Palladium-catalyzed domino coupling reactions of aryl halides with norbornene and norbornene derivatives — a simple route to polycyclic aromatic compounds

Palladium/Norbornene Cooperative Catalysis

Palladium/norbornene cooperative catalysis has emerged as a distinct approach to construct polyfunctionalized arenes from readily available starting materials. This Review provides a comprehensive overview of this field, including the early stoichiometric investigations, catalytic reaction developments, as well as the applications in the syntheses of bioactive compounds and polymers. The section of catalytic reactions is divided into two parts according to the reaction initiation mode: Pd(0)-initiated reactions and Pd(II)-initiated reactions.

Modular Approach to Reductive C(sp2)-H and C(sp3)-H Silylation of Carboxylic Acid Derivatives through Single-Pot, Sequential Transition Metal Catalysis

We report a modular approach to catalytic reductive Csp2-H and Csp3-H silylation of carboxylic acid derivatives encompassing esters, ketones, and aldehydes. Choice of either an Ir(I)/Rh(I) or Rh(I)/Rh(I) sequence leads to either exhaustive reductive ester or reductive ketone/aldehyde silylation, respectively. Notably, a catalyst-controlled direct formation of doubly reduced silyl ethers is presented, specifically via Ir-catalyzed exhaustive hydrosilylation. The resulting silyl ethers undergo Csp2-H and benzylic Csp3-H silylation in a single vessel.

Of the ortho effect in palladium/norbornene-catalyzed reactions: a theoretical investigation

Mechanistic questions concerning palladium and norbornene catalyzed aryl-aryl coupling reactions are treated in this paper: how aryl halides react with the intermediate palladacycles, formed by interaction of the two catalysts with an aryl halide, and what is the rational explanation of the "ortho effect" (caused by an ortho substituent in the starting aryl halide), which leads to aryl-aryl coupling with a second molecule of aryl halide rather than to aryl-norbornyl coupling. Two possible pathways have been proposed, one involving aryl halide oxidative addition to the palladacycle, the other passing through a palladium(II) transmetalation, also involving the palladacycle, as previously proposed by Cardenas and Echavarren. Our DFT calculations using M06 show that, in palladium-catalyzed reaction of aryl halides, not containing ortho substituents, and norbornene, the intermediate palladacycle formed has a good probability to undergo transmetalation, energetically favored over the oxidative addition leading to Pd(IV). The unselective sp(2)-sp(2) and sp(2)-sp(3) coupling, experimentally observed in this case, can be explained in the framework of the transmetalation pathway since the energetic difference between aryl attack onto the aryl or norbornyl carbon of the palladacycle intermediate is quite small. On the other hand, according to the experimentally observed "ortho effect", selective aryl-aryl coupling only occurs in the reactions of ortho-substituted metallacycles. The present work offers the first possible rationalization of this finding. These in situ formed palladacycles containing an ortho substituent could more easily undergo oxidative addition of an aryl halide rather than reductive elimination from the transmetalation intermediate as a result of a steric clash in the transition state of the latter. The now energetically accessible Pd(IV) intermediate, featuring a Y-distorted trigonal bipyramidal structure, can account for the reported selective aryl-aryl coupling through a reductive elimination which is easier than aryl-norbornyl coupling. Thus, the steric effect represents the main factor that dictates the energetic convenience of the system to follow the Pd(IV) or the transmetalation pathway. Ortho substituents cause a higher energy transition state for reductive elimination from the transmetalation intermediate than for oxidative addition to the metallacycle palladium(II) and the pathway based on the latter predominates.

Carbocyclization of Aromatic Iodides, Bicyclic Alkenes, and Benzynes Involving a Palladium-Catalyzed C−H Bond Activation as a Key Step

A facile palladium-catalyzed carbocyclization reaction of aromatic iodides, bicyclic alkenes (norbornadiene, norbornene and oxabenzonorbornadiene), and benzynes to furnish various annulated 9,10-dihydrophenanthrene derivatives is described. The carbocyclization products from oxabenzonorbornadiene were further converted to polyaromatic hydrocarbons via a Lewis acid mediated deoxyaromatization reaction. [reaction: see text].

