preparation such as hydride reduction of a, 8-unsaturated carbonyl compounds followed by esterification. The scope and limitations of the reaction have been investigated. The allylic acetoxylation proceeds via a TT-allylpalladium intermediate, and as a result, substituted and linear alkenes generally give several isomeric allylic acetates. With oxygen nucleophiles the reaction is quite general, and reactants and products are stable towards the reaction conditions. This is normally not yet the case with nitrogen nucleophiles, although one intramolecular palladium-catalyzed allylic amination mechanistically related to allylic acetoxylation has been reported. ... 

In a process analogous to the hydration of alkenes, water can be added to alkynes in a Markovnikov sense to give alcohols—in this case enols, in which the hydroxy group is attached to a double-bond carbon. As mentioned in Section 12-16, enols spontaneously rearrange to the isomeric carbonyl compounds. This process, called tautomerism, interconverts two isomers by simultaneous proton and double-bond shifts. The enol is said to tautomerize to the carbonyl compound, and the two species are called tautomers (tauto, Greek, the same mews, Greek, part). We shall look at tautomerism more closely in Chapter 18 when we investigate the behavior of carbonyl compounds. Hydration followed by tautomerism converts alkynes into ketones. The reaction is catalyzed by Hg(II) ions. 

In Summary a, -Unsaturated aldehydes and ketones are more stable than their noncon-jugated counterparts. Either base or acid catalyzes interconversion of the isomeric systems. Reactions typical of alkenes and carbonyl compounds are also characteristic of a,)8-unsaturated aldehydes and ketones. 

Homogeneous catalysts play an important role in industry as well as in research laboratories. Established applications include, for example, polymerization processes with zirconocene and its derivatives, rhodium- or cobalt-catalyzed hydroformylation of olefins, and enantioselective isomerization catalysts for the preparation of menthol. In contrast to heterogeneous catalysts, more experimental studies of reaction mechanisms are available and the active species can be characterized experimentally in some cases. Most catalysts are based on transition metal compounds, for which electronic structures and properties are well studied theoretically. A substantial number of elementary reactions, such as reductive elimination, oxidative addition, alkene or carbonyl migratory insertion, etc., have been experimentally Studied in detail by means of isotopic, NMR, and IR studie.s, as well as theoretically. ... 

---