Metathesis Reactions of Tris(adamantylimido)methylrhenium and Aldehydes and Imines

Article abstract of DOI:10.1021/om990462p

The tris(imido)methylrhenium compound CH₃Re(NAd)₃ (Ad = 1-adamantyl) was prepared and characterized. It reacts with aromatic aldehydes ArCHO forming the imines ArCH= NAd. The reaction occurs in three stages, during which CH₃Re(NAd)₂O and CH₃Re(NAd)O₂ could be detected. In the third and slowest stage CH₃ReO₃ (MTO) was formed, eventually in quantitative yield. The second-order rate constant for PhCHO in C₆D₆ at 298 K is 1.4 × 10^-4 L mol^-1 s^-1. Electron-donating substituents at the para-position of ArCHO cause a significant diminution in rate. Treated by the Hammett equation, the reaction constant is p = +0.90. The reactions between CH₃Re(NAd)₃ and linear aliphatic aldehydes occur much faster than do reactions of nonlinear aliphatic or aromatic aldehydes, indicating an important steric effect. Ketones do not react. The imidorhenium complex evidently undergoes a metathesis reaction with the aldehyde. Analogously, CH₃Re(NAd)₃ reacts with imines. Imine-imine metathesis is catalyzed by MTO homogeneously and by MTO supported on Nb₂O₅.

Full text of DOI:10.1021/om990462p

5170
Organometallics 1999, 18, 5170-5175
Meta th esis Rea ction s of
Tr is(a d a m a n tylim id o)m eth ylr h en iu m a n d Ald eh yd es a n d
Im in es
Wei-Dong Wang and J ames H. Espenson*
Ames Laboratory and Department of Chemistry,
Iowa State University of Science and Technology, Ames, Iowa 50011
Received J une 14, 1999
The tris(imido)methylrhenium compound CH3Re(NAd)3 (Ad ) 1-adamantyl) was prepared
and characterized. It reacts with aromatic aldehydes ArCHO forming the imines ArCHd
NAd. The reaction occurs in three stages, during which CH3Re(NAd)2O and CH3Re(NAd)O2
could be detected. In the third and slowest stage CH3ReO3 (MTO) was formed, eventually
in quantitative yield. The second-order rate constant for PhCHO in C6D6 at 298 K is 1.4 ×
10-4 L mol-1 s-1. Electron-donating substituents at the para-position of ArCHO cause a
significant diminution in rate. Treated by the Hammett equation, the reaction constant is
F ) +0.90. The reactions between CH3Re(NAd)3 and linear aliphatic aldehydes occur much
faster than do reactions of nonlinear aliphatic or aromatic aldehydes, indicating an important
steric effect. Ketones do not react. The imidorhenium complex evidently undergoes a
metathesis reaction with the aldehyde. Analogously, CH3Re(NAd)3 reacts with imines.
Imine-imine metathesis is catalyzed by MTO homogeneously and by MTO supported on
Nb2O5.
In tr od u ction
or proposed as intermediates in nitrene transfer.18,19
Toward this end, we have examined compounds of the
formula CH3Re(NR)3. Compounds such as those with
R ) t-Bu and 2,6-diisopropylphenyl20,21 are known;
however, their nitrene transfer capabilities have not
been investigated. Here, we would like to report our
studies of nitrene transfer reactions with the compound
CH3Re(NAd)3 (R ) 1-adamantyl).
Transfer of an oxygen atom from hydrogen peroxide
is catalyzed by methyltrioxorhenium (CH3ReO3, ab-
breviated as MTO). Numerous substrates are able to
accept the oxygen, including alkenes, organic sulfides,
and phosphines. This area has been reviewed.1-4 Re-
cently, the transfer of a sulfur atom, catalyzed by a
derivative of MTO, has been investigated.5
With this in mind, we invoked the isoelectronic
principle to explore the possibility of nitrene (NR)
transfer. In other contexts, nitrene transfer to olefins,6-9
as well as to phosphines,10-12 aldehydes,11,13,14 and
transition metal complexes,15-17 has been explored
previously. Transition metal imido species are involved
Exp er im en ta l Section
Ma ter ia ls. The aldehydes and aniline derivatives for this
study were used as received from commercial sources, except
butyraldehyde (redistilled) and 4-methoxyaniline (recrystal-
lized). 1-Adamantyl isocyanate was also used as received. The
solvents toluene, benzene-d6, and hexane were dried with
sodium/benzophenone and stored in a nitrogen-filled glovebox.
