ARTICLE pubs.acs.org/Organometallics
Oxidative Synthesis of Amides and Pyrroles via Dehydrogenative Alcohol Oxidation by Ruthenium Diphosphine Diamine Complexes Nathan D. Schley, Graham E. Dobereiner, and Robert H. Crabtree* Department of Chemistry, Yale University, 225 Prospect Street, P.O. Box 208107, New Haven, Connecticut 06520, United States
bS Supporting Information ABSTRACT: A series of ruthenium complexes can perform the acceptorless dehydrogenation of diols as well as the reaction of amines and alcohols to form ester, lactam, and amide products. The ligand criteria necessary for high catalytic activity are identified to guide future catalyst development for amide formation from amines and alcohols. These complexes can be employed in a dehydrogenative PaalKnorr pyrrole synthesis to give 2,5-dimethyl-N-alkylpyrroles.
I
n recent years, a new approach to the synthesis of carboxylic acid derivatives has been developed which employs the dehydrogenative coupling of alcohols with other functional groups.14 The need for atom-economical methods of amide bond formation has generated a particular interest in the generation of amides from amines and primary alcohols in the presence58 or absence915 of a hydrogen acceptor, the most notable example being the Milstein catalyst.9 The acceptorless case is especially interesting, because the oxidation of the alcohol takes place without the need for a stoichiometric oxidant. Instead, the reaction is driven by the extrusion of H2 gas from the solution at high temperatures. The dehydrogenative oxidation of alcohols can be performed by homogeneous complexes of rhodium,1619 ruthenium,2026 osmium,21,26 and iridium.2729 In the case of ruthenium, acceptorless oxidation has previously been applied to the conversion of diols to lactones, as well as the reaction of primary alcohols to form homocoupled esters.3032 Previous attempts at forming amides via dehydrogenation, however, generally gave alkylated amines exclusively unless a hydrogen acceptor was present.6 Despite recent progress in the field, acceptorless amide formation has only been demonstrated in a few specific cases.915 The most prominent examples feature either specialized pincer ligands9 or highly donating N-heterocyclic carbene ligands.1015 Though some reports have provided rationales for amide selectivity, these have so far been ligand specific, and a general mechanistic framework for the selective synthesis of amides remains elusive. In order to develop improved catalysts for acceptorless dehydrogenation, we have used readily tunable ligand sets. Ruthenium(II) diphosphine diamine complexes are a well-studied class of compounds which have been used extensively in hydrogenation and transfer hydrogenation reactions,3335 but few reports have explored their use in the dehydrogenative oxidation of alcohols or determined the aspects of the ligand set which are crucial for activity.25,36 Our group has previously shown that a ruthenium diphosphine diamine complex, RuCl2(2-(aminomethyl)pyridine)(1,4-bis(diphenylphosphino)butane) (1), catalyzes the oxidative r 2011 American Chemical Society
cyclization of 5-amino-1-pentanol to δ-valerolactam in high yield in the absence of oxidants.37 Amide formation by 1 proceeds in good selectivity from a variety of 1,5-amino alcohols but gives unsatisfactory selectivity and yields with other substrates. In this report we investigate the contribution of both the diamine and diphosphine ligands to the activity of the resulting complexes for the dehydrogenative oxidation of alcohols. A variety of substrates react under our catalytic conditions, providing the selective formation of lactams, lactones, amides, and pyrroles. Complexes incorporating diamine ligands with NH protons were superior catalysts for amide formation, consistent with prior computational work identifying an intramolecular hydrogen bond between amine ligand and bound substrate as a key factor in hydrogen loss from the metal center.37
’ RESULTS Catalyst Design. In a report examining the conversion of neat 1,4-butanediol to γ-butyrolactone at 205 °C, Hartwig and Zhao had success with a number of ruthenium(II) compounds.36 We prepared a family of related ruthenium diphosphine diamines (Figure 1) and screened the dehydrogenative lactonization reaction at moderate temperatures in order to select for the most active catalysts. From complex 1 as a starting point, we systematically modified the two principal ligands, the diphosphine and the diamine, to determine the contribution of each to dehydrogenation activity and to identify possible ways to improve catalysis. Complex 3 was prepared to compare the tethered diphosphine dppb of 1 with two individual triphenylphosphine ligands. 3 has been prepared previously as both the trans-dichloro and cis-dichloro isomers, and these have been shown to isomerize from trans to cis in refluxing toluene.38 The monodentate phosphines in 3 are mutually cis, but Received: June 2, 2011 Published: July 13, 2011 4174
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Figure 1. Ruthenium(II) complexes compared for dehydrogenation catalysis.
