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1、M.C. White/Q. Chen, Chem 153 Q this argues that the mechanism for -trimethylenemethane cyclizations is in fact stepwise.Trost ACIEE 1986 (25) 1.Silicon-carbon bond weakened by the proximal positive chargeM.C. White, Chem 153 -Allyl chemistry -362- Week of December 2, 2002 Asymmetric functionalizatio
2、n via deracemizationRRX*LnPd0enantiotopicends ofa meso -allyl intermediate.aRRNubRRNuDeracemizationRPd*LnRNuabRPd*Lnsyn/antiRNuanote that syn/anti isomerization results in an errosion in ees.Solutions:NOt-BuPh2P PdThe stronger -acceptor properties of phosphines vs. imines results in differential tra
3、ns-effectsbetween the two coordinating functionalities. The strong P-Pd bond translates into a long (weaker) Pd-C bond trans to it. This is thought to increase the ionic nature of the bond resultinginmorecarboniumcharacteratthe corresponding carbon and preferential attack by the nucleophile at that
4、site.RROAcRRMeO2CCO2MeClPd(3-allyl)2 1 mol%2 (2.5 mol%), DMF, 65oCCO2MeCO2MeNa2alkyl substituted 1,3 allyls give high ees:R = Me, 50% yield, 97% eei-Pr, 96% yield, 88% eePh, 99% yield, 99% eePfaltzs electronic disymmetry. Pfaltz ACIEE 1993 (32) 566.Challenge: creating an effective asymmetric environ
5、ment outside the coordination sphere of the metal opposite to the chiral ligand.Hayashis nucleophile directing tether: Hayashi TL 1986 (27) 191.FePPh2PPh2PdNONuHThe hydroxyl group on the ligand tether is thought to interact attractively with the incomingnucleophile via H-bonding anddirect it towards
6、 attack at one end of the -allyl group.ArArOAcO H1ClPd(3-allyl)2 0.5mol%1 (1 mol%), THFCO2MeCO2MeNaArArMeO2CCO2MeAr = Ph, 97% yield, 90% ee1-Np, 40% yield, 92% eeThe 1,3-diphenylallyl system favors the syn,syn isomer. Syn-anti isomerizations often lead to erosions in ees for acyclic substrates.M.C.
7、White, Chem 153 -Allyl chemistry -363- Week of December 2, 2002 Asymmetric functionalization via deracemizationChallenge: creating an effective asymmetric enviroment outside the coordination sphere of the metal opposite to the chiral ligand. Solutions:NHHNOOPPh2Ph2Pchiral backbonelinkerlinkerPPPdThe
8、primarychirality ofthe stereogenic centers of the backbone gets translated via the linker units into a secondary chirality at the phosphines that are closest to the Pd-allyl unit. Theconformationalbiasforedge-face stacking of the phosphine phenyl groups enforces chiral “propellers“ that reach out to
9、wards the allyl unit and sterically block the trajectory of nucleophilic attack from one side. The steric bulk of the diphenylphosphine units is also thought to retard the rate of syn/anti isomerization.Cat+ Nu-open quadrant3Trost Acc. Chem. Res. 1996 (29) 355.Trost JACS 1994 (116) 4089.Cyclic syste
10、ms: Because the rxn is performed in nonpolar media, the Nu exists as an ion pair. Trost has found that the molecular recognition event depends more on the nature of the cation than of the anion that ultimately attcks the -allyl unit. The larger the cation, the higher the ee.OAcCO2Me CO2MeCatClPd(3-a
11、llyl)2 2.5mol% 3 (7.5 mol%), THFCO2MeCO2Me+Cat%Yield% eeNa+ N(CH3)4+ N(n-C6H13)4+N(n-C6H13)4+ in CH2Cl2K+ Cs+Cs+ in CH2Cl277% 88% 92% 81%90% 76% 98%38% 41% 68% 98%51% 76% 99%All the ammonium cations were added as chloride salts.CH2Cl2results in an even tighter ion pair.CO2MeCO2Me 86% yield 96% eeCO2
12、MeCO2Me99% yield 93% eeNOO87% yield 94% eelarger metal ionbulkier ammonium ionResults under optimal ammonium salt conditions:M.C. White, Chem 153 -Allyl chemistry -364- Week of December 2, 2002 Asymmetric functionalization: regio-and stereoselective aminationMeOCO2R+PhNH2FePPh2PPh2NOHOH6 mol%Pd2(dba
13、)3 1.5 mol%, THF, 0oCMeHNPhMeHNPh+87% yield 97: 3 (branched: linear) 84% ee(91:9, E/Z)note:resultindicatesthat syn/anti isomerization is slow relative to nucleophilic attack.Me Pd*LnMe Pd*LnHHMeH*LnPdHMeLn*PdMeOCO2Me+50%MeOCO2Me50%conditions as aboveMeHNPh82%MeHNPh18%- isomerization is fast relative
14、 to nucleophilic attackHayashi TL 1990 (31) 1746.achiral substrateracemic substrateRXorRX*LnPd0RPd*Ln (aka 3-1-3) isomerizationRNuRNudiastereomericNuNu+RNuBoth a regio- and stereoselectivity challengeRLn*PdM.C. White/M.W. Kanan Chem 153 -Allyl chemistry -365- Week of December 2, 2002 Dynamic kinetic
15、 asymmetric transformation of 5-Acyloxy-2-(5H)-furanoneTrost JACS 1999 (121) 3543.OOEtOOH3COOHI OBocOOOOEtOOH3COOIOHONHNH PPh2PPh2OO+1.2 eq.racemic2.5 % Pd2dba3, 7.5% 1 30% Bu4NCl, CH2Cl21PdOBocOOPd0PPOOOOPdPPOOPdPPClP POOEtOOH3COO-IOOEtOOH3COOIOHOPd0PP+ Cl- Cl-+The addition of Cl- ion is thought to accelerate the equilibrationbetweenthetwo?-allyl diastereomers by promoting formation of an 1 intermediate.Since the kinetic discrimination between the two reac
