Palladium acetylene, insertion

Bis(triphenylphosphine)palladium dichloride [(Ph3P)2PdCl2] can also be used as a catalyst for the phase-transfer carbonylation of halides. However, considerably more drastic conditions [95°C, 5 atm] are required when compared with Co2(CO)8 (44). The carbonylation of allyl chlorides can be catalyzed by nickel tetracarbonyl, giving isomeric mixtures of bu-tenoic acids. There is evidence for the intermediacy of polynuclear nickel-ates in this phase-transfer process (45). Acetylene insertion did not occur... 

Heck-type reactions with enol carboxylates (e.g., vinyl acetate) are generally complex. Most common are reactions in which vinyl acetate is employed as an ethylene equivalent (see Scheme 24). However, an example of a preparatively useful reaction with an intact acetate function is given in entry 44.The reaction of vinyl triflates with vinyl phosphonates affords the corresponding conjugate dienylphosphonates (entry 45).f A new access to reactive nonaflates via a one-pot nonaflation-Heck reaction was recently reported (entry 46). " This reaction sequence starts from silyl enol ethers and provides functionalized 1,3-dienes in a simple manner, lodonium salts can be used as RPd precursors (entry 47). It is notable that the palladium(O) insertion preferentially occurs inbetween the iodonium ion and the vinylic, rather than the arylic sp -hybridized carbon (entry 47). Some years ago, Jeffery used acetylenic halides to achieve (JiJ-enynoates and (Ji)-enynones in fair yields at room temperature (entry 48). ... 

Insertion of the alkyne into the Pd-H bond is the first step in the proposed catalytic cycle (Scheme 8), followed by insertion of the alkene and /3-hydride elimination to yield either the 1,4-diene (Alder-ene) or 1,3-diene product. The results of a deuterium-labeling experiment performed by Trost et al.46 support this mechanism. 1H NMR studies revealed 13% deuterium incorporation in the place of Ha, presumably due to exchange of the acetylenic proton, and 32% deuterium incorporation in the place of Hb (Scheme 9). An alternative Pd(n)-Pd(iv) mechanism involving palladocycle 47 (Scheme 10) has been suggested for Alder-ene processes not involving a hydridopalladium species.47 While the palladium acetate and hydridopalladium acetate systems both lead to comparable products, support for the existence of a unique mechanism for each catalyst is derived from the observation that in some cases the efficacies of the catalysts differ dramatically.46... 

A unique bis-silylation system, in which a bis(silyl)palladium intermediate is generated via recombination of two Si-Si bonds, has been developed.8,97 A bis(disilanyl)dithiane reacts with alkynes in the presence of a palladium/ isocyanide catalyst, giving five-membered ring bis-silylation products in high yield with elimination of hexamethyl-disilane (Scheme 14). The recombination, that is, bond metathesis, is so efficient that no product derived from direct insertion of acetylene into the Si-Si bonds of the bis(silyl)dithiane is formed at all. 

Reaction of the stannylborane 9 with an allenyne gives a cyclization product, in which the boryl and stannyl groups are introduced to the acetylenic terminus and the allenic central carbon, respectively (Equation (104)).159 Based on the assumption that an unsaturated functionality initially inserts into the Pd-B bond of (boryl)(stannyl)palladium(n) species, it seems likely that the alkyne moiety is more reactive than the allene moiety in this reaction. 

Diphenylacetylene has also been reacted with iodouracil 431 having a formamidine moiety under similar conditions to afford the dehydrogenated product 432 and the deaminated product 433 via intermediates 435 and 436, respectively. The selectivity increased in the presence of lithium chloride, whereby 93% of 432 with a trace amount of 433 were obtained. The lithium cation prevents the insertion of palladium into intermediate 434 to form intermediate 436, which is necessary to form 433. The reaction of 431 with asymmetric acetylenes in the presence of lithium chloride afforded the dehydrogenated pyridopyrimidines 437 and 439. However, reaction of 422 with acetylenic compounds in the absence of lithium chloride afforded the deaminated pyridopyrimidines 438 and 440 (Equation 37) <2000TL5899>. 

