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Generation of Bis(Ferrocenyl)Silylenes from Siliranes

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Generation of Bis(Ferrocenyl)Silylenes from Siliranes molecules Article y Generation of Bis(ferrocenyl)silylenes from Siliranes Yang Pan 1, Shogo Morisako 2 , Shinobu Aoyagi 1 and Takahiro Sasamori 1,2,* 1 Graduate School of Science, Nagoya City University, Nagoya, Aichi 467-8501, Japan; [email protected] (Y.P.); [email protected] (S.A.) 2 Division of Chemistry, Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan; [email protected] * Correspondence: [email protected]; Tel.: +81-29-853-4412 Dedicated to Professor Yosuke Yamamoto on the occasion of his 65th birthday. y Academic Editor: M.-H. Whangbo Received: 23 November 2020; Accepted: 9 December 2020; Published: 14 December 2020 Abstract: Divalent silicon species, the so-called silylenes, represent attractive organosilicon building blocks. Isolable stable silylenes remain scarce, and in most hitherto reported examples, the silicon center is stabilized by electron-donating substituents (e.g., heteroatoms such as nitrogen), which results in electronic perturbation. In order to avoid such electronic perturbation, we have been interested in the chemistry of reactive silylenes with carbon-based substituents such as ferrocenyl groups. Due to the presence of a divalent silicon center and the redox-active transition metal iron, ferrocenylsilylenes can be expected to exhibit interesting redox behavior. Herein, we report the design and synthesis of a bis(ferrocenyl)silirane as a precursor for a bis(ferrocenyl)silylene, which could potentially be used as a building block for redox-active organosilicon compounds. It was found that the isolated bis(ferrocenyl)siliranes could be a bottleable precursor for the bis(ferrocenyl)silylene under mild conditions. Keywords: silirane; silylene; ferrocenyl group 1. Introduction Silylenes, i.e., divalent silicon analogues of carbenes, represent highly attractive reactive intermediates due to their considerable potential as building blocks for organosilicon compounds [1–7]. There are several examples of stable silylenes, which are usually stabilized thermodynamically by introduction of heteroatom-based substituents such as amino groups. For example, silicon analogues of N-heterocyclic carbenes (NHCs), N-heterocyclic silylenes (NHSis) [8–17], have been isolated as stable compounds (e.g., I [8,11] and II [9,13]) as well as the N-substituted cyclic silylene III [14] (Chart1). However, such thermodynamically stabilized silylenes are too stable, i.e., they usually do not exhibit considerable reactivity as, e.g., building blocks for organosilicon compounds. Conversely, carbon-substituted silylenes, which do not contain a stabilizing electron-donating ligand, are highly reactive due to their considerable electrophilicity on account of their low-lying LUMOs [18–24]. Such highly reactive silylenes can be stabilized kinetically by sterically demanding substituents and subsequently being isolated under an inert atmosphere [25–28]. For example, dicoordinated, carbon-substituted silylenes [29–32] such as IV [29] and V [32] that bear sterically demanding substituents have been isolated and well characterized (Chart1). However, it is very di fficult to synthesize and isolate such silylenes as they usually require laborious and technically exigent synthetic procedures. In this context, easily accessible and bottleable precursors for carbon-substituted silylenes would be an attractive research target. In most cases, silylenes can be generated by reduction of a dihalosilane or photolysis of a trisilane [4,7,33]. When the substituents on the central silicon atom offer insufficient steric protection, the products from the aforementioned reactions usually afford complex Molecules 2020, 25, 5917; doi:10.3390/molecules25245917 www.mdpi.com/journal/molecules Molecules 2020, 25, x 2 of 12 Molecules 2020, 25, x 2 of 12 Moleculessteric protection,2020, 25, 5917 the products from the aforementioned reactions usually afford complex mixtures2 of of 12 oligosilanes.steric protection, Conversely, the products most from cases the of aforementi the reductonedion reactionsof sterically usually hindered afford dihalosilanescomplex mixtures or the of photolysis of trisilanes that bear bulky substituents result in the facile formation of a disilene, i.e., the mixturesoligosilanes. of oligosilanes. Conversely, Conversely, most cases mostof the cases reduct of theion reduction of sterically of sterically hindered hindered dihalosilanes dihalosilanes or the formalphotolysis dimer of trisilanesof a silylene that [33–39]. bear bulky To preparesubstituents a stable result silylene in the precursor, facile formation it should of a thus disilene, be required i.e., the orto introduce the photolysis very ofbulky trisilanes substituents that bear on bulkythe silicon substituents atom to resultavoid facile in the dimerization facile formation of the of generated a disilene, i.e.,formal the dimer formal of dimer a silylene of a silylene[33–39]. [To33 –prepare39]. To a prepare stable silylene a stable precursor, silylene