Applications of Radical Reactions in Asymmetric Synthesis. Brandon Meyers Michigan State University Department of Chemistry November 19, 2008
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1 Applications of adical eactions in Asymmetric Synthesis Brandon Meyers Michigan State University Department of Chemistry ovember 19, 2008
2 utline Introduction Importance of radical reactions Challenges of stereochemistry Methods employed to achieve asymmetric control Chiral Auxiliary Chiral Acid rganocatalysis
3 General Bond Forming Conditions Acidic (Cationic) - F 3 B! F 3 B! n Basic (Anionic) a I!! I I a eutral (adical) 3 C Br 3 C Br Clayden, J.; Greeves,.; Warren, S.; Wothers, P. rganic Chemistry. xford: University Press. 2001
4 eversal of eactivity eterolytic Cleavage I electrophilic carbocation I omolytic Cleavage I nucleophilic radical LDA LDA! nucleophilic carbanion Bu 3 Sn AIB Bu 3 Sn!Br electrophilic radical Br Giese, B.; Tetrahedron, 1985, 41, 4025
5 adicals Stabilized by Electron Donating Groups ucleophilic adical adical SM Energy nonbonding lone pairs Parsons, A.F. An Introduction to Free adical Chemistry, xford: Blackwell Science, 2000, p. 40
6 adicals Stabilized by Electron Withdrawing Groups adical SM!" Energy Electrophilic adical nonbonding lone pairs!! Parsons, A.F. An Introduction to Free adical Chemistry, xford: Blackwell Science, 2000, p. 40
7 Early Problems of Stereochemistry Synthesis of Sativene & Capocamphene Br Bu 3 Sn C 3 tbu, h! benzene X X = (37%) X = C 2 - Sativene X X = (25%) X = C 2 - Capocamphene Bakuzis, P.; Campos,..S.; Bakuzis, M.L.F. J. rg. Chem., 1976, 41, 3261
8 Methods for Asymmetric Control Chiral Auxiliary, X C X C 1 X C * 1 Chiral Acid, A C A C 1 A C 1 * rganocatalysis 1!LG * 1
9 Sultam & xazolidinone Auxiliaries S ' M * * ' Dipole-dipole control Lewis acid chelation
10 Dipole-Dipole Chiral Auxiliary Me glyoxylic oxime ether Bn S 1.0 eq. Me 3 Al Cl(C 2 ) 2 Cl reflux 0.67 eq. (1)-(+)-2,10-camphorsultam Bn S 2 90% yield S 2 Bn 5.0 eq. -I, C 2 Cl eq. Bu 3 Sn 2.0 eq. BF 3 Et eq. Et 3 B, -78 C S 2 Bn aito, T; Miyabe,.; Ushiro, C.; Ueda, M.; Yamakawa, K. J. rg. Chem., 2000, 65,
11 Mechanism Bu 3 Sn I! Bu 3 Sn!I S Bn S Bn S Bn SnBu 3 S Bn
12 Substrate Scope S 2 Bn 5.0 eq. -I, C 2 Cl eq. Bu 3 Sn 2.0 eq. BF 3 Et eq. Et 3 B, -78 C S 2 1 Bn Entry I Product Isolated Yield (%) dr of 1 1 i-pr-i 1a : 4 2 Et-I 1b : 5 3 t-bu-i 1c 83 >98 : 2 4 i-bu-i 1d : 3 5 c-ex-i 1e : 4 aito, T; Miyabe,.; Ushiro, C.; Ueda, M.; Yamakawa, K. J. rg. Chem., 2000, 65,
