Luciano Cuesta

Orthometallation as a Strategy in Pd-mediated Organic Synthesis

The orthopalladated complexes are among the most representative Pd(II) compounds. They display a wide prospect of applications, but they are mainly known by their use as intermediates in metal-mediated organic synthesis. In this review we describe how palladacycles are used to build up new molecules through regioselective formation of C-halogen, C-O, C-N, C-P, C-S and C-C bonds, either under catalytic or stoichiometric conditions. This methodology is based on the selective incorporation of the Pd into the organic skeleton to be modified and subsequent reactivity of the Pd-C bond toward different substrates. In many cases this strategy is alternative or even competitive, improving standard organic methods. The most recent achievements of the last five years will be covered here, presenting only the most impressive results.

Cyclometallation of Heterocycles: A Reliable Strategy for Selective Functionalization

Heterocyclic compounds are of paramount importance in essentially all fields of chemistry and in our daily life. Therefore, the synthesis and modification of such compounds is a constantly expanding field in synthetic chemistry. Recent developments of functionalization of heterocycles by means of cyclometallation (also called chelation-assisted functionalization), both stoichiometric and catalytic, indicate that this strategy displays enormous potential. In particular, this method facilitates considerably the synthesis of functionalized heterocycles in a regioselective (site-selective) manner, and also provides novel types of synthetic transformations.

Binuclear organometallic ruthenium complexes of a Schiff base expanded porphyrin

The synthesis of binuclear organometallic ruthenium complexes of an expanded porphyrin-type macrocycle is reported; pyrrolic hydrogen bonding donors were found to interact with ancillary ligands in the primary coordination sphere and to stabilize coordinated dioxygen in an eta(2)-fashion.

Synthesis, Structure, and Reactivity of N-Benzoyl Iminophosphoranes Ortho Lithiated at the Benzoyl Group

Ortho lithiation of N-benzamido-P,P,P-triaryliminophosphoranes through deprotonation with alkyllithium bases was achieved with ortho-C═O and ortho-P═N chemoselectivity. However, the synthetic scope of these processes was rather limited. Ortho-lithiated N-benzamido-P,P,P-triphenyliminophosphorane 8 was efficiently prepared via lithium/halogen exchange of the corresponding ortho-brominated precursor with s-BuLi in THF at -90 °C. The reaction of 8 with a variety of electrophiles provides an easy and mild method for the regioselective synthesis of ortho-modified iminophosphoranes via C-C (alkylation and hydroxyalkylation) and C-X (X = I, Si, P, Sn, and Hg) bond-forming reactions. NMR characterization of 8 in THF solution showed that 8 exists as an equilibrium mixture of one monomer and two dimers. The Li atoms of these species become members of five-membered rings through chelation by the ortho-metalated carbon and the carbonyl oxygen. The dimers differ in the relative orientation of the two chelates with respect to the plane defined by the C(2)Li(2) core. The equilibrium between all species is established by splitting the dimers into monomers and subsequent recombination with formation of a different dimer.

Novel Cyclopalladated Imino-thiophenes: Synthesis and Reactivity Toward Alkynes and Carbon Monoxide

ortho-palladated complexes based on thiophene and benzothiophene ligands 1a and 1b have been synthesized by direct C-H activation under mild conditions. These species were fully characterized, including single-crystal X-ray diffraction analysis. The reactions of these novel complexes with internal alkynes afforded a variety of thieno[3,2-c]pyridinium salts substituted at the 6- and 7-positions. The thiophene-based complex 2a also reacts with carbon monoxide, in the presence of different alcohols, forming the corresponding esters by tandem alkoxycarbonylations. This latter reaction can be exploited for the unexpected, but straightforward, formation of the monomeric bis-cyclometallated complexes 6a and 6b from 2a or 2b, whose syntheses do not require the employment of transmetallating agents. The structures of these monomeric palladacycles were also fully elucidated by means of X-ray diffraction studies.

Novel Thiophene-Based Cycloruthenated Compounds: Synthesis, Characterization, and Reactivity

The reactions between a series of thiophene-based imines with [(η(6)-C(6)H(6))RuCl(μ-Cl)](2), in a basic medium, and in MeCN give a family of ruthenacycles of stoichiometry [Ru(C^N)(NCMe)(4)]PF(6) (C^N = orthometalated thiopheneimine). In these species, the C-H activation process is produced in most cases at the thiophene ring. When two C-H bonds are competing (thiophene vs aryl), the cyclometalation can be driven regioselectively to the thiophene unit or to the aryl ring as a function of the location of the iminic C=N bond. Cyclometalation can also be oriented to positions 2 or 3 of the thiophene depending on the situation of the imine in the heterocycle (3 or 2, respectively). In all studied cases, the η(6)-C(6)H(6) ligand was substituted by acetonitrile. The X-ray structures of two representative complexes have been determined. These thiophene-based metallacycles react with iodine under very mild conditions affording, after hydrolysis, substituted 3-iodo-2-formyl(benzo)thiophenes or substituted 2-iodo-3-formyl(benzo)thiophenes, as a function of the organometallic precursor.

CHAPTER 8:Coordination‐Directed Metallation Strategy for C–H Functionalization

This chapter deals with the use of ortho directing groups to achieve full regioselectivity in metal-catalysed C(aryl)–C and C(aryl)–X couplings through C–H bond activation, with particular emphasis on their applications in functionalization of organic materials. In the first part of the chapter we highlight the concepts and basic ideas behind a new generation of directing groups aimed to be multifunctional. Remarkable and useful examples are those containing oxidant moieties, those so-called “traceless” directing groups, removed after achieving the proper orientation, and those designed to be incorporated into the final molecule. After these considerations, examples of the most relevant processes developed during the 3–4 last years are presented. We have focused our attention on arylations, alkenylations, alkynilations, carbonylations, oxidations, aminations and amidations of arylic substrates catalysed by palladium, ruthenium, rhodium and copper, because these metals cover more than 90% of the literature published in this subject. Moreover, we have attempted to show the complementary chemical behaviour of these metals, comparing the results of the same type of processes catalysed by different metals. This includes surprising features, such as the meta-orientation shown by copper and palladium in special cases. As a whole, the utility of the directing groups has been firmly established, as well as their synthetic capabilities when different transition metals are used, although it is also clear that much remains to be done.

A new route for the synthesis of an alkylideneamido complex

The synthesis of the new alkylideneamido complex [Re{NC(Ph)CCPh}(CO)3(bpy)] is described. Analysis of its spectroscopic and structural data reveals the absence of π-donation from the nitrogen to the metal.