Synthesis of 3,4-Bis(benzylidene)cyclobutenes

Abstract

The syntheses of several derivatives of 3,4-bis(benzylidene)cyclobutene are reported. Previously unknown 1,2-dibromo-3,4-bis(benzylidene)cyclobutene was obtained through in situ generation of 1,6-diphenyl-3,4-dibromo-1,2,4,5-tetraene followed by electrocyclic ring closure. Ensuing reduction and metal-catalyzed cross-coupling provided additional derivatives. The effects of ring strain on the geometry and electronics of these derivatives were examined by X-ray crystallography and ^¹H NMR spectroscopy, respectively.

Key words

electrocyclic reactions - ring closure - allenes - highly strained ring systems - metal-catalyzed cross-coupling

References and Notes

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13

Dibromide 6 was found to rearrange to 5 (ca. 50%) and hydrolyze to 4 (ca. 50%) upon chromatography on silica gel and decomposed partially during recrystallization attempts.

16

Photolysis experiments were conducted in hexane using a Hg pen-lamp, without the use of optical filters.

17

Spartan B3LYP/6-31+g*, relative E (kcal/mol): 7 (in,out) = -0.4487, 7 (in,in) = 0, 7 (out,out) = 0.0056.

19

Attempts to purify compound 8 via column chromatography or recrystallization were unsuccessful due to its limited stability. Any impurities visible in Figure S1 are believed to be primarily high-molecular-weight decomposition products.