Figure 2. Crystal structures of 138 (left) and 2 (center), comparing their geometry about the metal center. An alternate view of 2 (right) shows the distorted P2RuCl1 angle.
unlike 1, they are not geometrically constrained by a linker and could isomerize to a trans-diphosphine complex in situ. Complex 2 replaces the flexible dppb of 1 with the more rigid dppf. This diphosphine has a similar bite angle but greatly reduced flexibility, owing to the incorporation of a metallocene into the backbone.39 Single-crystal X-ray diffraction on a sample of 2 confirms the cis-dichloro geometry and affords us an opportunity to compare the solid-state structures of 138 and 2 (Figure 2). The P1RuP2 angle in 2 is found to be 97.03(5)°, substantially larger than the reported bite angle of 92.92(2)° in 1.38 A similar difference can be observed in square-planar palladium(II) dichloride complexes, where the diphosphines adopt somewhat larger bite angles: 99.07(5)°40 for dppf and 94.36°41 for dppb. In addition to the difference in bite angle, the principal structural variation between 2 and 1 is the degree of steric congestion around the site trans to the primary amine. We have previously determined that this is the most thermodynamically favored site for a dihydrogen ligand in these systems.37 In the crystal structure of 2, the P2RuCl1 angle deviates to nearly 100°, owing to the projection of the bulky diphosphine into the axial site (Figure 2, right). We next prepared several complexes to investigate the role of the diamine ligand. Our previous computational work on the mechanism of alcohol dehydrogenation by 1 identified a hydrogen-bonding interaction of the aminomethylpyridine ligand with the substrate as an important factor in stabilizing key intermediates. However, ruthenium complexes lacking amine ligands have previously been shown to catalyze alcohol dehydrogenation at high temperatures.36 Amine ligands containing NH protons may participate in ligand bifunctionality that serves to greatly increase rates of hydrogen transfer in certain reactions,42,43 and this could contribute to a lower barrier to alcohol dehydrogenation (Scheme 1). In order to test these hypotheses, compounds of the fully methylated N,N-dimethylaminomethylpyridine and the related
Scheme 1. Common Motif for Ligand Bifunctionality in Hydrogen Transfer from Alcohols
secondary diamine N,N0 -dimethylethylenediamine were prepared. Complex 5, which lacks an amine ligand, was also synthesized. Complexes 1-Me2 and 2-Me2 were prepared as analogues to 1 and 2. Both 1-Me2 and 2-Me2 show a single pair of doublets in their respective 31P{1H} NMR spectra, indicating that they are both single isomers in solution. X-ray crystallographic analysis of 1-Me2 and 2-Me2 show that, unlike 1 and 2, both complexes adopt a trans-dichloro arrangement (Figure 3). Cis/trans isomerism is often seen in complexes of this type;44 however, 1-Me2 does not isomerize even on heating in mesitylene for 1 h at 160 °C, suggesting that the trans-dichloro isomer is thermodynamically preferred. When the synthesis of 1Me2 is conducted at room temperature instead of at reflux in toluene, the same trans isomer is obtained. The geometric difference between 1, 2 and 1-Me2, 2-Me2 may be significant, since the inner-sphere pathway for hydride abstraction from an alcohol requires two cis sites on the catalyst. Calculations on 1 suggest that both inner- and outer-sphere hydride transfer mechanisms are close in energy, indicating that trans complexes may still be active for catalysis by an outer-sphere mechanism. Additionally, the most thermodynamically favorable solid-state geometry of the dichloride complexes does not necessarily reflect the preferred geometry of any hydrido45 or alkoxo intermediates under the catalytic conditions in solution. Catalysis. Complexes 15 were initially screened for two related reactions involving alcohol dehydrogenation. The dehydrogenation 4175
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Figure 3. ORTEP drawings of crystal structures of 2 (left), 2-Me2 (center), and 1-Me2 (right) with ellipsoids shown at the 50% probability level. Solvent molecules and hydrogen atoms are omitted for clarity.
