rethinking cross-coupling: enantiospecific sp -sp3 … · rethinking cross-coupling:...
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Rethinking cross-coupling:
Enantiospecific sp2-sp3 coupling
Souvik Rakshit
Burke group Literature Seminar
July 19, 2014
A. Bonet, M. Odachowski, D. Leonori, S. Essafi, V. K. Aggarwal; Nature Chem. 2014, 6, 584–589
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Biscoe (2013)
Pd-catalysed cross-coupling reactions between isolable, optically active organometallic
nucleophiles and aryl halides
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E+
E
E
H. Sun, D.G. Hall; Nature Chem. 2014, 6, 562.
V. K. Aggarwal et al. The Chemical Record 2009, 9, 24.
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Related SEAr of electron rich aromatic group with achiral boranes
A. B. Levy; J. Org. Chem. 1978, 43, 4684 A. Suzuki et. al. Synthesis 1979, 146.
• Handling air-sensitive boranes
• Creating stereo-defined boranes
• Issue as to which group will migrate in non-symmetrical boranes
A. B. Levy et al. Tet. Lett. 1979, 20, 2313.
Difficulty associated with
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Synthesis of enantioenriched secondary and tertiary boronic esters
V. K. Aggarwal et al. Angew. Chem. Int. Ed. 2007, 46, 7491; Nature 2008, 456, 778; Angew. Chem. Int. Ed. 2010, 49, 5142;
J. Am. Chem. Soc. 2013, 135, 16054.
One-pot lithiation/borylation of Hoppe-type carbamates
Lithiation/borylation of enantioenriched secondary
carbamates.
R = BnCH2, R1 = Me, R2 = Et, R3 = pin
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Synthesis of enantioenriched secondary and tertiary boronic esters
Readily available
Stable
High enatiopurity
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V. K. Aggarwal, et al. J. Am. Chem. Soc. 2011, 133, 16794; Chem. Sci. 2013, 5, 602.
Aliphatic electrophilic substitution at sp3 carbon
Optimized Conditions for stereospecific sp2-sp3 coupling
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Solvent effect
V. K. Aggarwal Chem. Sci. 2013, 5, 602–607.
THF: SE2inv dominate
MeOH: SEAr dominate due to involvement of
cationic intermediate
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Effect of substituent
Berionni, G., Maji, B., Knochel, P., Mayr, H. Chem. Sci. 2012, 3, 878.
2 and 4-methoxyphenyllithium were not effective, whereas 3-methoxyphenyllithium was.
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Post-modification of functional group
Suzuki cross-couplings of aryltrimethylammonium salts
Blakey, S. B.; MacMillan, D. W. C. J. Am. Chem. Soc. 2003, 125, 6046.
Removal of dialkylamino-substituents from aromatic rings
Paras, N., Simmons, B.; MacMillan, D.W. C. Tetrahedron 2009, 65, 3232.
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Summary
A general method for coupling electron-rich aromatic/ heteroaromatic compounds
with enantioenriched secondary and tertiary boronic esters.
This methodology, which uses simple, readily available reagents, no transition
metals and user-friendly conditions, shows broad scope in the boronic esters and
shows complete stereospecificity.
Although its virtues are certainly more numerous, the method is not without its
limitations.
• The aryl halide coupling partner is limited to electron-rich or heteroaryl species
• The use of aryl lithium reactants, which are both strong bases and
nucleophiles, limits the types of functionality that can be tolerated under these
reaction conditions.