Steric and electronic effects in the dimerization of Wanzlick carbenes: The alkyl effect

Article abstract of DOI:10.1002/ejic.200700217

The steric and electronic influence of N-alkyl substituents on the dimerization energies ΔG° of Wanzlick carbenes (imidazolidin-2- ylidenes) was investigated experimentally and through DFT methods for a series of non-symmetrically substituted Wanzlick carbenes. A series of 3-alkyl-1-tert-butylimidazolidin-2-ylidenes with decreasing steric demand of the alkyl substituent (isopropyl, ethyl and methyl) were obtained in four steps from the commercially available N-alkylaminoethanol compounds. The carbenes are hydrolytically sensitive, colorless oils that can be distilled without decomposition and show no sign of dimerization to the respective enetetramines, even after prolonged heating. Calculations at the B98/ 6-31G(d) level confirm that the dimerization of all three carbenes is thermodynamically unfavorable. To separate the steric and electronic stabilization of Wanzlick carbenes by N-alkyl substituents, the formation energies of R,H₃ mono-alkyl enetetramines were used to derive electronic increments for the N-alkyl substituents. The computational data show that all alkyl substituents electronically stabilize Wanzlick carbenes vs. their dimerization products with increments ranging from 2.97 kcal mol^-1 (N-methyl) to as high as 6.28 kcal mol^-1 (N-tert-butyl). For combinations of N-methyl, N-ethyl and N-isopropyl substituents, the increments are additive and the dimerization energies were found to be free of noticeably steric effects. Significant steric strain was found for all tBu-substituted carbenes with strain energies of the dimerization products ranging from 6.92 kcal mol^-1 [formation of (E)-Me₂tBu₂-enetetramine] to 24.23 kcal mol^-1 (formation of tBu₄ enetetramine). The tert-butyl substituent thus assumes a unique position by strongly stabilizing the carbenes electronically as well as sterically. Wiley-VCH Verlag GmbH & Co. KGaA, 2007.

Full text of DOI:10.1002/ejic.200700217

FULL PAPER
DOI: 10.1002/ejic.200700217
Steric and Electronic Effects in the Dimerization of Wanzlick Carbenes:
The Alkyl Effect[‡]
Michael K. Denk,*[a] Azardokht Hezarkhani,[a] and Feng-Lan Zheng[a]
Keywords: Carbenes / Thiourea / Thermochemistry / Dimerization / Enetetramine / Steric hindrance / DFT calculations /
Steric effects / Electronic effects
The steric and electronic influence of N-alkyl substituents on
the dimerization energies ∆G° of Wanzlick carbenes (imid-
azolidin-2-ylidenes) was investigated experimentally and
through DFT methods for a series of non-symmetrically sub-
stituted Wanzlick carbenes. A series of 3-alkyl-1-tert-butyl-
imidazolidin-2-ylidenes with decreasing steric demand of the
alkyl substituent (isopropyl, ethyl and methyl) were obtained
in four steps from the commercially available N-alkylamino-
ethanol compounds. The carbenes are hydrolytically sensi-
tive, colorless oils that can be distilled without decomposition
and show no sign of dimerization to the respective enetetra-
mines, even after prolonged heating. Calculations at the B98/
the N-alkyl substituents. The computational data show that
all alkyl substituents electronically stabilize Wanzlick carb-
enes vs. their dimerization products with increments ranging
from 2.97 kcalmol–1 (N-methyl) to as high as 6.28 kcalmol–1
(N-tert-butyl). For combinations of N-methyl, N-ethyl and N-
isopropyl substituents, the increments are additive and the
dimerization energies were found to be free of noticeably ste-
ric effects. Significant steric strain was found for all tBu-sub-
stituted carbenes with strain energies of the dimerization
products ranging from 6.92 kcalmol–1 [formation of (E)-
Me2tBu2-enetetramine] to 24.23 kcalmol–1 (formation of tBu4
enetetramine). The tert-butyl substituent thus assumes a
6-31G(d) level confirm that the dimerization of all three carb- unique position by strongly stabilizing the carbenes electron-
enes is thermodynamically unfavorable. To separate the ste-
ically as well as sterically.
ric and electronic stabilization of Wanzlick carbenes by N-
alkyl substituents, the formation energies of R,H3 mono-alkyl (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
enetetramines were used to derive electronic increments for
Germany, 2007)
be the benzo-annellated carbene E that has recently been
obtained by Metallinos et al.[12]
1. Introduction
The crucial role of steric protection is particularly obvi-
ous from the behavior of Wanzlick carbenes D which dimer-
ize for R = phenyl, the substituent originally chosen by
Wanzlick’s group, but are thermodynamically stable
towards dimerization for R = tert-butyl[4] and mesityl.[13]
For alkyl substituents, even a slight relaxation of the steric
demand (R = isopropyl, ethyl, or methyl) the Wanzlick
carbenes dimerize although the dimerization is slow enough
to be observed by NMR methods.[4,5] The slow nature of
the dimerization has since been strikingly highlighted by
the successful isolation and structural characterization of a
series of thermodynamically unstable amino carbenes like
the tetramethyl analog of C,[10a] the tetraethyl analog of
C[10b] and even the monoamino carbene F (Scheme 1).[10c]
Irrespective of the kinetic aspect of the reaction, the pres-
ence of bulky substituents remains a prerequisite for the
design of thermodynamically stable carbenes. For Wanzlick
carbenes bearing identical N-substituents, the demarcation
line separating stable from unstable carbenes is now estab-
A limit to the steadily growing number of thermodynam-
ically stable diamino carbenes[1] obtained since the isolation
of a imidazol-2-ylidene (A, R = 1-adamantyl) in 1991 by
Arduengo et al.[2] is their dimerization[3–5] to the respective
enetetramines,[6] the very reaction preventing the isolation
of 1,3-diphenylimidazolidin-2-ylidene investigated by
Wanzlick’s group more than 40 years ago.[3] While dimeriza-
tion of the aromatically stabilized[7] carbenes of type A is
thermodynamically unfavorable even for small substituents
like R = Me, and has only been observed for the special
case of the covalently tethered bis-carbene (formation of
B=B)[8] all other diamino carbenes like C and D require
sterically demanding substituents to be thermodynamically
stable towards dimerization.[5–11] A notable exception may
[‡] Presented in part (experimental part) at the OMCOS confer-
ence, July 27–31, 2005, Geneva, Switzerland.
[a] Guelph–Waterloo Centre For Graduate Research in Chemistry,
Guelph Campus
[4]
lished as tBu2/iPr2 and Mes2/Ph2.[13] Wanzlick carbenes
50 Stone Road, Guelph, Ontario N1G 2W1, Canada
Fax: +1-519-766-1499
bearing only one bulky N-alkyl substituent should dimerize
to the (E)-enetetramines in which steric repulsion is mini-
mized due to the large-small combination of alkyl substitu-
ents facing each other across the double bond (Scheme 2).
