Facile palladium(0)-catalyzed ring expansion reactions of hydroxy methoxyallenyl cyclic compounds via hydropalladation

Palladium(0)-catalyzed one-atom ring expansion of various hydroxy methoxyallenyl compounds has been achieved in excellent yields without the use of aryl halides. Hydroxy methoxyallenylisoindolinones, -indanones, and -phthalans have been readily converted to the corresponding isoquinolones, naphthoquinones, and isochromanones in the presence of P(o-tolyl)(3).     

Medium-sized rings versus macrocycles through rhodium-catalyzed ring-expansion reactions of cyclic acetals

α-Diazo β-ketoesters and diketones react with cyclic acetals under Rh(II) catalysis to yield unprecedented polyoxygenated 8- and 9-membered rings in one pot. The reactions occur under mild conditions with yields up to 90%. A perfect regioselectivity is obtained, which can be rationalized through a mechanistic hypothesis that considers 1) the formation of an oxonium ylide, 2) its transformation into an unsaturated acyclic oxocarbenium electrophilic intermediate, and 3) an intramolecular nucleophilic attack in a direct application of Baldwin's rules.     

A new synthesis for thermolabile low-valent palladium complexes by electron transfer reactions from nickel(0) to palladium(II) compounds

The nickel(0) complex [Ni(bpy)(cod)] (bpy: 2,2′-bipyridine, cod: cycloocta-1,5-diene) was used as a mild reducing reagent for the synthesis of the extremely reactive low-valent palladium complexes [Pd₂X₂(cod)₂] (1: X = Cl, 2: X = Br), Pd(cod)₂ (3) and Pd(norbornene)₃ (4). The X-ray analysis of 1 showed that the two [Pd(cod)(Cl)] moieties are only connected by a short Pd(I)-Pd(I) bond (bond length: 2.5379(4) Å) with the chloride ions as monodentate ligands. The X-ray structure of 3 which is also known to be an extremely reactive compound could be determined by X-ray diffraction. As expected, the Pd(0) centre is surrounded by the two cod ligands to form a PdC₄ tetrahedron with typical Pd-C bond lengths. The crystal structure of 3 shows it to be very similar to the closely related complexes M(cod)₂ (M: Ni, Pt). The X-ray structure of 4 displays that the Pd(0) centre is in a trigonal planar environment of the three olefin groups. According to ¹H NMR measurements the complexes have the same structure in solution as found in the solid state.     

Reactions of silica chloride (SiO(2)Cl)/DMSO, a heterogeneous system for the facile regeneration of carbonyl compounds from thioacetals and ring-expansion annelation of cyclic thioacetals

Silica chloride (SiO(2)Cl)/DMSO, as a heterogeneous system, has been efficiently used for deprotection of thioacetals into aldehydes in dry CH(2)Cl(2) at room temperature. Thioketals without enolizable hydrogens adjacent to a sulfur atom are converted easily to the corresponding ketones in high yields under similar reaction conditions. However, thioketals with enolizable methyl and methylene groups undergo ring-expansion reactions to afford 1,4-dithiepins and 1,4-dithiins in dry CH(2)Cl(2) at room temperature in good yields.     

Some reactions of 3-halogenocinnolines catalysed by palladium compounds

3-Bromo- or 3-iodo-cinnoline (and 4-substituted analogues) are condensed with terminal alkynes in the presence of Pd and Cu compounds as catalysts to give the 3-alkynyl-derivatives. When 4-chloro- or 4-phenoxy-compounds are used, the products react with amines, in the presence of copper(I) iodide, to form pyrrolo[3,2-c]cinnolines and with hydrazines to give either the same ring system or a pyrazolo[4,3-c]cinnoline. Hydrolysis of 3-alkynyl-4-phenoxycinnoline to 3-alkynyl-4(1H)-cinnoline, followed by cyclisation, yields the furo[3,2-c]cinnoline. Attempts to condense 3-halogenocinnolines with alkenes gave variable results: 3-phenyl ethenyl- and 3(2-pyridylethenyl)-4-(1H)-cinnolinones were obtained but 3-bromocinnoline gave the 3,3′-bicinnolinyl. Action of palladium acetate in the presence of ethyl acrylate converted 3-bromo-4-phenoxycinnoline into benzofuro[3,2-c]cinnoline and 3-bromo-4-phenylaminocinnoline into indolo[3,2-c]cinnoline.     

