Synthesis, structure, and E-Z isomerization of beta-(hetero)aryl-alpha-nitro-alpha,beta-enals.
The first general methodology for the gram-scale preparation of previously overlooked beta-(hetero)aryl-alpha-nitro-alpha,beta-enals (3) is reported. Condensation of (hetero)aromatic aldehydes with 2-nitroethanol gave the E-isomers of uncommon beta-(hetero)aryl-alpha-hydroxymethyl-alpha,beta-unsatured-nitroalkenes (2), as determined by NOE and X-ray studies. alpha-Nitro-alpha,beta-enals 3 were subsequently obtained by hypervalent iodine oxidation of 2 as E-Z-mixtures in solid form. They showed varied stability and solvent-dependent thermal-promoted and photopromoted E-Z interconversion. Starting with furfural, experimental conditions were developed to prepare the corresponding nitroenal 3a enriched in either the E or the Z isomer: E-3a/Z-3a approximately 90/10 and 20/80, respectively. In contrast with other structurally related compounds, nitroenals 3 have their (hetero)aryl-vinyl unit and their formyl and nitro groups all in a planar arrangement, both in solid form and in solution; accordingly, they are colored compounds with predicted high dipole moments. As deduced from solution-NMR and X-ray data, the C=C and the C=O double bonds in 3 are exclusively s- cis-oriented; this disposition corresponds in fact to the DFT-computed most stable conformer.
Martínez-Bescos
Patricia
P
Departamento de Química Orgánica, Universidad de Santiago de Compostela, Santiago de Compostela, Spain.
Cagide-Fagín
Fernando
F
Roa
Luis F
LF
Ortiz-Lara
Juan Carlos
JC
Kierus
Krzysztof
K
Ozores-Viturro
Lidia
L
Fernández-González
Marta
M
Alonso
Ricardo
R
D000447:Aldehydes / Q000138:chemical synthesis* / Q000737:chemistry*; D008958:Models, Molecular; D015394:Molecular Structure; D013237:Stereoisomerism; D013997:Time Factors
D000447:Aldehydes