Chiral Brønsted Acid Catalysis
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1 Chiral Brønsted Acid Catalysis Aryl Aryl Aryl Aryl S CF 3 2 P Fe CF 3 CF 3 2 Jack Liu ov. 16, 2004 CF 3
2 Introduction Chiral Brønsted acid catalysis in nature: enzymes and peptides Chiral Brønsted acid catalysis is a special class of organocatalysis Brønsted Acids are also Lewis acids Advantages: Cost effective Catalysts and reaction conditions not sensitive to oxygen Catalysts can be easily purified after preparation Catalysts can be recycled o heavy-metal waste stream generated Acidic waste can be easily neutralized Challenges: ydrogen bond is non-directional Proton can be lost in solvent (salvation), causing loss of chiral information
3 Modes of Action Chiral Brønsted Acids as ydrogen Bond Donors u - X u * X * * X X * * * 2 Chiral Brønsted Acids as Proton Donors 3 * * 3 4 4
4 Al-Salen-Catalyzed Asymmetric Strecker eaction 1. C, cat. (5 mol %), toluene, -70 C, 15 h F 3 C 2. TFAA C entry yield (%) e.r. Catalyst: : p-c 6 4 p-brc :5 89:11 78:22 Al Cl 5 c-ex 77 68:32 Aliphatic imines are poor substrates for Al-Salen system Jacobsen, E.. et al. J. Am. Chem. Soc. 1998, 120, 5315.
5 Urea-Catalyzed Asymmetric Strecker eaction 1. C, cat. (2 mol %), 1 toluene, -70 C, 20 h F 3 C 1 2. TFAA C entry 1 1 allyl yield (%) 74 e.r. 97:3 Catalyst: p-c 6 4 p-brc 6 4 o-c 6 4 allyl allyl allyl :3 94:6 97: c-ex allyl allyl Bn :3 93:7 98:2 8 c-ex Bn 85 93:7 Urea system is superior to Al-Salen system for aliphatic aldimine substrates Jacobsen, E.. et al. Angew. Chem. Int. Ed. 2000, 39, elated process: Ti-peptide-catalyzed asymmetric Strecker eaction, see: oveyda, A..; Snapper, M. L. et al. J. Am. Chem. Soc. 1998, 121, 4
6 Asymmetric C Addition to Ketimines Bn C, cat. (2 mol %), Bn toluene, -75 C C entry t (h) yield (%) e.r. Catalyst: p-c 6 4 p-brc > 99 95:5 94:6 96:4 4 5 p-cf 3 C > :2 85:15 Enantioselectivity decreases dramatically if the smaller substituent of imine is larger than methyl Jacobsen, E.. et al. rg. Lett. 2000, 2, 867. elated process: thiourea-catalyzed hydrophosphonylation of imines, see: Jacobsen, E.. et al. J. Am. Chem. Soc. 2004, 126, 4102.
7 rigin of Asymmetric Induction Imine nitrogen binds urea protons of catalyst via hydrogen bonds Smaller substituent of imine extents into catalyst, while larger substituent extents into solvent Enantioselectivity eroded when smaller substituent becomes sterically demanding Cyanide adds to Si face of imine near cyclohexyl group Jacobsen, E.. et al. J. Am. Chem. Soc. 2002, 124, A: urea-imine complex. binding energy: 8.5 Kcal/mol both protons participate in binding B: urea-aminonitrile complex. binding energy: 5.0 Kcal/mol only one proton participates in binding Thiourea binds more strongly than urea
8 Thiourea-Catalyzed Asymmetric Mannich eaction Boc + TBS 1. cat. (5 mol %), -30 or -40 C, toluene, 48 h Boc i-pr 2. TFA, 2 min i-pr entry yield (%) e.r. Catalyst: p-c 6 4 p-fc 6 4 o-c thienyl :2 95:5 96:4 93:7 96:4 S 6 3-pyridyl 99 99:1 o background reaction at -40 C over 48 h in the absence of catalyst Jacobsen, E.. et al. J. Am. Chem. Soc. 2002, 124, elated process: asymmetric aza-enry reaction, see: Johnston, J.. et al. J. Am. Chem. Soc. 2004, 126, 3418.
9 osphate-catalyzed Asymmetric Mannich eaction + TBS Boc cat. (10 mol %), Et Et toluene, -78 C, 24 h E/Z = 87:13 entry yield (%) syn:anti e.r. Catalyst: 2 1 > 99 87:13 98: p-c 6 4 p-fc thienyl > 99 > :8 91:9 94:6 94:6 92:8 94:6 P 5 C=C 84 95:5 95:5 2 osphate- and thiourea-catalyzed Mannich reactions show comparable enantioselectivity Akiyama, T. et al. Angew. Chem. Int. Ed. 2004, 43, 1566.
10 Brønsted Acid-Catalyzed Baylis-illman eaction + C PBu 3 (10 mol %), co-cat. (10 or 20 mol %), TF, rt 1 h > 99% Co-catalysts: e.r. < 55:45 when ()-BIL used as a co-catalyst Ikegami, S. et al. Tetrahedron Lett. 2000, 41, 2165.
