Herein, the reversal of polarity of cationic bromine, organocatalytically, is presented. NBS, a proven bromocation source was converted to a superior bromoanion reagent by H/Br exchange with a secondary amine, substantiated with spectroscopic and computational evidences. The concept has further been used in a successful accelerated organocatalyzed dibromination of olefins, in a non-hazardous, commercially viable process with a wide substrate scope. The reactivity of key entities has been observed through NMR kinetics and reaction acceleration using 10 mol% of catalyst loading accounts for its major success. The nucleophilicity of the bromoanion was found to be superior in comparison to other nucleophiles such as MeOH, H2O etc. and the protocol dominates over competing allylic bromination reaction.
Herein a successful chemoselective either functionalization of the nucleophilic sites of prolinol by exploiting the relative acidity difference and inverted nucleophilicity of the corresponding conjugate bases, employing a suitable base is reported. An elaborate investigation into the overlooked sensitivity of reaction conditions to a highly utilized protocol has been reported. As an example, mono-Boc functionalization of prolinol for the exclusive synthesis of either NBoc/OBoc/Oxazolidinone derivatives is reported. Failing to emulate the former protocols, a mechanistic investigation was initiated which revealed that the rudimentary steps can be controlled by: a) a requisite base to recognize the differently acidic sites (NH and OH) for the formation of the conjugate base reacting to the electrophile, b) the disparity in nucleophilicity of the completely formed conjugate basic sites. This protocol has been extended to be successful with various other substrates, which might prove to be applicable as suitable catalysts in asymmetric reactions. Never-reported-before substrates such as O-Boc, O-CBz, O-Bz and O-ethyl carbonate derivatives of prolinol were synthesized in good to excellent yields along with other substrates.
Herein, organocatalytically achieved polarity reversal of cationic bromine is presented. The proven bromocation source N‐bromosuccinimide (NBS) was converted to a superior bromoanion reagent by H/Br exchange with a secondary amine, substantiated with spectroscopic and computational evidence. The concept has further been used in a successfully accelerated organocatalyzed dibromination of olefins in a non‐hazardous, commercially viable process with a wide range of substrate scope. The reactivity of key entities observed through NMR kinetics and reaction acceleration using only 10 mol % of catalyst account for its major success. The nucleophilicity of the bromoanion was found to be superior in comparison to other nucleophiles such as MeOH and H2O also the protocol dominates over the competing allylic bromination reaction.
An investigation into the sensitivity of reaction conditions to a highly utilized protocol has been reported, wherein the mono-Boc functionalization of prolinol could be controlled for the exclusive synthesis of either N-Boc, O-Boc, or oxazolidinone derivatives. Mechanistic investigation revealed that the elementary steps could possibly be controlled by (a) a requisite base to recognize the differently acidic sites (NH and OH) for the formation of the conjugate base, which reacts with the electrophile, and (b) the difference in nucleophilicity of the conjugate basic sites. Herein, a successful chemoselective functionalization of the nucleophilic sites of prolinol by employing a suitable base is reported. This has been achieved by exploiting the relative acidity difference of NH and OH along with the reversed nucleophilicity of the corresponding conjugate bases N– and O–. This protocol has also been used for the synthesis of several O-functionalized prolinol derived organocatalysts, few of which have been newly reported.
This review is devoted to underpinning the contributions of Indian researchers towards asymmetric organocatalysis. More specifically, a comprehensive compilation of reactions mediated by a wide range of non-covalent catalysis is illustrated. A detailed overview of vividly catalogued asymmetric organic transformations promoted by hydrogen bonding and Brønsted acid catalysis, alongside an assortment of catalysts is provided. Although asymmetric organocatalysis has etched itself in history, we aim to showcase the scientific metamorphosis of Indian research from baby steps to large strides within this field. 1 Introduction2 Non-Covalent Catalysis and Its Various Activation Modes3 Hydrogen-Bonding Catalysis3.1 Urea- and Thiourea-Derived Organocatalysts3.1.1 Thiourea-Derived Organocatalysts3.1.2 Urea-Derived Organocatalysts3.2 Squaramide-Derived Organocatalysts3.2.1 Michael Reactions3.2.2 C-Alkylation Reactions3.2.3 Mannich Reactions3.2.4 [3+2] Cycloaddition Reactions3.3 Cinchona-Alkaloid-Derived Organocatalysts3.3.1 Michael Reactions3.3.2 Aldol Reactions3.3.3 Friedel–Crafts Reactions3.3.4 Vinylogous Alkylation of 4-Methylcoumarins3.3.5 C-Sulfenylation Reactions3.3.6 Peroxyhemiacetalisation of Isochromans3.3.7 Diels–Alder Reactions3.3.8 Cycloaddition Reactions3.3.9 Morita–Baylis–Hilman Reactions4 Brønsted Acid Derived Organocatalysts4.1 Chiral Phosphoric Acid Catalysis4.1.1 Diels–Alder Reactions4.1.2 Addition of Ketimines4.1.3 Annulation of Acyclic Enecarbamates5 Conclusion
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