MCl2(ampy)(dppf) (M = Ru, Os): Multitasking Catalysts for Carbonyl

Article pubs.acs.org/Organometallics

MCl2(ampy)(dppf) (M = Ru, Os): Multitasking Catalysts for Carbonyl Compound/Alcohol Interconversion Reactions Elisabetta Putignano, Gianluca Bossi, Pierluigi Rigo, and Walter Baratta* Dipartimento di Chimica, Fisica e Ambiente, Università di Udine, Via Cotonificio 108, I-33100 Udine, Italy S Supporting Information *

ABSTRACT: The easily accessible complexes [MCl2(dppf)(ampy)] (M = Ru (cis-1), Os (trans-2); dppf = 1,1′bis(diphenylphosphino)ferrocene; ampy = 2-aminomethylpyridine) in the presence of base (NaOiPr, KOtBu) are efficient catalysts for several reactions involving carbonyl compounds and alcohols. The derivatives 1 and 2 catalyze the selective transfer hydrogenation of aldehydes and ketones to alcohols with 2propanol using 0.1−0.005 mol % of catalyst at 82 °C with TOF values up to 3.0 × 105 h−1. These compounds (0.1−0.02 mol %) promote the hydrogenation of aldehydes and ketones in EtOH or a MeOH/EtOH mixture at 30−90 °C (5 atm of H2) and the acceptorless dehydrogenation of alcohols to ketones in tBuOH at 130−145 °C (0.4 mol %). Complexes 1 and 2 (0.5 mol %) catalyze the racemization of chiral alcohols at 70 °C in 2propanol and the isomerization of allylic alcohols to ketones in tBuOH at 70−120 °C (1 mol %). In addition, 1 and 2 (0.5 mol %) promote the α alkylation of α-tetralone with primary alcohols (EtOH, nPrOH, and nBuOH) at 120 °C in a tBuOH/toluene mixture (1/2 v/v). Complex 2 is easily obtained in 93% yield from [OsCl2(PPh3)3], dppf, and ampy in toluene.

■

INTRODUCTION The search for highly efficient transition-metal catalysts is a current issue for applications in the synthesis of valuable organic compounds. In this context ruthenium has become one of the preferred metals because of its high performance and versatility for a variety of organic catalyzed reactions. High selectivity and productivity, which are crucial parameters in catalysis, can be achieved through an appropriate design of the ligands, resulting in the formation of structurally well-defined catalysts. Nevertheless, there is also a great interest in multitasking catalysts able to efficiently promote different organic transformations by a careful switching of the reaction parameters: namely temperature, solvent, and cocatalyst concentration. Moreover, to make catalysts appealing for industrial applications, a straightforward preparation of the catalysts is highly desirable. As regards the reduction of carbonyl compounds, outstanding results have been obtained by Noyori in the enantioselective hydrogenation1 and transfer hydrogenation2 of carbonyl compounds, leading to the isolation of the systems trans-[RuCl2(PP)(1,2-diamine)]3 (A; PP = diphosphine) and [(η6-arene)RuCl(Tsdpen)] (B),4 respectively (Figure 1). Although no universal catalyst exists for different substrates and for various organic transformations, the complexes [(η6arene)Ru(OTf)(Tsdpen)] (C)5 and [MCl2(PP)(ampy)] (D; M = Ru,6 Os;7 ampy = 2-aminomethylpyridine) and the related pincer [MCl(CNN)(PP)]8 (E; HCNN = 1-(6-arylpyridin-2yl)methanamine) were demonstrated to be efficient catalysts © 2012 American Chemical Society

Figure 1. Ru and Os catalysts for TH and HY reactions.

for both transfer hydrogenation (TH) and hydrogenation (HY) reactions. It is worth noting that the ampy ligand, having the NH2 function and the pyridine moiety, leads to the acceleration of both catalytic TH and HY reactions and allows reduction of bulky substrates (e.g., tert-alkyl ketones), on account of the flat pyridine ring that facilitates the approach of the ketone. Another example is system B, originally developed for the asymmetric TH of ketones, which was successfully applied for Received: November 29, 2011 Published: January 26, 2012 1133

dx.doi.org/10.1021/om201189r | Organometallics 2012, 31, 1133−1142

Organometallics

Article

Scheme 1. Catalytic Reactions Catalyzed by the Ruthenium and Osmium Complexes cis-1 and trans-2

ampy (2 h), affords the osmium derivative trans-[OsCl2(dppf)(ampy)] (2), which was isolated in 93% yield (eq 1).

enantioselective C−C bond formation reactions with a wide scope.9 Despite the great interest in the asymmetric reduction of ketones,10 the design of novel, highly productive, and easily accessible nonchiral catalysts for the preparation of primary and rac secondary alcohols from carbonyl compounds is a topic of industrial relevance. In this regard, only few catalysts have been reported to efficiently catalyze the chemoselective reduction of aldehydes via TH.11 It is worth noting that the complexes [MCl2(dppf)(en)] (M = Ru, Os; en = ethylenediamine), containing 1,1′-bis(diphenylphosphino)ferrocene diphosphine (dppf), have recently been found to be active in the acceptorless dehydrogenation (DHY) 12 of alcohols to ketones.13 The presence of the flexible ferrocenyl diphosphine allows the formation of catalytically active species which are thermally more stable and productive with respect to the analogous derivatives having diphosphines with an alkyl backbone. Therefore, it is expected that these catalysts, which can easily promote C−H activation, can enhance the alcohol reactivity, giving access to new reactions: i.e., via hydrogen borrowing.14 We report here that the complexes [MCl2(dppf)(ampy)] (M = Ru (cis-1), Os (trans-2)) which are easily obtained in one-pot reactions from commercially available compounds, are efficient catalysts for a range of organic transformations. The complexes 1 and 2 in the presence of a base catalyze the fast and productive TH and HY (5 atm of H2) of aldehydes and ketones (TOF up to 3.0 × 105 h−1) with 0.1−0.0005 mol % loading, the acceptorless DHY of alcohols, the racemization of secondary alcohols, the isomerization of allylic alcohols, and ketone αalkylation with primary alcohols (Scheme 1). The catalytic activities of the Ru and Os complexes are compared here.

