Over the past decade there has been a growing interest in the use of carbonyl ylides that were simply generated by addition of carbene or carbenoids onto the oxygen atom of a carbonyl group. Only a few ylides have been isolated and their existence in reactions have been verified indirectly. Thus, computational studies serve as a strong tool to explain the reaction mechanisms. The reactions of α,-unsaturated carbonyl ylides have attracted considerable attention as they provide a challenging route for the synthesis of dihydrofuran derivatives. The reactions of a few carbenes with cis E-enones gave rise to dihydrofuran intermediates, probably via 6-electrocyclization of the corresponding conjugated carbonylylide. Anac and Sezer have shown that the reaction of α,-unsaturated ketones with dimethyl diazomalonat (dmdm) in the presence of copper acetylacetonate (C(acac)2) give dihydrofuran type products while α,-unsaturated aldehydes give 1,3-dioksol derivatives. They also reported that the substituents of both enone and diazocarbonyl components had a beneficial influence on the reaction rate and favored the dihydrofuran formation. In this study, Cu(acac)2 catalyzed cycloaddition reaction of dmdm with 6,6-dimethyl-2-methylenebicyclo[3.1.1]heptan-3-on (P), a strained and cisoid-fixed bicyclic α,-conjugated enone, was studied to elucidate the preference of the reaction route leading to 1,3-dioxole type of products although, dihydrofuran derivatives were also the expected outcomes of the reaction. It was a surprising result since it was contrary to previous reports on other ketones which yielded dihydrofuran type of products. Geometry optimizations of reactants, products and the transition states were carried out by Density Functional Theory (DFT) method with empirically parametrized hybrid functional B3LYP at 6-31G* level. Calculations were performed by using Gaussian 03 software package. The equilibrium structures have been characterized by the lack of imaginary vibrations whereas transition states have been characterized by the presence of exactly one imaginary vibration belonging to the reaction coordinate and also verified by the IRC (Intrinsic Reaction Coordinate) analysis. The single-point electronic energies were corrected to Gibbs free energies at 298.15 Kº and 1 atm based on the unscaled harmonic frequencies obtained with the 6-31G* basis sets. The partial charges on the atoms were calculated by the Natural Bond Orbital (NBO) analysis. The reaction of dmdm with Cu(acac)2 gives copper(I) carbene complex. The calculations showed that the copper(I) carbene complex is a stable and chiral complex in which the carbene carbon is sp2 hybridized. This complex can also be considered as a Fischer type carbene because of the double bond character of Cu-C4 bond which is a result of the strong and interactions between them. Reaction of carbene complex with P can yield the ylide intermediate in one of the two possible geometries, namely E and Z, depending on the orientation of the substrate. We have shown that with the concerted mechanism, E-ylide would only give the 1,3-dioxole, whereas the dihydrofuran derivative would be the only product that can be obtained from the Z-ylide. In the latter mechanism, the olefinic C1=C2 double bond must lose its olefinic character and rotate so as to bring the reacting centers close to each other for an efficient overlap. The reacting center, C4 is at the center of two ester groups that have an extended delocalization, thus it will resist pyramidilization as it undergoes reaction. Additionally, ester groups will cause steric hindrance in the vicinity of reacting centers. These geometric hindrances that make the reaction difficult, cause an increase in the reaction barrier. On the other hand, the transition state in 1,3-dioxole formation is very early which results in a low barrier for the reaction. The reacting centers are oriented in a very favorable geometry for an efficient overlap in cyclization to a 1,3-dioxole product. Cyclization in 1,3-dioxole transition state geometries is not hindered by steric or pyramidilization effects. Thus, cyclization to 1,3-dioxole products does not require a dramatic change in the overall geometry of the reactant, which results in a much lower barrier. In conclusion, the reaction mechanisms leading to different products greatly depend on the conformations of copper-stabilized carbonyl ylides, which are treated as reactants in our calculations. The conformational effects and donor-acceptor type stabilizations between the catalyst and the carbonyl ylide observed in the reactants and the transition state geometries seem to be the main reasons for the favored 1,3-dioxole formation reaction. Keywords: DFT, reaction mechanisims, carbonyl ylides. Oksijenli beş üyeli heterosiklik yapılar 1,3-dioksol ve dihidrofuran bileşikleri gibi doğal ve bioaktif özelliklerinden dolayı önemli bileşiklerdir. Bu sebeple, bu bileşiklerin tek aşamada yüksek verimle sentezlenmeleri önemlidir. Bu çalışmada 6,6-dimetil-2-metilenbisiklo[3.1.1]heptan-3-on bileşiğinin bakır(II) asetilasetonat katalizörü varlığında dimetil diazomalonat ile reaksiyonuna ait olası tüm mekanizalar Yoğunluk Fonksiyoneli Teorisi (DFT) ile incelenmiştir. Bu reaksiyonlar için kabul gören mekanizma Doyle tarafından önerilmiştir ve bu mekanizma incelenen reaksiyon için uygulanmıştır. Hesaplamar sonucu, dihidrofuran ve 1,3-dioksol türevlerinin reaksiyon mekanizmalarının, reaktif moleküllerin konformasyonuna bağlı olduğu sonucuna varılmıştır. Cu atomu 1,3-dioksol oluşumu reaksiyonunun geçiş aşamalarını geri bağ ile kararlı hale getirmektedir. 1,3-dioksol oluşumu geçiş aşamaları birer erken geçiş aşaması şeklinde tanımlanabilmektedir. Bunun sebebi, geçiş aşamaları ile E yilidlerin geometrilerinin birbirlerine büyük oranda benzemesidir. Öte yandan, dihidrofuran geçiş aşamalarında C-C çift bağının uzayarak kapanmanın meydana gelmesi için gerekli geometriye ulaşabilmek için çift bağ etrafında dönmesi ve karben karbonuna yaklaşması gerekmektedir. Bakır atomu ise karben karbonuyla olan koordinasyonunu kaybetmek zorundadır ve karben karbonu, kendisine bağlı ester grupları ile piramidal bir geometriye ulaşmalıdır. Ester gruplarının karben karbonuyla sübstitüsyonu sonucu karben karbonunun elektron yoğunluğu ester grupları tarafından delokalizedir. Tüm bu geometrik ve elektronik etkiler sebebiyle dihidrofuran reaksiyonu 1,3-dioksol kapanmasına oranla büyük bir aktivasyon bariyeri gerektirmektdir. Yapılan hesaplamalara göre 6,6-dimetil-2-metilenbisiklo[3.1.1]heptan-3-on molekülünün tercihli kapanma ürünü 1,3-dioksol türevleridir. Anahtar Kelimeler: YFT, reaksiyon mekanizması, karbonil yilid