Understanding Sulfone Behavior in Palladium-Catalyzed Domino Reactions with Aryl Iodides

Unlike traditionally used acyclic 1,2-disubstituted alkenes, the reaction of alpha,beta-unsaturated phenyl sulfones with aryl iodides under Heck reaction conditions takes place mainly by means of a four-component domino process, involving one unit of the alkene and three units of the aryl iodide, affording substituted 9-phenylsulfonyl-9,10-dihydrophenanthrenes. We report here the results of a computational study on the mechanism of this domino arylation reaction. Based on these results we can explain why vinyl sulfones, unlike other electron-deficient alkenes such as enones, preferentially follow this domino pathway instead of the usual Heck pathway. The key step is a C-H activation process in which a five-membered palladacycle is formed. The greater ability of vinyl sulfones, relative to enones, to reach the transition state that leads to the formation of the initial palladacycle makes the difference.

Palladium-catalysed synthesis of nonsymmetrically disubstituted-1,1′-biphenyls from o-substituted aryl iodides through aryl coupling and delayed hydrogenolysis

In the presence of Pd(OAc)2 and norbornene as catalysts, two molecules of an ortho-substituted aryl iodide undergo aryl–aryl coupling to a palladium-bonded biphenylyl intermediate, which is hydrogenolyzed by benzyl alcohol to give 2,3′-disubstituted biaryl derivatives in satisfactory yields. A curious behaviour was observed with o-chloroiodobenzene, which gave activation of the C—Cl bond with four-membered ring formation.Key words: biphenyls, aryl–aryl coupling, multistep reactions, palladium, palladacycles, C-H activation, catalysis.

A RuH2(CO)(PPh3)3-Catalyzed Regioselective Arylation of Aromatic Ketones with Arylboronates via Carbon−Hydrogen Bond Cleavage

When the reaction of aromatic ketones with arylboronates (arylboronic acid esters) using RuH(2)(CO)(PPh(3))(3) (3) as a catalyst was conducted in toluene, the corresponding arylation product was obtained in moderate yields. In this case, a nearly equivalent amount of a benzyl alcohol derived from a reduction of an aromatic ketone was also formed. The use of aliphatic ketones, such as pinacolone and acetone, as an additive or a solvent dramatically suppressed the reduction of the aromatic ketones and, as a result, ortho-arylation products were obtained in high yield based on the aromatic ketones. In these reactions, the aliphatic ketone functioned as a scavenger of ortho-hydrogens of the aromatic ketones and the B(OR)(2) moiety of the arylboron compound (HB species). A variety of aromatic ketones, such as acetophenones, acetonaphthones, tetralones, and benzosuberone, could also be used in this coupling reaction. Several arylboronates containing electron-donating (NMe(2), OMe, and Me) and -withdrawing (CF(3) and F) groups were also applicable to this coupling reaction. Intermolecular competitive reaction using pivalophenone-d(0)() and -d(5) and intramolecular competitive reaction using pivalophenone-d(1) were carried out using 3 as a catalyst. The k(H)/k(D) value for the intermolecular competitive reaction was substantially different, compared with intramolecular competitive reaction. This strongly suggests the production of an intermediate where the ketone carbonyl is coordinated to the ruthenium involved in this catalytic reaction. (1)H and (11)B NMR studies using 2'-methylacetophenone, phenylboronate (2), and pinacolone (6) indicate that 6 functions effectively as a scavenger of the HB species.

Triarylation of eta 6-dihydronaphthalene-Cr(CO)3 complexes

With ease, eta 6-dihydronaphthalene tricarbonylchromium(0) complexes undergo a triple arylation with concomitant ring closure, when submitted to Heck reactions under 'Jeffery conditions'.

Tandem Suzuki Coupling−Norbornadiene Insertion Reactions. A Convenient Route to 5,6-Diarylnorbornene Compounds

This paper presents optimization studies on a palladium-catalyzed tandem norbornadiene insertion-Suzuki coupling reaction, which provides a one-pot procedure for the formation of diarylnorbornene derivatives. This procedure allows for the synthesis of these compounds from readily available aryl halides, arylboronic acids, and substituted norbornadienes.

