Rhodium Catalyzed Alkyl C-H Insertion Reactions Rh Rh Jeff Kallemeyn 5/17/05
1. Cyclopropanation The Versatile and Reactive Rhodium Carbene R + Et Rh 2 (Ac) 4 R C 2 Et N 2 2. [2,3] sigmatropic rearrangement R Me + Et N 2 Rh 2 (A c) 4 R Et 2 C M e Rh 2 (A c) 4 R M e C 2 Et 3. Aromatic cycloaddition Rh 2 (Ac) 4 Me Me N 2 4. C-H insertion R H + Et N 2 Rh 2 (Ac) 4 Et H R
Challenges for Site Selective Reactions H H H H H + E t N 2 Rh 2 ( 2 CR) 4 4 h addition 60 o C E t 2 C H C 2 Et H H C 2 E t C 2 Et Ligand structure influences selectivity Noels, Teyssie et al, Chem. Comm. 1981, 688. Noels, Teyssie et al, J. Mol. Cat. 1988, L13.
Improving Selectivity 30 min Formation of trans-5-membered rings favored Taber et al. JC 1982, 4808
Competition Experiments R 1 R 2 N 2 Et Rh 2 (Ac) 4 rt 1 h addition R 1 R C 2 Et R 2 R C 2 Et ratio 2.3 : 1 2.9 : 1 2.3 : 1 3.5 : 1 1 : 0 methine > methylene > allyl, benzyl Taber et al. JACS 1986, 7686
Diastereoselectivity 10 min rt >95:5 dr Taber et al. JACS 1996, 547
Mechanistic Models Taber Doyle H E Me Rh H Taber et al. JACS 1996, 547 Doyle et al. JACS 1993, 958
Computational Studies a) b) c) CH 4 H H Nakamura et al. JACS, 2002, 7181
Carbene Structural Characteristics Nakamura et al. JACS, 2002, 7181
C-H Insertion Transition State Nakamura et al. JACS, 2002, 7181
Computational Studies Summary 1. RDS is the decomposition of the diazo 2. Rh 2 acts as a bifunctional electron pool 3. Rh anchored and orientated by acetate ligands 4. Concerted, nonsynchronous 3-center TS -hydride transfer preceeds C-C and Rh-Rh bond formation 5. Carbonyl is not in conjuagtion with the carbene - the two faces of the carbene are nonequivalent Nakamura et al. JACS, 2002, 7181
Classes of Chiral Rhodium Catalysts Carboxylate Ligands Carboxamidate Ligands
Classes of Carbenoids Acceptor EWG = C 2 R, CR, N 2, P(R) 2, S 2 R Acceptor/Acceptor EWG = C 2 R, CR, N 2, S 2 R, CN Acceptor/Donor EWG = C 2 R, CR EDG = vinyl, alkynyl. aryl, heteroaryl H EWG EDG N 2 EWG N 2 EWG N 2 EWG Nitrogen extrusion by a variety of catalysts Nitrogen extrusion by active catalysts since diazo is stabilized Nitrogen extrusion by active catalysts since diazo is stabilized M H EWG M EWG EWG M EDG EWG Moderate electrophilicity High reactivity High electrophilicity Very high reactivity Attenuated electrophilicity Moderate reactivity
Acceptor Carbenoid H diketene, E t 3 N THF, r t 94% 1) Ms N 3, THF, rt 2)Li H, THF, H 2 79% N 2 + ~5% 10 h addition R 22 Doyle et al. JC, 1996, 9146
Stereoselection Doyle et al. JACS 1996, 8837; Inorg. Chem. 1996, 6064.
Stereoselection Doyle et al. JC, 1996, 9146.
Enantiomer Differentiation or 10 h addition 40 o C Doyle et al. JACS 1996, 8837.
Stereoselection Doyle et al. JACS 1996, 8837.
Competition with Cyclopropanation Doyle et al. Tett. Lett 2001, 3155.
Acceptor/Acceptor Carbenoids Intramolecular: Intermolecular: Reactive substrates provide low enantioselectivity
Intramolecular Acceptor/Donor Carbenoid Davies et al. rg. Lett 2001, 1475.
Intermolecular Acceptor/Donor 90 min addition 10 o C High enantioselectivity in intramolecular reactions Davies et al. JACS 1997, 9075. Davies et al. JACS 2000, 3063.
Selectivity Davies et al. JACS, 2000, 3063.
Benzylic Substitutions 50 % Why is the di-cyclopropanated product formed selectively? Davies et al. JC, 2002, 4165. Davies et al. Tett. Asymm. 2003, 941.
Hammett Study Davies et al. JC, 2002, 4165.
Allylic Substitutions Davies et al. rg Lett. 2001, 3587.
a-xygen Substitutions 60 min addition 10h -50 o C Davies et al. JACS 2000, 3063. Davies et al. Adv. Syn. Cat. 2003, 1133. Davies et al. JC, 2003, 6126.
a-xygen Substitutions TBS Ac + Me Br Rh 2 [(S)-DSP] 4 50 C N 2 1. What are the possible products (4)? 2. Which is the major product? 3. Why is this product favored? Davies et al. JC, 2003, 6126.
a-xygen Substitutions TBS Ac + Me Br Rh 2 [(S)-DSP] 4 50 C N 2 Davies et al. JACS 2000, 3063.
a-nitrogen Substitutions Davies, Angew. Chem. Int. Ed. 2002, 2197. Davies, JACS 2003, 6462.
C-H activation, Cope Rearrangement Davies et al. PNAS 2004, 5472.
Summary of Intermolecular Reactions Davies et al. Chem. Rev. 2003, 2861.
Conclusions and utlook 1. The wide variety of catalysts (ligands) allows for significant optimization opportunities. 2. An understanding of activation sites allows for a modulated synthetic advantage -protection of alcohol as silyl ether or acetate will change the preference for reaction 3. The C-H bond is quite inert to other reactions; allows for development of orthogonal reaction sequences. 4. Nitrene based C-H insertions being developed as mild methods for C-N bond formation. Du Bois, JACS 2004, 15378 Muller. Helv. Chim. Acta. 1997, 1087.