Marta Catellani

Catalytic Sequential Reactions Involving Palladacycle-Directed Aryl Coupling Steps

Catalytic methods are important tools for the synthesis of C-C bonds under mild and ambient conditions. Palladium chemistry predominates in this area because it offers the opportunity to form several different types of bonds in one pot. Palladium can also tolerate a variety of functional groups. Among the many investigations of catalytic aryl-aryl couplings, the most successful technique has been the Suzuki reaction, which uses an arylboronic acid to attack an aryl-Pd bond. This Account reports our methodology, based on the cooperative action of Pd and norbornene, that achieves selective aryl-aryl coupling through C-halide and C-H activation. We are primarily interested in Pd-catalyzed sequential reactions. These reactions combine palladium as an inorganic catalyst and a strained olefin such as norbornene as an organic catalyst and can lead to biphenyl derivatives. While the palladium facilitates C-C bond formation through C-halide and C-H activation, the norbornene contributes to the construction of a palladacycle, an intermediate structure that controls and directs the subsequent reaction steps selectively. To achieve regioselective arylation at the carbon ortho to the original C-halide bond, palladacycles require an additional ortho substituent (R(1)). The palladacycle opens, giving rise to a biphenylylnorbornylpalladium complex. Because of the steric hindrance exerted by the two ortho groups, norbornene deinsertion readily occurs to form a biphenylylpalladium complex. Thus, norbornene acts as a removable scaffold. We used this biphenylylpalladium species to form C-C (with olefins, alkynes, or arylboronic acids) or C-H bonds (by hydrogenolysis). Using nonidentical aryl or heteroaryl halides, we also formed a biaryl-bonded Pd species able to undergo the final termination reaction (C-C, C-N, or C-O bond formation) either inter- or intramolecularly. We used this method to synthesize a variety of aromatic and heteroaromatic compounds. We also obtained the key metallacycle able to selectively direct the reactions by replacing norbornene with an aryl-bonded aminocarbonyl group. This method provided a diverse series of condensed heterocycles.

Palladacycle formation by electrophilic aromatic substitution, as monitored by 1H NMR spectroscopy

Abstract The formation of alkylaromatic palladacycles proceeds through an electrophilic aromatic substitution, as shown by monitoring by 1 H NMR spectroscopy the reactions of variously substituted aromatic compounds.

Palladium-(II) and -(IV) complexes as intermediates in catalytic CC bond-forming reactions

Abstract Palladium-(II) and -(IV) species active as catalysts in CC bond-forming reactions have been stabilized by adding phenanthroline as a ligand. The complexes formed have been characterized by chemical and spectroscopic methods and their formation and subsequent fate have been monitored by 1 H NMR spectroscopy.

Conformational effects in elementary steps in catalytic reactions. Oxidative addition of 3-bromoprop-1-ene to a palladium(II) metallacyclic complex

Abstract The solution structure of an η 1 -allylpalladium(IV) complex formed by oxidative addition of bromopropene to a palladium(II) metallacyclic species has been determined by 1 H NMR spectroscopy. A cis attack from the less-hindered side is implied.

Palladium-catalyzed synthesis of 1,2,3,4,4a,12b-hexahydro-1,4-methanotriphenylenes

Abstract A catalytic synthesis of hexahydromethanotriphenylenes from bicyclo[2.2.1]hept-2-ene and aryl bromides is reported. Palladacycles are shown to be intermediates. Two modes of reaction of the latter with aryl bromides are observed.

A Sequential Pd/Norbornene-Catalyzed Process Generates o-Biaryl Carbaldehydes or Ketones via a Redox Reaction or 6H-Dibenzopyrans by C–O Ring Closure

o-Biaryl carbaldeydes and ketones are obtained through the one-pot reaction of o-aryl iodides with o-bromobenzyl alcohols under the catalytic action of Pd and norbornene, in the presence of a base. The same reaction can also give dibenzopyrans by Pd and norbornene catalysis with a different termination, leading to C-O ring closure. In both cases the process first leads to a five-membered palladacycle, which controls C-C coupling, then to a seven-membered oxapalladacycle, which gives aldehydes and ketones or dibenzopyrans.

Regioselective ring opening of a palladium(IV) alkylaromatic metallacycle by benzyl group migration from palladium to the aromatic carbon and X-ray structure of the resulting palladium(II) complex

Abstract The stereochemical course of a PdII → PdIV → PdII transformation is described for the first time through isolation of organometallic species containing the entire carbon skeleton.

A catalytic process based on sequential ortho-alkylation and vinylation of ortho-alkylaryl iodides via palladacycles

Abstract A new synthesis of vinylarenes selectively substituted in both their ortho positions with different alkyl groups is reported. The reaction occurs starting from ortho -alkyl substituted iodoarenes which are caused to react with aliphatic iodides, terminal olefins, K 2 CO 3 , AcOK and Pd(OAc) 2 in a one-pot reaction consisting of a series of elementary steps controlled by palladium in its oxidation states 0, II and IV. A correct balance of all the steps involved is required to obtain a catalytic reaction.

One-pot palladium-catalyzed synthesis of 2,3-disubstituted bicyclo[2.2.1]heptanes and bicyclo[2.2.1]hept-5-enes

Abstract A new and highly stereoselective palladium-catalyzed synthesis is reported, based on two subsequent insertions of the bicyclo[2.2.1]heptene system into an aryl or vinylpalladium bond, formed in situ from aryl or vinyl bromides.

A new palladium catalyzed synthesis of cis,exo-2,3-diarylsubstituted bicyclo[2.2.1]heptanes or bicyclo [2.2.1] hept-2-enes

Abstract The title bicycloheptanes or bicycloheptenes, containing a phenyl group and an aryl group in 2,3 positions, are obtained by oxidative addition of aryl bromides to palladium(0), followed by double bond insertion and coupling with a phenyl group of tetraphenyl borate anion.