Ambident reactivity of bis(diisopropylamino)carbene

Article abstract of DOI:10.1002/ejoc.201000074

Bis(diisopropylamino)carbene can display not only carbenelike reactivity but also enter 1, 3-addition reactions with the participation of the carbon and nitrogen atoms of the N-C-N unit.

Full text of DOI:10.1002/ejoc.201000074

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201000074
Ambident Reactivity of Bis(Diisopropylamino)carbene
Dmytro Poliakov[a] and Igor Shevchenko*[a]
Keywords: Carbenes / Zwitterions / Carbonyl compounds / Deoxygenation / Cyclization
Bis(diisopropylamino)carbene can display not only carbene-
like reactivity but also enter 1,3-addition reactions with the
participation of the carbon and nitrogen atoms of the N–C–
N unit.
would like to report on the first reaction of bis(diisopropyl-
amino)carbene in which the N–C–N carbon atom takes
part in the reaction together with one of the adjacent nitro-
gen atoms.
Introduction
Except steric protection, the stability of singlet carbenes
known to date is accounted for by π-electronic interaction
of the vacant pπ orbital of the carbene center with the lone
pair of electrons on adjacent phosphanyl and/or amino
groups.[1,2] For phosphanyl-stabilized carbenes this interac-
tion can be schematically represented by the superposition
of mesomeric structures A, B, and C. Theoretical calcula-
tions[3] and X-ray diffraction studies[4] revealed that the real
structure is better described by zwitterionic form B with
certain contributions of carbene A and phosphaacetylene
C. Due to their electronic structure, phosphanylcarbenes
can display the reactivity of carbenes as well as that of zwit-
terions with participation of the phosphorus atom.[2]
Results and Discussion
There are two examples known of the interaction of sing-
let nucleophilic diaminocarbenes with
a
carbonyl
group.[9,10] One of them is the reaction of bis(diisopropyl-
amino)carbene (2) with aroylimines.[9] Carbene 2 deoxyge-
nates aroylimine 1 first to give carbene-nitrile ylide 3. This
intermediate then reacts with a second equivalent of the
carbene to afford alkene 4. This is a rare example of the
untypical carbene-like behavior of 3. Compounds of this
type usually react as nitrile ylides entering different 1,3-ad-
dition reactions.[11]
We have found that together with alkene 4 the interaction
of aroylimine 1 with carbene 2 leads to the formation of
product 5 in 15% yield (Scheme 1). In this compound, both
the carbon atom of the starting carbene and the adjacent
nitrogen atom are enclosed in the heterocyclic five-mem-
bered ring. The formation of this [2+3] cycloaddition prod-
uct implies that carbene 2 and carbene-nitrile ylide 3 can
react with each other not only as two carbenes to give 4,
but formally also as two zwitterions.
Compound 5 was isolated by column chromatography as
a stable crystalline product. X-ray single-crystal analysis
gave the molecular structure shown in Figure 1. The C9–
C8 bond formed during the cyclization between the former
carbene center and the hexafluoroisopropyl unit is very
long (1.61 Å) and is comparable with the longest C–C
bonds known.[12] This can probably be accounted for by
the repulsion of the sterically demanding diisopropylamino
group and the two trifluoromethyl groups. The N2 nitrogen
atom that initially belonged to the starting diaminocarbene
is bound in compound 5 to the C7 atom of the former carb-
onyl group. This nitrogen atom has tetrahedral geometry
and its lone pair of electrons is not conjugated with the
adjacent C7–N1 double bond. Despite this and the presence
Unlike phosphorus, nitrogen possesses stronger σ-ac-
cepting ability. Besides this, nitrogen adopts the planar con-
figuration much more easily and the overlap of its lone pair
of electrons with the pπ orbital of the carbene carbon is
more efficient.[5,6] As a consequence, the amino group is a
more effective π-donor. The stabilization of diaminocarb-
enes can be represented mainly by mesomeric structure A.
The contribution of ylidic mesomeric form B was a matter
of discussion.[2,7] Alder and co-workers described the first
acyclic stable bis(diisopropylamino)carbene and found that
C–N bonds in this compound have as much double bond
character as those in the appropriate amidinium ion.[8]
However, in all reactions known so far diaminocarbenes
displayed only carbene-like behavior. In this paper, we
[a] Institute of Bioorganic Chemistry and Petrochemistry,
Murmanskaya Street 1, 02094 Kiev, Ukraine
Fax: +38-044-5732552
E-mail: igorshevchenko@yahoo.com
Eur. J. Org. Chem. 2010, 2449–2451
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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D. Poliakov, I. Shevchenko
SHORT COMMUNICATION
carbene 2 should undergo decomposition to give imine 6,
