Cu-catalyzed synthesis of symmetric group 6 (Fischer) bis-carbene complexes

The efficient copper oxidative homocoupling (CuCl/TMEDA/O2) of alkynyl Fischer carbene complexes yields bis-carbene complexes having pi-tethers. The bimetallic complexes are exceptional templates to prepare diverse organic symmetric structures connected by pi-extended fragments.     

Pd-catalyzed inter- and intramolecular carbene transfer from group 6 metal-carbene complexes.

The use of group 6 metal-carbene complexes in inter- and intramolecular carbene transfer reactions has been studied. Thus, pentacarbonyl[(aryl)(methoxy)carbene]chromium(0) and tungsten complexes, 10, efficiently dimerize at room temperature in the presence of diverse Pd(0) and Pd(II)/Et(3)N catalysts. The effect of additives (PPh(3), AsPh(3), or SbPh(3)) on the nature and the isomeric ratio of the reaction products is negligible. The nature of the reaction products is more catalyst-dependent for metal carbenes 12 bearing alkyl groups attached to the carbene carbon. In these cases, either carbene ligand dimerization or beta-hydrogen elimination reactions are observed, depending on the catalyst. The carbene ligand dimerization reaction can be used to prepare conjugated polyenes, including those having metal moieties at both ends of the polyene system, as well as enediyne derivatives. The intramolecular carbene ligand dimerization of chromium bis-carbene complexes 28 and 30 allows the preparation of mono- and bicyclic derivatives, with ring sizes from six to nine members. For bis-carbene derivatives the beta-hydrogen elimination reaction is inhibited, provided that both metal centers are tethered by an o-xylylene group. Other alkyl complexes 32 form new mononuclear carbene complexes 37 or decompose to complex reaction mixtures. The results obtained in these reactions may be explained by transmetalation from Cr(0) to Pd(0) and the intermediacy of Pd-carbene complexes. Aminocarbene-chromium(0) complexes 15, need harsher reaction conditions to transfer the carbene ligand, and this transfer occurs only in the presence of deactivated olefins. The corresponding insertion/hydrolysis products 48 resulted in these cases. A catalytic cycle involving transmetalation from a chromacyclobutane to a palladacyclobutane is proposed to explain these results.     

New cascade processes on group 6 Fischer-type carbene complexes: cyclopropanation and metathesis reactions

Alkynyl Fischer carbene complexes 1, which feature a pendant olefin group, undergo novel cascade processes triggered by [2+2], [3+2], and [4+2] cycloadditions. Competition between cyclopropanation and olefin metathesis is controlled by the substitution at the double bond. The method described can be used to obtain different kind of polycarbo- and -heterocycles. A mechanistic explanation by means of DFT computational modeling is provided.     

Lewis acid promoted phenylseleno group transfer tandem radical cyclization reactions

[reaction: see text] A new Lewis acid promoted phenylseleno group transfer tandem radical cyclization method was developed. In the presence of Lewis acids such as Yb(OTf)3 or Mg(ClO4)2, under photolysis condition at low temperature (-45 degrees C), various unsaturated alpha-phenylseleno beta-keto amides underwent radical cyclization reactions to give monocyclic or bicyclic products in a highly efficient, regioselective, and stereoselective manner.     

New mono- and bis-carbene samarium complexes: synthesis, X-ray crystal structures and reactivity

New samarium carbene complexes have been synthesized and characterized by X-ray diffraction analysis; the carbenic nature was assessed by reactivity studies.     

Pd-catalyzed asymmetric aza-Wacker-type cyclization reaction of olefinic tosylamides

The Pd-catalyzed asymmetric aza-Wacker-type cyclization reaction of the olefinic tosylamides with molecular oxygen as the green oxidant was developed. Under the optimized conditions, excellent catalytic activity and good enantioselectivity with up to 74% ee were obtained with quinolineoxazoline ligands. This reaction provides direct and easy access to chiral nitrogen-containing heterocycles which retain the olefin functionality.     

