Tridentate Lewis Acids Based on 1,3,5‐Trisilacyclohexane Backbones and an Example of Their Host–Guest Chemistry

Abstract

Directed tridentate Lewis acids based on the 1,3,5-trisilacyclohexane skeleton with three ethynyl groups [CH2Si(Me)(C2H)]3 were synthesised and functionalised by hydroboration with HB(C6F5)2, yielding the ethenylborane {CH2Si(Me)[C2H2B(C6F5)2]}3, and by metalation with gallium and indium organyls affording {CH2Si(Me)[C2M(R)2]}3 (M = Ga, In, R = Me, Et). In the synthesis of the backbone the influence of substituents (MeO, EtO and iPrO groups at Si) on the orientation of the methyl group was studied with the aim to increase the abundance of the all-cis isomer. New compounds were identified by elemental analyses, multi-nuclear NMR spectroscopy and in some cases by IR spectroscopy. Crystal structures were obtained for cis-trans-[CH2Si(Me)(Cl)]3, all-cis-[CH2Si(Me)(H)]3, all-cis-[CH2Si(Me)(C2H)]3, cis-trans-[CH2Si(Me)(C2H)]3 and all-cis-[CH2Si(Me)(C2SiMe3)]3. A gas-phase electron diffraction experiment for all-cis-[CH2Si(Me)(C2H)]3 provides information on the relative stabilities of the all-equatorial and all-axial form; the first is preferred in both solid and gas phase. The gallium-based Lewis acid {CH2Si(Me)[C2Ga(Et)2]}3 was reacted with a tridentate Lewis base (1,3,5-trimethyl-1,3,5-triazacyclohexane) in an NMR titration experiment. The generated host-guest complexes involved in the equilibria during this reaction were identified by DOSY NMR spectroscopy by comparing measured diffusion coefficients with those of the suitable reference compounds of same size and shape.

Hexadentate Poly‐Lewis Acids Based on 1,3,5‐Trisilacyclohexane

“…As expected, the corresponding carbon atoms in equatorial position (C(7)/C(11)/C(15)) have significantly larger distances of 7.738(4) to 8.139(4) Å to one each other. These are comparable to distances of the trisilacyclohexane derivative cyclo ‐(−Si(CCH)(CH 3 )−CH 2 −) 3 with all alkyne groups in equatorial positions (d(C−C): 7.648(1) to 7.849(1) Å) [17b] . Molecule 3 is deformed (for full structure of the asymmetric unit see S.I.…”

Tridentate Lewis Acids: Boron‐, Silicon‐ and Gallium‐Functionalised Tris(dimethylsilyl)methanes

“…The donor-free C 3 -symmetric scaffold 5 with freely rotatable ethynyl groups was treated, in analogy to earlier work by our group, [11,12] with GaMe 3 or GaEt 3 in alkane elimination reactions to afford compounds 8 and 9 in quantitative yields (Scheme 3). The reaction with GaMe 3 required 72 h for completion at room temperature, whereas that with GaEt 3 took 24 h under the same conditions.…”

“…In the majority of cases the rigid backbones used are anthracenes functionalised with boron, [4] aluminium, [5] gallium [5] and indium, [5] but also naphthalenes functionalised with mercury, [6] boron [7,8] and other Lewis acid atoms. [7,9,10] Recently we reported some tridentate Lewis acids based on 1,3,5-trisilacyclohexane and equipped with boron, [11,12] gallium, [11,12] indium [12] and tin [13] functions. Such molecules are semi-flexible poly-Lewis acids with directed functions because of the low barrier to ring inversion of the 1,3,5-trisilacyclohexanes.…”

“…[15] Interactions between the tridentate Lewis acid 1,3,5-tris[(diethylgallanyl)ethynyl]-1,3,5-trimethyl-1,3,5-trisilacyclohexane and the substrate 1,3,5-trimethyl-1,3,5-triazacyclohexane have been studied by using the same methodology. [12] As mentioned above, most poly-Lewis acids are based on flexible frameworks, for example, spherical [16] or cyclic poly-Lewis acids containing silicon, [17] tin [18] and mercury [19,20] functions. A prominent example of a mercury-based poly-Lewis acid was synthesised by Sartori and Golloch; [20] their tridentate Lewis acid is able to form complexes with carbonyl groups and acetylenes.…”

Hydrogen-bond-induced selectivity of a head-to-head photo-dimerisation of dialkynylanthracene – access to tetradentate Lewis acids

“…3f,9 However, while these difunctional receptors are comparatively simple, those with more than two binding sites are rather scarce. 10 Tetradentate derivatives, with a doubled number of functions, are in principle accessible in the form of the corresponding head-to-head anthracene photodimers B instead of their monomers A (Scheme 1). 11 However, the main problem associated with photo-dimerising 1,8-functional anthracenes is the formation of a mixture of two isomers, syn (head-to-head) and anti (head-to-tail).…”