Wittig Reaction - Phosphorous Ylides
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1 Wittig eaction - osphorous Ylides 3 P CC 3 3 C ylide tereoselectivity increases as the size of increases cis-olefin is derived from non-stabilized ylides Mechanism: Irreversible [22] cycloaddition P 3 3 C! "! 2!a 2!s cycloaddition 3 C P 3 group of aldehyde far away from ylide C 3 3 C 3 P 3 C 3 P amd P 3 Me Me Chem Ber. 1976, 1694.
![Irreversible [22] cycloaddition P 3 3 C! "! 2!a 2!](/docs-images/47/21098343/images/page_1.jpg "s cycloaddition 3 C P 3 group of aldehyde far away from ylide C 3 3 C 3 P 3 C 3 P")
2 E-selective Wittig eactions 3 P C 2 P 3 Li Li P 3 1 eq. Cl P 3 ylide "chlosser" Wittig equilibrates to the more stable oxaphosphetane ACIEE, 1966, E:Z C 3 C :1 C 5 11 C 3 96:4 C 3 99:1 2 tabilized Ylides are much less reactive than alkyl ylides; the react with aldehydes, but only slowly with ketones Br P 3 3 P a 2 C 3 3 P A A C slow P 3 P 3 tabilize ylides thus form E alkenes as major products reversible C 2 major kinetic C 2 minor kinetic fast slow thermodynamic product: major C 2 minor C 2
3 orner-wadsworth-emmons Wittig: E-selective osphonate esters are easily deprotonated and are more basic/nucleophilic than stabilized ylides; they react with both ketones and aldehydes ynthesis: Br PEt) 3 Arbuzov reaction Et P Et Mechanism of lefin Formation: Et Et D P C 3 Claisen Condensation: 1. LDA 2. ' Et Et P Et 3 C ' TL, 1976, 2829 Et P Et Et a Et P Et Et eversible C a P(Et) 2 C 2 Et a P(Et) 2 C 2 Et Good for: Et P Et JC, 1961, 1733 W W= C, C 2, C, C 2, P(Et) 2 water soluble phosphate can be removed in aqueous workup fast - P(Et)2 P(Et) 2 C 2 Et - C 2 Et slow thermodynamic product: major C 2 minor C 2
4 Modifications to the orner-emmons Wittig Masamune and oush: for Base-sensitive substrates, use LiCl/tertuary amine (Et 3, DBU, ipr 2 Et) BzC Et P BzC Et TL, 1984, 2183 C 3 LiCl, ipr 2 Et C 3 C, 23 C C 3 JC, 1989, 896 metal ion coordination lowers pka further: M Et P Et 3 Both hindered phosphonates and hindered aldehydes increase E-selectivity: C 3 C 3 Bn Bn C 2 P 3 =CC 2 Et (ipr) 2 PC 2 C 2 Et/KtBu (Me) 2 PC 2 C 2 Me/KtBu 7 :1 E:Z 95:5 E:Z 1:3 E:Z TL, 1981, 3873.
5 Modifications to the orner Emmons Wittig, continued Z-selective olefin synthesis: till modified phosphonate: TL, 1983, 4407 F 3 C 2 C P F 3 C 2 C C 3 KMD, 18-crown-6 ' C 2 Me Z:E >10:1 F 3 C 2 C P F 3 C 2 C C 3 KMD, 18-c-6 C 2 Me C C 3 2 Me Bn 84%, 11:1 Z:E F 3 C 2 C P C 2 Me C 3 F 3 C 2 C K, TF Bn Tetrahedron, 1987, 2369 Et P C 2 Me Et a, TF C 3 Bn C 2 Me 83%, 12:1 E:Z
6 Trisubstituted lefins: C 3 P C 3 tbuk, TF 2 C 3 C 3 C 2 2 C 3 C 3 C E:Z C 3 C 3 5:95 C 3 Et 10:90 Et Et 40:60 ipr Et 90:10 ipr ipr 95:5 F 3 C 2 C P F 3 C 2 C C 3 C 3 KMD, 18-crown-6 C 3 C 2 Me Z:E >10:1 Z:E >50:1 >50:1 TL, 1983, 4403 >50:1
7 Peterson lefination: An alternative to the Wittig eaction 2-step procedure: Addition to aldehyde (non-stereoselective) and silanol elimination (stereospecific) JC, 1968, 781 Me 3 i M M=Li, Mg irreversible Isolate and separate silanol diastereomers Li 2 C Me 3 i Me 3 i Me 3 i nondiastereoselective 2 2 Elimination tep is tereospecific: anti elimination 2 2 Me 3 i 3 Me 3 i 2 cis control geometry of olefin with conditions for elimination! 2 a Me 3 i a Me 3 i (base) syn elimination trans
8 tereoselective Additions in the Peterson lefination: threo product favored by small i 3 (Me 3 i) erythro product favored by large i 3 (t-bu 2 i) 3 i small i 3 large i 3 3 i - 3 i - threo maintain an anti relationship between aldehyde and largest substitutent on the silicon reagent 3 i erythro Elimination tep is tereospecific: anti elimination tbu 2 i 3.0 K Z:E Me 92:8 85:15 vinyl 95:5 tbu 2 i BF 3 Et 2 Bu Bu E:Z = 99:1 ynthesis, 2000, 1223
9 Julia lefination: E-selective synthesis mixture of diastereomers: 2 2 on stereoselective 2 2 Ac 2 Ac 2 2 a/g reductive fragmentation major 2 TL 1973, Ac Me BuLi Me 1. MsCl, Et 3 radical intermediate prefers and 2 trans Me 76% TB C 5 11 C TB C a/g TB C BuLi C 2 mi 2 TF 2 2 TL, 1990, 7105 see also: JC, 1995, 3194 rg. Lett. 2005, 2373.
10 Tebbe eagent: Cp 2 Ti AlMe 2 Cl eacts with aldehydes, ketones, esteres, lactones, amides to give methylene compounds: Tebbe C 2 JAC, 1978, 3611 X X Tebbe C 2 C 2 Tebbe Et Et Tebbe C 2 Tebbe C 2 see also: Petasis reagent: Cp 2 TiMe 2 JAC, 1990, 6392.
11 Corey-Winter eaction: Vicinal Diol Elimination Carbene: ' Cl Cl ' P(Et) 3 heat ' (Et) 3 P -C 2 syn elimination ' hapiro eaction: JAC, 1963, 2677 JAC,1965, 934 Ts 2 Ts 2 MeLi Ts TL, 1975, vinyl anion -I
12 hapiro eaction: BuBr Ts 2 2 BuLi Ts Li 3 Acc. Chem. es. 1983, 55.
13 Burgess eagent: Dehydration of alcohols to form alkenes Et 3 for 2 and 3 alcohols only JAC, 1970, 5224 JC, 1973, 26 ' Burgess Exothermic ' Et 3 cis elimination ' D Burgess D D Burgess Burgess JAC, 1990, 8433 C 3 C 3
14 Martin ulfurane: C(CF 3 ) 2 C(CF 3 ) 2 for 2 and 3 alcohols only JAC, 1971, 4327 JC, 1973, 26 Martin ulfurane ' ' ' ' Eliminations for 1 alcohols: Grieco method 2 ec 2 MsCl Ms e Et 3 ab 4 JC, 1975, ther selenide eliminations: see JAC, 1973, 5813 JC, 1975, 542. retro-hetero-ene reaction - e 2
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