Synthesis of 3,6-Dihydro-2H-[1, 2]-Oxazines from Nitroarenes and Conjugated Dienes, Catalyzed by Palladium/Phenanthroline Complexes and Employing Phenyl Formate as a CO Surrogate

Article abstract of DOI:10.1002/cctc.201801223

Palladium/phenanthroline catalyzed reduction of nitroarenes by in situ-generated carbon monoxide, from the decomposition of phenyl formate, affords the corresponding nitrosoarenes. The latter are trapped by conjugated dienes to give the corresponding 3,6-dihydro-2H-[1, 2]-oxazines (hetero Diels-Alder adducts). Many functional groups are well tolerated. Yields are higher than those obtainable by any previously reported method, including the direct reaction of the diene with the pure nitrosoarene. The reaction can be performed in a single standard glass pressure tube, without the need for autoclaves or high-pressure CO lines.

Full text of DOI:10.1002/cctc.201801223

Accepted Article
Title: Synthesis of 3,6-Dihydro-2H-[1,2]-Oxazines from Nitroarenes
and Conjugated Dienes, catalyzed by Palladium/Phenanthroline
Complexes and employing Phenyl Formate as a CO Surrogate
Authors: Mohamed A. EL-Atawy, Dario Formenti, Francesco Ferretti,
and Fabio Ragaini
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: ChemCatChem 10.1002/cctc.201801223
Link to VoR: http://dx.doi.org/10.1002/cctc.201801223
A Journal of
www.chemcatchem.org

10.1002/cctc.201801223
ChemCatChem
FULL PAPER
Synthesis of 3,6-Dihydro-2H-[1,2]-Oxazines from Nitroarenes and
Conjugated Dienes, catalyzed by Palladium/Phenanthroline
Complexes and employing Phenyl Formate as a CO Surrogate
Mohamed A. EL-Atawy,[a] Dario Formenti,[b] Francesco Ferretti,[b] and Fabio Ragaini[b]*
Abstract: Palladium/phenanthroline catalyzed reduction of
nitroarenes by the carbon monoxide generated in situ by
decomposition of phenyl formate affords the corresponding
nitrosoarene. The latter are trapped by conjugated dienes to give the
corresponding 3,6-dihydro-2H-[1,2]-oxazines (hetero Diels-Alder
adducts). Many functional groups are well tolerated. Yields are
higher than those obtainable by any previously reported method,
including the direct reaction of the diene with the pure nitrosoarene.
The reaction can be performed in a single standard glass pressure
tube, without the need for autoclaves or high-pressure CO lines.
more convenient alternative is represented by the domino
reaction sequence in which the nitrosoarene intermediate is
catalytically produced in-situ from more stable and easily
available starting materials and subsequently trapped by the
diene in the cycloaddition reaction. Oxidation of hydroxylamines
(Scheme 1, Path (A)) is typically employed to this aim when
acylnitroso compounds are desired, but the low stability of N-
arylhydroxylamines makes this approach less convenient for the
latter substrates and we are aware of only one paper in which
the synthesis of N-aryloxazines was pursued by this protocol.[8]
In other cases, N-aryloxazines were obtained by this way only in
the contest of mechanistic studies (to prove the formation of free
nitrosoarenes).[9] A more interesting approach involves the use
of arylamines and organic or inorganic peroxides (Path (B)).[10]
Introduction
Heterocyclic compounds belonging to the oxazines family are
structural motifs present in various natural products, life science
molecules and, in general, biologically active compounds.[1]
Within this family, 1,2-oxazines represent an interesting class,
2]
for both their roles as bioactive molecules[1a, and synthetic
intermediates. In fact, the weakness of the N-O bond makes the
1,2-oxazine scaffold useful for further functionalization, allowing
to obtain numerous other structures. The synthesis and
reactivity of 1,2-oxazines have been reviewed many times,[1a, 2a,
3]
so that a complete list of the investigated reactions is not
possible here. We just mention that the selective transformation
of 1,2-oxazines to cyclic or acyclic aminoalcohols or
aminosugars,[4] oxazepinones,[5] and pyrroles[3b, 6] are significant
examples of how these heterocycles play a pivotal role in
synthetic chemistry.
The hetero Diels-Alder [4+2] cycloaddition reaction between
nitrosoarenes and dienes (Path (D), Scheme 1) is the classic
approach for the preparation of 1,2-oxazines bearing an aryl ring
on the nitrogen atom and the field has extensively been
reviewed.[1a, 3a-g, 7] However, this transformation is limited by the
reduced availability and relatively low stability of nitroso
compounds, which are also well-known cancer promoters. A
Scheme 1. Synthetic approaches for the preparation of the 1,2-
oxazine scaffold..
The preparation of 1,2-oxazines from nitroarenes can be a
valuable approach because of the wide commercial availability
and stability of the substrates (Path (C), Scheme 1). Several
years ago one of us reported that both ruthenium/Ar-BIAN (Ar-
BIAN
=
bis(aryl)acenaphthenequinonediimine)[11]
1,10-phenanthroline)[12] complexes
and
[a]
Dr. Mohamed A. EL-Atawy
palladium/Phen (Phen
=
Chemistry Department, Faculty of Science, Taibah University,
Yanbu 46423,( Saudi Arabia) and Chemistry Department, Faculty of
Science, Alexandria University, P.O. 426 Ibrahemia, Alexandria
21321 (Egypt)
catalyze this reaction when CO is used as reductant for the nitro
group.[13] Although ruthenium complexes afforded only moderate
yields due to the competitive formation of allylic amines, very
good selectivities towards 1,2-oxazines could be obtained by
using the palladium/phenanthroline catalyst. However, in both
cases, the use of pressurized CO is a drawback since it requires
the use of high-pressure equipment and CO lines and the
presence of the corresponding safety measures, limiting its
[b]
Dr. Dario Formenti, Dr. Francesco Ferretti, and Prof. Dr. Fabio
Ragaini*
Dipartimento di Chimica, Università degli Studi di Milano, Via Golgi
19, 20133 Milano (Italy). e-mail: fabio.ragaini@unimi.it
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/cctc.201801223
ChemCatChem
FULL PAPER
application as a practical synthetic method by most research
groups.
group removed by Birch reduction to give the N-H pyrrole in high
yields.[6b]
In the last decades, much attention has been paid to the
development of methods that allow to avoid the use of gaseous
CO in carbonylation reactions, employing molecules capable of
releasing CO in- or ex-situ.[14] Very recently, we reported that
phenyl formate can be used as a stable and effective CO source
for the reductive cyclization of various nitro compounds affording
important heterocycles.[15] No elaborate catalyst or ligand was
necessary and commercially available or easily prepared
Pd(CH3CN)2Cl2 and phenanthroline were employed. In that
paper we focused our attention to the preparation of indoles;
however we demonstrated that the proposed protocol can be as
well applied to the synthesis of other heterocycles. Herein, we
report the application of this reaction to the synthesis of a variety
of 3,6-dihydro-2H-[1,2]-oxazines, highlighting how phenyl
formate allows obtaining higher yields than those previously
achieved with any of the previously reported synthetic methods.
On the contrary, only a low isolated yield was achieved when
using 4-nitrobenzamide (1h, entry 8). It should be recalled that
nitroarenes are readily carbonylated to diaryl ureas when
reacted with arylamines[13a, 13e, 17], a reaction that is efficiently
catalyzed by palladium/phenanthroline complexes,[18] and that
amides have been shown to be able to enter this reaction in
place of amines, affording acyl ureas.[19] So a competition with
another reaction is expected in this case, which may also give
oligomeric products if the nitro and amido groups of the so
formed acyl urea are involved in further reactions of the same
kind.
When the reaction was performed under the standard conditions
starting from 4-nitrobenzoic acid (1i), a product was isolated in a
moderate yield that turned out to be the phenyl ester of the
expected oxazine (3ia', entry 9). Clearly, the phenol formed as a
byproduct of the formate decomposition reentered the reaction.
In order to isolate the oxazine having the free carboxylic acid
(3ia), we repeated the reaction and subjected the crude reaction
mixture to a basic hydrolysis step before the work-up. However,
most of the nitroarene remained unreacted. Suspecting a
deactivation of the triethylamine, required to decompose phenyl
formate,[15] due to its protonation by the carboxylic group, we
repeated the reaction by adding an amount of triethylamine such
that it could completely deprotonate the carboxylic group and
still leave an unreacted amine amount equal to the standard one
(entry 10). A better 40% yield of 3ia was obtained, which was
anyway lower than that achieved for the corresponding methyl
ester 3ga. The reason appears to be the deprotonation of the
carboxylic group, which transform it from an electronwithdrawing
to an electrondonating group, thus retarding the reaction and
decreasing its efficiency (see later). 1,4-Dinitrobenzene (1j) may
in principle yield a bis-oxazine, but under the present reaction
conditions only the mono oxazine (3ja) could be isolated as a
clean compound in 88% yield, without further reduction of the
second nitro group (entry 11).
Results and Discussion
The single experiment for the synthesis of an oxazine by the
palladium/phenanthroline catalyzed reaction of nitrobenzene and
2,3-dimethylbutadiene
reported
in
our
preliminary
communication on the use of phenyl formate as a CO source[15]
had given an excellent yield (97%), but a relatively large excess
of the diene (16 equiv. with respect to nitrobenzene) had been
used. Thus, we first attempted decreasing the amount of diene
and we were pleased to find that the excess diene can be
reduced fourfold without essentially affecting the isolated yield of
the oxazine (96 %, Table 1, entry 1). Given this excellent result,
we decided not to attempt to further optimize the experimental
conditions and used these to investigate the substrate scope of
the reaction (Table 1).