13 Preferred Site of Addition ote: = S Bn anti, s-cis S Bn S Bn anti, s-trans syn, s-cis syn, s-trans Bn S aito, T; Miyabe,.; Ushiro, C.; Ueda, M.; Yamakawa, K. J. rg. Chem., 2000, 65,
14 eduction of xime Ether and emoval of Chiral Auxiliary Bn 0.7 eq. Mo(C) 6 S 2 2, MeC, reflux S 2 2 = i-pr, 88% yield S Li-TF (1:4) 2 = i-pr, 88% yield Synthesis of D-Valine 55% overall yield, 4 steps aito, T; Miyabe,.; Ushiro, C.; Ueda, M.; Yamakawa, K. J. rg. Chem., 2000, 65,
15 Lewis Acid Chelation to Chiral Auxiliary C 2 Et 1. (CCl) 2, TF 2. C 2 Et n-buli, -78 C 87% yield, two steps C 2 Et i-pr!i (10 eq) Sm(Tf) 3 (1.0 eq) Bu 3 Sn (6.0 eq) C 2 Et Et 3 B (3.0 eq), 2 C 2 Cl 2 /TF (4:1), -78 C 95% yield dr = 29:1 Sibi, M.P.; Liu, P.; Ji, J.; ajra, S.; Chen, J.-X. J. rg. Chem., 2002, 67,
16 Stereoselective Model Activates!"carbon to imide carbonyl Tf Tf Sm Tf i-pr C 2 Et Blocks Si-face i-pr Sibi, M.P.; Liu, P.; Ji, J.; ajra, S.; Chen, J.-X. J. rg. Chem., 2002, 67,
17 Synthetic Application: ( )-Enterolactone Previous Synthesis: Chenevert,.; et al. 7 steps, 35% yield Key Step: Enzyme-catalyzed esterification Chenevert,.; Mohammadi-Ziarani, G.; Caron, D.; Dasser, M.; Can. J. Chem., 1999, 77, 223
18 Lewis Acid Chelation to Chiral Auxiliary Me C 2 Et Br C 3 (10 eq) Sm(Tf) 3 (1.0 eq) Bu 3 Sn (6.0 eq) Et 3 B (3.0 eq), 2 C 2 Cl 2 /TF (4:1), -78 C C 2 Et 71% yield BEt 3 2 Et 2 B Et Et C 2 Et 17% yield Sibi, M.P.; Liu, P.; Ji, J.; ajra, S.; Chen, J.-X. J. rg. Chem., 2002, 67,
19 Total Synthesis of ( )-Enterolactone Me Me Me C 2 Et 3-MeC6 4 -C 2 I amds, TF -78 C to -54 C C 2 Et Li 2 2 C2 Et 1 Me 50% yield 88% yield Me eq. B 3 /TF -15 C 2. PPTS, reflux Me 4.0 eq. BBr 3 0 C to -18 C Me 78% yield, two steps 88% yield Sibi, M.P.; Liu, P.; Ji, J.; ajra, S.; Chen, J.-X. J. rg. Chem., 2002, 67,
20 Methods for Asymmetric Control Chiral Auxiliary, X C X C 1 X C * 1 Chiral Acid, A C A C 1 A C 1 * rganocatalysis 1!LG * 1
21 Types of Chiral Acids Lewis Acids MgI 2 MgBr 2 Brønsted Acid C 3 2 P 2 Quinine, QP
22 Chiral Lewis Acid Chelation S 2 Bn 5.0 eq. -I, C 2 Cl eq. Bu 3 Sn 2.0 eq. BF 3 Et eq. Et 3 B, -78 C S 2 Bn = i-pr 80% yield, dr = 96:4 Me Bn MgBr 2 i-pr-i, Bu 3 Sn BEt 3, -78 C Me Bn 97% yield, 52% ee aito, T; Miyabe,.; Ushiro, C.; Ueda, M.; Yamakawa, K. J. rg. Chem., 2000, 65,
23 Model for Selectivity Mg Bn Me e-face open aito, T; Miyabe,.; Ushiro, C.; Ueda, M.; Yamakawa, K. J. rg. Chem., 2000, 65,
24 Chiral Brønsted Acids Quaternary Ammonium Salts of ypophosphorous Acid C 3 C 3 2 P 2 2 P 2 Quinine, QP Quinidine, QDP Cho, D.K.; Jang, D.. Chem. Commun., 2006,
