Tamara Bolaño

POP–Pincer Ruthenium Complexes: d6 Counterparts of Osmium d4 Species

A wide range of ruthenium complexes stabilized by the POP-pincer ligand xant(P(i)Pr2)2 (9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene) were prepared starting from cis-RuCl2{κ-S-(DMSO)4} (1; DMSO = dimethyl sulfoxide). Treatment of toluene solutions of this adduct with the diphosphine under reflux leads to RuCl2{xant(P(i)Pr2)2}(κ-S-DMSO) (2), which reacts with H2 in the presence of a Brønsted base. The reaction in the presence of Et3N affords RuHCl{xant(P(i)Pr2)2}(κ-S-DMSO) (3), whereas NaH removes both chloride ligands to give RuH2{xant(P(i)Pr2)2}(κ-S-DMSO) (4). The stirring of 3 in 2-propanol under 3 atm of H2 for a long time produces the elimination of DMSO and the coordination of H2 to yield the dihydrogen derivative, RuHCl(η(2)-H2){xant(P(i)Pr2)2} (5). In contrast to H2, PPh3 easily displaces DMSO from the metal center of 3 to afford RuHCl{xant(P(i)Pr2)2}(PPh3) (6), which can be also obtained starting from RuHCl(PPh3)3 (7) and xant(P(i)Pr2)2. In contrast to 3, complex 4 does not undergo DMSO elimination to give RuH2(η(2)-H2){xant(P(i)Pr2)2} (8) under a H2 atmosphere. However, the latter can be prepared by hydrogenation of Ru(COD)(COT) (9; COD = 1,5-cyclooctadiene and COT = 1,3,5-cyclooctatriene) in the presence of xant(P(i)Pr2)2. A more efficient procedure to obtain 8 involves the sequential hydrogenation with ammonia borane of the allenylidene derivative RuCl2(═C═C═CPh2){xant(P(i)Pr2)2} (10), which is formed from the reaction of 2 with 1,1-diphenyl-2-propyn-1-ol. The hydrogenation initially gives RuCl2(═C═CHCHPh2){xant(P(i)Pr2)2} (11), which undergoes the subsequent reduction of the Ru-C double bond to yield the hydride-tetrahydroborate complex, RuH(η(2)-H2BH2){xant(P(i)Pr2)2} (12). The osmium complex, OsCl2{xant(P(i)Pr2)2}(κ-S-DMSO) (13), reacts with 1,1-diphenyl-2-propyn-1-ol in a similar manner to its ruthenium counterpart 2 to yield the allenylidene derivative, OsCl2(═C═C═CPh2){xant(P(i)Pr2)2} (14). Ammonia borane also reduces the Cβ-Cγ double bond of the allenylidene of 14. However, the resulting vinylidene species, OsCl2(═C═CHCHPh2){xant(P(i)Pr2)2} (15), is inert. Complex 12 is an efficient catalyst precursor for the hydrogen transfer from 2-propanol to ketones, the α-alkylations of phenylacetonitrile and acetophenone with alcohols, and the regio- and stereoselective head-to-head (Z) dimerization of terminal alkynes.

POP-pincer osmium-polyhydrides: head-to-head (Z)-dimerization of terminal alkynes

A wide range of osmium-polyhydride complexes stabilized by the POP-pincer ligand xant(P(i)Pr2)2 (9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene) have been synthesized through cis-OsCl2{κ-S-(DMSO)4} (1, DMSO = dimethyl sulfoxide). Treatment of toluene solutions of this adduct with the diphosphine, under reflux, leads to OsCl2{xant(P(i)Pr2)2}(κ-S-DMSO) (2). The reaction of 2 with H2 in the presence of Et3N affords OsH3Cl{xant(P(i)Pr2)2} (3), which can be also prepared by addition of xant(P(i)Pr2)2 to toluene solutions of the unsaturated d(4)-trihydride OsH3Cl(P(i)Pr3)2 (5). Complex 3 reductively eliminates H2 in toluene at 90 °C. In the presence of dimethyl sulfoxide, the resulting monohydride is trapped by the S-donor molecule to give OsHCl{xant(P(i)Pr2)2}(κ-S-DMSO) (6). The reaction of 2 with H2 is sensible to the Brønsted base. Thus, in contrast to Et3N, NaH removes both chloride ligands and the hexahydride OsH6{xant(P(i)Pr2)2} (7), containing a κ(2)-P-binding diphosphine, is formed under 3 atm of hydrogen at 50 °C. Complex 7 releases a H2 molecule to yield the tetrahydride OsH4{xant(P(i)Pr2)2} (8), which can be also prepared by reaction of OsH6(P(i)Pr3)2 (9) with xant(P(i)Pr2)2. Complex 8 reduces H(+) to give, in addition to H2, the oxidized OsH4-species [OsH4(OTf){xant(P(i)Pr2)2}](+) (10, OTf = trifluoromethanesulfonate). The redox process occurs in two stages via the OsH5-cation [OsH5{xant(P(i)Pr2)2}](+) (11). The metal oxidation state four can be recovered. The addition of acetonitrile to 10 leads to [OsH2(η(2)-H2)(CH3CN){xant(P(i)Pr2)2}](2+) (12). The deprotonation of 12 yields the osmium(IV) trihydride [OsH3(CH3CN){xant(P(i)Pr2)2}](+) (13), which is also formed by addition of HOTf to the acetonitrile solutions of 8. The latter is further an efficient catalyst precursor for the head-to-head (Z)-dimerization of phenylacetylene and tert-butylacetylene. During the activation process of the tetrahydride, the bis(alkynyl)vinylidene derivatives Os(C≡CR)2(=C═CHR){xant(P(i)Pr2)2} (R = Ph (14), (t)Bu (15)) are formed.

Nazarov Type Cyclization on an Osmium-Dienylcarbene Complex

The Nazarov reaction is an acid-catalyzed 4pi-electrocyclic ring closure of dienylketones, which affords cyclopentenones. This type of cyclization has been increasing in interest over the years, due to the importance of the construction of five-membered rings in the synthesis of natural products. However, one potential problem is that the carbonyl group necessary for the cyclization to occur may not be required in the final synthetic target and can sometimes be difficult to remove or modify. One possible solution is to design analogous reactions which do not suffer the carbonyl dependence.