The palladium(II)-catalyzed Nazarov reaction

The PdCl(2)-catalyzed cyclization of alpha-alkoxy dienones leads to 2-hydroxycyclopentenones, whereas the Pd(OAc)(2)-catalyzed reaction leads to cross-conjugated cyclopentenones through an oxidative process. [reaction: see text]     

Cu(II)-catalyzed oxidation of sulfides

A variety of sulfides and disulfides were converted into the corresponding sulfoxide derivatives with 70% t-BuOOH (water) as the oxidant in the presence of catalytic quantity of CuBr₂. The method described does not involve cumbersome work-up, has wide range of applicabilities, exhibits chemoselectivity, and proceeds under mild reaction conditions, and the resulting products are obtained in good yields within reasonable time.     

The chemistry of binuclear palladium(II) and platinum(II) complexes

The chemistry of binuclear palladium(II) and platinum(II) complexes has been reviewed. This review deals with complexes derived from various classes of ligands and covers various aspects, viz. synthesis, spectroscopic and structural features and chemical reactivity, of these complexes. Applications of these complexes are briefly described in the respected sections.     

New palladium(II)-catalyzed asymmetric 1,2-dibromo synthesis

[reaction: see text] The oxidation of olefins by chiral monometallic and bimetallic Pd(II)-Cu(II) catalysts in bromide-containing aqueous-THF reaction mixtures produced chiral 1,2-dibromides. With alpha-olefins, the ee's were about 95% while most of the internal alkenes gave somewhat lower enantioselectivities ( approximately 80%).     

Direct palladium(II)-catalyzed synthesis of arylamidines from aryltrifluoroborates

A fast and convenient synthesis of arylamidines starting from readily available potassium aryltrifluoroborates and cyanamides is reported. The coupling was achieved by Pd(II)-catalysis in a one step 20 min microwave protocol using Pd(O(2)CCF(3)), 6-methyl-2,2'-bipyridyl, TFA, and MeOH, providing the corresponding arylamidines in moderate to excellent yields.     

Mild and efficient palladium(II)-catalyzed racemization of allenes

Allenes undergo racemization in the presence of catalytic amounts of Pd(OAc)2/LiBr under mild conditions; the reaction proceeds via a bromopalladation-debromopalladation sequence and tolerates various functional groups.     

The chemistry of tri- and high-nuclearity palladium(II) and platinum(II) complexes

This review gives an overview of the progress on tri- and high-nuclearity palladium(II) platinum(II) complexes and discusses recent developments in the chemistry of these complexes. Three or more square-planar metal atoms can be assembled in several ways resulting into myriad geometric forms of the resulting complexes. These square planes may be sharing a corner, an edge and two edges or even separated by ligands having their donor atoms incapable of forming chelates yielding dendrimers and self-assembled molecules. A variety of ligands have been used to stabilize these complexes. The chemistry of these complexes has been dealt based on nuclearity of the complexes. Synthetic, spectroscopic, structural aspects and applications of these complexes are described in this review.     

Cationic palladium(II)-catalyzed stereoselective glycosylation with glycosyl trichloroacetimidates

The development of a new method for stereoselective glycosylation with glycosyl trichloroacetimidate donors employing cationic palladium(II), Pd(CH(3)CN)(4)(BF(4))(2), is described. This process employs Pd(CH(3)CN)(4)(BF(4))(2) as an efficient activator, providing access to a variety of disaccharides and glycopeptides. This reaction is highly stereoselective and proceeds under mild conditions with low catalyst loading. Interestingly, this palladium catalysis directs beta-glucosylations in the absence of classical neighboring group participation.     

Carbon-carbon bond formation in palladium(II)-catalyzed allylic oxidation: a novel oxidative carbocyclization of allene-substituted olefins

A new efficient palladium(II)-catalyzed oxidative carbocyclization has been developed. It was found that allene-substituted olefins 1 cyclized in the presence of 1 mol % Pd(O2CCF3)2 and p-benzoquinone (2 equiv) to give bicyclic ring systems 2 in good to excellent yields. The cyclization constitutes a new type of carbon-carbon bond forming reaction between an allene and an olefin under oxidative conditions.     

Convenient preparation of trans-arylalkenes via palladium(II)-catalyzed isomerization of cis-arylalkenes

A convenient method for the isomerization of cis-arylalkenes to their trans isomers using a palladium(II) catalyst is described. The reaction conditions are mild and general across a range of arylalkenes. The synthesis of a trans-resveratrol derivative from a mixture of alkene isomers was also completed.     

