Pyrrolidine catalyzed reactions of cyclopentadiene with α,β-unsaturated carbonyl compounds: 1,2- versus 1,4-additions

Abstract

A systematic study of the reactions of cyclopentadiene with α,β-unsaturated carbonyl compounds in the presence of catalytic pyrrolidine-H2O revealed that the reactions can either proceed with a Michael attack at the β-carbon of enone, or 1,2-addition to the carbonyl, leadingeither to 4-cyclopentadienyl-2-butanones or 6-vinylfulvenes. The former can be isolated and/or converted to the corresponding 1,2-dihydropentalenes with base (or in one-pot at longer reaction times). Substitution pattern on the enones on the competing pathways have been studied and consistent mechanisms are proposed.

Synthesis of a Tetraphenyl-Substituted Dihydropentalene and Its Alkali Metal Hydropentalenide and Pentalenide Complexes

“…The annulation of cyclopentadienes with chalcones based on a 1,4 nucleophilic attack followed by a ring-closing 1,2 attack and final water elimination is shown in Figure 2. 28 2.1. Synthesis of 1,3,4,6-Tetraphenyl-1,2-dihydropentalene.…”

“…54 The addition of MeOH to toluene as the reaction solvent was found to be important, as protic solvents likely facilitate the various proton-shuffling steps involved in the reaction (Figure 2). 28 The use of pure alcohol led to low product yields, mainly due to the poor solubility of 1,4-Ph 2 CpH in MeOH. Whereas Griesbeck's synthesis of 1,3-Ph 2 PnH 2 was performed at room temperature, it was found that heating at 70 °C was required to achieve high yields of 1,3,4,6-Ph 4 PnH 2 , which is probably due to the decreased nucleophilicity and increased steric demand of 1,4-Ph 2 CpH compared to CpH.…”

“…Another synthetic approach to accessing 1,3,4,6-Ph 4 PnH 2 from 1,4-Ph 2 CpH is by first converting it to 1,3,6triphenylfulvene 31 and then reacting this species with acetophenone similar to the method described by Coskun et al 28 (Figure 5). Although featuring one step more than the direct reaction of cyclopentadienes with chalcones (Figure 2), this approach bypasses the synthesis of the enone and potentially allows for easier variation of the substituents in the 1 and 3 positions.…”

“…NMR (500 MHz, THF-d 8 ) δ 7.03 (d,3 J HH = 8 28. Hz, 8H, o-Ph), 6.90 (t, 3 J HH = 7.31 Hz, 8H, m-Ph), 6.78 (s, 2H, H a ), 6.48 (t, 3 J HH = 7.24 Hz, 4H, p-Ph); 13 C{ 1 H} NMR (125 MHz, THF-d 8 ) δ 143.3, 127.3, 126.6, 123.2 (identified by HMBC), 117.8, 115.4, 111.1.■ ASSOCIATED CONTENT* sı Supporting InformationThe Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.organomet.1c00495.Alternative synthesis protocols; Additional analytical data (NMR, UV−vis, XRD) (PDF) h i b i t e d .…”

Two Reaction Mechanisms via Iminium Ion Intermediates: The Different Reactivities of Diphenylprolinol Silyl Ether and Trifluoromethyl‐Substituted Diarylprolinol Silyl Ether

“…Thus, the appropriate selection of acid is advisable. In general, a diphenylprolinol silyl ether is the superior catalyst in type A reactions, whereas a trifluoromethyl‐substituted diarylprolinol silyl ether is preferred in type B reactions 29…”