title: Diels-Alder reactions of 4-halo masked o-benzoquinones. Experimental and theoretical investigations.
abstract: The studies on [4 + 2] cycloaddition of 4-halo derivatives of 6,6-dimethoxycyclohexa-2,4-dienones known as orthoquinone monoketals/masked o-benzoquinones are described. The 4-fluoro, 4-chloro- and 4-iodo-masked o-benzoquinones were stable enough for their isolation and characterization. These conjugated dienones cycloadded with several electron-deficient and electron-rich dienophiles in a highly regio- and stereo-selective manner to afford the corresponding halo bicyclo[2.2.2]octenone derivatives in high to excellent chemical yields. The halo masked o-benzoquinones did not undergo dimerization under the reaction conditions. To evaluate the observed selectivities of these Diels-Alder reactions, we have performed quantum mechanical calculations for the reactions between halo masked o-benzoquinones and methyl vinyl ketone and ethyl vinyl ether at the B3LYP/6-31G** theory level. The differences in HOMO and LUMO energy gaps suggest that these reactions can be classified as inverse electron-demand Diels-Alder reactions. The calculated transition state energies and global electronic indexes supported the experimentally observed selectivities of the reaction in many cases.
authors: Department of Chemistry, Indian Institute of Technology, Roorkee-247 667, Uttarakhand, India. rkpedfcy@iitr.ernet.in ramakpeddinti@gmail.com.
authors: Seshi Reddy
authors: SR
authors: Surasani
authors: Santosh Kumar Reddy
authors: SK
authors: Parumala
authors: Rama Krishna
authors: RK
authors: Peddinti