The effect of changing the substituents on the nitroarene was
first tested by employing 2,3-dimethylbutadiene as a constant
counterpart. 4-Fluoro (1b), chloro (1c) and bromo (1d)
substituted nitrobenzene gave the corresponding oxazine in
almost quantitative yield (entries 2-4). This was not an obvious
results because palladium/phenanthroline complexes are known
to catalyze the Heck reaction.[16] Thus, a competitive activation
of the carbon-halogen bond may have occurred, at least in the
case of the bromo derivative, also taking into account that the
presence of the strongly electronwithdrawing nitro group
activates the C-X bond towards oxidative addition.
Moderately donating alkyl groups (methyl (1k), ethyl (1l), and
isopropyl (1m), entries 12-14) were well tolerated, but a strongly
donating methoxy group (1n) was not (entry 15). This was not
completely unexpected, since a poor or no yield of the final
product was obtained when a methoxy group was present on the
aryl ring for other amination and heterocyclization reactions of
20]
nitroarenes in several cases,[11a,
including the synthesis of
12]
oxazines,[11b,
although good results were obtained in other
cases.[21] The rationale for this behavior is based on two points:
a) The initial reduction step of nitroarenes by metal complexes
has been shown to be an electron transfer from the metal
complex to the nitro group in all the cases in which this step has
been experimentally analyzed.[22] Electrondonating substituents
make this reduction more difficult and slow down the reaction in
all cases in which the initial reduction is the r.d.s. of the cycle.
This has been unequivocally shown to be the case by kinetic
measurements in at least two cases,[11a, 21c] but is likely a much
more frequent eventuality, if not the rule, for reductive cyclization
reactions of nitroarenes in the presence of carbon monoxide.
Nitroarenes 3,5-disubstituted with electronwithdrawing chloro
(1e) or trifluoromethyl (1f) groups also gave the oxazine in
excellent yields (entries
5
and 6). Among other
electronwithdrawing groups, methoxycarbonyl (1g) was also
well-tolerated (entry 7). This result is interesting because the 4-
methoxycarbonylphenyl group can be removed by Birch
reduction. Relevant to the chemistry here described, the oxazine
derived from the reaction of isoprene and 4-methoxycarbonyl-
nitrosobenzene has been previously dehydrated to the
corresponding pyrrole by irradiating it with UV light and the aryl
This article is protected by copyright. All rights reserved.

10.1002/cctc.201801223
ChemCatChem
FULL PAPER
Table 1. Substrate scope.
a]
Experimental conditions: nitroarene 0.54 mmol, Pd(CH3CN)2Cl2 1 mol %,
Phen 2.5 mol %, diene 4 equiv, HCOOPh 2.2 mmol, Et3N 0.27 mmol, 140 °C.
Reactions were performed in an ACE Pressure Tube. [b] Yields refer to isolated
[c]
products.
Et3N 0.81 mmol, t = 6 h. The reaction mixture was hydrolyzed
[d]
before the chromatographic purification. The ratio between the two isomers
was determined by 1H NMR on the isolated product.
employed with respect to 1b. 6 equiv. of 2i were employed with respect to
1b.
[e]
8 equiv. of 2h were
[f]
This article is protected by copyright. All rights reserved.

10.1002/cctc.201801223
ChemCatChem
FULL PAPER
enantioselective version of the present reaction may be
developed in the future.
Having investigated in depth the effect of substituents on the
nitroarene, we investigated the effect of changing the diene. 4-
b) The following cyclization proceeds from the intermediately
formed nitrosoarene, which acts as an electrophile.[23]
Electrondonating groups usually make even this step less
efficient. A favorable effect of a methoxy group on the cyclization
step has been attributed at least in one case to the high
stabilizing effect of this group on radical species that may be
formed as intermediates during the cyclization.[24]
In the present case, the first point is surely operating, since a lot
of unreacted 1n was still present at the end of the reaction.
Moreover, even the second aspect is important, as the
nitrosoarene is involved in a hetero Diels-Alder reaction and the
latter is well known to proceed the better the lower is the level of
the orbitals of the dienophile.[25] The presence of a para methoxy
group on the aryl ring of nitrosobenzene allows a quinonoid
resonance form for 1n, where the nitroso group gain a negative
charge and loses its double bond, making it unsuitable to
undergo a Diels-Alder reaction (Scheme 2).[26]
Fluoronitrobenzene (1b) was employed as
a
reference
nitroarene.
Two isomers (distal and proximal) can be obtained for the
oxazines derived from isoprene (2b) (Scheme 3). The formation
of these isomers has been previously investigated both from an
experimental[3c, 3f] and a theoretical[29] point of view.
Scheme 3. Resonance forms of 4-nitrosoanisole.
Both isomers were obtained with
a prevalence of the
electronically favored one (3bbA/3bbB = 5.0:1, entry 22). A
similar result was obtained when using myrcene (2c) as the
diene (3bcA/3bcB = 2.9:1, entry 23), but the two isomers were
formed in closer amounts.
Scheme 2. Resonance forms of 4-nitrosoanisole.
A diene in which one of the two olefinic double bonds is internal,
1,3-pentadiene (2d) still allowed the reaction to proceed
efficiently (entry 24). Again two isomers were formed, but in this
case the proximal isomer prevailed (3bdA/3bdB = 0.75:1) in
accord with what expected based on the literature for related
oxazines.[7c, 30] However, when dienes were employed in which
both double bonds are internal (2e-g) only trace amounts of the
corresponding oxazines could be detected (entries 25-27). One
problem is that in these cases the formation of the oxazine is
reversible at high temperature[11b] and the accumulated
nitrosobenzene can be involved in side reactions, among which
the formation of azoxyarenes appears to predominate in our
system. However, a second problem was the low nitroarene
conversion when these dienes were employed as substrates.
We will come back to this point in the following.
Although no resonance structure can be drawn in the case of 4-
nitrobenzoate anion, it is clear that the deprotonation of 1i by
triethylamine both decreases the reactivity of the nitro compound
and the tendency of the intermediately formed nitroso arene to
be involved in a hetero Diels-Alder reaction, thus explaining the
low yield obtained with this substrate.
The problem of getting an oxazine having an oxygen atom on
the para position of the aryl ring could be partly solved by
employing an acetylated nitrophenol (1o, entry 16). In this case,
the desired oxazine was obtained, albeit in a low yield.
Very interestingly, the protocol was compatible with the
presence of two functional groups, cyano (1p) and formyl (1q),
which may themselves be involved into a Diels-Alder reaction
(entries 17-18). In particular, the formation of 3qa is noteworthy.
This product would likely not be obtainable starting from the
corresponding aniline under oxidizing conditions and the
corresponding nitroso compound, though known, requires
several delicate synthetic steps to be prepared.[27]
The presence of a sterically large bromide atom in the ortho
position of the nitroarene (1r) still allowed the reaction to afford
high yields of the oxazine (entry 19).
As far as heteroaromatic nitro compounds are concerned, the
electron poor pyridine ring (1s) allowed the reaction to proceed
efficiently, but the electron rich thiophene (1t) did not (entries 20
and 21). The low reactivity of 1t can be explained by the same
reasons discussed above for electron rich nitroarenes. It is worth
recalling that 2- nitrosopyridine has been employed is several
We were glad to find that our protocol is compatible with a
functionalized diene such as 2,3-dimethoxydiene (2h, entry 28).
In the corresponding oxazine (3bh) all four carbon atoms of the
diene moiety are substituted with an oxygen or nitrogen atom,
which may easily be subject to further elaboration to give a
variety of products, such as azasugars analogues.[4c]
.
Finally, even unsubstituted butadiene can be employed (2i, entry
29). Given the gaseous nature of this diene at ambient
temperature and pressure, the reaction protocol had to be
slightly changed (see the experimental part). The
diene/nitroarene ratio was also increased to 6, to account for the
fact that a larger fraction of it is in the gas phase during the
reaction.
Having investigated the substrate scope, we performed some
additional experiments in order to assess the potentialities of the
reaction.
cases as
a
substrate in enantioselective reactions of
nitrosoarenes with different substrates, including the asymmetric
3g, 25c, 28]
hetero Diels-Alder reaction.[3e,
The possibility of
First, the synthesis of 3aa was repeated working on a ten-fold
larger scale. In this case, we used a Teflon-coated autoclave
having a c.a. 140 mL free volume space to perform the reaction.
obtaining the same kind of products starting from the
commercially available nitropyridine suggests that an
This article is protected by copyright. All rights reserved.

10.1002/cctc.201801223
ChemCatChem
FULL PAPER
The isolated yield (871 mg, 95%) is essentially the same as in
the smaller scale experiment, showing that the reaction can be
scaled-up without losses in efficiency.
at any time and this surely prevents significant
accumulation of dimers.
3) The yields obtained with our method also favorably
compare with those obtained for the corresponding
products by the previously mentioned oxidative protocol
starting from arylamines and hydrogen peroxide, in the
presence of molybdenum[10b] or selenium[10c] catalysts (e.g.
for 3ca NMR yields of 95% and 90% were reported with the
two systems, which however respectively lowered to 51%
and 63% upon isolation of the product, compared to a 98%
isolated yield in our case). However, we acknowledge that
the oxidative protocol works at room temperature and this,
analogously to the direct reaction of nitrosoarenes and
dienes, allows obtaining the hetero Diels-Alder adducts of
dienes such as 1,4-diphenylbutadiene and cyclohexadiene
for which our protocol failed due to reversibility of the
adduct formation at high temperature.[33]
Since, depending on the respective substitution patterns, dienes
can be more expensive than nitroarenes, we also performed a
reaction in which a twofold excess of nitroarene was present
with respect to the diene. The desired oxazine was again
obtained, although with a somewhat reduced yield of 68%.