E-mail: mdenk@uoguelph.ca
michaeldenk@mac.com
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
Eur. J. Inorg. Chem. 2007, 3527–3534
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3527

M. K. Denk, A. Hezarkhani, F.-L. Zheng
FULL PAPER
Scheme 1. Dimerization of aminocarbenes.
2. Results and Discussion
The synthesis of Wanzlick carbenes 4 bearing dissimilar
substituents R, RЈ on the nitrogen atoms is of obvious
interest to understand the fine interplay of steric and elec-
tronic effects in the Wanzlick equilibrium but is also desir-
able to broaden the repertoire of N-heterocyclic carbenes
used as steering ligands in catalysis. While the symmetri-
cally substituted ethylenediamines RNH-CH2CH2-NHRЈ
(R = RЈ) are commercially available and are also readily
accessible from 1,2-dibromoethane and primary amines,[15]
ethylenediamines RNH-CH2CH2-NHRЈ bearing dissimilar
substituents are less well explored and accessible only
through multistep reactions.[16] Except for R, RЈ = Me,H
the diamines are not commercially available.
Among the number of possible synthetic approaches
tested in our group, only the reaction of primary amines
with 2-chloroethylamines[17] proved to be the most useful
(Scheme 3). Apart from being adaptable to a great variety
of substituents R,RЈ the reactions are readily scaled up and
deliver acceptable yields and purities of the diamines.
The disadvantage of using the rather toxic 2-chloroeth-
ylamines is diminished by the fact that the stable and non-
Scheme 2. Steric matching in non-symmetric Wanzlick carbenes.
To examine this prediction experimentally, we have ob- volatile hydrochloride salts can be used instead of the vola-
tained a series of 3-alkyl-1-tert-butylimidazolidin-2-ylidenes tile free bases.
with alkyl substituents of decreasing size (R: iPr, Et and
Reaction of 2-(tert-butylamino)ethanol with SOCl2 fol-
Me). Much to our surprise, all three carbenes are perfectly lowed by reaction with the respective primary amine thus
stable and do not show any sign of dimerization even for gave the N-tert-butyl-NЈ-(alkyl)ethylenediamines (1). To ob-
the smallest substituent (Me). While surprising, the ob- tain good yields of the respective ethylenediamines, the use
served lack of dimerization is in good agreement with the of an excess of the primary amine (5 equiv. or more) as well
behavior of 1,3,4-trisubstituted Wanzlick carbenes E and as high reaction temperatures (180 °C) were found to be
F (Scheme 2) which have been obtained via a new elegant crucial. Lowering the amount of primary amine lead to the
synthetic protocol by Hahn’s group while this work was in predominant formation of N,NЈ-di-tert-butylpiperazine
progress.[14]
through the competing self-condensation of the 2-chlo-
The observed lack of dimerization is nevertheless hard to roamine, while lower reaction temperatures were found to
rationalize on purely steric grounds so that a more detailed be inadequate to achieve complete conversion (detection of
computational examination of the steric and electronic in- the free 2-chloroamine by GC-MS). The synthesis of the
fluence of N-alkyl groups on the dimerization of Wanzlick respective thioureas from the ethylenediamines turned out
carbenes seemed desirable.
to be an additional obstacle. As we noted previously for
3528
www.eurjic.org
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2007, 3527–3534

Dimerization of Wanzlick Carbenes
FULL PAPER
Scheme 3. Synthesis of 3-alkyl-1-tert-butylimidazolidin-2-ylidenes from 2-(tert-butylamino)ethanol.
ethylenediamines bearing two bulky N-alkyl substituents, neither of the two possible dimerization products could be
the respective thioureas can only be obtained in poor yields observed is therefore hard to justify on purely steric
via the textbook CS2 route or its many variations.[18] The grounds and points towards the importance of electronic
reaction of the ethylenediamines 1a–1c with CS2 gave like- effects in the stabilization of 4. To clarify this issue, the ste-
wise only trace amounts (GC-MS) of the thioureas 3a–3c. ric and electronic influence of different N-alkyl substituents
The presence of only one sterically demanding substituent on the dimerization energies of Wanzlick carbenes was in-
(tert-butyl) is obviously sufficient to block the CS2 route.[18] vestigated through DFT calculations.
To circumvent these problems, the thioureas 3a–3c were ob-
tained by oxidation of the respective formaldehyde aminals
(imidazolidines) 2a–2c with elemental sulfur, S8, a two-step
Computational Studies
one-pot approach recently developed in our group to over-
come the problems of the CS2 method.[18] Reductive
desulfurization of the thioureas with potassium (Kuhn
method)[19] gave the respective carbenes in high yields (85–
95%).
The carbenes were obtained as spectroscopically pure
(1H NMR, 13C NMR) oils that are readily distilled without
decomposition. In line with the previously examined
Wanzlick and Arduengo carbenes[20] the carbenes 4a–4c
were found to be unreactive towards dry oxygen but readily
hydrolyze in air.[21] Most remarkably, the carbenes did not
show any sign of dimerization either as such or in C6D6 at
room temperature or at 100 °C even after 12 months. The
dimerization of the carbenes 4a–4b can in principle lead
to the formation of either the respective (E)- or the (Z)-
enetetramines (Scheme 4).
DFT calculations of symmetrically substituted diamino
carbenes have already generated significant insight into the
nature of the dimerization reaction,[5] and have recently
been reviewed by Alder et al.[1a] Of particular importance
is the conclusion that calculated dimerization energies ∆G°
are in good qualitative agreement with the experimentally
observed dimerization behavior.[1a] The precise balance of
steric and electronic effects on the stability of the carbenes
towards dimerization on the other hand is less clear. DFT
calculations at the B98/6-31G(d) level were carried out to
establish if the dimerization of the carbenes 4a–4c has a
thermodynamic basis and to quantify steric and electronic
effects of the respective N-alklyl substituents. The B98 func-
tional was selected because it offers improved thermochemi-
cal accuracy over the B3LYP method without adding com-
putational cost.[23] To obtain a benchmark for the accuracy
of the DFT calculations, the dimerization energy of the hy-
pothetical unsubstituted carbene 4H was calculated with
the high precision CBS-Q[26] and G3[27] methods. All calcu-
lations were carried out with the Gaussian03 suite of pro-
grams.[22] Local minima were confirmed through the ab-
sence of virtual frequencies. The geometry selected for the
enetetramines 5 is that found experimentally for all solid-
state structures of N-alkyl-substituted enetetramines.[28]
The surprisingly strong effect of N-alkyl substituents on
the dimerization of Wanzlick carbenes is readily apparent
Scheme 4. Potential formation of enetetramines 5 from carbenes
4a–4c.
