Kinetic Studies of the Cyclization of Singlet Vinylchlorocarbenes

Cyclobutenylidene: A Multifaceted Two-Coordinate Carbon Species Obtained via Skeletal Editing of a Cyclopropenylidene

C₄H₄ isomers not only serve as a basis to understand the chemical properties of hydrocarbons but are possible intermediates in combustion and organic reactions in outer space. Cyclobutenylidene (CBY), an elusive C₄H₄ isomer, is often proposed as a key intermediate in transition-metal-catalyzed metathesis and cycloaddition reactions between carbon-carbon multiple bonds. The geometrical structure of cyclobutenylidene predicted by calculations had been debated as whether it should be regarded as a carbocyclic carbene or a strained bridgehead alkene. Here, we report the synthesis of a crystalline cyclobutenylidene derivative, namely, a 3-silacyclobut-2-en-4-ylidene (SiCBY) via "carbene-to-carbene ring-expansion" reaction of an isolable diaminocyclopropenylidene induced by a silicon analogue of a carbene (silylene). The SiCBY exhibits multifaceted electronic properties which are corroborated by its extremely strong electron-donating properties and ambiphilic reactivity toward small gaseous molecules and C-H bonds. This result introduces an exciting strategy as well as a molecular motif to access low-valent carbon species with unusual electronic properties.

Synthesis of vinyl germylenes

Reaction of a disilylated germylene phosphane adduct with phenylacetylene unexpectedly occurs stereo- and regioselectively as insertion into the Si–Ge bond.

Excited state carbene formation from UV irradiated diazomethane

The laser flash photolysis process of diazomethane has been studied by using a real time propagation time-dependent density functional theory (RTP-TDDFT) combined with molecular dynamics. The activation energy barrier for disintegrating diazomethane into nitrogen (N(2)) and carbene (CH(2)) molecules significantly decreases in the electronic excited S(1) state compared to that in the S(0) ground state. Furthermore, the produced carbene molecule can be in the electronic excited state of (1)CH(2) ((1)B(1)) instead of the lowest state among singlet states (1)CH(2) ((1)A(1)), which is evident in the wave function characteristics of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) throughout the disintegration. This is regarded as the initial stage of the rearrangement in the excited state (RIES), the evidence of which has been given by experiments in the past decade. In the RIES mechanism scheme, we suggest that the photoreaction in the S(1) state contributes considerably to the photochemistry of carbene formation. The passing near the S(1)/S(0) conical intersection, which allows the transition to ground state diazomethane producing the lowest singlet state carbene molecule, is considered a rare event from our molecular dynamics, although this has been regarded as the dominant mechanism in previous theoretical studies.

Ultrafast Study of p-Biphenylyldiazoethane. The Chemistry of the Diazo Excited State and the Relaxed Carbene

Ultrafast photolysis of p-biphenylyldiazoethane (BDE) produces an excited state of the diazo compound in acetonitrile, cyclohexane, and methanol with lambdamax = 490 nm and lifetimes of less than 300 fs. The decay of the diazo excited state correlates with the growth of singlet carbene absorption at 360 nm. The optical yields of diazo excited states produced by photolysis of p-biphenylyldiazomethane (BDM) and BDE are the same; however, the optical yield of singlet p-biphenylylmethylcarbene (1BpCMe) is 30-40% less than that of p-biphenylylcarbene (1BpCH) in all three solvents. The results are explained by rearrangement in the excited state (RIES) of BDE to form p-vinylbiphenyl (VB) in parallel with extrusion of nitrogen to form 1BpCMe in reduced yield. This interpretation is consistent with product studies (ethanol-OD in cyclohexane) which indicate that there is an approximately 25% yield of VB that is formed by a mechanism that bypasses the relaxed singlet carbene. The decay of 1BpCMe is biexponential, and that of 1BpCH is monoexponential. This is attributed either to efficient relaxation of vibrationally excited 1BpCMe by 1,2 migration of hydrogen to form VB (minor) or to the increased number of low-frequency vibrational modes provided by the methyl group (major). A methyl group retards the rate of intersystem crossing (ISC), relative to a hydrogen atom, and ISC is more rapid in nonpolar solvents. Reaction of 1BpCMe with methanol is much faster than spin equilibration. Both the lifetime of 1BpCMe and 1BpCH are the same in cyclohexane and in cyclohexane-d12. This demonstrates that spin equilibration is faster than reaction of either carbene with the solvent. The lifetimes of 1BpCMe and 1BpCMe-d3 are the same in cyclohexane. This indicates that 1,2 hydrogen migration of 1BpCMe to form VB is slower than spin equilibration in cyclohexane. In acetonitrile, however, the lifetime of 1BpCMe-d3 is 1.5 times longer than that of 1BpCMe in the same solvent. Thus, in acetonitrile, where ISC is slow, the rate of 1,2 hydrogen shift of 1BpCMe is competitive with ISC. In cyclohexene, the lifetime of 1BpCH is shortened relative to that in cyclohexane. The lifetime of 1BpCMe is the same in cyclohexene and cyclohexane. The data indicate that spin relaxation is slow relative to reaction of 1BpCH with neat alkene but that spin relaxation is fast for 1BpCMe relative to reaction with neat cyclohexene.