Preparation of organosilicon-substituted borazenes

Stereospecific synthesis of silicon-stereogenic optically active silylboranes and general synthesis of chiral silyl Anions

Silicon-stereogenic optically active silylboranes could potentially allow the formation of chiral silyl nucleophiles as well as the synthesis of various chiral silicon compounds. However, the synthesis of such silicon-stereogenic silylboranes has not been achieved so far. Here, we report the synthesis of silicon-stereogenic optically active silylboranes via a stereospecific Pt(PPh3)4-catalyzed Si-H borylation of chiral hydrosilanes, which are synthesized by stoichiometric and catalytic asymmetric synthesis, in high yield and very high or perfect enantiospecificity (99% es in one case, and >99% es in the others) with retention of the configuration. Furthermore, we report a practical approach to generate silicon-stereogenic silyl nucleophiles with high enantiopurity and configurational stability using MeLi activation. This protocol is suitable for the stereospecific and general synthesis of silicon-stereogenic trialkyl-, dialkylbenzyl-, dialkylaryl-, diarylalkyl-, and alkylary benzyloxy-substituted silylboranes and their corresponding silyl nucleophiles with excellent enantiospecificity (>99% es except one case of 99% es). Transition-metal-catalyzed C-Si bond-forming cross-coupling reactions and conjugate-addition reactions are also demonstrated. The mechanisms underlying the stability and reactivity of such chiral silyl anion were investigated by combining NMR spectroscopy and DFT calculations.

Dismutation of Tricyanoboryllead Compounds: The Homoleptic Tetrakis(tricyanoboryl)plumbate Tetraanion

A series of unprecedently air-stable (tricyanoboryl)plumbate anions was obtained by the reaction of the boron-centered nucleophile B(CN)3 2- with triorganyllead halides. Salts of the anions [R3 PbB(CN)3 ]- (R=Ph, Et) were isolated and found to be stable in air at room temperature. In the case of Me3 PbHal (Hal=Cl, Br), a mixture of the anions [Me4-n Pb{B(CN)3 }n ]n- (n=1, 2) was obtained. The [Et3 PbB(CN)3 ]- ion undergoes stepwise dismutation in aqueous solution to yield the plumbate anions [Et4-n Pb{B(CN)3 }n ]n- (n=1-4) and PbEt4 as by-product. The reaction rate increases with decreasing pH value of the aqueous solution or by bubbling O2 through the reaction mixture. Adjustment of the conditions allowed the selective formation and isolation of salts of all anions of the series [Et4-n Pb{B(CN)3 }n ]n- (n=2-4) including the homoleptic tetraanion [Pb{B(CN)3 }4 ]4- .

Synthesis and Reactions of Borazines

Given the wide array of current applications of borazine-based materials, synthetic access to these compounds is of importance. This review summarizes the many ways of preparing borazines and its carbo-substituted analogues. In addition, the functionalization of borazines is covered. The synthesis of molecules incorporating more than one borazine units as well as aspects of unsymmetrically substituted borazines are not included. The literature has been covered comprehensively until the end of 2020.

1 Introduction: Structure and Properties of Borazine

2 Synthesis of Parent Borazine

3 Synthesis of N-Substituted Borazines

4 Synthesis of B-Halo/B-Halo-N-Substituted Borazines

5 Synthesis of B-Substituted Borazines

6 Synthesis of Polycyclic Borazines Containing One Borazine Ring

7 Modifications or Hydrolysis of the Borazine Ring

8 Borazine Metal Complexes

9 Outlook and Conclusion

Selective Route to Stable Silicon-Boron Radicals and Their Corresponding Cations

Herein, we report on a facile and selective one-pot synthetic route to silicon-boron radicals. Reduction of Br₂BTip (Tip = 2,4,6-ⁱPrC₆H₂) with KC₈ in the presence of LSi-R affords LSi(^tBu)-B(Br)Tip (1) and LSi(N(TMS)₂)-B(Br)Tip (2) [L = PhC(N^tBu)₂]. These first examples of silicon-boron isolated radical species feature spin density on the silicon and boron atoms. 1 and 2 exhibit extraordinary stability to high temperatures under inert conditions in solution and air stability in the solid state. Both radicals have been isolated and fully characterized by electron paramagnetic resonance spectroscopy, SQUID magnetometry, mass spectrometry, cyclic voltammetry, single-crystal X-ray structure analysis, and density functional theory calculations. Moreover, compound 1 exhibits one-electron transfer when treated with 1 equiv of AgSO₃CF₃ and [Ph₃C]⁺[B(C₆F₅)₄]^-, respectively, resulting in the corresponding cations [LSi(^tBu)-B(Br)Tip]⁺[CF₃SO₃]^- (3) and [LSi(^tBu)-B(Br)Tip]⁺[B(C₆F₅)₄]^- (4). Compounds 3 and 4 have been characterized with multinuclear NMR and mass spectrometry.

General Synthesis of Trialkyl- and Dialkylarylsilylboranes: Versatile Silicon Nucleophiles in Organic Synthesis

Compared to carbon-based nucleophiles, the number of silicon-based nucleophiles that is currently available remains limited, which significantly hampers the structural diversity of synthetically accessible silicon-based molecules. Given the high synthetic utility and ease of handling of carbon-based boron nucleophiles, silicon-based boron nucleophiles, i.e., silylboranes, have attracted considerable interest in recent years as nucleophilic silylation reagents that are activated by transition-metal catalysts or bases. However, the range of practically accessible silylboranes remains limited. In particular, the preparation of sterically hindered and functionalized silylboranes remains a significant challenge. Here, we report the use of rhodium and platinum catalysts for the direct borylation of hydrosilanes with bis(pinacolato)diboron, which allows the synthesis of new trialkylsilylboranes that bear bulky alkyl groups and functional groups as well as new dialkylarylsilylboranes that are difficult to synthesize via conventional methods using alkali metals. We further demonstrate that these compounds can be used as silicon nucleophiles in organic transformations, which significantly expands the scope of synthetically accessible organosilicon compounds compared to previously reported methods. Thus, the present study can be expected to inspire the development of efficient methods for novel silicon-containing bioactive molecules and organic materials with desirable properties. We also report the first ¹¹B{¹H} and ²⁹Si{¹H} NMR spectroscopic evidence for the formation of i-Pr₃SiLi generated by the reaction of i-Pr₃Si-B(pin) with MeLi.

Synthesis and structures of lithium alkoxytris(dimethylphenylsilyl)borates

The X-ray crystallography showed that the structures of the silyl-substituted borates are affected by the alkoxy groups.

New generation of organosilyl radicals by photochemically induced homolytic cleavage of silicon-boron bonds

Unlike bis(diethylamino)organosilylboranes, bis(diisopropylamino)organosilylboranes, which have UV absorption at longer wavelength than 300 nm, undergo photolysis to afford pairs of an organosilyl radical and a bis(diisopropylamino)boryl radical by homolytic scission of the silicon-boron bonds. Generation of organosilyl radical and organoboryl radical was confirmed by trapping experiments using TEMPO (2,2,6,6-tetramethylpiperidine N-oxyl). The organosilyl radical thus generated induces not only silylation of mono-olefins and silylative cyclization of dienes but also polymerization to afford polymers bearing organosilyl termini. On the other hand, the bis(diisopropylamino)boryl radical generated is not incorporated into the olefin adducts.