precursor, it should itthus should be required thus be silylenesto introduce [33]. very On bulky the other substituents hand, too on bulkythe silicon substitu atoments to avoidcan be facile expected dimerization to negatively of the generatedaffect the requiredinherent toreactivity introduce of verythe bulkyresulting substituents silylene. onIn theorder silicon to find atom the to avoidright facilebalance dimerization in this trade-off of the generatedsilylenes [33]. silylenes On the [33 other]. On hand, the other too bulky hand, toosubstitu bulkyents substituents can be expected can be to expected negatively to negatively affect the relationship,inherent reactivity transient of silylenesthe resulting with asilylene. combination In order of very to bulky find andthe lessright hindered balance groupsin this have trade-off been adesigned.ffect the inherent Our group reactivity has already of the reported resulting the silylene. design Inand order synthetic to find utility the right of the balance sterically in this demanding trade-off relationship, transienttransient silylenessilylenes withwith aa combination ofof very bulky and less hindered groups have been 2,5-bis(3,5-di-designed. Ourt -butylphenyl)-1-ferrocenylgroup has already reported the(Fc*) design group and [40–42],synthetic andutility this of thegroup sterically should demanding be an designed.appropriate Our substituent group has fo alreadyr bis(ferrocenyl)silylene reported the design 1 and(Chart synthetic 1), which utility bears of the Fc* sterically and ferrocenyl demanding (Fc) 2,5-bis(3,5-di-tt-butylphenyl)-1-ferrocenyl-butylphenyl)-1-ferrocenyl (Fc*) (Fc*) group group [40– 42[40–42],], and thisand group this should group be should an appropriate be an groupsappropriate on the substituent silicon center for [43].bis(ferrocenyl)silylene Silylene 1 can be expected 1 (Chart to 1), (i) which be very bears reactive Fc* dueand toferrocenyl its low-lying (Fc) substituentLUMO level, for and bis(ferrocenyl)silylene (ii) show intriguing redox1 (Chart activity1), which on account bears Fc* of andits two ferrocenyl ferrocenyl (Fc) moieties. groups on the silicongroups centeron the [silicon43]. Silylene center1 [43].can Silylene be expected 1 can to be (i) expected be very reactiveto (i) be duevery to reactive its low-lying due to LUMOits low-lying level, andLUMO (ii) level, show and intriguing (ii) show redox intriguing activity redox on account activity of on its twoaccount ferrocenyl of its two moieties. ferrocenyl moieties. tBu target tBu Dip Me Si SiMe PR 3 3 3 Fe N N tBu N target tBu Dip Me3Si SiMe3 PR3 Si: Si: Si: Si: Si: Fe Si: N N N Dtp N N N Si: Si: Si: Si: Si: SiMe3 Si:Dtp tBu Dip Me3Si PR Dtp N N tBu N 3 Fe Dtp IIIIIIDip Me Si IVSiMe3 V PR 1 tBu tBu 3 3 Fe Dip = 2,6-diisopropylphenyl R = Ph or tol Dtp = 3,5-di(t-butyl)phenyl IIIIII IV V 1 Dip = 2,6-diisopropylphenyl R = Ph or tol Dtp = 3,5-di(t-butyl)phenyl Chart 1. Examples of isolable dicoordinated silylenes. Chart 1. Examples of isolable dicoordinated silylenes. Chart 1. Examples of isolable dicoordinated silylenes. 2. Results and Discussion 2. Results and Discussion 2. ResultsReaction and of Discussion ferrocenyl (Fc) lithium with SiCl4 in toluene resulted in the formation of FcSiCl3 (2) Reaction of ferrocenyl (Fc) lithium with SiCl in toluene resulted in the formation of FcSiCl [44], which was used in the following reactions withou4 t further purification due to its lability toward3 (2)[44Reaction], which of was ferrocenyl used in the (Fc) following lithium reactionswith SiCl without4 in toluene further resulted purification in the dueformation to its lability of FcSiCl toward3 (2) moisture. Subsequently, 2 was treated in Et2O at r.t. with Fc*Li (3) [40–42], which was prepared by moisture.[44], which Subsequently, was used in the2 was following treated reactions in Et2O withou at r.t. witht further Fc*Li purification (3)[40–42], due which to its was lability prepared toward by the reaction of Fc*Br (Fc* = 2,5-bis(3,5-di(t-buthyl)phenyl)-1-ferrocenyl) with n-BuLi in Et2O, to give themoisture. reaction Subsequently, of Fc*Br (Fc* 2= was2,5-bis(3,5-di( treated int -buthyl)phenyl)-1-ferrocenyl)Et2O at r.t. with Fc*Li (3) [40–42], with whichn-BuLi was in EtpreparedO, to give by the corresponding bis(ferrocenyl)dichlorosilane (4) as a stable orange solid in 44% yield (Scheme2 1). the correspondingreaction of Fc*Br bis(ferrocenyl)dichlorosilane (Fc* = 2,5-bis(3,5-di(t-buthyl)phenyl)-1-ferrocenyl) (4) as a stable orange solid with in n 44%-BuLi yield in Et (Scheme2O, to give1). Although single crystals suitable for an X-ray diffraction (XRD) analysis could not be obtained due Althoughthe corresponding single crystals bis(ferrocenyl)dichlorosilane suitable for an X-ray diff (raction4) as a stable (XRD) orange analysis solid could in not44% be yield obtained (Scheme due 1). to to partial hydrolysis that furnished Fc*FcSi(OH)2 [45], 4 was characterized by spectroscopic partialAlthough hydrolysis single crystals that furnished suitable Fc*FcSi(OH) for an X-ray diffraction[45], 4 was characterized(XRD) analysis by could spectroscopic not be obtained techniques.
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