Table 1. Comparison of Ruthenium Complexes for the Dehydrogenation of 1,4-Butanediola
cat.
1 1
TOF (h )
10
1-Me2 5.9
2 12
2-Me2 1.5
3 5.0
4 5.0
Table 2. Comparison of Ruthenium Complexes for the Dehydrogenation of N-Butyl-5-amino-1-pentanola
5
cat. 1
0.4
a
TOF (h )
1
1-Me2
2
2-Me2
3
4
5
>120
13
>120
23
40
78
9
Conditions: 2 mmol of diol, 0.10 mmol of KOH, 0.02 mmol of catalyst, 1 mL of toluene. The reaction mixture was heated to 125 °C under a slow flow of N2 for 4 h. The TOF was determined by the NMR yield of butyrolactone after 4 h with respect to an internal standard (1,3,5trimethoxybenzene).
Conditions: 1 mmol of amino alcohol, 0.06 mmol of KOH, 0.02 mmol of catalyst, 1 mL of toluene. The reaction mixture was heated to 125 °C under a slow flow of N2 for 20 min. The TOF was determined by the NMR yield of the lactam after 20 min with respect to an internal standard (1,3,5-trimethoxybenzene).
of 1,4-butanediol to γ-butyrolactone has been used previously to assess dehydrogenation activity;36 however, we perform the reaction at a temperature lower than that previously reported. All compounds tested showed activity in refluxing toluene over 4 h (Table 1), but large differences in apparent rates are seen, depending on the ligand set. The chelating diphosphine ligand in 1 clearly plays a strong role, as 3 shows a reduced rate. The dppf derivative 2 proved to be a minor improvement on 1, and both complexes are significantly more active than all others tested. The effect of the amine ligand on catalysis is even more profound. Methylation of the amine (1 vs 1-Me2, 2 vs 2-Me2) is observed to dramatically decrease the rate of lactone formation from butanediol. Complex 5, which lacks the diamine entirely, is nearly inactive for the dehydrogenative oxidation of the diol, turning over fewer than 2 times in 4 h. A useful gauge for the dehydrogenative amide-forming activity of 15 is the intramolecular conversion of N-butyl-5-amino-1pentanol to N-butylvalerolactam. Our previous work has shown that this substrate is oxidized much more rapidly than the unsubstituted 5-amino-1-pentanol and the reaction is higher yielding than the intermolecular reaction of an amine and a primary alcohol. The rates are presented in turnovers per hour, calculated after 20 min in refluxing toluene (Table 2). Overall this reaction is much more rapid than the lactone formation reported above. 1 and 2 both display a rate in excess of 120 t.o./h, proceeding to near completion in 20 min. Their methylated analogues 1-Me2 and 2-Me2 show substantially lower rates: 13 and 23 t.o./h, respectively.