which then enters a 1,3-addition reaction with nitrile ylide
3 to give cyclic product 5.
Scheme 1.
Scheme 2.
However, there are some arguments because of which the
initial decomposition of carbene 2 before reacting with 3
seems to be less probable. First, carbene 2 has been investi-
gated in different reactions as a reagent or a catalyst and
has displayed no tendency to such decomposition. Second,
we did not detect a noticeable amount of compounds of
type 5 when carbene 2 reacted with aroylimine 1 containing
the p-nitrophenyl group (Ar = p-NO2C6H4) or the more
sterically demanding mesityl group (Ar = Mes). Only the
corresponding alkenes of type 4 were formed.[9] Such a de-
pendency of the formation of 5 on the nature of the Ar
substituent can be observed only if the elimination of one
isopropyl group occurs after the formation of the C9–C8
bond and not before it. Thus, it is probably more likely that
the reaction begins with the formation of the C9–C8 bond
to give intermediate compound 7 (mechanism B). Taking
into account the considerable length of this bond, the pro-
cess may have reversible character. The positively charged
immonium center in 7 initiates a 1,3-proton shift that is
followed by evolution of propene and closure of the ring.
The impossibility of the cyclization of 7 if Ar = p-NO2C6H4
or Mes is probably accounted for by the decreasing nucleo-
philicity of the C7 atom or by steric factors, respectively.
of bulky substituents, the central five-membered ring is al-
most planar. It is interesting that the N2 atom in the start-
ing bis(diisopropylamino)carbene was connected with two
isopropyl groups but only one of them remained in product
5. This means that the mechanism of the reaction should
include elimination of one of the isopropyl groups before
the formation of the N2–C7 bond. Such a transformation
occurs probably through migration of one proton from the
isopropyl group to the C9 atom with evolution of propene.
A similar decomposition of the diisopropylamino group has
been already observed in several carbenes.[13–15]
Conclusions
Figure 1. Crystal structure of 5. Selected bond lengths [Å] and
angles [°]:C8–C9 1.610(2), N2–C7 1.367(2), C7–N1 1.287(2), C9–
N2 1.4882(19), C9–N3 1.419(2), N2–C18 1.481(2); N1–C7–N2
118.16(14), C7–N2–C9 108.81(12), C10–C6–C1 107.16(12), N1–
C7–N2–C9 –2.32, N1–C8–C9–N2 4.68, C9–C8–N1–C7 –6.18.
Reacting with each other, both diaminocarbene 2 and
nitrile ylide 3 display ambident chemical behavior. The for-
mation of compounds 5 and 4 means that diaminocarbene
2 does not only display carbene-like reactivity and that ni-
trile ylide 3 can also react like a mesomeric carbene species.
One can propose two possible reaction mechanisms for
the formation of compound 5 depending on whether trans-
formation of the diisopropylamino group with evolution of
propene takes place before the formation of the C9–C8
Experimental Section
General: All operations were performed under an atmosphere of
bond or after it (Scheme 2). In the first case (mechanism A) nitrogen in a dry box. The solvents were dried by the usual pro-
2450 Eur. J. Org. Chem. 2010, 2449–2451
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© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Ambident Reactivity of Bis(Diisopropylamino)carbene
cedures. The NMR spectra were recorded with Varian Gemini
400 MHz and JEOL FX-90Q spectrometers. The 1H and 13C chem-
ical shifts are referenced to tetramethylsilane (TMS), and the 19F
shifts are referenced to CFCl3. The 31P chemical shifts were mea-
sured by using 85% aqueous H3PO4 as an external standard.
Acknowledgments
The authors thank Prof. A. Chernega and Dr. E. Rusanov (Insti-
tute of Organic Chemistry, Kiev, Ukraine) for the X-ray diffraction
studies, as well as Prof. A. Rozhenko, (Institute of Organic Chemis-
try, Kiev, Ukraine) for fruitful discussions.
Compound 5: To a frozen (–196 °C) solution of 2 (84 mg, 0.4 mmol)
in Et2O (1 mL) was dropwise added a chilled solution of N-(per-
fluoroprop-2-ylidene)benzamide (1; 52 mg, 0.19 mmol) in Et2O
(0.4 mL) so that the mixture remained solid. The mixture was
warmed to room temperature over 20 min whilst stirring. After
evaporation of ether in vacuo, the residue was extracted with hex-
ane (1 mL) at 20 °C with stirring. The hexane solution was sepa-
rated and stored overnight at –15 °C. After separation of the pre-
cipitate, the mother liquor was concentrated and purified by col-
umn chromatography (Rf = 0.6, silica gel 60, hexane/diethyl ether,
10:1). Colorless crystals of 5 suitable for X-ray analysis were grown
from hexane at 20 °C. Yield after recrystallization: 6 mg (7.5%).
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Received: January 20, 2010
Published Online: March 19, 2010
Eur. J. Org. Chem. 2010, 2449–2451
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
2451