While the (Z)-enetetramine is likely to suffer from signifi- from a comparison of the dimerization energies ∆G° of
cant steric front-strain due to the interaction of the tert- N,NЈ-dimethylimidazolidin-2-ylidene (4e) with that of the
butyl groups and might be unstable for steric reasons, the unsubstituted carbene 4H (Scheme 5). The dimerization en-
argument cannot be applied to the (E)-isomer. The fact that ergies reveal a net stabilizing effect caused by the presence
Eur. J. Inorg. Chem. 2007, 3527–3534
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
3529

M. K. Denk, A. Hezarkhani, F.-L. Zheng
FULL PAPER
Scheme 5. Influence of N-methyl substitution on the dimerization energy of imidazolidin-2-ylidene. Dimerization energies ∆G° (in
kcalmol–1) at the B98/6-31G(d), CBS-Q and G3 level.
of four methyl groups ca. 13 kcalmol–1. The high accuracy amined (Scheme 7). The presence of only one alkyl substitu-
of the B98/6-31G(d) approach can be inferred by the ex- ent in the enetetramines 5h–5l eliminates the possibility of
cellent agreement of the dimerization energy of 1H steric effects resulting from alkyl–alkyl repulsion. The varia-
(–26.4 kcalmol–1) with the respective G3 (–27.8 kcalmol–1) tion in the dimerization energies will thus reflect the elec-
and CBS-Q data (–28.7 kcalmol–1).
tronic effect of the N-alkyl substituent.
The lack of dimerization observed for the carbenes 4a–
4c can in principle have thermodynamic reasons (∆G°Ͼ0)
or kinetic reasons (high ∆G‡). The kinetic stabilization of
Wanzlick carbenes bearing only one sterically demanding
substituent may in fact be expected from the orbital sym-
metry required non-least-motion pathway.[24] The calcula-
tions do however reveal that the dimerization of the carb-
enes 4a–4d is in fact thermodynamically unfavorable: di-
merization is predicted to be thermodynamically unfavor-
able for 4b and 4c and close to thermoneutral for 4a
(Scheme 6). The stable carbene 4d which was obtained ear-
lier in our group[4] has been included for comparison as the
sterically most crowded member of the series.
Scheme 7. Formation of mono-substituted enetetramines 5h–5l
from the respective carbenes 4H, 4f–4i. Energies ∆G° at the B98/6-
31G(d) level.
Comparison of the dimerization energies in Scheme 7
with those obtained for the unsubstituted enetetramine 5H
(Scheme 5, ∆G° = –26.4 kcalmol–1) leads to electronic in-
crements of the N-alkyl substituents as +2.97 kcalmol–1
(Me), +3.31 kcalmol–1 (Et), +4.04 kcalmol–1 (iPr) and
+6.28 kcalmol–1 (tBu). While all four N-alkyl substituents
electronically stabilize the carbene vs. the dimer, the effect
increases along the series Me Ͻ EtϽiPrϽtBu. Although
the electronic influence of the individual alkyl substituents
may appear small, it must be born in mind that the overall
effect is anything but negligible for the fully substituted en-
etetramines. The electronic effect of replacing four methyl
Scheme 6. Dimerization energies ∆G° (in kcalmol–1) of imid-
azolidin-2-ylidenes 4a–4d at the B98/6-31G(d) level of theory.
A comparison of the individual reaction energies is in- substituents with four tert-butyl substituents amounts to
structive. While the dimerization energies ∆G° show only 13 kcalmol–1, a large caloric change considering that the
small incremental changes from 4a–4c a steep rise of ca. dimerization reactions are close to equilibrium.
20 kcalmol–1 occurs from 4c to 4d. While it is tempting to
The electronic increments obtained above can now be
assign this discontinuous change in the dimerization energy used to separate electronic and steric effects of the N-alkyl
to the onset of steric crowding, a rational, quantitative sep- substituents.
aration of steric and electronic factors is desirable.
Addition of four electronic increments to the base value
To reveal the electronic effects of N-alkyl substituents on corresponding to the formation of unsubstituted enetetram-
the dimerization reaction, the formation of the mono-alkyl ine 5H from the unsubstituted carbene 4H (–26.36 kcal) will
enetetramines 5h–5l from the respective carbenes were ex- give the strain-free, electronic component ∆Gel of the dimer-
3530
www.eurjic.org
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2007, 3527–3534

Dimerization of Wanzlick Carbenes
FULL PAPER
ization energies. Comparison of ∆Gel with the fully opti-
mized values ∆GFC leads to the quantitative estimate of
strain energies ∆Gst according to ∆GFC = ∆Gel + ∆Gst.
Scheme 8 shows the results for the symmetrically substi-
tuted carbenes.
Scheme 9. Dimerization energies ∆G° (in kcalmol–1) of non-sym-
metrically substituted Wanzlick carbenes at the B98/6-31G(d) level.
Comparison of fully optimized energies (∆GFC) with those derived
from the addition of electronic alkyl increments. Steric contri-
butions ∆Gst derived according to ∆GFC = ∆Gel + ∆Gst.
Table 1. Dimerization of Wanzlick carbenes: comparison of 13C
NMR shifts and calculated dimerization energies ∆G° of carbenes
4 with calculated double bond lengths and valence vibrations of
the corresponding (E)-enetetramines 5.
Scheme 8. Dimerization energies ∆G° (in kcalmol–1) of symmetri-
cally substituted Wanzlick carbenes at the B98/6-31G(d) level.
Comparison of fully optimized energies (∆GFC) with those derived
from the addition of electronic alkyl increments (∆Gel). Steric con-
tributions ∆Gst derived according to ∆GFC = ∆Gel + ∆Gst.
