Osmium a Promising Alternative to Ruthenium for the Synthesis of

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Direct Access to POP-Type Osmium(II) and Osmium(IV) Complexes: Osmium a Promising Alternative to Ruthenium for the Synthesis of Imines from Alcohols and Amines Miguel A. Esteruelas,* Nicole Honczek, Montserrat Oliv
an, Enrique O~nate, and Marta Valencia Departamento de Química Inorg
anica-Instituto de Ciencia de Materiales de Arag
on, Universidad de Zaragoza-CSIC, 50009 Zaragoza, Spain

bS Supporting Information ABSTRACT: An easy and direct access to POP-type osmium(II) and osmium(IV) complexes, including OsH4{dbf(PiPr2)2} (dbf(PiPr2)2 = 4,6bis(diisopropylphosphine)dibenzofuran), is reported. This tetrahydride derivative is an efficient catalyst for the selective formation of imines from alcohols and amines with liberation of H2, proving that osmium is a promising alternative to ruthenium for catalysis.

I

mines are useful intermediates in organic synthesis, which act as electrophilic reagents in many different transformations, including additions, condensations, and cycloadditions.1 Their traditional preparation implies the reactions of ketones or aldehydes with amines. Imines have been also synthesized by self-condensation of amines upon oxidation,2 transition-metalpromoted hydrogen transfer from secondary amines,3 direct reaction of nitroarenes and primary alcohols in the presence of an Ir Pd heterodimetallic catalyst,4 and cross-coupling between amines and alcohols catalyzed by heterogeneous metallic systems under an oxygen atmosphere.5 The alkylation of amines with alcohols is a ruthenium- and iridium-promoted procedure, which takes place via imine intermediates.6 However, the selective preparation of the imines has been not achieved until very recently, when Milstein and coworkers reported a homogeneous PNP-type ruthenium pincer complex that, under argon, stops the reaction at the imine.7 This environmentally benign method for the synthesis of imines, in contrast to those using heterogeneous metallic systems under an oxygen atmosphere, occurs with liberation of molecular hydrogen, which is the most efficient and clean energy carrier known.8 Osmium is more reducing than ruthenium, prefers to be saturated by coordination, and forms redox isomers with more metal carbon bonds.9 These characteristics have been argued to justify the versatility of the stoichiometric osmium chemistry and its poorer catalytic activity in comparison with ruthenium.10 On the other hand, we have recently shown that osmium complexes are more efficient catalysts than tungsten, ruthenium, and rhodium systems for the regioselective 7-endo heterocyclization of aromatic alkynols into benzoxepines.11 Pincer ligands are having a tremendous impact in homogeneous catalysis.12 As a consequence of the disposition of their donor atoms, they develop marked abilities to stabilize metal r 2011 American Chemical Society

complexes capable of activating inert bonds.13 The most commonly encountered linker groups consist of either a metalated aryl ring in anionic PCP ligands or uncharged PNP pyridines or neutral POP ethers.14 In contrast to the remaining platinumgroup metals, osmium pincer chemistry has received little attention,15 in particular that of POP ligands. In the search for osmium pincer starting materials, we have recently synthesized 4,6-bis(diisopropylphosphino)dibenzofuran (dbf(PiPr2)2), which has been used to prepare the osmium(III) derivative OsCl3{dbf(PiPr2)2} from OsCl3 3 3H2O.16 We have now synthesized a POP-type osmium tetrahydride pincer compound (Scheme 1), which stops the alkylation of amines with alcohols at the imine, liberating molecular hydrogen, and is more efficient than the PNP-type ruthenium pincer compound previously reported. Treatment of 2-propanol solutions of the tetrasolvento adduct OsCl2(DMSO)4 (1) with 1.0 equiv of dbf(PiPr2)2 under reflux for 18 h leads to OsCl2{dbf(PiPr2)2}(DMSO) (2), as a result of the displacement of three DMSO molecules by the pincer ligand. This complex, which is isolated as a yellow solid in 86% yield, has been characterized by X-ray diffraction analysis. The coordination polyhedron around the osmium atom can be rationalized as being derived from a distorted octahedron, with the POP group occupying mer positions and the chloride ligands disposed trans (Cl(1) Os Cl(2) = 171.61(5)°). The P(1) Os P(2) bite angle of 156.86(5)° compares well with that of OsCl3{dbf(PiPr2)2} (159.95(3)°). A singlet at 6.3 ppm in the 31P{1H} NMR spectrum in dichloromethane-d2 is also characteristic of this compound. Complex 2 reacts with molecular hydrogen under 3 atm, in the presence of a Brønsted base. The products of the reactions Received: April 4, 2011 Published: April 11, 2011 2468