MTO22 and the organic imines23 were prepared according to
the literature. MTO on niobia was available from a previous
investigation.24
(1) Roma˜o, C. C.; Ku¨hn, F. E.; Herrmann, W. A. Chem. Rev. 1997,
97, 3197-3246.
(2) Espenson, J . H.; Abu-Omar, M. M. Adv. Chem. Ser. 1997, 253,
99-134.
(3) Espenson, J . H. J . Chem. Soc., Chem. Commun. 1999, 479-488.
(4) Gable, K. P. Adv. Organomet. Chem. 1997, 41, 127-161.
(5) J acob, J .; Espenson, J . H. J . Chem. Soc., Chem. Commun. 1999,
1003.
(6) Evans, D. A.; Faul, M. M.; Bilodeau, M. T. J . Am. Chem. Soc.
1994, 116, 2742.
CH3Re(NAd )3. This previously unreported compound was
prepared by refluxing a toluene solution of MTO with 3 equiv
of AdNCO (Ad ) 1-adamantyl), analogous to the literature
procedure.20 After removing most of the solvent by evaporation,
the concentrated solution was cooled to -20 °C, whereupon a
yellow solid was obtained. After filtration and washing with
(7) Li, Z.; Quan, R. W.; J acobsen, E. N. J . Am. Chem. Soc. 1995,
117, 5889.
(8) Muller, P. Adv. Catal. Processes 1997, 2, 113.
(9) J eon, H.-J .; Zhou, H.; Nguyen, S. T. In Book of Abstracts, 216th
ACS National Meeting; American Chemical Society, Washington, D.C.
1998; p INOR-635.
(10) Sleiman, H. F.; Mercer, S.; McElwee-White, L. J . Am. Chem.
Soc. 1989, 111, 8007.
(11) Moubaraki, B.; Murray, K. S.; Nichols, P. J .; Thomson, S.; West,
B. O. Polyhedron 1994, 13, 485.
(12) Leung, W.-H.; Hun, T. S. M.; Hou, H.-W.; Wong, K.-Y. J . Chem.
Soc., Dalton Trans. 1997, 237.
(16) Lapointe, A. M.; Schrock, R. R.; Davis, W. M. Organometallics
1995, 14, 2699.
(17) Gray, S. D.; Thorman, J . L.; Adamian, V. A.; Kadish, K. M.;
Woo, L. K. Inorg. Chem. 1998, 37, 1.
(18) Nugent, W. A.;; Mayer, J . M. Metal-Ligand Multiple Bonds;
J ohn Wiley & Sons: New York, 1988.
(19) Wigley, D. E. Prog. Inorg. Chem. 1994, 42, 239.
(20) Cook, M. R.; Herrmann, W. A.; Kiprof, P.; Takacs, J . J . Chem.
Soc., Dalton Trans. 1991, 797.
(13) Schrock, R. R.; Rocklage, S. M. J . Am. Chem. Soc. 1980, 102,
7808.
(14) Nugent, W.; Harlow, R. L. J . Chem. Soc., Chem. Commun. 1978,
579.
(21) Herrmann, W. A.; Ding, H.; Ku¨hn, F. E.; Scherer, W. Organo-
metallics 1998, 17, 2751.
(22) Herrmann, W. A.; Kratzer, R. M.; Fischer, R. W. Angew. Chem.,
Int. Ed. Engl. 1997, 36, 2652.
(15) J olly, M.; Mitchell, J . P.; Gibson, V. C. J . Chem. Soc., Dalton
Trans. 1992, 1331.
(23) Koleva, V.; Dudev, T.; Wawer, I. J . Mol. Struct. 1997, 412, 153.
