title: Electronic regioselectivity of diarylalkynes in cobalt-mediated Pauson-Khand reaction: an experimental and computational study with para- and meta-substituted diarylalkynes and norbornene.
abstract: Both steric and electronic factors of substituted alkynes are known to guide α/β-cyclopentenone regioselectivity in the cobalt-mediated Pauson-Khand reaction (PKR). In synthetic applications of the PKR, the steric factors can often override or render possible electronic effects. This study examined alkyne-dependent electronic regioselectivity of cyclopentenone formation in PKR with norbornene and sterically equivalent, but electronically unsymmetrical, meta- and para-substituted diarylethynyls to unveil the role of electronic effects alone. In agreement with the literature reports, EDG para-substituted aryls, to some extent, favored the cyclopentenone α-regioisomer, while the EWG-substituted aryls correspondingly preferred the β-regioisomer. The cooperation of EGW and EDG in diaryl-substituted alkynes did not lead to any increased regioselectivities that could be expected by a "push-pull" effect. Both EWG and EDG meta-substituted aryls preferred the β-regioisomer, which was demonstrated by 3,5-dimethoxy- and 3,5-bis(trifluoromethyl)-1-phenylethynyls that yielded 1/1.6 and 1/2.0 α/β-regioselectivities, respectively. Theoretically, inspection of Hammett values of α-alkyne carbons gave qualitatively satisfactory prediction for para-substituted aryls but correlated only weakly with meta-substituted effects. Computational investigations at the DFT level revealed a correlation between NBO charges and the regioselectivity. Overall, the results suggest that the polarity of an alkyne, also designated by the relative polarization of aryl α-carbons, dictates the regioselectivity in the absence of steric effects.
authors.affiliation: Laboratory of Organic Chemistry, Department of Chemistry, University of Helsinki, A. I. Virtasen aukio 1, P.O. Box 55, Helsinki, Finland.
authors.forename: Erika
authors.initials: E
authors.lastname: Fager-Jokela
authors.forename: Mikko
authors.initials: M
authors.lastname: Muuronen
authors.forename: Michael
authors.initials: M
authors.lastname: Patzschke
authors.forename: Juho
authors.initials: J
authors.lastname: Helaja
mesh_terms: D000480:Alkynes / Q000737:chemistry*; D003035:Cobalt / Q000737:chemistry*; D004583:Electrons; D015394:Molecular Structure; D009636:Norbornanes / Q000737:chemistry*; D009942:Organometallic Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D011789:Quantum Theory; D013237:Stereoisomerism
chemical_list: D000480:Alkynes; D009636:Norbornanes; D009942:Organometallic Compounds; D003035:Cobalt