Beilstein J. Org. Chem.2022,18, 1217–1224, doi:10.3762/bjoc.18.127
motivated to explore their reactivity as they apparently have many active functionalities. In view of the earlier reported results of the Diels–Alders (DA) reaction across the >C=P– functionality in phosphaalkenes [6], phosphaketenes [6], heterophospholes [7], phosphinines [8], and azaphosphinines [9] (a
of theory wherein it was revealed that the nitrogen lone-pair is transferred effectively into the azaphospole ring. In compound 1 (R1 = Me, R2 = CO2Me) the EWG at the 3-position emphasizes this effect further. As a result, the >C=P– functionality becomes electron-rich and does not undergo a DA
reaction with an electron-rich diene such as DMB. However, in the case of compound 1 (R1 = R2 = CO2Me), the EWG at the 1-position functions as electron-sink between the nitrogen lone-pair and the >C=P– functionality. As a result, the latter retains its electron-deficient character and undergoes DA reaction
PDF
Graphical Abstract
Figure 1:
Structures of 2-phosphaindolizine (1) and indolizine (2).
Beilstein J. Org. Chem.2013,9, 392–400, doi:10.3762/bjoc.9.40
, Germany 10.3762/bjoc.9.40 Abstract The Diels–Alder reaction of the 2-phosphaindolizine-η1-P-aluminium(O-menthoxy) dichloride complex with dimethylbutadiene was investigated experimentally and computationally. The >C=P– functionality of the complex reacts with 2,3-dimethylbutadiene with complete
diastereoselectivity to afford [2 + 4] cycloadducts. Calculation of the model substrate, 3-methoxycarbonyl-1-methyl-2-phosphaindolizine-P-aluminium(O-menthoxy) dichloride (7a), at the DFT (B3LYP/6-31+G*) level reveals that the O-menthoxy moiety blocks the Re face of the >C=P– functionality, due to which the activation
barrier of the Diels–Alder reaction of 7a with 1,3-butadiene, involving its attack from the Si face, is lower. It is found that in this case, the exo approach of the diene is slightly preferred over the endo approach.
Keywords: aluminium(O-menthoxy) dichloride; asymmetric synthesis; >C=P– functionality
PDF
Graphical Abstract
Scheme 1:
Diels–Alder reaction of 2-phosphaindolizines.