(24) Zhu, Z.; Espenson, J . H. J . Mol. Catal. A 1997, 121, 139.
10.1021/om990462p CCC: $18.00 © 1999 American Chemical Society
Publication on Web 11/22/1999

Tris(adamantylimido)methylrhenium
Organometallics, Vol. 18, No. 24, 1999 5171
Supporting Information, the RCHdNAd moiety has charac-
teristic 1H and 13C chemical shifts,23 which were measured
1
relative to the residual H and 13C resonances of the solvent
(δH 7.16, δC 128.39 ppm).
Resu lts
CH3Re(NAd )3. This compound proved to be moisture
sensitive, but is not affected by oxygen, even in solution.
The sample shows an absorption maximum at 374 nm
1
(ꢀ 720 L mol-1 cm-1). The H spectrum showed that the
three imido groups are equivalent. Although the chemi-
cal shifts for the protons on the C2 and C3 groups are
coincidentally the same, the carbons have different 13C
chemical shifts. The C1 carbon was observed in the
regular proton-decoupled 13C spectrum, which allowed
the differentiation of C2 and C3 by an APT (attached
proton test) experiment. The 13C chemical shift of the
methyl group on rhenium is -6.49 ppm, quite different
from the value 17.4 ppm of MTO, but similar to the
value of -6.08 ppm for CH3Re(N-t-Bu)3.25
A single-crystal X-ray analysis was performed on CH3-
Re(NAd)3, but no crystal was found that allowed a
quantitative resolution of the structure owing to disor-
der problems and the small crystal size. The large
R-factor precludes a meaningful comparison of bond
lengths and angles with the reported structure of CH3-
Re(NAr)3.21 Nonetheless, the low-precision results clearly
show that the compound is a monomer with a structure
analogous to that of CH3Re(NAr)3.
F igu r e 1. 1H-13C 2-D NMR spectrum of CH3Re(NAd)3 (Ad
) 1-adamantyl) in C6D6 at 298 K.
cold toluene, the yellow solid was dried under vacuum. Fine,
needlelike yellow crystals were obtained through recrystalli-
zation from hexane. The compound was characterized by
elemental analysis and spectroscopy. Anal. Found (calcd for
1
C
31H48N3Re): C 57.24 (57.38), H 7.63 (7.46), N 6.48 (6.48). H
NMR (C6D6): 1.99 (s, 3 H), 2.06 (s, 27 H), 1.58 (m, 18 H). 13C
NMR (C6D6): -6.39 (CH3Re), 30.80 (CH), 37.02 (CH2), 46.14
(CH2), 68.72 (CH0). EI MS: m/z 649. The 2D-NMR spectrum
of CH3Re(NAd)3 in C6D6 at room temperature is presented in
Figure 1.
Rea ction s w ith Ald eh yd es. The reaction between
4-nitrobenzaldehyde (δCHO 9.30 ppm) and CH3Re(NAd)3
1
(δCH3 1.99 ppm) was studied with H NMR. The loss of
intensity of the aldehyde proton and the buildup of the
characteristic imine proton of 4-NO2C6H4CHdNAd were
followed with time. Concurrent with these changes, a
new signal grew in at δ 1.91 ppm, assigned as the
methyl resonance of CH3Re(NAd)2O; see below. The rate
constant for reaction 1 with 4-nitrobenzaldehyde is k1
) 7 × 10-4 L mol-1 s-1 at 298 K. When a large excess
of the aldehyde was used, the buildup of CH3Re(NAd)-
(O)2 was detected at δ 1.75 ppm. When that solution
was allowed to stand for an even longer period, 40 h
and more, the buildup of MTO itself was noted at δ 1.21
ppm. This suggests these reactions:
Kin etics Stu d ies. Kinetics measurements, thermostated
at 298 K, were carried out in C6D6 under N2 by means of an
NMR spectrometer. The initial rate method was used to
determine the rate constants for the reactions of CH3Re(NAd)3
with CH3CH2CH2CHO and with 4-NO2C6H4CHO. The inte-
