Catalytic Borylative Opening of Propargyl Cyclopropane, Epoxide, Aziridine, and Oxetane Substrates: Ligand Controlled Synthesis of Allenyl Boronates and Alkenyl Diboronates

Zhao, Jian; Szabó, Kálmán J.

doi:10.1002/anie.201510132

Cited by 78 publications

(25 citation statements)

References 71 publications

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“…11,12 To the best of our knowledge, there is no extant catalytic protocol for preparation of enantiomerically enriched trisubstituted allenyl boronates (i.e., α -addition to furnish vi , Scheme 1b). 13 An attractive way to synthesize trisubstituted allenyl boronates ( viii , Scheme 2a) enantioselectively would be by 1,3-enyne hydroboration, 14 which might proceed via Cu–allenyl species vii . We wished to investigate whether, despite the relative robustness of a Cu–allenyl bond (vs the formerly studied Cu–alkyl bonds 3g ), there might be cases where the stereochemical integrity of a Cu–allenyl intermediate might be jeopardized, and whether conditions may be modified for achieving high enantioselectivity.…”

Section: Introductionmentioning

confidence: 99%

Enantioselective Synthesis of Trisubstituted Allenyl–B(pin) Compounds by Phosphine–Cu-Catalyzed 1,3-Enyne Hydroboration. Insights Regarding Stereochemical Integrity of Cu–Allenyl Intermediates

et al. 2018

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Catalytic enantioselective boron-hydride additions to 1,3-enynes, which afford allenyl-B(pin) (pin = pinacolato) products, are disclosed. Transformations are promoted by a readily accessible bis-phosphine-Cu complex and involve commercially available HB(pin). The method is applicable to aryl- and alkyl-substituted 1,3-enynes. Trisubstituted allenyl-B(pin) products were generated in 52-80% yield and, in most cases, in >98:2 allenyl:propargyl and 92:8-99:1 enantiomeric ratio. Utility is highlighted through a highly diastereoselective addition to an aldehyde, and a stereospecific catalytic cross-coupling process that delivers an enantiomerically enriched allene with three carbon-based substituents. The following key mechanistic attributes are elucidated: (1) Spectroscopic and computational investigations indicate that low enantioselectivity can arise from loss of kinetic stereoselectivity, which, as suggested by experimental evidence, may occur by formation of a propargylic anion generated by heterolytic Cu-C cleavage. This is particularly a problem when trapping of the Cu-allenyl intermediate is slow, namely, when an electron deficient 1,3-enyne or a less reactive boron-hydride reagent (e.g., HB(dan) (dan = naphthalene-1,8-diaminato)) is used or under non-optimal conditions (e.g., lower boron-hydride concentration causing slower trapping). (2) With enynes that contain a sterically demanding o-aryl substituent considerable amounts of the propargyl-B(pin) isomer may be generated (25-96%) because a less sterically demanding transition state for Cu/B exchange becomes favorable. (3) The phosphine ligand can promote isomerization of the enantiomerically enriched allenyl-B(pin) product; accordingly, lower ligand loading might at times be optimal. (4) Catalytic cross-coupling with an enantiomerically enriched allenyl-B(pin) compound might proceed with high stereospecificity (e.g., phosphine-Pd-catalyzed cross-coupling) or lead to considerable racemization (e.g., phosphine-Cu-catalyzed allylic substitution).

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Section: Introductionmentioning

confidence: 99%

Enantioselective Synthesis of Trisubstituted Allenyl–B(pin) Compounds by Phosphine–Cu-Catalyzed 1,3-Enyne Hydroboration. Insights Regarding Stereochemical Integrity of Cu–Allenyl Intermediates

et al. 2018

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“…Four major components were identified by 1 H NMR spectroscopy in the reaction mixture being unreacted starting material S1 (12 %), diborylated 1,4‐diene 1 (15 %), α‐hydroxyallene 17 (45 %) and the monoborylated allene 41 (≈28 %, cf. α‐borylation of the alkyne unit; for details, see the Supporting Information) [16] . These data illustrate that an excess of B 2 pin 2 was required to favor the formation of the diborylated diene, and that the monoborylated allene 41 is a likely precursor to 1 .…”

Section: Resultsmentioning

confidence: 77%

“…a-borylation of the alkyne unit;f or details,s ee the Supporting Information). [16] These data illustrate that an excess of B 2 pin 2 was required to favor the formation of the diborylated diene,a nd that the monoborylated allene 41 is al ikely precursor to 1.I nt he absence of as ufficiently large excess of B 2 pin 2 alternative parasitic pathways could be favored. Thef ormation of a-hydroxyallene 17 as the major compound under these reaction conditions suggested that alcohol-promoted protodeborylation of 41 is competitive.…”

Section: Angewandte Chemiementioning

confidence: 81%

Copper‐Mediated Dichotomic Borylation of Alkyne Carbonates: Stereoselective Access to (E)‐1,2‐Diborylated 1,3‐Dienes versus Traceless Monoborylation Affording α‐Hydroxyallenes

Guo

Kleij

2021

Angewandte Chemie

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A mild copper‐mediated protocol has been developed for borylation of alkynyl cyclic carbonates. Depending on the nature of the borylating reaction partner, either stereoselective diborylation of the propargylic surrogate takes place, providing convenient access to (E)‐1,2‐borylated 1,3‐dienes, or traceless monoborylation occurs, which leads to α‐hydroxyallenes as the principal product. The dichotomy in this borylation protocol has been scrutinized by several control experiments, illustrating that a relatively small change in the diboron(4) reagent allows for competitive alcohol‐assisted protodemetalation to forge an α‐hydroxyallene product under ambient conditions.

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“…Szabò reported copper‐catalyzed borylative ring openings of three‐ and four‐membered rings bearing an adjacent triple bond [77] . The reaction afforded allenylboronates using less basic and hindered phosphines, such as tri‐1‐naphthylphosphine.…”

Section: Diboron Compounds and Silaboranesmentioning

confidence: 99%

Boron Reagents and Catalysts for the Functionalization of Strained Heterocycles

Pineschi

2021

Adv Synth Catal

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The particular nature of boron compounds allows an ample modularity of their properties ranging from Lewis acids, C‐nucleophiles, B‐nucleophiles, or even conjunctive reagents for new synthetic manipulations. Moreover, the increasing demand for functionalized boron derivatives for pharmaceutical or material science applications requires the development of new synthetic methods for boron introduction in organic compounds. This review summarizes the possible combinations of boron derivatives with a variety of strained heterocycles reported in the most recent literature.

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Catalytic Borylative Opening of Propargyl Cyclopropane, Epoxide, Aziridine, and Oxetane Substrates: Ligand Controlled Synthesis of Allenyl Boronates and Alkenyl Diboronates

Cited by 78 publications

References 71 publications

Enantioselective Synthesis of Trisubstituted Allenyl–B(pin) Compounds by Phosphine–Cu-Catalyzed 1,3-Enyne Hydroboration. Insights Regarding Stereochemical Integrity of Cu–Allenyl Intermediates

Enantioselective Synthesis of Trisubstituted Allenyl–B(pin) Compounds by Phosphine–Cu-Catalyzed 1,3-Enyne Hydroboration. Insights Regarding Stereochemical Integrity of Cu–Allenyl Intermediates

Copper‐Mediated Dichotomic Borylation of Alkyne Carbonates: Stereoselective Access to (E)‐1,2‐Diborylated 1,3‐Dienes versus Traceless Monoborylation Affording α‐Hydroxyallenes

Boron Reagents and Catalysts for the Functionalization of Strained Heterocycles

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