All four of the complexes containing diamine ligands with NH protons (14) show rates at least double that of the rates obtained for those complexes without NH protons (1-Me2 and 2-Me2). Complexes 3 and 1-Me2, which had similar activity in the oxidation of 1,4-butanediol (Table 1), differ in rate by a factor of 3 for lactam formation, consistent with the ligand NH protons playing a role in catalysis. A high rate is also obtained for the secondary diamine diphosphine derivative 4. The lack of the diphosphine in 3 appears to hinder the activity versus 1 for lactam formation, just as it does for lactone formation. Amide Formation. As shown previously, 1 is an excellent catalyst for the dehydrogenative formation of lactams from 1,5amino alcohols but gives low conversion and selectivity when used for the corresponding intermolecular reaction between an aliphatic alcohol and amine under similar conditions.37 Subsequent experimental work using complex 2 has revealed that the yield and selectivity depend strongly on the concentration of the amine component. When the reaction is performed neat in an excess of the amine, good yields and turnover numbers can be obtained (Table 3). More hindered or less nucleophilic amines such as anilines gave poor yields, even under these optimized conditions. Reactions driven by release of a gas are most often run in refluxing solvent to reduce the solubility of gaseous products;46 however, the slow passage of nitrogen gas through the headspace of the reaction was sufficient to remove hydrogen even when no component of the reaction was near its boiling point (Table 3, entry 5).
a
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Table 3. Intermolecular Amide Formation by Complex 2a
entry
alcohol
amine
loading, %
yield, %b
1
benzyl alcohol
1-hexylamine
2
42
2
benzyl alcohol
1-hexylamine
4
74
3
benzyl alcohol
piperidine
2
42
4
benzyl alcohol
piperidine
4
89
5 6
benzyl alcohol 2-phenethanol
1-decylamine piperidine
4 4
58 55
Scheme 2. Amide Formation from an Amine and Alcohol
a
Conditions: 1 mmol of alcohol, 6 mmol of amine, 0.15 mmol of KOH (4% loading of 2) or 0.1 mmol KOH (2% loading of 2), catalyst loading with respect to alcohol substrate, heated to 125 °C for 3.5 h. b Isolated yields of the corresponding secondary or tertiary amides.
Table 4. Dehydrogenative Pyrrole Formationa
entry
amine
1
1-hexylamine
2
1-decylamine
3
1-decylamine
loading, %
yield, %b
2
1.5
45
2
1.5
48
4
1.5
46c
1.5
37
cat.
4
1-decylamine
1-Me2
5
1-decylamine
none
0
a
Conditions: 3.0 mmol of 2,5-hexanediol, 2.5 mmol of amine, 0.23 mmol of sodium formate, catalyst loading with respect to amine substrate. The reaction mixture was heated to 125 °C under a slow flow of N2 for 16 h. b Isolated yields of pyrrole. c NMR yield with respect to an internal standard (1,3,5-trimethoxybenzene).
Pyrrole Synthesis. In our previous report examining 1, imines were sometimes obtained as byproducts of amide formation by condensation of the amine with aldehyde generated in situ.37,47 By using a secondary alcohol in place of a primary alcohol, we could selectively generate ketimines and employ them in the oxidative synthesis of N-heterocycles. We have found that the dehydrogenative oxidation of 2,5-hexanediol by a suitable catalyst in the presence of a primary alkylamine gives the N-alkyl-2,5dimethylpyrroles in a dehydrogenative PaalKnorr pyrrole synthesis (Table 4). Several of the catalysts tested gave similar yields with the same loading, and increased catalyst loading did not result in additional product being formed. This reaction likely proceeds by metal-catalyzed oxidation of the diol to the dione followed by a series of uncatalyzed condensation reactions leading to the pyrrole product.48 Sodium formate gave better results than potassium hydroxide when used as an activator for the ruthenium complexes in this reaction, and other formate salts tested were comparable, including cesium formate and zinc formate. The primary diol 1,4butanediol gave less satisfactory results under the same conditions, as a myriad of products including the pyrrole, imide,49 and lactone were formed competitively.
