Polarity Inversion Catalysis by the 1,4-Addition of N-Heterocyclic Carbenes

Polarity inversion is the hallmark of N-heterocyclic carbene (NHC) organocatalysis, with the generation and reaction of acyl anion equivalents known for more than 70 years. In contrast, polarity inversion through 1,4-addition of NHCs to conjugate acceptors was first applied in a catalytic reaction in 2006. This sub-field of NHC-organocatalysis has developed steadily over the subsequent years, enabling novel coupling reactions, enantioselective cycloisomerizations, polymerizations, and other reactions. In this review, this emerging area of NHC-organocatalysis is discussed with comprehensive coverage. In addition, notes regarding the use of other Lewis base catalysts for related reactions, and comments regarding NHC selection for this type of catalysis, are provided.

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Studies on Thiourea Catalysed Bromocycloetherification and Bromolactonisations

Australian Journal of Chemistry ◽

10.1071/ch20184 ◽

2020 ◽

Vol 73 (12) ◽

pp. 1292

Author(s):

Venkatachalam Pitchumani ◽

David W. Lupton

Keyword(s):

Heterocyclic Carbenes ◽

Lewis Base ◽

Base Catalysts ◽

Lewis Base Catalysts

Lewis base catalysed halofunctionalisation reactions of alkenes are well established and allow access to, among others, various oxygen containing heterocycles. By exploiting the known conversion of N-heterocyclic carbenes into the corresponding thioureas it has been possible to prepare and study a range of chiral and non-chiral Lewis base catalysts for such reactions. Although all thiourea catalysts were found to mediate bromocycloetherification and bromolactonisation reactions, they could not be achieved with enantioselectivity.

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Developing a Methodology for Catalytic Asymmetric Crotylation of Aldehydes

Synlett ◽

10.1055/s-0040-1706659 ◽

2021 ◽

Author(s):

Andrei V. Malkov ◽

Aleksandr E. Rubtsov

Keyword(s):

Kinetic Resolution ◽

Lewis Base ◽

Chiral Auxiliaries ◽

Organometallic Reagents ◽

Total Syntheses ◽

Base Catalysts ◽

Homoallylic Alcohols ◽

Stereogenic Centers ◽

Long Time ◽

Lewis Base Catalysts

AbstractAsymmetric crotylation has firmly earned a place among the set of valuable synthetic tools for stereoselective construction of carbon skeletons. For a long time the field was heavily dominated by reagents bearing stoichiometric chiral auxiliaries, but now catalytic methods are gradually taking center stage, and the area continues to develop rapidly. This account focuses primarily on preformed organometallic reagents based on silicon and, to some extent, boron. It narrates our endeavors to design new and efficient chiral Lewis base catalysts for the asymmetric addition of crotyl(trichloro)silanes to aldehydes. It also covers the development of a novel protocol for kinetic resolution of racemic secondary allylboronates to give enantio- and diastereomerically enriched linear homoallylic alcohols. As a separate topic, cross-crotylation of aldehydes by using enantiopure branched homoallylic alcohols as a source of crotyl groups is discussed. Finally, the synthetic credentials of the developed methodology are illustrated by total syntheses of marine natural products, in which crotylation plays a key role in setting up stereogenic centers.1 Introduction2 Pyridine N-Oxides as Lewis Base Catalysts3 Bipyridine N,N′-Dioxides as Lewis Base Catalysts4 Chiral Allylating Reagents5 Synthetic Applications6 Concluding Remarks

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