A comparison between the oxazine yields obtained with the
present protocol and those previously obtained by other
procedures clearly shows the advantages of our method. In
particular:
1)
Yields are higher than those previously reported by us
using the nitroarene + diene approach and with pressurized
carbon monoxide as the reductant.[11b, 12] This is especially
noteworthy when the comparison with a very similar
palladium/phenanthroline catalytic system is made.[12] The
only exception to this rule is the reaction run in the
presence of an excess of diene with respect to the
nitroarene. In this case, it is likely that the lower yield in the
present work is due to a partial polymerization of the diene,
which decreases the amount of the limiting reagent. This
side reaction was less important in the previous work
because of the lower temperature employed (100 °C
instead of 140 °C) and of the higher effective CO pressure
(10 bar in the previous work. The actual CO pressure
during the reaction under the presently employed conditions
is not known, but it is surely much lower). Carbon monoxide
competes with the diene for coordination to the metal and
should inhibit the palladium-catalyzed polymerization of the
olefin.
As far as the reaction mechanism is concerned, we recall that
we had provided kinetic and mechanistic evidence that the allylic
amination of unfunctionalized olefins by nitroarenes under
carbon monoxide pressure and catalyzed by ruthenium
complexes with Ar-BIAN ligands involves the initial coordination
of the olefin before the rate-determining reaction with the
nitroarene.[11a] Preliminary coordination of the olefin to the metal
was also identified by Srivastava and Nicholas when studying
the activity of [CpFe(CO)2]2 for the same reaction.[34] We
proposed the same olefin pre-coordination for the analogous
reaction of dienes, which, in addition to allylic amines, also
produced oxazines.[11b] In the case of the palladium-catalyzed
reaction, the absence of any allylic amine and the close ratio
between the distal, A, and proximal, B, isomers of the oxazine
derived from the catalyzed reaction of isoprene and
nitrobenzene and that observed after a traditional reaction
between the same diene and nitrosobenzene had pushed us to
suggest that free nitrosobenzene was involved as the aminating
agent and that the diene was entering the reaction only at the
hetero Diels-Alder stage.[12] However, the slower catalytic
reaction when sterically hindered dienes are employed warrants
a reconsideration of this view. Indeed, if the diene only enters
the reaction at a later stage with respect to a rate-determining
reaction of the metal complex with the nitroarene, than a
complete nitroarene conversion should be observed in all cases
and steric hindrance should only affect selectivity. This is not the
case. Thus a mechanism in which coordination of the diene
precede the reaction with the nitroarene seems more likely.
To further elucidate the role of the diene during the reaction, we
reinvestigated the distal/proximal selectivity by comparing the
outcome of a reaction run with the present protocol with that of a
standard hetero Diels-Alder reaction employing a nitrosoarene
as reagent. To this aim, we performed a catalytic reaction
between isoprene and nitrobenzene to give 3abA/3abB and
compared the obtained selectivity with that achieved by reacting
nitrosobenzene and isoprene in the same solvent and at the
same temperature (the same pressure tube was employed to
avoid the boiling of the solvent), but in the absence of any other
reagent. To prevent any selective loss of one of the two isomers,
2)
Except for the cases in which the present protocol failed,
the yields obtained employing it are, to the best of our
knowledge, always higher than those obtained by direct
reaction of the corresponding nitrosoarenes with dienes.
For example, the highest isolated yield reported in the
literature for the reaction of nitrosobenzene with 2a to give
3aa is 80%.[31] It may appear odd that a higher yield is
obtained by generating nitrosoarenes in situ than by using
the pure compound. However, it should be recalled that
nitrosoarenes are in equilibrium in solution with their dimeric
form and that the dimer is not inactive. A common
byproduct is the azoxyarene derived from the starting
nitrosoarene. This may derive from a reduction reaction of
the dimer (azoarenedioxide). We also observed the
formation of this byproduct in those experiments that gave
poor yields. Moreover, some evidence has been provided
already more than 50 years ago that the main byproduct of
the reaction between m- or p-nitro-nitrosobenzene and 2a
is the hetero Diels-Alder adduct of the corresponding
nitrosoarene dimer with the diene.[32] Although the identity
of this byproduct has not been reinvestigated in more
recent years, the importance of the monomer-dimer
equilibrium on the selectivity even of other reactions
involving nitrosoarenes has been stressed more recently.[3e]
In our protocol, the nitrosoarene is slowly generated and
immediately consumed, so that its concentration is very low
This article is protected by copyright. All rights reserved.

10.1002/cctc.201801223
ChemCatChem
FULL PAPER
in this case the reaction mixture was analyzed by NMR before
any purification (Scheme 4).
should be noted that the instability of type D complexes implies
that they are highly reactive. Thus, a direct involvement of such
complexes in the cycloaddition step cannot be dismissed at the
moment. Mechanistic studies are in progress to definitely assess
this point, though the instability of zerovalent carbonyl
complexes with nitrogen ligands makes the design of
experiments giving a clear-cut conclusion difficult.
Scheme 4. Comparison between the distal/proximal selectivities
observed with the present protocol and a standard hetero Diels-
Alder reaction. For the experimental conditions of the former
reaction, see the caption to Table 1. The nitrosobenzene
reaction was run under the same conditions, but without the
catalyst, formate and Et3N. Both yields and ratio between the
1
two isomers were determined by H NMR on the crude reaction
mixture.
3abA and 3abB were formed in a 88:12 ratio from the catalytic
reaction and in a 87:13 ratio from the standard hetero Diels-
Alder reaction. Such close results suggests that the
cycloaddition step is occurring outside the coordination sphere
of the metal even under the conditions of the present study.
At this stage and also considering the known coordination
chemistry of nitrosoarenes,[3e, 35] the most likely reaction pathway
is that shown in scheme 5.
The complex [Pd(Phen)Cl2] (A) is formed from [Pd(CH3CN)2Cl2]
and Phen upon mixing in solution.[36] Reduction of this complex
by carbon monoxide, probably with the involvement of trace
amounts of water and the help of triethylamine, produces a
zerovalent palladium carbonyl complex (B).[37] To justify the
effect of the olefin on the reaction rate, the latter should be in
equilibrium with an olefin complex (C), which is the species
reacting with the nitroarene. This is exactly what it has been
shown to occur in the ruthenium[11] and iron systems[34]
discussed before. As previously mentioned, the activation of
nitroarenes occurs by a single electron transfer from the metal to
the nitroarene. Complex C is surely more electron rich than
complex B and this fact explains the slower observed rate when
the diene is too sterically hindered to stably coordinate to the
metal. The reaction is then proposed to proceed by formation of
a nitrosoarene complex (D). Although all previous attempts to
isolate a palladium nitrosoarene complex with phenanthroline or
other nitrogen ligands have failed,[38] all known [formally]
Scheme 5. Proposed reaction mechanism.
Conclusions
In the present work we have shown that 3,6-dihydro-2H-[1,2]-
oxazines, versatile substrates for further transformations, can be
obtained by reduction of nitroarenes in the presence of
conjugated dienes, employing phenyl formate as a CO surrogate.
The methods presents several advantages with respect to
previously reported alternatives:
1) No need exists to synthesize toxic and unstable precursors
such as nitrosoarenes or arylhydroxylamines. On the
contrary, nitroarenes are the cheapest and more widely
available nitrogen-containing aromatic compounds.
zerovalent
palladium
nitrosoarene
complexes
are
tricoordinate.[35b, 35c, 35g] This implies that the diene must leave
the coordination sphere of the metal. Release of the
nitrosoarene in solution and its off-metal hetero Diels-Alder
reaction with the diene complete the cycle. This hypothesis
explains both the rate acceleration by non-sterically hindered
dienes and the almost coincidence of the A/B ratios when the
outcome of a catalytic reaction is compared with that of a
traditional reaction starting from nitrosobenzene. However, it
2) Yields are higher than those obtained by any previous
synthetic strategy including the direct reaction of
nitrosoarene with the diene and the use of nitroarenes
under pressurized CO.
a
3) No autoclave or pressurized CO lines are necessary.
Pressure tubes or flasks are cheap and available in
volumes from 1 mL to at least half a liter.
This article is protected by copyright. All rights reserved.

10.1002/cctc.201801223
ChemCatChem
FULL PAPER
Catalytic reactions
4) A variety of substituents is tolerated, including easily
oxidizable ones, such a free formyl group, which would not
be compatible with protocols employing strongly oxidizing
agents.
