Palladium cycloalkenes

Arylation of cycloalkenes.1 Aryl halides undergo Heck coupling with cy-cloalkenes in the presence of a palladium catalyst. The reaction involves addition of an arylpalladium intermediate to the double bond followed by elimination of a palladium hydride. 

Particularly interesting is the reaction of enynes with catalytic amounts of carbene complexes (Figure 3.50). If the chain-length between olefin and alkyne enables the formation of a five-membered or larger ring, then RCM can lead to the formation of vinyl-substituted cycloalkenes [866] or heterocycles. Examples of such reactions are given in Tables 3.18-3.20. It should, though, be taken into account that this reaction can also proceed by non-carbene-mediated pathways. Also Fischer-type carbene complexes and other complexes [867] can catalyze enyne cyclizations [267]. Trost [868] proposed that palladium-catalyzed enyne cyclizations proceed via metallacyclopentenes, which upon reductive elimination yield an intermediate cyclobutene. Also a Lewis acid-catalyzed, intramolecular [2 + 2] cycloaddition of, e.g., acceptor-substituted alkynes to an alkene to yield a cyclobutene can be considered as a possible mechanism of enyne cyclization. 

Placement occurs through an isomerization process similar to that responsible for 3,1-placement in propene polymerization (Sec. 8-5c-l). 1,3-Placement is also observed with nickel and palladium a-diimine initiators [Sacchi et al., 2001] (Sec. 8-8b). 1,3-Placement has not been reported for other cycloalkene polymerizations. 

Durch Wittig-Olefinierung lassen sich 1-Nitromethyl-cycloalkene mit Triphenyl- oder Tri-butyl-phosphan in Gegenwart von Tetrakis-[triphenylphosphan]-palladium(0) und an-schlieBender Basen-katalysierter Reaktion mit Aldehyden in einer Eintopfreaktion zu 1 -(1-Alkenyl)-cycloalkenen ( >95%) umwandeln3 ... 

Most of the studies to date have employed either palladium [222—229] or platinum [220,224,226,228—235], commonly as Adams reduced platinum oxide, although nickel [228,236,237], rhodium [238,239], ruthenium [239], iridium [239], iron [237] and tungsten [237] have also been used. Many of these studies have been concerned with the stereochemistry of the hydrogenation of disubstituted cycloalkenes. Table 32 shows some typical results for the platinum- and palladium-catalysed hydrogenation of disubstituted cyclohexenes. Table 33 shows comparative results for the hydrogenation of 1,4-dialkylcyclohexenes over palladium, platinum and rhodium catalysts. 

Nickel has been reported to show behaviour similar to platinum [236], Further, in the reactions of cycloalkenes with deuterium, the product cycloalkanes are much more extensively exchanged over palladium than over nickel or platinum [236]. Such behaviour is not unexpected by comparison with the results obtained in the hydrogenation of linear alkenes (Sect. 3, p. 25). 

Cycloalkenes, into if-allyl palladium complexes, 8, 363 Cycloalkenyl rings, metal complex conformational interconversions, 1, 414 Cycloalkynes, in nickel complexes, 8, 147 (Cyclobutadiene)cyclopentadienyl complexes, with cobalt, polymercuration, 2, 435 Cyclobutadienes... 

Four-membered cycloalkenes, into r]3-allyl palladium complexes, 8, 363 Four-membered rings with G-G bonds, 3, 703... 

Larock, R. C. Yum, E. K. Yang, H. Palladium-catalyzed intermolecular arylation of function-ally-substituted cycloalkenes by aryl iodides. Tetrahedron 1994, 50, 305-321. 

The asymmetric allylic acetoxylation of cycloalkenes has also been reported. In this case, the catalyst is a bimetallic palladium(II) complex bearing a chiral bisox-azoline or a chiral diphosphine (DIOP). The reaction is performed in acetic acid/ sodium acetate under oxygen atmosphere at room temperature. Under these conditions, acetoxylation products of cyclohexene and cydopentene are obtained with 55 % and 78 % ee, respectively, albeit in low yields [39a]. 

Tetrahexylammonium phthalimide, generated in situ from potassium phthalimide and tetra-hcxylammonium bromide, aminates the racemic 3-(acyloxy)cycloalkenes 9 to imides (S)-10 with extremely high enantioselectivity in the presence of a palladium(0) complex prepared from dimeric n-allylpalladium chloride and the chiral ligand (/ ,/ )-G78. (5)-10c has been converted to a protected derivative of (S)-2-aminopimelic acid. 

The palladium-catalyzed hydrogenations of 1,2-cyclononadiene and 1,2-cyclodecadiene form substantial amounts of trans- as well as the ct s-cycloalkene. - The only unhindered approaches to either double bond of these dienes lead to a cis isomer. Because the trans is the less stable geometrical isomer in rings with fewer than 11 carbon atoms, its formation implies that an intermediate is produced which is capable of yielding both cis- and trans-cycloalkenes at comparable rates. Moore proposed that the intermediate has the syn-anti- n- y structure (43 equation 35). 