Pd-catalyzed stereoselective tandem ring-opening amination/cyclization of vinyl γ-lactones: access to caprolactam diversity

A stereoselective amination/cyclization cascade process has been developed that allows for the preparation of a series of unsaturated and substituted caprolactam derivatives in good yields. This conceptually novel protocol takes advantage of the easy access and modular character of vinyl γ-lactones that can be prepared from simple precursors. Activation of the lactone substrate in the presence of a suitable Pd precursor and newly developed phosphoramidite ligand offers a stereocontrolled ring-opening/allylic amination manifold under ambient conditions. The intermediate (E)-configured ε-amino acid can be cyclized using a suitable dehydrating agent in an efficient one-pot, two-step sequence. This overall highly chemo-, stereo- and regio-selective transformation streamlines the production of a wide variety of modifiable and valuable caprolactam building blocks in an operationally attractive way.

A Pd-mediated stereoselective tandem process has been developed towards a large variety of functional caprolactam synthons.     

Synthesis of tricyclic and tetracyclic sultones by Pd-catalyzed intramolecular cyclization

A new efficient synthesis of aromatic six-membered ring sultones by the implementation of ligand-free Pd-catalyzed intramolecular cyclization of aromatic sulfonates derived from various bromo phenols and naphthols is described.     

Stereochemistry as a tool in deciphering the processes of a tandem iminium cyclization and Smiles rearrangement

To understand the detailed mechanism of a recently reported tandem iminium cyclization and Smiles rearrangement, the reaction processes of a chiral substrate were investigated by monitoring its stereochemical courses. Under the tandem reaction conditions, chiral aldehyde 1 derived from l-prolinol led to two surprising results. First, the iminium cyclization gave a diastereomeric mixture with the cis-configured product as the predominant one. Second, Smiles rearrangement of both cis- and trans-2 led to the same product 3a directly derived from the trans isomer. The former was rationalized by the postulation of a Cram's chelate transition state leading to the cis product as kinetically favored. The latter was due to the equilibration between the trans/cis pair involving a carbocation intermediate and the steric hindrance, which prevented the cis isomer from undergoing the intramolecular nucleophilic substitution. This hypothesis was further supported by the results of a competition experiment in which the addition of 1 equiv of p-methoxyaniline in the rearrangement step led to a significant amount of anilinyl-exchanged rearrangement product.     

Mercuric triflate-catalyzed tandem cyclization leading to polycarbocycles

[reaction: see text] We developed Hg(OTf)2-catalyzed cyclization of (E)-1,3-dimethoxy-5-(4-methyl-3-nonen-7-ynyl)benzene leading to the formation of (4aS,10aS)-3,4,4a,9,10,10a-hexahydro-5,7-dimethoxy-1,4a-dimethylphenanthrene in 98% yield with up to 100 catalytic turnovers. This is the first mercuric salt-catalyzed biomimetic tandem cyclization.     

Asymmetric hydroamination/cyclization catalyzed by group 4 metal complexes with chiral biaryl-based ligands

Group 4 metal complexes based on chiral biaryl ligands are readily prepared by a convenient amine elimination procedure, and they are efficient catalysts for the asymmetric hydroamination/cyclization of aminoalkenes. The biaryl-based ligands are highly modular enabling facile tuning of the catalyst reactivity and selectivity. The corresponding heterocyclic products have been obtained in excellent enantiomeric excesses (up to 93%) using sterically hindered C₂-symmetric titanium and zirconium mesitoylamidate complexes such as [1,1′-(C₁₀H₁₀)₂-2,2′-{NCO(2,4,6-Me₃C₆H₂)}₂]M(NMe₂)₂ (M = Ti, Zr). These results are presented in this short review.     

New cationic group 4 metal alkyl complexes

Cationic group 4 metal alkyl complexes containing tetradentate Schiff base ligands, (acen) Zr(R)⁺ and (F6-acen) Zr(R)⁺, are prepared by protonolysis of suitable neutral dialkyl precursors. These complexes display electrophilic behavior and are moderately active ethylene polymerization catalysts in the presence of 1 molar equivalent of AlR₃.     

Development of a 5-exo, 6-exo tandem radical cyclization to access bisabosqual A

Substrates were designed to undergo tandem radical cyclizations to afford the cis, cis-fused hexahydrobenzofurobenzopyran ring system of the bisabosquals. Studies with these revealed that different initiating systems gave different yields and somewhat different diastereomeric ratios. A photocatalytic cyclization was also successful with the bisabosqual substrate.     