grated intensities of aldehydes (CHO) and imines (CHN) were
converted into concentrations using the known initial concen-
tration of the aldehyde and mass conservation law. The
concentration-time profile for the first 10% of the reaction is
linear, and its slope is the initial rate (mol L-1 s-1). A
competition method was used to evaluate the rate constants
for the reactions with other aromatic aldehydes. The rate
constants, relative to that for 4-NO2C6H4CHO, were calculated
according to the following equation:
CH3Re(NAd)3 + ArCHO f
CH3Re(NAd)2O + ArCHdNAd (1)
kArCHO
[ArCHdNAd]∞
CH3Re(NAd)2O + ArCHO f
)
×
k
[4-NO2C6H4CHdNAd]∞
4-NO2C6H4CHO
CH3Re(NAd)O2 + ArCHdNAd (2)
[4-NO2C6H4CHO]0
[ArCHO]0
CH3Re(NAd)O2 + ArCHO f
CH3ReO3 + ArCHdNAd (3)
The integrated intensities for CHO of aldehydes and CHN of
imines were used in the above calculations. For terephthala-
ldehyde, the statistical factor of 2 and the intensity of the CHO
protons contributed from two identical CHO groups were
considered. The reactions with aliphatic aldehydes were not
studied by the competition method because of the rapid
exchange between aliphatic aldehydes and their imines.
P r od u ct An a lysis. The products were characterized by
NMR and mass spectrometry with Bruker DRX-400 and a
Finnigan Magnum ion trap GC-mass spectrometer, in com-
parison with authentic samples obtained by refluxing 1-NH2-
Ad and RCHO in toluene for 3 h, during which time water
was removed by a Dean-Stark trap. As summarized in the
1
The H signal attributed to CH3Re(NAd)2O in the first
stage of this experiment matched exactly that from the
product of this reaction:
CH3ReO3 + 2CH3Re(NAd)3 f 3CH3Re(NAd)2O (4)
For example, when a C6D6 solution of 10 mM CH3ReO3
was added into a solution containing 2 equiv of CH3-
(25) Herrmann, W. A.; Weichselbaumer, G.; Paciello, R. A.; Fischer,
R. A.; Herdtweck, E.; Okuda, J .; Marz, D. W. Organometallics 1990,
9, 489.

5172 Organometallics, Vol. 18, No. 24, 1999
Wang and Espenson
Ta ble 1. Ra te Con sta n ts a n d Ha m m ett Su bstitu en t
Con sta n ts Accor d in g to Eq 1 for th e Rea ction of
Ar om a tic Ald eh yd es w ith CH3Re(NAd )3 in C6D6 a t
298 K
reactive than MeRe(NAr)3 toward 4-NO2C6H4CHO. It
is worth noting that a mixture of MeRe(NAr)3 and
MeRe(NAr)2O was observed from the above sample after
2 days; however, the corresponding imine 4-NO2C6H4-
CHdNAr was not observed. The formation of MeRe-
(NAr)2O is presumed to result from the reaction of
MeRe(NAr)3 with MTO, according to eqs 1-3. The
redistribution reactions between MeRe(NAr)3 and MTO
have been reported by Herrmann and co-workers.21
Rea ction w ith Im in es. Addition of PhCHdNPh
(δCHdN 8.13 ppm) to a solution of CH3Re(NAd)3 in C6D6
gave rise to a new imine resonance at 8.47 ppm,
assigned as PhCHdNAd. Likewise, use of (4-NMe)2C6H4-
CHdNC6H4(4-OMe) (δ 8.40 ppm) gave rise to a new
resonance for (4-NMe2)C6H4CHdNAd at 8.32 ppm.
The reactions of CH3Re(NAd)3 with imines are slower
than those of the aldehydes. Since CH3Re(NAd)3 is
moisture sensitive, it is reasonable to ask whether the
reaction really involves AdNH2, a hydrolysis product of
the parent. It should be noted that neither AdNH2 nor
H2O was observed in the spectra taken during the
reactions of CH3Re(NAd)3 with imines. An attempted
reaction of AdNH2 (25 mM) with (4-OMe)C6H4CHd
NC6H4(4-t-Bu) required many weeks to reach equilib-
rium (K298 ≈ 2.7).