11 8 -BIL-Catalyzed Asymmetric Baylis-illman eaction + PEt 3, cat. (10 mol %), TF, -10 C, 48 h Catalyst: entry yield (%) e.r. 1 2 C 2 C 2 c-ex :5 98: i-pr C=C :3 89:11 90:10 Cf.: La-catalyzed version: Chen, K. et al. J. rg. Chem. 2003, 68, 915. = C 3, CF 3 Schaus, S. E. et al. J. Am. Chem. Soc. 2003, 125, elated process: asymmetric Michael addition to nitroolefins, see: Takemoto, Y. et al. J. Am. Chem. Soc. 2003, 125,
12 Proposed Catalytic Cycle B* + PEt 3 B* B* PEt 3 PEt 3 oles of Brønsted Acid: Activation of enone Stabilization of zwitterionic intermediates
13 Brønsted Acid Activation of Dienophiles (x mol %) CDCl 3, rt, 10 min catalyst loading (mol %) 40 0 conversion (%) 90 3 Proposed active species 2 2 Kelly, T.. et al. Tetrahedron Lett. 1990, 31, 3381.
14 Brønsted Acid-Catalyzed Diels-Alder eactions TMS + 2 (10 equiv), BF 4 (0.2 equiv),, -40 C, 30 min = Aryl, C=C 87-98% Akiyama, T. et al. Tetrahedron Lett. 1999, 40, TBS butanol, rt 2. AcCl, -78 C 35-78% awal, V.. et al. J. Am. Chem. Soc. 2002, 124, 9662.
15 TADDL-Catalyzed etero-diels-alder eaction 1. Aryl Aryl Aryl Aryl TBS 2 + (10 mol %), toluene, -78 to -40 C, 24 h 2. AcCl, -78 C, 15 min Aryl = 1-naphthyl 52-97% e.r. > 93:7 CF 3 70% e.r. > 99:1 68% e.r. 97:3 68% e.r. 97:3 64% e.r. 93:7 52% e.r. 97:3 o background reaction in the absence of TADDL awal, V.. et al. ature 2003, 424, 146.
16 TADDL-Catalyzed Diels-Alder eaction TBS + 2 Aryl = 1-naphthyl 1. Aryl Aryl Aryl Aryl (20 mol %), toluene, -80 C, 2 d 2. LiAl 4, Et 2, -78 C to rt, 2 h 3. F, C, 0 C to rt, 0.5 h 77-83% e.r. > 86:14 i-pr 77% e.r. 86:14 83% e.r. 95:5 81% e.r. 96:4 82% e.r. 94:6 awal, V.. et al. Proc. at. Acad. Sci. USA 2004, 101, 5846.
17 Proposed rigin of Asymmetric Induction Structure based on X-ray sructure of 1-naphthyl-TADDL Intramolecular hydrogen bond enhances the acidity of the free hydroxyl proton substrate binds to catalyst via hydrogen bond p-p interaction possible between naphthyl group and e-face of substrate Si-face of substrate accessible to dienes awal, V.. et al. Proc. at. Acad. Sci. USA 2004, 101, 5846.
18 Enantioselective Addition of Thiol to Ketenes LiS (1.5 equiv), TF, -55 C Li S (1 equiv, slow addition over 3 h), -55 C, 3.5 h S 84% e.r. 97:3 ow to transform this into a catalytic process? Fehr, C. et al. Angew. Chem. Int. Ed. Engl. 1993, 32, 1044.
19 Catalytic Enantioselective Addition of Thiols to Ketenes Proposed catalytic cycle 1. Li (5 mol %), TF 2. S (1 equiv, slow addition over 3 h), -55 C, 3.5 h S 84% e.r. 97:3 S SLi Li + S Li S + Fehr, C. et al. Angew. Chem. Int. Ed. Engl. 1993, 32, 1044.
20 Catalytic Enantioselective Addition of Amines to Ketenes C + cat. (2 mol %), toluene, rt C C 93%, e.r. 95:5 Proposed catalytic cycle Catalyst: C cat C + C + cat Fe cat Fu, G. C. et al. J. Am. Chem. Soc. 2002, 124, elated process: enantioselective addition of alcohols to ketenes, see: Fu, G. C. et al. J. Am. Chem. Soc. 1999, 121, 2637.
21 chanistic Insights C C cat + cat C C + Catalyst readily deprotonates pyrrole Anion of pyrrole attacks ketene and generate enolate in toluene at room temperature Catalyst inert to ketone in toluene at room temperature ate = k[catalyst] 1 [ketene] 1 [pyrrole] 0 For pyrrole, k /k D 5; protonation of enolate is likely turn-over limiting Fu, G. C. et al. J. Am. Chem. Soc. 2002, 124, cat
22 Chemist elping Chemist Lewis Acid-Assisted Brønsted Acid TMS 1. (1.1 equiv), cat. (2 mol %), SnCl 4 (1.1 equiv), toluene, -80 C, 1 h 100% conversion e.r. 95:5 Catalyst: SnCl 4 TMS TMS (1.1 equiv), cat. (10 mol %), SnCl 4 (8 mol %), toluene, -80 C, 1 h 2. Cl 100% conversion e.r. 95:5 Yamamoto,. et al. J. Am. Chem. Soc. 1996, 118, Yamamoto,. et al. J. Am. Chem. Soc. 2000, 122, 8120.