The reaction was performed under dinitrogen atmosphere. To avoid
weighing very small amounts of PdCl2(CH3CN)2 and phenanthroline,
stock solutions of these reagents were prepared by dissolving
respectively 28 mg of the former reagent in 20 mL dry CH3CN (to give a
5.410-3 M solution) and 48.7 mg Phen in 20 mL dry CH3CN (to give a
1.3510-2 M solution). An 18 mL ACE pressure tube (see above) was
placed in a Schlenk tube having a wide mouth and the tube was
evacuated and filled with dinitrogen three times. Then, in a nitrogen flush,
were added at RT the nitroarene (0.540 mmol), the diene (2.16 mmol),
phenyl formate (240 µl, 2.20 mmol), the catalyst stock solution (1.00 mL,
corresponding to a catalyst amount of 1.40 mg, 0.00540 mmol), the Phen
stock solution (1.00 mL, corresponding to a ligand amount of 2.44 mg,
0.0135 mmol), acetonitrile (8.0 mL) and finally triethylamine (40 µL). The
pressure tube was closed and transferred to an oil bath preheated at
140 °C. When 1,3-butadiene was used, it was condensed into a Schlenk
flask containing 15 mL of CH3CN by cooling with a dry-ice/acetone bath
and kept cold, avoiding freezing of the solvent. The amount of condensed
diene (1.80 g, 33.2 mmol) was determined by weighing the flask before
and after condensation. The butadiene stock solution (1.5 mL
corresponding to a diene amount of 3.32 mmol) was transferred while
cold by mean of a syringe to the reactor immediately before closing it.
5) Phenyl formate and the produced phenol are little toxic if
compared with most alternative CO surrogates. The
availability of phenyl formate even on large scale makes its
use compatible with larger scale preparations even from an
economic point of view.
6) The catalyst and ligand are commercially available and
cheap when compared respectively with other palladium
sources and most phosphine ligands typically employed in
other catalytic systems.
These features collectively make the presently described
protocol a useful tool for the synthesis of 1,2-oxazines and of the
products derived thereof both at a laboratory and the production
scale.
Experimental Section
o
The reaction mixture was heated at 140 C with continuous stirring for 4
hrs. Then the tube was lifted from the oil bath and allowed to cool to
room temperature. The reaction mixture was evaporated to dryness
General procedures
Unless otherwise stated, all reactions and manipulations were performed
under a dinitrogen atmosphere using standard Schlenk apparatus. All
glassware and magnetic stirring bars were kept in an oven at 120 °C for
at least two hours and let to cool under vacuum before use. CH3CN and
Et3N were dried by distillation over CaH2. Phenyl formate was prepared
following a procedure reported in the literature.[14j] Isoprene, myrcene and
nitrobenzene were distilled before use. The two dienes were then stored
under dinitrogen at 4 °C. 1,10-Phenanthroline (Phen) was purchased as
hydrate (Sigma-Aldrich). Before use, it was dissolved in CH2Cl2, dried
over Na2SO4 followed by filtration under a dinitrogen atmosphere and
evaporation of the solvent in vacuo. Phen was weighed in the air but
stored under dinitrogen to avoid water uptake. Deuterated solvents were
purchased from Sigma-Aldrich. CDCl3 was filtered on basic alumina and
stored under dinitrogen over 4 Å molecular sieves. All other reagents
were commercially available and were used as received. 1H NMR, 13C
NMR, and 19F NMR spectra were recorded at room temperature on a
Bruker AC 300 FT, an Avance Bruker DPX 300, or on a Bruker Avance
DRX 400 spectrometers.
using
a rotary evaporator and the crude purified by column
chromatography on silica. In general, any dimer or oligomer of the diene
eluted first, followed by the oxazine. Only in the case of the reaction
between 1b and 2h did 4-fluoroazoxybenzene eluted before the oxazine.
Products characterization
4,5-Dimethyl-2-phenyl-3,6-dihydro-2H-[1,2]-oxazine (3aa). Obtained
as a colorless solid after column chromatography (gradient elution from
hexane to hexane
(400 MHz, CDCl3) δ 7.33 (t, J = 10.6 Hz, 2H), 7.19 – 7.11 (m, 2H), 7.02 (t,
J = 9.7 Hz, 1H), 4.35 (s, 2H), 3.69 (s, 2H), 1.76 (s, 3H), 1.67 (s, 3H) ppm.
13C NMR (101 MHz, CDCl3) δ 150.85, 129.19, 125.38, 122.75, 122.66,
116.29, 72.44, 56.75, 16.33, 14.06 ppm. C12H15NO requires C, 76.16; H,
7.99; N, 7.40% Found: C, 76.40; H, 7.89; N, 7.20%
1
̸
2-(4-Fluorophenyl)-4,5-dimethyl-3,6-dihydro-2H-[1,2]-oxazine (3ba).
Obtained as a colorless solid after column chromatography (gradient
elution from hexane to hexane
̸
Reactor
1
99%. H NMR (300 MHz, CDCl3) δ 7.16 – 7.07 (m, 2H), 7.05 – 6.93 (m,
2H), 4.32 (s, 2H), 3.60 (s, 2H), 1.73 (s, 3H), 1.64 ppm (s, 3H). 13C NMR
(75 MHz, CDCl3) δ 159.08 (d, J = 240.8 Hz), 147.15, 125.36, 122.69,
118.25 (d, J = 7.8 Hz), 115.72 (d, J = 22.4 Hz), 72.50, 57.42, 16.40,
14.01 ppm. 19F NMR (282 MHz, CDCl3) δ -121.74 ppm. C12H15NO
requires C, 76.16; H, 7.99; N, 7.40% Found: C, 76.40; H, 7.89; N, 7.20%.
C12H14FNO requires: C, 69.55; H, 6.81; N, 6.76%. Found: C, 69.80; H,
6.90; N, 6.60%.
The reactor in which the reaction is performed has a special importance.
We routinely employed a 18 mL ACE pressure tube. This tube, also
available in many other dimensions or as flasks for larger volumes, is
provided either with a "front" or with a "back" "seal". The former
corresponds to a FETFE® O-ring sitting below the threads of the PTFE
bushing. In the latter, the O-ring sits on top of the threads. In both cases,
the weak point of the system is the O-ring itself. With the solvent and
reagents employed in our study, the O-ring completely degraded after at
most a few reactions if not at the first use. Changing the O-ring material
to Viton or even to Karlez did not solve the problem. However, we found
that if a "front seal" setting is employed, it is possible to remove the O-
ring and still no leakage is observed. This way, the solvent only comes in
contact with glass or PTFE. Other brands also commercialize pressure
tubes that may be conveniently employed.
2-(4-Chlorophenyl)-4,5-dimethyl-3,6-dihydro-2H-[1,2]-oxazine (3ca).
Obtained as a colorless solid after column chromatography (gradient
elution from hexane to hexane
98%. H NMR (300 MHz, CDCl3) δ 7.25 (d, J = 9.0 Hz, 2H), 7.05 (d, J =
9.0 Hz, 2H), 4.32 (s, 2H), 3.63 (s, 2H), 1.73 (s, 3H), 1.64 ppm (s, 3H). 13
̸
1
NMR (75 MHz, CDCl3) δ 149.32, 129.13, 127.63, 125.38, 122.43, 117.53,
72.43, 56.55, 16.30, 14.05 ppm. C12H14ClNO requires: C, 64.43; H, 6.31;
N, 6.26%. Found: C, 64.14; H, 6.52; N, 6.30%.
This article is protected by copyright. All rights reserved.

10.1002/cctc.201801223
ChemCatChem
FULL PAPER
2-(4-Bromophenyl)-4,5-dimethyl-3,6-dihydro-2H-[1,2]-oxazine
acetonitrile and excess diene were evaporated in vacuo. The residue
was dissolved in ethanol (10 mL) and 3 mL of an aqueous 2 M KOH
solution was added. The reaction mixture was stirred overnight and then
washed with CH2Cl2 (3  10 mL). The aqueous layer was separated,
neutralized with conc. HCl (0.5 mL) and the formed solid was collected
by filtration. The pure product was obtained as a colorless solid after
column chromatography (methylene chloride/methanol = 98:2 as the
eluent). Yield = 40%. 1H NMR (400 MHz, CDCl3) δ 8.03 (d, J = 8.7 Hz,
2H), 7.11 (d, J = 8.7 Hz, 2H), 4.34 (s, 2H), 3.77 (s, 2H), 1.76 (s, 3H), 1.65
(s, 3H). 13C NMR (101 MHz, CDCl3) δ 171.45, 154.59, 131.93, 125.30,
122.12, 121.86, 114.17, 72.39, 54.62, 16.22, 14.11. C13H15NO3 requires:
C, 66.94; H, 6.48; N, 6.00%. Found: C, 66.60; H, 6.78; N, 6.80%.
(3da).Obtained as
(gradient elution from hexane to hexane
a
colorless solid after column chromatography
ethyl acetate 98:2 + 1% Et3N).
̸
Yield = 99%. 1H NMR (400 MHz, CDCl3) δ 7.39 (d, J = 9.0 Hz, 1H), 7.00
(d, J = 9.0 Hz, 1H), 4.31 (s, 2H), 3.62 (s, 2H), 1.73 (s, 3H), 1.64 ppm (s,
3H). 13C NMR (101 MHz, CDCl3) δ 149.44, 131.64, 124.98, 122.01,
117.44, 114.63, 72.01, 55.96, 15.90, 13.66 ppm. C12H14BrNO requires: C,
53.75; H, 5.26; N, 5.22%. Found: C, 53.90; H, 5.00; N, 5.01%.
2-(3,5-Dichlorophenyl)-4,5-dimethyl-3,6-dihydro-2H-[1,2]-oxazine
(3ea). Obtained as a colorless solid after column chromatography
(gradient elution from hexane to hexane ̸ethyl acetate 98:2 + 1% Et3N).
Yield = 88%. 1H NMR (400 MHz, CDCl3) δ 6.97 (d, J = 1.8 Hz, 2H), 6.93
(t, J = 1.8 Hz, 1H), 4.30 (s, 2H), 3.63 (s, 2H), 1.73 (s, 3H), 1.64 ppm (s,
3H). 13C NMR (75 MHz, CDCl3) δ 152.29, 135.55, 125.36, 122.02,
121.79, 114.04, 72.49, 55.53, 16.23, 14.07 ppm. C12H13Cl2NO requires:
C, 55.83; H, 5.08; N, 5.43%. Found: C, 56.03; H, 5.30; N, 5.24%.
