Diazirines triplet carbene reactions

Most of these reactions have been carried out in the gas phase. At higher pressure triplet carbenes are involved. However it is not clear if intersystem crossing occurrs already in the diazirine or in the carbene formed 63>. 

To determine whether room-temperature supramolecular constraint of carbene 64 could alter its reactivity by yielding ring-expanded allene 65, triplet carbene iso-64, or benzocyclopropene 68, diazirine 63 was prepared,166 and then included within CyDs and FAUs prior to irradiation.167 The photolysis of diazirine 63 is expected to form relatively long-lived carbene 64, which usually decomposes via intermol-ecular reactions (Scheme 14). 

The thermolysis (- 10 to -t- 25°C) or photolysis (2 > 300 nm, - 10°C) of 3//-diazirine-3-car-bonitriles generates cyanocarbenes under very mild reaction conditions (see Houben-Weyl, Vol. E19b, pl206). Their addition to electron-rich alkenes yielded cyclopropane carbonitriles in moderate yields (Table 3). The 3//-diazirines-3-carbonitriles were prepared from 3-halodiazirines, mostly in situ, by an exchange reaction with tetrabutylammonium cyanide. The cyclopropanation of (Z)- or ( )-but-2-ene proceeded nonstereospecifically, which is explained by the participation of a triplet carbene. 

In the 1950s and 1960s chemists learned how to generate carbenes from haloform, diazirine and diazo compound precursors as transient intermediates in solution. [1,5,6] The chemists of this era identified the products of carbene reactions and learned how to make these reactions synthetically useful. They postulated a framework for carbenes (Scheme 1) with ground or low lying triplet states which is still the starting point in all mechanistic discussions. 

The reaction of triplet diphenylcarbene with pyridine has been well studied, and a mechanism proposed from kinetic data (90TL953). The carbenes generated from laser flash photolysis of alkylbromo- and alkylfluoro-diazirines were trapped by pyridine to form the pyridinium ylides... 

---