Ring-opening mechanism of disilacyclopropylidenoids and trisilacyclopropylidenoid: A theoretical study Akın Azizoglu a, * , Cem Burak Yıldız a, b a Laboratory of Computational Chemistry, Department of Chemistry, University of Balikesir, TR-10145 Balikesir, Turkey b Department of Chemistry, University of Aksaray, TR-68100 Aksaray, Turkey article info Article history: Received 28 November 2011 Received in revised form 10 May 2012 Accepted 14 May 2012 Keywords: Silaallene Silacyclopropylidene Reaction mechanism Theoretical calculation MP2 abstract Ab-initio and density functional theory calculations have been performed to investigate the ring-opening reactions of 1-bromo-1-lithiodisilirane (5), 3-bromo-3-lithiodisilirane (6), and 1-bromo-1-lithiotrisilirane (7) to 1,2-disilaallene (9), 1,3-disilaallene (10), and trisilaallene (11), respectively. Formally, the ring-opening mechanism of silacyclopropylidenoids may be either concerted or stepwise involving the intermediacy of a free silacyclopropylidene or cyclopropylidene. The ring-opening of 5 to 9 can proceed in both concerted and stepwise mechanism, where high activation energy barriers need to be overcome in order to open the silacyclopropylidene ring and to generate 9. In contrast, the ring-opening reactions of 6 and 7 can occur in a concerted fashion. The activation energy barrier for the isomerization of 6 to the complex of 10 with LiBr was determined to be only 2.3 kcal/mol at the B3LYP/6-31G(d) level, and the reaction is highly exothermic, by 37.0 kcal/mol, which makes this reaction for a promising strategy for the synthesis of 1,3-disilaallenes. However, the barrier for the conversion of 7 to 11 is calculated to be quite high, 27.5 kcal/mol, and in this case the reaction is endothermic, by 8.8 kcal/mol. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction Silaallenes have attracted considerable attention since their synthesis in 1993 by West [1,2], and have recently been the focus of intensive investigations [3e6]. Studies have revealed that silaallenes exhibit the unique structures and the broad differences in their properties compared to the corresponding carbon compounds [7]. The isolated 1-silaallenes were stabilized by an extremely large steric hindrances around the Si]C]C moiety and characterized by X-ray techniques, revealing that they are slightly bent (173.5  and 174.2  ) [2,8], in contrast to the carbon analogue allene, which is linear. However, 2-silaallenes have never been isolated but only postulated as transient species in some chemical reactions [9e11]. Besides the synthesis, silaallenes have been the subject of several theoretical investigations [12e19]. The important one from the group of Apeloig proposed that the ring-opening of disilacyclopropylidenes via DoeringeMooreeSkattebøl reaction [20e27] provides a possible route to 1,3-disilaallenes, R 2 Si]C]SiR 2 , still unknown experimen- tally [17]. The first trisilaallene was synthesized in 2003 by Kira and co- workers [28]. Interestingly, trisilaallene has a flexible Si]Si]Si skeleton, in which a bending angle is 136.5  . Recently, Apeloig and co- workers have performed another theoretical quantum-mechanical study on trisilaallene, and reported that unlike H 2 C]C]CH 2 which is linear, H 2 Si]Si]SiH 2 is highly bent at the central silicon atom, with a SiSiSi bending angle of 69.4  . However, boryl-substituted tri- silaallenes have linear classical allenic-type structures [19]. More recently, we have performed a series of ab-initio and DFT calculations on the ring-openings of lithium bromosilacyclopro- pylidenoids via DoeringeMooreeSkattebøl reaction; 1-bromo-1- lithiosilirane (1) and 2-bromo-2-lithiosilirane (2) to 2-silallene (3) and 1-silaallene (4), respectively (Scheme 1) [18]. The theoretical results indicate that the ring-opening reaction of 1-bromo-1- lithiosilirane (1) to the complex of 2-silaallene (3) can occur in a stepwise fashion with the intermediacy of a free silacyclopropy- lidene, whereas that of 2-bromo-2-lithiosilirane (2) to the complex 1-silaallene (4) can proceed in a concerted fashion. Moreover, the ring-opening reaction of 1 is endothermic by 11.0 kcal/mol, while that of 2 is highly exothermic, by 44.3 kcal/mol. On the other hand, the ring-opening reaction for 2 is thermodynamically more favourable by 55.3 kcal/mol than the ring-opening of 1 . Hence, further thorough investigations on silaallenes and silacyclopropy- lidenoids will be of important theoretical and practical values. In the present paper, we report the results of theoretical calculations on 1-bromo-1-lithiodisilirane (5), 3-bromo-3- lithiodisilirane (6), 1-bromo-1-lithiotrisilirane (7), and their corre- sponding free divalent ones (12, 13, and 14)(Scheme 2). We also * Corresponding author. Tel.: þ90 266 6121000/4113; fax: þ90 266 6121215. E-mail address: azizoglu@balikesir.edu.tr (A. Azizoglu). Contents lists available at SciVerse ScienceDirect Journal of Organometallic Chemistry journal homepage: www.elsevier.com/locate/jorganchem 0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jorganchem.2012.05.007 Journal of Organometallic Chemistry 715 (2012) 19e25