Synthesis and Properties of Isolable Unsaturated Organosilicon Compounds Protected by a Bidentate Alkyl Group Featuring 3,5-Di-tert-butyl-4-methoxyphenyl Group

概要

Whether silicon can form compounds similar to carbon or not has been a question for scientists since the development of the periodic table.1-4 In contrast to the organic compounds, most of unsaturated silicon compounds (e.g. silicon analogues of alkenes (disilenes), ketones (silanones) etc.) are highly reactive because the multiple bonds that involve silicon atom are weaker than carbon.5 Thus, the isolation of such silicon compounds had been regarded as impossible task until the early 1980’s. However, West demonstrated the isolation of disilene 1 by utilizing steric protection in 1981 (Figure 1).6 Since this seminal study, various low-coordinate and unsaturated silicon compounds such as disilene, silylene and disilyne stabilized by kinetic stabilization and/or thermodynamic stabilization have been synthesized.7-10 These isolable silicon compounds enable us to reveal their characteristic properties and understand the nature of silicon compounds. Thus, the development of elaborate molecular design has been continued to open up new chemistry. It is very important task though challenging.

Our group isolated unsaturated silicon compounds such as silylene 4 by utilizing 1,1,4,4-tetrakis(trimethylsilyl)butane-1,4-diyl group A.11 4 is essentially stabilized by the steric demand of A, and hence 4 possesses unperturbed two-coordinate silicon atom that exhibit intrinsic properties of silylene.12 Furthermore, protecting group A enable us to isolate related silicon unsaturated compounds and to reveal their structure and properties (Figure 2).13 Though A successfully provided many insights into the unsaturated silicon compounds, there is a room to improve; for instance, the steric demand of A is not enough in the case of isolable silanones (silicon analogues of ketones).14

Inspired by the design of protecting group A, the author came up with the utilizing 3,5-di-tert-butyl-4-methoxy phenyl group that are more bulky and robust than silyl group. In this work, the author developed new protecting group: 1,1,4,4-tetrakis(3,5-di-tert-butyl-4-methoxy phenyl)butane-1,4-diyl group B and applied it for the isolation of unsaturated silicon compounds 10 to 13 that have not been isolated yet (Figure 3). Through the successful synthesis of unsaturated silicon compounds 10 to 13 by utilizing protecting group B and the exploration of their nature, the author clearly showed the usefulness of new protecting group B and confirmed that the elaborate molecular design open up new chemistry.

In chapter 2, the author describes the successful synthesis and structure analysis of the first isolable dialkylsilanone 11 that contains a genuine Si=O bond protected by B. The background, molecular design, and detailed synthesis of 11 from silylene 10 were described in this chapter.

In chapter 3, the author describes the reactions of silanone 11 which reflect the highly polarized nature of the genuine Si=O bond in 11.

In chapter 4, the synthesis and properties of an NHC-stabilized disilavinylidene 12 were described. The author proved that the designed protecting group B is applicable for the isolation of another unsaturated silicon compounds such as 12. During this study, a new tetrasilicon analogue of bicyclo[1.1.0]but-1(3)-ene, 13, were also obtained.

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