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Reduce Derivatives
Published in Aidé Sáenz-Galindo, Adali Oliva Castañeda-Facio, Raúl Rodríguez-Herrera, Green Chemistry and Applications, 2020
Revathi Kottappara, Shajesh Palantavida, Baiju Kizhakkekilikoodayil Vijayan
Organolithium reagents are highly reactive to electrophilic functional groups like keto carbonyl groups (Schlosser, 2001; Whisler et al., 2004). There are reports on the generation of organolithium species in the presence of ketones but are generally quenched in situ by the ketone carbonyl group (Rutherford and Hawkins, 2007). Thus, in a chemical transformation, it is necessary to protect the ketone functional group if it is not involved in the desirable transformation, before an organolithium reaction. It is reliable to avoid such protection steps for greener synthesis. A notable report by Kim et al. demonstrated a flow-microreactor approach to the organolithium reactions without using the protecting step (Mason et al., 2007). In a flow microreactor, the reaction time is defined by the residence time between the reagent inlet and the quencher inlet. They successfully demonstrated the generation of aryl lithium species bearing ketone groups by iodine-lithium exchange reactions of the corresponding aryl iodides with mesityl lithium followed by a reaction with various electrophiles by strictly controlling the residence time to 0.003 seconds or less. They also reported the application of this method for the formal synthesis of Pauciflorol F, a natural product isolated from a stem bark. Even though the flow reactor concept is still in its infancy, it is undoubtedly a scalable process for industrial use (Kim et al., 2011).
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.