25 Quinine esults Bn 2 eq. QP, 5 eq. -I 0.5 eq. BEt 3, 2 C 2 Cl 2 / 2 (1:1) 4 h, rt Bn Et Bn 1 2 2a Entry I Product Isolated Yield (%) 2a Yield (%) er of 2 : S C 3 1 i-pr-i 2b : 79 2 c-ex-i 2c : 79 3 t-bu-i 2d : > Ad-I 2e : >99 5 n-ct-i 2f : 60 2 P 2 Quinine, QP Cho, D.K.; Jang, D.. Chem. Commun., 2006,
26 Quinidine esults Bn 2 eq. QDP, 5 eq. -I 0.5 eq. BEt 3, 2 C 2 Cl 2 / 2 (1:1) 4 h, rt Bn Et Bn 1 2 2a Entry I Product Isolated Yield (%) 2a Yield (%) er of 2 : S 1 i-pr-i 2b >62 : 38 2 c-ex-i 2c 82 9 >72 : 28 3 t-bu-i 2d >99 : Ad-I 2e >99 : 1 5 n-ct-i 2f >58 : 42 C 3 Quinidine, QDP 2 P 2 Cho, D.K.; Jang, D.. Chem. Commun., 2006,
27 Model for Enantioselectivity Quinine versus Quinidine C 3 C 3 2 P 2 2 P 2 Si-face open e-face open Cho, D.K.; Jang, D.. Chem. Commun., 2006,
28 Conjugate adical Addition Enantioselective addition to α'-hydroxy enone I 0.3 eq. Chiral L.A. 2.0 eq. Bu 3 Sn 3.0 eq. Et 3 B/ 2 C 2 Cl 2, C, 24 h 66% yield, 75% ee Mg(Tf 2 ) 2 Chiral Lewis Acid Sibi, M.P.; Lee, S.; Lim, C.J.; Kim, S.; Subramaniam,.; Zimmerman, J. rg. Lett., 2006, 8,
29 Scope of α'-ydroxy Enone and Alkyl alide Substrates eq. 2 -I 0.3 eq. Chiral L.A. 2.0 eq. Bu 3 Sn 3.0 eq. Et 3 B/ 2 C 2 Cl 2, C, 24 h = C 2 C 2 2 = 1a-d 2a-d Entry 1 2 Product Isolated Yield (%) ee (%) 1 C 2 C 2 t-bu 1a C 2 C 2 i-pr 1b C 2 C 2 n-pr 1c C 2 C 2 Et 1d t-bu 2a i-pr 2b n-pr 2c Et 2d Mg(Tf 2 ) 2 Chiral Lewis Acid Sibi, M.P.; Lee, S.; Lim, C.J.; Kim, S.; Subramaniam,.; Zimmerman, J. rg. Lett., 2006, 8,
30 Model for Enantioselectivity 2 Mg X X 1 e-face open X=Tf 2 Sibi, M.P.; Lee, S.; Lim, C.J.; Kim, S.; Subramaniam,.; Zimmerman, J. rg. Lett., 2006, 8,
31 Synthetic Application: (+)-icciocarpin A Previous Synthesis: Krishe, M.; Agapiou, K. 6 steps, 14% yield Key step: Michael cycloisomerization (+)-icciocarpin A Agapiou, K.; Krishe, M. rg. Lett., 2003, 5,
32 adical Conjugate Addition MgI 2 Cl Bn Bn 5.0 eq. Cl Br 5.0 eq. Bu 3 Sn 5.0 eq. Et 3 B / 2, -78 C 84% yield, 97% ee Mg Bn icciocarpin A Sibi, M.P.; e, L. rg. Lett., 2004, 6,
33 Preparation of Aldehyde Intermediate Cl Sm(Tf) 3 C 3 Cl Bn 95% Me Bn ai Acetone I Me LiMDS -78 C to rt Me 98% Bn 97% Bn 1. Pd() 2 / 2, ex/etac, -10 C 2. TEMP, KBr, acl, std. ac 3, 0 C 76% over two steps C Me icciocarpin A Sibi, M.P.; e, L. rg. Lett., 2004, 6,
34 Synthesis of icciocarpin A Me (i-pr) 3 Ti 2.0 eq. C Solvent, -78 C 2.0 eq. s-buli 85% icciocarpin A 41% overall yield 5.7 : 1 Sibi, M.P.; e, L. rg. Lett., 2004, 6,
35 Methods for Asymmetric Control Chiral Auxiliary, X C X C 1 X C * 1 Chiral Acid, A C A C 1 A C 1 * rganocatalysis 1!LG * 1
36 MacMillan Enamine Chemistry Me 1 2 Me 1 2 SET Me 1 2 Me 1 2 Me 1 2 TMS * Beeson, T.D.; Mastracchio, A.; ong, J.B.; Ashton, K.; MacMillan, D.W.C. Science, 2007, 316,
