Abstract:is weakly coupled to five vicinal hydrogens and would be expected to appear as a broad, ill-resolved band. In confirmation of the postulate that the resonance from the hydrogen at C-3 is shifted upfield and appears at 1.48 ppm, irradiation of this part of the spectrum in a double resonance experiment42 resulted in the transformation of the triplet centered at 2.26 ppm to a doublet centered at that point. Thus, the nmr spectrum appears to be more readily accommodated to structure 15 than to structures 12 or 13.
“…This result does not change upon dilution of the alkene with less reactive cosolvents 9b. This result and the work of Baer and Gutsche indicate that spin equilibration of PC is rapid in solution relative to the rate of its bimolecular reactions. Typically, interception of the higher energy, more reactive singlet state predominates (Scheme ), as shown by Moss and Tomioka who have chemically intercepted 3 PC, but at −196°, where the equilibrium greatly favors the ground triplet state. 9b, 1 …”
Laser flash photolysis (LFP) (XeCl, 308 nm, 17 ns) of
phenyldiazomethane and pentafluorophenyldiazomethane
releases phenylcarbene (PC) and pentafluorophenylcarbene (PFPC),
respectively. In acetonitrile solvent the
carbenes react rapidly to form nitrile ylides which have convenient
absorption maxima for optical detection
(λmax = 350 nm). Phenylcarbene and
pentafluorophenylcarbene each react with acetonitrile with an
absolute
rate constant of 2.4 × 106 M-1
s-1 in
CF2ClCFCl2 (Freon-113) at ambient
temperature. The lifetimes of
spin-equilibrated PC and PFPC are 190 and 500 ns in Freon-113,
respectively, and the lifetime of each carbene
is deduced to be 22 ns in neat acetonitrile. LFP of phenyl and
pentafluorophenyldiazomethane in the presence
of pyridine leads to the expected pyridine ylide. The observed
absolute rate constants of reaction of spin
equilibrated PC and PFPC with pyridine are 1.9 × 107 and
5.1 × 107 M-1
s-1, respectively, in Freon-113
at
ambient temperature. From this data, one can deduce
singlet−triplet splittings (ΔG
ST, 298 K) of
2.3 and 3.1
kcal/mol for PC and PFPC, respectively, which is the difference in
energy between the triplet minimum and
the point of intersection of the singlet and triplet carbene plus
pyridine surfaces. LFP of phenyldiazomethane
and pentafluorophenyldiazomethane in acetonitrile containing carbene
scavengers (e.g., alcohols, alkenes,
and silanes) gives reduced yields of nitrile ylides. Analysis of
the yield of ylide as a function of quencher
by the Stern−Volmer method gives k
Qτ values
of carbene quenching. Low-temperature (77 K) photolysis
of
pentafluorophenyldiazomethane generates the persistent EPR spectrum of
triplet pentafluorophenylcarbene
which is the ground state of this carbene. However, photolysis of
pentafluorophenyldiazomethane at ambient
temperature generates stable reaction products derived from capture of
singlet pentafluorophenylcarbene. Thus
the properties of pentafluorophenylcarbene are remarkably similar to
those of phenylcarbene. Each carbene
has a triplet ground state but reacts in solution at ambient
temperature through a low-lying excited singlet
state. The lack of a fluorine substituent effect on the behavior
of phenylcarbene is compared to that in
singlet phenylnitrene.
