Reactions of grignard reagents with nitrous oxide

Article abstract of DOI:10.1021/om500333y

The chemical activation of nitrous oxide (N₂O) can be achieved by organocalcium, organosodium, and organolithium compounds. Grignard reagents, on the other hand, are believed to be inert. We demonstrate that this generalization is not correct. Some aliphatic Grignard reagents undergo a rapid conversion when subjected to an atmosphere of N₂O. Hydrazones are the main reaction products.

Full text of DOI:10.1021/om500333y

Communication
pubs.acs.org/Organometallics
Reactions of Grignard Reagents with Nitrous Oxide
Alexander G. Tskhovrebov, Euro Solari, Rosario Scopelliti, and Kay Severin*
́
Institut des Sciences et Ingen
́
ierie Chimiques, Ecole Polytechnique Fed
́
er
́
ale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland
S
* Supporting Information
ABSTRACT: The chemical activation of nitrous oxide (N2O) can be achieved by organocalcium,
organosodium, and organolithium compounds. Grignard reagents, on the other hand, are believed to be
inert. We demonstrate that this generalization is not correct. Some aliphatic Grignard reagents undergo a
rapid conversion when subjected to an atmosphere of N2O. Hydrazones are the main reaction products.
Scheme 1. Reaction of iPrMgCl and BnMgCl with N2O
itrous oxide is a very inert molecule. Chemical activation
Nunder ambient conditions is possible, but highly reactive
compounds are typically needed to achieve this goal.1 In most
cases, these reactions proceed by oxygen atom transfer and
liberation of dinitrogen.1,2 Examples of reactions in which
nitrogen atoms are incorporated into the final product are
scarce. The incorporation of intact N2O groups was observed
for frustrated Lewis pairs,3 for N-heterocyclic carbenes,4 and for
some transition-metal complexes.5,6 The utilization of N2O as a
source of NO ligands has also been reported.7 Of industrial
importance is the formation of sodium azide from sodium
amide and N2O.8 Organocalcium,9 organosodium,10 and
organolithium compounds11 are known to react with N2O to
give N-containing products. In contrast, organomagnesium
compounds are believed to be inert toward N2O. An early
attempt to combine a Grignard reagent with N2O was reported
by Zerner in 1913.12 He observed that solutions of MeMgI in
diethyl ether did not react with N2O, even upon heating. The
unreactivity of Grignard reagents toward N2O was later
confirmed by Beringer et al.11g and is now generally accepted
knowledge, as evidenced by statements in research reports9 and
a review article.1b We have recently demonstrated that, in the
presence of transition-metal catalysts, Grignard reagents react
rapidly with N2O to give oxidative C−C coupling products.13
Control reactions in the absence of metal catalysts showed that
phenylmagnesium chloride was indeed very reluctant to react
with N2O, in line with the reports mentioned above. Tests with
THF solutions of aliphatic Grignard reagents, however, gave a
surprising result: rapid conversion was observed for several
compounds. Details about these reactions are reported below.
Subjection of a solution of iPrMgCl in THF (2.1 M) to an
atmosphere of N2O resulted in the warming of the reaction
mixture along with gas formation. After 1 h, the mixture was
quenched with water and analyzed by GC-MS. The data
showed that hydrazone 1 had formed in 51% yield (Scheme 1).
The reaction was repeated in THF-d8. In situ analysis by
1H NMR spectroscopy after 1 h revealed the formation of
propane (∼29%) along with minor amounts of propene (<1%)
and some other, unidentified products. Furthermore, the
spectrum showed peaks which can be attributed to the
deprotonated form of hydrazone 1 (Supporting Information,
Figure S2). We assume that the rapid transformation of
iPrMgCl into mainly hydrazone and propane is a reflection of
its intrinsic reactivity toward N2O. The participation of trace
amounts of transition-metal impurities in the Grignard reagent
appears unlikely, because the common 3d transition metals lead
to oxidative homocoupling as described earlier.13
The primary aliphatic Grignard reagent BnMgCl was found
to react with N2O in a similar fashion. Addition of N2O to a
THF solution of BnMgCl (2.0 M) resulted in the fast
formation of hydrazone 2, which was isolated in 37% yield in
the form of its hydrochloride salt (Scheme 1).14 The latter was
comprehensively characterized, including a single crystal X-ray
analysis (Supporting Information). GC-MS analysis of the
reaction mixture revealed the presence of toluene (44%) and
1,2-diphenylethane (18%) as the main side products.
