Diazo chemistry controlling the selectivity of olefin ketonisation by nitrous oxide

Article abstract of DOI:10.1039/b704351a

The thermal reaction of olefins with nitrous oxide was recently put forward as a promising synthetic ketone source. The 1,3-dipolar cycloaddition of N ₂O to the C=C double bond, forming a 4,5-dihydro-[1,2,3]oxadiazole intermediate, was predicted to be the first elementary reaction step. This oxadiazole can subsequently decompose to the desired carbonyl product and N ₂ via a hydrogen shift. In this contribution, Potential Energy Surfaces are constructed at the reliable G2M level of theory and used to evaluate thermal rate constants by Transition State Theory. Compelling theoretical and experimental evidence is presented that an oxadiazole intermediate not only can undergo a hydrogen shift, but eventually also a methyl- or even an alkyl-shift. Special emphasis is also given on a hitherto neglected decomposition of the oxadiazole via a concerted C-C and N-O cleavage. For some substrates, such as internal olefins, this diazo route is negligibly slow, compared to the ketone path, leaving no marks on the selectivity. For cyclopentene the diazo cleavage was however found to be nearly as fast as the desired ketone route. However, the diazo compound, viz. 5-diazopentanal, reconstitutes the oxadiazole much faster upon ring-closure than it is converted to side-products. Therefore, a pre-equilibrium between the diazoalkanal and the oxadiazole is established, explaining the high ketone yield. On the other hand, for primary alkenes, such a concerted C-C and N-O cleavage to diazomethane is identified as an important side reaction, producing aldehydes with the loss of one C-atom. For these substrates, the bimolecular back-reaction of the C _n-1 aldehyde and diazomethane is too slow to sustain an equilibrium with the oxadiazole; diazomethane rather reacts with the substrate to form cyclopropane derivatives. The overall selectivity is thus determined by a combination of H-, methyl- or alkyl-shift, and the eventual impact of a diazo cleavage in the oxadiazole intermediate. the Owner Societies.

Full text of DOI:10.1039/b704351a

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www.rsc.org/pccp | Physical Chemistry Chemical Physics
Diazo chemistry controlling the selectivity of olefin ketonisation by
nitrous oxidew
a
a
b
a
a
Ive Hermans,* Bart Moens, Jozef Peeters, Pierre Jacobs and Bert Sels*
Received 22nd March 2007, Accepted 18th April 2007
First published as an Advance Article on the web 15th May 2007
DOI: 10.1039/b704351a
The thermal reaction of olefins with nitrous oxide was recently put forward as a promising
synthetic ketone source. The 1,3-dipolar cycloaddition of N O to the CQC double bond, forming
2
a 4,5-dihydro-[1,2,3]oxadiazole intermediate, was predicted to be the first elementary reaction
2
step. This oxadiazole can subsequently decompose to the desired carbonyl product and N via a
hydrogen shift. In this contribution, Potential Energy Surfaces are constructed at the reliable
G2M level of theory and used to evaluate thermal rate constants by Transition State Theory.
Compelling theoretical and experimental evidence is presented that an oxadiazole intermediate not
only can undergo a hydrogen shift, but eventually also a methyl- or even an alkyl-shift. Special
emphasis is also given on a hitherto neglected decomposition of the oxadiazole via a concerted
C–C and N–O cleavage. For some substrates, such as internal olefins, this diazo route is
negligibly slow, compared to the ketone path, leaving no marks on the selectivity. For
cyclopentene the diazo cleavage was however found to be nearly as fast as the desired ketone
route. However, the diazo compound, viz. 5-diazopentanal, reconstitutes the oxadiazole much
faster upon ring-closure than it is converted to side-products. Therefore, a pre-equilibrium
between the diazoalkanal and the oxadiazole is established, explaining the high ketone yield. On
the other hand, for primary alkenes, such a concerted C–C and N–O cleavage to diazomethane is
identified as an important side reaction, producing aldehydes with the loss of one C-atom. For
these substrates, the bimolecular back-reaction of the C
aldehyde and diazomethane is too
nÀ1
slow to sustain an equilibrium with the oxadiazole; diazomethane rather reacts with the substrate
to form cyclopropane derivatives. The overall selectivity is thus determined by a combination of
H-, methyl- or alkyl-shift, and the eventual impact of a diazo cleavage in the oxadiazole
intermediate.