■

The 31P{1H} NMR spectrum (CD2Cl2) of 2 displays two doublets at δ −7.0 and −17.9 with 2J(P,P) = 19.7 Hz. In the 1H NMR spectrum the two protons of the ampy CH2 group lead to one signal at δ 4.39, whereas the resonance for the NH2 protons is at δ 3.59, which is consistent with a complex of trans configuration. Notably, the 1H and 13C NMR data of 2 are much the same as those of the corresponding trans[RuCl2(dppf)(ampy)], which is the kinetic product in the formation of the cis-1 derivative.13 Attempts to prepare cis[OsCl2(dppf)(ampy)] by heating a mesitylene solution of 2 at 150 °C (3 h) results in the partial conversion of 2 and formation of two uncharacterized Os dppf species, as inferred from 31P NMR measurements.15 Transfer Hydrogenation of Aldehydes and Ketones. The ruthenium and osmium complexes 1 and 2 are extremely efficient catalysts for the TH of both aldehydes and ketones in basic 2-propanol (eq 2).

RESULTS AND DISCUSSION

Synthesis of the Osmium Complex 2. The ruthenium complex cis-[RuCl2(dppf)(ampy)] (1) was easily prepared in high yield from [RuCl2(PPh3)3], dppf, and ampy, according to the literature method.13 Treatment of [OsCl2(PPh3)3] with 1 equiv of dppf in toluene at 50 °C (2 h), followed by addition of 1134


Organometallics

Article

Table 1. TH of Aldehydes and Ketones (0.1 M) with 1 and 2 with NaOiPr (2.0 mol %) in 2-Propanol at Reflux Temperature

a

The conversion and TOF (moles of aldehyde or ketone converted into alcohol per mole of catalyst per hour at 50% conversion) were determined by GC analysis. bNaOiPr 1.0 mol %. c(+)-neomenthol/(−)-menthol =3.2. d(+)-neomenthol/(−)-menthol =2.

show the catalytic potential of these complexes, p-anisyl alcohol (1.74 g) has been isolated in 90% yield, starting from panisaldehyde and using 0.05 mol % of 1 (30 min at 82 °C), whereas citronellol (1.92 g) has been prepared in 88% yield with 2 (2 h). The ketones acetophenone, 4-methoxyacetophenone, and the bulkier 2,2-dimethylpropiophenone have also been reduced to secondary alcohols with 1 and 2 at 0.1−0.002 mol %, affording TOF = 103−105 h−1. It is worth noting that employment of the isomer trans-[RuCl2(dppf)ampy] (0.002 mol %) leads to TH of acetophenone with only 43% conversion after 20 h, whereas 1 gives 92%, indicating that the cis derivative is more active than the trans analogue, in accordance with the literature data.6a The diaryl ketone benzophenone has also quantitatively reduced to benzhydrol with both 1 and 2 (0.005 mol %), the Os complex being more efficient. Also, the cyclic derivative (−)-menthone was rapidly reduced with 1 and 2 (0.05 mol %) in 10 min. With 1 the ratio (+)-neomenthol/(−)-menthol was 3.2, whereas with 2 the ratio was 2. Finally, complexes 1 and 2 (0.05 mol %) catalyze the selective TH of the unsaturated ketone 5-hexen-2-one with the

Using a catalyst loading of 0.1−0.0005 mol % and in the presence of NaOiPr (2 mol %), quantitative conversion of several carbonyl compounds has been achieved in a short time, achieving TOF values up to 3 × 105 h−1 at 82 °C (Table 1). With 0.005 mol % of 1 benzaldehyde and p-anisaldehyde are efficiently reduced to the corresponding primary alcohols in 2 and 1 h, respectively (TOF up to 5.4 × 104 h−1). The latter substrate is also reduced (91%, 6 h) using a remarkably low amount of 1 (0.0005 mol %). Interestingly, the osmium complex 2 (0.005 mol %) displays a higher activity for these aromatic aldehydes, leading to quantitative conversion in 5 min with TOF values up to 3.0 × 105 h−1. Aliphatic aldehydes such as hexanal and 2-methylbutanal are also promptly converted into alcohols using 1 and 2 (0.05−0.005 mol % TOF up to 1.5 × 105 h−1). Under these catalytic conditions, the unsaturated aldehydes citronellal and trans-cinnamaldehyde have selectively been reduced at the CO bond using 1 and 2, without hydrogenation or isomerization of the CC bond (TOF = 103−104 h−1). These results indicate that the Os complex efficiently catalyzes the reduction of aldehydes, with rates comparable to or even higher than those of the Ru complex. To 1135


Organometallics

Article

Table 2. Influence of the Base on the TH of Aldehydes (0.1 M) with Complex 1 in 2-Propanol at Reflux Temperature

a

Conversion into the alcohol was determined by GC analysis.

alcohol and p-anisyl alcohol at 50 °C, using 0.02 mol % of 1 in 10 and 5 min, respectively (TOF = 7.5 × 104 and 5.0 × 104 h−1) (Table 3). With the osmium complex 2 (0.1 mol %) quantitative conversion to alcohols has been achieved at 90 °C in 10 and 30 min (TOF up to 1.0 × 104 h−1). Phenylacetaldehyde is efficiently converted into 2-phenylethanol (98% in 10 min) using 1 (0.1 mol %), whereas with 2 at 90 °C, 91% conversion is achieved after 8 h. The substrate trans-cinnamaldehyde is reduced to cinnamyl alcohol, using 1 (0.02 mol %) at 50 °C in 60 min (TOF = 2.8 × 104 h−1) and with 2 (0.1 mol %) at 90 °C, with no hydrogenation at the CC bond. In addition, 2phenylethanol (1.04 g) was isolated in 85% yield using 0.02 mol % of 2 at 90 °C under 5 atm of H2 (8 h). In the HY of ketones the ruthenium 1 is catalytically active at 30−50 °C, leading to high conversion of acetophenone, 4methoxyacetophenone, and 4-chlorobenzophenone in 1−5 h (93−98%). It is worth noting that 1 leads to 1-phenylethanol (93%) in 60 min, whereas the analogous trans-[RuCl2(dppf)(ampy)] displays a lower activity, with 94% conversion in 20 h, under the same catalytic conditions. With osmium complex 2, quantitative HY of these ketones has been achieved in 10−30 min at 90 °C (TOF up to 9.0 × 103 h−1). As regards cyclic ketones, cyclohexanone was reduced to cyclohexanol with 1 at 50 °C in 1 h, whereas with 2 at 90 °C the reaction was complete after 15 min. The HY of (−)-menthone with 1 at 50 °C leads to poor conversion (35%, 3 h), whereas with 2 at 90 °C, 90% conversion is attained in 2 h, leading to (+)-neomenthol/(−)-menthol = 2. These data suggest that osmium is a valid complement to ruthenium for both HY and TH reactions, taking into account that osmium is more robust and generally requires a higher temperature to become catalytically active. Dehydrogenation of Alcohols. Catalytic alcohol dehydrogenation represents a straightforward route to prepare carbonyl compounds through the direct formation of H2 without the need for oxidizing agents.12 Several ruthenium complexes were described to be efficient catalysts for both oxidation of alcohols to ketones18 and production of hydrogen.19 The derivatives 1 and 2 in the presence of KOtBu display high catalytic activity in the dehydrogenation of secondary alcohols to ketones in tert-butyl alcohol or a tert-butyl alcohol/ toluene mixture at reflux and in an open system (eq 4).