Cyclometallation reactions of N-(benzylidene)benzylamines with palladium compounds

The cyclometallation of p-RC₆H₄CHNCH₂C₆H₂, (R = H, Cl, NO₂) by PdX₂ (X = Cl, AcO) has been studied.In every case the cyclometallation occurs with formation of a five-membered ring containing the methine group. The structure of these compounds [$\text{PdX(}\text{p}\text{-RC}{}_6\text{H}{}_3\text{CHN}$CH₂C₆H₅)]₂, derived from ¹H NMR spectra, are different from those reported previously. Reaction of these compounds with PEt₃ gives the compounds [PdX(p-RC₆H₃CHNCH₂C₆H₅)(PEt₃)₂] but with an excess of PPh₃ only the complexes [$\text{PdX(}\text{p}\text{-RC}{}_6\text{H}{}_3\text{CHN}$CH₂C₆H₅)(PPh₃)] are formed.     

Solid supported synthesis of phosphinates via palladium (0) catalysed coupling reactions

In order to gain a broad access to phosphinic acid derivatives, a palladium catalysed coupling reaction of aryl iodides with hypophosphorous acid derivatives has been developed on the solid phase. The resulting arylphosphorous acids (or esters) were derivatised using addition reactions with aldehydes, imines and isocyanates, to give phosphinic acids (or esters) with alpha-hydroxy, alpha-amino or aminoacyl groups attached to the aryl phosphorus moiety. This approach provided a broad chemical entry into a class of polar phosphinates compounds which were rather difficult to handle using normal solution phase synthesis. The synthetic potential of this solid phase based methodology was demonstrated by the synthesis of targeted libraries against the enzyme dihydrodipicolinate synthase (DHDPS).     

New coordination compounds of palladium(II) with iminodiacetic acid

Summary Iminodiacetic acid (IDA) forms 1:1 or 1:2 complexes with Pdll at room temperature and pH 6.0. The solid compounds have been characterized by elemental analysis, titrimetry, magnetic measurements, i.r. spectroscopy and thermal studies. The far i.r. spectra show peaks that can be assigned to Pd-Cl, Pd-N and Pd-O vibrations. On the basis of these studies tentative structures for the isolated substances are proposed.     

Regiochemical studies of the ring expansion reactions of hydroxy azides with cyclic ketones

The regiochemistry of ring expansions of 2-substituted cyclic ketones using 1,2-azidoethanol and 1,3-azidopropanol was examined. It was determined that the reactions of ketones with an adjacent methyl or ethyl group are generally unselective, but that bulkier substituents lead to preferential migration of the more highly substituted carbon. In addition, it was found that ketones bearing inductively electron-withdrawing substituents (OMe, Ph, Br) undergo selective migration of the less highly substituted carbon. For some substrates, alternative reaction pathways were also identified.     

Oxidative addition and insertion reactions of cyclic alkyne-platinum(0) complexes

The reactions of various alkyne-platinum(0) complexes with methyl iodide and with iodine have been studied. The 3-hexyne complex Pt(C₂H₅C₂C₂H₅)(PPh₃)₂ gives alkyne-free oxidative addition products PtI(CH₃) (PPh₃)₂ and PtI₂ (PPh₃)₂ exclusively. In contrast, the strained cyclic alkyne complexes Pt(C₆H₈)(PPh₃)₂, Pt(C₇H₁₀)(PPh₃)₂, Pt(C₆H₈) (dppe) and Pt(C₇H₁₀) (dppe)¹ react with methyl iodide to give mainly 2-methylcycloalkenyiplatinum(II) complexes, e.g. PtI(C₆H₈CH₃) (PPh₃)₂, formed by electrophilic attack on the metal-alkyne bond. Iodine reacts similarly with Pt(C₆H₈) (PPh₃)₂ and Pt(C₇H₁₀) (PPh₃)₂ to give 2-iodocycloalkenylplatinum(II) complexes but, in the case of the corresponding dppe complexes, PtI₂(dppe) is the main product. The insertion reaction of methyl iodide with Pt(C₆H₈)(PPh₃)₂ proceeds via an oxidative addition intermediate PtI(CH₃) (C₆H₈) (PPh₃)₂ which can be isolated. Trifluoromethyl iodide reacts with Pt(C₆H₈)(PPh₃)₂ to give a 2-iodocyclohexenyl complex Pt(CF₃) (C₆H₈I) (PPh₃)₂ and with Pt(C₇H₁₀) (PPh₃)₂ to give PtI(CF₃) (PPh₃)₂. ³¹P NMR data are given and discussed.     

New aerobic oxidation of benzylic compounds: efficient catalysis by N-hydroxy-3,4,5,6-tetraphenylphthalimide (NHTPPI)/CuCl under mild conditions and low catalyst loading

Efficient aerobic oxidation of benzylic compounds using NHTPPI, a new NHPI analogue, as a key catalyst combined with CuCl, have been achieved under mild conditions and using as little as 1 mol% catalyst.     