OD intermediate via a H-shift yields the desired carbonyl
product and N (Scheme 1). The hypothesis that initially
Introduction
2
The exploration of sustainable and economically viable routes
to ketones and aldehydes remains an ongoing quest. A pro-
mising reaction is the direct ketonization of olefins with
formed epoxides would isomerise to the carbonyl products
had to be discarded as these compounds are inert under the
2a
reaction conditions. The scarce theoretical studies on the
nitrous oxide (N O) in the liquid phase. Although discovered
2
1
quite a time ago, this chemistry was recently reinvestigated
rate-determining cycloaddition of N
2
O report barriers con-
2
siderably higher than for other dipoles such as fulminic acid
7–9
3
–4
and shown to offer large scale potential.
5
severe greenhouse gas is usefully converted to harmless N ,
2
Interestingly, a
and nitrones.
2
N O, in contrast to other dipoles, behaves as
an electrophile and the dipolarophile as a nucleophile; this
and, rather than starting from expensive alcohols, a relatively
cheap olefin feedstock is used. Moreover, the thermal stability
of nitrous oxide up to 800 1C, and the non-radical character of
its oxidation reactions, allows the design of safe and selective
processes. Preliminary theoretical calculations predicted that
type of cycloaddition is referred to as inverse electron demand
8
1
,3-cycloaddition.
the 1,3-dipolar cycloaddition of N O to the unsaturated bond
2
of the olefin is the first elementary reaction step, forming a
6
,5-dihydro-[1,2,3]oxadiazole (OD). Decomposition of the
4
a
Centre for Surface Chemistry and Catalysis, K.U.Leuven,
Kasteelpark Arenberg 23, B-3001 Heverlee, Belgium. E-mail:
ive.hermans@biw.kuleuven.be; bert.sels@biw.kuleuven.be; Fax: +32
16 321998; Tel: +32 16 321648
b
Department of Chemistry, K.U.Leuven, Celestijnenlaan 200F, B-3001
Heverlee, Belgium. Fax: +32 16 327992; Tel: +32 16 327382
w Electronic supplementary information (ESI) available: Energy and
ro-vibrational data of key structures. See DOI: 10.1039/b704351a
Scheme 1 Formation of the 4,5-dihydro-[1,2,3]oxadiazole (OD) and
its unimolecular decomposition to ketone under elimination of N
can be an alkyl group or an H-atom).
. (R
2 3
This journal is ꢀc the Owner Societies 2007
Phys. Chem. Chem. Phys., 2007, 9, 4269–4274 | 4269

1
4
Table 1 Ketonisation of several olefin types by N
.5 MPa N O, STP-conditions); reaction rate and initial product
distribution at 5–10% conversion
2
O (453 K and
with the Gaussian 03 software. Interesting to note is that all
unrestricted calculations automatically defaulted to restricted
calculations, indicating that no open-shell singlet states are
involved. Thermal reaction rates, at 453 K, were evaluated by
2
2
À1
Entry
1
Substrate
Rate/mM h
Selectivity (%)
1
5
a
Transition State Theory (TST), based on the ro-vibrational
PES data obtained at the G2M level of theory. Additionally,
these first-principles rate constants were used to estimate
branching fractions when different pathways were identified
for a given intermediate.
160
a
2
3
15
The reactions were studied in 50 mL stainless steel reactors,
magnetically stirred and equipped with a thermocouple. Be-
fore it was pressurized to 2.5 MPa N
2
O, a filled reactor was
O to remove O in
8
flushed several times with medical grade N
2
2
order to exclude possible radical-chain autoxidation reactions.