Hydrazone formation likely proceeds via diazotates (A),
obtained by insertion of N2O into the Mg−C bond (Scheme
2). The formation of intermediate diazotates is in line with
what has been suggested for Li-, Na-, and Ca-organic
compounds.9−11 Deprotonation, cleavage of the N−O bond,
and coupling with another 1 equiv of RR′CHMgCl results in
Scheme 2. Proposed Mechanism for the Formation of
Hydrazonides by Reaction of Aliphatic Grignard Reagents
with N2O
Received: March 31, 2014
© XXXX American Chemical Society
A
dx.doi.org/10.1021/om500333y | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
the formation of hydrazone anions (B). The order of these
processes is presently not clear, but it is conceivable that the
reactions involves diazo compounds R(R′)CN2, which react
with RR’CHMgCl to give hydrazone anions of type B.15 The
proposition that part of the Grignard reagents act as a base is
substantiated by the fact that the main side products for the
reactions with iPrMgCl and BnMgCl are propane and toluene.
Evidently, the simple mechanism depicted in Scheme 2 does
not reflect the complexity of the real systems, because other
side products such as 1,2-diphenylethane are not accounted for.
The possibility of concurrent reaction pathways was also
proposed for organolithium compounds.1b
Another simplification in Scheme 2 is the depiction of the
salt byproducts as “MgO” and “MgCl2”. These compounds can
form solvated magnesium oxohalide clusters.16 Evidence for the
relevance of such clusters was obtained by X-ray crystallog-
raphy. Placing a THF solution of iPrMgCl under an
atmosphere of N2O and cooling it to −35 °C led to the
formation of colorless single crystals, which were analyzed by
X-ray diffraction. The analysis revealed that the complex
[Mg5Cl4O(C6H12N2)2(THF)5] (3) had formed (Figure 1).
being consumed after 10 h. The observation that the methyl
Grignard reagent is the least reactive of all primary Grignard
reagents tested is in line with what has been reported for
reactions with substrates other than N2O.17 Still, our results are
in apparent contradiction to the reported unreactivity of
MeMgI.12 However, we were able to confirm that solutions of
MeMgI in diethyl ether are indeed inert toward N2O: no
noticeable conversion was observed after 24 h at room
temperature. Apparently, the solvent has a pronounced effect
on the reactivity. In line with this assumption, we found that
the conversion of BnMgCl was slowed down substantially when
the reaction was performed in Et2O: upon addition of N2O,
only 28% conversion of the starting material was observed after
24 h (THF: t1/2 = 26 min).
Further, we examined the reactivity of a tertiary aliphatic
Grignard reagent toward N2O. The reaction of tBuMgCl in
THF-d8 resulted in the formation of 2-methylpropane (∼43%)
and 2-methylpropene (∼32%) according to an analysis by
1H NMR spectroscopy after 15 h. The formation of N-
containing products was not observed in this case.
Having established that primary and secondary aliphatic
Grignard reagents are suited reagents for reactions with N2O,
we have investigated whether we could use this transformation
for synthetic purposes. Aliphatic hydrazones are very sensitive
compounds which decompose rapidly.18,19 Therefore, we
decided to target the hydrolysis products by adding an acidic
workup step. Using this procedure, it was possible to convert
iPrMgCl into the corresponding hydrazinium salt (iPr-
NH2NH2)Cl (4) in 44% yield (Scheme 3). The Grignard
reagents CyMgCl, EtrMgCl, and Ph(CH2)2MgCl react in a
related fashion, and we were able to isolate the salts 5−7 with
yields between 42 and 63% (Scheme 3).