Intermolecular reactions of chlorohydrine anions: acetalization of carbonyl compounds under basic conditions

[reaction: see text] Nonenolizable aldehydes and ketones react with 2-chloroethanol and 3-chloropropanol under basic conditions (t-BuOK, DMF/THF) with formation of 2-substituted 1,3-dioxolanes and 1,3-dioxanes, respectively. Conversion of the two-step addition-alkylation process depends on the electrophilicity of the carbonyl group that governs the equilibrium of addition of chloroalkoxides. This method of protection of carbonyl groups in the form of cyclic acetals under kinetically controlled conditions is complementary to the acid-catalyzed reaction with diols.     

New copper(II)/cyclic tetrapeptide system that easily oxidizes to copper(III) under atmospheric oxygen

A 13-membered ring cyclic tetrapeptide was synthesized by the solid-phase peptide synthesis method, and its copper(II) coordination properties were analyzed by optical spectroscopy, mass spectrometry, and electrochemistry. All collected data strongly support the presence, at alkaline pH, of a stable peptide/copper(III) complex that is formed in solution by atmospheric dioxygen oxidation. On the basis of previous studies on cyclic peptide/copper systems, we suggest that the copper(III) ion is at the center of the ligand's cavity being coordinated to four deprotonated amide nitrogen atoms. This donor set would greatly lower the redox potential for the CuIII/CuII couple, thus allowing easy oxidation of the coordinated copper(II) by atmospheric oxygen.     

Highly efficient phosphine-catalyzed aza-Michael reactions of α,β-unsaturated compounds with carbamates in the presence of TMSCl

Aza-Michael reactions of enones with carbamates took place efficiently in the presence of a catalytic amount of phosphine and TMSCl to afford the total products in high yields. The new catalytic system was also efficient in the aza-Michael reaction of chalcone, which was difficult to react with carbamates by transition metal salts catalysts.     

Phosphine-free Sonogashira coupling: reactions of aryl halides catalysed by palladium(II) complexes of azetidine-derived polyamines under mild conditions

Readily synthesised, water-stable pyridylazetidine-based Pd(II) complexes have been studied as catalysts for the Sonogashira coupling reaction. Under low catalyst loadings, various aryl bromides and chlorides were efficiently coupled with phenylacetylene at only moderately elevated temperatures (50-70 °C) and, in some cases, even at room temperature. Not only was the catalysis efficient under mild conditions but it was also operative in aerated, partly aqueous and phosphine-free media.     

Suzuki-Miyaura reactions of arenediazonium salts catalyzed by PD(0)/C. One-pot chemoselective double cross-coupling reactions

The Suzuki-Miyaura cross-coupling of arenediazonium tetrafluoroborate salts with boronic acid partners catalyzed by Pd(0)/C is described as a practical and efficient alternative to classical homogeneous conditions. Reactions conducted in alcoholic solvents proved to be extremely fast using mild conditions. Additionally, we developed a chemoselective double Suzuki-Miyaura cross-coupling in a single reaction vessel allowing the synthesis of unsymmetrical terphenyls.     

Reactions of tris(dimethylsilyl)methane and polymers containing si—h groups with various hydroxy compounds under aerobic and mild conditions

Tris(dimethylsilyl)methane, (HMe₂Si)CH, reacts with n‐propyl, iso‐propyl, n‐butyl, s‐butyl, iso‐butyl, n‐pentyl, s‐pentyl, n‐hexyl alcohol, and phenol in the presence of chloroplatinic acid (H₂PtCl₆ċ6H₂O) in air to give products (ROMe₂Si)₃CH (where R is n‐propyl, iso‐propyl, n‐butyl, s‐butyl, iso‐butyl, n‐pentyl, s‐pentyl, n‐hexyl, phenyl). Similar reactions with benzyl alcohol and acetic acid lead to the unexpected products, (PhCH₂)₂O and HC(Me₂SiOSiMe₂)₃CH, respectively. The compound (n‐BuOMe₂Si)₃CSiMe₂H was made from (n‐BuOMe₂Si)₃CH by treatment with lithium diisopropylamide, followed by quenching with HMe₂SiCl. The homopolymer poly{(4‐chloromethyl)styrene} and copolymers with styrene (in 1:1 and 1:3 mol ratio) were synthesized by free radical polymerization, and (HMe₂Si)₃C groups were linked to them by nucleophilic substitution reactions between (HMe₂Si)₃CLi and the chloromethyl groups of the polymer side chains. The resulting functional polymers containing Si—H bonds reacted with various alcohols in the presence of chloroplatinic acid under heterogeneous conditions. © 2008 Wiley Periodicals, Inc. Heteroatom Chem 19:365–376, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20440