R
δ N2C
[ppm]
ν(C=C) d(C=C)
∆G°
[cm–1]
5e 1771
5f 1765
[pm]
[kcalmol–1]
Me,Me 4e 239.8
Et,Et
iPr,iPr
135.23
135.28
135.80
136.45
136.43
136.70
137.47
–13.21
–12.73
–9.65
4f 237.7
4g 236.8
The dimerization energies obtained from full calculations
(∆GFC) and those obtained through the increment approach
5g 1746
5a 1715
5b 1710
5c 1704
5d 1671
tBu,Me 4a 239.7
–0.94
(∆Gel) are practically identical for R = Me, Et, and iPr tBu,Et 4b 238.8
+2.13
+4.54
+25.47
tBu,iPr 4c 237.6
tBu,tBu 4d 238.3
which rules out the presence of significant steric strain for
these alkyl substituents. For R = tBu, the pronounced dif-
ference between ∆GFC (+25.47 kcalmol–1) and ∆Gel
(+1.24 kcalmol–1) confirms a strong steric destabilization
The increase in double bond lengths and decrease in the
C=C stretching frequencies of the enetetramines 5 corre-
lates well with the dimerization energies of 4. The 13C NMR
shifts (N2C) of the carbenes 4 on the other hand show only
a negligible variation regardless of whether the respective
carbene dimerizes (4e, 4f, 4g) or not (4a–4d). It therefore
seems likely, that the variation in dimerization energies is
predominantly caused by a progressive destabilization of
the enetetramines 5 rather than a stabilization of the carb-
enes 4. The thermodynamic stability of Wanzlick carbenes
4 towards dimerization may thus reflect the stability of the
dimers 5 rather than those of the carbenes 4.[25]
(+24.23 kcalmol–1) of the enetetramine. The same approach
can now be used to check if the enetetramines 5a–5c are
indeed free from steric strain as the large-small size match
of the alkyl substituents (Scheme 2) seems to suggest.
The quantitative analysis (Scheme 9) contradicts this
simplistic pictorial prediction by revealing strain energies
∆Gst ranging from +6.92 kcalmol–1 (tBu,Me) to
+10.25 kcalmol–1 (tBu,iPr). Steric strain is in fact a crucial
factor for the thermodynamic stability of the carbenes 4a–
4c because dimerization would occur otherwise (∆Gel Ͻ0).
The thermodynamic stability of Wanzlick carbenes 4
towards dimerization can in principle be ascribed to an in-
herent stabilization of the carbenes 4, a destabilization of
the enetetramines 5 or a combination of both. While dimer-
ization energies, experimental or computational, cannot dif-
ferentiate between these two possibilities, a comparison of
other relevant data with the dimerization energies may nev-
ertheless provide some insight. Table 1 compares the influ-
ence of the N-alkyl substituent on the 13C NMR shifts and
dimerization energies of Wanzlick carbenes 4, with the cal-
A comparable analysis of how the respective activation
energies of the dimerization are influenced by steric and
electronic factors would be very desirable and will be the
subject of future studies.
3. Conclusions
The stability of Wanzlick carbenes towards dimerization
culated length of the carbon–carbon double bond and the is the result of a complex interplay between electronic and
associated valence vibrations in the dimers 5 arranged in steric effects of the nitrogen substituents. There is a smooth
order of decreasing carbene dimerization energies ∆G°.
increase of stability along the series R = Me, Et, iPr, tBu,
Eur. J. Inorg. Chem. 2007, 3527–3534
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
3531

M. K. Denk, A. Hezarkhani, F.-L. Zheng
FULL PAPER
N-tert-Butyl-NЈ-methyl-1,2-diaminoethane (1a): Colorless oil, b.p.
151 °C, 12.9 g (34%). H NMR (CDCl3): δ = 1.09 [s, 9 H, C(CH3)
the tert-butyl substituent occupies a special position by
strongly stabilizing the carbenes electronically as well as ste-
rically. A comparison of spectroscopic and computational
data suggests that the dimerization energies of Wanzlick
carbenes may in fact reflect variations in the stabilities of
the enetetramines rather than that of the carbenes them-
selves.
1
3], 2.43 [s, 3 H, CH3], 2.68 [s, 2 H + 2 H, identical shift, NHCH2]
ppm. 13C NMR (CDCl3): δ = 29.1 [C(CH3)3], 36.4 [NCH3], 41.8
[tBuNCH2], 50.1 [C(CH3)3], 52.6 [MeNCH2] ppm. GC-MS: tr =
5.2 min. m/z (rel. int.%) = 131 (100), 115 (23), 100 (8), 86 (55), 72
(23), 58 (74), 44 (76).
N-tert-Butyl-NЈ-ethyl-1,2-diaminoethane (1b): Colorless oil, b.p.
165 °C, 27 g (64%), 1H NMR (CDCl3): δ = 1.10 [s, 9 H, C(CH3)3],
1.11 [t, 3J(H,H) = 7 Hz, 3 H, CH2CH3], 2.65 [q, 2 H, CH2CH3],
2.71 [m, 4 H, NHCH2 CH2] ppm. 13C NMR (CDCl3): δ = 15.3
[CH2CH3], 29.0 [C(CH3)3], 42.1 [tBuNCH2], 44.1 [EtNCH2], 50.2
[C(CH3)3], 50.3 [CH2CH3] ppm. GC-MS: tr = 6.1 min, m/z (rel.
int.%) = 145 (100), 129 (18), 101 (10), 86 (50), 72 (85), 58 (92).
4. Experimental Section
Melting points were recorded in sealed capillaries and are uncor-
rected. EI-MS spectra were obtained with a Varian CP-3800 GC/
Saturn 2000 MS combination at an ionizing voltage of 70 eV and a
30ϫ0.25 mm Varian CP 5860 low-bleed phenyl(dimethyl)siloxane
column (5% phenyl). Temperature program: 4 min 50 °C; 10 min
constant heating rate of 20 °C/min; 6 min 250 °C. NMR spectra
were recorded with Bruker 400 MHz (1H) and 500 MHz (13C)
NMR spectrometers at room temperature. All NMR spectra are
referenced vs. tetramethylsilane (internal). 2-(tert-Butylamino)etha-
nol, isopropylamine, ethylamine, methylamine, sulfur, paraformal-
dehyde and potassium were obtained from Aldrich Inc., thionyl
chloride from Johnson Mathey. All operations were carried out un-
der inert gas (99.999% Argon) unless indicated otherwise. [2-(tert-
butylamino)ethyl chloride] hydrochloride was obtained from 2-
(tert-butylamino)ethanol and thionyl choride according to ref.[17]
N-tert-Butyl-NЈ-isopropyl-1,2-diaminoethane (1c): Colorless oil, b.p.
181 °C, 32 g (70%). 1H NMR (CDCl3): δ = 1.06 [d, 3J(H,H) =
6 Hz, 6 H, CH(CH3)2], 1.10 [s, 9 H, C(CH3)3], 2.68 [m, 4 H, NCH2],
2.78 [sept, 3J(H,H) = 6, Hz, 1 H, CH(CH3)2] ppm. 13C NMR
(CDCl3): δ = 23.1 [CH(CH3)2], 29.2 [C(CH3)3], 42.6 [tBuNCH2],
48.2 [iPrNCH2], 48.8 [CH(CH3)2], 50.2 [C(CH3)3] ppm. GC-MS: tr
= 7.4 min, m/z (rel. int.%) = 159 (28), 143 (5), 100 (18), 86 (75), 72
(100), 57 (32).
