Note pubs.acs.org/Organometallics
Ambient-Temperature Carbon−Oxygen Bond Cleavage of an α‑Aryloxy Ketone with Cp2Ti(BTMSA) and Selective Protonolysis of the Resulting Ti−OR Bonds Addison N. Desnoyer,† Beata Fartel,† K. Cory MacLeod,† Brian O. Patrick,‡ and Kevin M. Smith*,† †
Department of Chemistry, University of British Columbia Okanagan, 3333 University Way, Kelowna, British Columbia, Canada V1V 1V7 ‡ Department of Chemistry, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z1 S Supporting Information *
ABSTRACT: Reaction of Cp2Ti[η2-(CSiMe3)2] with an αaryloxy ketone produces a Ti(IV) enolate aryloxide complex. Selective protonolysis of the enolate ligand or both Ti−OR bonds can be achieved with various acids. The reaction of the enolate aryloxide with 1-phenyl-2-phenoxyethanol is catalyzed by a mixture of NEt3 and [HNEt3]X (X = OTf, BPh4).
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earth-abundant transition metal with low inherent toxicity.7 The synthesis and reactivity of Cp2Ti complexes have been extensively investigated for the Ti(IV), Ti(III), and Ti(II) oxidation states.8 These oxophilic early-transition-metal organometallic compounds can be remarkably water tolerant,7,9 and subtle modifications of the ancillary Cp ligands can have dramatic reactivity consequences.10 Importantly, Gansäuer has demonstrated the use of lutidinium chloride to break TiIV−OR bonds under catalytic conditions,11 potentially allowing for release of acetophenone and phenol from the Cp2Ti fragment.
he development of selective new catalysts for the cleavage of carbon−oxygen bonds is a critical challenge for applications from biomass conversion1 to organic synthesis.2 Recently reported reactions based on ruthenium,3 nickel,2,4 and vanadium5 typically require high temperatures for effective catalysis.2−5 For example, Scheme 1a illustrates the mechanism Scheme 1
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RESULTS AND DISCUSSION Cp2Ti(BTMSA) (5; BTMSA = bis(trimethylsilyl)acetylene) is a well-defined organometallic complex used extensively as a source of Cp2TiII.12 Complex 5 reacted readily with α-aryloxy ketone 2 at ambient temperature to cleanly form the desired Ti(IV) enolate phenoxide 4 (eq 1).13 As shown in eq 2, 4 was
independently synthesized from Cp2Ti(OPh)Cl (6) via the in situ generated triflate 7. Luinstra has previously demonstrated the utility of Cp2Ti(X)(OTf) precursors for the synthesis of TiIV−OR complexes via salt metathesis reactions.14 Following Luinstra’s general procedure,14 pure 7 was readily isolated from the comproportionation reaction of Cp2Ti(OTf)2 and Cp2Ti-
proposed for the Ru-catalyzed conversion of 1-phenyl-2phenoxyethanol (1), a model for the β-O-4 linkage in lignin,1 to acetophenone (3) and phenol at 135 °C.3 Notably, the C−O bond-cleavage step is preceded by the Ru-catalyzed dehydrogenation of 1 to α-aryloxy ketone 2. We hypothesized that, with an appropriate source of “Cp2Ti”, α-aryloxy ketone 2 might react via intermediate A to give the Ti(IV) enolate phenoxide 4 (Scheme 1b).6 Titanium is particularly attractive for these applications as an inexpensive, © 2012 American Chemical Society
Received: October 10, 2012 Published: October 31, 2012 7625
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or various [HNR3]X reagents also cleanly generate acetophenone and the corresponding Cp2Ti(OPh)X complexes.15 The selectivity of the [HNR3]X salts is consistent with the previously noted sensitivity of Ti−enolate complexes to protonolysis.17,21 In contrast, the Cp2Ti(OPh)X products are expected to be more robust, since 6 and related Ti(IV) aryloxides can be prepared from Cp2TiCl2, NEt3, and HOAr.18,22 However, the analogous reactions employing HOCH(Me)Ph and other secondary alcohols can lead to displacement of a Cp ligand and formation of CpTi(OCHR2)Cl2 compounds.23 As shown in eq 3, the Cp2TiIV alkoxide 8 derived from secondary alcohol 1 was obtained through the use of DBU. In
(OPh)2. The 1H NMR spectrum of isolated enolate phenoxide 4 prepared according to eq 2 was identical with that synthesized from Cp2Ti(BTMSA) and 2.15 The molecular structure of 4 was confirmed by single-crystal X-ray diffraction (Figure 1a). In comparison to mono(alkoxide)
contrast, the use of NEt3 resulted in 8 contaminated with Cp2TiCl2, even after recrystallization.24 The molecular structure of 8 determined by X-ray diffraction is shown in Figure 1b. In comparison to 4, the Ti−OR bonds in mono(alkoxide) 8 are significantly shorter for both independent molecules in the unit cell, at 1.8440(9) and 1.8438(9) Å.16 The ether oxygen does not significantly interact with the Ti center in 8, with Ti1···O2 and Ti2···O4 distances both above 4.0 Å.17 Although 4 does not react directly with 1 over 2 days to produce acetophenone, the reaction does proceed at ambient temperature in the presence of [HNEt3]X salts (X = OTf, BPh4),25 as shown in Scheme 2. High conversions of 4 and 1 to
Figure 1. Molecular structures (50% probability ellipsoids) of 4 (left) and 8 (right). For 8, only one of the two independent molecules in the unit cell is shown.
