9 Õèìèÿ, ãîä. ÕX, êí. 2 (2011) Chemistry, Vol. 20, Iss. 2 (2011) DFT STUDY ON CYCLOPENTA-2,4-DIENONES AND CYCLOPENTA-2,4-DIENETHIONES AS DIENOPHILES FOR REACTIVITY PREDICTION IN DIELS-ALDER REACTIONS Banjo SEMIRE, Olusegun Abayomi ODUNOLA Ladoke Akintola University of Technology, NIGERIA Abstract. The reactivity of both cyclopenta-2,4-dienones and cyclopenta- dienethiones were discussed with special emphasis on the use of cyclopentadienethione dienophiles in Diels-Alder type reactions. The ù values obtained for the molecules with electron-donating group (–CH 3 ) increased the nucleophilicity power whereas substitution with electron-withdrawing groups (such as –NO 2 and –CO 2 CH 2 CH 3 ) increased the electrophilicity power, indicating an increase of reactivity towards a nucleophiles. The cyclopentadienethiones would be more reactive towards nucleophiles than the cyclopentadienone analogues due to low values of ç and high values of ù. This was supported by local ionization potential energy surfaces and Mulliken's charges on the cardonyl carbon and oxygen on cyclopentadienone and thienyl carbon and sulphur on cyclopentadienethiones. A relationship was established for the C-S bond and electrophicity index ( ù) of cyclopentadienethiones. Keywords: cyclopenta-2,4-dienones, cyclopenta-2,4-dienethiones, reactivity pre- diction Introduction Cyclopentadienones have been widely recognized as having a high degree of synthetic potential [1-5]. Unfortunately, their propensity to dimerize [1,2] has limited their use in synthesis to masked cyclopentadienones or to those containing multiple aromatic rings [3-5]. A new method has been developed for the synthesis of cyclopentadienes, which involved (3+2) cycloaddition of cyclopropenones and alkynes E 9