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Making a Cyclotrigermene from a Digermene Kiera McNeice, Vladimir Ya. Lee,* and Akira Sekiguchi* Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan
bS Supporting Information ABSTRACT: A straightforward method for the preparation of the cyclotrigermene cyclo-[(R3Si)4Ge3] 2 (R = SiMetBu2), based on the thermolysis of the digermene (R3Si)2Ged Ge(SiR3)2 1, is presented.
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n organic chemistry, cyclopropene and its derivatives represent a synthetically important class of unsaturated cyclic hydrocarbons. The very high strain energy of cyclopropene (52.2 kcal/ mol) gives rise to the extraordinary instability and reactivity of cyclopropene derivatives.1 By contrast, the analogues of cyclopropenes of the heavy group 14 elements cyclo-[R2E0 E(R)dE(R)] (E, E0 = Si, Ge, Sn) are remarkably stable, especially when substituted with electropositive σ-donating silyl groups (R = SiR3), because of the attractive πEdEσ*E0 R orbital mixing resulting in effective stabilization of the heavy cyclopropene’s HOMO (πEdE).2,3 Synthetically, heavy analogues of cyclopropenes only recently became available, and twenty stable derivatives have been isolated to date.2 Previously we synthesized a novel cyclotrigermene, cyclo-[(tBu2MeSi)4Ge3] (2) by the reaction of 1,1-dilithiogermane (tBu2MeSi)2GeLi24 with the tetrachlorodigermane tBu2MeSiGeCl2GeCl2SiMetBu2.5 Given the synthetic usefulness of 2 on one hand and its low yield (22%) and complicated experimental procedure on the other hand, we were motivated to develop an alternative method for the preparation of 2. Here we describe a simple approach to 2 from the readily available digermene (tBu2MeSi)2Ged Ge(SiMetBu2)2 (1).6 Preliminary results proved the stability of 1 at room temperature, implying the integrity of its GedGe bond that does not dissociate into germylenes >Ge:, unlike many other digermenes.3,6 However, upon solid-state thermolysis of 1 in an evacuated sealed tube for 1 h, we made the following observations: (1) below 150 °C no spectroscopically observable changes take place, (2) at 170 °C digermene 1 was converted into the cyclotrigermene 2, along with hydrosilane tBu2MeSiH (3) and tris(silyl)germyl radical (tBu2MeSi)3Ge• (4),7 formed in a molar ratio of 3:3:2 (Scheme 1). Vacuum distillation of 3 followed by sublimation of 4 and glovebox column chromatography of the residue gave pure 2 in 48% yield (Scheme 1).8 Given its evident simplicity, ease of separation, and higher yield of 2, this new method represents a very attractive alternative to the previously reported more complicated and lower-yielding procedure.5 Apart from its undoubted synthetic benefits, the r 2011 American Chemical Society
novel method is interesting from the viewpoint of the reaction pathways and intermediates involved in this unusual thermal transformation. Among several pathways leading to the formation of the observed products 2, 3, and 4, the following can be proposed as one of the most reasonable, as depicted in Scheme 2. The first step of the reaction is the breaking of the GedGe bond of digermene 1, forming a pair of germylene species (tBu2MeSi)2Ge: (5).9 The thermal generation of germylenes was unambiguously proved by the isolation of germacyclopentene 6, as a [1+4] cycloaddition product formed upon the co-thermolysis of 1 in the presence of 2,3-dimethylbuta-1,3-diene, an effective germylene-trapping reagent.8 In the absence of germylene traps, 5 may undergo partial disproportionation, forming transient monovalent germylyne (tBu2MeSi)Ge•: (7), along with the germyl radical 4 and hydrosilane 