Cp2TiIV complexes,16 both of the Ti−O bonds in 4 are quite long, at 1.9269(16) and 1.9325(17) Å for Ti1−O1 and Ti1− O2, respectively. A similar trend has previously been observed for Cp2Ti(OR)2 structures, where OR = enolate,17 aryloxide.18 The C7−C8 distance of 1.337(3) Å indicates significant C−C double-bond character for the enolate ligand.17 Prompted by the demonstrated stability of the Cp2TiΙV framework to triflic acid,14 the protonolysis reactions of 4 with HOTf, HOTs, and HCl were investigated (Figure 2).
Scheme 2
Figure 2. Reactivity of enolate phenoxide 4 with various protonolysis reagents, HX.
9 and 3 can be achieved with a mixture of 0.5 equiv of NEt3 and 0.1 equiv of HOTf. As described in the Supporting Information,15 the conversion of 4 to 9 is sensitive to both the nature of the added base and the identity of the anion of the conjugate acid. The phenoxide alkoxide 9 can be independently synthesized from 7, NEt3, and 1.
Triflic acid reacts with 4 to cleanly generate Cp2Ti(OTf)2 and 3. Although the presence of water can lead to unexpected outcomes in Cp2Ti chemistry,19 the reactions of 4 with ptoluenesulfonic acid monohydrate or aqueous HCl also provided Cp2TiX2 and acetophenone. While 4 did not react appreciably with Me3SiCl under strictly anhydrous conditions, intentional addition of 2 equiv of H2O to the reaction mixture led to Cp2TiCl2 and 3. Similar reactivity was recently reported for CrIII−OR complexes20d and may be significant in other multicomponent catalytic reactions that employ Me3SiCl to cleave metal−OR bonds.20 The use of milder acids permits the selective protonolysis of the Ti−enolate bond of 4 while retaining the phenoxide group (Figure 2). Reaction of 4 with lutidinium chloride produced Cp2Ti(OPh)Cl (6) and 3.11 Similar reactions of 4 with other lutidinium salts, commercial collidinium tosylate or [HNEt3]Cl,
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CONCLUSION The anion sensitivity of the formation of 9 is not unexpected, given the critical role played by weakly coordinating anions in polymerization catalysts based on cationic group 4 metallocenes.26 More surprising is that all the reactions presented here proceed readily at ambient temperature, including the crucial C−O bond cleavage step (eq 1), which is complete within 20 min. The sequential reactions of Cp2TiCl2 with Mg and BTMSA,12,27 alkyne complex 5 with 2 (eq 1), and enolate phenoxide 4 with aqueous HCl (Figure 2) constitute a stoichiometric cycle for the conversion of the α-aryloxy ketone 7626
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to acetophenone and phenol.28 Similarly, deprotonation of 8 and loss of chloride should also generate enolate phenoxide 4 through intermediate A, although our initial attempts to achieve this transformation have been unsuccessful. Finally, elimination of PhOH from 9 would also be expected to regenerate 4 (Scheme 2). Developing a catalytic system capable of operating at ambient temperatures will require an ancillary ligand framework that can address the reactivity challenges identified in this study.