dx.doi.org/10.1021/om200290u | Organometallics 2011, 30, 2468–2471

Organometallics Scheme 1

depend upon the base and the experimental conditions. Triethylamine produces the abstraction of a chloride ligand. Thus, the trihydride OsH3Cl{dbf(PiPr2)2} (3) is formed after 60 h, in toluene, at 90 °C, and in the presence of the amine. On the other hand, sodium hydride removes both chloride ligands. As a result of this, the tetrahydride OsH4{dbf(PiPr2)2} (4) is obtained after 60 h, in tetrahydrofuran, at 50 °C, and in the presence of the salt. Complexes 3 and 4 are isolated as pale yellow solids in 50% and 65% yields, respectively, and have been characterized by X-ray diffraction analysis. The geometry around the osmium atom of 3 can be rationalized as a distorted pentagonal bipyramid, with the phosphorus atoms of the POP group occupying axial positions (P(1) Os P(2) = 158.99(5)°) and the chloride situated between the oxygen atom and the hydride H(03). The B3W91 optimized structure confirms the trihydride character of the OsH3 unit. The H(01) H(02) and H(02) H(03) separations are 1.611 and 1.555 Å, respectively. At room temperature, the 1H NMR spectrum in dichloromethane-d2 shows a hydride resonance at 11.86 (t, JH P = 9.4 Hz) ppm, which is consistent with the operation of two thermally activated site exchange processes between the hydrides. At about 243 K, decoalescence occurs and, between 240 and 187 K, two signals are observed at 12.2 at 14.0 ppm in a 2:1 intensity ratio. In agreement with the trihydride character of the complex, at 223 K, T1(min) values of 68 ( 1 and 70 ( 1 ms were found for these resonances. The 31 1 P{ H} NMR spectrum contains a singlet at 54.2 ppm. The geometry around the osmium atom of 4 is similar to that of 3 and can be rationalized as a distorted pentagonal bipyramid with the phosphorus atoms of the POP group occupying axial positions (P(1) Os P(2) = 158.84(2)°) and the hydride H(04) in the position of the chloride ligand of 3. The B3W91 optimized structure confirms the tetrahydride character of the OsH4 unit. The H(01) H(02), H(02) H(03), and

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Scheme 2

H(03) H(04) separations are 1.816, 1.651, and 1.849 Å, respectively. As expected for two inequivalent hydride positions, two broad resonances centered at 3.3 and 14.3 ppm are observed at 193 K in the high-field region of the 1H NMR spectrum in toluene-d8. At about 223 K, coalescence between them takes place. This behavior indicates that the hydride ligands H(01) and H(02) (or H(04) and H(03)) undergo a thermally activated position exchange. A ΔGq223 value of 9.8 kcal mol 1 can be estimated for the process. The 31P{1H} NMR spectrum contains a singlet at 64.9 ppm. The tetrahydride complex 4 reacts with 1.0 equiv of benzyl alcohol in toluene under reflux to afford after 7 h the hydride aryl carbonyl derivative OsH(Ph)(CO){dbf(PiPr2)2} (5), which is isolated as a yellow solid in 53% yield, according to eq 1. The behavior of 4 is in agreement with the tendency shown by osmium polyhydrides to dehydrogenate alcohols17 and with our previous observation that the tetrahydride species OsH4(PiPr3)2, generated from OsH6(PiPr3)2 by thermal activation, reacts with benzaldehyde to give OsH(Ph)(CO)2(PiPr3)2 via a hydride acyl intermediate.10c The presence of a hydride in 5 is revealed by a triplet (JH P = 19.5 Hz) at 8.07 ppm in the 1H NMR spectrum, which agrees well with that of OsH(Ph)(CO)2(PiPr3)2 (δ 6.59), where the hydride and carbonyl are also disposed trans. In the 13C{1H} NMR spectrum the carbonyl and metalated aryl resonances appear at 189.9 (t, JP C = 7 Hz) and 157.6 (t, JP C = 8.5 Hz) ppm, respectively. The 31P{1H} NMR spectrum contains a singlet at 56.8 ppm.