Reaction products were analysed with GC-FID (CP Sil-5 GC
column or a BPX 70 (SGE) column); peak areas were cor-
rected according to the sensitivity coefficients using the inter-
nal standard technique.
4
5
90
65
Results and discussion
1
.
Potential energy surfaces
In this work, the mechanistic problem one is faced with is
addressed by high level quantum-chemical calculations, vali-
add
dated for the reaction type discussed. In TS (Scheme 1), the
N
2
OÁ Á ÁC and ONNÁ Á ÁC bonds are still very weak (distances
H-shift
˚
Z 2 A), and a similar problem arises with TS
. As DFT
fails to describe long-range interactions accurately, results
obtained with the popular B3LYP-DFT method should be
handled with care. At least, crucial diffuse and polarized basis-
set functions to describe the bond formation process under
consideration should be included. Unfortunately, the appro-
priate ab initio levels of theory, in combination with large basis
sets are beyond current computational resources for large
systems. Therefore, feasible levels of theory have to be com-
a
The reactivity difference between cC5 and cC6 olefins results from
the highly different ring-strain relaxation upon N O addition: G2M
2
À1
addition barriers of 101.6 versus 116.5 kJ mol , respectively.
Despite the theoretical investigations, it remains unclear
why substantial C–C cleavage, or even skeletal rearrangements
occur with some substrates, whereas others can be ketonized in
a very high selectivity (Table 1 and ref. 2). The aim of this
work is to elucidate a generic molecular mechanism, rationa-
lizing the experimental observations. A sound, comprehensive
scheme should benefit the rational development of this novel
chemistry.
1
3
pared with benchmark theories such as CBS-QB3 and
1
2
G2M(large) for small analogous systems, such as the ethy-
lene plus N O reaction (Table 2). Apparently, the DFT results
deviate significantly from the average of the reference
methods, even if very large basis-set is used, i.e.
-311++G(3df,3pd). On the other hand, the single-point
2
a
6
G2M method clearly provides a sound description of the
PES. Intrinsic Reaction Coordinate (IRC) analysis of the
cycloaddition step demonstrates that it proceeds via a very
Experimental and theoretical methods
weak, pre-reactive complex between N O and the olefin. This
2
Potential Energy Surfaces (PESs) of relevant reaction steps were
complex results from weak electrostatic interactions between
the dipole and the olefin, and should be considered as the first
step in the cycloaddition reaction. However, it will not appear
in the rate expression as it is in fast thermal pre-equilibrium
with the reactants, strongly shifted towards the latter because
1
constructed at the DFT level, using the Becke three-parameter
0
hybrid exchange functional, combined with the Lee–Yang–Parr
1
1
non-local correlation functional: B3LYP-DFT.
The 6-
311++G(d,p) basis set was used to account for long-range
interactions. Single-point energies were refined using a variant
of the G2M scheme, based on UCCSD(T)/6-31G(d) single-
point calculations and MP2/6-311++G(3df,3pd) basis set ex-
of entropic reasons: the ketonization rate is only determined
add
by the relative position of TS
and the reactants level.
However, the pre-reactive complex can induce regioselective
12
trapolations. Zero-Point Energy (ZPE) corrections were made
at the B3LYP/6-311++G(d,p) level of theory. For a smaller
model system, the G2M results were compared with several
addition of N
below.
