Shi Asymmetric Epoxidation
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1 Shi Asymmetric Epoxidation Chiral dioxirane strategy: R xone, ph 10.5, K 2 C 3, H 2, C R 3 formed in situ catalyst (10-20 mol%) is prepared from D-fructose, and its enantiomer from L-sorbose oxone, the stoichiometric oxidant, is a 2:1:1 mixture of KHS 5, KHS 4, and K 2 S 4 H 2 2 / C may also be used as stoichiometic oxidant. a ph of 10.5 is an optimum balance between oxone decomposition and Baeyer-Villiger rearrangement of dioxirane intermediate R 3 R2 R 3 R 2 spiro TS electronically favored n=>!* planar TS Higher ee's observed with smaller and larger R 3 substituents
2 Effect: smaller R1 beneficial Shi Asymmetric Epoxidation R 3 R 2 79% ee 81%ee 98%ee 26% ee Effect: larger R3 beneficial Compare: H 3 C H 3 C C 10 H 21 H 3 C H 3 C 76% ee 86% ee 91% ee 76% ee 97% ee ote that the substituent size preferences reflect interactions in the spiro TS. R 3 Proposed Catalytic Cycle: R 3 HS 5 - H S - 3 S S - 3
3 Shi Asymmetric Epoxidation Substrate Product Yield ee 73% 95% Cl Cl 61% 93% 91% 93% C 10 H 21 C 10 H 21 94% 89% JACS, 1996, 9806 JACS, 1997, Monoepoxidation of dienes occurs at the more e - rich or less sterically hindered olefin 25mol% 1, xone TBS K 2 C 3, C TBS 81% yield 96% ee 25mol% 1, xone K 2 C 3, C 65% yeild 89% ee
4 Shi Asymmetric Epoxidation Trisubstituted olefins are selectively epoxidized because they are more e-rich TMS 25mol% 1, xone K 2 C 3, C SiMe 3 JC, 1998, %, 92% ee Epoxidation of enynes occurs selectively at the C-C double bond: SiMe 3 25mol% 1, xone K 2 C 3, C SiMe 3 64%, 94%ee TL, 1998, 4425 JC, 1999, ,1-disubstituted epoxides can be prepared from trisubstituted vinyl silanes by epoxidation and desilylation: SiMe 3 25mol% 1, xone K 2 C 3, C 74%, 94% ee SiMe 3 TBAF 82% 94%ee JC, 1999, 7675
5 Shi Asymmetric Epoxidation A modified catalyst is used for epoxidation of cis-disubstituted olefins and styrenes Boc 2 JACS, 2000, xone, K 2 C 3, DME 82%, 91%ee 2 xone, K 2 C 3, DME 100%, 81%ee rg. Lett. 2001, 1929 JC, 2002, 2435!-substituent prefers to be proximal to the spiro oxazolidinone: Boc R!
6 Shi Asymmetric Epoxidation Kinetic Resolution of racemic 1,3- and 1,6-disubstituted cyclohexenes provides optically enriched allylic Silyl ethers TMS 35 mol% 1 49% conv. TMS + TMS TBS 35 mol% 1 70% conv. 96% ee TBS 99% ee + 95% ee TBS 81%ee The original Shi catalyst decomposes faster than it reacts with electron-deficient unsaturated esters. A second-generation catalyst, incorporating electron-withdrawing acetate groups, slows the decomposition: JACS, 1999, 7718 Baeyer-Villiger reaction - C 2 Et Ac Ac 73% yield, 96% ee C 2 Et JACS, 2002, 8792
7 Upjohn Dihydroxylation
8 riginal UpJohn Procedure: Sharpless Asymmetric Dihydroxylation Reaction R 2 Cat. s 4, 1eq. M 8:1 acetone : water H R 2 H TL, 1976, 1973 M= - Catalytic Cycle: s L 2 H 2, 2H - s L L s L VI H 4 s H H + L VIII turnover is achieved with stoichiometric oxidants: K 3 Fe(C) 6, M M is found to be deleterious to the enantioselective process Addition is likely a 3+2 cycloaddition rather than a 2+2 cycloaddition/rearrangement: 2 Fe(C) Fe(C) 6 3- JC, 1990, 766 [3+2] s L s L TL, 1996, 4899 JACS, 1997, 9907