1-tert-Butyl-3-methylimidazolidine (2a): Colorless oil, no CAS. 1H
NMR (CDCl3): δ = 1.10 [s, 9 H, C(CH3)3], 2.37 [s, 3 H, NCH3],
2.76 [m, 2 H, MeNCH2], 2.92 [m, 2 H, tBuNCH2], 3.50 [NCH2N]
ppm. 13C NMR (CDCl3): δ = 26.0 [C(CH3)3], 39.9 [tBuNCH2], 45.8
[MeNCH2], 52.8 [C(CH3)3], 55.2 [CH3], 71.6 [NCH2N] ppm. GC-
MS: tr = 6.19 min, m/z (rel. int.%) = 141 (100), 127 (48), 112 (47),
85 (73), 57 (18), 42 (37).
Synthesis of N,NЈ-Disubstituted Ethylenediamines 1a–c: 2-(tert-Bu-
tylamino)ethyl chloride hydrochloride (50 g, 0.29 mol), the primary
amine (1.45 mol) and water (250 mL) were sealed in a 1-L stainless-
steel autoclave and heated to 180 °C for 48 h. After cooling the
autoclave to room temperature, water (100 mL) and NaOH pellets
(75 g) were added to the autoclave. The resulting biphasic crude
solutions were extracted with Et2O (3ϫ100 mL). The combined
organic phases were dried by addition of NaOH pellets and de-
canting from the freshly forming aqueous phase until the NaOH
pellets remained undissolved. After removal of the solvent in vacuo,
the pure N,NЈ-dialkylethylenediamines were obtained by fractional
distillation over a 20 cm Vigreux column under normal pressure.
1-tert-Butyl-3-ethylimidazolidine (2b): Colorless oil, no CAS. 1H
NMR (CDCl3): δ = 1.09 [s, 9 H, C(CH3)3], 1.10 [t, 3J = 7 Hz, 3 H,
CH2-CH3], 2.52 [q, 3J = 7 Hz, 2 H, CH2CH3], 2.76 [m, 2 H,
CH2CH3], 2.88 [m, 2 H, CH2CH2], 3.50 [s, 2 H, NCH2N] ppm.
13C NMR (CDCl3): δ = 13.9 [CH2CH3], 26.0 [C(CH3)3], 45.0 [tBu-
NCH2], 49.0 [EtNCH2], 52.4 [C(CH3)3], 52.8 [CH2-CH3], 69.6
[NCH2N] ppm. GC-MS: tr = 7.0 min, m/z (rel. int.%) = 155 (100),
141 (25), 126 (13), 100 (5), 99 (52), 84 (43), 42 (25).
Synthesis of 3-Alkyl-1-tert-butylimidazolidines 2a–c: A mixture of
the respective N,NЈ-disubstituted ethylenediamine (1a, 1b or 1c,
30 mmol), ether (15 mL), and paraformaldehyde (1.2 g, 40 mmol)
was stirred at room temperature for 24 h. The resulting turbid solu-
tions were stirred with MgSO4 (ca. 1 g, 24 h) and the imidazolidines
isolated by filtration and subsequent evaporation of the solvent in
vacuo. Yield quantitative.
1-tert-Butyl-3-isopropylimidazolidine (2c): Colorless oil, no CAS.
1H NMR (CDCl3): δ = 1.08 [d, 3J(H,H) = 7.0 Hz, 6 H, CH-
(CH3)2], 1.09 [s, 9 H, C(CH3)3], 2.41 [sept, J(H,H) = 7.0 Hz, 1 H,
3
CH(CH3)2], 2.84 [m, 4 H, NCH2CH2], 3.54 [NCH2N] ppm. 13C
NMR (CDCl3): δ = 21.9 [CH(CH3)2], 26.0 [C(CH3)3], 45.2
[tBuNCH2], 51.1 [iPrNCH2], 52.2 [CH(CH3)2], 54.1 [C(CH3)3], 68.6
[N2CH2] ppm. GC-MS: tr = 8.4 min, m/z (rel. int.%) = 169 (100),
155 (22), 140 (6), 113 (81), 98 (8), 84 (25), 71 (26), 55 (13).
Synthesis of 3-Alkyl-1-tert-butylimidazolidine-2-thiones 3a–b: Ele-
mental sulfur S8 (2.21 g, 8.64 mmol), the respective imidazolidine
(34.5 mmol) and ether (15 mL) were sealed in a stainless steel auto-
clave and heated to 160 °C for 12 h. The cold reaction mixtures
were extracted with methanol (3ϫ10 mL), filtered, and the thio-
ureas precipitated by dropwise addition of water (ca. 30 mL) to the
methanol solution. Crystallization was typically completed after
standing at room temperature for 24 h. A second crop of the thio-
ureas could be obtained by cooling the mother liquor to –20 °C.
After vacuum drying for 24 h, both fractions were adequate for the
subsequent reduction to the carbenes.
1-tert-Butyl-3-methylimidazolidine-2-thione (3a): Colorless crystals,
1
m.p. 96.5–97 °C (hexane). Yield 2.37 g, (40%). No CAS. H NMR
(CDCl3): δ = 1.60 [s, 9 H, C(CH3)3], 3.09 [s, 3 H, NCH3], 3.43 [m,
4 H, NCH2], 3.60 [m, 4 H, NCH2CH2] ppm. 13C NMR (CDCl3):
δ
=
27.9 [C(CH3)3], 35.0 [N-CH3], 44.7 [tBuNCH2], 48.2
[MeNCH2], 56.4 [C(CH3)3], 183.1 [CS] ppm. GC-MS: tr
=
10.7 min, m/z (rel. int.%) = 172 (100), 157 (20), 116 (55), 83 (12),
44 (30).
1-tert-Butyl-3-ethylimidazolidine-2-thione (3b): Colorless crystals,
m.p. 57.5–58 °C (hexane). Yield 0.96 g (15%) No CAS. 1H NMR
Synthesis of 3-Alkyl-1-tert-butylimidazolidin-2-ylidenes 4a–c:
A
3
solution of the respective 3-alkyl-1-tert-butylimidazolidine-2-thione
(75 mmol) in THF (200 mL) was boiled with potassium (8.80 g,
226 mmol) for 2 h. The slightly yellowish solutions were filtered
through a medium-porosity glass frit and the carbenes isolated by
removal of the solvent in vacuo.