2
O to unsymmetrical CQC sites as discussed
epox
The barrier of the concerted epoxidation, via TS
was
1
3
12
benchmark levels such as CBS-QB3 and G2M(large),
demonstrating its reliability. All calculations were performed
found to be much higher than that of the cycloaddition
6b
channel, in line with previous studies. Contrary to a
4
270 | Phys. Chem. Chem. Phys., 2007, 9, 4269–4274
This journal is ꢀc the Owner Societies 2007

À1
Table 2 ZPE-corrected relative energies (kJ mol ) of the stationary points on the PES of the ethylene plus N
theory
2
O reaction at various levels of
a
b
Structure
O + ethylene
Pre-reactive complex
B3LYP/6-31G(d)
B3LYP/6-31 1+ +G(3df,3pd)
G2M
G2M(full)
CBS-QB3
N
2
0.0
À2.5
0.0
À0.2
125.0
2.8
84.6
78.5
0.0
0.0
À5.4
0.0
À5.8
À4.2
119.2
À19.7
64.6
add
TS
OD
105.0
À28.9
76.6
116.3
À25.9
64.0
117.1
À18.2
72.6
H-shift
TS
TS
diazo
68.6
À282.4
71.1
À302.9
68.6
À305.4
75.5
À297.9
Acetaldehyde + N
CH O + CH
O + ethene
2
À279.7
2
2 2
N
10.5
0.0
9.6
0.0
16.9
0.0
15.9
0.0
20.9
0.0
N
2
epox
TS
Epoxide
198.7
203.3
À158.5
205.8
203.8
207.9
À184.5
À162.3
G2M refers to the energy calculated as E[CCSD(T)/6-31G(d)//B3LYP/6-311++G(d,p)]
À188.7 À191.6
+ {E[MP2/6-31 1+ +G(3df,3pd)//B3LYP/
a
b
6
-31 1+ +G(d,p)] À E[MP2/6-31G(d)//B3LYP/6-31 1+ +G(d,p)]} + ZPE(B3LYP/6-31 1+ +G(d,p). G2M(large) refers to the energy calculated as
E[CCSD(T)/6-31 1+ +G(d,p)//B3LYP/6-31 1+ +G(3df,3pd)]+{E[MP2/6-31 1+ +G(3df,3pd)//B3LYP/6-31 1+ +G(3df,3pd)]ÀE[MP2/6-31 1+ +G(d,p)//
B3LYP/6-31 1+ +G(3df,3pd)]} + ZPE(B3LYP/6-31 1+ +G(3df,3pd).
6
b
suggestion in the literature, this reaction mode cannot
compete at all with the 1,3-dipolar addition, even at high
temperatures (e.g. at 1000 K, the formation of the OD is still
It might appear puzzling that, despite the low barrier for
À1
C–C cleavage (81.7 kJ mol , i.e. barely higher than the
À1
81.2 kJ mol for the H-shift, Table 3), the selectivity remains
4
favoured by a factor E 10 ). Therefore, this direct epoxidation
channel is not discussed in this paper.
high for cyclopentene (entry 1, Table 1). Indeed, the resulting
1
7
4-diazopentanal can eliminate N
2
,
forming a primary car-
bene which is able to isomerise to 4-pentenal. Interestingly,
2
.
Decomposition of the oxadiazole
The core objective of this work is the fate of the OD inter-
mediate and its influence on the selectivity. Inspired by earlier
1
6
work, we examined the concerted NN–O and C–C cleavage
in the OD, forming a diazo compound (‘‘diazo-route’’), as a
possible competitor for the conventional H-shift channel,
which results in the desired ketonization (Scheme 2). The
decomposition of the OD to the corresponding epoxide faces
1
6b
a much higher barrier
as a result of increasing ring-strain
and is therefore not considered in this paper.
2
.1 Cyclopentene and cyclohexene. Competition between
the H-shift and the diazo cleavage was firstly investigated for
two cyclic model substrates, cyclopentene and cyclohexene, in
Fig. 1 and Table 3. It must be noted that for these cyclic
olefins, the ‘‘diazo-route’’ results in a single product molecule,
i.e. a 1-diazoalkanal. Whereas for cyclohexene the TST-eval-
uated rates show the diazo-route to be negligible (ratio of rates
H-shift/diazo channel = 9000), the rate constant of the
hitherto overlooked diazo channel for cyclopentene is found
to be as large as for the desired H-shift. It is the higher ring
strain in the bicyclic OD of cyclopentene, compared to that of
cyclohexene, that induces this vast difference.
2
Fig. 1 Potential energy surface of the reaction of N O with cyclo-
pentene (upper) and cyclohexene (lower), constructed at the G2M level
(see Table 3).
