Bis dibenzylideneacetone palladium O

The catalyst reported by Drent [48] was generated in situ by mixing a palladium source with the ligand. A palladium source is broadly defined as a complex or any form of palladium metal whereby upon mixing with the ligand an active catalyst is formed. Many palladium sources are possible, but the sources exemplified by Drent aretris(dibenzylideneacetone)dipalladium(0)(Pd2(dba)3),bis(dibenzylideneacetone) palladium(O) (Pd(dba)2), or palladium(II) acetate. 

Methods (i) and (ii) require palladium(II) salts as reactants. Either palladium acetate, palladium chloride or lithium tetrachloropalladate(II) usually are used. These salts may also be used as catalysts in method (iii) but need to be reduced in situ to become active. The reduction usually occurs spontaneously in reactions carried out at 100 °C but may be slow or inefficient at lower temperatures. In these cases, zero valent complexes such as bis(dibenzylideneacetone)palladium(0) or tetrakis(triphenylphos-phine)palladium(O) may be used, or a reducing agent such as sodium borohydride, formic acid or hydrazine may be added to reaction mixtures containing palladium(II) salts to initiate the reactions. Triarylphosphines are usually added to the palladium catalysts in method (iii), but not in methods (i) or (ii). Normally, 2 equiv. of triphenylphosphine, or better, tri-o-tolylphosphine, are added per mol of the palladium compound. Larger amounts may be necessary in reactions where palladium metal tends to precipitate prematurely from the reaction mixtures. Large concentrations of phosphines are to be avoided, however, since they usually inhibit the reactions. 

The non-hydrocarbon substrates most often employed in dimerization or oligomerization reactions are cyclopropenone acetals, due to their ease of preparation. 3,3-Dimethoxycyclo-propene has been treated with various palladium(O) complexes. In the absence of phosphane, bis(dibenzylideneacetone)palladium leads to cyclodimerization, with 3,3,6,6-tetramethoxy-exu-tricyclo[3.1.0.0 ]hexane (30) being formed in 74 /o yield. No cyclotetramers, as in the reaction of 3,3-dialkyl- or 3,3-diarylcyclopropenes, are formed. ... 

With palladium catalysts, such as bis(dibenzylideneacetone)palladium/triisopropylphosphane (1 1) or tris(acetylacetonato)palladium/triisopropylphosphane/ethoxydiethylaluminum (1 1 2), 3,3-dimethylcyclopropene can be reacted in methyl acrylate as solvent to yield small amounts (about 10%) of cotrimers constisting of two molecules of the cyclopropene and one molecule of methyl acrylate, namely, a 1 1 mixture of methyl 3-[2,2-dimethyl-3-(2,2-dimethyl-cyclopropyl)cyclopropyl]prop-2-enoate isomers 40 and methyl 3,3,8,8-tetramethyltri-cyclo[5.1.0.0 ]octane-5-carboxylate (41). These compounds can be obtained in far better yield using trialkylphosphane-modified nickel(O) catalysts (see above). Major products of the palladium-catalyzed reaction are the homo-cyclodimer, 3,3,6,6-tetramethyl-exo-tricy-clo[3.1.0.0 ]hexane (55%), and the homo-cyclotrimer, 3,3,6,6,9,9-hexamethyl-crtt/o,e.TO-tetra-cyclo[6.1.0.0 - .0 ]nonane (33.5%). 

Sbderberg et al. have developed a sequence of Stille coupling followed by palladium (O)-catalyzed reductive N-heteroannulation for the synthesis of tetrahydrocarbazo-lones 238 (Scheme 57) [216, 217]. Palladium(0)-catalyzed coupling of 2-nitroar-ylstannanes 236 with 2-iodo-2-cyclohexenones 235 afforded the 2-(2-nitrophenyl)-2-cyclohexenones 237. Cyclization with carbon monoxide in the presence of catalytic amounts of bis(dibenzylideneacetone)palladium, l,3-bis(diphenylphos-phino)propane (dppp), and 1,10-phenanthroline led to the corresponding tetrahy-drocarbazolones 238 which can be transformed to the carbazoles 32 by Wolff-Kishner reduction and subsequent aromatization. This approach has been applied to the formal synthesis of murrayaquinone A [216]. 