formation of 5-hexen-2-ol, without reduction at the CC bond, 2 displaying a higher rate with respect to 1. The effect of the base in the TH of aldehydes has also been investigated. With NaOiPr (2 mol %) and in the absence of 1, the aromatic p-anisaldehyde undergoes reduction (96% conversion) after 20 h, whereas with K2CO3 only 47% conversion is obtained (Table 2). Addition of 1 (0.005 mol %), in the presence of NaOiPr, increases the rate of the TH with production of the p-anisyl alcohol in 1 h. As regards the aliphatic aldehydes, hexanal in the presence of NaOiPr and without 1 is poorly converted to alcohol (3%) in 20 h. In the presence of 1 (0.005 mol %) the conversions were 12 and 81% (20 h) with 0.5 and 1 mol % of NaOiPr, respectively. The use of K2CO3, which is moderately soluble in 2-propanol, leads to a lower conversion of alcohol (9%). With 1, 2-methylbutyraldehyde is rapidly reduced to the alcohol in the presence of NaOiPr (99%) in 10 min, whereas with K2CO3 92% conversion was obtained in 20 h. These results indicate that while aromatic aldehydes can be reduced in basic 2-propanol, the addition of 1 dramatically increases the rate of conversion. In contrast, aliphatic aldehydes necessitate the presence of 1 with NaOiPr, the base K2CO3 being less efficient. It is likely that the TH of carbonyl compounds catalyzed by 1 and 2 involves the formation of the [MHX(dppf)(ampy)] (M = Ru, Os; X = H, OR′) species, via a β-hydrogen elimination reaction,16 similarly to the Ru pincer complexes [RuX(CNN)(PP)], for which a reversible βhydrogen elimination has been observed.17 On account of the easy cleavage of the alcohol α-C−H bond with 1 and 2, a broadening of the alcohol reactivity is expected with these complexes (see below). Hydrogenation of Aldehydes and Ketones. The complexes 1 and 2 are catalytically active in the HY of aldehydes and ketones at low hydrogen pressure (5 atm) using 0.1−0.02 mol % loading, affording TOF values up to 7.5 × 104 h−1 (eq 3).

With 1 (0.1−0.02 mol %) a rapid HY occurs in ethanol at 30−50 °C in the presence of NaOEt (2 mol %), whereas with 2 (0.1 mol %) good performances are achieved in a methanol/ ethanol mixture (3/1, v/v), using KOtBu (2 mol %) at 90 °C. Benzaldehyde and p-anisaldehyde are quickly reduced to benzyl 1136


Organometallics

Article

Table 3. HY of Carbonylic Compounds (0.5 M) with the Ru and Os Catalysts 1 and 2 (0.1 mol %) under 5 atm of H2

a

NaOEt (2 mol %) in ethanol. bKOtBu (2 mol %) in methanol/ethanol (3/1, v/v). cThe conversion and TOF (moles of aldehyde or ketone converted into alcohol per mole of catalyst per hour at 50% conversion) were determined by GC analysis. dCatalyst 0.02 mol %. eReaction with trans-[RuCl2(dppf)(ampy)]. f(+)-neomenthol/(−)-menthol =2.

Table 4. DH of Alcohols with the Ru and Os Catalysts 1 and 2 (0.4 mol %) with KOtBu (2 mol %) in tert-Butyl Alcohol

a

The conversion and TOF (moles of alcohol converted into ketone per mole of catalyst per hour at 50% conversion) were determined by GC analysis. bThe conversion and TOF were determined by NMR analysis of cholesterol (0.21 M) with 0.8 mol % of catalyst and KOtBu (4 mol %) in tert-butyl alcohol/toluene (2/1, v/v).

With 1 (0.4 mol %) at 130 °C, α-tetralol (1.25 M) in tertbutyl alcohol was efficiently converted into α-tetralone (3 h) with TOF = 600 h−1 (Table 4). It is worth noting that the analogous diamine derivative [RuCl2(dppf)(en)] afforded dehydrogenation of α-tetralone with a lower rate (TOF = 200 h−1).13

The osmium catalyst 2 shows a lower activity with respect to 1, leading to 87% conversion in 20 h. The substrate 1-(4methoxyphenyl)ethanol alcohol is oxidized to 4-methoxyacetophenone in 4 h, whereas with 2 a longer reaction time is required (20 h). The oxidation of sterols is a significant reaction for the synthesis of 4-en-3-one steroidal compounds. Several 1137


Organometallics

Article

through a reversible hydrogen transfer process. In the racemization, no formation of side products such as ketones was observed, since the reaction was carried out in 2-propanol, which acts as a hydrogen donor. Isomerization of Allylic Alcohols. The catalytic isomerization of allylic alcohols to ketones is an interesting route for the preparation of carbonyl compounds. Among the several transition-metal catalysts which have been developed for this transformation,25 particular attention has been devoted to ruthenium, which led to the most active systems.26 Recently, cyclopentadienyl osmium complexes with pendant amine ligands were found to be active in the alcohol allylic isomerization.27 Complexes 1 and 2 (1 mol %) with KOtBu (2 mol %) in tert-butyl alcohol catalyzes the isomerization of monosubstituted aliphatic allylic alcohols (eq 6).

