Abstract
The cobalt/chromium-catalyzed three-component coupling of aryl iodides, allenes, and aldehydes has been developed to afford multi-substituted homoallylic alcohols in a diastereoselective manner. Control experiments for understanding the reaction mechanism reveal that the cobalt catalyst is involved in the oxidative addition and carbometalation steps in the reaction, whereas the chromium salt generates highly nucleophilic allylchromium intermediates from allylcobalt species, without the loss of stereochemical information, to allow the addition to aldehydes.
Highlights
Carbon–carbon bond formation is the fundamental and central transformation of synthetic organic chemistry
Suitable reaction conditions were investigated for the three-component coupling reaction between iodobenzene (1a), 5-phenylpenta-1,2-diene (2a), and 4-methylbenzaldehyde (3a) in the presence of CoBr2 (10 mol %), CrCl3 (20 mol %), and manganese powder (2.0 equiv), using trimethylsilyl chloride (TMSCl, 1.2 equiv) as a trapping reagent [7]
The absence of a ligand afforded the homoallyl alcohol 4a in 25% yield as a syn/anti (80:20) mixture of diastereomers, the ratio of which was determined by the coupling constant between the two protons at the C1- and C2-positions of 4a; a coupling value of 3J = ca. 5.0 Hz indicated the syn-form, and a coupling value of 3J = ca. 8.0 Hz indicated the anti-form (Table 1, entry 1) [17]
Summary
Carbon–carbon bond formation is the fundamental and central transformation of synthetic organic chemistry. As part of our continuing work on the cobalt-catalyzed functionalization of carbon–carbon unsaturated bonds, a three-component coupling method is reported for the direct synthesis of highly diastereoselective multi-substituted homoallyl alcohols employing a cobalt/chromium hybrid catalyst (Scheme 5). Scheme 5: Three-component coupling of aryl iodides, arenes, and aldehydes using Co/Cr catalyst (this work).
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