’ DISCUSSION The chelating aminomethylpyridine ligand appears to have a crucial role in certain reactions presented above. Though the
Figure 4. Schematic of hydrogen bonding between a bound hemiaminal and either the ligand NH protons or a dihydrogen ligand.37
presence of the primary amine ligand does not appear to have a large effect on the efficiency of the lactonization reaction, for the catalytic cyclization of N-butyl-5-amino-1-pentanol the amine ligand is the most important factor governing reactivity. 1 and 2 turn over more than 40 times in 20 min, respectively, while complexes 1-Me2 and 2-Me2 are nearly inactive by comparison. This reaction is thought to proceed by initial oxidation of the amino alcohol to an amino aldehyde, followed by formation of a metal-bound hemiaminal and subsequent oxidation to the amide (Scheme 2). In a previous computational study of 1, we identified a hydrogen-bonding interaction between a bound hemiaminal and either a ligand NH proton or a bound dihydrogen ligand (Figure 4).37 The two isomers were calculated to be close in energy, but a crucial difference between the two is the magnitude of the barrier to loss of the dihydrogen ligand from the metal center. The isomer in which the hemiaminal is hydrogen-bonded to the amine ligand has a substantially lower barrier for loss of hydrogen. The difference in the barrier to hydrogen loss could account for the difference in reaction rate between complexes containing amine ligands with NH protons and those without. The high rates of lactam formation by 2 do not extend directly to the intermolecular case without modification of the conditions. The low yields obtained in the absence of a large excess of the amine component of the reaction highlight the important role of the substrate amine in the catalytic cycle. The need for high concentrations of amine was also observed in a recent report of amide formation by a ruthenium N-heterocyclic carbene catalyst.14 From our computational work on the intramolecular amide bond formation, we suggest that the aldehyde intermediate must be trapped by the amine while still bound to the metal center (assuming an inner-sphere pathway) to give a metalbound hemiaminal which can undergo further oxidation. If attack by the amine is slow, as would be the case at lower amine concentrations or with weakly nucleophilic amines, the aldehyde can potentially diffuse away and undergo an uncatalyzed imine condensation, leading to undesired products, or contribute to catalyst deactivation by decarbonylation.20 The superior activity of 1 and 2 versus that of other complexes tested highlights the importance of both the diphosphine and primary aminomethylpyridine ligand in the determining the rate of dehydrogenative catalysis. Minor changes such as employing two monophosphines instead of the diphosphine or methylation of the diamine ligand result in profoundly lower rates, particularly 4177
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in lactam formation. As the factors which enable ruthenium complexes to catalyze dehydrogenative amide formation are still not well understood, subsequent work on new catalysts could usefully benefit from this study by incorporation of bulky diphosphines and amine ligands which are capable of acting as hydrogen bond donors for bound substrates.
