Nucleophilic Homogeneous Hydrogenation by Iridium Complexes

Abstract

Catalytic homogeneous hydrogenation of 7-methoxy-3-phenylchromone and other substrates was achieved in the presence of cationic iridium complexes and base as co-catalyst. Contrary to common alkene hydrogenation, which is inactivated by base, the hydrogenation of the above set of electron-deficient alkenes turned out to be base-activated.

Key words

hydrogenation - homogenous catalysis - iridium - al-kenes - complexes

References and Notes

• 1 The Handbook of Homogeneous Hydrogenation   de Vries JG. Elsevier CJ. Wiley-VCH; Weinheim: 2007. 
• 2a Comprehensive Asymmetric Catalysis I   Jacobsen EN. Pfaltz A. Springer; Berlin: 1999. 
• 2b Jacobsen EN. Pfaltz A. Comprehensive Asymmetric Catalysis II   Jacobsen EN. Pfaltz A. Springer; Berlin: 1999. 
• 2c Comprehensive Asymmetric Catalysis III   Jacobsen EN. Pfaltz A. Springer; Berlin: 1999. 
• 2d Comprehensive Asymmetric Catalysis, Supplement   Jacobsen EN. Pfaltz A. Yamamoto H. Springer; Berlin, New York: 2004. 
• 3 Asymmetric Catalysis on Industrial Scale: Challenges, Approaches and Solutions   Blaser HU. Schmidt E. Wiley-VCH; Weinheim: 2004.  p.454 
• 4a Oro LA. Carmona D. Rhodium, In The Handbook of Homogeneous Hydrogenation   Vol. 1:  de Vries JG. Elsevier CJ. Wiley-VCH; Weinheim: 2007.  p.3-30  
• 4b Schrock RR. Osborn JA. J. Am. Chem. Soc.  1976,  98:  2134 
• 4c Schrock RR. Osborn JA. J. Am. Chem. Soc.  1976,  98:  2143 
• 4d Schrock RR. Osborn JA. J. Am. Chem. Soc.  1976,  98:  4450 
• 5a Crabtree RH. Iridium, In The Handbook of Homogeneous Hydrogenation   Vol. 1:  de Vries JG. Elsevier CJ. Wiley-VCH; Weinheim: 2007.  p.31-44  
• 5b Crabtree RH. Felkin H. Morris GE. J. Organomet. Chem.  1977,  141:  205 
• 6 Blackmond DG. Lightfoot A. Pfaltz A. Rosner T. Schnider P. Zimmermann N. Chirality  2000,  12:  442 
• 7 Xue D. Chen Y.-C. Cui X. Wang Q.-W. Zhu J. Deng J.-G. J. Org. Chem.  2005,  70:  3584 
• 8 Lightfoot A. Schnider P. Pfaltz A. Angew. Chem.  1998,  37:  2897 
• 9 Kallstrom K. Munslow I. Andersson PG. Chem. Eur. J.  2006,  12:  3194 
• 10a Frasinyuk MS. Khilya VP. Chem. Heterocycl. Compd. (Engl. Transl.)  1999,  35:  20 
• 10b Kazakov AL. Khilya VP. Mezheritskii VV. Litkei Y. Natural and Modified Isoflavonoids (in Russian)   Izdat. Rostovsk University; Rostov na Donu: 1985.  p.184 
• 14 Ugo R. Aspects of Homogeneous Catalysis: A Series of Advances   Manfredi; Milano: 1970. 
• 15a Jiang Z. Sen A. J. Am. Chem. Soc.  1990,  112:  9655 
• 15b Jiang Z. Sen A. Organometallics  1993,  12:  1406 
• 16 Munakata M. Yan S.-G. Maekawa M. Akiyama M. Kitagawa S. J. Chem. Soc., Dalton Trans.  1997,  4257 
• 17 Barsan F. Karam AR. Parent MA. Baird MC. Macromolecules  1998,  31:  8439 
• 19 Sosnovskikh VY. Irgashev RA. Barabanov MA. Synthesis  2006,  2707 
• 21 Yokoe I. Maruyama K. Sugita Y. Harashida T. Shirataki Y. Chem. Pharm. Bull.  1994,  42:  1697 

11

Typical Experimental Procedure: Catalyst (1-2 mg) was weighed into a Teflon vessel, then a calculated amount of substrate (catalyst/substrate = 1:100), a few drops of thiophene (to suppress possible heterogeneous process), base (0.07 mL) and degassed solvent (5 mL, distilled under an N₂ atmosphere) were added. All operations were quickly carried out under air and the vessel was sealed up in stainless steel autoclave. The autoclave was first purged with H₂ (3 ×) and then charged to 100 bar. After 8 h of hydrogenation at r.t., the autoclave was unsealed and the reaction mixture was subjected to gas chromatography with mass-spectral analyser (GCMS). GC conversion was calculated from correspondent peak areas and was not corrected with an external standard. If needed, the reaction mixture was evaproated in vacuo, the residue dissolved in CDCl₃ or DMSO-d ₆ and subjected to ^¹H NMR analysis, which showed the same conversion.