Scheme 3. Reactions of Aliphatic Grignard Reagents with
a
N2O
Figure 1. Synthesis of complex 3 and its molecular structure in the
crystal. Hydrogen atoms are omitted for clarity.
a
The yields are calculated on the basis of the assumption that 2 equiv
of RMgCl is needed to generate the hydrazinium salts.
The pentanuclear cluster features an oxido ligand which is
bound to four Mg2+ ions, three bridging and one terminal
chlorido ligand, and five THF ligands, as well as two doubly
deprotonated hydrazone ligands. Each of these last two ligands
is coordinated to three Mg2+ ions via both of its N atoms (Mg−
N = 2.0690(15)−2.1777(15) Å) and via one C atom (Mg−C =
2.3487(19) Å, Mg−C′ = 2.4001(18) Å). A noteworthy
structural feature of cluster 5 is the presence of rather long
N−N bonds (1.4911(18) and 1.4913(19) Å). The bond
elongation is likely a consequence of the presence of two
negative charges on the ligand.
The atom economy of this process is not very good. Part of
the RMgCl starting material is lost during the reaction with
N2O in the form of RH, and the hydrolytic workup results in
further loss of material. However, this new reaction allows the
conversion of Grignard reagents in a straightforward fashion
into hydrazines. One should also note that the standard
procedure for synthesizing hydrazines (reductive amination of a
carbonyl compound with Boc-protected hydrazine, followed by
deprotection)20 is not very atom economical either. The
formation of alkylhydrazines by reaction of hydrazine with alkyl
halides is known, but it is not often used for the synthesis of
monoalkylhydrazines because of problems with overalkyla-
tion.21
To conclude: for about a century, Grignard reagents have
been believed to be inert toward N2O. We show that this
generalization is not correct. While some organomagnesium
compounds are in fact rather reluctant to react with N2O, there
are others which react within minutes at ambient temperature.
In particular, THF solutions of primary (with the exception of
Next, we turned our attention to the simple primary
Grignard reagents EtMgCl and MeMgI. As for iPrMgCl and
BnMgCl, we observed a conversion when the reaction flask was
filled with N2O. The relative reactivity of all four compounds
1
was determined by H NMR spectroscopy using THF-d8 (all
compounds: 0.3 M, THF). The fastest reaction was observed
for EtMgCl (t1/2 ≈ 5 min), followed closely by iPrMgCl (t1/2
≈
8 min) and then BnMgCl (t1/2 = 26 min). The reaction of
MeMgI was much slower, with half of the starting material
B
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Communication
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MeMgI) and secondary aliphatic Grignard reagents undergo a
rapid conversion when subjected to an atmosphere of N2O. As
the main reaction products, we have identified hydrazones and
alkanes derived from protonation of RMgX. When the reaction
of Grignard reagents with N2O is combined with an acidic
workup, it is possible to obtain alkylhydrazinium salts on a
preparative scale. These conversions are rare examples of
reactions in which N2O is used as an N-donor in synthetic
organic chemistry.
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ASSOCIATED CONTENT
* Supporting Information
Text, figures, a table, and CIF files giving experimental details
and characterization data for the compounds prepared in this
paper and crystallographic data for 1 + 1·O2, 2, and 3. This
material is available free of charge via the Internet at http://
pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*E-mail for K.S.: kay.severin@epfl.ch.
Notes
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(7) (a) Cavigliasso, G.; Criddle, A.; Kim, H.-S.; Stranger, R.; Yates, B.
F. Dalton Trans. 2014, 43, 4631−4634. (b) Tskhovrebov, A. G.; Solari,
E.; Scopelliti, R.; Severin, K. Inorg. Chem. 2013, 52, 11688−11690.
(c) Reeds, J. O.; Yonke, B. L.; Zavalij, P. Y.; Sita, L. R. J. Am. Chem.
Soc. 2011, 133, 18602−18605. (d) Cherry, J.-P. F.; Johnson, A. R.;
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The work was supported by the Ecole Polytechnique Fed
de Lausanne (EPFL) and the Swiss National Science
Foundation. A.G.T. thanks Dr. Pascal Mieville for support
with NMR measurements.
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