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(dd, J = 9.6, 6.8 Hz, 1H), 3.95 (dd, J = 9.6, 4.8 Hz, 1H). 13C NMR (100 MHz, CD2Cl2): δ 159.37, 142.74, 130.07, 128.75, 128.07, 127.01, 121.30, 117.38, 117.32, 115.04, 90.51, 73.74. Anal. Calcd for C24H23O2ClTi: C, 67.55; H, 5.43. Found: C, 67.25; H, 5.45. Synthesis of Cp2Ti[OCH(Ph)CH2OPh](OPh) (9). To a solution of AgOTf (41.8 mg, 0.163 mmol) in 3 mL of toluene was added a solution of 6 (50.0 mg, 0.163 mmol) dissolved in 3 mL of toluene, resulting in an immediate color change to dark red and formation of a pale precipitate. The mixture was wrapped in foil and stirred for 1 h, after which the mixture was filtered through Celite, removing the white precipitate and giving a dark red solution of 7. A solution of 1 (34.9 mg, 0.163 mmol) in 3 mL of toluene and NEt3 (45 μL, 0.323 mmol) were added to the filtrate, and the mixture was stirred for 72 h, resulting in an orange solution. The solvent was removed in vacuo to give an orange residue. The residue was extracted with Et2O and filtered through Celite. Recrystallization from 1/2 Et2O/hexanes at −30 °C yielded a dark orange powder of 9 (38.0 mg, 48%). NMR analysis indicated that the product obtained by recrystallization was contaminated with a small amount (∼7%) of the intermediate phenoxide triflate 7. 1H NMR (400 MHz, CD2Cl2): δ 7.48 (dt, J = 0.8, 8.0 Hz, 2H), 7.41 (t, J = 7.6 Hz, 2H), 7.26−7.34 (m, 4H), 7.07 (dd, J = 7.2, 8.4 Hz, 2H), 6.93−6.97 (m, 4H), 6.67 (dd, 7.6, 8.0 Hz, 1H), 6.26 (s, 5H), 6.23 (s, 5H), 5.70 (dd, J = 4.4, 7.2 Hz, 1H), 4.09 (dd, J = 7.2, 9.6 Hz, 1H), 4.01 (dd, J = 4.4, 9.6 Hz, 1H). 13C NMR (100 MHz, CD2Cl2): δ 170.81, 159.63, 144.12, 130.04, 129.401, 128.77, 127.94, 127.27, 121.17, 118.55, 118.37, 115.94, 115.77, 115.05, 88.60, 68.65. General Procedure for 1H NMR Monitoring of Protonolysis Reactions of Cp2Ti[OC(CH2)Ph](OPh) (4). In an inert-atmosphere glovebox, 4 (8.0 mg, 0.020 mmol) and the internal standard 1,3,5-tBu3C6H3 (2.0 mg, 0.0081 mmol) were dissolved in CD2Cl2. An appropriate amount of an acid (Figure 2) and/or 1-phenyl-2phenoxyethanol (1) was dissolved in CD2Cl2. The solutions were combined in a J. Young or Wilmad screw-cap NMR tube and allowed to react before recording a 1H NMR spectrum. The yields of acetophenone (3) and the Cp2Ti product(s) were determined at various reaction times. X-ray Diffraction Studies: Crystal Data. 4: C24H22O2Ti, monoclinic, space group P21/c, a = 13.3250(19) Å, b = 9.4540(13) Å, c = 14.454(2) Å, β = 92.884(4)°, V = 1830.0(4) Å3, Z = 4, μ = 4.84 cm−1, F000 = 816, 11 999 reflections collected, 4030 unique reflections, Rint = 0.0431, GOF = 1.022, R1 (I > 2σ(I)) = 0.0465, wR2 (all data) = 0.1209. 8: C24H23O2ClTi, triclinic, space group P1̅, a = 9.1320(11) Å, b = 13.5283(16) Å, c = 16.404(2) Å, α = 88.597(3)°, β = 83.995(3)°, γ = 85.320(3)°, V = 2008.5(5) Å3, Z = 4, μ = 5.76 cm−1, F000 = 888, 42 998 reflections collected, 11 812 unique reflections, Rint = 0.0236, GOF = 1.05, R1 (I > 2σ(I)) = 0.0303, wR2 (all data) = 0.0794.
