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The photo-and thermochemistry of diazo(2-fury1)methane (3) and diazo(3-fury1)methane (6) was investigated using the matrix isolation technique. Photolysis of 3 results in the formation of (Z)-pent-2-en-4-yn-l-a1 (2) in a clean reaction, in agreement with the observation of Shechter et al. Photolysis or pyrolysis of diazo compound 6 yields (s-Z)-(a-formy1)methylenecyclopropene (s-Z-13) as the only detectable product. Carbene 14 and cyclopropene 15 are proposed as intermediates, but not stable under the reaction conditions and thus not directly observed. The reaction sequence 6 + 13 provides a novel and simple route to methylenecyclopropenes.2-Furfurylidene (1) is a highly reactive carbene which rapidly rearranges under opening of the furane ring to give pent-2-en-4-yn-1 -a1 (2). The thermochemistry of diazo(2-fury1)methane (3) and several derivatives as precursors of 1 has been extensively studied by Hoffman and Shechter ['-3]. The labile 2-2, which could be stored at low temperature but rapidly decomposed at room temperature, was identified as the main product of the rearrangement of 1. The authors concluded that the 1 -+ 2 isomerization is stereospecific, and the E isomer is formed by secondary EIZ isomerization of 2 under the conditions of the thermolysis. Trapping experiments revealed that the carbene exists as a discrete intermediate. There was no evidence for the formation of cyclopropene 4 or carbene 5. The photolysis of l under a variety of conditions gave the same results. Recently, Herges analyzed the topology of the 1 -+ 2 rearrangement and concluded that this is a symmetry-allowed, concerted reaction[4]. Scheme 1 4In contrast to the 2-substituted derivative 3, thermolysis of diazo(3-fury1)methane (6) did not result in the ring opening of the furan system. Again, the formation of the corresponding carbene I was demonstrated by isolating the dimer and trapping products, while rearranged products such as carbene 8 or cyclopropene 15 were not detected.The related diazo(2-thieny1)methane (9) and diazo(3-thieny1)methane (lo), also investigated by Shechter et al., exhibit a very similar thermo-and photochemistry as the fury1Recently, we described the novel photochemical rearrangement of (3-thieny1)methylene (11) to give 12, which provides a novel entry to methylenecy~lopropenes [~].Here we report on the thermo-and photochemistry of diazo compound 6, demonstrating the usefulness of diazo (3-hetary1)methanes as precursors of methylenecyclopropenes. 11 Diazo(2-fury1)methane (3)Diazo compound 3 was generated by warming the sodium salt of the corresponding tosyl hydrazone to 65 "C and trapping the products directly in argon at 10 K (Scheme 2). In the UV spectrum a strong absorption with I, , , = 233 nm and in the IR a strong band at 0 = 2082 cm-' are characteristic of 3, while products of the thermal decomposition of 3 were not observed. Photolysis (h > 435 nm, argon, 10 K) of 3 leads to 2 -2 with strong IR absorptions at 3 = 3325 and 2109 cm-', characteristic of a terminal acetylene group, at P = 1697 cm-', assi...

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The photochemical rearrangement of 4,4-dimethylcyclohexa-2,5-dienylidene (3) to 4,4-dimethylbicyclo-[3.1 .O]hexa-1(6),2-diene (5) was investigated by means of the matrix isolation technique. Carbene 3 was generated in argon matrices at 10 K by photolysis of the corresponding diazo compound (A > 550 nm) and was characterized by IR and UV/Vis spectroscopy and its characteristic thermal reaction with triplet oxygen. Long-wavelength irradiation (A>515 nm) induced an irreversible rearrangement of carbene 3 to the highly strained cyclopropene 5. This is in contrast to the previously investigated 1 H-bicyclo(3.1 .O]hexa-3,5-dien-2-one (I), which is thermally labile even at 10 K and