Palladium-arene interactions in catalytic intermediates: an experimental and theoretical investigation of the soft rearrangement between eta1 and eta2 coordination modes

A series of dichloro-bridged arylbicycloheptylpalladium complexes have been synthesized and characterized by means of NMR spectroscopy. The compound [(C16H19)PdCl]2*CH2Cl2 with ortho and para methyl substituents at the arene has been characterized by means of X-ray diffraction techniques. The C(ipso) atom of the arene lies almost at the fourth planar coordination site of the metal [Pd-C(ipso) = 2.22(1) A (average)], and due to the arene's tilting, the substituted C(ortho) atom is relatively close to the metal atom [2.54(1) A (average)]. The coordinated C(ipso)-C(ortho) linkage, in a seemingly dihapto coordination, is anti with respect to the CH2 bridge of the bicycloheptyl unit. Variable-temperature NMR experiments for the para-substituted dimer 9 reveal restricted rotation of the two aryl groups about the corresponding C-C(ipso) bonds (DeltaE < or =17 kcal x mol(-1)). DFT-B3LYP calculations have been carried out on the known and similar monomer (phenylbicycloheptenyl)Pd(PPh3)I (4) and its related substituted derivatives. The essential results are as follows: (i) The potential energy surface for twisting the phenyl ring away from the symmetric eta(1) coordination in 4 is very flat (DeltaE < or = 1 kcal x mol(-1)) whereas an Atoms in Molecules analysis excludes the existence of an actual Pd-C(ortho) bond in the seemingly eta2-type conformer. (ii) Complete rotation of the unsubstituted phenyl ring is not facile but feasible. A significant strain affects the transition-state structure featuring a Pd-HC(aryl) agostic-type bond. The calculated destabilization of 10.3 kcal x mol(-1), with respect to the ground state, can be compared to the experimental barrier of the dimer 9. (iii) Various methyl-substituted derivatives of 4 have been optimized, and their structural and energetic trends are discussed. An almost ideal eta1 coordination is shown by the anti conformer of the C(ortho)-substituted complex due steric effects. For all of the other cases, a slipped eta2 coordination may be described. As a general conclusion, the unsaturated metal center receives pi electron density of the arene mainly through its C(ipso) atom. The effect may be slightly improved if the C(ortho) atom also gets closer to the metal, but in no case, does the slipped eta2 coordination seem to be crucial for the stability of the system.

A novel palladium-catalyzed synthesis of phenanthrenes from ortho-substituted aryl iodides and diphenyl- or alkylphenylacetylenes

[reaction--see text] 1,5-Selectively disubstituted 9,10-diphenyl- or alkylphenylphenanthrenes are obtained by reaction of ortho-substituted aryl iodides and diphenyl- or alkylphenylacetylenes in the presence of palladium and norbornene as catalysts. The reaction outcome is controlled by the steric effect exerted by the ortho substituent in the aryl iodide.

Synthesis, Structure, and Photophysical Studies of a Pair of Novel Rigid Bichromophoric Systems Bearing a Methyl Viologen Acceptor Unit

The rigid bichromophoric systems7a.2BF(4) and7b.2BF(4), comprising either a dimethoxybenzene (a) or dimethoxynaphthalene (b) unit, each covalently linked through a six-bond norbornylogous bridge to a methyl viologen unit, were synthesized with a view to studying long-range intramolecular energy (EnT) and/or electron (ET) transfer. X-ray crystallography has been used to illustrate that 7b.2BF(4) has the ability to undergo self-organization on crystallization to form extended linear arrays within the crystal lattice. Preliminary photophysical studies on 7b.2BF(4) indicate the presence of an additional rapid nonradiative process (rate constant > 2.5 x 10(10) s(-)(1)) in the bichromophore compared to a model dimethoxynaphthalene unit, and possible explanations for this process are presented.