37 Previous rganocatalytic esearch Iminium & Enamine rganocatalysis Enamine Activation M catalysis Iminium Activation LUM catalysis e - aldehyde amine catalyst These two modes of catalyst activation have provided more than 60 asymmetric metric methodologies over the past 7 years. - Dr. David MacMillan Beeson, T.D.; Mastracchio, A.; ong, J.B.; Ashton, K.; MacMillan, D.W.C. Science, 2007, 316,
38 ypothesis - SM Activation Et SET Butanal IP = 9.8 ev Pyrrolidine IP = 8.8 ev Et Enamine IP = 7.2 ev Et SM-activated Me Me Beeson, T.D.; Mastracchio, A.; ong, J.B.; Ashton, K.; MacMillan, D.W.C. Science, 2007, 316,
39 ypothesis - Enantioselectivity Density Functional Theory Model of Imidazolidinone Catalyst Me Beeson, T.D.; Mastracchio, A.; ong, J.B.; Ashton, K.; MacMillan, D.W.C. Science, 2007, 316,
40 Applications of SM-rganocatalysis α-substitution of Aldehydes Allylation Enolation * 1 xyamination Alkylation Vinylation
41 α-allylation of Aldehydes General eaction aldehyde 1 SiMe 3 CA (2.5 equiv.) ac 3, 24 h DME, -20 C 2.5 equiv. CF 3 C allylsilane 20 mol% cat. 1 product 1 CA = Ceric Ammonium itrate ( 4 ) 2 Ce( 3 ) 6 Beeson, T.D.; Mastracchio, A.; ong, J.B.; Ashton, K.; MacMillan, D.W.C. Science, 2007, 316,
42 rganocatalytic Allylation: Scope of Aldehyde Substrate 20 mol% SiMe 3 TFA CA (2.5 eq.), -20 C ac 3, DME, 24 h 81% yield, 91% ee 75% yield, 92% ee 72% yield, 87% ee Bz Boc 75% yield, 94% ee 72% yield, 95% ee 70% yield, 93% ee Beeson, T.D.; Mastracchio, A.; ong, J.B.; Ashton, K.; MacMillan, D.W.C. Science, 2007, 316,
43 rganocatalytic Allylation: Scope of Allylsilane Substrate 1 TFA 20 mol% C 6 13 SiMe 3 CA (2.5 eq.), -20 C ac 3, DME, 24 h 1 C 6 13 C 6 13 C 6 13 C 6 13 C 2 Et 88% yield, 91% ee 77% yield, 88% ee 87% yield, 90% ee 81% yield, 90% ee Beeson, T.D.; Mastracchio, A.; ong, J.B.; Ashton, K.; MacMillan, D.W.C. Science, 2007, 316,
44 α-eteroarylation & lefin Cyclization of Aldehydes C Boc 20 mol% TFA CA (2.5 eq.), -20 C ac 3, DME, 24 h Boc 85% yield 84% ee TFA 20 mol% CA (2.5 eq.), -10 C LiCl, TF, 24 h Cl 85% yield dr >8:1 95% ee Beeson, T.D.; Mastracchio, A.; ong, J.B.; Ashton, K.; MacMillan, D.W.C. Science, 2007, 316,
45 Mechanistic Investigation Bn t-bu radical Bn t-bu C 6 13 C 6 13 Me Me 2.5 equiv. cation Bn t-bu C 6 13 Me Beeson, T.D.; Mastracchio, A.; ong, J.B.; Ashton, K.; MacMillan, D.W.C. Science, 2007, 316,
46 Mechanistic Investigation Bn t-bu radical Bn t-bu C C 6 13 Me C 6 13 Me 65% yield Me Bn t-bu C 6 13 Me cation Bn t-bu C 6 13 Me C Me not observed Beeson, T.D.; Mastracchio, A.; ong, J.B.; Ashton, K.; MacMillan, D.W.C. Science, 2007, 316,
