A Hydride‐Substituted Homoleptic Silylborate: How Similar is it to its Diborane(6)‐Dianion Isostere?

Abstract

The B-nucleophilic 9H-9-borafluorene dianion reacts with 9-chloro-9-silafluorene to afford air-and moisture-stable silylborate salts M[Ar 2 (H)BÀ Si(H)Ar 2 ] (M[HBSiH], M = Li, Na). Li[HBSiH] and Me 3 SiCl give the BÀ pyridine adduct Ar 2 (py)BÀ Si-(H)Ar 2 ((py)BSiH) or the chlorosilane Li[Ar 2 (H)BÀ Si(Cl)Ar 2 ] (Li[HBSiCl]) in C 6 H 6 -pyridine or THF. In both cases, the first step is H À abstraction at the B center. The resulting free borane subsequently binds a py or thf ligand. While the py adduct is stable at room temperature, the thf adduct undergoes a 1,2-H shift via the cyclic B(μ-H)Si intermediate BHSi, which is afterwards attacked at the Si atom by a Cl À ion to give Li[HBSiCl]. DFT calculations were employed to support the proposed reaction mechanism and to characterize the electronic structure of BHSi. Treatment of Li[HBSiCl] with the N-heterocyclic carbene IMe introduces the neutral donor at the Si atom and leads to Ar 2 (H)BÀ Si(IMe)Ar 2 (HBSi(IMe)), a donor-acceptor-stabilized silylene.

Catalyst-free diboration and silaboration of alkenes and alkynes using bis(9-heterofluorenyl)s

“…17 On the BSi side, the thf adduct 2(thf) of the elusive 2 can straightforwardly be generated from Na[4] through H − -abstraction with Me 3 SiBr at low temperatures ( Scheme 2 ; 2 C (thf) → 2 C + THF; Δ G diss = 10.5 kcal mol −1 ). 15,17 …”

“…With the aim of replacing the in situ generated 2 with an isolable analog of precisely controllable composition and purity, we switched from the Na[ 4 ]/Me 3 SiBr system to the chlorosilane 9 (thf) (Scheme 4). 15 As further asset, nucleophilic Cl-substitution reactions on the chlorosilyl-containing products could be included, in addition to hydrosilylation reactions, in the pool of possible late-stage derivatizations. Similar to 2 , treatment of 9 (thf) with ethylene under ambient conditions resulted in 1,2-silaboration within minutes to afford 10 (thf).…”

“…17 On the BSi side, the thf adduct 2(thf) of the elusive 2 can straightforwardly be generated from Na[4] through H − -abstraction with Me 3 SiBr at low temperatures (Scheme 2; 2 C (thf) / 2 C + THF; DG diss = 10.5 kcal mol −1 ). 15,17 According to our experience, the reactivity of anionic aryl boranes is oen strongly inuenced by the nature of their counter cations. 15,18 We have therefore conducted all transformations involving the [1] − anion with both Li [3] and K [3].…”

“…15,17 According to our experience, the reactivity of anionic aryl boranes is oen strongly inuenced by the nature of their counter cations. 15,18 We have therefore conducted all transformations involving the [1] − anion with both Li [3] and K [3]. Since in the present context the counter-cation dependence turned out to be marginal, we will henceforth refer only to the K + salts, as these gave the qualitatively best crystallographic analyses (all results obtained with the Li + salts are provided in the ESI †).…”

“…For the following reasons, we relied on our recently reported borafluorenyl derivative 2 to answer this question ( Fig. 1a ): 15 (i) the B center of 2 is part of a formally antiaromatic borole ring and therefore a particularly good electron acceptor, and (ii) it is exposed to its environment due to the planarized 2,2′-biphenylylene ligand. Moreover, the silafluorenyl part of 2 renders it a structural analog of corresponding bis(9-borafluorenyl) anions [1] − ( Fig.…”