23 chanistic Considerations Proposed catalytic cycle: Aryl * 1 2 *SnCl 3 + TMSCl TMS TMS Aryl 2 1 * SnCl 4 The protonation is not believe to involve Sn enolate Control experiment: silyl enol ether inert to 1 equiv of SnCl 4 at -78 C over 2.5 h Yamamoto,. et al. J. Am. Chem. Soc. 1996, 118, Yamamoto,. et al. J. Am. Chem. Soc. 2000, 122, 8120.
24 Structural Aspect and rigin of Asymmetric Induction ptimized structure of biphenol- SnCl 4 Proposed transition state structure Ǻ Ǻ Ǻ Ǻ BIL oxygens binds to equatorial sites of Sn - bond adapts pseudoaxial orientation due to static interaction with apical Cl Charge on acidic proton: (cf. phenol: 0.489) Aryl group of substrate stacks on naphthyl group of catalyst due to p-p interaction Enantioselectivity reduced if 1 is large Protonation occurs on e face of silyl enol ether Yamamoto,. et al. J. Am. Chem. Soc. 2000, 122, 8120.
25 Summary igh enantioselectivity and efficiency of chiral Brønsted acid catalysts have been demonstrated in following systems Trough hydrogen bonding and activation Urea-Catalyzed asymmetric Strecker reaction Thiourea- and phosphate-catalyzed asymmetric Mannich reactions 8 -BIL-catalyzed asymmetric Baylis-illman reaction TADDL-catalyzed asymmetric Diels-Alder reaction Through protonation Ephedrine- and PPY-catalyzed asymmetric addition to ketenes Lewis acid-asissted Brønsted acid-catalyzed asymmetric protonation of silyl enol ether and ketene disilyl acetals Many other Lewis base-catalyzed and Brønsted acid promoted processes have the potential to be developed into chiral Brønsted acid-catalyzed reactions
26 Asymmetric Strecker eaction: Library Development Library 4 Varying sterically hindered amino acid side chains and 2 of salicylaldehyde Library Size: 70 Compounds Jacobsen, E.. et al. J. Am. Chem. Soc. 1998, 120, Jacobsen, E.. et al. Angew. Chem. Int. Ed. 2000, 39, 1279.
27 Efficient Synthesis of D-tert-Leucine Aldimine Cost of D-tert-Leucine: $214/g from Aldrich/Fluka Jacobsen, E.. et al. Angew. Chem. Int. Ed. 2000, 39, 1279.
28 Al-Salen-Catalyzed Asymmetric Strecker eaction Ti(i-Pr) 4 ligand (5 mol %), TMSC, i-pr toluene, 4 C, 20 h C entry yield (%) e.r. Ligand: p-c >99 98:2 97:3 88:12 oveyda, A...; Snapper, M. L. et al. J. Am. Chem. Soc. 1998, 121, 4284
29 Thiourea-Catalyzed ydrophosphonylation of Imines 77-93% 81-90% ee 1b: S Jacobsen, E.. et al. J. Am. Chem. Soc. 2004, 126, 4102.
30 Efficient Synthesis of a-thyl enylglycine Jacobsen, E.. et al. rg. Lett. 2000, 2, 867.
31 Model Driven ptimization of Catalyst Jacobsen, E.. et al. J. Am. Chem. Soc. 2002, 124, 10012
32 Asymmetric Aza-enry eaction Johnston, J.. et al. J. Am. Chem. Chem. Soc. 2004, 126, 3418.
33 BF 4 -Catalyzed Mannich eaction Akiyama, T. et al. Adv. Syn. Catal. 2002, 344, 338.
34 La-Catalyzed Asymmetric Baylis-illman eaction Catalyst: La(Tf) 3 2 C C 2 Chen, K. et al. J. rg. Chem. 2003, 68, 915.
35 Thiourea-Catalyzed Michael Addition to itroolefins F 3 C 2 S CF 3 1a Takemoto, Y. et al. J. Am. Chem. Soc. 2003, 125,
36 Catalytic Enantioselective Addition of Alcohols to Ketenes Proposed catalytic cycle Catalyst: A pathway involving Brønsted base is also possible. Fu, G. C. et al. J. Am. Chem. Soc. 1999, 121, 2637.
37 X-ay Structure of 1-aphthyl-TADDL Aryl Aryl Aryl Aryl Seebach, D. et al. J. rg. Chem. 1995, 60, 1788.
38 Efficient Synthesis of Ibuprophen 1. Catalyst: i-bu TMS TMS (1.1 equiv), cat. (10 mol %), SnCl 4 (8 mol %), toluene, -80 C, 1 h 2. Cl i-bu SnCl 4 80% e.r. 98:2 Yamamoto,. et al. J. Am. Chem. Soc. 1996, 118, Yamamoto,. et al. J. Am. Chem. Soc. 2000, 122, 8120.
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