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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).

Literatur:

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):

4715–4719.

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.