Diels–Alder reaction of 2-phosphaindolizines catalysed by organoaluminium reagent: theoretical and experimental results

Synthesis of a variety of organophosphorus compounds using 2-phosphaindolizines as precursors

1,3-Azaphospholo[1,5-a]pyridines named as 2-phosphaindolizines were obtained from [4+1] cyclocondensation and 1,5-electrocyclization methods. These compounds incorporate several reactive functionalities and undergo electrophilic substitution, Diels-Alder reaction and 1,2-addition with hydrogen sulfide and sulfur across the >C=P– functionality and form η1-P coordination compounds with metal carbonyls. Thus, 2-phosphaindolizines could be used as precursors for accessing a variety of organophosphorus compounds.

Synergy between Experimental and Theoretical Results of Some Reactions of Annelated 1,3-Azaphospholes

Computational calculations have been used successfully to explain the reactivity of the >C=P- functionality in pyrido-annelated 1,3-azaphospholes. Theoretical investigation at the Density Functional Theory (DFT) level shows that the lone pair of the bridgehead nitrogen atoms is involved in extended conjugation, due to which electron density increases considerably in the five-membered azaphosphole ring. The electron density in the azaphosphole is further enhanced by the presence of an ester group at the 3-position making the >C=P- functionality electron-rich. Thus, 1,3-azaphospholo[5,1-a]pyridine, i.e., 2-phosphaindolizine having ester group at the 3-position only does not undergo Diels-Alder (DA) reaction with an electron rich diene, such as 2,3-dimethyl-1,3-butadiene (DMB). However, an ester group at 1-position acts as electron-sink, due to which transfer of the electron density to the >C=P- moiety is checked and DA reaction occurs across the >C=P- functionality. The coordination of the Lewis acid to the carbonyl group at the 3-position raises the activation barrier, while it is lowered remarkably when it is coordinated to the P atom. Furthermore, the attack of 1,3-butadiene on the Si face of the P-coordinated (o-menthoxy)aluminum dichloride complex is a lower activation energy path. Fukui functions could be used to explain relative reactivities of indolizine and 2-phosphaindolizine towards electrophilic substitution reactions.

Asymmetric Diels-Alder reaction with >C=P- functionality of the 2-phosphaindolizine-η(1)-P-aluminium(O-menthoxy) dichloride complex: experimental and theoretical results

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.