Past one decade has witnessed the tremendous growth in the field of carbenes stabilized low-valent silicon compounds owing to very exciting properties of these molecules. Herein, we have employed a bicyclic (alkyl)(amino)carbene (MeBICAAC) to explore the low-valent chemistry of silicon compounds. The reduction of bicyclic (alkyl)(amino)carbene-SiCl4 complex, [(MeBICAAC)SiCl4] (1) with KC8 afforded their low-valent Si complexes, including Si(III) radical [(MeBICAAC)SiCl3] (2) and a complex with silicon center in a formal zero-valent state [(MeBICAAC)2Si] (3). Similarly, the reduction of in-situ generated MeBICAAC adduct of Me2SiCl2 with one equivalent of KC8 leads to the formation of [(MeBICAAC)SiMe2Cl] (4) complex having an unpaired electron. All these complexes have been characterized by IR, UV-Vis., NMR (for 1), HRMS and their solid-state structures were also elucidated by single crystal X-ray crystallography. Further, DFT calculations revealed the lower energy singlet state for complexes 1, 3 and doublet state for complexes 2, 4. The magnetic moment measurements and EPR studies were also carried out to confirm the presence of radicals in complexes 2 and 4.
Abstract The novel tellurato complexes (I) and (II) are obtained by reaction of H 2 TeO 3 , Fe(NO 3 ) 3 , and Li 2 SO 4 in weak or strong alkaline medium, respectively.
Nitrosamines are in the cohort of concern (CoC) as determined by regulatory guidance. CoC compounds are considered highly potent carcinogens that need to be limited below the threshold of toxicological concern, 1.5 μg/day. Nitrosamines like NDMA and NDEA require strict control, while novel nitrosamine drug substance-related impurities (NDSRIs) may or may not be characterized as potent carcinogens. A risk assessment based on the structural features of NDSRIs is important in order to predict potency because they lack substance-specific carcinogenicity. Herein, we present a quantum mechanical (QM)-based analysis on structurally diverse sets of nitrosamines to better understand how structure influences the reactivity that could result in carcinogenicity. We describe the potency trend through activation energies corresponding to α-hydroxylation, aldehyde formation, diazonium intermediate formation, reaction with DNA base, and hydrolysis reactions, and other probable metabolic pathways associated with the carcinogenicity of nitrosamines. We evaluated activation energies for selected cases such as N-nitroso pyrrolidines, N-nitroso piperidines, N-nitroso piperazines, N-nitroso morpholines, N-nitroso thiomorpholine, N-methyl nitroso aromatic, fluorine-substituted nitrosamines, and substituted aliphatic nitrosamines. We compare these results to the recent framework of the carcinogenic potency characterization approach (CPCA) proposed by health authorities which is meant to give guidance on acceptable intakes (AI) for NDSRIs lacking substance-specific carcinogenicity data. We show examples where QM modeling and CPCA are aligned and examples where CPCA both underestimates and overestimates the AI. In cases where CPCA predicts high potency for NDSRIs, QM modeling can help better estimate an AI. Our results suggest that a combined mechanistic understanding of α-hydroxylation, aldehyde formation, hydrolysis, and reaction with DNA bases could help identify the structural features that underpin the potency of nitrosamines. We anticipate this work will be a valuable addition to the CPCA and provide a more analytical way to estimate AI for novel NDSRIs.
A silicon-aluminum heterocycle LAl(SiH2SiH2)2AlL (L = PhC(NtBu)2) (1) was prepared. Compound 1 exhibits a unique (N2Al)2(SiH2)4 centrosymmetric six-membered ring structure with a chair conformation, which is comparable with that of cyclohexane. Furthermore, two intermediate analogues, silylene-alane adduct LSi(AlMe3)-Si(AlMe3)L (2) and silylene-alane oxidative product [LAlHSiH2Mes]2 (3) were obtained. Compound 3 has an interesting arrangement of an Al-H and an SiH2 unit, which are in close vicinity to each other. 3 might be important to function as a catalyst, due to the already activated bridging Al-H bonds.
Abstract A comprehensive DFT (M06‐L‐D3(SMD)/BS2//M06‐L/BS1 level) investigation has been carried out to explore in detail the mechanism of the transmetalation and reductive elimination reactions of abnormal N‐heterocyclic carbene (aNHC) palladium(IV) complexes within the framework of Suzuki–Miyaura cross‐coupling reactions. Emphasis was placed on the role of base and the effect of countercations on the critical transmetalation and reductive elimination events involving palladium(IV) complexes. Of the two competing roles of the base, the route involving boronate formation followed by halide exchange prevails over that of direct halide exchange for the intermediates [Pd IV (aNHC)(OMe) 2 Cl] − Na + (pathway A), [Pd IV (aNHC)(OMe)(Cl) 2 ] − Na + (pathway B), and [Pd IV (aNHC)Cl 3 ] − Na + (pathway C) emanating from the oxidative addition reaction. The results of the calculations are in accordance with our previous theoretical findings of favorable energetics for palladium intermediates incorporating two coordinated methoxy groups. The negative role played by the countercation in the transmetalation step is mainly due to the overstabilization of the pre‐transmetalation intermediate, which is in line with experimental kinetic results. The anionic complexes exhibit greater affinity for the transmetalation and reductive elimination reactions than the neutral variants.
Past one decade has witnessed the tremendous growth in the field of carbenes stabilized low-valent silicon compounds owing to very exciting properties of these molecules. Herein, we have employed a bicyclic (alkyl)(amino)carbene (MeBICAAC) to explore the low-valent chemistry of silicon compounds. The reduction of bicyclic (alkyl)(amino)carbene-SiCl4 complex, [(MeBICAAC)SiCl4] (1) with KC8 afforded their low-valent Si complexes, including Si(III) radical [(MeBICAAC)SiCl3] (2) and a complex with silicon center in a formal zero-valent state [(MeBICAAC)2Si] (3). Similarly, the reduction of in-situ generated MeBICAAC adduct of Me2SiCl2 with one equivalent of KC8 leads to the formation of [(MeBICAAC)SiMe2Cl] (4) complex having an unpaired electron. All these complexes have been characterized by IR, UV-Vis., NMR (for 1), HRMS and their solid-state structures were also elucidated by single crystal X-ray crystallography. Further, DFT calculations revealed the lower energy singlet state for complexes 1, 3 and doublet state for complexes 2, 4. The magnetic moment measurements and EPR studies were also carried out to confirm the presence of radicals in complexes 2 and 4.
An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.
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