routes were reported, including the use of Cr(VI) and Mn(IV) oxidants,20 Oppenauer oxidation,21 and hydrogen transfer with the Shvo catalyst using acetone.22 The acceptorless dehydrogenation of cholest-5-en-3β-ol with 1 at 0.8 mol % at 145 °C in a tert-butyl alcohol/toluene mixture (2/1, v/v), leads to cholest4-en-3-one quantitatively in 3 h. A faster reaction has been observed with the osmium catalyst 2, for which the formation of the steroid compound has been achieved in 1 h, affording a remarkably high rate (TOF = 210 h−1). These data show that the ampy complexes 1 and 2 display a significantly higher activity for dehydrogenation, with respect to the analogous diamine compounds [MCl2(dppf)(en)] (M = Ru, Os) (TOF = 15 h−1 for cholest-5-en-3β-ol) and the CNN pincer complexes [MCl(CNN)(PP)], with an increase of the reaction rate of up to 1 order of magnitude.13 Racemization of Secondary Alcohols. Chiral alcohols can be converted to racemates by several transition-metal complexes. This catalytic transformation is particularly relevant for dynamic kinetic resolution in which a lipase is combined with a ruthenium catalyst for the preparation of chiral alcohols.23 Recently, the pincer complexes [MCl(CNN)(PP)] (M = Ru, Os) were found to be active in the racemization of chiral alcohols.24 Since 1 and 2 are efficient catalysts for both TH and DHY reactions, these complexes were investigated for the racemization of chiral alcohols (eq 5).

Thus, with 1 3-buten-2-ol (0.33 M) gave 2-butanone in 64, 90, and 94% conversion after 1, 5, and 10 min at 70 °C, respectively, affording TOF = 6000 h−1 (Table 6). The substrates 1-penten-3-ol and 1-hepten-3-ol are isomerized to 3-pentanone and 3-heptanone in 30 min with TOF values of 480 and 440 h−1, respectively. The substrate 1-phenyl-2-propen-1-ol is isomerized to propiophenone in 2 h at 120 °C, while at 70 °C the reaction is sluggish. With the osmium complex 2, the substrates 3-buten2-ol, 1-penten-3-ol, 1-hepten-3-ol, and 1-phenyl-2-propen-1-ol were converted into ketones at 120 °C with TOF values up to 460 h−1. These data clearly indicate that osmium complex 2 displays catalytic activity at higher temperature with respect to the ruthenium catalyst 1. The GC and NMR data for these reactions showed that the yield is much the same as the conversion and no other products were detected, indicating that the isomerization occurs with high selectivity. α-Alkylation of α-Tetralone. The ruthenium complexes [RuCl2(PPh3)3]28 and [Ru(DMSO)4Cl2]29 and the iridium system [Ir(COD)Cl]2/PPh330 have been described to catalyze the α-alkylation of ketones with primary alcohols. This reaction, which occurs through a step-economical synthesis, is an attractive route which avoids the use of toxic alkylating (e.g., organohalides) reagents. The transformation can be envisaged as a sequence of oxidation of alcohol/aldol condensation/ reduction of the unsaturated ketone (Scheme 2).12a Complexes 1 and 2 (0.5 mol %) efficiently catalyze the alkylation of α-tetralone with several primary alcohols at 120 °C and in the presence of KOtBu (30 mol %) (eq 7). Reaction

Reaction of (R)-1-phenylethanol (0.33 M) in 2-propanol at 70 °C with 1 (0.5 mol %) and in the presence of NaOiPr (2.0 mol %), leads to 18% ee in 5 min, whereas complete racemization is achieved after 10 min (Table 5). Table 5. Racemization of Chiral Alcohols (0.33 M) with the Ru and Os Catalysts 1 and 2 (0.5 mol %) with NaOiPr (2.0 mol %) in 2-Propanol at 70 °C

a

The conversion was determined by chiral GC analysis.

Complex 2 is also catalytically active, affording 54% ee in 10 min and 0% ee in 30 min. The aliphatic alcohol (S)-2-butanol with 1 gives 46 and 22% ee in 5 and 10 min, respectively, and complete racemization in 30 min. With 2 the reaction is slower (64 and 21% ee in 10 and 30 min), leading to the racemate in 1 h. It is worth noting that the analogous chiral derivatives cis[RuCl2(Josiphos)(R-ampy)]6b were found to be extremely active catalysts for the asymmetric TH of methyl aryl ketones, giving alcohols with up to 99% ee. These data suggest that the alcohol racemization as well as the enantioselective ketone reduction, catalyzed by the ampy Ru and Os complexes, occur

of α-tetralone with EtOH (3 equiv) in the presence of 1 leads to 96% of the alkylated ketone in 4 h (Table 7). With nPrOH and nBuOH the conversions were 87 and 92% in 2 and 3 h, respectively, with TOF values up to 420 h−1. The alkylated products were characterized by NMR analysis, which revealed that the alkylation occurs selectively, and the data of the compounds were compared with those reported in the literature.28b,31Apparently, no example of alkylation of ketones with simple alcohols (EtOH and nPrOH) was previously 1138


Organometallics

Article

Table 6. Isomerization of Allylic Alcohols (0.33 M) with the Ru and Os Catalysts 1 and 2 (1 mol %) with KOtBu (2 mol %) in tert-Butyl Alcohol

a

The conversion and TOF (moles of allylic alcohol converted into ketone per mole of catalyst per hour at 50% conversion) were determined by GC analysis.

Scheme 2. α-Alkylation of Ketones Using Primary Alcohols

double bond or via reduction of the CO bond, followed by isomerization of the allylic alcohol.

