Rearrangements of 2‐ and 3‐Furfurylidene

Abstract

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...

Photochemistry of Furyl- and Thienyldiazomethanes: Spectroscopic Characterization of Triplet 3-Thienylcarbene

“…In accord with the results of earlier studies, 17 irradiation of diazo compound 4 ( λ > 613 nm; Ar, 10 K) causes fragmentation of the furan ring, yielding Z -pent-2-en-4-ynal ( 17 ) (Figure S19). Subsequent irradiation at λ > 444 nm gives rise to E -pent-2-en-4-ynal ( 16 ) (Scheme 9, Figure S20).…”

“…In the current study, we focus on the isomeric furyl- and thienylcarbenes, which have eluded detection and characterization to the present time. 16,17 These fundamental studies of reactive thienyl intermediates are also germane to an understanding of the electronic structure of doped states of thiophene derivatives—materials that have achieved wide use in conducting polymers and other electronic applications. 18–20 …”

Rearrangements of 2-Furfurylidenes − Synthesis of Acylsilanes and Acylstannanes

“…Thus, photolysis of phenyldiazomethane in solid argon at 10 K produces phenylcarbene, which under these conditions is a completely stable compound 8. In contrast, photolysis of matrix‐isolated furfuryldiazomethanes ( 3 , R′ = H) does not result in furfurylidenes 1 , but rather in rearranged products 9,10…”

Rearrangements of 2-Furfurylidenes − Synthesis of Acylsilanes and Acylstannanes

“…[1Ϫ7] Thus, photolysis of phenyldiazomethane in solid argon at 10 K produces phenylcarbene, which under these conditions is a completely stable compound. [9,10] The ring-opening reactions of furfurylidenes 1 were investigated by Hoffman and Shechter almost 30 years ago. [9,10] The ring-opening reactions of furfurylidenes 1 were investigated by Hoffman and Shechter almost 30 years ago.…”

“…[9] Irradiation of 2furylchlorodiazirine, on the other hand, resulted in chloro(2-furyl)carbene 1 (R ϭ H, RЈ ϭ Cl), which, on subsequent visible irradiation, rearranged to the corresponding alkyne 2. [9] Irradiation of 2furylchlorodiazirine, on the other hand, resulted in chloro(2-furyl)carbene 1 (R ϭ H, RЈ ϭ Cl), which, on subsequent visible irradiation, rearranged to the corresponding alkyne 2.…”