(CDCl3): δ = 1.15 [t, J(H,H) = 7 Hz, 3 H, CH2CH3], 1.61 [s, 9 H,
C(CH3)3], 3.38–3.43 [m, 2 H, NCH2], 3.43 [m, 2 H, NCH2], 3.57
[m, 2 H, NCH2], 3.66 [q, 3J(H,H) = 7 Hz, CH2CH3] ppm. 13C
NMR (CDCl3):
δ = 11.8 [CH2CH3], 28.0 [C(CH3)3], 42.0
[tBuNCH2], 44.8 [NCH2], 45.0 [NCH2], 56.4 [C(CH3)3], 182.3 [CS]
3532
www.eurjic.org
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2007, 3527–3534

Dimerization of Wanzlick Carbenes
FULL PAPER
W. Wanzlick, H.-J. Kleiner, Angew. Chem. 1961, 73, 493; f) H.-
W. Wanzlick, Angew. Chem. 1962, 74, 128–134; g) H.-W.
Wanzlick, H.-J. Kleiner, Angew. Chem. Int. Ed. Engl. 1964, 3,
65; Angew. Chem. 1963, 75, 1204; h) H.-W. Wanzlick, B. Lach-
mann, Z. Naturforsch., Teil B 1969, 24, 574–576; i) B. Lach-
mann, H. Steinmaus, H.-W. Wanzlick, Tetrahedron 1971, 27,
4085–4090.
a) M. K. Denk, A. Thadani, K. Hatano, A. J. Lough, Angew.
Chem. Int. Ed. Engl. 1997, 36, 2607–2609; Angew. Chem. 1997,
109, 2719–2721; b) M. K. Denk, K. Hatano, M. J. Ma, Tetrahe-
dron Lett. 1999, 40, 2057–2060.
For computational studies of the dimerization, see: a) D. C.
Graham, K. J. Cavell, B. F. Yates, J. Phys. Org. Chem. 2005,
18, 298–309; b) D. C. Graham, B. F. Yates, Aust. J. Chem. 2004,
57, 359–364; c) J. Cheng, C.-L. Lai, C.-H. Hu, Mol. Phys. 2004,
102, 2617–2621; d) R. W. Alder, M. E. Blake, J. M. Oliva, J.
Phys. Chem. A 1999, 103, 11200–11211; e) R. W. Alder, M. E.
Blake, L. Chaker, J. N. Harvey, F. Paolini, J. Schutz, Angew.
Chem. Int. Ed. 2004, 43, 5896–5911; f) J. M. Oliva, Chem. Phys.
Lett. 1999, 302, 35–42.
a) D. Ülkü, M. N. Tahir, B. Çetinkaya, I. Özdemir, Acta Crys-
tallogr., Sect. C 1997, 53, 240–241; b) A. Schönberg, E. Singer,
W. Stephan, Chem. Ber. 1987, 120, 1581–1588; c) Schönberg,
E. Singer, W. Stephan, Chem. Ber. 1987, 120, 1589–1591; d)
Schönberg, E. Singer, W. Stephan, Chem. Ber. 1983, 116, 2068–
2073; for reviews on enetetramines see: e) E. Haug, W.
Kantlehner, in: Methoden Org. Chem., Houben-Weyl, 4th ed.
1952, vol. E15, 2898–2904, Thieme, Stuttgart, 1993; f) M. Re-
gitz, Methoden Org. Chem. (Houben-Weyl), 4th ed. 1952, vol.
E19b, Thieme, Stuttgart, 1989; g) M. F. Lappert, J. Organomet.
Chem. 1988, 358, 185–214; h) B. Çetinkaya, E. Çetinkaya, J. A.
Chamizo, P. B. Hitchcock, H. A. Jasim, H. Küçükbay, M. F.
Lappert, J. Chem. Soc. Perkin Trans. 1 1998, 2047–2054; i) B.
Çetinkaya, E. Çetinkaya, P. B. Hitchcock, M. F. Lappert, I.
Özdemir, J. Chem. Soc. Dalton Trans. 1997, 1359–1362; j) E.
Çetinkaya, P. B. Hitchcock, H. Küçükbay, M. F. Lappert, S.
Al-Juaid, J. Organomet. Chem. 1994, 481, 89–95; k) E. Çetin-
kaya, P. B. Hitchcock, H. A. Jasim, M. F. Lappert, K. Spyro-
poulos, J. Chem. Soc. Perkin Trans. 1 1992, 561–567; l) P. B.
Hitchcock, M. F. Lappert, P. L. Pye, J. Chem. Soc. Dalton
Trans. 1977, 2160–2172; m) M. F. Lappert, P. L. Pye, J. Chem.
Soc. Dalton Trans. 1977, 2172–2181; n) B. Çetinkaya, P.
Dixneuf, M. F. Lappert, J. Chem. Soc. Chem. Commun. 1973,
1827–1833; o) J. Hocker, R. Merten, Angew. Chem. 1972, 84,
1022; Angew. Chem. Int. Ed. Engl. 1972, 11, 964–973; p) N.
Wiberg, Angew. Chem. 1968, 80, 809–833; Angew. Chem. Int.
Ed. Engl. 1968, 7, 766–779; q) R. W. Hoffmann, Angew. Chem.
Int. Ed. Engl. 1967, 7, 754–765; Angew. Chem. 1968, 80, 823;
r) H. Quast, S. Hünig, Chem. Ber. 1966, 99, 2017–2038.
ppm. GC-MS: tr = 11.0 min, m/z (rel. int.%) = 186 (100), 171 (13),
157 (5), 129 (45), 115 (43), 56 (13), 42 (16).
1-tert-Butyl-3-isopropylimidazolidine-2-thione (3c): Colorless crys-
tals, m.p. 102.5–103.5 °C (hexane). Yield 2.84 g (41%) No CAS. 1H
NMR (CDCl3): δ = 1.14 [d, 3J(H,H) = 7 Hz, 6 H, CH(CH3)2], 1.61
[s, 9 H, C(CH3)3], 3.33 [m 2 H, NCH2], 3.56 [m, 2 H, NCH2], 4.98
[sept, 3J(H,H) = 6.8 Hz, 1 H, CH(CH3)2] ppm. 13C NMR (CDCl3):
δ = 20.0 [CH(CH3)2], 28.1 [C(CH3)3], 40.0 [tBuNCH2], 44.9
[iPrNCH2], 46.3 [CH(CH3)2], 56.4 [C(CH3)3], 181.8 [CS] ppm. GC-
MS: tr = 11.8 min, m/z (rel. int.%) = 200 (71), 185 (78), 157 (5),
143 (43), 129 (100), 111 (8), 102 (7), 84 (10), 70 (12).
