Microwave-Assisted Domino Hydroformylation without Syngas

Abstract

Hydroformylation is a powerful reaction that has suffered for some negative prejudices related to the use of gaseous H₂ and CO. Now it is possible to carry out hydroformylation and different cyclohydrocarbonylations, even on complex substrates, using aqueous formalin as H₂ and CO surrogate in few minutes under microwave irradiation. The catalytic system developed by Morimoto (Rh/BINAP for decomposition of formaldehyde and Rh/Xantphos for hydroformylation) is compatible with microwave dielectric heating and with complex substrates containing ligand atoms allowing rapid domino hydroformylation cyclization reactions without using the external supply of gaseous H₂ and CO (gas cylinder) and without any particular safety limitation or device.

Key words

microwaves - hydroformylation - homogeneous catalysis - domino reactions

Reference and Notes

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13

(3 S ,8a R )-3-Phenylhexahydro-5 H -[1,3]oxazolo[3,2- a ]-pyridin-5-one (20) - General Procedure
A MW vessel was charged with [RhCl(cod)]₂ (2.5 mg, 0.005 mmol), BIPHEP (5.2 mg, 0.01 mmol), and Nixantphos (5.5 mg, 0.01 mmol) under nitrogen (the reaction was carried out without degassing the solution, an experiment done on a degassed solution, gave comparable results). After adding toluene (3 mL), alkene 11 (103 mg, 0.5 mmol), and formalin (37%, 205 µL, 2.5 mmol), the mixture was heated for 30
min at 90 ˚C by microwave irradiation at 250 W (value previously settled on the microwave oven, model Discover from CEM). The solvent was removed in vacuo and the product purified by column cromatography (eluent hexane-EtOAc, 4:1) and isolated as a waxy material (85 mg, 79% yield, dr > 98:2). Characterization as in ref. 12. Compounds 24-26 were described in ref. 7d and compound 28 in ref. 5. New compounds isolated in this work:
1-Oxaspiro[4.5]decan-2-ol (26) ^¹H NMR (400 MHz, CDCl₃): δ = 5.41 (s-like, 1 H), 3.85 (br s, 1 H), 2.11-1.90 (m, 4 H), 1.58-1.40 (m, 10 H). ^¹³C NMR (100 MHz, CDCl₃): δ = 100.7, 89.9, 38.7, 35.5, 34.4, 32.2, 25.8, 23.7). ESI-LRMS: m/z = 179 [M + Na]⁺, 157 [M + H]⁺.
5-Hydroxy-5-(6-hydroxytetrahydro-2 H -pyran-2-yl)-pentanal (27) ^¹H NMR (400 MHz, CDCl₃): δ = 9.77 (s, 1 H_a), 9.74 (t, J = 1.6 Hz, 1 H_b), 5.48 (s, 1 H_a, 1 H_b), 4.07-4.02 (m, 2 H_a, 2 H_b), 2.54-2.50 (m, 2 H_a), 2.48-2.44 (m, 2 H_b), 1.88-1.45 (m, 10 H_a, 10 H_b). ^¹³C NMR (100 MHz, CDCl₃): δ = 201.9, 102.1, 101.4, 78.8, 78.1, 43.2, 34.3, 30.5, 29.8, 28.0, 27.3, 23.9, 19.2, 17.7. ESI-LRMS: m/z = 257 [M + MeOH + Na]⁺.