Synthesis of Naphthalene Derivatives through Platinum(II)-Catalyzed Reaction of 2-Alkynylbenzoates with Vinyl Ethers

Abstract

A concise method for the preparation of 1-acyl-4-alkoxy- or 1-acyl-4-alkylsulfanylnaphthalenes has been developed by the PtCl₂-catalyzed reaction of o-ethynylbenzoates or benzothioates with vinyl ethers. Treatment of o-alkynylbenzoate derivatives with PtCl₂ generated the platinum(II)-containing carbonyl ylides as a key reactive intermediate and these species reacted with vinyl ethers to give substituted naphthalenes in good yields. The presence of the platinum-containing carbonyl ylide was confirmed by NMR analyses of the mixture of o-ethynylbenzoate and [PtCl₂(H₂C=CH₂)]₂. Furthermore, it was suggested that the naphthalene derivatives were produced by [3+2] cycloaddition of the ylide species with vinyl ethers, followed by 1,2-alkyl migration of the generated platinum-carbene intermediates.

Key words

platinum - naphthalene - carbonyl ylide - carbene - [3+2]-cycloaddition

References

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12

In marked contrast with the platinum-catalyzed reaction, treatment of isopropyl o-ethynylbenzoate (1a) and butyl vinyl ether with a catalytic amount of preformed W(CO)₅(thf) did not produce the expected adduct 2a at all, and the starting material 1a remained intact.

13

To clarify the reason for this result, NMR measurement of the mixture of the amide 1g and platinum(II) complex was performed. Formation of the dimethylamino-substituted platinum-containing carbonyl ylide was confirmed even at 230 K. However, this ylide did not react even with excess amounts of n-butyl vinyl ether and was gradually decomposed at 273 K, resulting in a complex mixture at 300 K. Probably the ylide intermediate is increasingly stabilized by the electron-donation from the dimethylamino group to react with the vinyl ether.

15

Since PtCl₂ is barely soluble in CDCl₃, [PtCl₂(H₂C=CH₂)]₂ was used in this study.

16

This platinum satellite with a coupling constant of 39.4 Hz (² J _Pt,H) was observed more clearly in the ¹H NMR spectra obtained using a 300 MHz spectrometer than that using a 500 MHz spectrometer (Figure [1] , c).

19

When N₂ gas was bubbled into a mixture of the benzoate 1a, platinum(II) complex and 10 equiv of vinyl ether in CDCl₃ at 273 K, the ylides A and B almost disappeared and the intermediate C was observed as the major species. This phenomenon strongly suggests that the structure of intermediate C is a platinum-containing carbonyl ylide having butyl vinyl ether on platinum as shown in Figure [5] .