9 Ligands: C2-symmetric, pseudo-enantiomeric Sharpless Asymmetric Dihydroxylation Reaction Et Et Et Et Me Me Me Me (DHQD) 2 -PHAL ligand for AD-mix-! (DHQ) 2 -PHAL Ligand for AD-mix-" slightly less enantioselective AD-mix reagents are commercially available: 1.4 g Ad-mix-! will oxidize 1mmol olefin 0.98 g K 3 Fe(C) 6 (3 mmol) 0.41g K 2 C 3 (3 mmol) g (DHQD) 2 -PHAL (0.01 mmol) g K 2 s 2 (H) 4 (0.002 mmol) ˆJC, 1992, 2768 Corey Proposes a U- shape binding pocket: Me Me s H TL, 1995, 3481
10 Sharpless Asymmetric Dihydroxylation Reaction: Ligand Accelerated Catalysis 4 of 6 olefin classes are successfully dihydroxylated: tetra tri trans-di gem-di mono cis-di Mnemonic: slightly hindered (DHQD) 2 -PHAL! R S R M Application of Mnemonic: H 3 C C 5 H 11 C 2 Et attractive area: good for aromatic and alkyl substituents H 3 C Et 2 C R L H " (DHQ) 2 -PHAL C 5 H 11 very hindered AD-mix-! R H 3 C H H AD-mix-! H S R C 5 H 11 Et 2 C H R AD-mix-! CH 2 H H C 8 H 17 AD-mix-! C 8 H 17 H R CH 2 H
11 Generality of Sharpless Asymmetric Dihydroxylation AD-mix-! AD-mix-" (DHQD) 2 -PHAL (DHQ) 2 -PHAL % ee, config % ee, config H 3 C 98, R 95, S 99, R, R 97, S, S C 5 H 11 C 2 Et 99, 2S, 3R 96, 2R, 3S >99.5, R,R >99.5, S,S 94, R 93, S C 8 H 17 84, R 80, S 97, R 97, S JC, 1992, 2768
12 Generality of Sharpless Asymmetric Dihydroxylation Cis-Disubstituted lefins are poor substrates; with a modified catalyst, DHQD-ID, good ee s can be obtained: ee at 0 C 72 (1R, 2S) Et JACS, 1992, 7568 H Me C 2 ipr 80 (2S, 3R) DHQD-ID (DHQD) 2 AQ is often a superior ligand: DHQD (DHQD) 2 AQ DHQD Cl 90% ee vs. 63% ee with (DHQD) 2 PHAL 88%ee vs. 77% ee with (DHQD) 2 PHAL ACIEE, 1996, % ee vs. 44% ee with (DHQD) 2 PHAL
13 Good substrates Allylic 4-methoxybenzoates Generality of Sharpless Asymmetric Dihydroxylation Me Me But: H TIPS H 3 C AD-mix-! (DHQ) 2 PHAL >99%e, 93% yield H H 3 C H H 3 C 18%ee H 3 C 13%ee AD, (DHQD) 2 PYDZ >99% yield, 98%ee PYDZ = Me 98% yield, 97% ee JACS, 1995, Me 96%, 91% ee Me
14 Generality of Sharpless Asymmetric Dihydroxylation Regioselectivity of AD with dienes: in general, AD is selective for more electron-rich doible bonds Substrate Product % yield, % ee H 78, 93 C 2 Et H H C 2 Et 78, 92 JACS, 1992, 7570 H 73, 98 H H 70,98 H H
15 Use of AD with Chiral lefins H H Me "anti" Me H + H "syn" Me conditions anti:syn s 4, M 88% yield (mixture) 1.9:1 matched (DHQ) 2 PHAL 86% yield (anti) 54:1 TL, 1997, 5941 mismatched (DHQD) 2 PYDZ 86% yield (syn) 1:35
16 H xidative Kinetic Resolution KR H + R 2 chiral catalyst R 2 R 2 [] racemic non-racemic It was found that combination of a palladium source, (-) sparteine as a chiral ligand, and 2 could effect Efficient oxidative kinetic resolution under defined conditions H Pd(nbd)Cl 2, 5 mol% (-)-sparteine 20 mol% + H racemic R 2 MS 3Å, 2, 80 C R 2 R 2 non-racemic enriched alcohol isolated yield ee RH H H JACS,2001, H H
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