Cycloadditions palladium complexes

A variety of triazole-based monophosphines (ClickPhos) 141 have been prepared via efficient 1,3-dipolar cycloaddition of readily available azides and acetylenes and their palladium complexes provided excellent yields in the amination reactions and Suzuki-Miyaura coupling reactions of unactivated aryl chlorides <06JOC3928>. A novel P,N-type ligand family (ClickPhine) is easily accessible using the Cu(I)-catalyzed azide-alkyne cycloaddition reaction and was tested in palladium-catalyzed allylic alkylation reactions <06OL3227>. Novel chiral ligands, (S)-(+)-l-substituted aryl-4-(l-phenyl) ethylformamido-5-amino-1,2,3-triazoles 142,... 

For example, cycloaddition of nitrone (643, R1 =Ph, R2 = Me) to DIO, catalyzed by chiral phosphine-palladium complexes (Fig. 2.42), gave isoxazolidines (644) in high yield with high enantioselectivity (794). 

Murakami and co-workers have shown that phenyl- and vinyl-substituted vinylallenes react in a palladium-catalyzed intermolecular [4+ 4]-cycloaddition in the presence of a palladium complex to give the cyclooctadiene cycloadducts in moderate to good yields (Scheme 29).103 In a method reported by Lee and Lee, bicyclo[6.4.0]-dodecatrienes are prepared in good overall yields via a two-step, one-flask procedure that involves a serial palladium-catalyzed cross-coupling/[4 + 4]-cycloaddition followed by [4 + 2]-cycloaddition (Scheme 30). Overall, this two-step process impressively brings together five simple components to form relatively complex bicyclic products.1... 

The first synthesis of 1,3-dioxolium-4-olates (here defined as oxamiinchnones) was reported in 1980 by Berk et al. (64) but it was work of Hamaguchi and Nagai (65,66) that demonstrated the accessibility and utility of these new mesoionic heterocycles in cycloaddition reactions. Thus, reaction of diazoacetic benzoic anhydrides 108 with a 7t-allyl palladium complex affords oxamiinchnones 109. 

In conjunction with this, Jeong reported the cycloadditions of bis(allyl) and bis(homoallyl) acetals of alkynals leading to bicyclic lactols. Smaltz extended its utility to the synthesis of carbocyclic nucleoside by coupling with nucleophilic substitution of a 7r-allylic palladium complex (Equation (46)). ... 

The addition of the trimethylenemethane-palladium complex to alkenes may proceed by a concerted process or via a stepwise mechanism in which the anion of the 1,3-dipole attacks Michael-fashion to generate an intermediate anion which collapses to form a five-membered ring by attack on the allylpalla-dium complex. This [3 + 2] cycloaddition reaction has been reviewed.128 A number of additional reports of its use have appeared recently.129-134... 

The palladium complex derived from TolBINAP catalyzes cycloaddition of vinyl epoxide to a carbodiimide with high enantioselectivity (Eq. 8E.12) [166], The stereochemical course of the cycloaddition of the epoxide with the heterocumulene was noticeably influenced by the structure of... 

The presence of five-membered rings such as cyclopentanes, cyclopentenes, and dihydrofurans in a wide range of target molecules has led to a variety of methods for their preparation. One of the most successful of these is the use of trimethylenemethane [3 + 2] cycloaddition, catalysed by pal-ladium(O) complexes. The trimethylenemethane unit in these reactions is derived from 2-[ (trimethylsilyl)methyl]-2-propen- 1-yl acetate which is at the same time an allyl silane and an allylic acetate. This makes it a weak nucleophile and an electrophile in the presence of palladium(0). Formation of the palladium 7t-allyl complex is followed by removal of the trimethylsilyl group by nucleophilic attack of the resulting acetate ion, thus producing a zwitterionic palladium complex that can undergo cycloaddition reactions. 

The selective synthesis of the 2-allyltetrazoles 55 by the three-component coupling reaction of the cyano compounds 54, allyl methyl carbonate 5b, and trimethylsilyl azide 42 was accomplished in the presence of Pd2(dba)3.CHCl3 and P(2-furyl)3 (Scheme 19) [55,56]. Most probably, the formation of (r)3-allyl)( ]5-tetrazoyl)-palladium complex 56 took place through [3 + 2] dipolar cycloaddition of 7r-allylpalladium azide 44 with the nitrile 54. The complex 56 thus formed would undergo reductive elimination to form the products 55. 

