Diels–Alder Reactions with the >C=P– Functionality of Annelated Azaphospholes

Abstract

The >C=P– functionality in annelated azaphospholes acts as a dienophile and undergoes Diels–Alder reactions with dienes. 1,3-Bis(alkoxycarbonyl)-2-phosphaindolizines react with dienes even in the absence of a catalyst; however, in the presence of an oxidizing agent such as sulfur or selenium, the reaction is much faster. 3-Alkoxycarbonyl-2-phosphaindolizines with an electron-withdrawing group exclusively at position 3 undergo the Diels–Alder reaction only in the presence of a Lewis acid. Density functional theory calculations reveal strong hyperconjugative interactions between the nitrogen lone pair and the >C=P– functionality of the 3-alkoxycarbonyl-2-phosphaindolizine, making it electron-rich. However, in 1,3-bis(alkoxycarbonyl)-2-phosphaindolizines, the ester group at the position 1 acts as a sink for the lone pair of electrons of nitrogen leaving the >C=P– functionality sufficiently electron-deficient to undergo the Diels–Alder reaction.

1 Background and Introduction

2 Scope and Limitations

3 Synthetic Methods

3.1 [4+1] Cyclocondensation Method

3.2 1,5-Electrocyclization Method

4 Energetics of the Diels–Alder Reaction with the >C=P– Functionality

5 Diels–Alder Reactions of Annelated 1,3-Azaphospholes

5.1 Diels–Alder Reactions of 1,3-Azaphospholo[1,5-a]pyridines, i.e., 2-Phosphaindolizines

5.2 Theoretical Investigation of the Dienophilic Reactivity of the >C=P– Functionality in 2-Phosphaindolizines

5.3 Theoretical Investigation of the Catalytic Effect of AlCl₃ on the ­Dienophilic Reactivity of >C=C< and >C=P– Functionalities in ­Indolizine and 2-Phosphaindolizine

5.4 Organo-aluminum Chloride Catalyzed Diels–Alder Reactions of 2-Phosphaindolizines

5.5 Diels–Alder Reaction of 1,3-Azaphospholo[5,1-b]benzothiazole

6 Diels–Alder Reactions of Diazaphospholes

6.1 Diels–Alder Reactions of 1,4,2-Diazaphospholo[4,5-a]pyridines

6.2 Diels–Alder Reactions of Thiazolo[3,2-d][1,4,2]diazaphospholes and their 5,6-Dihydro and Benzo Derivatives

7 Conclusions

Key words

asymmetric catalysis - asymmetric synthesis - Diels–Alder reaction - >C=P– functionality - azaphospholes - DFT calculations - natural bond orbital

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