Cyclopropenylidene 2a ++ 2b is of considerable theoretical interest"]. Until now, only the diphenyl[*] and bis(dia1-kylamino) derivatives[31 had been detected indirectly in trapping reactions. We report here the generation of cyclopropenylidene 2 by flash thermolysis and its matrix isolation.
2a
b 34 5
tiThe quadricyclane derivative 114] was heated to 523 K under conditions of preparative vacuum flash pyrolysis at 1 torr. The products, which were trapped at 77 K, underwent exothermic polymerization upon thawing; in addition, benzene 3 was isolated in almost quantitative yield. If water was not completely excluded from the reaction, cyclopropenol 4 and dicyclopropenyl ether 5 were formed [']. This indicates that in the pyrolysis of 1 a C,H2-fragment having an intact cyclopropene ring is cleaved off.If the product of the pyrolysis is condensed with a large excess of argonL6] onto a window at 10 K immediately after it leaves the hot zone, the IR spectrum shows, apart from the bands from benzene, four additional absorptions (Table 1). Within the normal variations of calculations of this type (the calculated frequencies are ca. 10% too high), the frequencies and relative intensities are consistent with those of the four most intense IR bands predicted for 2['"]. Four other bands are too weak to be observed under our experimental conditions. The presence of cyclopropenylidene 2 is confirmed by the following experiment: Upon irradiation (A > 360 nm) of the matrix-isolated pyrolysis product of 1, only the four IR bands belonging to 2 disappear. Simultaneously, new absorptions at 3390,3366,412,405,261, and 248 cm-' appear, whose frequencies correspond to the known values for matrix-isolated propynylidene 6"l. The photolytic conversion of 2 into 6 can also be observed using ESR spectroscopy. In accord with the singlet ground-state of 2"], the ESR spectrum of the matrix-isolated product of pyrolysis exhibits no triplet signal. After irradiation, the known spectrum[" of the triplet carbene 6 is obtained, consistent with the results of IR studies.2 does not react when the matrix is tempered for several hours at 35-40 K. After evaporation of the argon, a brown, polymeric residue remains. Evidence for the formation of the interesting dimer of 2, 1,l-bicyclopropenylidene, could not be obtained.Summary: Cyclopropenylidene 2 is stable in an argon matrix at 10 K and has, as predicted"], a singlet groundstate. (1982) 801) flanged directly onto the vacuum shroud of the cryostat (Displex Closed Cycle Refrigerations System CSA 202, Air Products). 1 was cooled to 223 K in a flask attached directly to the pyrolysis tube lo-' torr), pyrolyzed at 820-Y20 K within 5 h, and condensed with a large excess of argon onto the C'sl window (10 K, distance from end to the hot zone ca. 5 cm).[7] C.