Scheme 2 Possible decomposition channels for the oxadiazole inter-
mediate in the ketonization of cyclic olefins with N O.
2
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Phys. Chem. Chem. Phys., 2007, 9, 4269–4274 | 4271

Table 3 ZPE-corrected relative energies of the stationary points on the PES of the cyclopentene and cyclohexene plus N
level
2
O reaction at the G2M
À1
À1
Cyclopentene system
O + cyclopentene
G2M/kJ mol
Cyclohexene system
O + cyclohexene
G2M/kJ mol
N
2
0.0
101.7
À47.3
33.9
N
2
0.0
116.5
À32.2
33.0
add
add
TS
OD
TS
OD
H-shift
H-shift
TS
TS
TS
TS
diazo
diazo
34.4
À314.6
68.2
À310.4
Cyclopentanone + N
-Diazopentanal
2
Cyclohexanone + N
5-Diazohexanal
2
5
À19.5
À2.2
traces of this compound have been detected by GC-MS. The
cleavage (Scheme 3, route a), yielding a diazoalkanone. A
second path is the shift of the methyl group (route b), yielding
high ketone selectivity is due to the unimolecular re-cyclisation
2
0
of the 4-diazopentanal to the OD, being much faster than the
À1
2-methylcyclohexanone. In order to explain the formation of
cyclopentyl methyl ketone (entry 3, Table 1) one also has to
21
elimination of N
2
(at 453 K, E10 s for N
2
elimination versus
4
À1
18
E10 s for re-cyclisation). Therefore, a fast (pre-)equili-
brium between the OD and the 4-diazopentanal intermediate
is established, in a computed 1 : 1 ratio. The H-shift constitutes
consider a backbone rearrangement (route c). Important for
the chemistry is that these methyl and alkyl shifts face a higher
À1
barrier (83.3 and 79.0 kJ mol , respectively) than the H-shift
À1
4
À1
a very fast and irreversible sink of the OD (1.3 Â 10 s ),
in the OD of cyclohexene (i.e. 65.2 kJ mol , see Table 3),
À1
À4
significantly reducing its lifetime (less than 10 s). This short
making the diazo route (barrier of 94.0 kJ mol ) competitive
lifetime also readily explains why the OD could not yet be
for this OD. Analogous to the cyclopentene case, the OD can
thus establish a (pre-)equilibrium with the diazo-compound.
However, in this case, the diazo/OD equilibrium ratio at 453 K
is calculated to be as high as 260, i.e. much higher than for
cyclopentene. Therefore, subsequent reactions of the diazo
compound become significantly more important here. The
observed experimentally. As the N -elimination from 4-diazo-
2
1
7
À1
pentanal is rather slow (E10 s ), only a very small fraction
is converted to ring-opened by-products.
Next, a number of relevant cases are addressed where the
diazo-route puts its mark on the olefin plus N
distribution.
2
O product
ratio of irreversible decomposition rates of the diazo-com-
diazo OD
pound and the OD can be estimated as k
4
/k  [Diazo]eq
/
2
.2 1-Methyl-1-cyclohexene. Apparently, the substitution
OD
[
OD]eq E {10/(3.5 Â 10 )} Â 260 E 0.07, with k the sum of
of a methyl group for an H-atom at the unsaturated CQC
bond in cyclohexene results in a significant drop in selectivity
the rate constants of the methyl- and alkyl-shift. Thus, on
taking into account that this OD isomer is only formed in
about 65% selectivity, the isomerisation product of the car-
bene, hept-6-en-2-one, is predicted to be produced for about
(
entry 3, Table 1). The rationalisation behind this enigmatic
behaviour starts with the unsymmetrical reaction site in the
-methyl-1-cyclohexene substrate. Indeed, N O can add in two
1
2
5
%, in quantitative agreement with experimental observation
entry 3, Table 1). A smaller fraction (about 35%, vide supra)
of the N O molecules will add with the O-atom to the
regioselective ways. We predict theoretically that addition with
(
the O-atom of N O to the substituted C-atom is slightly faster
2
2
add
than addition to the unsubstituted site (E[TS ] = 114.2
unsubstituted site, yielding, after a fast H-shift, some addi-
tional 2-methylcyclohexanone.