“…This result does not change upon dilution of the alkene with less reactive cosolvents 9b. This result and the work of Baer and Gutsche indicate that spin equilibration of PC is rapid in solution relative to the rate of its bimolecular reactions. Typically, interception of the higher energy, more reactive singlet state predominates (Scheme ), as shown by Moss and Tomioka who have chemically intercepted 3 PC, but at −196°, where the equilibrium greatly favors the ground triplet state. 9b, 1 …”
Laser flash photolysis (LFP) (XeCl, 308 nm, 17 ns) of
phenyldiazomethane and pentafluorophenyldiazomethane
releases phenylcarbene (PC) and pentafluorophenylcarbene (PFPC),
respectively. In acetonitrile solvent the
carbenes react rapidly to form nitrile ylides which have convenient
absorption maxima for optical detection
(λmax = 350 nm). Phenylcarbene and
pentafluorophenylcarbene each react with acetonitrile with an
absolute
rate constant of 2.4 × 106 M-1
s-1 in
CF2ClCFCl2 (Freon-113) at ambient
temperature. The lifetimes of
spin-equilibrated PC and PFPC are 190 and 500 ns in Freon-113,
respectively, and the lifetime of each carbene
is deduced to be 22 ns in neat acetonitrile. LFP of phenyl and
pentafluorophenyldiazomethane in the presence
of pyridine leads to the expected pyridine ylide. The observed
absolute rate constants of reaction of spin
equilibrated PC and PFPC with pyridine are 1.9 × 107 and
5.1 × 107 M-1
s-1, respectively, in Freon-113
at
ambient temperature. From this data, one can deduce
singlet−triplet splittings (ΔG
ST, 298 K) of
2.3 and 3.1
kcal/mol for PC and PFPC, respectively, which is the difference in
energy between the triplet minimum and
the point of intersection of the singlet and triplet carbene plus
pyridine surfaces. LFP of phenyldiazomethane
and pentafluorophenyldiazomethane in acetonitrile containing carbene
scavengers (e.g., alcohols, alkenes,
and silanes) gives reduced yields of nitrile ylides. Analysis of
the yield of ylide as a function of quencher
by the Stern−Volmer method gives k
Qτ values
of carbene quenching. Low-temperature (77 K) photolysis
of
pentafluorophenyldiazomethane generates the persistent EPR spectrum of
triplet pentafluorophenylcarbene
which is the ground state of this carbene. However, photolysis of
pentafluorophenyldiazomethane at ambient
temperature generates stable reaction products derived from capture of
singlet pentafluorophenylcarbene. Thus
the properties of pentafluorophenylcarbene are remarkably similar to
those of phenylcarbene. Each carbene
has a triplet ground state but reacts in solution at ambient
temperature through a low-lying excited singlet
state. The lack of a fluorine substituent effect on the behavior
of phenylcarbene is compared to that in
singlet phenylnitrene.
“…Singlet to triplet intersystem crossing (ISC) of diarylcarbenes proceeds on a time scale of hundreds of picoseconds. ISC is typically fast 13 and reversible in aryl- and diarylcarbenes relative to bimolecular reactions of either the singlet or lower energy triplet state . Given these facts, it seems clear that spin equilibration of carbene 2 is complete in Freon-113 and in hexafluorobenzene and is faster than WR to form ketene 3 .…”
Photolysis of methyl 2-diazo(2-naphthyl)acetate releases singlet 2-naphthyl(carbomethoxy)carbene.
The singlet carbene relaxes to the lower energy triplet state within 350 ps−1 ns. Singlet to triplet carbene
intersystem crossing is much faster than Wolff rearrangement to the corresponding ketene. The barrier to
Wolff rearrangement of the spin-equilibrated carbene is 3.4 kcal/mol in hexafluorobenzene. In this system,
ketene is formed from the carbene and is not formed to a significant extent from an excited state of the diazo
compound.
“…Our enthusiasm was whetted by the suspicion that, because of its "slow" spin state equilibration,5 the temperature dependence of the reactions of 1 with alkenes might be rather different from that of diphenylcarbene,4,13 for which spin state equilibration appears to be rapid relative to intermolecular reaction, and also different from the behavior of phenylcarbene,11 for which rapid spin state equilibration has been suggested. 14 The results, described below, demonstrate that the chemistry of 1 at low temperature does indeed involve a substantially enhanced triplet component, and that, in parallel with the behavior of other arylcarbenes,11-13 the hallmark of low temperature triplet fluorenylidene chemistry is abstraction from rather than addition to alkenes.…”
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