■

CONCLUDING REMARKS

We have found that the Ru and Os complexes [MCl2(dppf)(ampy)] (M = Ru (cis-1), Os (trans-2)), bearing the 2aminomethylpyridine (ampy) ligand, efficiently catalyze a variety of organic transformations involving ketones and alcohols, namely (a) fast and productive reduction of aldehydes and ketones to alcohols via both transfer hydrogenation with 2propanol and hydrogenation with H2, (b) dehydrogenation of alcohols (cholesterol) to ketones, (c) racemization of chiral alcohols, (d) isomerization of allylic alcohols to ketones, and (e) α-alkylation of α-tetralone with primary alcohols. The fast rate of the transfer hydrogenation with 2-propanol (TOF up to 105 h−1) indicates that complexes 1 and 2 efficiently promote the cleavage of the α-C−H bond of the alcohols, leading to a broad alcohol reactivity. The comparison of the activity of the Ru and Os complexes 1 and 2 shows that 2 usually requires a higher temperature to catalyze the reactions and can lead to better performance with respect to 1. The straightforward synthesis of 1 and 2 makes these efficient catalysts attractive for applications in several organic transformations.

described; most of the reactions involved the use of nBuOH or PhCH2OH.12a The relatively low loading of catalyst and the high rate indicates that 1 is an efficient system for this reaction. Surprisingly, the osmium derivative 2 displays a higher activity compared to 1. Thus, the alkylation of α-tetralone with EtOH, nPrOH, and nBuOH is attained in 1 h, 30 min, and 10 min, respectively, with TOF values up to 2800 h−1. No, example of the use of Os in the α-alkylation of ketones has been reported. It is worth noting that the reduction of the unsaturated ketone intermediate may occur through hydrogenation at the CC

Table 7. α-Alkylation of α-Tetralone (0.33 M) with Primary Alcohols (3 equiv) using the Ru and Os Catalysts 1 and 2 (0.5 mol %) with KOtBu (30 mol %) in tert-Butyl Alcohol/Toluene (1/2, v/v) at 120 °C

The conversion and TOF (moles of ketone converted into α-alkylated ketone per mole of catalyst per hour at 50% conversion) were determined by NMR analysis. a

1139


Organometallics

■

Article

filtered over a short silica pad, and the conversion was determined by GC analysis (Ru 0.02 mol %, NaOEt 2 mol %, substrate 0.5 M). Typical Procedure for Catalytic Hydrogenation of Aldehydes and Ketones with 2. Complex 2 (2.0 mg, 2.2 μmol) was dissolved in methanol (2 mL). The substrate (2.0 mmol), KOtBu (4.5 mg, 0.04 mmol), and 1.82 mL of the catalyst solution (2.0 μmol) were added to a methanol (1.2 mL)/ethanol (1 mL) mixture (final volume of the solution 4 mL; MeOH/EtOH 3/1, v/v). The solution was transferred into a thermostated reactor, and the reduction was performed by introducing H2 at 5 atm. The reaction was sampled by removing an aliquot of the reaction mixture, and diethyl ether was added (1/1, v/v). The solution was filtered over a short silica pad, and the conversion was determined by GC analysis (Os 0.1 mol %, KOtBu 2 mol %, substrate 0.5 M). Preparation of 2-Phenylethanol by Catalytic Hydrogenation with 2. Complex 2 (18.5 mg, 0.02 mmol) was dissolved in 10 mL of a MeOH/EtOH mixture (3/1, v/v). After addition of phenylacetaldehyde (1.20 g, 10 mmol) and KOtBu (22.4 mg, 0.2 mmol) the solution was transferred into a thermostated reactor at 90 °C for 8 h and the reduction was performed by introducing H2 at 5 atm. Diethyl ether was added to the solution (1/1, v/v), the mixture was filtered through a silica pad, and the solvent was evaporated to afford the product (1.04 g, 85% yield). Typical Procedure for Catalytic Dehydrogenation of Alcohols. The complex 1 or 2 (0.01 mmol) was dissolved in 2 mL of tBuOH. After addition of the alcohol substrate (2.5 mmol) and KOtBu (0.05 mmol) the solution was heated at 130 °C (bath temperature) under argon in an open system. The reaction was sampled by removing an aliquot of the reaction mixture, and diethyl ether was added (1/1, v/v). The solution was filtered over a short silica pad, and the conversion was determined by GC analysis (Ru or Os 0.4 mol %, KOtBu 2 mol %, alcohol 1.25 M). Typical Procedure for Catalytic Dehydrogenation of Cholesterol. The complex 1 or 2 (5.0 μmol), cholesterol (242 mg; 0.626 mmol), and KOtBu (25.0 μmol) were dissolved in 2 mL of tBuOH and 1 mL of toluene. The solution was heated at 145 °C (bath temperature) under argon in an open system. The reaction was sampled by removing an aliquot of the reaction mixture, and diethyl ether was added (1/1, v/v). The solution was filtered over a short silica pad, the solvent was evaporated, and the conversion was determined by NMR analysis (Ru or Os 0.8 mol %, KOtBu 4 mol %, sterol 0.21 M). Typical Procedure for the Racemization of Chiral Alcohols. The complex 1 or 2 (1.6 μmol), NaOiPr (66 μL, 0.1 M solution in 2propanol, 6.6 μmol), and the chiral alcohol (0.33 mmol) were dissolved in 2-propanol (1 mL) under argon. The solution was stirred at 70 °C. The reaction was sampled by removing an aliquot of the reaction mixture, and diethyl ether was added (1/1 in volume). The solution was filtered over a short silica pad, and the conversion was determined by GC analysis (Ru or Os mol 0.5 mol %, NaOiPr 2 mol %, chiral alcohol 0.33 M). Typical Procedure for the Isomerization of Allylic Alcohols. The complex 1 or 2 (0.01 mmol), the allylic alcohol (1 mmol), and KOtBu (2.2 mg, 0.02 mmol) were dissolved in 3 mL of tBuOH. The solution was heated at 70 °C (Ru) or 120 °C (Os, bath temperature) under argon. The reaction was sampled by removing an aliquot of the reaction mixture, and diethyl ether was added (1/1, v/v). The solution was filtered over a short silica pad, and the conversion was determined by GC analysis (Ru or Os 1 mol %, KOtBu 2 mol %, allylic alcohol 0.33 M). Typical Procedure for the α-Alkylation of α-Tetralone with Primary Alcohols. The complex 1 or 2 (5.0 μmol), α-tetralone (146.2 mg, 1.0 mmol), the primary alcohol (3.0 mmol), and KOtBu (34 mg, 0.30 mmol) were dissolved in 1 mL of tBuOH and 2 mL of toluene. The solution was heated at 120 °C (bath temperature) under argon. The reaction was sampled by removing an aliquot of the reaction mixture, and diethyl ether was added (1/1, v/v). The solution was filtered over a short silica pad, the solvent was evaporated, and the conversion was determined by NMR analysis (Ru or Os 0.5 mol %, KOtBu 30 mol %, α-tetralone 0.33 M).

