Cyclopentadienes (Cps) are highly useful synthetic intermediates in the field of organic and organometallic chemistry. They are useful not only as a reactive diene component in the Diels-Alder reaction [1] but also as a precursor for the preparation of transition-metal complexes with Cp-type ligands.[2] However, the preparation of well-defined, highly substituted cyclopentadienes is not necessarily easy owing to the absence of general methods [3] and also to the facile migration of the endocyclic double bonds.[4] Herein we describe a novel method for the preparation of highly substituted cyclopentadiene derivatives based on the Pt II -catalyzed cyclization of 1,2,4-trienes in which platinumcarbene intermediates play an important role in determining the reaction pathways.We expected that treatment of 1,2,4-trienes with appropriate electrophilic transition-metal complexes [5] would generate a,b-unsaturated carbene complexes III through cyclization of p complex I [6] and/or pentadienyl cationic complex II, and that the produced carbene complex III would further undergo useful transformations to give cyclopentadienes or their derivatives [Eq. (1)].On the basis of these considerations, we initiated our study to find optimal conditions for this reaction using 5-(cyclohex-1-enyl)-1-phenylhepta-3,4-diene (1 a) as a substrate, with a catalytic amount of various electrophilic transition-metal complexes. The expected cyclization of 1 a proceeded most effectively with 0.05 equivalents of PtCl 2 [7,8] in dichloroethane at room temperature, and the corresponding cyclopentadiene 2 a was obtained in 86 % yield with about 95 %
Transition-metal-containing zwitterionic intermediates generated through the electrophilic activation of alkynes toward nucleophilic attack by a carbonyl oxygen atom or imino nitrogen atom can be used in addition reactions with nucleophiles or cycloaddition reactions with alkenes or alkynes to prepare synthetically useful heterocyclic compounds. [1][2][3][4][5] However, in almost all cases, it is necessary to employ rigid aromatic substrates for the effective generation of the zwitterionic species. Herein, we report a catalytic method for the generation of nonconjugated platinum ( [6] was found to be very effective. Thus, the treatment of 1 a with platinum(II) chloride (10 mol %) in the presence of n-butyl vinyl ether at room temperature gave the 8-oxabicyclo[3.2.1]octane derivative 2 a in 78 % yield as a single diastereoisomer with the butoxy substituent in an exo orientation.We propose the following mechanism for the formation of 2 a: The electrophilic activation of the alkyne moiety in 1 a by platinum(II) chloride induces the attack of the carbonyl oxygen atom onto the alkyne in a 6-endo manner to generate the platinum-containing carbonyl ylide 3 a, which then undergoes a [3+2] cycloaddition reaction with n-butyl vinyl ether to give the nonstabilized carbene complex 4 a. Finally, a 1,2-hydrogen-atom shift to the carbene carbon atom gives the product 2 a with regeneration of the catalyst (Scheme 1).
[reaction: see text] A concise method for the preparation of 1-acyl-4-alkoxy- or 1-acyl-4-alkylsulfanylnaphthalenes has been developed by the reaction of o-ethynylbenzoates or benzothioates with vinyl ethers, in the presence of a catalytic amount of PtCl(2). It is proposed that the reaction proceeds through [3 + 2]-cycloaddition of the platinum-containing carbonyl ylides followed by 1,2-alkyl migration.
A Pt(II)-catalyzed [3 + 2] cycloaddition reaction of silyl propadienyl ethers and alkenyl ethers has been developed as the first example of the utilization of allenes as a three-carbon unit in a transition-metal-catalyzed intermolecular cycloaddition reaction. Pt(II)-containing 1,3-dipole equivalents generated by electrophilic activation of silyl propadienyl ethers using a Pt(II) catalyst reacted with various electron-rich alkenes to give synthetically useful functionalized cyclopentene derivatives in high yield with wide generality.
Novel tungsten-containing carbonyl ylides 7, generated by the reaction of the o-alkynylphenyl carbonyl derivatives 1 with a catalytic amount of W(CO)(5)(thf), reacted with alkenes to give polycyclic compounds 5 through [3 + 2]-cycloaddition reaction followed by intramolecular C-H insertion of the produced nonstabilized carbene complex intermediates 8. In the presence of triethylsilane, these tungsten-containing carbene intermediates 8 were smoothly trapped intermolecularly by triethylsilane to give silicon-containing cycloadducts 17 with regeneration of the W(CO)(5) species. By this procedure, the scope of alkenes employable for this reaction was clarified. The presence of the tungsten-containing carbonyl ylide 7c was confirmed by direct observation of the mixture of o-ethynylphenyl ketone 1c and W(CO)(5)(thf-d(8)). Careful analysis of the intermediate by 2D NMR, along with the observation of the direct coupling with tungsten-183 employing the (13)C-labeled substrate, confirmed the structure of the ylide 7c. Examination using (E)- or (Z)- vinyl ether revealed that the [3 + 2]-cycloaddition reaction proceeded in a concerted manner and that the facial selectivity of the reaction differed considerably depending on the presence or absence of triethylsilane. These results clarified the reversible nature of this [3 + 2]-cycloaddition reaction.
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