The palladium catalysed reductive insertion of acetylenes is also viable through the use of formate ions as hydride equivalent (c.f 3.24.), The ring closure of /V-acetylenic-2-iodoanilines gave the corresponding 3-alkylideneindolines in a selective manner (3.27.), demonstrating that the... 

The insertion of acetylene derivatives might also be utilised in the preparation of six membered rings. A characteristic distinction between such processes and olefin insertion is the fact, that the intermediate formed by the insertion of an acetylene into the palladium-carbon bond is unable to undergo /2-hydride elimination, therefore the concluding step of these processes is usually reductive elimination. 

The analogous palladium catalyzed reaction of internal acetylenes, 2-iodophenol and carbon monoxide leads to the selective formation of coumarins. The heterocyclic analogues of o-iodophenol are also effective. The o-iodopyridone shown in 4.16. for example gave rise to the formation of azacoumarin in 70% yield.18 In these processes the insertion of the acetylene derivative occurs in advance of the insertion of CO. Interestingly, the change of the acetylene to an alkene reverses the insertion order and leads to flavone formation.19... 

As described above for the palladium(0)-catalyzed reactions, carbon-carbon bonds can be obtained by insertion of an olefin into a palladium-vinyl bond (vinylpalladation). This approach has been applied in palladium(II)-catalyzed exchange reactions of olefins by generating the vinylpalladium species from chloropalladation of an acetylene [115,116]. This technique for generating vinylpalladium was later applied to the palladium-catalyzed 1,4-oxidation of conjugated dienes [117]. Thus, the use of substrate 91 in a palladium(II)-catalyzed oxidation in the presence of LiCl afforded 94 in 65% yield [Eq.(49)]. The... 

It is well known that palladium chloride is an active catalyst for the cyclization of acetylene to form cyclobutadiene as well as benzene derivatives. In this reaction an intermediate complex was isolated which has a palladium carbon a-bond, the formation of which was explained by an insertion mechanism, not by concerted cyclotrimerization. When this complex obtained from butyne and palladium chloride was decomposed by various means, 5-vinyl-l,2,3,4,5-penta-methylcyclopentadiene and 5-(l-chlorovinyl)-l,2,3,4,5-pentamethylcyclopenta-diene were obtained in addition to hexamethylbenzene... 

The key to clarifying the catalytic cycle is to determine whether or not hydride-palladium (H-Pd) is the active species. Taking advantage of hydride exchange between D2O and H-Pd to generate D-Pd species [69,70], the reactions under excess D2O (600 mol%) conditions were examined (Scheme 16). Consequently, the deuterated product at the vinylic position was obtained without diminution of catalytic activity under either polar or less polar conditions. These results indicate that the hydride(deuteride)-palladium is the active species. The possible catalytic cycle would be completed as shown in Scheme 17 the initially formed D-Pd coordinates to acetylene (B ), followed by D-Pd addition (H), insertion (cyclization) (E), and P-H elimination to give the product (9) and regenerate the H-Pd species. 

In some cases the insertion reactions are reversible. This is the case with the palladium complexes for carbon monoxide and sulfur dioxide. With acetylenes, preliminary studies indicate that the insertion products can be made to eliminate the acetylene on photolysis. The remaining A-frames shown in Figure 5.16 are formed irreversibly. 

Samuel, E.G. and Norton, J.R. (1984) Mechanism of acetylene and olefin insertion into palladium arbon sigma-bonds. J.Am. Chem. Soc., 106, 5505-12. 

The insertion reactions of acetylene compounds with di-p-iodo-bis(lV, -dimethyl-l-naphthylamine-CAO-dipalladium form substituted heterocyclic products 7.29 with formation of palladium metal and the loss of one IV-methyl groups by refluxing in chlorobenzene (Eq. (7.26)) [79,82,84,85]. The heterocyclic compounds are synthesized directly by the reaction of iododimethylaminonaphthalenes with acetylenes in the presence of a catalytic amount of the 1-dimethylaminonaphthalene palladacycle, as shown in Eq. (7.27) [85]. 

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