EXPERIMENTAL SECTION

All reactions were carried out under an argon atmosphere using standard Schlenk techniques. The solvents were carefully dried by standard methods and distilled under argon before use. The diphosphine dppf and all other chemicals were purchased from Aldrich and used without further purification. The compounds cis[RuCl2(dppf)(ampy)] (1)13 and [OsCl2(PPh3)3]32 were prepared according to the literature procedure. NMR measurements were recorded on a Bruker AC 200 spectrometer, and the chemical shifts, in ppm, are relative to TMS for 1H and 13C{1H} and 85% H3PO4 for 31 1 P{ H}. Elemental analysis (C, H, N) was carried out with a Carlo Erba 1106 elemental analyzer, whereas the GC analyses were performed with a Varian GP-3380 gas chromatograph equipped with a MEGADEX-ETTBDMS-β chiral column. Synthesis of trans-[OsCl2(dppf)(ampy)] (2). [OsCl2(PPh3)3] (100 mg, 0.095 mmol) and dppf (58 mg, 0.105 mmol) were treated with toluene (1.5 mL), and the suspension was stirred for 2 h at 50 °C. After addition of ampy (11 μL, 0.107 mmol) the suspension was stirred for 2 h at 50 °C and then concentrated to about 0.5 mL. Addition of heptane (2 mL) afforded a yellow precipitate, which was washed with heptane (3 × 1 mL) and diethyl ether (3 × 1 mL) and dried under reduced pressure. Yield: 82 mg (93%). 1H NMR (200.1 MHz, CD2Cl2, 20 °C): δ 8.54 (m, 1H; o-C5H4N), 8.08 (pseudo t, J(H,H) = 8.7 Hz, 4H; aromatic protons), 7.60−7.13 (m, 18H; aromatic protons), 6.58 (pseudo t, J(H,H) = 6.6 Hz, 1H; aromatic proton), 4.77 (m, 2H; C5H4), 4.39 (m, 2H; CH2N), 4.22 (m, 2H; C5H4), 4.18 (m, 2H; C5H4), 4.05 (m, 2H; C5H4), 3.59 (m, 2H; NH2). 13 C{1H} NMR (50.3 MHz, CD2Cl2, 20 °C): δ 152.7 (d, J(C,P) = 3.0 Hz; NCH), 138.1−117.3 (m, aromatic carbons), 74.1 (d; J(C,P) = 8.2 Hz; C5H4), 72.0 (d; J(C,P) = 7.4 Hz; C5H4), 68.3 (d; J(C,P) = 6.1 Hz; C5H4), 65.4 (d; J(C,P) = 6.3 Hz; C5H4), 46.9 (s, CH2). 31P{1H} NMR (81.0 MHz, CD2Cl2, 20 °C): δ −7.0 (d, J(P,P) = 19.7 Hz), −17.9 (d, J(P,P) = 19.7 Hz). Anal. Calcd for C40H36Cl2FeN2OsP2: C, 52.02; H, 3.93; N, 3.03. Found: C, 51.36; H, 3.88; N, 2.90. Typical Procedure for the Catalytic Transfer Hydrogenation of Aldehydes and Ketones. Complex 1 or 2 (1.2 μmol) was dissolved in 3 mL of 2-propanol. The aldehyde or ketone (1 mmol) was dissolved in 9 mL of 2-propanol, and the solution was refluxed (100 °C bath temperature) under argon. After addition of 200 μL of NaOiPr in 2-propanol (0.1 M; 0.02 mmol), 125 μL of the catalyst solution (0.05 μmol), and 2-propanol (final volume of the solution 10 mL), the reduction of the aldehyde or the ketone started immediately. The reaction was sampled by removing an aliquot of the reaction mixture, and diethyl ether was added (1/1, v/v). The solution was filtered over a short silica pad, and the conversion was determined by GC analysis (Ru or Os 0.005 mol %, NaOiPr 2 mol %, aldehyde or ketone 0.1 M). Preparation of p-Anisyl Alcohol by Catalytic Transfer Hydrogenation with 1. Complex 1 (5.8 mg, 7.0 μmol) was dissolved in 50 mL of 2-propanol. After addition of p-anisaldehyde (1.7 mL, 14.0 mmol) and NaOiPr (0.1 M, 2.8 mL, 0.28 mmol) the solution was refluxed under argon for 30 min. Diethyl ether was added to the solution (1/1, v/v), the mixture was filtered through a silica pad and the solvent was evaporated to afford the product (1.74 g, 90% yield). Preparation of (±)-Citronellol by Catalytic Transfer Hydrogenation with 2. (±)-β-Citronellol was obtained following the procedure used for p-anisyl alcohol, by employment of (±)-citronellal in place of p-anisaldehyde, complex 2 in place of 1, and reflux of the solution for 2 h (1.92 g, 88% yield). Typical Procedure for Catalytic Hydrogenation of Aldehydes and Ketones with 1. Complex 1 (2.0 mg, 2.4 μmol) was dissolved in ethanol (2 mL). The substrate (2.0 mmol), 0.16 mL of NaOEt in ethanol (0.25 M, 0.04 mmol), and 0.33 mL of the catalyst solution (0.4 μmol) were added to ethanol (final volume of the solution 4 mL). The solution was transferred into a thermostated reactor, and the reduction was performed by introducing H2 at 5 atm. The reaction was sampled by removing an aliquot of the reaction mixture, and diethyl ether was added (1/1, v/v). The solution was 1140