Tetracarbonyl group 6 complexes containing secondary and tertiary phosphines

The mixed ligand tetracarbonyl derivatives, cis-M(CO)₄(PPh₂H)(PPh₃) (M  Cr, Mo, W) and cis-W(CO)₄(PPh₂H)(L) (L  PEt₃, PEt₂Ph, PEtPh₂) have been prepared from the reaction of M(CO)₅PPh₂H with L in THF in the presence of potassium t-butoxide. These reactions are accompanied in most instances by the formation of [W(CO)₅PPh₂]⁻, [(OC)₅M(μ-PPh₂)M(CO)₅]⁻, [(OC)₅M(μ-PPh₂)-M(CO)₄(PPh₂H)]⁻, [(OC)₄M(μ-PPh₂)₂M(CO)₄]²⁻, (OC)₄M(μ-PPh₂)₂M(CO)₄, and cis-M(CO)₄(PPh₂H)₂.     

Palladium-catalyzed tandem cyclization of fluorinated imidoyl chlorides with 2-bromophenylboronic acid: Synthesis of 6-fluoroalkyl-phenanthridines

An efficient method has been developed to synthesize 6-fluoroalkyl-phenanthridines via the palladium-catalyzed tandem cyclization of fluorinated imidoyl chlorides with 2-bromophenylboronic acid. This methodology facilitates the rapid synthesis of 6-fluoroalkyl-phenanthridines through dual C–C bond formation in an oxidant-free one-pot manner.     

Syntheses and characterizations of group 6 metal cyanotrihydroborate complexes

The anions, [M(CO)_6-n(NCBH₃)_n]ⁿ⁻ (n=2, M=Cr(1); n=3, M=Cr(2), Mo(3), W(4)), were prepared either from the reactions of sodium cyanotrihydroborate with group 6 transition metal hexacarbonyls, M(CO)₆ (M=Cr, Mo, W), or through the reactions of M(CO)₃(CH₃CN)₃ (M=Cr, W) with sodium cyanotrihydroborate. The cyanotrihydroborate ligand bonds to the metal through a nitrogen atom, which was confirmed by the Infrared, proton and boron NMR spectroscopies. Crystal structures of the above complexes were determined by single crystal X-ray diffraction analyses. A cis configuration is found in 1. Molecular structures of 2, 3, and 4 are similar and a facial configuration is observed.     

One-pot synthesis of 6H-2,2a1,3-triazaaceanthrylen-6-ones and 6H-2,2a1,4-triazaaceanthrylen-6-ones via tandem cyclization strategies

Two operationally simple one-pot protocols have been developed for the synthesis of 6H-2,2a¹,3-triazaaceanthrylen-6-ones and 6H-2,2a¹,4-triazaaceanthrylen-6-ones. The first Pd-catalyzed tandem cyclization of imidazo[1,2-a]pyrimidines/imidazo[1,2-a]pyrazines with 2-chlorobenzaldehydes could proceed in aqueous medium under air, affording the desired products in moderate to good yields. The molecular structures of products 3i and 5b were confirmed by X-ray crystallographic analysis.     

Regiospecific palladium catalysed tandem cyclisation-anion capture processes. Stereospecific group transfer from organotin reagents.

Vinyl-, alkyl- and π-allyl-palladium species arising from regiospecific palladium catalysed 5- or 6-exo-trig cyclisations onto proximate alkynes, alkenes or dienes can be intercepted by organotin reagents with stereospecific transfer of an organic group from tin leading to carbo- and hetero-cyclic compounds in moderate to good yield.     

Exploring the oxidative cyclization of substituted N-aryl enamines: Pd-catalyzed formation of indoles from anilines

The direct Pd-catalyzed oxidative coupling of two C-H-bonds within N-aryl-enamines 1 allows the efficient formation of differently substituted indoles 2. In this cross-dehydrogenative coupling, many different functional groups are tolerated and the starting material N-aryl-enamines 1 can be easily prepared in one step from commercially available anilines. In addition, the whole sequence can also be run in a one-pot fashion. Optimization data, mechanistic insight, substrate scope, and applications are reported in this full paper.