Complex-induced proximity effects: the effect of varying directing-group orientation on carbamate-directed lithiation reactions.
A series of selected bicyclic carbamates in which the range of accessible angles and distances between the carbonyl group and the proton removed in an alpha-lithiation reaction are structurally defined have been investigated. Oxazolidinones 7-10 undergo stereoselective lithiation-substitution reactions to provide cis-18-27 and cis-31-35 as the major diastereomers. Two series of competition experiments show that the conformationally restricted carbamates 7, 10, 11, and 15 undergo lithiation via complexes more efficiently than Boc amines 4-6. These results along with semiempirical calculations suggest that a small dihedral angle and a calculated distance of 2.78 A between the carbamate carbonyl oxygen and the proton to be removed are favorable for a carbamate-directed lithiation. A series of tin-lithium exchange experiments on cis- and trans-18 and (S)-39 indicate that the configurational stability of a carbamate-stabilized organolithium species may be enhanced by restrictive geometry.
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Contribution from the Department of Chemistry, Roger Adams Laboratory, University of Illinois at Urbana-Champaign, 61801, USA.
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D001643:Bridged Bicyclo Compounds / Q000737:chemistry; D002219:Carbamates / Q000737:chemistry*; D008024:Ligands; D008094:Lithium / Q000737:chemistry*; D008958:Models, Molecular; D015394:Molecular Structure; D023303:Oxazolidinones / Q000737:chemistry; D013237:Stereoisomerism; D014001:Tin / Q000737:chemistry
D001643:Bridged Bicyclo Compounds; D002219:Carbamates; D008024:Ligands; D023303:Oxazolidinones; D014001:Tin; D008094:Lithium