The formation of 5 can be rationalized through the intermediates A D shown in Scheme 2. The deprotonation of the alcohol by the basic tetrahydride 4 could afford the trihydride alkoxy intermediate A, which should give the unsaturated species B by dissociation of molecular hydrogen. A β-elimination reaction in the alcoholate group could lead to the dihydride 2469

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Organometallics Table 1. Direct Synthesis of Imines from Alcohols and Amines Catalyzed by POP-Type Osmium Complex 4a

a

Complex 4 (0.02 mmol), alcohol (10 mmol), amine (10 mmol), KOH (0.1 mmol), and toluene (1 mL) were heated at 150 °C in a Schlenk tube under an argon atmosphere. b Yields of products were determined by 1H NMR spectroscopy.

benzaldehyde C, which should generate the unsaturated hydride acyl intermediate D. The formation of the latter could take place by heterolytic C H bond activation of the aldehyde, promoted by one of the hydride ligands, or alternatively by reductive elimination of molecular hydrogen and subsequent homolytic C H bond activation of the aldehyde. Finally, the deinsertion of the phenyl group should give 5. Intermediate D is similar to that proposed for the formation of OsH(Ph) (CO)2(PiPr3)2.10c Intermediate C can dissociate the aldehyde to afford an equilibrium mixture with the unsaturated dihydride E. Thus, when the reaction of 4 and benzyl alcohol is carried out in the presence of aniline, the generation of N-benzylideneaniline is observed. According to this, the latter is catalytically formed in

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98% yield after 3 h, by treatment of equimolecular amounts of benzyl alcohol and aniline in toluene under reflux and in the presence of 0.2 mol % of 4. Because osmium has a tendency higher than that of ruthenium to be saturated by coordination and to activate C H bonds, in contrast to the PNP-type ruthenium pincer system, 1 mol % of KOH is also necessary in order to increase the nucleophilicity of the medium. This facilitates the amine attack, preventing the decarbonylation of the aldehyde, which produces the catalytic deactivation of the system. The catalytic cycle is closed with the addition of the O H bond of the alcohol to E. Complexes 2 and 3 also catalyze the reaction but are significantly less active than 4 (49% and 54% yield, respectively, after 3 h). Complex 4 catalyzes the selective formation of a variety of imines from alcohols and amines with liberation of molecular hydrogen, under an argon atmosphere (eq 2). The reactions include the formation of aliphatic imines, which are inherently more challenging because of their instability. High yields are obtained after 24 h (Table 1), while in the presence of the PNP-type ruthenium pincer system, the reactions need between 48 and 56 h.

Benzyl alcohol reacts with anilines and aliphatic amines, including cyclohexylamine. The position of the substituent of the anilines has a marked influence on the reaction rates (Table 1, runs 2 and 3). While N-benzylidene-p-methylaniline is formed after 3 h in 92% yield, along with a small amount of secondary amine (5%), N-benzylidene-o-methylaniline is obtained in 87% yield after 24 h. The reactions with the aliphatic amines are slower. After 24 h, cyclohexylamine (run 4), benzylamine (run 5), and dodecylamine (run 6) lead to the corresponding imines in yields between 61% and 82%. Cyclohexylmethanol reacts more slowly than benzyl alcohol. After 24 h, N-cyclohexylmethylene-p-methylaniline (run 7), N-cyclohexylmethylenecyclohexylamine (run 8), and N-cyclohexylmethylenedodecylamine (run 9) are formed in 55%, 65% and 37% yields, respectively. The reactions between linear alcohols and aliphatic amines are also very efficient (runs 10 13). Both butan-1-ol and octan-1-ol react with cyclohexylamine and dodecylamine to afford selectively the corresponding aliphatic imine in high yields (80 94%). The secondary cyclohexanol and dodecylamine give N-cyclohexylidenedododecylamine in 30% yield, after 24 h (run 14). Small amounts of secondary amine are also formed from the reactions of dodecylamine with cyclohexylmethanol (11%, run 9) and octan-1-ol (7%, run 13). The current lack of development of the osmium chemistry in comparison with that of ruthenium is certainly in part a consequence of the scarce effort to find starting materials. These results show an easy and direct access to POP-type osmium(II) and osmium(IV) pincer complexes and prove that osmium is a promising alternative to ruthenium for the direct synthesis of imines, including aliphatic imines, from alcohols and amines with liberation of molecular hydrogen.

’ ASSOCIATED CONTENT

bS

Supporting Information. Text, figures, tables, and CIF files giving experimental procedures regarding the synthesis,

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dx.doi.org/10.1021/om200290u |Organometallics 2011, 30, 2468–2471

Organometallics characterization data for the complexes and the imines, X-ray data for complexes 2 4, and computational details and Cartesian coordinates for the calculated structures. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT Financial support from the MICINN of Spain (CTQ200800810 and Consolider Ingenio 2010 CSD2007-00006), the Diputaci
on General de Arag
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