Ruthenium complexes heterocyclic

In 2008, Grisi et al. reported three ruthenium complexes 65-67 bearing chiral, symmetrical monodentate NHC ligands with two iV-(S)-phenylethyl side chains [74] (Fig. 3.26). Three different types of backbones were incorporated into the AT-heterocyclic moiety of the ligands. When achiral triene 57 was treated with catalysts 65-67 under identical reaction conditions, a dramatic difference was observed. As expected, the absence of backbone chirality in complex 65 makes it completely inefficient for inducing enantioselectivity in the formation of 58. Similarly, the mismatched chiral backbone framework of complex 66 was not able to promote asymmetric RCM of 57. In contrast, appreciable albeit low selectivity (33% ee) was observed when the backbone possessed anti stereochemistry. 

Further improvements in activity of the ruthenium carbene complexes were achieved by incorporation of methyl groups in 3,4-position of imidazol-2-ylidene moiety. Introduction of sulfur in the trara-position to the N-heterocyclic carbene leads to increased stability of the resulting ruthenium complexes. The synthesis and the first applications of these new rathenium complexes are described herein. 

Ruthenium complex catalyzes reductive IV-heterocyclization of y-nitroketones to give pyr-roline derivatives (Eq. 10.72).108... 

The proposed mechanisms are similar in both cases and involve in particular an (aryl)(hydrido)ruthenium intermediate in which the ruthenium is additionally coordinated by an in. ( ////-generated /V-phenylimine moiety tethered to the same Ru-bound aromatic ring. The C-C bond-forming step for the construction of the corresponding heterocyclic framework proceeds via insertion of the C=N double bond into the C-Ru bond with transfer of the (hydrido) ruthenium complex to the now phenylamine nitrogen. The desired heterocycles 158 and 159 were obtained after successive reductive elimination, deamination, and dehydrogenation. 

Pericyclic reactions involving thiophenes have been utilized to prepare a variety of complex heterocycles. The intramolecular Diels-Alder reaction of 2-vinylbenzo[i]thiophene 92 produced a pair of tetracyclic adducts 93 and 94 <02TL3963>. Coupling of Fischer carbene 96 with 3-alkynylthiophene 95 led to the formation of thieno[2,3-c]pyran-3-one 97 in one step <02JOC4177>. An intramolecular cycloaddition of 97 then afforded tetracyclic adduct 98. A ruthenium-catalyzed cyclodimerization reaction involving bis-thienyl acetylene derivatives was... 

A relatively large number of organic lumiphores and luminescent polypyridyl ruthenium complexes have been developed as luminescent pH probe molecules (82-84). The heterocyclic-substituted platinum-1,2-enedithiolates have also been developed in this regard (18, 19). 

RCM has attracted much attention and has seen a tremendous increase in synthetic applications over the last decade <2000CR2963, 2006JOM(691)5129>. In this reaction, two C-C multiple bonds, such as double and double, or double and triple in the same molecule, are converted to unsaturated carbocycles or heterocycles in the presence of a metal carbene complex. The versatility of Schrock s molybdenum catalyst and Grubbs ruthenium complexes 68 and 69 (Scheme 10) in carbo- and heterocyclizations, respectively, of very different ring sizes were demonstrated <2000CR2963, 2006JOM(691)5129>. 

Quite recently, novel cyclization reactions involving CO to give carbocydic and heterocyclic compounds, which are characteristic for mthenium catalysts, have been developed. Ruthenium complexes provide new avenues for cydization reactions. In addition, CO is often used as a reducing agent, and reductive carbonylations of nitro compounds catalyzed by mthenium complexes are very attractive reactions that provide phosgene-free processes [3]. 

Let us examine first the principal sites of tagging ruthenium complexes (Scheme 1.41) they include covalent ligands A [phosphines, N-heterocyclic carbenes (NHCs) or pyridine], anionic ligands B, or the carbene ligand C (either in the aromatic moiety or at the alkoxy ligand). 

Since the diphosphine is appreciably more electron-rich than is BINAP, the major ruthenium complex is a more active hydrogenation catalyst than the parent. Increased electron-rich ligation may be the reason for the success of heterocyclic analogues of BINAP in which the binaphthalene is replaced by a bi(ben-zothiophene) or biindolyl the resulting Ru complexes are effective both in terms of enantioselectivity and reactivity [139]. Readers of the related Chapter 6.1 on the asymmetric hydrogenation of carbonyl compounds will encounter the Ru complexes of ligands in the DUPHOS family, where the ease of modification of the alkyl substituents of the phospholane enhances the power of the system, since it permits the easy optimization of ee for any substrate [140]. 

A different type of metallation, directed by an acyl group at either the pyridine 3- or 4-position, uses a catalytic ruthenium complex and results in a reductive Heck-type substitution, as illustrated below. The mechanism involves insertion of the metal into a C-H bond. The process is non-polar and works equally well with electron-rich heterocycles, for example indole. ... 

The transition-metal-catalyzed intramolecular [5 + 2]-cycloaddition of 154 bearing a heteroatom between an alkynyl and a cyclopropylalkenyl group provides the heterocycles 155 bearing a seven-membered carbocycle (Scheme 53).129 Rhodium and ruthenium complexes have been used as catalysts of this [5 + 2]-cycloaddition. Trost and Shen extended this reaction for constructing the tricyclic heterocycle 157 from 156 (Scheme 54).129i... 

Reviews have appeared of light-induced electron-transfer reactions of ruthenium complexes containing nitrogen heterocycles. 

---