À1
versus 114.6 kJ mol , respectively; TST calculated ratio of
rate constants E 1.8 at 453 K, taking into account entropic
effects). Therefore, it can be estimated that the OD depicted in
1
9
2.3 Linear olefins. Linear internal olefins are selectively
converted to two equimolar ketone isomers (entry 4, Table 1).
This is, again, consistent with the TST calculated ratio of the
ketonisation and diazo channel rates of about 80 at 453 K
Scheme 3 will be produced in about 65% selectivity. As this
OD lacks an aH-atom at the substituted site, a H-shift as in
the case of cyclohexene is impossible for this OD. However,
three other reaction paths can be identified (Scheme 3). The
first one is the diazo route by concerted C–C and N–O
add
À1
diazo
À1
(
E[TS ] = 73.3 kJ mol and E[TS
] = 90.4 kJ mol ,
respectively). As for cyclohexene, the diazo channel is thus
also negligible for the linear internal olefins, explaining the
excellent selectivity. Inspired by the high ketonisation effi-
ciency of such simple model substrates, we recently proposed
this new N O chemistry to ketonise unsaturated renewables,
2
ranging from purified methyl esters, over commercial bio-
2
diesel, to triacylglycerides.
2
In the ketonisation of 1-alkenes, the formation of aldehydes
with the loss of one C-atom is observed in high yields (entry 5,
Table 1). Up to now, it was not yet clear whether this was
2d
2 2 2
caused by the formation of :CH or CH N species. Calcula-
tions on 1-hexene as a model substrate (Scheme 4) reveal that
Scheme 3 Reaction paths of the major oxadiazole (OD) formed after
N O addition to the substituted site of 1-methyl-1-cyclohexene: (a) the
2
diazo cleavage, (b) the methyl-shift, and (c) the backbone rearrange-
ment. G2M calculated barrier heights are indicated.
the addition of the terminal N-atom of N O to the primary
2
C-atom is slightly faster than addition with the O-atom
add
(E[TS ] = 112.5 versus 117.7 kJ mol ; TST calculated ratio
À1
4
272 | Phys. Chem. Chem. Phys., 2007, 9, 4269–4274
This journal is ꢀc the Owner Societies 2007

2
stitutes the OD faster than it eliminates N leading to a
reactive carbene, an equilibrium will be established between
OD and the diazo intermediate. This is for instance the case
for cyclopentene, for which the intramolecular addition, in
4
-diazopentanal, of the diazo functionality to the aldehyde
group is very fast. For 1-alkenes however, the diazo cleavage
gives rise to CH
plus a CnÀ1 aldehyde, and the rather
slow bimolecular re-association of both fragments to regener-
ate the OD cannot compete with the reaction of CH with
2
N
2
2 2
N
the olefin substrate, which results in a significant yield of
cyclopropane derivatives. Nevertheless, even in the cases
where the OD can equilibrate with the diazo compound, by-
products can arise if the equilibrium is strongly shifted to-
wards the diazo compound. The bottom-line conclusion of this
work should therefore be that the subtle role of the hitherto
overlooked diazo compound governs the selectivity of this
emerging olefin to carbonyl chemistry. The mechanistic in-
sights provided by this work should be highly useful in future
assessments of the potential of a certain substrate, prior to
experimental testing.
2
Scheme 4 Ketonization of 1-hexene with N O. The reported branch-
ing fractions are based on the TST evaluated rate constants (based on
G2M-PES data).
of rate constants E 2.9, taking into account the entropic
effects), again induced by a small difference in the charges
on the two unsaturated C-atoms. Therefore it is estimated that
the first attack will account for about 75%. Diazo cleavage in
this dominant OD A is responsible for the production of
H-
Acknowledgements
substantial amounts of diazomethane (see Scheme 4; E[TS
shift
À1
diazo
À1
]
= 83.7 kJ mol versus E[TS
] = 89.1 kJ mol ).