EXPERIMENTAL SECTION
Synthesis of Cp2Ti[OC(CH2)Ph](OPh) (4). Method A: From Cp2Ti(OPh)Cl (6). To an orange solution of Cp2Ti(OPh)Cl (50.3 mg, 0.164 mmol) in 6 mL of Et2O was added AgOTf (42.3 mg, 0.165 mmol) in 3 mL of Et2O, resulting in a rapid color change to pink and the formation of a pale precipitate. The mixture was protected from light by wrapping with foil and was stirred for 45 min. The mixture was then filtered through Celite, stranding a pink solid and giving a dark red filtrate. The solvent was removed in vacuo to give a dark red residue, which was then taken up in 3 mL of THF. A solution of KOC(CH2)Ph (28.6 mg, 0.181 mmol) in 3 mL of THF was then added, resulting in an instant color change from red to light orange. The solution was stirred for 14 h before the solvent was removed in vacuo. The residue was extracted with Et2O, filtered through Celite, and then again taken to dryness in vacuo. Recrystallization of this crude product from 2 mL of 1/1 Et2O/hexanes at −30 °C yielded red X-ray-quality crystals of 4 (34.6 mg, 54%). 1H NMR (400 MHz, CD2Cl2): δ 7.60 (d, J = 7.2 Hz, 2H), 7.35 (t, J = 7.6 Hz, 2H), 7.30 (d, J = 7.2 Hz, 1H) 7.21 (dd, J = 8.4, 7.6 Hz, 2H), 6.78 (tt, J = 7.2, 1.2 Hz, 1H), 6.60 (dd, J = 8.4, 1.2 Hz, 2H), 6.35 (s, 10H), 4.72 (s, 1H), 4.09 (s, 1H). 13C NMR (100 MHz, CD2Cl2): δ 171.04, 140.03, 129.59, 128.61, 128.18, 125.77, 119.19, 118.45, 116.57, 116.36, 86.72. Anal. Calcd for C24H22O2Ti: C, 73.86; H, 5.68. Found: C, 73.81; H, 5.66. Method B: From Cp2Ti(BTMSA) (5). A sample of Cp2Ti(BTMSA) in C6D6 was found to contain 0.0232 mmol of Cp2Ti(BTMSA) via 1H NMR with 1,3,5-tBu3C6H3 as an internal standard. To this yellowbrown solution was added 2 (6.0 mg, 0.0283 mmol), resulting in an immediate color change to red-orange. Subsequent NMR analysis showed 80% conversion of Cp2Ti(BTMSA) to 4 after 20 min at ambient temperature. Synthesis of Cp2Ti(OPh)(OTf) (7). Cp2Ti(OTf)2 (130.7 mg, 0.275 mmol) and Cp2Ti(OPh)2 (99.9 mg, 0.274 mmol) were dissolved in 10 mL of toluene. The mixture was stirred for 24 h, and the dark cherry red solution was then filtered through Celite. The solvent was removed in vacuo, and the residue was dissolved in a minimum amount of toluene. Storage at −30 °C for 2 days resulted in very dark red crystals of 7 (89.9 mg, 39%). 1H NMR (400 MHz, CD2Cl2): δ 7.28 (dd, J = 7.6, 8.8 Hz, 2H), 6.92 (tt, J = 7.6, 1.36 Hz, 1H), 6.61 (dd, J = 8.8, 1.24 Hz, 2H), 6.54 (s, 10H). 13C NMR (100 MHz, CD2Cl2): δ 171.91, 129.87, 121.65, 119.40, 116.80. Anal. Calcd for C17H15F3O4STi: C, 48.59; H, 3.60. Found: C, 48.94; H, 3.70. Synthesis of Cp2Ti[OCH(Ph)CH2OPh]Cl (8). To a suspension of Cp2TiCl2 (162.9 mg, 0.654 mmol) in 50 mL of toluene were added 1 (141.8 mg, 0.662 mmol) and DBU (48 μL, 0.321 mmol), and the mixture was stirred at room temperature for 10 min, during which time a pale precipitate formed. More DBU (50 μL, 0.334 mmol) was then added, and the orange mixture was stirred for an additional 80 min before a final aliquot of DBU (10 μL, 0.067 mmol) was added. This bright yellow mixture was then stirred for 75 min before being filtered through Celite. The solvent was removed in vacuo, the resulting solid residue was extracted with 3 mL of 10/1 Et2O/hexanes, and the extract was filtered through Celite and then cooled to −30 °C for 15 min, during which time an oily residue formed. The amber supernatant was decanted and diluted with 1 mL of Et2O. Chilling this solution at −30 °C yielded 8 (132.5 mg, 48%) as orange crystals isolated in two different fractions. X-ray-quality crystals were grown from Et2O. 1H NMR (400 MHz, CD2Cl2): δ 7.40−7.32 (m, 4H), 7.30 (dd, J = 8.8, 7.6 Hz, 3H), 6.96 (dt, J = 7.6, 1.2 Hz, 1H), 6.92 (dt, J = 7.6, 1.2 Hz, 2H), 6.33 (s, 5H), 6.28 (s, 5H), 5.76 (dd, J = 6.8, 4.8 Hz, 1H), 4.03
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ASSOCIATED CONTENT
S Supporting Information *
Text, tables, figures, and CIF files giving crystallographic data for 4 and 8, complete experimental details, and characterization data. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
We are grateful to the Natural Sciences and Engineering Research Council of Canada (NSERC), the Canadian Foundation for Innovation, and the University of British Columbia for financial support. K.M.S. thanks Anita Lam and 7627
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Wen Zhou for assistance with the single-crystal X-ray diffraction studies.
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