’ CONCLUSIONS A variety of ruthenium diamine diphosphine complexes have been shown to catalyze the dehydrogenative oxidation of diols to lactones and amino alcohols to lactams. The most active catalyst tested can convert primary alcohols and amines to amides, operating best under conditions with excess of the amine component. Complexes containing amine ligands with NH protons are superior catalysts, possibly due to a hydrogen-bonding interaction between the ligand and bound substrate which lowers the barrier to hydrogen loss from the metal. The future design of catalysts for net dehydrogenative processes may well benefit from consideration of hydrogen-bonding interactions in proposed catalytic intermediates beyond the initial dehydrogenation step. ’ EXPERIMENTAL SECTION RuCl2(2-aminomethylpyridine)(dppb) (1),38 RuCl2(2-aminomethylpyridine)(dppf) (2),25 RuCl2(PPh3)2(2-aminomethylpyridine) (3),38 RuCl2(N,N0 -dimethylethylenediamine)(dppf) (4),35 RuCl2(PPh3)(dppb) (5),50 and N-butyl-5-amino-1-pentanol37 were prepared according to the literature procedures. All solvents were of commercial grade and dried over activated alumina using a Grubbs-type solvent purification system prior to use.51 NMR spectra were recorded on a 400 or 500 MHz Bruker or Varian spectrometer and referenced to the residual solvent peak (δ in ppm and J in Hz). Elemental analyses were performed by Atlantic Microlabs Inc. (Norcross, GA). Crystals of 2 suitable for X-ray diffraction were grown by diffusion of diethyl ether into a saturated methylene chloride solution. 2-(N,N-Dimethylamino)methylpyridine.52 2-Aminomethylpyridine (4.0 mL, 39 mmol) was combined with formic acid (7.2 mL, 190 mmol) and formaldehyde (37 wt %, 8.8 mL, 120 mmol) under nitrogen in a 50 mL round-bottom flask with an attached reflux condenser. The reaction mixture was heated to reflux for 18 h and then cooled and treated with a 2 M NaOH solution to liberate the free base. The solution was extracted with four 150 mL portions of methylene chloride, and the combined organic layers were dried over MgSO4 and concentrated on a rotary evaporator to yield a dark brown oil. Chromatography on basic alumina (Aldrich, Brockmann I, standard grade) with 20% EtOAc/80% hexanes gave the pure product as a yellow oil. Yield: 2.60 g (49%). 1H NMR (400 MHz, CDCl3): δ 8.56 (d, J = 4.2 Hz, 1H), 7.66 (m, 1H), 7.38 (d, J = 7.8 Hz, 1H), 7.16 (m, 1H), 3.58 (s, 2H), 2.29 (s, 6H).
solution. 1H NMR (500 MHz, CD2Cl2): δ 8.33 (d, J = 5.2 Hz, 1H), 7.84 (t, J = 8.2 Hz, 4H), 7.74 (t, J = 8.3 Hz, 4H), 7.43 (td, J = 7.6, 1.5 Hz, 1H), 7.347.28 (m, 2H), 7.287.19 (m, 6H), 7.147.07 (m, 5H), 6.58 (t, J = 6.3 Hz, 1H), 4.26 (br s, 2H), 2.992.80 (m, 4H), 2.10 (s, 6H), 1.71 (br s, 4H). 31P NMR (202 MHz, CD2Cl2): δ 39.43 (d, J = 40.0 Hz), 29.20 (d, J = 40.8 Hz). 13C NMR (126 MHz, CD2Cl2): δ 157.34, 140.47 (d, J = 34.4 Hz), 139.33 (d, J = 36.5 Hz), 136.18, 134.53 (d, J = 8.1 Hz), 134.35 (d, J = 8.1 Hz), 129.18, 127.89 (d, J = 8.3 Hz), 127.73 (d, J = 8.6 Hz), 121.71, 121.19, 71.78, 51.11, 27.76 (d, J = 27.1 Hz), 27.18 (d, J = 27.4 Hz), 23.71 (d, J = 3.3 Hz), 22.73 (d, J = 2.7 Hz). Anal. Calcd for C36H40Cl2N2P2Ru: C, 58.86; H, 5.49; N, 3.81. Found: C, 59.09; H, 5.48; N, 3.70.