4,5-Dimethyl-2-(4-nitrophenyl)-3,6-dihydro-2H-[1,2]-oxazine
Obtained as a pale yellow solid after column chromatography (gradient
(3ja).
elution from hexane to hexane
88%. H NMR (300 MHz, CDCl3) δ 8.15 (d, J = 9.3 Hz, 2H), 7.05 (d, J =
9.3 Hz, 2H), 4.33 (s, 2H), 3.79 (s, 2H), 1.75 (s, 3H), 1.65 ppm (s, 3H). 13
̸
1
C
NMR (75 MHz, CDCl3) δ 154.78, 141.34, 125.71, 125.24, 121.71, 113.54,
72.48, 53.81, 16.14, 14.13 ppm. C12H14N2O3 requires: C, 61.53; H, 6.02;
N, 11.96%. Found: C, 61.33; H, 6.22; N, 11.73%.
2-(3,5-Bis(trifluoromethyl)phenyl)-4,5-dimethyl-3,6-dihydro-2H-[1,2]-
oxazine (3fa). Obtained as
a
colorless solid after column
ethyl acetate
chromatography (gradient elution from hexane to hexane
̸
98:2 + 1% Et3N). Yield = 96%. 1H NMR (300 MHz, CDCl3) δ 7.49 (s, 2H),
7.44 (s, 1H), 4.35 (s, 2H), 3.74 (s, 2H), 1.77 (s, 3H), 1.67 (s, 3H) ppm.
13C NMR (75 MHz, CDCl3) δ 151.63, 132.48 (q, 2JF-C = 33.0 Hz), 125.45,
4,5-Dimethyl-2-(p-tolyl)-3,6-dihydro-2H-[1,2]-oxazine (3ka). Obtained
as a colorless solid oil after column chromatography (gradient elution
from hexane to hexane ̸
ethyl acetate 98:2 + 1% Et3N). Yield = 72%. 1H
NMR (400 MHz, CDCl3) δ 7.13 (d, J = 8.3 Hz, 2H), 7.06 (d, J = 8.6 Hz,
2H), 4.34 (s, 2H), 3.64 (s, 2H), 2.32 (s, 3H), 1.75 (s, 3H), 1.65 ppm (s,
3H). 13C NMR (101 MHz, CDCl3) δ 148.19, 131.91, 129.33, 124.97,
122.47, 116.32, 72.02, 56.82, 20.68, 15.95, 13.65 ppm. C13H17NO
requires: C, 76.81; H, 8.43; N, 6.89%. Found: C, 76.91; H, 8.55; N,
6.79%.
1
123.84 (q, JF-C 272.7 Hz).121.78 , 115.72 – 114.66 (m), 72.58 , 55.39 ,
16.08
,
13.94 ppm. 19F NMR (282 MHz, CDCl3)
δ
-63.37 ppm.
C14H13F6NO requires: C, 51.70; H, 4.03; N, 4.31%. Found 52.10; H, 4.25;
N, 4.15%.
Methyl
(3ga). Obtained as a colorless solid after column chromatography
(gradient elution from hexane to hexane ethyl acetate 98:2 + 1% Et3N).
4-(4,5-dimethyl-3,6-dihydro-2H-[1,2]-oxazin-2-yl)benzoate
̸
Yield = 86%. 1H NMR (400 MHz, CDCl3) δ 7.96 (d, J = 9.0 Hz, 2H), 7.07
(d, J = 9.0 Hz, 2H), 4.31 (s, 2H), 3.86 (s, 3H), 3.72 (s, 2H), 1.72 (s, 3H),
1.63 ppm (s, 3H). 13C NMR (101 MHz, CDCl3) δ 166.95, 153.64, 130.75,
124.89, 122.70, 121.77, 113.91, 54.48, 51.76, 15.81, 13.68 ppm.
C14H17NO3 requires: C, 68.00; H, 6.93; N, 5.66%. Found: C, 68.30; H,
7.15; N, 5.50%.
2-(4-Ethylphenyl)-4,5-dimethyl-3,6-dihydro-2H-[1,2]-oxazine
Obtained as a colorless solid after column chromatography (gradient
elution from hexane to hexane ethyl acetate 98:2 + 1% Et3N). Yield =
87%. 1H NMR (300 MHz, CDCl3) δ 7.25 – 7.04 (m, 4H), 4.37 (s, 2H), 3.68
(s, 2H), 2.66 (q, J = 7.6 Hz, 2H), 1.77 (s, 3H), 1.68 (s, 3H), 1.27 ppm t, J
= 7.6 Hz, 3H). 13C NMR (75 MHz, CDCl3) δ 148.83, 138.74, 128.57,
125.40, 122.91, 116.76, 72.47, 57.22, 28.61, 16.38, 16.19, 14.09 ppm.
C14H19NO requires: C, 77.38; H, 8.81; N, 6.45%. Found: C, 77.70; H,
8.95; N, 6.35%.
(3la).
̸
4-(4,5-Dimethyl-3,6-dihydro-2H-[1,2]-oxazin-2-yl)benzamide
(3ha).
Obtained as a colorless solid after column chromatography (gradient
elution from hexane to hexane
25%. H NMR (300 MHz, CDCl3) δ 7.55 (d, J = 8.7 Hz, 2H), 7.09 (d, J =
8.8 Hz, 2H), 4.32 (s, 2H), 3.73 (s, 2H), 1.75 (s, 3H), 1.65 ppm (s, 3H). 13
̸
1
2-(4-Isopropylphenyl)-4,5-dimethyl-3,6-dihydro-2H-[1,2]-oxazine
(3ma). Obtained as a colorless solid after column chromatography
NMR (75 MHz, CDCl3) δ 153.44, 133.54, 125.35, 121.88, 120.01, 114.83,
104.00, 72.43, 54.39, 16.21, 14.11 ppm. C13H16N2O2 requires: C, 67.22;
H, 6.94; N, 12.06%. Found: C, 67.33; H, 6.98; N, 11.85%.
(gradient elution from hexane to hexane ̸ethyl acetate 98:2 + 1% Et3N).
Yield = 95%. 1H NMR (300 MHz, CDCl3) δ 7.18 (d, J = 8.6 Hz, 2H), 7.08
(d, J = 8.7 Hz, 2H), 4.33 (s, 2H), 3.64 (s, 2H), 2.88 (p, J = 6.9 Hz, 1H),
1.74 (s, 3H), 1.65 (s, 3H), 1.25 (s, 3H), 1.23 ppm (s, 3H). 13C NMR (75
MHz, CDCl3) δ 148.79, 143.47, 127.09, 125.37, 122.90, 116.73, 72.47,
57.22, 33.83, 24.49, 16.35, 14.06 ppm. C15H21NO requires: C, 77.88; H,
9.15; N, 6.05%. Found: C, 78.12; H, 9.46; N, 5.85%.
Phenyl
(3ia’). Obtained as a colorless solid after column chromatography
(gradient elution from hexane to hexane ethyl acetate 98:2 + 1% Et3N).
4-(4,5-dimethyl-3,6-dihydro-2H-[1,2]-oxazin-2-yl)benzoate
̸
Yield = 32%. 1H NMR (400 MHz, CDCl3) δ 8.13 (d, J = 8.8 Hz, 2H), 7.42
(t, J = 7.8 Hz, 2H), 7.30 – 7.09 (m, 5H), 4.36 (s, 2H), 3.79 (s, 2H), 1.77 (s,
3H), 1.66 ppm (s, 3H). 13C NMR (75 MHz, CDCl3) δ 154.50, 151.52,
131.88, 129.80, 126.03, 125.33, 122.23, 122.13, 114.28, 72.41, 54.69,
16.25, 14.12 ppm. Three quaternary carbon were not detected.
C19H19NO3 requires: C, 73.77; H, 6.19; N, 4.53%. Found: C, 73.60; H,
6.23; N, 4.80%.
4-(4,5-Dimethyl-3,6-dihydro-2H-[1,2]-oxazin-2-yl)phenyl acetate (3oa).
Obtained as a colorless solid after column chromatography (gradient
elution from hexane to hexane
̸
1
15%. H NMR (300 MHz, CDCl3) δ 7.13 (d, J = 9.0 Hz, 2H), 7.02 (d, J =
9.0 Hz, 2H), 4.32 (s, 2H), 3.64 (s, 2H), 2.28 (s, 3H), 1.73 (s, 3H), 1.64 (s,
3H) ppm. 13C NMR (75 MHz, CDCl3) δ 125.36, 122.64, 122.09, 117.29,
72.45, 57.01, 21.52, 16.32, 14.06 ppm. C14H17NO requires: C, 68.00; H,
6.93; N, 5.66%. Found: C, 68.31; H, 6.99; N, 5.50%.
4-(4,5-Dimethyl-3,6-dihydro-2H-[1,2]-oxazin-2-yl)benzoic acid (3ia).
For this reaction, 1.54 equiv. of Et3N were added instead of 0.54 to
neutralize the starting acid and guarantee activation of phenyl formate.
The reaction time was also increased to 6 h. At the end of the reaction,
4-(4,5-Dimethyl-3,6-dihydro-2H-[1,2]-oxazin-2-yl)benzonitrile (3pa).
Obtained as a colorless solid after column chromatography (gradient
This article is protected by copyright. All rights reserved.

