An exceptionally simple and efficient synthesis of 6-methyl-6-vinylfulvene, and its oxidative transformations

Palladium(II)-Catalyzed Oxidative Decarboxylative [2 + 2 + 1] Annulation of Cinnamic Acids with Alkynes: Access to Polysubstituted Pentafulvenes

An unprecedented palladium(II)-catalyzed oxidative decarboxylative [2 + 2 + 1] annulation of cinnamic acids with alkynes has been developed for the synthesis of polysubstituted pentafulvenes. Ag₂CO₃ and DMSO are essential for the reaction. This protocol features readily available starting materials, a wide substrate scope, and moderate to excellent yields. Moreover, various significant frameworks can be easily obtained from the late-stage transformations of pentafulvenes via oxidation, reduction, and Scholl-type reaction.

Palladium-Catalyzed Regio- and Stereoselective Synthesis of ( E)-1,3-Bissilyl-6-arylfulvenes from Aryl Iodides and Silylacetylenes

An efficient synthetic route to ( E)-1,3-bissilyl-6-arylfulvenes has been developed. The reaction of aryl iodides with trimethylsilylacetylene in the presence of a catalytic amount of PdBr₂ gives 6-aryl-1,3-bis(trimethylsilyl)fulvenes in good to excellent yields with complete regio- and stereoselectivity. The reaction involves trimerization of trimethylsilylacetylene and cleavage of one silyl group. The silylated fulvenes obtained could be converted into halogenated fulvenes by site-selective halodesilylation. The halogenated fulvenes underwent the Stille coupling leading to the corresponding arylated fulvenes.

Kinetics and Mechanism of Pyrrolidine Buffer-Catalyzed Fulvene Formation

Rapid synthesis of fulvenes is achieved using pyrrolidinium/pyrrolidine buffers in anhydrous acetonitrile. Time-dependent UV-vis absorption and NMR spectroscopy reveal that the rate and yield of fulvene formation depend strongly on both the presence of acid in the medium and the choice of solvent, and they are negatively affected by water. Kinetic data have been collected for various substrates, and the synthetic benefits of the adjusted reaction conditions are showcased. Enhancements of reaction rates are found in comparison to literature procedures. α-Unsaturated fulvenes that were previously difficult to access can now be obtained in good yields.

Imidazolidin-4-ones via (3+2) cycloadditions of aza-oxyallyl cations onto (E)-Narylideneanilines

In the course of our studies on the chemistry of oxyallyl species we uncovered a new (3+2) cycloaddition of aza-oxyallyl systems, generated in situ from N-benzyloxy-2-chloroamides in the presence of NEt₃, onto N-arylimines yielding imidazolidin-4-ones in moderate to good yields. The cycloadditions are regioselective. Computational modelling using DFT at the M062×/6-311+G** level is in support the observed regioselectivities. Although the path to the trans imidazolin-4-one is favored, the cis product is preferred by almost 8 kcal/mol and could be formed by base-catalyzed epimerization. All products were isolated by chromatography and characterized by means of their FTIR, NMR and HRMS data.

Unusual hydroxyl effect on fulvene endoperoxide decompositions

The thermal decomposition of fulvene endoperoxides ordinarily proceeds via an allene oxide intermediate affording oxepin-2(3H)-one derivatives. We have now uncovered new, unusual pathways in these decompositions where the presence of a hydroxyl group on the alkyl or aryl attached to the fulvene exocyclic double bond has a profound effect on the fate of the reactive intermediates derived from the unstable endoperoxides. Computational work supports the proposed mechanistic pathways.

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

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.

Effect of allylic groups on S(N)2 reactivity

The activating effects of the benzyl and allyl groups on S_N2 reactivity are well-known. 6-Chloromethyl-6-methylfulvene, also a primary, allylic halide, reacts 30 times faster with KI/acetone than does benzyl chloride at room temperature. The latter result, as well as new experimental observations, suggests that the fulvenyl group is a particularly activating allylic group in S_N2 reactions. Computational work on identity S_N2 reactions, e.g., chloride^– displacing chloride^– and ammonia displacing ammonia, shows that negatively charged S_N2 transition states (tss) are activated by allylic groups according to the Galabov–Allen–Wu electrostatic model but with the fulvenyl group especially effective at helping to delocalize negative charge due to some cyclopentadienide character in the transition state (ts). In contrast, the triafulvenyl group is deactivating. However, the positively charged S_N2 transition states of the ammonia reactions are dramatically stabilized by the triafulvenyl group, which directly conjugates with a reaction center having S_N1 character in the ts. Experiments and calculations on the acidities of a variety of allylic alcohols and carboxylic acids support the special nature of the fulvenyl group in stabilizing nearby negative charge and highlight the ability of fulvene species to dramatically alter the energetics of processes even in the absence of direct conjugation.

An expedient synthesis of 6-vinylfulvene

6-Vinylfulvenes constitute a class of fulvenes that are difficult to access due to the lack of a general method for their synthesis. In particular, the unsubstituted parent system has been very difficult to obtain by existing methods. In this communication we describe a convenient 3-step protocol for the synthesis of the title compound by way of sulfide oxidation and subsequent sulfoxide elimination.

Temperature-dependent, competitive 1,3-acyl shift versus decarbonylation of a cyclopropanone intermediate

Photooxygenation of 1,1,3-trimethyl-1,2-dihydropentalene gives an unstable endoperoxide which upon decomposition delivers a bicyclic cyclopropanone intermediate; this species either extrudes CO to give a cycloheptadienone or undergoes a 1,3-acyl shift, both processes occurring most likely in a stepwise manner via diradical intermediates. Alternatively, C3a-C4 cleavage in the dioxygen diradical derived from the endoperoxide yields a 2-cyclopropyl substituted cyclopentadienone epoxide.

An efficient catalytic method for fulvene synthesis

The effects of the nature and amount of base, substrate structure, amount of added water and solvent on the condensation of carbonyl compounds with cyclopentadiene in the presence of secondary amines were investigated. Based on these studies, a new efficient and green synthesis of fulvenes was developed.

1,2-Dihydropentalenes from fulvenes by [6 + 2] cycloadditions with 1-isopropenylpyrrolidine

In situ generated acetone pyrrolidine enamine undergoes [6 + 2] cycloadditions with fulvenes to give 1,2-dihydropentalenes. This ring annulation method works particularly well with 6-monosubstituted fulvenes and is subject to steric hindrance at C-6 of the fulvene. On the basis of mechanistic studies, optimal conditions have been developed for a one-pot synthesis of 1,2-dihydropentalenes using catalytic amounts of pyrrolidine.