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Organometallic Compounds as Heterogeneous Catalysts
Published in Varun Rawat, Anirban Das, Chandra Mohan Srivastava, Heterogeneous Catalysis in Organic Transformations, 2022
Garima Sachdeva, Monu Verma, Varun Rawat, Ved Prakash Verma, Manish Srivastava, Sudesh Kumar, Singh Vanshika
Organolithium compounds contain a bond between lithium and a carbon atom. Due to the electropositive nature of lithium, charge density is on carbon; hence it behaves as a carbanion, and the organolithium compound acts as both nucleophile and base. They are essential intermediates and effective reagents for organic transformations due to their exceptionally high reactivity. Organolithium compounds are thermally more stable as compared to other alkali metals. They are found to make coordination complexes with ethers, neutral Lewis base, amines, and alkoxides and display diverse features depending on the complexing reagent [24]. Organolithium compounds are found as oligomers in solutions, and separation occurs when they react with an electrophile. The degree of aggregation and reactivity can be controlled with the help of chelating ligands such as tetramethylene diamine and hexamethyl phosphoric triamide.
Organometallic Compounds
Published in Michael B. Smith, A Q&A Approach to Organic Chemistry, 2020
Organolithium reagents are characterized by a C—Li bond, which is polarized similarly to the C—Mg bond of a Grignard reagent (the C—Li has the polarization δ–C-Liδ+). Organolithium reagents will, therefore, function as nucleophiles in the presence of an electrophilic species such as acetone or other ketones and aldehydes or as bases in the presence of a suitable acid. What solvent is used to form organolithium reagents?
Synthesis and mesomorphic properties of 4,4”-dialkynyl-2’,3’-difluoro-p-terphenyls – the influence of C≡C acetylene linking bridge
Published in Liquid Crystals, 2019
Marta Pytlarczyk, Przemysław Kula
Presented synthetic approach is based on 4-(alkyl-1-yn-1-yl)phenyl-4-yl units as a nonpolar terminal endcap of the molecule. Synthesis was started from selective Sonogashira reaction between 1-chloro-4-iodobenzene or 1-bromo-4-iodobenzene and terminal acetylene. The reactivity difference of bromine or chlorine vs. iodine, in the case of Sonogashira reaction, is sufficient to ensure effective mono-functionalisation from the iodo-side. 4-(Alkyl-1-yn-1-yl)bromo/chlorobenzenes with longer alkyl part (for alkyls longer than four carbon atoms) described in [39,40] were obtained via direct Sonogashira reaction of 1-bromo-4-iodobenzene and pent-1-yne or hex-1-yne with good yields. The situation becomes more complicated for shorter homologues since the terminal alkynes are gases at normal conditions and direct Sonogashira protocol cannot be easily employed. In this case at the beginning we obtained 1-bromo/chloro-4-ethynylbenzenes with two-step synthesis (Scheme 1). The reaction of 1-bromo-4-iodobenzene with 2-methyl-3-butyn-2-ol (Method I) leads to the formation of protected phenylacetylene derivative. Next synthetic step is the hydrolysis of intermediate product by catalytic amount of sodium hydride in toluene as a solvent. Finally, the 1-bromo-4-ethynylbenzene was obtained in good yield and was used to synthesise homologue longer by one carbon atom [41]. Unfortunately, reaction of deprotonation-alkylation of this compound (reaction with excess of n-BuLi as a base as well as insufficiency equivalent amount of methyl iodide as an electrophile) gives a mixture of by-products (debromianted substrate and debrominated product), see Table 1. It shows that this organolithium reagent is too reactive for bromine/lithium exchange which occurs along with wanted deprotonation of terminal acetylene -C≡ CH. Purification of this mixture is too complicated, so we decided to replace bromine atom to chlorine. Method II is Sonogashira reaction of 1-chloro-4-iodobenzene with ethynyltrimethylsilane and second step is deprotection of trimethylsilyl group of alkyne [42]. Reaction of 1-chloro-4-ethynylbenzene with n-BuLi and methyl iodide gives only one expected product and we decided to use this approach to formation 1-chloro-4-(but/prop-1-yn-1-yl)benzenes.