NHC-Silyliumylidene Cation-Catalyzed Hydroboration of Isocyanates with Pinacolborane.

The low-oxidation-state silicon-catalyzed hydroboration of isocyanates with pinacolborane (HBpin) using the NHC-silyliumylidene cation catalyst [(IMe)2SiH]I (1, IMe = :C{N(Me)C(Me)}2) is described. In the catalysis, the Si lone pair electrons activate isocyanates, and the latter react with HBpin to form N-boryl formamides at room temperature. Catalyst 1 further activates N-boryl formamides at 70 °C, the intermediates of which react with HBpin to form N-boryl methylamines and (pinB)2O.

A Versatile NHC-Parent Silyliumylidene Cation for Catalytic Chemo- and Regioselective Hydroboration.

This study describes the first use of a silicon(II) complex, NHC-parent silyliumylidene cation complex [(IMe)2SiH]I (1, IMe = :C{N(Me)C(Me)}2) as a versatile catalyst in organic synthesis. Complex 1 (loading: 10 mol %) was shown to act as an efficient catalyst (reaction time: 0.08 h, yield: 94%, TOF = 113.2 h-1; reaction time: 0.17 h, yield: 98%, TOF = 58.7 h-1) for the selective reduction of CO2 with pinacolborane (HBpin) to form the primarily reduced formoxyborane [pinBOC(═O)H]. The activity is better than the currently available base-metal catalysts used for this reaction. It also catalyzed the chemo- and regioselective hydroboration of carbonyl compounds and pyridine derivatives to form borate esters and N-boryl-1,4-dihydropyridine derivatives with quantitative conversions, respectively. Mechanistic studies show that the silicon(II) center in complex 1 activated the substrates and then mediated the catalytic hydroboration. In addition, complex 1 was slightly converted into the NHC-borylsilyliumylidene complex [(IMe)2SiBpin]I (3) in the catalysis, which was also able to mediate the catalytic hydroboration.

Donor-Acceptor Stabilized Tetra(silanimine).

The synthesis of an oligo(silanimine) is described. The reaction of the amidinato silylene [LSiN(SiMe3)2] (1, L = PhC(NtBu)2) with SiI4 in toluene afforded a mixture of the silanimine [LSi(I)NSiI3] (2), SiMe3I, and Si2I6. The mechanistic studies showed that 1 reacts with SiI4 to form the silyl ionic intermediate "{LSi(I)N(SiMe3)2}+{SiI3}-", which then eliminates SiMe3I and "SiI2" to form the silanimine intermediate "LSi(I)NSiMe3". It further undergoes a substitution with another molecule of SiI4 to form a mixture of 2 and SiMe3I. In addition, "SiI2" undergoes an oxidative addition with SiI4 to form Si2I6. Subsequently, compound 2 reacted with [LiN(SiMe3)Ar] to form the silanimine [LSi(I)NSiI2N(SiMe3)Ar] (6, Ar = 2,6-iPr2C6H3), which was then treated with KC8 to give the donor-acceptor stabilized tetra(silanimine) [LSiN(SiMe3)SiNAr]2 (7). It comprises four formal silanimine ">Si═N-" units, which are linked together. Compounds 2, 6, and 7 were characterized by NMR spectroscopy and X-ray crystallography.

A Dimeric NHC-Silicon Monotelluride: Synthesis, Isomerization, and Reactivity.

The reaction of the NHC-disilicon(0) complex [(IAr )Si=Si(IAr )] (1, IAr =:C{N(Ar)C(H)}2 , Ar=2,6-iPr2 C6 H3 ) with two equiv of elemental Te in toluene at room temperature for three days afforded a mixture of the first dimeric NHC-silicon monotelluride [(IAr )Si=Te]2 (2) and its isomeric complex [(IAr )Si(µ-Te)Si(IAr )=Te] (3). When the same reaction was performed for ten days, only 3 was isolated from the reaction mixture. Compound 1 reacted with four equiv of elemental Te in toluene for four weeks, which proceeded through the formation of 2, 3 and the NHC-disilicon tritelluride complex [{(IAr )Si(=Te)}2 Te] (5-Te), to form the dimeric NHC-silicon ditelluride [(IAr )Si(=Te)(µ-Te)]2 (4). The reactions are in line with theoretical mechanistic studies for the formation of 4. Compound 3 reacted with one equiv of elemental sulfur in toluene to form the first NHC-disilicon sulfur ditelluride complex [{(IAr )Si(=Te)}2 S] (5-S).

Trapping a Silicon(I) Radical with Carbenes: A Cationic cAAC-Silicon(I) Radical and an NHC-Parent-Silyliumylidene Cation.

The trapping of a silicon(I) radical with N-heterocyclic carbenes is described. The reaction of the cyclic (alkyl)(amino) carbene [cAACMe ] (cAACMe =:C(CMe2 )2 (CH2 )NAr, Ar=2,6-iPr2 C6 H3 ) with H2 SiI2 in a 3:1 molar ratio in DME afforded a mixture of the separated ion pair [(cAACMe )2 Si:. ]+ I- (1), which features a cationic cAAC-silicon(I) radical, and [cAACMe -H]+ I- . In addition, the reaction of the NHC-iodosilicon(I) dimer [IAr (I)Si:]2 (IAr =:C{N(Ar)CH}2 ) with 4 equiv of IMe (:C{N(Me)CMe}2 ), which proceeded through the formation of a silicon(I) radical intermediate, afforded [(IMe )2 SiH]+ I- (2) comprising the first NHC-parent-silyliumylidene cation. Its further reaction with fluorobenzene afforded the CAr -H bond activation product [1-F-2-IMe -C6 H4 ]+ I- (3). The isolation of 2 and 3 confirmed the reaction mechanism for the formation of 1. Compounds 1-3 were analyzed by EPR and NMR spectroscopy, DFT calculations, and X-ray crystallography.

Reactivity of an amidinato silylene and germylene toward germanium(ii), tin(ii) and lead(ii) halides.

The coordination chemistry of an amidinato silylene and germylene toward group 14 element(ii) halides is described. The reaction of the amidinato silicon(ii) amide [LSiN(SiMe3)2] (1, L = PhC(NtBu)2) with SnCl2 and PbBr2 afforded the amidinato silylene-dichlorostannylene and -dibromoplumbylene adducts [L{(Me3Si)2N}SiEX2] (E = Sn, X = Cl (2); E = Pb, X = Br (3)), respectively, in which there is a lone pair of electrons on the Sn(ii) and Pb(ii) atoms. X-ray crystallography, NMR spectroscopy and theoretical studies show conclusively that the Si(ii)-E(ii) bonds are donor-acceptor interactions. Similar electronic structures were found in the amidinato germylene-dichlorogermylene and -dichlorostannylene adducts [L{(Me3Si)2N}GeECl2] (E = Ge (5), Sn (6)), which were prepared by treatment of the amidinato germanium(ii) amide [LGeN(SiMe3)2] (4) with GeCl2·dioxane and SnCl2, respectively.