A much more frequently used reaction is the cleavage of unsaturated compounds to aldehydes (equations 98 and 99). Alkenes and cycloalkenes that possess one or two hydrogens at the double bonds are oxidized by ozone to ozonides, which have to be reduced to prevent a subsequent oxidation to acids by the excess oxygen atom. Reductions are carried out, usually without isolation of the ozonides, by catalytic hydrogenation over palladium catalyst [80, 81,1106] or Raney nickel [55] or by treatment with... 

Copper-, rhodium-, palladium-, and ruthenium-catalyzed cyclopropanation with diazoacetic esters is possible for a wide range of electron-rich alkenes, including alkylated acyclic alkenes, cycloalkenes, styrenes, 1,3-dienes, enol ethers, enol acetates, and ketene acetals (examples are given in this section, in Houben-Weyl Vol.E19b, ppl099-1155 and in refs 2, 152, 155 and 184). Furthermore, the construction of cyclopropanes with additional strain is possible, for example ... 

Photochemical decomposition of diazo(trimethylsilyl)methane (1) in the presence of alkenes has not been thoroughly investigated (see Houben-Weyl Vol. E19b, p 1415). The available experimental data [trimethylsilylcyclopropane (17% yield) and la,2a,3j8-2,3-dimethyl-l-trimethylsilylcyclopropane (23% yield)] indicate that cyclopropanation occurs only in low yield with ethene and ( )-but-2-ene. In both cases the formal carbene dimer is the main product. In reactions with other alkenes, such as 2,3-dimethylbut-2-ene, tetrafluoroethene or hexafluoro-propene, no cyclopropanes could be detected.The transition-metal-catalyzed decomposition of diazo(trimethylsilyl)methane (1) has been applied to the synthesis of many different silicon-substituted cyclopropanes (see Table 3 and Houben-Weyl Vol.E19b, p 1415) 3.20a,b,2i.25 ( iQp. per(I) chloride has been most commonly used for carbene transfer to ethyl-substituted alkenes, cycloalkenes, styrene, and related arylalkenes. For the cyclopropanation of acyl-substituted alkenes, palladium(II) chloride is the catalyst of choice, while palladium(II) acetate was less efficient, and copper(I) chloride, copper(II) sulfate and rhodium(II) acetate dimer were totally unproductive. The cyclopropanation of ( )-but-2-ene represents a unique... 

Cyclopentene yields mixtures of ROMP and double-bond polymerization with some Ti and V initiators. ROMP occurs exclusively with molybdenum and tungsten initiators, as well as Re, Nb, and Ta initiators. The relative amounts of cis and trans structures vary with the initiator and temperature [DalTAsta et al., 1962 Pampus and Lehnert, 1974]. Metallocene initiators polymerize cyclopentene through the double bond, but the polymer structure consists of cis 1,3-placement (Coates, 2000 Kaminsky, 2001 Kelly et al., 1997]. 1,3-Placement occurs through an isomerization process similar to that responsible for 3,1-placement in propene polymerization (Sec. 8-5c-l). 1,3-Placement is also observed with nickel and palladium a-diimine initiators [Sacchi et al., 2001] (Sec. 8-8b). 1,3-Placement has not been reported for other cycloalkene polymerizations. 

To appreciate how important alkyl reversal is in the reactions being considered, the reactions of monosubstituted Ce cycles with deuterium are informative the results obtained with carbon-supported metals after 25% addition are given in Table 7.7 Remembering that deuterium numbers M of the cycloalkane greater than two mean more hydrogen exchange than cycloalkene exchange, and vice versa, the results are broadly in line with the characteristics of the three metals as exposed in the earlier sections. A certain amount of alkyl reversal must occur in all cases, but alkene desorption is only important with palladium. With Pt/C... 

Since strained alkenes and cycloalkenes are particularly good ligands for palladium, even an ort/io-allenyl-substituted arylpalladium iodide will first carbopalladate a norbomene intermolecularly before the allenyl unit intercepts the relay norbomylpalladium species... 

A cascade Heck reaction with termination by nucleophiles is considered to start with an oxidative addition of a heteroatom-carbon bond (starter) onto a palladium(O) species (startup reaction), followed by carbopalladation of a nonaromatic carbon-carbon double or triple bond without subsequent dehydropalladation (relay), a second and possibly further carbopalladation of a carbon-carbon double or triple bond (second etc. relay). The terminating step is a displacement of the palladium residue by an appropriate nucleophile. It is crucial for a successful cascade carbopalladation that no premature dehydropalladation takes place, and that can be prevented by using alkynes and 1,1-disubstituted alkenes (or certain cycloalkenes) as relay stations since they give kinetically stable alkenyl- or neopentylpalladium intermediates, respectively. In addition, reaction of haloalkenes with alkenes in certain cases may form rr-allyl complexes, which are then trapped by various nucleophiles. 

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