3 as the end products.10 This disproportionation is expectedly endothermic [ΔH = +24.1 kcal/mol for the isodesmic reaction 2 (tBu2MeSi)2Ge: (5) f (tBu2MeSi)Ge•: (7) + (tBu2MeSi)3Ge• (4)],11 given the intrinsically high instability of the open-shell germylyne 7. The overall endothermicity agrees with the high temperature required for the process, with no signs of the reaction below 150 °C and complete transformation only at 170 °C. Dimerization of the germylyne species 7, leading to digermyne 8 (or its valence isomer), may result in the final formation of cyclotrigermene 2 upon the [1+2] cycloaddition of germylene to a digermyne. The stoichiometry of the digermene thermolysis agrees reasonably well with the reaction scheme proposed above. In organic chemistry, the analogous thermal transformation of an alkene into a cyclopropene is unprecedented, given the major differences between the alkenes/carbenes and their germanium (or other heavy group 14 element E) analogues: the ease of the dissociation of the >EdE< bond into heavy carbene analogues >E: and the tendency of the latter to undergo disproportionation
Received: July 11, 2011 Published: August 08, 2011 4796
dx.doi.org/10.1021/om200619v | Organometallics 2011, 30, 4796–4797
Organometallics
NOTE
Scheme 1. Thermolysis of the Digermene 1 Leading to the Cyclotrigermene 2
Scheme 2. Proposed Reaction Pathways Involved in the Formation of the Cyclotrigermene 2
(8) Experimental procedures and spectral data of compounds 2 and 6: see the Supporting Information. (9) This process should be facilitated by the extreme twisting of the GedGe bond in 1 (see ref 6) because of its very bulky silyl substituents, resulting in a remarkable decrease in the bond dissociation energy. (10) Disproportionation of the transient germylenes/stannylenes forming germylynes/stannylynes was proposed several decades ago, although such monovalent species have been neither directly observed nor chemically trapped: (a) Davidson, P. J.; Hudson, A.; Lappert, M. F.; Lednor, P. W. J. Chem. Soc., Chem. Commun. 1973, 829. (b) Hudson, A.; Lappert, M. F.; Lednor, P. W. J. Chem. Soc., Dalton Trans. 1976, 2369. (c) Sita, L. R.; Kinoshita, I. J. Am. Chem. Soc. 1991, 113, 1856. (11) Geometry optimizations and energy calculations for the compounds 4, 5, and 7 were performed at the UB3LYP/6-31G(d) level with the GAUSSIAN 03 program package.
versus the rigidity/stiffness of the >CdC< bond and reluctance of the carbene species >C: to disproportionate.
’ ASSOCIATED CONTENT
bS
Supporting Information. Experimental procedures and spectral data for compounds 2 and 6. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected];
[email protected]. ac.jp.
’ REFERENCES (1) Wiberg, K. B. In Reactive Intermediates Chemistry; Moss, R. A.; Platz, M. S.; Jones, M., Jr., Eds.; Wiley: Hoboken, NJ, 2004; Chapter 15. (2) G€oller, A.; Heydt, H.; Clark, T. J. Org. Chem. 1996, 61, 5840. (3) Lee, V. Ya.; Sekiguchi, A. Organometallic Compounds of LowCoordinate Si, Ge, Sn and Pb: From Phantom Species to Stable Compounds; Wiley: Chichester, 2010; Chapter 5. (4) (a) Sekiguchi, A.; Izumi, R.; Ihara, S.; Ichinohe, M.; Lee, V. Ya. Angew. Chem., Int. Ed. 2002, 41, 1598. (b) Lee, V. Ya.; Sekiguchi, A. Inorg. Chem. 2011(Forum Article), published online June 1, 2011 http://dx.doi.org/ 10.1021/ic2006106. (5) Lee, V. Ya.; Yasuda, H.; Ichinohe, M.; Sekiguchi, A. Angew. Chem., Int. Ed. 2005, 44, 6378. (6) Lee, V. Ya.; McNeice, K.; Ito, Y.; Sekiguchi, A. Chem. Commun. 2011, 3272. (7) Sekiguchi, A.; Fukawa, T.; Nakamoto, M.; Lee, V. Ya.; Ichinohe, M. J. Am. Chem. Soc. 2002, 124, 9865. 4797
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