47 Applications of SM-rganocatalysis α-substitution of Aldehydes Allylation Enolation * 1 xyamination Alkylation Vinylation
48 α-xyamination of Aldehydes 4.0 eq. 1. Catalyst, Cp 2 FeBF 4 2. ab 4, rt Zn(Ac) 2 enantioselective 1,2-diol synthesis Sibi, M.P.; asegawa, M. J. Am. Chem. Soc., 2007, 129,
49 Catalyst ptimization 4.0 eq. 1. Catalyst, TF (1.0 M), rt 1.0 eq. Cp 2 FeBF eq. ab 4, rt TEMP BF Tf C 2 Entry Catalyst mol % time, h Isolated Yield (%) ee (%) 1 none Sibi, M.P.; asegawa, M. J. Am. Chem. Soc., 2007, 129,
50 Scope of Aldehydes 4.0 eq mol% 3, DMF (1.0 M) 0.1 eq. FeCl 3, 0.3 eq. a 2 temp., time eq. ab 4, rt TEMP Entry Temp, C time, h Isolated Yield (%) ee (%) 1 C 6 5 r.t C 6 5 C 2 r.t C 6 5 C 2 C 2 r.t Me-C 6 4 C 2 C 2 r.t C 6 4 C 2 C 2 r.t (C 3 ) 2 C r.t BF 4 3 Sibi, M.P.; asegawa, M. J. Am. Chem. Soc., 2007, 129,
51 Model for Enantioselectivity Si-face open Sibi, M.P.; asegawa, M. J. Am. Chem. Soc., 2007, 129,
52 Applications of SM-rganocatalysis α-substitution of Aldehydes Allylation Vinylation * 1 xyamination Alkylation Enolation * 1
53 α-enolation of Aldehydes TMS 1 1!-substituted 1,4-dicarbonyl Importance of eaction: ne step reaction to form umpolung polarity without going through anionic mechanism Jang,.Y.; ong, J.B.; MacMillan, D.W.C. J. Am. Chem. Soc., 2007, 129,
54 rganocatalytic Enolation: Scope of Aldehyde Substrate TMS 20 mol% CA (2 eq.), DTBP (2 eq.) acetone, 2, 24 h, -20 C TFA hexyl 85% yield 90% ee 7 92% yield 92% ee 74% yield 93% ee 77% yield 91% ee 2 71% yield 90% ee 84% yield 95% ee Bn Boc Jang,.Y.; ong, J.B.; MacMillan, D.W.C. J. Am. Chem. Soc., 2007, 129,
55 rganocatalytic Enolation: Scope of Enolsilane Substrate TFA hexyl Si mol% CA (2 eq.), DTBP (2 eq.) DME, 2, 24 h, -20 C hexyl 1 Enolsilane Product Enolsilane Product TMS hexyl 77% yield 92% ee TBS t-bu hexyl t-bu 74% yield 96% ee TMS hexyl 77% yield 92% ee TBS t-bu hexyl 55% yield 92% ee Jang,.Y.; ong, J.B.; MacMillan, D.W.C. J. Am. Chem. Soc., 2007, 129,
56 Applications of SM-rganocatalysis α-substitution of Aldehydes Allylation Vinylation * 1 xyamination Alkylation 1 Enolation * 1
57 rganocatalytic α-vinylation of Aldehydes KF 3 B 1 20 mol% TFA 1 aldehyde vinyl-bf 3 K enantioenriched!-vinyl aldehyde Importance of eaction: Form β,γ-unsaturated aldehydes without olefin transpostion Kim,.; MacMillan, D.W.C. J. Am. Chem. Soc., 2008, 130,
58 rganocatalytic Vinylation of Aldehydes KF 3 B 1 20 mol% TFA 1 aldehyde vinyl-bf 3 K enantioenriched!-vinyl aldehyde BF 3 K - 1 e - 1 KF 3 B 1 KF 3 B 1 Kim,.; MacMillan, D.W.C. J. Am. Chem. Soc., 2008, 130,