, is
This work was performed in the framework of an IAP project
Diazomethane, as well as its decomposition product :CH
2
(
federal government), of IDECAT (European government)
2
3
known to react very fast with CQC bonds, forming the
observed cyclopropane derivatives (Table 1, entry 5). Note
that the yield of CnÀ1-aldehyde indeed matches that of the
and CECAT (K. U. Leuven). I. H. thanks the FWO-Vlaan-
deren for a research position.
2 2
cyclopropane derivative. Also, the fast CH N reaction with
References
the substrate will far outrun its reaction with the CnÀ1
aldehyde to regenerate the OD (here in an unfavourable
bimolecular process instead of an intramolecular reaction for
cyclic olefins, vide supra). Consequently, no equilibrium can be
established between the OD and CH N . Therefore, the ratio
1
(a) F. S. Bridson Jones, G. D. Buckley, L. H. Cross and A. P. Driver,
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(a) G. I. Panov, K. A. Dubkov, E. V. Starokon and V. N. Parmon,
React. Kinet. Catal. Lett., 2002, 76, 401; (b) G. I. Panov, K. A.
Dubkov, E. V. Starokon and V. N. Parmon, React. Kinet. Catal.
Lett., 2002, 77, 197; (c) E. V. Starokon, K. A. Dubkov, D. E.
Babushkin, V. N. Parmon and G. I. Panov, Adv. Synth. Catal.,
2
2
2
of rate constants of the H-shift channel and the diazo-cleavage
represents the flux ratios (i.e. ratio E 2.4 estimated by TST).
In this case, the diazo cleavage is thus very effective in
generating by-products. The less favourable OD B will decom-
HÀshift
2
004, 346, 268; (d) S. V. Semikolenov, K. A. Dubkov, E. V.
pose almost exclusively via a H-shift to hexanal (E[TS
À1
]
Starokon, D. E. Babushkin and G. I. Panov, Russ. Chem. Bull.
Int. Ed., 2005, 54, 948; (e) E. V. Starokon, K. A. Dubkov, V. N.
Parmon and G. I. Panov, React. Kinet. Catal. Lett., 2005, 84, 383.
diazo
À1
=
76.1 kJ mol versus E[TS
] = 90.8 kJ mol ). The
theoretically predicted product distribution (Scheme 4) is not
only in line with our experimental results on 1-octene (entry 5,
Table 1), but also with those reported in the literature for
3
J. Teles, B. Ro
¨
ssler, R. Pinkos, T. Genger and T. Preiss, Pat. WO
2005/030689, 2005 and Pat. WO 2005/030690, 2005.
4 Chem. Eng. News, 2006, 84(15), 30.
5 (a) K. Sato, M. Aoki and R. Noyori, Science, 1998, 281, 1646; (b)
W. C. Trogler, Coord. Chem. Rev., 1999, 187, 303; (c) A. Scott,
Chem. Week, 1998, Feb. 18; (d) R. Ben-Daniel, L. Weiner and R.
Neumann, J. Am. Chem. Soc., 2002, 124, 8788.
2d
other primary olefins.
Conclusions
6 (a) V. I. Avdeev, S. Ph. Ruzankin and G. M. Zhidomirov, Chem.
Commun., 2003, p. 42; (b) V. I. Avdeev, S. Ph. Ruzankin and G. M.
Zhidomirov, Kinet. Catal., 2005, 46(2), 177.
Summarizing, this work elucidates the key-parameters which
7
(a) L. T. Nguyen, F. De Proft, A. K. Chandra, T. Uchimaru, M. T.
Nguyen and P. Geerlings, J. Org. Chem., 2001, 66, 6096; (b) L. T.
