Reactions of carbenes in solidified organic molecules at low temperature

Studies on reactions of carbenes in reactive organic glasses at low temperatures clearly reveal that solution results and liquid phase mechanistic rules cannot be readily extrapolated to matrix conditions. Thus, the usual course of reaction of a carbene with an alkene in solution results in the formation of a cyclopropane for both the singlet and triplet states although a one-step addition possible for singlet carbene produces the cyclopropane stereospecifically and a stepwise pathway with the triplet state affords two possible stereoisomers of the cyclopropane. In a sharp contrast, the formal insertion products into the allylic C-H bonds of alkenes are produced at the expense of the cyclopropane when carbene is generated in alkene matrix at low temperature. Similar results are obtained in the reaction with alcohols, where the C-H insertion products are formed in low temperature alcoholic matrices at the expense of the O-H insertion products which are predominant products in the reaction with alcoholic solution at ambient temperature. The ¹³C labelling experiments as well as deuterium kinetic isotope effects suggest that these C-H insertion products are most probably produced from the triplet carbene, not from the singlet, by abstraction of H atom from the matrix followed by the recombination of the resulting radical pairs. Kinetic studies using ESR and laser flash photolysis techniques demonstrate that the mechanism of a H-atom transfer reaction changes from a completely classical process in a soft warm glass to a completely quantum mechanical tunneling process in a cold hard glass. Thus, as the reaction temperature is lowered, the classical reaction rate decreases, and eventually becomes much slower than decay by hydrogen atom tunneling. The members of the radical pairs which usually diffuse apart in a fluid solution are not able to diffuse apart owing to the limited diffusibility within a rigid matrix and therefore recombine with high efficiency to give the CH “insertion” products. A rather surprising and intriguing difference between the C-H insertion undergone by singlet carbenes in fluid solution at ambient temperatures and one by triplet carbenes in matrix at low temperature is noted. Thus, a marked increase in the primary and secondary C-H insertion over the tertiary is observed in the matrix reaction indicating that triplet carbenes tend to abstract H from less crowded C-H bonds. This is interpreted to indicate that the distance between carbenic center and tunneling H becomes important in H atom tunneling process. More surprisingly, the C-H insertion by triplet carbene by the abstraction-recombination mechanism in a rigid matrix proceeds with retention of the configuration, suggesting that the solid state prevents motion of the radicals to the extent that does not allow racemization to occur. Reactions with heteroatom substrates such as ethers, amines, alkyl halides and ketones are also subject to the matrix effects and the C-H insertion products increase at the expense of singlet carbene reaction products resulting from the interaction with the heteroatoms. Stereoselectivities of cyclopropanation to styrenes are also shown to be affected by the matrix effects. t-Butyl alcohol matrix is shown to be unreactive toward carbenes and thus can be used as a “solvent” in matrix carbene reactions presumably due to a large inert guest cavity provided by bulky tertiary alcohol which binds a molecular aggregate inside it. H atom tunneling in the matrix is also shown to compete with very efficient intramolecular migration of hydrogen to the carbenic center. Migration aptitude as well as stereochemistry are also found to be subject to the matrix effects.