59 rganocatalytic Vinylation: Scope of Aldehyde Substrate KF 3 B 20 mol% CA (2.5 eq.), -50 C ac 3, DME, 2, 24 h TFA Me 72% yield 94% ee 4 78% yield 95% ee 82% yield 96% ee Et 79% yield 93% ee 2 78% yield 93% ee 76% yield 96% ee Bn Boc Kim,.; MacMillan, D.W.C. J. Am. Chem. Soc., 2008, 130,
60 rganocatalytic Vinylation: Scope of Vinyl Trifluoroborate Salt hexyl KF 3 B 1 1 =, Me 20 mol% CA (2.5 eq.), -50 C ac 3, DME, 2, 24 h TFA hexyl 1 hexyl 81% yield 94% ee hexyl Me 78% yield 95% ee hexyl 6 82% yield 89% ee Cl Me hexyl 77% yield 95% ee hexyl 61% yield 95% ee hexyl 84% yield 90% ee Kim,.; MacMillan, D.W.C. J. Am. Chem. Soc., 2008, 130,
61 Applications of SM-rganocatalysis α-substitution of Aldehydes Allylation Enolation * 1 xyamination Alkylation * * 1 1 * 2 Vinylation 1
62 α-alkylation of Aldehydes Br 1 2 rganocatalysis otoredox Catalysis 1 2 aldehyde racemic!-bromocarbonyl enantioenriched!-alkylated "-ketoaldehyde Me Tf Me Me Me Me u 2+ rganocatalyst otoredox Catalyst icewicz, D.A.; MacMillan, D.W.C. Science, 2008, 322, 77-80
63 Catalytic Cycles 1 ()C Br! Br SET u(bpy) 3 + (3) reductant t-bu 11 catalyst 6 t-bu aldehyde 7 u(bpy) 3 2+ photoredox catalyst 1 otoredox Catalytic Cycle SET rganocatalytic Cycle 8 t-bu photon source * u(bpy) 3 2+ (2) oxidant 1 9 t-bu C Si-face open icewicz, D.A.; MacMillan, D.W.C. Science, 2008, 322, 77-80
64 2.0 eq. rganocatalytic Alkylation: Scope of Aldehyde Substrate hexyl Et C 2 Et C 2 Et Br 93% yield 90% ee Et 20 mol% Et C 2 Et 4 C 2 Et Me TFA Me 86% yield 90% ee Me Me Me 0.5 mol% u(bpy) 3 Cl eq 2,6-lutidine, DMF fluorescent light, 23 C C 2 Et C 2 Et C 2 Et C 2 Et 83% yield 95% ee C 2 Et C 2 Et C 2 Et C 2 Et 92% yield 90% ee C 2 Et 63% yield 93% ee C 2 Et 66% yield 91% ee Boc icewicz, D.A.; MacMillan, D.W.C. Science, 2008, 322, 77-80
65 rganocatalytic Alkylation: Scope of α-bromocarbonyl Substrate ex 2.0 eq. Br 1 20 mol% Me TFA Me Me Me Me 0.5 mol% u(bpy) 3 Cl eq 2,6-lutidine, DMF fluorescent light, 23 C ex 1 ex 84% yield 96% ee ex C 2 CF 3 80% yield 92% ee ex C 2 Et C 2 Et 80% yield 88% ee 2 Me t-bu 2 C ex 84% yield 95% ee ex 87% yield 96% ee ex 70% yield 5:1 dr, 99% ee icewicz, D.A.; MacMillan, D.W.C. Science, 2008, 322, 77-80
66 To Sum Up Asymmetric control is difficult in radical synthesis Fast reactivity Planar structure Methods that have been used to control asymmetry Chiral Auxiliary Chiral Acid Chelation rganocatalysis
67 Acknowledgements Dr. Jetze Tepe Dr. Babak Borhan Group Membes: Brandon, Chris, Daljinder, Jason, Mike, ahman, Samantha, Thu, Amanda Arvind, Camille, Carmin
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