Amine- and phosphine-free palladium(II)-catalyzed homocoupling reaction of terminal alkynes

An efficient, amine- and phosphine-free palladium(II)-catalyzed homocoupling of terminal alkynes has been developed. In the presence of PdCl₂, CuI, Me₃NO, and NaOAc, homocoupling of various terminal alkynes underwent smoothly to afford the corresponding diynes in moderate to high yields without any phosphine ligands. In contrast, the presence of a phosphine ligand (PPh₃) disfavored this palladium-catalyzed homocoupling procedure. Bases, solvents, and CuI have fundamental influence on the palladium-catalyzed homocoupling of terminal alkynes.     

Scope of enantioselective palladium(II)-catalyzed aerobic alcohol oxidations with (-)-sparteine

Evaluation of the substrate scope for Pd(II)/ (-)-sparteine catalyzed aerobic oxidative kinetic resolution of secondary alcohols is disclosed. An improved system is found with use of tert-butyl alcohol solvent in which benzylic and aliphatic alcohols as well as alcohols containing olefins are effectively oxidatively resolved. For substrates that successfully undergo oxidative kinetic resolution, k(rel) values are generally between 10 and 20. Successful scale-up of various substrates to 10-mmol scale is described. Extension to oxidative desymmetrization of 1,3-meso-diols is successful with enantiomeric excesses ranging from 78 to 85%.     

Snapshot of the palladium(II)-catalyzed oxidative biaryl bond formation by X-ray analysis of the intermediate diaryl palladium(II) complex

Pd(II) caught in the act: The diaryl Pd(II) intermediate of a Pd(II)-catalyzed oxidative biaryl bond formation proceeding via a double C-H bond activation has been isolated and fully characterized, including an X-ray crystal structure analysis. Stabilization due to chelation by adjacent pivaloyloxy and acetyl groups has allowed the isolation of this long-sought crucial intermediate. On gentle warming, the complex is transformed into a carbazole product, and the catalytically active Pd(II) species is regenerated by oxidation with Cu(II).     

The coordination chemistry of simple phospholes: Complexes of nickel(II), palladium(II) and platinum(II)

The reactions between four very simply substituted phospholes and the chlorides of Ni(II), Pd(II) and Pt(II) are described. The phospholes 1-phenylphosphole, 3-methyl-1-phenyl-phosphole and 3,4dimethyl-1-phenylphosphole all readily form bis-complexes of formula L₂MCl₂ [L = phosphole ligand and M = Ni(II), Pd(II) or Pt(II)] or tris-complexes of formula L₃MCI₂. 1-n-Butyl-3,4-dimethylphosphole appears to form stable complexes only with Ni(II). Evidence is put forward which indicates that the L₂MCl₂ complexes exist in a four-coordinate, square-planar monomeric/five coordinate equilibrium while the L₃MCl₂ complexes are primarily the ionic species [L₃MCl]⁺ Cl^? in solution. Comparisons are made with the behaviour of other simple phospholes which do not form Ni(II) complexes and the results are discussed briefly in terms of both aromatic and non-aromatic phosphole models.     

Diversity synthesis using the complimentary reactivity of rhodium(II)- and palladium(II)-catalyzed reactions

Rhodium(II)-catalyzed reactions of aryldiazoacetates can be conducted in the presence of iodide, triflate, organoboron, and organostannane functionality, resulting in the formation of a variety of cyclopropanes or C-H insertion products with high stereoselectivity. The combination of the rhodium(II)-catalyzed reaction with a subsequent palladium(II)-catalyzed Suzuki coupling offers a novel strategy for diversity synthesis.     

Phosphine-free palladium(II)-catalyzed arylation of naphthalene and benzene with aryl iodides

A phosphine-free arylation of naphthalene and benzene with aryl iodides to give biaryls in moderate to good yields is carried out in the presence of catalytic Pd(OAc)2 and stoichiometric CF3CO2 Ag in TFE or/and TFA.     

Self-assembly in palladium(II) and platinum(II) chemistry: the biomimetic approach