trans-RuCl2(dppf)(2-(N,N-dimethylamino)methylpyridine) (2-Me2). Complex 2-Me2 was prepared by variation on the published
procedure for complex 1.38 A flame-dried 100 mL Schlenk flask was placed under nitrogen and RuCl2(PPh3)4 (0.663 g, 0.543 mmol) and 2-(N,Ndimethylamino)methylpyridine (0.100 g, 0.734 mmol) were added, followed by 25 mL of dry, degassed toluene. The mixture was heated to 110 °C for 1 h and then cooled. 1,10 -Bis(diphenylphosphino)ferrocene (0.302 g, 0.545 mmol) was then added at once, and the reaction mixture was heated to 110 °C for an additional 19 h. After the flask had cooled to room temperature, 50 mL of degassed pentane was added. The solution was cooled in an ice bath and then filtered quickly in air. The deep orange supernatant was reduced to approximately 10 mL on a rotary evaporator, treated with an additional 50 mL of pentane, and then filtered once again to afford the product as an orange solid. Yield: 0.292 g (62%). Crystals suitable for X-ray diffraction were grown by diffusion of diethyl ether into a saturated benzene solution. The product is relatively insensitive to air and can be handled in solution for a period of minutes to hours without decomposition. The NMR spectra of 2-Me2 are broad at room temperature; therefore, the spectra were recorded at 20 °C in order to resolve the coupling constants. 1H NMR (500 MHz, CD2Cl2, 20 °C): δ 9.56 (m, 1H), 8.37 (dd, J = 11.8, 8.2 Hz, 1H), 7.85 (d, J = 5.8 Hz, 1H), 7.65 (t, J = 8.9 Hz, 1H), 7.61 7.55 (m, 2H), 7.50 7.06 (m, 14H), 6.99 (t, J = 7.6 Hz, 1H), 6.84 (t, J = 7.5 Hz, 1H), 6.62 (t, J = 8.4 Hz, 1H), 6.56 (t, J = 7.6 Hz, 1H), 6.45 (t, J = 6.6 Hz, 1H), 5.97 (1H), 5.58 (d, J = 14.1 Hz, 1H), 5.05 (1H), 4.58 (1H), 4.28 (1H), 4.21 (1H), 3.97 (1H), 3.92 (dd, J = 3.7, 2.4 Hz, 1H), 2.95 (d, J = 3.1 Hz, 1H), 2.93 (s, 1H), 2.58 (s, 3H), 1.59 (s, 3H). 31P NMR (202 MHz, CD2Cl2, 20 °C): δ 48.12 (d, J = 38.3 Hz), 30.09 (d, J = 38.3 Hz). Anal. Calcd for C42H40Cl2N2P2FeRu: C, 58.48; H, 4.67; N, 3.25. Found: C, 58.91; H, 4.89; N, 3.43.
’ ASSOCIATED CONTENT
bS
Supporting Information. Text, tables, figures, and CIF files giving catalytic procedures, characterization data for the products of catalysis, and details of the X-ray analysis of 1-Me2, 2, and 2-Me2. This material is available free of charge via the Internet at http://pubs.acs.org.
trans-RuCl2(dppb)(2-(N,N-dimethylamino)methylpyridine) (1-Me2). Complex 1-Me2 was prepared by variation of the published
’ AUTHOR INFORMATION
procedure for complex 1. A flame-dried 50 mL Schlenk flask was placed under nitrogen, and RuCl2(PPh3)4 (0.832 g, 0.681 mmol) and 2-(N,Ndimethylamino)methylpyridine (0.115 g, 0.844 mmol) were added, followed by 10 mL of dry, degassed toluene. The mixture was heated to 105 °C for 1 h and then cooled. 1,4-Bis(diphenylphosphino)butane (0.290 g, 0.680 mmol) was then added at once, and the reaction mixture was heated to 110 °C for an additional 19 h. After the flask had cooled to room temperature, 40 mL of degassed pentane was added. The resulting precipitate was collected by vacuum filtration in air, washed with 10 mL of diethyl ether, and then dried in vacuo to give the product as a pure orange solid. Yield: 0.380 g (76%). Crystals suitable for X-ray diffraction were grown by diffusion of diethyl ether into a saturated benzene solution. The product is stable to air in the solid state but is somewhat sensitive to air in
Corresponding Author
38
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We acknowledge funding from the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy, through Grant DEFG02-84ER13297. We thank Dr. Ainara Nova (Institute of Chemical Research of Catalonia, Tarragona, Spain) and Prof. Odile Eisenstein (Universite Montpellier 2, Montpellier, France) for valuable discussions. 4178
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Organometallics
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