Attempts to prepare an all-carbon indigoid system.

Şeref Yildizhan,Henning Hopf,Peter G Jones

doi:10.3762/bjoc.11.42

Şeref Yildizhan, Henning Hopf + Show 1 more

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https://doi.org/10.3762/bjoc.11.42

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Abstract

First attempts are described to prepare a precursor for an all-carbon analog of indigo, the tetracyclic triene 4. Starting from indan-2-one (9) the α-methylene ketone 13 was prepared. Upon subjecting this compound to a McMurry coupling reaction, it dimerized to the bis-indene derivative 17, rather than providing the tetramethyl derivative of 4, the hydrocarbon 14. In a second approach, indan-1-one (18) was dimerized to the conjugated enedione 21 through the bis-1-indene dimer 19. All attempts to methylenate 21 failed, however. When 19 was treated with the Tebbe reagent, the dimer 23 was produced, presumably through a Cope reaction of the intermediately generated isomer 22. The bis-indene derivative 23 can be alkylated with 1,2-dibromoethane to produce a 1:1 mixture of the spiro compounds 24 and 25. Although 9 could be reductively dimerized to 30, the conversion of this olefin to 14 failed.

Highlights

Cross-conjugated organic molecules are defined as unsaturated systems containing two π-electron systems that are in direct conjugation, whereby a third such system is excluded from interaction [3]
Many of the organic compounds that played an important role in the dawn of organic chemistry are cross-conjugated systems or were converted into cross-conjugated organic salts during the color-generating process
Our final attempt for the present to prepare a precursor hydrocarbon for an all-carbon indigoid system rests on the observation that the Thiele condensation of indene (6) with acetone in pyrrolidine/methanol yields the benzofulvene 29 (Scheme 6)

Summary

Introduction

Cross-conjugated organic molecules are defined as unsaturated systems containing two π-electron systems (or lone pairs) that are in direct conjugation, whereby a third such system is excluded from interaction [3]. Our first attempt to prepare hydrocarbon 4 started from indene (6, Scheme 2). Subjecting 13 to a McMurry coupling reaction (TiCl4/HgCl2 in THF) did not result in the formation of the expected dimer 14, a tetramethyl derivative of 4, but furnished dimer 17 in low yield (10%, Scheme 3).

Results

Conclusion