Triplet carbenes persistent

As to other carbenes, some react as triplets, some as singlets, and others as singlets or triplets, depending on how they are generated. There are, however, molecules that generate persistent triplet carbenes. 

Another interesting allenic intermediate is generated when di(9-anthryl)diazo-methane (256) is deazotized by irradiation in benzene (Scheme 5.39). The carbene produced, 257, belongs to the long-sought persistent triplet carbenes, although its... 

It should also be noted that persistent triplet carbenes have been discovered by Tomioka.12... 

Since this discovery, a few persistent triplet carbenes have been prepared and other types of stable singlet carbenes have been isolated. Triplet carbenes are discussed in Chapter 9 by H. Tomioka in this volume, and therefore this chapter will be focused on singlet carbenes. 

Much time and effort has been spent in recent years seeking long-lived carbenes.92 This flurry of activity has met limited success. Persistent triplet carbenes,93 such as triplet bis(10-phenylanthracen-9-yl)carbene (11) (t /2 — 19 min, D/hc — 0.105 cm-1) (Fig. 7),92a 93a b 94 constitute one area of promising research. Ylide-like singlet carbenes, such as Breslow thiazol-2-ylidenes (12),95 Arduengo imidazolin-2-ylidenes (13),92f 96,97 Bertrand phosphinocarbenes (14),920,98 and Wanzlick imidazolidin-2-ylidenes (15),97,99 comprise another (Fig. 8).100 However, supramolecular inclusion of more typical carbenes within suitable hosts may offer distinct advantages. 

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