10.1002/cctc.201801223
ChemCatChem
FULL PAPER
elution from hexane to hexane
82%. H NMR (400 MHz, CDCl3) δ 7.54 (d, J = 9.0 Hz, 2H), 7.08 (d, J =
9.0 Hz, 2H), 4.31 (s, 2H), 3.73 (s, 2H), 1.74 (s, 3H), 1.65 ppm (s, 3H). 13
̸
4H), 1.73 (s, 3H), 1.65 (s, 3H). Expect for the peaks at 4.43 and 3.76
ppm, all the other signals are ascribed to both the isomers. 13C NMR
(101 MHz, CDCl3) δ 158.76 (d, JF-C = 241.5 Hz, both isomers), 146.93
1
1
NMR (101 MHz, CDCl3) δ 153.05, 133.13, 124.94, 121.49, 119.59,
114.44, 103.60, 72.02, 53.99, 15.79, 13.70 ppm. C13H14N2O requires: C,
72.87; H, 6.59; N, 13.07%. Found: C, 72.65; H, 6.43; N, 13.18%.
(both isomers), 137.50 (minor isomer), 134.66 (major isomer), 132.22
(both isomers), 123.64 (both isomers), 119.36 (major isomer), 117.96 (d,
3
3JF-C = 7.8 Hz, major isomer), 117.83 (d, JF-C = 7.6 Hz, minor isomer),
2
116.50 (minor isomer), 115.38 (d, JF-C = 22.3 Hz, major isomer), 114.97
2
(d, JF-C = 22.1 Hz, minor isomer), 71.35 (minor isomer), 68.68 (major
4-(4,5-Dimethyl-3,6-dihydro-2H-[1,2]-oxazin-2-yl)benzaldehyde (3qa).
isomer), 55.88 (major isomer), 52.72 (minor isomer), 34.47 (major
isomer), 32.56 (minor isomer), 26.16 (both isomers), 25.75 (both
isomers), 17.81 (both isomers) ppm. 19F{1H} NMR (376 MHz, CDCl3) δ -
121.43 (major isomer), -121.54 (minor isomer) ppm. C16H20FNO requires:
C, 73.35; H, 7.71; N, 5.36%. Found: C, 73.69; H, 7.57; N, 5.48%.
Obtained as a tan solid after column chromatography (gradient elution
from hexane to hexane ̸
ethyl acetate 98:2 + 1% Et3N). Yield = 77%. 1H
NMR (300 MHz, CDCl3) δ 9.85 (s, 1H), 7.80 (d, J = 8.8 Hz, 2H), 7.13 (d,
J = 8.7 Hz, 2H), 4.33 (s, 2H), 3.78 (s, 2H), 1.75 (s, 3H), 1.65 ppm (s, 3H).
13C NMR (75 MHz, CDCl3) δ 191.27, 154.96, 131.70, 130.10, 125.27,
121.98, 114.27, 72.42, 54.20, 16.20, 14.13 ppm. C13H15NO2 requires: C,
71.87; H, 6.96; N, 6.45%. Found: C, 71.93; H, 6.82; N, 6.30%.
2-(4-Fluorophenyl)-3-methyl-3,6-dihydro-2H-[1,2]-oxazine (3beA) and
2-(4-fluorophenyl)-6-methyl-3,6-dihydro-2H-1,2-oxazine
(3bdB).
Obtained as
a
pale yellow oil after column chromatography
2-(2-Bromo-5-methylphenyl)-4,5-dimethyl-3,6-dihydro-2H-[1,2]-
(hexane:AcOEt = 9:1 + 1% Et3N). Yield = 89%. H{19F} NMR (300 MHz,
CDCl3) δ 7.18 – 6.90 (m, 4H, both isomers), 6.01 – 5.75 (m, 2H, both
isomers), 4.81 – 4.66 (m, 1H, major isomer), 4.57 – 4.27 (m, 2H, minor
isomer), 4.07 – 3.92 (m, 1H, minor isomer), 3.87 – 3.55 (m, 1H, major
isomer), 1.34 (d, J = 6.7 Hz, 3H, major isomer), 1.06 ppm (d, J = 6.6 Hz,
1
oxazine (3ra). Obtained as
chromatography (gradient elution from hexane to hexane
a
colorless solid after column
ethyl acetate
̸
98:2 + 1% Et3N). Yield = 76%. 1H NMR (300 MHz, CDCl3) δ 7.43 (d, J =
8.1 Hz, 1H), 7.27 (d, J = 1.5 Hz, 1H), 6.83 (dd, J = 8.3, 1.7 Hz, 1H), 4.39
(s, 2H), 3.60 (s, 2H), 2.32 (s, 3H), 1.73 (s, 3H), 1.66 ppm (s, 3H). 13C
NMR (75 MHz, CDCl3) δ 148.63, 138.62, 133.36, 127.49, 124.99, 123.16,
121.35, 114.43, 72.95, 57.31, 21.59, 16.37, 14.10 ppm. C13H16BrNO
requires: C, 55.33; H, 5.72; N, 4.96%. Found: C, 55.01; H, 5.53; N,
4.88%.
1H, minor isomer). 13C NMR (75 MHz, CDCl3) δ 158.97 (d, JF-C = 241.1
1
Hz, minor isomer), 158.67 (d, 1JF-C = 240.6 Hz, major isomer), 147.01 (d,
4JF-C = 2.5 Hz, major isomer), 145.24 (d, JF-C = 2.6 Hz, minor
4
isomer),131.24 (major isomer), 129.57 (minor isomer), 125.07 (minor
isomer), 122.68 (major isomer), 119.66 (d, JF-C = 7.8 Hz, minor isomer),
3
117.67 (d, 3JF-C = 7.8 Hz, major isomer), 115.46 (d, 4JF-C = 22.3 Hz, minor
isomer), 115.42 (d, 2JF-C = 22.4 Hz, major isomer), 74.18 (major isomer) ,
68.90 (minor isomer), 56.59 (minor isomer), 52.24 (major isomer), 19.05
(major isomer), 15.10 ppm (minor isomer). 19F{1H} NMR (282 MHz,
CDCl3) δ -121. 91(both isomers) ppm. C11H12FNO requires: C, 68.38; H,
6.26; N, 7.25%. Found: C, 68.00; H, 5.89; N, 7.33%.
4,5-Dimethyl-2-(pyridin-2-yl)-3,6-dihydro-2H-[1,2]-oxazine
Obtained as a colorless solid after column chromatography (gradient
elution from hexane to hexane ethyl acetate 98:2 + 1% Et3N). Yield =
58%. 1H NMR (300 MHz, CDCl3) δ 8.25 (d, J = 4.9 Hz, 1H), 7.69 – 7.53 (t,
1H), 7.20 (d, J = 8.4 Hz, 1H), 6.94 – 6.70 (t, 1H), 4.34 (s, 2H), 4.06 (s,
2H), 1.77 (s, 3H), 1.64 ppm (s, 3H). 13C NMR (75 MHz, CDCl3) δ 161.34,
147.82, 137.96, 124.18, 123.12, 116.80, 110.38, 72.12, 52.02, 16.18,
14.11 ppm. C11H14N2O requires: C, 69.45; H, 7.42; N, 14.73%. Found: C,
69.35; H, 7.34 N, 14.81%.
(3sa).
̸
2-(4-Fluorophenyl)-4,5-dimethoxy-3,6-dihydro-2H-[1,2]-oxazine (3bh).
Obtained as a colorless solid after column chromatography (gradient
elution from hexane to hexane ̸ethyl acetate 98:2 + 1% Et3N). Yield =
79%. 1H{19F} NMR (400 MHz, CDCl3) δ 7.08 (dd, J = 9.1, 4.8 Hz, 2H),
7.00 (t, J = 8.7 Hz, 2H), 4.39 (s, 2H), 3.77 (s, 2H), 3.75 (s, 3H), 3.75 ppm
(s, 3H). 13C NMR (101 MHz, CDCl3) δ 158.96 (d, J = 242.0 Hz), 145.94,
135.81, 133.64 , 118.04 (d, J = 7.7 Hz), 115.44 (d, J = 22.4 Hz), 66.76 ,
58.96 , 58.82 , 52.89 ppm. 19F{1H} NMR (376 MHz, CDCl3) δ -120.79
ppm. C12H14FNO3 requires: C, 60.24; H, 5.90; N, 5.85. Found: C, 60.64;
H, 5.91; N, 5.85
2-(4-Fluorophenyl)-5-methyl-3,6-dihydro-2H-[1,2]-oxazine (3bbA) and
2-(4-fluorophenyl)-4-methyl-3,6-dihydro-2H-2-(4-fluorophenyl)-4-
methyl-3,6-dihydro-2H-[1,2]-oxazine-oxazine (3bbB). Obtained as a
pale yellow oil after column chromatography (gradient elution from
hexane to hexane ̸ethyl acetate 98:2 + 1% Et3N). Yield = 78% (6.3:1).