4-XC6H4CHO, X )
k1/10-4 L mol-1 s-1
σX
4-NO2
CHO
MeOC(O)
H
7.0a
5.6b
3.2b
1.4b
0.78
0.45
0.45
0
-0.27
-0.83
b
MeO
0.42b
Me2N
0.28
Determined directly. b Determined in competition experiments
a
relative to the value for 4-nitrobenzaldehyde.
Re(NAd)3, the predominant resonance in 5 min was from
CH3Re(NAd)2O. As reported for MeRe(NAr)2O,21 com-
pound CH3Re(NAd)2O could also be prepared by reflux-
ing a toluene solution of MTO and 2 equiv of AdNCO.
Also, the spectrum of CH3Re(NAd)O2 obtained from the
second stage of the reaction is the same as that from
this reaction:
2CH3ReO3 + CH3Re(NAd)3 f 3CH3Re(NAd)O2 (5)
The chemical shifts of the methyl group bound to
rhenium change systematically with the number of
oxygen atoms. For CH3Re(NAd)3-xOx, the values are δ
1.99, 1.91, 1.75, and 1.21 ppm for the series with x ) 0
to 3, respectively. For the same series, the 13C chemical
shifts are -6.49, 0.80, 8.10, and 17.40 ppm.
Other reagents that do not react with CH3Re(NAd)3
at 298 K in C6D6 are the N,N-dimethylhydrazones of
2-pyridinecarboxaldehyde and cyclohexanecarboxalde-
hyde.
To overcome the complications from the succession
of reactions in eqs 1-3, the rate constants for the first
stage of the reactions between CH3Re(NAd)3 and aro-
matic aldehydes were evaluated by a competition method.
Details about data collection and analyses are given in
the Experimental Section. These reactions were carried
out using a mixture of 4-nitrobenzaldehyde (for which
k1 was independently determined) and a given ArCHO.
The intensities of the aldehydes and imines were
measured at least three times during the reaction of eq
1. From these measurements, the values of k1 could be
determined. The data are presented in Table 1. It is
clear that a change in substituent X of the compound
4-XC6H4CHO has a substantial effect on the rate, with
electron-donating X groups slowing the reaction and vice
versa.
Rea ction s w ith Oth er Don or s. Cyclic ketones,
acetophenone, nitrosobenzene, and methylphenylsul-
foxide do not react with CH3Re(NAd)3. Nor do cis- and
trans-stilbene oxides, styrene oxide, and pyridine-N-
oxide. Indeed, MTO is the best oxygen transfer system,
as first reported for CH3Re(NAr)3, Ar ) 2,6-diisopropy-
lphenyl.21 Thus CH3Re(NAd)3 reacts with MTO to give
CH3Re(NAd)2O and/or CH3Re(NAd)O2, according to the
proportions used.
Reactions between CH3Re(NAd)3 and phosphines
were attempted, since certain imido metal complexes
produce phosphinimines.11,12 This met with no success
for PPh3, PPhMe2, and P(n-Bu)3.
Im in e-Im in e Meta th esis. Although the products of
the metathesis reaction between organic imines and
CH3Re(NAd)3 have been observed, this rhenium com-
pound does not catalyze the metathesis reaction be-
tween two imines.
On the other hand, MTO itself does catalyze the
imine-imine metathesis process, both as the dissolved
complex in benzene and MTO supported on Nb2O5, the
preparation of which has been described.24 This uncata-
lyzed reaction was very slow:
The reactions of linear aliphatic aldehydes with CH3-
Re(NAd)3 proceed much more rapidly than those of
aromatic aldehydes. The reaction of t-BuCHO takes
place slowly, and the imine formed is stable. However,
the other aliphatic organic imines produced are not
3
stable. When 2 mM n-C3H7CHO (δCHO 9.26 ppm; J HH
1.6 Hz) was added to a 1 mM C6D6 solution of CH3Re-
(NAd)3, a triplet (δ 7.47 ppm, 3J HH 4.8 Hz) was observed,
corresponding to n-C3H7CHdNAd. The rate constants
PhCHdNPh + ArCHdNAr′ a
PhCHdNAr′ + ArCHdNPh (6)
for the first and second stages are k1 ) 3.2 × 10-2
L
mol-1 s-1 and k2 ) 6.4 × 10-4 L mol-1 s-1 at 298 K.