The unusual complex tetrakis(tert-butyl isocyanide)di-/i-chloro-di palladium(I) chlorobenzene has been prepared by Otsuka et al The preparation utilized a novel coupling reaction between bis(rert-butyl isocyanide)palladium(O) and cis-bis(rert-butyl isocyanide)dichloro-palladium(II) in cold chlorobenzene. Although the reported yield for the final step of this synthesis was good (70%), the preparation of the precursor bis(tert butyl isocyanide)palladium(O) from (i7 -allylX> cyclopenta-dienyl)palladium is time-consuming and is accomplished in only S0% yield. We have developed a greatly improved preparation of the title complex (chlorobenzene-free), which utilizes readily available Pd(0) and Pd(II) compounds as starting materials, namely bis(dibenzylideneacetone)palladium(0), [Pd(dba)2], and trans-bis(benzonitrile)dichloropalladium(II). In addition, since full details of the preparation of Pd(dba)2 have not been reported, we give a complete account of the preparation here.H... 

Cyclocarbonylation of o-iodophenols 503 with isocyanates or carbodiimides and carbon monoxide in the presence of a catalytic amount of a palladium catalyst (tris(dibenzylideneacetone)dipalladium(O) Pd2(DBA)3) and l,4-bis(di-phenylphosphino)butane (dppb) resulted in formation of l,3-benzoxazine-2,4-diones 504 or 2-imino-l,3-benzoxazin-4-ones 505 (Scheme 94). The product yields were dependent on the nature of the substrate, the catalyst, the solvent, the base, and the phosphine ligand. The reactions of o-iodophenols with unsymmetrical carbodiimides bearing an alkyl and an aryl substituent afforded 2-alkylimino-3-aryl-l,3-benzoxazin-4-ones 505 in a completely regioselective manner <1999JOC9194>. On the palladium-catalyzed cyclocarbonylation of o-iodoanilines with acyl chlorides and carbon monoxide, 2-substituted-4f/-3,l-benzoxazin-4-ones were obtained <19990L1619>. 

The choice of catalyst is important, for instance the use of tetrakis(triphenylphos-phane)palladium(O) complex results in the quantitative cyclotrimerization of 3,3-dimethylcy-elopropene.17 In similar fashion 3,3-dimethoxycyclopropene cyciodimerizes to 3 (R = OMe, 74%) using bis(dibenzylideneacetone)paUadium(0) [Pd(dba)2] complex.18 The trisubstituted cyclopropene 4 is transformed to the head-to-head dimer 5 in the presence of copper(I) iodide.19... 

Nemoto, Yamamoto, and Cai[30a] later modified the preparation of their water-soluble carborane to include the attachment of a tumor seeking uracil moiety (Scheme 5.8). Key transformations allowing the synthesis of this unique dendritic carborane (33) included construction of masked uracil allyl carbonate 34 and its subsequent connection to the benzyl protected o-carborane cascade 35, the intermediate precursor to tetraol 30, via palladium bis(dibenzylideneacetone) [Pd(dba)2] and l,2-bis(diphenylphosphino)ethane (dppe) mediation. 

Oxatrimethylenemethanepalladium complexes can also be generated by oxidative addition of palladium(O) to 5-methylene-l,3-dioxolan-2-ones and subsequent decarboxylation. Again, reaction with norbornene, norbornadiene and dicyclopentadiene yields polycyclic cyclopropyl ketones in medium to high yield (Table 19). In this case, tetrakis(triphenylphosphane)pal-ladium(O) was the best catalyst found, whereas tris(dibenzylideneacetone)palladium(0)-chloro-form/triphenylphosphane (see above) and bis(cycloocta-l,5-diene)nickel/triphenylphosphane (used in stoichiometric amounts) proved less efficient. 

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