1H{19F} NMR (300 MHz, CDCl3) δ 7.11 (m, 2H, both isomers), 7.02 (m,
2H, both isomers), 5.63 (s, 2H, both isomers), 4.47 (s, 2H, distal isomer),
4.38 (s, 2H, proximal isomer), 3.73 (s, 2H, proximal isomer), 3.63 (s, 2H,
distal isomer), 1.81 (s, 3H, distal isomer), 1.72 (s, 3H, proximal isomer)
ppm. 13C NMR (75 MHz, CDCl3) δ 159.10 (d, J = 240.9 H, both isomers),
147.11 (both isomers), 133.90 (proximal isomer), 131.04 (distal isomer),
2-(4-Fluorophenyl)-3,6-dihydro-2H-[1,2]-oxazine (3bi). Obtained as a
colorless solid after column chromatography (hexane:AcOEt = 9:1 + 1%
Et3N). Yield = 87%. 1H{19F} NMR (400 MHz, CDCl3) δ 7.16 – 7.06 (m,
2H), 7.06 – 6.91 (m, 2H), 6.06 – 5.84 (m, 2H), 4.72 – 4.41 (m, 2H), 3.88 –
3.68 ppm (m, 2H). 13C NMR (100 MHz, CDCl3) δ 158.82 (d, 1JF-C = 240.9
Hz), 146.95 (d, 4JF-C = 2.5 Hz), 126.22, 123.15, 117.91 (d, 3JF-C = 7.8 Hz),
3
120.15 (distal isomer), 118.28 (d, JF-C = 7.8 Hz, proximal isomer),
3
118.24 (d, J = JF-C = 7.8 Hz, distal isomer), 117.45 (proximal isomer),
2
2
115.74 (d, JF-C = 22.5 Hz), 115.73 (d, JF-C = 22.1 Hz), 72.52 (proximal
isomer), 68.99 (distal isomer), 57.12 (distal isomers), 53.06 (proximal
isomer), 20.59 (distal isomers), 18.45 (proximal isomer) ppm. 19F{1H}
NMR (282 MHz, CDCl3) δ -121. 41 (both isomers) ppm. C11H12FNO
requires: C, 68.38; H, 6.26; N, 7.25%. Found: C, 68.59; H, 6.37; N,
7.28%.
2
115.46 (d, JF-C = 22.4 Hz), 69.26, 53.07 ppm. 19F{1H} NMR (376 MHz,
CDCl3) δ -121.39 ppm. C10H10FNO requires: C, 67.03; H, 5.63; N, 7.82%.
Found: C, 67.13; H, 5.70; N, 7.99.
1,2-Bis(4-fluorophenyl)diazene 1-oxide (bis-4-fluoroazoxybenzene).
1H NMR (400 MHz, CDCl3) δ 8.32 (dd, J = 9.2, 4.8 Hz, 2H), 8.26 (dd, J =
9.2, 5.4 Hz, 2H), 7.18 (q, J = 8.0 Hz, 4H). Spectroscopic data are in
accordance with those published in the literature.[39]
2-(4-Fluorophenyl)-4-(4-methylpent-3-en-1-yl)-3,6-dihydro-2H-[1,2]-
oxazine (3bcA) and 2-(4-fluorophenyl)-5-(4-methylpent-3-en-1-yl)-
3,6-dihydro-2H-[1,2]-oxazine (3bcB). Obtained as a pale yellow oil after
column chromatography (gradient elution from hexane to hexane ̸ethyl
acetate 98:2 + 1% Et3N). Yield = 62% (2.5:1). 1H{19F} NMR (400 MHz,
CDCl3) δ 7.13 (m, 2H), 7.06 – 6.90 (m, 2H), 5.73 – 5.50 (s, 1H), 5.16 (s,
1H), 4.51 (s, 2H), 4.43 (s, 2H), 3.76 (s, 2H), 3.68 (s, 2H), 2.26 – 2.04 (m,
This article is protected by copyright. All rights reserved.

10.1002/cctc.201801223
ChemCatChem
FULL PAPER
Ferretti, D. Formenti, F. Ragaini, Rend. Fis. Acc. Lincei 2017, 28, 97-
115; c) F. Ragaini, S. Cenini, E. Gallo, A. Caselli, S. Fantauzzi, Curr.
Org. Chem. 2006, 10, 1479-1510; d) B. C. G. Soderberg, Curr. Org.
Chem. 2000, 4, 727-764; e) S. Cenini, F. Ragaini, Catalytic Reductive
Carbonylation of Organic Nitro Compounds, Kluwer Academic
Publishers, Dordrecht, The Netherlands, 1996.
Acknowledgements
We thank the Italian Ministero dell’Università e della Ricerca
(MiUR) for financial support (PRIN 20154X9ATP). F.F. thanks
the Università degli Studi di Milano for a postdoctoral fellowship.
A special thank is due to Ms. Doaa R. M. Ramadan for
performing some experiments.
[14] a) L. Wu, Q. Liu, R. Jackstell, M. Beller, Angew. Chem. Int. Ed. 2014,
53, 6310-6320; b) P. Gautam, B. M. Bhanage, Catal. Sci. Technol.
2015, 5, 4663-4702; c) S. D. Friis, A. T. Lindhardt, T. Skrydstrup, Acc.
Chem. Res. 2016, 49, 594-605; d) H. Konishi, K. Manabe, Synlett 2014,
25, 1971-1986; e) J. B. Peng, X. X. Qi, X. F. Wu, Synlett 2017, 28, 175-
194; f) J. Cao, Z.-J. Zheng, Z. Xu, L.-W. Xu, Coord. Chem. Rev. 2017,
336, 43-53; g) V. V. Grushin, H. Alper, Organometallics 1993, 12, 3846-
3850; h) S. N. Gockel, K. L. Hull, Org. Lett. 2015, 17, 3236-3239. i) T.
Fujihara, T. Hosoki, Y. Katafuchi, T. Iwai, J. Terao, Y. Tsuji, Chem.
Commun. 2012, 48, 8012-8014; j) T. Ueda, H. Konishi, K. Manabe, Org.
Lett. 2012, 14, 3100-3103.
Keywords: Palladium • Hetero-Diels-Alder reaction •
Nitroarenes • CO surrogate • N-ligands
[1]
a) M. Balasubramanian, in Comprehensive Heterocyclic Chemistry III,
Vol. 8 (Eds.: C. A. Ramsden, E. F. V. Scriven, R. J. K. Taylor), Elsevier,
Oxford, 2008, pp. 333-371; b) L. Lazar, F. Fülöp, in Comprehensive
Heterocyclic Chemistry III, Vol. 8 (Eds.: C. A. Ramsden, E. F. V.
Scriven, R. J. K. Taylor), Elsevier, Oxford, 2008, pp. 373-459.
a) X. Wan, M. M. Joullié, Front. Chem. China 2009, 4, 249-258; b) S.
Carosso, M. J. Miller, Org. Biomol. Chem. 2014, 12, 7445-7468.
a) J. Hamer, M. Ahmad, 1, 4 - Cycloaddition Reactions, the Diesl-Alder
Reaction in Heterocyclic Synthesis, Academic Press, New York, 1967;
b) G. Kresze, J. Firl, Fortschr. Chem. Forsch. 1969, 11, 245-284; c) D.
L. Boger, S. M. Weinreb, Hetero Diels-Alder Methodology in Organic
Synthesis, Academic Press (New York), 1987; d) J. Streith, A. Defoin,
Synthesis-Stuttgart 1994, 1107-1117; e) H. Yamamoto, N. Momiyama,
Chem. Commun. 2005, 3514-3525; f) S. M. Weinreb, R. R. Staib,
Tetrahedron 1982, 38, 3087-3128; g) H. Yamamoto, M. Kawasaki, Bull.
Chem. Soc. Jpn. 2007, 80, 595-607; h) P. F. Vogt, M. J. Miller,
Tetrahedron 1998, 54, 1317-1348; i) G. W. Kirby, Chem. Soc. Rev.
1977, 6, 1-24; j) C. Kibayashi, S. Aoyagi, Synlett 1995, 873-&; k) B. S.
Bodnar, M. J. Miller, Angew. Chem., Int. Ed. 2011, 50, 5630-5647.
a) F. Pfrengle, H.-U. Reissig, Chem. Soc. Rev. 2010, 39, 549-557; b) S.
Scholz, B. Plietker, Org. Chem. Front. 2016, 3, 1295-1298; c) J. Streith,
A. Defoin, Synlett 1996, 189-200.
[15] D. Formenti, F. Ferretti, F. Ragaini, ChemCatChem 2018, 10, 148-152.
[16] W. Cabri, I. Candiani, A. Bedeschi, R. Santi, J. Org. Chem. 1993, 58,
7421-7426.
[2]
[3]
[17] a) F. Ragaini, in Reference Module in Chemistry, Molecular Sciences
and Chemical Engineering, Elsevier, 2016. doi: 10.1016/B978-0-12-
409547-2.12210-3; b) F. Paul, Coord. Chem. Rev. 2000, 203, 269-323;
c) A. M. Tafesh, J. Weiguny, Chem. Rev. 1996, 96, 2035-2052; d) F.
Ragaini, Dalton Trans. 2009, 6251-6266.
[18] a) F. Ragaini, M. Gasperini, S. Cenini, Adv. Synth. Catal. 2004, 346,
63-71; b) M. Gasperini, F. Ragaini, C. Remondini, A. Caselli, S. Cenini,
J. Organomet. Chem. 2005, 690, 4517-4529.
[19] E. Drent, P. E. Prillwitz, Eur. Pat. Appl. EP 225673 A1, 1987.
[20] a) J. H. Smitrovich, I. W. Davies, Org. Lett. 2004, 6, 533-535; b) S.
Cenini, F. Ragaini, S. Tollari, D. Paone, J. Am. Chem. Soc. 1996, 118,
11964-11965.
[21] a) B. C. Soderberg, J. A. Shriver, J. Org. Chem. 1997, 62, 5838-5845;
b) B. C. Soderberg, A. C. Chisnell, S. N. O'Neil, J. A. Shriver, J. Org.
