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Theoretical Investigation of Electrocyclic Reactions of Siloles

Fig. 1: Formation of silole 2 (and subsequent isomerisation to silole 3) which can dimerise to three different products.

Fig. 2: Barrierless Diels-Alder reaction of the silole (red) with 2,3-dimethylbutadiene can lead to two different products.

Siloles are five-membered heterocyclic dienes, in which a silicon atom replaces one of the carbon atoms of cyclopentadiene. Silole derivatives find application as building units for σ- and π-conjugated compounds for use in organic electroluminescent devices,[1]  and their synthesis, reactions, and properties have been subject to intensive studies.[2]  In an earlier investigation, the 1-chlorosilacyclopentene 1 has been obtained starting from perchlorinated silanes, Li[SitBu3], and 2,3-dimethylbutadiene[3]  and siloles can readily be obtained from this compound by deprotonation and LiCl elimination. In a recent combined experimental and theoretical study we have shown that the resulting siloles are very reactive and can isomerise, dimerise or be trapped by reagents like cyclohexene or 2,3-dimethylbutadiene, and the mechanisms underlying these reactions have been investigated in detail by means of double-hybrid density functional theory calculations.[4] Silole 2 can easily isomerise to the more stable silole 3 (Figure 1) via hydrogen shift. Alternatively, it can also dimerise to yield several products: two dimers are formed via Diels-Alder reactions (2+3 and 2+2) and the last one is an head-to-head dimer of 2 and its isomer 3. This latter, more unusual product is accessible via both singlet and triplet intermediates (Figure 1). Further, compound 2 can be trapped prior to isomerisation or dimerisation by addition of excess cyclohexene yielding the racemate of the [4+2] cycloadduct. Under similar conditions, the reaction of 2 with 2,3-dimethylbutadiene yields the [2+4] and [4+2] cycloadducts and hence, 2 acts as silene (dienophile) as well as diene. Our results show that both reaction steps occur without activation barriers (Figure 2).


1. S. Yamaguchi and K. Tamao (1998), J. Chem. Soc. Dalton Trans. 22: 3693–3702.

2. J. Dubac, A. Laporterie, and G. Manuel (1990), Chem. Rev., 1 (90): 215–263.

3. F. Meyer-Wegner, A. Nadj, M. Bolte, N. Auner, M. Wagner, M.C. Holthausen, and H.-W. Lerner (2011), Chem. Eur. J., 17 (17):


4. F. Meyer-Wegner, J.H. Wender, K. Falahati, T. Porsch, T. Sinke, M. Bolte, M. Wagner, M.C. Holthausen, and H.-W. Lerner (2014), Chem. Eur. J., 16 (20): 4681–4690.