Abstract:Benzylchlorocarbene (1, BCC) was generated
photochemically from benzylchlorodiazirine (2)
in
isooctane, methylcyclohexane (MCH), and tetrachloroethane (TCE) at
temperatures from ∼30 to
−75 °C. At −70 °C in isooctane, the identified products
included Z/E-β-chlorostyrenes 4
(46.6%),
α-chlorostyrene 5 (2.4%), 1,1-dichloro-2-phenylethane
6 (1.9%), a BCC-isooctane insertion product
8 (5.5%), carbene dimers 9 (3.8%), and azine
3 (30%). The significant incursion of
intermolecular
products 3, 8, and 9 implies that
las… Show more
“…Thus, not all of the carbene can be diverted to cyclopropane, even at infinite concentration of TME (Scheme ). Furthermore, some dependence of the intercept of plots of [ 3 /adduct (e.g., 4 )] versus 1/[alkene] on the nature of the alkene was found, as predicted by the carbene−alkene complex mechanism . Subsequently, it was found that the E / Z ratio of β-chlorostyrenes varies with [TME], which is also consistent with Scheme .…”
Section: Introductionsupporting
confidence: 82%
“…Subsequently, it was found that the E / Z ratio of β-chlorostyrenes varies with [TME], which is also consistent with Scheme . The kinetics and spectroscopy of benzylchlorocarbene have also been studied by laser flash photolysis techniques. , 1 2 …”
The curved plots of (carbene adduct)/(carbene-rearrangement product) versus carbene trapping agent,
tetramethylene [TME], reported with benzylchlorodiazirine 1 have been reproduced. However, with the use
of a non-nitrogenous precursor, plots of this type are approximately linear over the range of [TME] employed.
Thus, any complex formed between benzylchlorocarbene and TME must collapse to form cyclopropane faster
then it can fragment with rearrangement to β-chlorostyrene and TME. Diazirine 1 does photoisomerize to
diazo compound 7, but this process is inefficient (φ = 0.075) and is not likely to be responsible for the curvature
in plots of adduct/styrene versus [TME] observed with the diazirine precursor. Thus, the second, noncarbene,
pathway to β-chlorostyrene is neither a carbene−olefin complex nor a diazo intermediate. It is proposed that
the second pathway involves a rearrangement in the excited state of the diazirine, although other explanations
cannot be discarded.
“…Thus, not all of the carbene can be diverted to cyclopropane, even at infinite concentration of TME (Scheme ). Furthermore, some dependence of the intercept of plots of [ 3 /adduct (e.g., 4 )] versus 1/[alkene] on the nature of the alkene was found, as predicted by the carbene−alkene complex mechanism . Subsequently, it was found that the E / Z ratio of β-chlorostyrenes varies with [TME], which is also consistent with Scheme .…”
Section: Introductionsupporting
confidence: 82%
“…Subsequently, it was found that the E / Z ratio of β-chlorostyrenes varies with [TME], which is also consistent with Scheme . The kinetics and spectroscopy of benzylchlorocarbene have also been studied by laser flash photolysis techniques. , 1 2 …”
The curved plots of (carbene adduct)/(carbene-rearrangement product) versus carbene trapping agent,
tetramethylene [TME], reported with benzylchlorodiazirine 1 have been reproduced. However, with the use
of a non-nitrogenous precursor, plots of this type are approximately linear over the range of [TME] employed.
Thus, any complex formed between benzylchlorocarbene and TME must collapse to form cyclopropane faster
then it can fragment with rearrangement to β-chlorostyrene and TME. Diazirine 1 does photoisomerize to
diazo compound 7, but this process is inefficient (φ = 0.075) and is not likely to be responsible for the curvature
in plots of adduct/styrene versus [TME] observed with the diazirine precursor. Thus, the second, noncarbene,
pathway to β-chlorostyrene is neither a carbene−olefin complex nor a diazo intermediate. It is proposed that
the second pathway involves a rearrangement in the excited state of the diazirine, although other explanations
cannot be discarded.