Nguyen, F. De Proft, V. L. Dao, M. T. Nguyen and P. Geerlings,
J. Phys. Org. Chem., 2003, 16, 615.
steer the selectivity in the ketonisation of olefins with N O. A
2
first important factor is the regioselective addition of N
2
O to
unsymmetrical CQC sites: the addition of the O-atom of N
2
O
to the most positively charged C-atom is slightly favoured.
Subsequently, all possible decomposition routes of the oxa-
diazole intermediate should be considered, i.e. the known H-
shift, methyl- and alkyl-rearrangements, but also the concerted
C–C and N–O cleavage. For some substrates, the latter
channel faces a higher barrier, making it completely un-
important, while for others the fate of the resulting diazo
compound should be detailed. If the diazo compound recon-
8 P. Perez, L. R. Domingo, M. Jose Aurell and R. Contreras,
´ ´
Tetrahedron, 2003, 59, 3117.
9
(a) M.-D. Su, H.-Y. Liao, W.-S. Chung and S.-Y. Chu, J. Org.
Chem., 1999, 64, 6710; (b) V. Polo, J. Andres, R. Castillo, S. Berski
and B. Silvi, Chem.–Eur. J., 2004, 10, 5165.
10 See ESIw.
1
1 (a) A. D. Becke, J. Chem. Phys., 1992, 96, 2115; (b) A. D. Becke, J.
Chem. Phys., 1992, 97, 9173; A. D. Becke, J. Chem. Phys., 1993, 98,
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This journal is ꢀc the Owner Societies 2007
Phys. Chem. Chem. Phys., 2007, 9, 4269–4274 | 4273

À2
À1 À1
s for the addition of
diazoethane to cyclopentene; given [cyclopentene] = 11.3 M, the
1
1
1
2 A. M. Mebel, K. Morokuma and M. C. Lin, J. Chem. Phys., 1995,
17 TST predicts k(453 K) E 4 Â 10
M
103, 7414.
À1
3 J. A. Montgomery, Jr, M. J. Frisch, J. W. Ochterski and G. A.
Petersson, J. Chem. Phys., 2000, 112, 6532.
pseudo-first order rate constant can be estimated at E 0.5 s . On
the other hand, the unimolecular N -elimination from diazopenta-
nal is endothermal for about 114.6 kJ mol , but can benefit from a
very loose variational TS, putting the estimated rate constant at
2
À1
4 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A.
Robb, J. R. Cheeseman, J. A. Montgomery, Jr., T. Vreven, K. N.
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W. Gill, B. G. Johnson, W. Chen, M. W. Wong, C. Gonzalez and
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À1
k(453 K) E 10 s . Clearly, the formation of carbene will be
favoured.
18 Note that the alternative ring-closure, i.e. addition of the C-atom
2
group to the carbonyl O-atom was reported earlier
to face a higher barrier than the addition with the terminal N-atom
to the carbonyl O-atom, reconstituting the OD.
19 This moderate regioselectivity is induced by the charges on N
of the –CH–N
16b
.
2
2
O
(NQN–O: À0.07/+0.64/À0.57, respectively) and on the sp -C
atoms (substituted C: +0.06, non-substituted C: À0.22); calculated
Mulliken charges at the UCCSD(T)/6-31G(d)//B3LYP-DFT/
6-31 1+ +G(d,p)-level.
20 A methyl shift was already suggested in the ketonization of
1
b
2,3-dimethylbut-2-ene by Buckley, in order to explain the for-
mation of 3,3-dimethylbutan-2-one.
21 Such a rearrangement of the backbone has also been observed by
Panov et al. during the gas phase ketonization of cyclohexene with
2e
2
N O at higher temperature. Indeed, for cyclohexene this channel
15 J. I. Steinfeld, J. S. Francisco and W. L. Hase, Chemical Kinetics
and Dynamics, Prentice Hall, New Jersey, 1989.
is usually much slower than the competing H-shift; only at elevated
temperatures can competition take place.
1
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4
274 | Phys. Chem. Chem. Phys., 2007, 9, 4269–4274
This journal is ꢀc the Owner Societies 2007