12

To a solution of 8-hydroxyquinoline (23.4 mg, 0.162 mmol) in Et₂O (1 mL) n-BuLi (0.1 mL, 0.16 mmol, 1.6 M hexane solution) was added. After stirring for 10 min Ph₂PCl (36.5 mg, 0.162 mmol) was added and stirred for 2 h. The Et₂O was evaporated, and [IR (cod)Cl]₂ (54.2 mg, 0.081 mmol) in CH₂Cl₂ (2 mL) added. The mixture obtained was refluxed for 2 h, cooled, and sodium tetrakis[3,5-bis(trifluoromethyl)-
phenyl]borate (NaBARF; 143.2 mg, 0.162 mmol) was added. After overnight stirring the iridium [8-(diphenyl-phosphinooxy)quinoline](1,5-cyclooctadiene) tetrakis-
[3,5-bis(trifluoromethyl)phenyl]borate was separated by silica gel column chromatography (R _f 0.9; CH₂Cl₂). This complex decomposed after its formation, and could not be isolated in pure form at r.t. This product was characterized by ^³¹P NMR, having only one peak at δ = +107 ppm. During 1 d it could catalyze nucleophilic or electrophilic hydrogenation. In the latter case we checked the hydrogenation of stilbene in CH₂Cl₂.

13

To a solution of [IR(cod)Cl]₂ (30.1 g, 0.044 mmol) and NaBARF (77.4 mg, 0.088 mmol) in CH₂Cl₂ (1.5 mL) freshly distilled 1.5-cyclooctadiene (0.1 mL) was added. The mixture was stirred for 30 min, then Na₂SO₄ was added to adsorb the formed NaCl. The mixture was filtrated, thoroughly washed by CH₂Cl₂, evaporated to 0.25 mL and pentane (10 mL) was added. The formed crystalline iridium bis(1,5-cyclooctadiene)tetrakis [3,5-bis(trifluoromethyl)-
phenyl]borate ([IR(cod)₂]BARF) was collected by filtration, washed from the filter by CH₂Cl₂ and dried under vacuum. ^¹H NMR (400 MHz, CDCl₃/TMS): δ = 2.26 (m, 8 H, CH₂), 2.41 (m, 8 H CH₂), 5.00 (s, 8 H, CH_COD), 7.56 (s, 4 H,
4-H_BARF), 7.71 (s, 8 H, 2-H_BARF and 6-H_BARF).
Under nitrogen to a suspension of 1,3-dimethyl-1H-imidazol-3-ium iodide (23.1 mg, 0.103 mmol) in THF (5 mL) n-BuLi (0.064 mL, 0.103 mmol, 1.6 M solution in hexane) was added and stirred for 2 h. [IR(cod)₂]BARF (65.5 Mg, 0.0516 mmol) was added and stirred for 28 h ar r.t. Reaction was quenched by MeOH and adsorbed on silica gel. Column chromatography on silica gel (9 g; under air, eluent: CH₂Cl₂; R _f 0.9) gave orange crystalline iridium bis(imidazol-3-ene)(1,5-cyclooctadiene)tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (30 mg, 42%). ^¹H NMR (400 MHz, CDCl₃/TMS): δ = 1.95 (m, 4 H, ch_2cod), 2.21 (m, 4 H, ch_2cod), 3.76 (s, 4 H, ch_cod), 3.79 (s, 12 H, Me), 6.73 (s, 4 H, CH_imidazolene), 7.52 (s, 4 H, 4-H_BARF), 7.71 (s, 8 H,
2-H_BARF, 5-H_BARF). ^¹³C NMR (100 MHz, CDCl₃/TMS): δ = 31.14 (CH₂), 37.49 (Me), 76.78 (ch_cod), 117.50 (br s,
4-CH_BARF), 122.60 (CH_imidazolene), 124.53 (q, J = 272.3 Hz, CF₃), 128.9 (br q, J = 30.0 Hz, 3-CCF₃), 134.80 (br s,
2-CH_BARF, 6-CH_BARF), 161.74 (q 1:1:1:1, J _CB = 50.5 Hz, CB_BARF), 177.70 (C_carbene). ^¹¹B NMR (128 MHz, CDCl₃/BF₃Et₂O): δ = -6.9. ^¹¹F NMR (376 MHz, CDCl₃/CFCl₃): δ = -62.8.

18

Typical Preparative Procedure: DIPEA was distilled from ninhydrine, then twice from LiH. Substrate (50 mg) was weighed into a Teflon vessel, then a calculated amount of catalyst (to achieve the desired catalyst/substrate ratio), DIPEA (500 equiv relative to Ir complex) and degassed toluene (15 mL, distilled under an N₂ atmosphere) were added. Further operations were similar to the experimental procedure given above. Products were purified by silica gel column chromatography, eluting with Et₂O.

20

3-Trifluoroacetylchromone, mixture of ketone and hydrate, was obtained as a gift from A. Kotljatrov and V. Iaroshenko, or synthesized by the known procedure. [²¹] This mixture (0.8 g) was stirred with P₂O₅ (4.7 g) in CH₂Cl₂ (50 mL) for 30 h (control by ^¹9F NMR), and then filtrated under nitrogen. The filtrate was evaporated and dried under high vacuum, to give 10 (0.3 g). ^¹H NMR data were similar to those published. [¹9] ^¹³C NMR (100 MHz, CDCl₃/TMS): δ = 115.73 (q, J = 289.4 Hz, CF₃), 118.34 (8-CH), (119.14, 3-CH), 124.87 (10-C), 126.65 (5-CH), 127.09 (6-CH), 135.04 (7-CH), 155.551
(9-C), 163.07 (q, J = 2.2 Hz, 2-CH), 172.65 (4-C), 179.38 (q, J = 39.2 Hz, COCF₃). ^¹9F NMR (376 MHz, CDCl₃/CFCl₃): δ = -74.81.