Organometallics

■

Article

Vinci, D.; Ruan, J.; Xiao, J. Angew. Chem., Int. Ed. 2006, 45, 6718. (c) Mebane, R. C.; Mansfield, A. J. Synth. Commun. 2005, 35, 1. (d) Naskar, S.; Bhattacharjee, M. J. Organomet. Chem. 2005, 690, 5006. (e) Miecznikowski, J. R.; Crabtree, R. H. Organometallics 2004, 23, 629. (f) Yamada, I.; Noyori, R. Org. Lett. 2000, 2, 3425. (12) For reviews on DHY see: (a) Johnson, T. C.; Morris, D. J.; Wills, M. Chem. Soc. Rev. 2010, 39, 81. (b) Dobereiner, G. E.; Crabtree, R. H. Chem. Rev. 2010, 110, 681. (c) Friedrich, A.; Schneider, S. ChemCatChem. 2009, 1, 72. (d) Kubas, G. J. J. Organomet. Chem. 2009, 694, 2648. (e) Loges, B.; Junge, H.; Spilker, B.; Fischer, C.; Beller, M. Chem. Ing. Tech. 2007, 79, 741. (13) Baratta, W.; Bossi, G.; Putignano, E.; Rigo, P. Chem. Eur. J. 2011, 17, 3474. (14) Nixon, T. D.; Whittlesey, M. K.; Williams, J. M. J. Dalton Trans. 2009, 753. (15) 31P{1H} NMR: doublets at δ −3.3 and −11.2 with 2J(P,P) = 20.2 Hz and δ −5.1 and −9.8 with 2J(P,P) = 20.0 Hz. (16) (a) Pàmies, O.; Bäckvall, J. E. Chem. Eur. J. 2001, 7, 5052. (b) Aranyos, A.; Csjernyik, G.; Szabó, K. J.; Bäckvall, J. E. Chem. Commun. 1999, 351. (17) (a) Baratta, W.; Ballico, M.; Esposito, G.; Rigo, P. Chem. Eur. J. 2008, 14, 5588. (b) Baratta, W.; Siega, K.; Rigo, P. Chem. Eur. J. 2007, 13, 7479. (18) (a) van Buijtenen, J.; Meuldijk, J.; Vekemans, J. A. J. M.; Hulshof, L. A.; Koojiman, H.; Spek, A. L. Organometallics 2006, 25, 873. (b) Adair, G. R. A.; Williams, J. M. J. Tetrahedron Lett. 2005, 46, 8233. (c) Zhang, J.; Gandelman, M.; Shimon, L. J. W.; Rozenberg, H.; Milstein, D. Organometallics 2004, 23, 4026. (d) Zhang, J.; Gandelman, M.; Shimon, L. J. W.; Milstein, D. Dalton Trans. 2007, 107. (19) (a) Nielsen, M.; Kammer, A.; Cozzula, D.; Junge, H.; Gladiali, S.; Beller, M. Angew. Chem., Int. Ed. 2011, 50, 9593. (b) Junge, H.; Loges, B.; Beller, M. Chem. Commun. 2007, 522. (c) Junge, H.; Beller, M. Tetrahedron Lett. 2005, 46, 1031. (d) Morton, D.; Cole-Hamilton, D. J.; Utuk, I. D.; Paneque-Sosa, M.; Lopez-Poveda, M. J. Chem. Soc., Dalton Trans. 1989, 489. (e) Morton, D.; Cole-Hamilton, D. J. J. Chem. Soc., Chem. Commun. 1988, 1154. (20) (a) Fieser, L. F.; Fieser, M. Steroids; Reinhold: New York, 1959. (b) Rasmusson, G. H.; Arth, G. E. In Organic Reactions in Steroid Chemistry; Fried, J., Edwards, J. A., Eds.; Van Nostrand: New York, 1972; Vol. 1, p 222. (c) Karlsson, U.; Wang, G. Z.; Bäckvall, J. E. J. Org. Chem. 1994, 59, 1196. (21) (a) Djerassi, C. Org. React. 1951, 6, 207. (b) Skoda-Földes, R.; Kollár, L. Chem. Rev. 2003, 103, 4095. (22) Almeida, M. L. S.; Kočovský, P.; Bäckvall, J. E. J. Org. Chem. 1996, 61, 6587. (23) (a) Pamies, O.; Bäckvall, J. E. Chem. Rev. 2003, 103, 3247. (b) Ahn, Y.; Ko, S.-B.; Kim, M.-J.; Park, J. Coord. Chem. Rev. 2008, 252, 647. (c) Pellissier, H. Tetrahedron 2008, 64, 1563. (d) Karvembu, R.; Prabhakaran, R.; Muthu Tamizh, M.; Natarajan, K. C. R. Chim. 2009, 12, 951. (24) Bossi, G.; Putignano, E.; Rigo, P.; Baratta, W. Dalton Trans. 2011, 40, 8986. (25) Uma, R.; Crévisy, C.; Grée, R. Chem. Rev. 2003, 103, 27. (b) van der Drift, R. C.; Bouwman, E.; Drent, E. J. Organomet. Chem. 2002, 650, 1. (26) (a) García-Á lvarez, J.; Gimeno, J.; Suárez, F. J. Organometallics 2011, 30, 2893. (b) Liu, P. N.; Ju, K. D.; Lau, C. P. Adv. Synth. Catal. 2011, 353, 275. (c) Cadierno, V.; Crochet, P.; Gimeno, J. Synlett 2008, 1105. (d) Bouziane, A.; Carboni, B.; Bruneau, C.; Carreaux, F.; Renaud, J.-L. Tetrahedron 2008, 64, 11745. (e) Cadierno, V.; GarcíaGarrido, S. E.; Gimeno, J.; Varela-Á lvarez, A.; Sordo, J. A. J. Am. Chem. Soc. 2006, 128, 1360. (f) Martin-Matute, B.; Bogar, K.; Edin, M.; Kaynak, F. B.; Bäckvall, J.-E. Chem. Eur. J. 2005, 11, 5832. (27) Batuecas, M.; Esteruelas, M. A.; García-Yebra, C.; Oñate, E. Organometallics 2010, 29, 2166. (28) (a) Cho, C. S.; Kim, B. T.; Kim, T.-J.; Shim, S. C. Tetrahedron Lett. 2002, 43, 7987. (b) Cho, C. S.; Kim, B. T.; Kim, T.-J.; Shim, S. C. J. Org. Chem. 2001, 66, 9020.

ASSOCIATED CONTENT

S Supporting Information *

Text giving characterization data of the isolated alcohols and αalkylated ketones. This material is available free of charge via the Internet at http://pubs.acs.org.

