Synthesis of Double Bonds
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1 Synthesis of Double Bonds Wittig eaction!!! Georg Wittig: Nobel Prize 1979 For their development of the use of boronand phosphorous-containing compounds respectively, into important reagents in organic synthesis. (shared prize with. C. Brown) P X Li P ylide P P 84%
2 General Principles!!! 3 P C Y 3 P Y - can get high selectivity for either E or Z olefin depending on: 1. ylide type 2. type of C= 3. reaction conditions Ylide Types & utcomes stabilized ylides: have strongly conjugating functionality (C 2, CN, S 2 ) favor formation of trans olefin moderately stabilized ylides: have less strongly conjugating functionality (allyl, aryl) show little stereoselectivity non stabilized ylides: have no conjugating substituents favor formation of cis olefin
3 General Principles!!! early example Br P 3 Ac Na, Me 98% vitamin A BASF 1956 ylide preparation X Δ X strong base - bases include: Na, K nbuli, Li LDA, LiMDS, NaMDS, KMDS tbuk C 3 (S=)C 2 -
4 Nonstabilized Ylides!!! original mechanism - stepwise, ionic process (betaine mechanism) anti (E) syn (Z) betaine oxaphosphatane 80 kcal/mol 130 kcal/mol
5 Nonstabilized Ylides!!! alternate mechanism - synchronous cycloddition process ([22] mechanism) [22]? P 3 consistent with available evidence, but does not rule out other possibilities!
6 Nonstabilized Ylides!!! counterion dependence - generally consider nonstabilized ylides to give cis olefins - BUT, reaction is complex! X 1. base 2. C LiMDS 50 : 50 KMDS 98 : 2 - use salt free conditions - NaX, KX less soluble; ppt out of reaction medium
7 Nonstabilized Ylides!!! Schlosser modification - Li catalyzes oxaphosphatane opening (stabilizes betaine) - excess salt promotes equilibration and formation of E alkene - Na, K counterions do not have sufficient Lewis acidity to cleave oxaphosphatane (not recommended for Schlosser). -78 C Li -78 C β-oxido ylide tbu oxaphosphatane warm to rt (E) X 1. LiMDS 2. C 3. Li, -78 C 4. tbu --> rt 4 : 96
8 Nonstabilized Ylides!!! Trisubstituted lefins - reaction with ketones typically not stereoselective - rare exceptions large steric difference between ketone groups Me Me Et Me Me Me Me ~ 1 : 1 Me Me
9 Nonstabilized Ylides!!! Trisubstituted lefins - reaction with α-alkoxy ketones can be stereoselective TP Me TP TP - branching at α position of ketone increases selectivity 41 : 1 TP Me TP TP 200 : 1 - branching at α' position of ketone not tolerated TP Me TP TP 1 : 1
10 Nonstabilized Ylides!!! Trisubstituted lefins - reaction with α-alkoxy ketones can be stereoselective - application to α-santalol P 3 1. Me 2. Ac (aq) > 99 : 1
11 Stabilized Ylides!!! C 2 Me C - corresponding phosphonium salt can be deprotonated with weaker base - trans selective - will not react with ketones Bz Bz C C C benzene, Δ C 2 Me C 2 Me buffer ph 8.4 C 60%
12 osphonates!!! orner-wadsworth-emmons eaction Et P Et W - orner described use of phosphine oxides in Wittig type reactions in : Wadsworth and Emmons described the increased reactivity of phosphonate stabilized carbanions with α-electron withdrawing groups. - no alkene produced if W = alkyl Et P Et C 2Et C 2 Et Na
13 osphonates!!! orner-wadsworth-emmons eaction preparation: Arbuzov reaction (Et) 3 P Br C 2 Et (Et) 3 P C 2 Et Br - EtBr Et P Et C 2Et deprotonation: nbuli, tbuk, Na
14 orner-wadsworth-emmons eaction!!! mechanism P - C 2Me C P kinetic product C 2Me slow P C 2 Me (Z) C 2 Me C P C 2Me fast DS P C 2 Me (E) C 2 Me
15 orner-wadsworth-emmons eaction!!! application to prostaglandin synthesis prostaglandin F 2α C 2 Me Me C
16 orner-wadsworth-emmons eaction!!! application to prostaglandin synthesis Et P Et C 5 7 prostaglandin F 2α Ac C Na Ac 1. Zn(B 4 ) 2 2. separate Ac 1. K 2 C 3 (aq) 2. DP, pts 3. DIBAL TP TP C 2 - Ac TP TP
17 orner-wadsworth-emmons eaction!!! Trisubstituted lefins - reaction with ketones typically not stereoselective - rare exceptions large steric difference between ketone groups TBS Et P C 2Et Et Na TBS Et TBS 4 : 1 Et Et P Et C 2Et Et 2 C C 2 Et Na 9 : 1
18 orner-wadsworth-emmons eaction!!! Trisubstituted lefins - reaction with ketones typically not stereoselective - can get useful levels of selectivity with α-substituted phosphonates C Et P C 2Et Et Na C 2 Et 14 : 1 E : Z
19 orner-wadsworth-emmons eaction!!! Trisubstituted lefins Still-Gennari modification - cis selective! - selectivities from 10 to 50 : 1 CF 3 C 2 P CF 3 C 2 C 2Me CF 3 C 2 P CF 3 C 2 C 2Me CF 3 C 2 P CF 3 C 2 C 2Me 18-cr-6 KMDS C 2 Me CF 3 C 2 P CF 3 C 2 C 2Me 18-cr-6 KMDS C 2 Me
20 orner-wadsworth-emmons eaction!!! Trisubstituted lefins Still-Gennaro modification - cis selective! - selectivities from 10 to 50 : 1 C CF 3 C 2 P CF 3 C 2 C 2Me 18-cr-6 KMDS C 2 Me 78% >50:1 (Z:E)
21 osphorous Ylides & osphonates Selectivity Summary!! Unstabilized Ylides C Wittig C TP Schlosser TP Still
22 osphorous Ylides & osphonates Selectivity Summary!! Stabilized Ylides C 2 C " " P X C 2 C C C 2 with stabilized ylide or phosphonate C C 2 Still-Gennari (X = ; " = CF 3 C 2 ) C 2 Still-Gennari (X = Me; " = CF 3 C 2 )
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