[4]
[5]
1-tert-Butyl-3-methylimidazolidin-2-ylidene (4a): 8.7 g (83%). Col-
orless oil, b.p. 65 °C/0.1 Torr 1H NMR (C6D6): δ = 1.35 [s, 9 H,
C(CH3)3], 2.80 [m, 2 H, NCH2], 2.98 [m, 2 H, NCH2], 3.01 [s, 3 H,
CH3]. 13C (C6D6): δ = 30.0, [C(CH3)3], 37.9 [NCH3], 45.3
[tBuNCH2], 50.8 [CH3NCH2], 53.7 [C(CH3)3], 239.7 [N2C:] ppm.
1-tert-Butyl-3-ethylimidazolidin-2-ylidene (4b): 10.5 g (91%) Color-
less oil, b.p. 67 °C/0.1 Torr. 1H NMR (C6D6): δ = 1.07 [t, 3J = 7 Hz,
3 H, CH2-CH3], 1.35 [s, 9 H, C(CH3)3], 2.90 [m, 2 H, NCH2], 3.03
[6]
3
[m, 2 H, NCH2], 3.46 [q, J = 7 Hz, 2 H, CH2CH3]. 13C (C6D6): δ
= 14.8 [CH2CH3], 30.1 [C(CH3)3], 44.9 [tBuNCH2], 45.7 [EtNCH2],
48.1 [CH2CH3], 53.9 [C(CH3)3], 238.8 [N2C:] ppm.
1-tert-Butyl-3-isopropylimidazolidin-2-ylidene (4c): 11.7 g (93%).
Colorless oil, b.p. 70 °C/0.1 Torr. 1H NMR (C6D6): δ = 1.16, [d,
6.7 Hz, 6 H, CH(CH3)2], 1.36 [s, 9 H, C(CH3)3], 2.93 [m, 2 H,
NCH2], 2.99 [m, 2 H, NCH2], 4.00 [sept, 6.7 Hz, 1 H, CH(CH3)2].
13C (C6D6): δ = 22.1 [CH(CH3)2], 29.9 [C(CH3)3], 44.2 [tBuNCH2],
44.9 [iPrNCH2], 50.8 [CH(CH3)2], 53.8 [C(CH3)3], 237.6 [N2C:]
ppm.
Supporting Information (see also the footnote on the first page of
this article): A table listing the energies, point groups and lowest
frequencies of the calculated diamino carbenes 4 and enetetramines
5.
Acknowledgments
We thank the Natural Sciences and Engineering Research Council
of Canada (NSERC) for support of this work and Professor Gord
Lange for valuable comments.
[7]
a) M.-D. Su, S.-Y. Chu, Chem. Phys. Lett. 1999, 308, 283–288;
b) J. F. Lehmann, S. G. Urquhart, L. E. Ennis, A. P. Hitchcock,
K. Hatano, S. Gupta, M. K. Denk, Organometallics 1999, 18,
1862–1872; c) S. Urquhart, A. P. Hitchcock, M. K. Denk, Or-
ganometallics 1998, 17, 2352–2360; d) C. Boehme, G. Frenking,
J. Am. Chem. Soc. 1996, 118, 2039–2046; e) C. Heinemann, T.
Müller, Y. Apeloig, H. Schwartz, J. Am. Chem. Soc. 1996, 118,
2023–2038; f) R. R. Sauers, Tetrahedron Lett. 1996, 37, 149–
152; g) C. Heinemann, W. Thiel, Chem. Phys. Lett. 1994, 217,
11–16; h) J. Cioslowski, Int. J. Quantum Chem. 1993, 27, 309–
319.
a) T. A. Taton, P. Chen, Angew. Chem. 1996, 108, 1098–1100;
Angew. Chem. Int. Ed. Engl. 1996, 35, 1011–1013; b) Z. Shi, V.
Goulle, R. G. Thummel, Tetrahedron Lett. 1996, 37, 2357–
2360; c) G. V. Tormos, M. G. Bakker, P. Wang, M. V. Lakshmi-
kantham, M. P. Cava, R. M. Metzger, J. Am. Chem. Soc. 1995,
117, 8528–8535.
[1] For reviews on stable carbenes, see: a) R. Alder, M. E. Blake,
L. Chaker, J. N. Harvey, F. Paolini, J. Schutz, Angew. Chem.
Int. Ed. 2004, 43, 5896–5911; b) D. Bourissou, O. Guerret, F. P.
Gabbaï, G. Bertrand, Chem. Rev. 2000, 100, 39–91; c) G. Ber-
trand, C. Buron, H. Gornitzka, V. Romanenko, G. Bertrand,
Science 2000, 288, 834–836; d) A. J. Arduengo, Acc. Chem. Res.
1999, 32, 913–921; e) A. J. Arduengo, R. Krafczyk, Chem. Un-
serer Zeit 1998, 32, 6–14; f) W. A. Herrmann, C. Köcher, An-
gew. Chem. Int. Ed. Engl. 1997, 36, 2163–2187; g) M. Regitz,
Angew. Chem. Int. Ed. Engl. 1996, 35, 725–728; Angew. Chem.
1996, 108, 791–794; h) W. Sander, Angew. Chem. Int. Ed. Engl.
1990, 29, 344–354.
[8]
[9]
[2] A. J. Arduengo, R. L. Harlow, M. Kline, J. Am. Chem. Soc.
1991, 113, 361–362.
[3] a) H.-W. Wanzlick, F. Esser, H.-J. Kleiner, Chem. Ber. 1963,
96, 1208–1212; b) H.-W. Wanzlick, B. Lachmann, E. Schikora,
Chem. Ber. 1965, 98, 3170–3177; c) H.-W. Wanzlick, H.-J.
Kleiner, I. Lasch, H. U. Füldner, Angew. Chem. Int. Ed. Engl.
1966, 5, 126–127; Angew. Chem. 1966, 78, 115–116; H.-W.
Wanzlick, E. Schikora, Angew. Chem. 1960, 72, 494; d) H.-W.
Wanzlick, E. Schikora, Chem. Ber. 1961, 94, 2389–2393; e) H.-
a) R. W. Alder, P. R. Allen, M. Murray, A. G. Orpen, Angew.
Chem. 1996, 108, 1211–1213; Angew. Chem. Int. Ed. Engl. 1996,
35, 1121–1122; b) R. W. Alder, M. E. Blake, Chem. Commun.
1997, 1513–1514; c) R. W. Alder, M. E. Blake, C. Bortolotti, S.
Eur. J. Inorg. Chem. 2007, 3527–3534
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
3533

M. K. Denk, A. Hezarkhani, F.-L. Zheng
FULL PAPER
Bufali, C. Butts, E. Linehan, J. M. Oliva, A. G. Orpen, M. J.
Quayle, Chem. Commun. 1999, 241–242; J. M. Oliva, Chem.
Phys. Lett. 1999, 302, 35–42.
Montgomery Jr, T. Vreven, K. N. Kudin, J. C. Burant, J. M.