Rea ctivity of MeRe(NAd )3 a n d MeRe(NAr )3. Re-
distribution reactions between MeRe(NAd)3 and MeRe-
(NAr)3 were not observed. The only imine noted was
4-NO2C6H4CHdNAd after a large excess of 4-NO2C6H4-
CHO was added to a C6D6 solution of MeRe(NAd)3 and
MeRe(NAr)3 (1:5). We assumed that the other possible
imine, 4-NO2C6H4CHdNAr, was formed, but it amounted
to less than 5% of 4-NO2C6H4CHdNAd and thus was
not detectable by the 1H NMR technique. One can
conclude that MeRe(NAd)3 is at least 100 times more
Upon addition of MTO (11 mM), equilibrium was
reached in 40 h at room temperature and in >60 h when
1.8 mM MTO was used, Figure 3. The intensities of the
resonances for the reaction at equilibrium lead to a
value of the equilibrium constant of K6 ) 1.1 at room
temperature in benzene.
The heterogeneous catalyst, MTO supported on nio-
bium oxide,24,26 proved much more active. Equilibrium
was attained in reaction 6 within 2 h when 5 mg of
MTO/Nb2O5 was added to 0.7 mL of C6D6 containing

Tris(adamantylimido)methylrhenium
Organometallics, Vol. 18, No. 24, 1999 5173
Sch em e 1
ing PhCHdNPh and ArCHdNCH2Ar′. Besides the four
imines shown in eq 6, the following two imines were
found as well: PhCHdNCH2Ph and ArCHdNCH2Ph.
It seems that the amine/imine exchange reaction is
governed not only by the basicity of the amine but also
by its steric demand. The sterically less hindered
amines, PhCH2NH2 and 4-MeOC6H4NH2, react with
4-MeO-C6H4CHdNC6H4(4-t-Bu) much faster than ster-
ically demanding amines such as 1-NH2Ad and 2,6-
Me2C6H3NH2. These equilibrium constants were ob-
tained for reaction 7 for 4-MeO-C6H4CHdNC6H4(4-t-
Bu). The least favorable reactions required ∼3 weeks
to attain equilibrium, and the values of K7(C6D6, 298
K) given below may be less precise:
F igu r e 2. Correlation of the rate constants for the reaction
between CH3Re(NAd)3 (Ad ) 1-adamantyl) and para-
substituted benzaldehydes, with the Hammett substituent
constant σ.
2,6-Me2-
amine:
PhCH2NH2 4-MeOC6H4NH2 AdNH2 C6H3NH2
K7(rt, C6D6)
27
5
∼2.7
∼0.04
Discu ssion
Ald eh yd e Rea ction s: Kin etics. The triphasic steps
(reactions 1-3) are not well enough separated in time
to allow a simple resolution of the kinetics. The times
at which the spectroscopic intensities appeared were
sufficient to allow one to state that the steps 1-3
decrease in rate in succession. As a typical example, the
reactions of n-C3H7CHO for stages 1 and 2 are in the
ratio 50:1. Because of the concurrent nature of this
reaction trio, resolution of the rate constants, even for
the first stage, may lead to large and perhaps systematic
errors.
With the series of benzaldehydes, the rate constants
were determined for the first stage, as given in Table
1. It is clear that there is a systematic effect of the para
substituent of benzaldehydes, such that electron-
withdrawing groups accelerate the reaction. This series
was examined by Hammett’s linear free-energy correla-
tion. Figure 2 depicts a plot of log k versus the sub-
stituent constants σ. The data define a reasonable
straight line (correlation coefficient 0.976) with a slope
that gives the reaction constant F ) +0.90 ( 0.10; the
relatively low precision arises from the nature of the
kinetic data and analysis. In any event, this value of F
implicates a mechanism in which the reaction center,
here the carbonyl carbon, becomes less positive in the
transition state as compared to the starting material.