The self-assembly of complex cationic structures by combination of cis-blocked square planar palladium(II) or platinum(II) units with bis(pyridyl) ligands having bridging amide units has been investigated. The reactions have yielded dimers, molecular triangles, and polymers depending primarily on the geometry of the bis(pyridyl) ligand. In many cases, the molecular units are further organized in the solid state through hydrogen bonding between amide units or between amide units and anions. The molecular triangle [Pt(3)(bu(2)bipy)(3)(mu-1)(3)](6+), M = Pd or Pt, bu(2)bipy = 4,4'-di-tert-butyl-2,2'-bipyridine, and 1 = N-(4-pyridinyl)isonicotinamide, stacks to give dimers by intertriangle NH.OC hydrogen bonding. The binuclear ring complexes [[Pd(LL)(mu-2)](2)](CF(3)SO(3))(4), LL = dppm = Ph(2)PCH(2)PPh(2) or dppp = Ph(2)P(CH(2))(3)PPh(2) and 2 = NC(5)H(4)-3-CH(2)NHCOCONHCH(2)-3-C(5)H(4)N, form transannular hydrogen bonds between the bridging ligands. The complexes [[Pd(LL)(mu-3)](2)](CF(3)SO(3))(4), LL = dppm or dppp, L = PPh(3), and 3 = N,N'-bis(pyridin-3-yl)-pyridine-2,6-dicarboxamide, and [[Pd(LL)(mu-4)](2)](CF(3)SO(3))(4), LL = dppm, dppp, or bu(2)bipy, L = PPh(3), and 4 = N,N'-bis(pyridin-4-yl)-pyridine-2,6-dicarboxamide, are suggested to exist as U-shaped or square dimers, respectively. The ligands N,N'-bis(pyridin-3-yl)isophthalamide, 5, or N,N'-bis(pyridin-4-yl)isophthalamide, 6, give the complexes [[Pd(LL)(mu-5)](2)](CF(3)SO(3))(4) or [[Pd(LL)(mu-6)](2)](CF(3)SO(3))(4), but when LL = dppm or dppp, the zigzag polymers [[Pd(LL)(mu-6)](x)](CF(3)SO(3))(2)(x) are formed. When LL = dppp, a structure determination shows formation of a laminated sheet structure by hydrogen bonding between amide NH groups and triflate anions of the type NH-OSO-HN.     

The renaissance of aluminum chemistry

The preparation and reactions of stable monomeric and tetrameric organoaluminum(I) compounds are described. Especially, their conversion to the more stable oxidation state (III) is discussed. The synthesis of unusual and unique molecules such as LAl(OH)(2), LAl(SH)(2), LAl(SeH)(2), and LAl(NH(2))(2) is reported. A new synthetic approach to organometallic hydroxides and amides was discovered, when N-heterocyclic carbenes are used as hydrogen chloride acceptors. The synthesis of LGeOH demonstrates that this method can also be applied for other elements in the periodic system. The successful preparation of carbaalanes and aluminum nitride clusters resulted in the generation of several model compounds for the fixation of organometal substituents on carbide and nitride surfaces. It is shown that AlH(3).NMe(3) is a powerful hydroalumination reagent for acetylenes, nitriles, and isonitriles.     

Versatile palladium(II)-catalyzed Negishi coupling reactions with functionalized conjugated alkenyl chlorides

Under palladium catalysis, we found that organozincate reagents, generated in situ, by reaction of Grignard compounds with less than molar amounts of zinc chloride, in the presence of conjugated alkenyl chlorides, give rapidly and cleanly the corresponding coupling product in high yields. In this way, aryl as well as primary and secondary alkyl substituents have been introduced successfully. The selectivity of the reaction allows to prepare various functionalized conjugated enynes and dienes from chloroenyne and chlorodiene derivatives bearing a functional group.     

Palladium(II)-catalyzed oxidation of aldehydes and ketones. 1. Carbonylation of ketones with carbon monoxide catalyzed by palladium(II) chloride in methanol.

Unsubstituted or alkyl-substituted cyclic ketones react with PdCl2 in methanol under a CO atmosphere to give mainly acyclic diesters along with some acyclic chloro-substituted monoesters. The monosubstituted cyclic ketones, 2-hydroxy- and 2-methoxycyclohexanone, do not give ring cleavage but rather produce 2-(carbomethoxy)cyclohex-2-en-1-one. 13CO labeling experiments indicate one CO is inserted in forming the diester product so the second ester group must arise from the original ketone group. Two mechanisms are possible for the diester reaction. One involves initial Pd(II)-CO2CH3 insertion across the double bond of the enol form of the ketone while the second involves initial addition of Pd(II)-OCH3 followed by CO insertion into the new Pd(II)-carbon bond formed. Pd(II) elimination and acid-catalyzed ring cleavage produce the second methyl ester group in both routes. The chloro-substituted monoester is formed by initial Pd(II)-Cl insertion across the double bond followed by the acid-catalyzed ring cleavage. The 2-(carbomethoxy)cyclohex-2-en-1-one must result from elimination of water or methanol from the alpha-ketoester product formed by the initial methoxycarbonylation of the enol form of the ketone. As expected, the acyclic ketone, 2-decanone, formed methyl acetate and a mixture of methyl nonanoate and 1-chlorooctane as products.