Copper-Catalyzed Enantioselective Allylic Substitution Reactions with Organoaluminum and Boron Based Reagents Promoted by Chiral Sulfonate Bearing N-Heterocyclic Carbenes

Abstract

Chapter 1. A Review of Catalytic Enantioselective Allylic Substitution (EAS) with Chiral Sulfonate Containing N-heterocyclic Carbenes (NHC). A comprehensive review of enantioselective allylic substitution reactions, which are promoted by a chiral N-heterocyclic carbene metal complex that features a unique sulfonate motif, is provided in this chapter. Reactions are classified into two categories. One class of transformations is catalyzed by a series of easily modifiable sulfonate bearing NHC-Cu complexes, with which a range of nucleophilic organometallic reagents (i.e., organozinc-, aluminum-, magnesium- and boron-based) that carry different carbon-based units are readily utilized in efficient and highly selective C-C bond forming processes. Another set of reactions exclude the use of a copper salt; catalytic amount of a sulfonate containing imidazolinium salt is capable of promoting additions of alkyl Grignard, zinc and aluminum species to easily available allylic electrophiles in a site- and enantioselective fashion. The mechanistic scenarios of both catalytic systems that account for the observed experimental data are discussed in detail. Chapter 2. Cu-Catalyzed Enantioselective Allylic Substitutions with Aryl- and Heteroarylaluminum Reagents. In this chapter, the first examples of EAS reactions of aryl- and heteroaryl-substituted dialkylaluminum reagents to a wide range of trisubstituted allylic phosphates are demonstrated through a facile and selective catalysis rendered possible by an in situ generated sulfonate containing NHC-Cu complex, delivering enantiomerically enriched olefin products that bear an all carbon quaternary stereogenic center. The requisite organometallic species are easily prepared from either the corresponding aryl- and heteroaryl halides, or through efficient and site selective deprotonation at the C-2 position of furan and thiophene; such aluminum entities are readily used in situ without the requirement of purification. Application to small molecule natural product synthesis is also carried out to illustrate the utility of the present protocol. Chapter 3. Cu-Catalyzed Enantioselective Allylic Substitutions with Alkenylaluminum Reagents. This chapter focuses on our research towards construction of enantioenriched tertiary and quaternary stereogenic centers that are substituted with two further functionalizable alkenes. The first combination of the study involves the addition of stereochemically well-defined trisubstituted alkenylaluminum reagents to disubstituted allylic phosphates; the transformation commences with a silyl-directed stereoselective hydroalumination and finishes with an enantioselective Cu-catalyzed EAS promoted by a sulfonate bearing NHC. Such reactions deliver molecules that feature silicon containing trisubstituted olefin adjacent to the tertiary stereogenic center; subsequent conversion of the versatile silicon group to a proton reveals the first set of examples that incorporate pure Z alkene in Cu-catalyzed EAS. The stereoselective and concise synthesis of naturally occurring small molecule nyasol demonstrates the utility of the above method. On a different front, Ni-catalyzed site-selective hydroalumination of terminal alkynes has opened new possibility of introducing 1,1-disubstituted olefins in Cu-catalyzed EAS in the formation of tertiary stereogenic center containing enantioenriched organic building blocks. Such catalytic hydrometallation procedure also allows efficient access to alkenylaluminums that are derived from the conventionally problematic aromatic alkynes. The importance of efficient and selective synthesis of terminal aryl-substituted alkenylaluminum species is showcased in NHC-Cu-catalyzed EAS reactions that construct all-carbon quaternary stereogenic centers; a three-step convergent synthesis of natural product bakuchiol in enantiomerically enriched form highlights the potential of the current protocol in chemical synthesis. Chapter 4 Cu-Catalyzed Enantioselective Allylic Substitutions with Alkenylboronic Acid Pinacol Ester Reagents and Applications in Natural Product Synthesis. Within this chapter, we disclose the efficient utilization of alkenylboron reagents in Cu-catalyzed EAS reactions, which lead to highly site and enantioselective formations of molecules that contain both tertiary and quaternary carbon stereogenic centers. Unlike their aluminum-based counterparts, the use of boron-based reagents allows effective delivery of sensitive organic function groups, such as a carbonyl, which would be incompatible in the hydrometallation process with dibal-H. Our efforts accumulate to the first report of incorporation of all carbon quaternary centers that are substituted with unsaturated ester and aldehyde units in the EAS products; such a method facilitates the concise diastereo- and enantioselective synthesis of Pummerer's ketone and it's trans isomer. Further development of the above protocol towards the construction of tertiary stereogenic centers requires the design of new chiral sulfonate-containing imidazolinium salts as the ligand precursors and has lead to the employment of a broader range of alkenylboron species, which feature readily functionalizable motifs. Subsequent demonstrations in enantioselective synthesis of a variety of small molecule natural products showcase the utility.