Millam, S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M.
Cossi, G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji,
M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M.
Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene,
X. Li, J. E. Knox, H. P. Hratchian, J. B. Cross, V. Bakken, C.
Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yaz-
yev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, P. Y.
Ayala, K. Morokuma, G. A. Voth, P. Salvador, J. J. Dannen-
berg, V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C.
Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K. Raghavach-
ari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clif-
ford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P.
Piskorz, I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A.
Al-Laham, C. Y. Peng, A. Nanayakkara, M. Challacombe,
P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong, C. Gonzalez,
and J. A. Pople, Gaussian, Inc., Wallingford CT, 2004.
[23] M. Bienati, C. Adamo, V. Barone, Chem. Phys. Lett. 1999, 311,
69–76.
[10] a) M. Otto, S. Conejero, Y. Canac, V. D. Romanenko, V. Rud-
zevitch, G. Bertrand, J. Am. Chem. Soc. 2004, 126, 1016–1017;
b) R. W. Alder, L. Chaker, F. P. V. Paolini, Chem. Commun.
2004, 2172; c) V. Lavallo, J. Mafhouz, Y. Canac, B. Donnadieu,
W. W. Schoeller, G. Bertrand, J. Am. Chem. Soc. 2004, 126,
8670–8671.
[11] a) F. E. Hahn, L. Wittenbecher, D. L. Van, R. Fröhlich, Angew.
Chem. Int. Ed. 2000, 39, 541–544; b) F. E. Hahn, L. Witten-
becher, R. Boese, D. Bläser, Chem. Eur. J. 1999, 5, 1931–1935;
c) F. E. Hahn, M. Foth, J. Organomet. Chem. 1999, 585, 241–
245; d) Y. Liu, P. E. Lindner, D. M. Lemal, J. Am. Chem. Soc.
1999, 121, 10626–10627.
[12] C. Metallinos, F. B. Barrett, J. L. Chaytor, M. E. A. Heska,
Org. Lett. 2004, 6, 3641–3644. DFT calculations at the B98/6-
31G(d) level carried out in our group suggest that the dimeriza-
tion is exothermic (∆G° = –6.1 kcalmol–1).
[13] A. J. Arduengo, R. Goerlich, W. J. Marshall, J. Am. Chem. Soc.
1995, 117, 11027–11028.
[14] a) F. E. Hahn, D. Le Van, M. Paas, R. Fröhlich, Dalton Trans.
2006, 860–864; b) F. E. Hahn, M. Paas, D. L. Van, T. Lügger,
Angew. Chem. Int. Ed. 2003, 42, 5243–5246; c) F. E. Hahn, M.
Paas, D. L. Van, R. Fröhlich, Chem. Eur. J. 2005, 11, 5080–
5085.
[15] M. K. Denk, M. J. Krause, D. F. Niyogi, N. K. Gill, Tetrahe-
dron 2003, 59, 7565–7570.
[16] a) Y. Hirokawa, H. Harada, T. Yoshikawa, N. Yoshida, S. Kato,
Chem. Pharm. Bull. 2002, 50, 941–959; b) G. Gelbard, P.
Rumpf, Bull. Soc. Chim. Fr. 1969, 1161–1170.
[17] P. Foster, J. C. W. Chien, M. D. Rausch, J. Organomet. Chem.
1997, 545–546, 35–38.
[18] M. K. Denk, S. Gupta, J. Brownie, S. Tajammul, A. J. Lough,
Chem. Eur. J. 2001, 7, 4477–4486.
[19] a) N. Kuhn, E. Niquet, M. Steimann, I. Walker, Z. Natur-
forsch., Teil B 1999, 54, 1181–1187; b) N. Kuhn, T. Kratz, Syn-
thesis 1993, 561–562.
[20] a) M. K. Denk, J. Rodezno, S. Gupta, A. J. Lough, J. Or-
ganomet. Chem. 2001, 617–618, 242–253; b) M. K. Denk, J.
Rodezno, J. Organomet. Chem. 2001, 617–618, 737–740.
[21] The outcome of the hydrolysis reaction does not seem to be
controlled by the N-alkyl substituent: examination of the hy-
drolysis mixtures of 4a–c reveals the presence of the respective
N-formyl and NЈ-formyl products in similar quantities (1H
NMR).
[24] R. Hoffmann, R. Gleiter, F. B. Mallory, J. Am. Chem. Soc.
1970, 92, 1460–1466.
[25] Hydrogenation energies of Wanzlick carbenes (4 + H2 Ǟ 2)
at the B98/6-31G(d) level showed only a small variation (ca.
1 kcalmol–1) with the N-alkyl substituent.
[26] a) M. R. Nyden, G. A. Petersson, J. Chem. Phys. 1981, 75,
1843–1862; b) G. A. Petersson, M. A. Al-Laham, J. Chem.
Phys. 1991, 94, 6081–6090; c) G. A. Petersson, T. Tensfeldt,
J. A. Montgomery, J. Chem. Phys. 1991, 94, 6091–6101; d) J. A.
Montgomery, J. W. Ochterski, G. A. Petersson, J. Chem. Phys.
1994, 101, 5900–5909.
[27] a) L. A. Curtiss, K. Raghavachari, P. C. Redfern, V. Rassolov,
J. A. Pople, J. Chem. Phys. 1998, 109, 7764–7776; b) L. A. Cur-
tiss, P. C. Redfern, K. Raghavachari, J. A. Pople, Chem. Phys.
Lett. 1999, 313, 600–607.
[28] In solution, rapid nitrogen inversion equilibrates this geometry
with other isomers (singlet for the ethylene ring protons of en-
etetramines 5).[4,20] For X-ray data of enetetramines 5, see: a)
P. B. Hitchcock, M. F. Lappert, P. L. Pye, J. Chem. Soc. Dalton
Trans. 1977, 2160; b) P. B. Hitchcock, Acta Crystallogr., Sect.
A 1978, 34, S102; c) E. Çetinkaya, P. B. Hitchcock, H. A. Ja-
sim, M. F. Lappert, K. Spyropoulos, J. Chem. Soc. Perkin
Trans. 1 1992, 561; d) P. B. Hitchcock, J. Chem. Soc. Dalton
Trans. 1979, 1314; e) E. Çetinkaya, P. B. Hitchcock, H. A. Ja-
sim, M. F. Lappert, K. Spyropoulos, J. Chem. Soc. Perkin
Trans. 1 1992, 561.
Received: February 16, 2007
[22] Gaussian 03, Revision D.01, M. J. Frisch, G. W. Trucks, H. B.
Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A.
Published Online: June 14, 2007
3534
www.eurjic.org
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2007, 3527–3534