■ ■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

ACKNOWLEDGMENTS This work was supported by the Ministero dell’Università e della Ricerca (MIUR) and the Regione Friuli Venezia Giulia. We thank Johnson-Matthey/Alfa Aesar for a generous loan of ruthenium and Mr. P. Polese for carrying out the elemental analysis.

■

REFERENCES

(1) (a) The Handbook of Homogeneous Hydrogenation; de Vries, J. G., Elsevier, C. J., Eds.; Wiley-VCH: Weinheim, Germany, 2007; Vols. 1− 3. (b) Asymmetric Catalysis on Industrial Scale; Blaser, H. U., Schmidt, E., Eds.; Wiley-VCH: Weinheim, Germany, 2004. (c) Comprehensive Asymmetric Catalysis; Jacobsen, E. N., Pfaltz, A., Yamamoto, H., Eds.; Springer: Berlin, 2004; Supplements 1 and 2. (2) (a) Morris, R. H. Chem. Soc. Rev. 2009, 38, 2282. (b) Baratta, W.; Rigo, P. Eur. J. Inorg. Chem. 2008, 4041. (c) Wang, C.; Wu, X.; Xiao, J. Chem. Asian J. 2008, 3, 1750. (d) Ikariya, T.; Blacker, A. J. Acc. Chem. Res. 2007, 40, 1300. (e) Morris, D. J.; Wills, M. Chem. Today (Chim. Oggi) 2007, 25, 11. (f) Samec, J. S. M.; Bäckvall, J. E.; Andersson, P. G.; Brandt, P. Chem. Soc. Rev. 2006, 35, 237. (g) Gladiali, S.; Alberico, E. Chem. Soc. Rev. 2006, 35, 226. (3) Doucet, H.; Ohkuma, T.; Murata, K.; Yokozawa, T.; Kozawa, M.; Katayama, E.; England, A. F.; Ikariya, T.; Noyori, R. Angew. Chem., Int. Ed. 1998, 37, 1703. (4) Haack, K. J.; Hashiguchi, S.; Fujii, A.; Ikariya, T.; Noyori, R. Angew. Chem., Int. Ed. Engl. 1997, 36, 285. (5) (a) Ohkuma, T.; Utsumi, N.; Tsutsumi, K.; Murata, K.; Sandoval, C.; Noyori, R. J. Am. Chem. Soc. 2006, 128, 8724. (b) Ohkuma, T.; Tsutsumi, K.; Utsumi, N.; Arai, N.; Noyori, R.; Murata, K. Org. Lett. 2007, 9, 255. (6) (a) Baratta, W.; Herdtweck, E.; Siega, K.; Toniutti, M.; Rigo, P. Organometallics 2005, 24, 1660. (b) Baratta, W.; Chelucci, G.; Herdtweck, E.; Magnolia, S.; Siega, K.; Rigo, P. Angew. Chem., Int. Ed. 2007, 46, 7651. (c) Ohkuma, T.; Sandoval, C. A.; Srinivasan, R.; Lin, Q.; Wei, Y.; Muñiz, K.; Noyori, R. J. Am. Chem. Soc. 2005, 127, 8288. (d) Sandoval, C. A.; Li, Y.; Ding, K.; Noyori, R. Chem. Asian J. 2008, 3, 1801. (7) Baratta, W.; Ballico, M.; Del Zotto, A.; Siega, K.; Magnolia, S.; Rigo, P. Chem. Eur. J. 2008, 14, 2557. (8) (a) Baratta, W.; Benedetti, F.; Del Zotto, A.; Fanfoni, L.; Felluga, F.; Magnolia, S.; Putignano, E.; Rigo, P. Organometallics 2010, 29, 3563. (b) Baratta, W.; Fanfoni, L.; Magnolia, S.; Siega, K.; Rigo, P. Eur. J. Inorg. Chem. 2010, 1419. (c) Baratta, W.; Chelucci, G.; Magnolia, S.; Siega, K.; Rigo, P. Chem. Eur. J. 2009, 15, 726. (d) Baratta, W.; Ballico, M.; Chelucci, G.; Siega, K.; Rigo, P. Angew. Chem., Int. Ed. 2008, 47, 4362. (e) Baratta, W.; Ballico, M.; Baldino, S.; Chelucci, G.; Herdtweck, E.; Siega, K.; Magnolia, S.; Rigo, P. Chem. Eur. J. 2008, 14, 9148. (f) Baratta, W.; Siega, K.; Rigo, P. Adv. Synth. Catal. 2007, 349, 1633. (9) (a) Ikariya, T.; Gridnev, I. D Top. Catal. 2010, 53, 894. (b) Ikariya, T.; Murata, K.; Noyori, R. Org. Biomol. Chem. 2006, 4, 393. (10) (a) Shang, G.; Li, W.; Zhang, X. In Catalytic Asymmetric Synthesis, 3rd ed.; Ojima, I., Ed.; Wiley: Hoboken, NJ, 2010; Chapter 7. (b) New Frontiers in Asymmetric Catalysis; Mikami, K., Lautens, M., Eds.; Wiley: Hoboken, NJ, 2007. (11) (a) Zhao, M.; Yu, Z.; Yan, S.; Li, Y. Tetrahedron Lett. 2009, 50, 4624. (b) Wu, X.; Liu, J.; Li, X.; Zanotti-Gerosa, A.; Hancock, F.; 1141


Organometallics

Article

(29) (a) Martínez, R.; Brand, G. J.; Ramón, D. J.; Yus, M. Tetrahedron Lett. 2005, 46, 3683. (b) Martínez, R.; Ramón, D. J.; Yus, M. Tetrahedron 2006, 62, 8988. (30) Taguchi, K.; Nakagawa, H.; Hirabayashi, T.; Sakaguchi, S.; Ishii, Y. J. Am. Chem. Soc. 2004, 126, 72. (31) (a) Adamczyk, M.; Watt, D. S.; Netzel, D. A. J. Org. Chem. 1984, 49, 4226. (b) Pažický, M.; Semak, V.; Gásp̌ ár, B.; Bílešová, A.; Sališová, M.; Bohác,̌ A. ARKIVOC 2008, 225. (32) Elliott, G. P.; McAuley, N. M.; Roper, W. R. Inorg. Synth. 1989, 26, 184.

1142


MCl2(ampy)(dppf) (M = Ru, Os): Multitasking Catalysts for Carbonyl

Recommend Documents