A nonconcerted mechanism, involving a least-hindeted approach of the cycloaddends and a stepwise addition of the palladium complex to the p-caibon of the a.p-unsaturated alkene, followed by cycliza-tion without loss of alkene geometry and/or competitive rotation about the Ca-Cp bond prior to cycliza tion with loss of stereochemistry, provides a reasonable account of the cycloaddition (Scheme 5). ... 

Dipolar cycloaddition of nitrones to olefins is also catalyzed by chiral cationic palladium complexes [66]. When nitrone 119 is treated with 3-(( )-2-butenoyl)-l,3-oxazoli-din-2-one (120) under the influence of (5)-BINAP PdCla (10 mol %) and AgBF4 (20 mol %), isoxazolidine 121 is obtained in 61 % yield as a 45 55 mixture of endo and exo forms. The endo isomer is obtained in 91 % ee (Sch. 32). The chemical yield and endo selectivity are improved by employing (5 )-p-Tol-BINAP as chiral ligand. 

Additions to nonactivated olefins and dienes are important reactions in organic synthesis [1]. Although cycloadditions may be used for additions to double bonds, the most common way to achieve such reactions is to activate the olefins with an electrophilic reagent. Electrophilic activation of the olefin or diene followed by a nucleophilic attack at one of the sp carbon atoms leads to a 1,2- or 1,4-addition. More recently, transition metals have been employed for the electrophilic activation of the double bond [2]. In particular, palladium(II) salts are known to activate carbon-carbon double bonds toward nucleophilic attack [3] and this is the basis for the Wacker process for industrial oxidation of ethylene to acetaldehyde [41. In this process, the key step is the nucleophilic attack by water on a (jt-ethylene)palladium complex. 

Acetoxymethyl)-3-(trimethylsilyl)propene (1) is used as a trimethylenemethane precursor in palladium(0)-catalvzed [3 + 2] cycloaddition with electron-deficient alkenes, such as enones, acrylonitriles and a,/ -unsaturaled esters23,26. Mechanistically, this reaction, which gives methylenecyclopentane derivatives 2. is interpreted to proceed via trimethylenemethane-palladium complexes 27,28. 

Cyclopropenes and mcthylcnccyclopropanes serve as multifunctional reagents in transition metal catalyzed reactions22. Methylenecyclopropanes, via C-C bond cleavage, are also used as trimethylenemethane precursors in transition metal catalyzed [3 + 2] cycloadditions for selective five-membered-ring formation. Low-valent nickel and palladium complexes are used as catalysts. This method has been extensively reviewed 22 and stereoselective applications are fully described in Section D.1.6.1.2.3. 

Tetracarbonylnickel and other nickel(O) compounds, as well as palladium complexes, catalyze the [2 + 2 + 1] cycloaddition of allylic systems with alkenes or alkynes and carbon monoxide to form cyclopentanones or cyclopentenones. This reaction type resembles stoichiometric zirconium- and cobalt-mediated [2 + 2 + 1] cycloadditions (vide supra), mechanistically, however, it proceeds via transition metal 7r-allyl complexes. 

The success of this carboxylative trimethylenemethane cycloaddition extends to the addition to cyclohexenone. In contrast to the poorly yielding process involving the unsubstituted TMM -Pd complex, a respectable yield of 49 % is obtained here. This is explained by the reduced basicity of the silylated complex, thus leading to fewer side reactions. The reaction is also considered to have a greater degree of concertedness and this becomes apparent in the discussion of chiral Z- and f-olefins in Section 1.6.1.2.3.2. The failure of in situ derived palladium complexes to yield the desired product is attributed to the basic conditions employed which result in double-bond migrations to the endocyclic, conjugated system. 

Cycloaddition. A new trapping partner for the trimethylene-palladium complex is carbon dioxide. 3-Methyl-2-buten-4-olide is formed. A,A -Di-r-butyl-l,2-diaziridinone behaves as a 1,3-dipolar species in the presence of (Ph3P)4Pd and such is trapped by alkenes. ... 

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