Chem. 1999, 64, 9731-9734; c) I. W. Davies, J. H. Smitrovich, R. Sidler,
C. Qu, V. Gresham, C. Bazaral, Tetrahedron 2005, 61, 6425-6437; d) J.
T. Kuethe, A. Wong, I. W. Davies, Org. Lett. 2003, 5, 3975-3978; e) A.
Penoni, K. M. Nicholas, Chem. Commun. 2002, 484-485; f) D. K. O'Dell,
K. M. Nicholas, Heterocycles 2004, 63, 373-382; g) R. S. Srivastava, K.
M. Nicholas, Chem. Commun. 1998, 2705-2706.
[4]
[5]
[6]
K. Okuro, D. Tuan, K. Khumtaveeporn, H. Alper, Tetrahedron Lett.
1996, 37, 2713-2716.
a) F. Tripoteau, L. Eberlin, M. A. Fox, B. Carboni, A. Whiting, Chem.
Commun. 2013, 49, 5414-5416; b) R. S. Givens, D. J. Choo, S. N.
Merchant, R. P. Stitt, B. Matuszewski, Tetrahedron Lett. 1982, 23,
1327-1330; c) V. Krchňák, K. R. Waring, B. C. Noll, U. Moellmann, H.-
M. Dahse, M. J. Miller, J. Org. Chem. 2008, 73, 4559-4567.
[22] a) R. S. Berman, J. K. Kochi, Inorg. Chem. 1980, 19, 248-254; b) S. J.
Skoog, W. L. Gladfelter, J. Am. Chem. Soc. 1997, 119, 11049-11060;
c) Y. A. Belousov, T. A. Kolosova, Polyhedron 1987, 6, 1959-1970; d) F.
Ragaini, J. S. Song, D. L. Ramage, G. L. Geoffroy, G. A. P. Yap, A. L.
Rheingold, Organometallics 1995, 14, 387-400; e) P. H. Liu, H. Y. Liao,
C. H. Cheng, J. Chem. Soc., Chem. Comm. 1995, 2441-2442; f) F.
Ragaini, S. Cenini, F. Demartin, Organometallics 1994, 13, 1178-1189.
[23] a) A. G. Leach, K. N. Houk, J. Am. Chem. Soc. 2002, 124, 14820-
14821; b) I. W. Davies, V. A. Guner, K. N. Houk, Org. Lett. 2004, 6,
743-746; c) A. Penoni, G. Palmisano, Y.-L. Zhao, K. N. Houk, J.
Volkman, K. M. Nicholas, J. Am. Chem. Soc. 2009, 131, 653-661; d) L.
M. Kogan, Russ. Chem. Rev. 1986, 55, 1164-1174.
[7]
a) S. B. Needleman, M. C. Chang Kuo, Chem. Rev. 1962, 62, 405-431;
b) K. C. Nicolaou, S. A. Snyder, T. Montagnon, G. Vassilikogiannakis,
Angew. Chem., Int. Ed. 2002, 41, 1668-1698; c) L. Brulíková, A.
Harrison, M. J. Miller, J. Hlaváč, Beilstein J.Org. Chem. 2016, 12, 1949-
1980.
[8]
[9]
C. P. Frazier, A. Bugarin, J. R. Engelking, J. Read de Alaniz, Org. Lett.
2012, 14, 3620-3623.
a) R. S. Srivastava, K. M. Nicholas, J. Org. Chem. 1994, 59, 5365-
5371; b) C. M. Ho, T. C. Lau, New J. Chem. 2000, 24, 859-863; c) A. A.
Lamar, K. M. Nicholas, Tetrahedron 2009, 65, 3829-3833.
[24] F. Ragaini, A. Rapetti, E. Visentin, M. Monzani, A. Caselli, S. Cenini, J.
Org. Chem. 2006, 71, 3748-3753.
[10] a) K. F. McClure, S. J. Danishefsky, J. Org. Chem. 1991, 56, 850-853;
b) E. R. Moller, K. A. Jorgensen, J. Org. Chem. 1996, 61, 5770-5778; c)
Z. Dongbo, J. Mikael, B. Jan‐E., Eur. J. Org. Chem. 2007, 2007, 4431-
4436.
[25] a) G. Kresze, J. Firl, H. Zimmer, U. Wollnik, Tetrahedron 1964, 20,
1605-1611; b) A. G. Leach, K. N. Houk, Chem. Commun. 2002, 1243-
1255; c) Y. Yamamoto, H. Yamamoto, Eur. J. Org. Chem. 2006, 2031-
2043.
[11] a) F. Ragaini, S. Cenini, S. Tollari, G. Tummolillo, R. Beltrami,
Organometallics 1999, 18, 928-942; b) F. Ragaini, S. Cenini, E. Borsani,
M. Dompe, E. Gallo, M. Moret, Organometallics 2001, 20, 3390-3398.
[12] F. Ragaini, S. Cenini, D. Brignoli, M. Gasperini, E. Gallo, J. Org. Chem.
2003, 68, 460-466.
[26] L. Greci, G. Tommasi, R. Petrucci, G. Marrosu, A. Trazza, P.
Sgarabotto, L. Righi, A. Alberti, Perkin 2 2000, 2337-2342.
[27] a) C. Karakus, P. Zuman, J. Electrochem. Soc. 1995, 142, 4018-4025;
b) G. A. Molander, L. N. Cavalcanti, J. Org. Chem. 2012, 77, 4402-
4413.
[13] For some reviews on the use of CO as a reductant for nitroarenes
applied to fine chemical synthesis see: a) A. A. Tsygankov, M.
Makarova, D. Chusov, Mendeleev Commun. 2018, 28, 113-122; b) F.
[28] a) A. T. Tran, P. Liu, K. N. Houk, K. M. Nicholas, J. Org. Chem. 2014,
79, 5617-5626; b) Y. Yamamoto, H. Yamamoto, J. Am. Chem. Soc.
This article is protected by copyright. All rights reserved.

10.1002/cctc.201801223
ChemCatChem
FULL PAPER
2004, 126, 4128-4129; c) Y. Yamamoto, H. Yamamoto, Angew. Chem.,
West, G. B. Richter-Addo, Chem. Rev. 2002, 102, 1019-1065; f) P.
Zuman, B. Shah, Chem. Rev. 1994, 94, 1621-1641; g) P. Fantucci, M.
Pizzotti, F. Porta, Inorg. Chem. 1991, 30, 2277-2282; h) M. Cameron, B.
G. Gowenlock, G. Vasapollo, Chem. Soc. Rev. 1990, 19, 355-379; i) C.
J. Jones, J. A. McCleverty, A. S. Rothin, J. Chem. Soc., Dalton Trans.
1985, 401-403.
Int. Ed. 2005, 44, 7082-7085; d) Y. Yamamoto, N. Momiyama, H.
Yamamoto, J. Am. Chem. Soc. 2004, 126, 5962-5963.
[29] a) A. G. Leach, K. N. Houk, J. Org. Chem. 2001, 66, 5192-5200; b) M.
A. McCarrick, Y. D. Wu, K. N. Houk, J. Org. Chem. 1993, 58, 3330-
3343.
[30] a) G. Kresze, J. Firl, Tetrahedron Lett. 1965, 6, 1163-1170; b) G.
Galvani, R. Lett, C. Kouklovsky, Chem. Eur. J. 2013, 19, 15604-15614.
[31] J. Baran, H. Mayr, J. Org. Chem. 1989, 54, 5774-5783.
[32] J. Hamer, R. E. Bernard, J. Org. Chem. 1963, 28, 1405-1406.
[36] G. R. Newkome, V. K. Gupta, H. C. R. Taylor, F. R. Fronczek,
Organometallics 1984, 3, 1549-1554.
[37] F. Ragaini, M. Gasperini, S. Cenini, L. Arnera, A. Caselli, P. Macchi, N.
Casati, Chem. Eur. J. 2009, 15, 8064-8077.
[33]
A
comparison with the yields obtained by oxidation of N-
[38] a) E. Gallo, F. Ragaini, S. Cenini, F. Demartin, J. Organomet. Chem.
1999, 586, 190 - 195; b) F. Ferretti, M. Rimoldi, F. Ragaini, P. Macchi,
Inorg. Chim. Acta 2018, 470, 284-289.
arylhydroxylamines is not possible because the only two products
previously obtained by this technique[8] both derive from a reaction of
cyclohexadiene.
[39] T. Ueda, H. Konishi, K. Manabe, Org. Lett. 2012, 14, 3100-3103.
[40] E. Voutyritsa, A. Theodorou, M. G. Kokotou, C. G. Kokotos, Green
Chem. 2017, 19, 1291-1298.
[34] R. S. Srivastava, K. M. Nicholas, Organometallics 2005, 24, 1563-1568.
[35] a) S. Kundu, S. C. E. Stieber, M. G. Ferrier, S. A. Kozimor, J. A. Bertke,
T. H. Warren, Angew. Chem., Int. Ed. 2016, 55, 10321-10325; b) B. R.
Barnett, L. A. Labios, C. E. Moore, J. England, A. L. Rheingold, K.
Wieghardt, J. S. Figueroa, Inorg. Chem. 2015, 54, 7110-7121; c) N. C.
Tomson, L. A. Labios, T. Weyhermꢀller, J. S. Figueroa, K. Wieghardt,
Inorg. Chem. 2011, 50, 5763-5776; d) B. G. Gowenlock, G. B. Richter-
Addo, J. Chem. Ed. 2008, 85, 1243-1245; e) J. Lee, L. Chen, A. H.
This article is protected by copyright. All rights reserved.