Tris borate ruthenium complexes

The vast majority of tris(pyrazolyl)borate ruthenium complexes prepared until now contains the parent Tp ligand. In contrast to other transition metals, ruthenium compounds which bear bulky Tp ligands remain still rare and are, thus far, limited to Tp, Tp Tp and Tp as shown in Chart 1. 

The regioselective anti-Markovnikov addition of benzoic acid to phenyl-acetylene has also been carried out with success at 111 °C in the presence of ruthenium complexes containing a tris(pyrazolyl)borate (Tp) ligand [RuCl(Tp)(cod), RuCl(Tp)(pyridine), RuCl(Tp)(N,N,Ar,AT-tetramethylethyl-enediamine )] with a stereoselectivity in favour of the (E)-enol ester isomer [22]. The o-enynyl complex Ru(Tp)[PhC=C(Ph)C=CPh](PMe/-Pr2) (C) efficiently catalyses the regioselective cyclization of a,cu-alkynoic acids to give en-docyclic enol lactones [23] (Eq. 2). 

Catalytic transformations involving the C-H bonds of thiophene are rare, but recently there has been a report on the catalytic addition of the C(2)-H bond of thiophene across ethylene to form 2-ethylthiophene <20040M5514>. Reaction of the ruthenium complex TpRu(CO)(NCMe)(Me) (where Tp = hydrido tris(pyrazolyl)borate) with thiophene produces the 2-thienyl complex 249 and methane. This complex catalyzes the formation of 2-ethylthiophene from a solution of thiophene and ethylene (Equation 121). The mechanism of this reaction has been explored. 

Cyano-derivatives can be readily obtained by a ruthenium-catalyzed addition of various hydrazines to terminal alkynes [89] in which the cyano carbon atom arises from the terminal alkyne carbon atom. The tris(pyrazolyl)borate (Tp) complex RuCl(Tp)(PPh3)2 (1 mol%) was found to be the most active catalyst, and N,N-dimethylhydrazine (5 equiv.) the best nitrogen source. The proposed mechanism involves the nucleophilic attack of the nitrogen nucleophile on the a-carbon of a vinylidene intermediate (Scheme 8.27). Proton migration in the resulting a-hydrazi-nocarbene, followed by deamination, would give the nitrile derivative and regenerate the catalytic species. 

From a coordination point of view, Complex 75 and the related ruthenium complex [(ij6-p-cymene)Ru(pz)2(Hpz)], 76 (7), are comparable to protonated polypyrazolylborates RB(pz)2(pzH) (R = H or pz) (12). The formal similarity between the tris(pyrazolyl)borate anion and the deprotonated form of the iridium complex 75 suggested the preparation of heterodinuclear complexes by using 75 as a building block. Thus, heterodinuclear (n-pz)2 complexes of the formula [(C5Me5)(pz)Ir(/i-pz)2M(PPh3)] (M = Cu, 77 Ag, 78 Au, 79) were obtained (96). 

Scheme 12.1 Ruthenium complexes with tris(pyrazolyl)borate (Tp) anionic ligands.

The regioselective anri-Markovnikov addition of benzoic acid to phenylacetylene has also been carried out with success at 111°C in the presence of ruthenium complexes containing a tris(pyrazolyl)borate (Tp) ligand, [RuCl(Tp)(cod) (C),... 

The formation of a ruthenium vinylidene is proposed as the key intermediate in the regioselective addition of hydrazine to terminal alkynes [55]. This reaction, which proceeds via addition of the primary amino group of a 1,1-disubstituted hydrazine followed by deamination, provides an unprecedented access to a variety of aromatic and aliphatic nitriles. The tris(pyrazolyl)borate complex RuCl(Tp)(PPh3)2 gave the best catalytic activity in the absence of any chloride abstractor (Scheme 10.17). 

Organometallic Chemistry of Group 8 Tris(pyrazolyl)borate Complexes 177 F. Ruthenium-Carbon Double and Triple Bonds... 

Since its preparation, reactions of RuCl[k -HB(pz)3](PPh3)2 have been investigated in a number of papers. The lability of both the phosphines and the chloride hgand allow functional group transformations to be carried out in order to provide a wide range of ruthenium hydro-tris(pyrazol-l-yl)borate complexes. A straightforward preparation of the complex on a large scale (10 g) is described here. 

Jacquot de RouviUe H-P, ViUenave D, Rapenne G. Synthesis of a photoswitchable azobenzene-fiinctionalized tris(indazol-l-yl)borate ligand and its ruthenium(II) cyclopentadienide complex. Tetrahedron. 2010 66 1885-1891. 

Farmer JD, Man WY, Fox MA, et al. Syntheses, strucmres and redox properties of tris(pyrazolyl)borate-capped ruthenium vinyl complexes. J Organomet Chem. 2012 721-722 173-185. 

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