rganic Tutorials 3 rd Year Michaelmas 2010 Transition Metals in rganic Synthesis: (General paper level) Reading 1. Lecture Course, and suggested references from this. 2. Clayden, Greaves, Warren and Wothers. 3. Norman & Coxon 4. Carey & Sundberg vol B. 5. Hegedus Transition Metals in the Synthesis of Complex Molecules very good for the option paper (many copies in the RSL) Transition metals are widely used to facilitate carbon-carbon bond forming reactions in synthesis and to catalyse stereoselective oxidation and reduction processes. Things to understand: Palladium Catalysis You need to understand the fundamental concepts of oxidative addition migratory insertion transmetallation reductive elimination as well as being able to count the number of electrons and the oxidation state of a given metal in a complex. Reactions of importance: Heck, Suzuki, Sonagashira, Stille understand the mechanisms and catalytic cycles; how is Pd(II) reduced in situ to Pd(0) with NEt 3 or PPh 3? The various -allyl complexes; regio- and stereoselectivity. The Wacker Reaction; mechanism and synthetic applications. Metathesis Preparation of catalysts. Mechanism understand how metathesis reactions work and the difference between ring closing metathesis, cross metathesis, enyne metathesis etc. Understand (in principle) how asymmetric variants of the reaction work. As always do not get bogged down learning too many names (although it always helps to distinguish between the different reaction mechanisms); try and understand the fundamental concepts of each of the major reaction types as this is the main thing to understand. ther rgano-metallics: Please also remember to revise the preparation and use of non-transition metal reagents such as organo-lithium, -magnesium, -copper, -cadmium and zinc compounds etc.! 1!
PRBLEMS 1. Give mechanisms and illustrate the catalytic cycle for the following Heck reactions. (a) I C 2 Me Pd(Ac) 2 (10 mol%) NEt 3 PPh 3,! C 2 Me How is inactive Pd(II) reduced to the active Pd(0) catalyst? (b) I H Pd(Ac) 2 (5 mol%) KAc PPh 3,! Account for the transformation of the alcohol to the ketone Ph (c) I Pd(Ac) 2 (5 mol%) KAc, Bu 4 NCl PPh 3,! Ph In the presence of silver salts, such as Ag 2 C 3 what would the product of the reaction be? (d) Tf Pd(Ac) 2 (5 mol%) Na 2 C 3 Et 4 NCl, PPh 3,! C 2 Me C 2 Me 2. Give mechanisms and catalytic cycles for the following coupling reactions: Br SnBu 3 Pd 2 (dba) 3 or Pd(PPh 3 ) 4 CuCl, DMS (i). LDA (ii). PhN(S 2 CF 3 ) 2 Tf Bu 3 Sn Stille Couplings tbu tbu Pd(PPh 3 ) 4 (5 mol%) LiCl, THF tbu MEM MEM I (i). Pd(PPh 3 ) 4, toluene,! SnBu 3 (ii). HCl (aq), THF H H! 2!
Br B(H) 2 Pd(Ac) 2 PPh 3, Na 2 C 3! HC HC Suzuki Couplings Br Ph B Pd(PPh 3 ) 4 NaEt! Ph SiMe 3 Tf H SiMe 3 Tf CuI, PdCl 2 (PPh 3 ) 2, Bu 4 NI, NEt 3 SiMe 3 2 N Br Br H C 5 H 11 CuBr, Pd(PPh 3 ) 4, NEt 3 2 N C 5 H 11 Br Sonagashira H 2 N Br Br H C 5 H 11 CuBr, Pd(PPh 3 ) 4, NEt 3 H 2 N Br C 5 H 11! 3!
3. Give mechanisms for the following reactions: Me 2 C C 2 Me (i). Pd(PPh 3 ) 4 (cat), THF Me 3 Si Ac (ii). 3, CH 2 Cl 2, MeH then Me 2 S Me 2 C C 2 Me Ac (i). Pd(PPh 3 ) 4 (cat), THF H N Ph N H Ph H Me 2 C C 2 Me Pd(Ac) 2 (5mol%) H C 2 Me C 2 Me Br Pd(PPh 3 ) 4 (cat), PPh 3, NaH, THF H C 2 H H (-)-lactone Me 2 C C 2 Me C 2 Me C 2 Me syn- but racemic Ac Na H + NaAc Pd catalyst Me N 2 Me N 2 4. Suggest a catalytic cycle for the ring closing metathesis reaction below. 2 mol % Ru PCy 3 catalyst Cl N N + C 2 H 4 catalyst: Cl Ru CHPh Me Me PCy 3 Cy = cyclohexyl! 4!
5 Rationalise the product of each of the following reactions catalytic Cl 2 (Cy 3 P) 2 Ru=CHPh SiEt 3 catalytic SiEt 3 Cl 2 (Cy 3 P) 2 Ru=CHPh Ac catalytic Cl 2 (Cy 3 P) 2 Ru=CHPh Ac N Ts N Ts 6. Rationalise why syn-alkene 1 gives product in 92% e.e. upon metathesis with diallyl ether and chiral catalyst, whereas the anti-isomer 2 polymerises. H 1 Me Mo catalyst, C 5 H 12 5% H H Me Me Me Me Me catalyst t Bu t Bu NAr Mo R Me 2 H 7. Give a mechanism for the Tebbe olefination below: Ph Ti CH 2Al Me Cl Me Ph CH 2! 5!
8. Rationalise the following transformations: Ph Pd(0) catalyst Ac Me 2 C C 2 Me Me 2 C C 2 Me R PhMgBr R Pd(0) catalyst R H Ac Pd(0) catalyst H Ph H PhMgBr Minor Ph Major C 2 Me (i). PdCl 2, CuCl 2, 2, H 2, DMF C 2 Me (ii). LDA! 6!