“…For the second criterion, the ratio A H /A D was 0.28 (A H = 97.9, A D = 344.2), much less than 0.7, which implies tunneling. This suggestion is supported by ample precedent for tunneling in 1,2-H shifts, particularly in carbenes (17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27), and also in cyclopentadienes (14). The barriers in these reactions vary from ca.…”
Hydrogen atom transfer reactions between the aldose and ketose are key mechanistic features in formose chemistry by which formaldehyde is converted to higher sugars under credible prebiotic conditions. For one of these transformations, we have investigated whether hydrogen tunneling makes a significant contribution to the mechanism by examining the deuterium kinetic isotope effect associated with the hydrogen transfer during the isomerization of glyceraldehyde to the corresponding dihydroxyacetone. To do this, we developed a quantitative HPLC assay that allowed us to measure the apparent large intrinsic kinetic isotope effect. From the Arrhenius plot of the kinetic isotope effect, the ratio of the preexponential factors A H /A D was 0.28 and the difference in activation energies E a(D) − E a(H) was 9.1 kJ·mol −1 . All these results imply a significant quantum-mechanical tunneling component in the isomerization mechanism. This is supported by multidimensional tunneling calculations using POLYRATE with small curvature tunneling.formose reaction | quantum tunneling | hydride shift | prebiotic reactions W e have described a new mechanism for the formose reaction ( Fig. 1) (1), essentially the same as we had proposed earlier (2) except that the isomerizations of aldose to ketose and the reversal involve a hydride shift rather than an enolization (3-7). Our evidence came from the finding that 2-deuteroglyceraldehyde was converted to 1-deuterodihydroxyacetone under conditions of the formose reaction, with catalysis by Ca 2+ at pH 12. In 2001 Nagorski and Richard had reported an extensive study of the interconversion of glyceraldehyde and dihydroxyacetone by either enolization or hydride shift and had seen that with Zn 2+ the hydride shift mechanism was the exclusive process, by a mechanism closely related to our more recent one (8). We also showed that the isomerization of the ketose erythrulose to the aldose aldotetrose in D 2 O did not lead to deuterium incorporation, as it would have in an enolization process, so it also uses the hydride shift mechanism (1). The first study of the formose reaction in D 2 O was performed by Benner, who saw that no deuterium was incorporated in the intermediates; for some reason he did not invoke hydride shift mechanisms (6).It should be mentioned that we saw that the presence of formaldehyde in the formose reaction in D 2 O led to trapping of any enols formed; without formaldehyde, as in our previous study, there is subsequent deuterium incorporation in dihydroxyacetone, but not significant in glyceraldehyde. We saw that the glyceraldehyde was present almost entirely as its hydrate, a gem diol, but dihydroxyacetone was present mainly as the ketone. This reverses the normally accepted enolization relative rates.
Materials and MethodsIn a hydride shift the distance traveled by the proton is small, comparable to the range of its wave character, so we have investigated the possibility that there is a quantum-mechanical tunneling process involved (9). We find that there is indeed tunnelin...
“…Azine can be formed by several mechanisms including the reaction of a carbene with its diazirine precursor. This reaction is very fast and complicates analysis of the kinetics of benzylchlorocarbene …”
Laser flash photolysis (Nd:YAG laser, 355 nm, 35 mJ, 150 ps) of
dimethyldiazirine and
dimethyldiazirine-d
6 produces dimethylcarbene
(DMC) and dimethylcarbene-d
6
(DMC-d
6), respectively. The
carbenes were trapped with pyridine to form ylides which absorb around
364 nm. It was possible to resolve
the growth of the ylides as a function of pyridine concentration in
Freon-113, α,α,α-trifluoromethylbenzene,
and perfluorohexane as a function of temperature. The observed
rate constant (k
obs) of ylide formation
was
linearly dependent on the concentration of pyridine in all solvents and
at all temperatures. From plots of
k
obs
versus [pyridine] it was possible to extract values of
k
pyr (the absolute rate constant of reaction of
the carbene
with pyridine) and τ, the carbene lifetime in the absence of
pyridine, and their associated Arrhenius parameters.
In Freon-113 and α,α,α-trifluoromethylbenzene the carbenes
decay both by rearrangement and by reaction
with solvent. In perfluorohexane the carbene decay appears to be
predominantly unimolecular. The experimental
results are compared with ab initio molecular orbital
calculations. The experimentally determined barrier
to
disappearance of DMC in perfluorohexane (2.56 ± 0.05 kcal/mol) is
much smaller than that calculated (7.4
± 2 kcal/mol) using ab initio molecular orbital theory.
The Arrhenius parameters and isotope effects indicate
that the rearrangement of DMC in perfluorohexane has a large component
of quantum mechanical tunneling.
The activation energy for the disappearance of
DMC-d
6 in perfluorohexane (5.63 ± 0.03
kcal/mol) is consistent
with calculations which indicate that QMT makes only a minor
contribution to the deuterated system under
the conditions of this study.
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