Enantioselective organocatalytic conjugate addition of ...

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Mar 3, 2017 - References. [1] (a) Komnenos T. 218, 145; 1883;. (b) Michael A. J Prakt Chem/Chem-Ztg. 1887;35:349;. (c) Michael A. Am Chem J. 1887;9:115;.
Tetrahedron Letters 58 (2017) 1535–1544

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Enantioselective organocatalytic conjugate addition of organoboron nucleophiles Truong N. Nguyen, Jeremy A. May ⇑ Department of Chemistry, University of Houston, 112 Fleming Building, Houston, TX 77204-5003, United States

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Article history: Received 14 November 2016 Revised 15 February 2017 Accepted 16 February 2017 Available online 3 March 2017

a b s t r a c t This review covers the use of organoboron nucleophiles in enantioselective conjugate additions catalyzed by organic-based catalysts. It is divided into sections based on the type of nucleophile, with each section arranged in roughly chronological order. The categories of nucleophiles are alkynyl, alkenyl, and aryl boronates or borates. The principle modes of catalysis, iminium formation and boron chelation, are covered. Ó 2017 Elsevier Ltd. All rights reserved.

Keywords: Conjugate addition Organocatalysis Organoboronates Enantioselective catalysis

Contents Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The enantioselective organocatalytic conjugate addition of alkynyl boronates The enantioselective organocatalytic conjugate addition of alkenyl boronates The enantioselective organocatalytic conjugate addition of aryl boronates . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Introduction Stabilized carbon anions have been added to electron deficient olefins for over 130 years.1 Gilman’s seminal report of organocuprates adding in a 1,4-addition to ‘‘benzalacetophenone” (i.e., chalcone)2 established an alternative method to approach conjugate addition carbon–carbon bond formation that was later made catalytic. In the years since those discoveries, diastereoselective and enantioselective versions of those transformations have been developed. Approaches include enone activation via Lewis acids,3 Brønsted acids,4 or iminium5 catalysis as well as catalytic control of the nucleophile, principally via asymmetric organocatalysis6 or copper,7 rhodium,8 and palladium9 complexes. Many of the latter ⇑ Corresponding author. E-mail address: [email protected] (J.A. May). http://dx.doi.org/10.1016/j.tetlet.2017.02.061 0040-4039/Ó 2017 Elsevier Ltd. All rights reserved.

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approaches take advantage of the readily available and extraordinarily practical unsaturated organoboron nucleophiles, but their use in combination with organocatalysis has been significantly more limited. While the conjugate addition of these nucleophiles has been reviewed for organometallic catalysis, this review will address their use in organocatalytic conjugate additions. Because of the Lewis acidic nature of many of the organoboron intermediates in organocatalytic reactions, intriguing and novel reactivity is often seen along with outstanding stereocontrol. Background The strategy for the use of Lewis acidic nucleophiles with organocatalysis has roots in studies by H.C. Brown and Akira Suzuki conducted fifty years ago in the 1960’s.10 They found that alkyl boranes, readily generated via the hydroboration of alkenes,

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add to a,b-unsaturated ketones and aldehydes to form b-alkylated products (3, Scheme 1). Doing so when the electrophile had prior b-substitution (including cyclic enones) was difficult, but finding that the mechanism of the addition was radical-based allowed them to overcome this obstacle by using catalytic oxygen, diacyl peroxide, or photoactivation. Suzuki later showed in the 1990’s that vinyl boronates underwent a Lewis acid catalyzed conjugate addition to enones, but with a polar mechanism (Scheme 2A).11 BF3OEt2 was the acid of choice, and Suzuki postulated that one of the alkoxy ligands on the boronate ester was replaced with fluoride, which greatly increased the Lewis acidity of the nucleophile for the reaction (Scheme 2C). Coordination of this Lewis acid 14 to the enone then brought these two species into close proximity for C–C bond formation. Cyclic enones did not serve as electrophiles because of a mechanistic need for an

Scheme 1. Radical-based Conjugate Addition of Organoboranes.

s-cis conformation of the enone in the transition state (see 15). That effort was shortly followed by a report of the conjugate addition of alkynyl boronic acids, which required a lower temperature to avoid nucleophile decomposition (Scheme 2B).12 Suzuki also showed that boronic acids could be used with cyanuric fluoride as an activating agent (Scheme 3A).13 The cyanuric fluoride was proposed to generate monofluoroboronate 24, which was Lewis acidic enough to react with the enone in a similar manner to 14 above (Scheme 3B). Csákÿ has more recently reported the use of TFA instead of cyanuric fluoride to promote this racemic transformation.14 The enantioselective organocatalytic conjugate addition of alkynyl boronates Alkynes are an extremely useful synthetic functional group, as they may be oxidized or reduced, may undergo nucleophilic or electrophilic substitution, or may be substrates for cycloadditions. In 2000, about a decade after Suzuki’s initial report of BF3OEt2 catalyzed conjugate addition, Chong reported the use of 3,30 -diphenylBINOL (27) as a stoichiometric boron ligand for the first enantioselective addition of alkynyl boronate esters 28 to enones 30 (Scheme 4).15 The ligated complex 29 was formed by the addition of the BINOL to the trialkoxy ate complex 28 that had been

Scheme 2. Lewis Acid Promoted Boronate Ester Conjugate Addition.

Scheme 3. Cyanuric Fluoride Promoted Boronic Acid Conjugate Addition.

T.N. Nguyen, J.A. May / Tetrahedron Letters 58 (2017) 1535–1544

Scheme 4. Stoichiometric BINOL as Ligand.

derived from lithium acetylide addition to triisopropyl borate. Excess BF3OEt2 was then needed for the dissociation of isopropoxide from 29 to activate the ligated boron as a Lewis acid.16 This was proposed to coordinate to the enone to form complex 31 in a similar fashion to Suzuki’s proposed mechanism, followed by C–C bond formation via alkyne attack on the enone p-system. While catalytic turnover of the ligand was not achieved in this report, high stereoselectivities were seen for b-aryl enones, where the ee obtained was usually 90%. With the exception of a b-t-Bu-enone, however, the b-alkyl enones provided