The mechanism proposed for the reaction between
CH3Re(NAd)3 and aldehydes, Scheme 1, is analogous
to the olefin metathesis. The involvement of a four-
center transition state is generally accepted for many
related (2+2) exchange reactions.18,19 The final step is
irreversible since no tris(imido)metal complex was
formed from CH3Re(NAd)2O and an excess of any
imine.28 In support of the structure proposed for the
reaction intermediate, two imidomethylrhenium com-
F igu r e 3. 1H spectra taken of reactions between PhCHd
NPh (10 mM) and (4-MeO)C6H4CHdNC6H4(4-t-Bu) (20
mM) in the presence of (a) 1.8 mM MTO, (b) 11 mM MTO,
and (c) in the absence of MTO.
10 mM PhCHdNPh and 20 mM ArCHdNAr′. It has
been established that MTO/Nb2O5 is superior to the
homogeneous catalyst for olefin metathesis.26
Am in e-Im in e Exch a n ge. Such a reactions, eq 7,
might be considered as a method for preparing imines
of sterically hindered ketones.27
RNH2 + R′′CHdNR′ a R′CHdNR + R′NH2 (7)
To see if amines can initiate imine-imine exchange
reactions, PhCH2NH2 was added to a solution contain-
(26) Buffon, R.; Choplin, A.; Leconte, M.; Basset, J .-M.; Touroude,
R.; Herrmann, W. A. J . Mol. Catal. 1992, 72, L7.
(27) Dayagi, S. In The Chemistry of the Carbon-Nitrogen Double
Bond; Patai, S., Ed.; Interscience Publishers: New York, 1970; p 61.

5174 Organometallics, Vol. 18, No. 24, 1999
Wang and Espenson
plexes with bridging oxo ligands have been identified.21
A similar intermediate can be postulated for the imido-
oxo exchange reactions between CH3Re(NAd)3 and
MTO.
Sch em e 2
In the reaction of CH3Re(NAd)3 and RCHO, neither
a nitroso compound, AdNO, nor a rhenium carbene, Red
CHAr, was observed. Thus the reaction is regiospecific.
The electrophilic carbon of the aldehyde interacts with
the nitrogen atom of the imidorhenium compound,
forming the cyclic intermediate shown in Scheme 1.
Quite likely, the oxygen atom initially attacks the Re-
(VII) center. The large substituent effect is consistent
with a polar cycloaddition mechanism, as observed in
Wittig reactions.29 The positive value of F suggests that
electrophilic attack of the nitrogen of the imido group
by the carbon of the aldehyde is rate-controlling. As it
turns out, CH3Re(NAd)3 is a much weaker Lewis acid
than MTO. Pyridine, for example, which binds ap-
preciably to MTO (K ) 2 × 102 L mol-1),30 does not
coordinate to the imido complex to a detectable extent.
Although an ortho substituent of an aromatic alde-
hyde does not lead to a steric effect, the reactivity of
sterically hindered aliphatic aldehydes is significantly
diminished. Thus, n-C3H7CHO is much more reactive
than (CH3)3CCHO. We suggest that ketones do not react
with CH3Re(NAd)3 for steric reasons. Whereas MTO
reacts with CH3Re(NAd)3, CH3Re(NAr)3 does not be-
cause of the steric hindrance in forming an intermediate
with µ-NAd and µ-NAr bridges.
Although reaction 10 is not practical at room temper-
ature due to the low activities of the catalysts, this
water-free method to prepare imines may be useful at
elevated temperature. For example, 4-NO2C6H4CHd
NAd was obtained quantitatively when a solution of
AdNCO (1 mmol), 4-NO2C6H4CHO (1 mmol), and MTO
(0.02 mmol) was refluxed in toluene for 4 h. Organic
imine was not observed when the reaction was carried
out in the absence of MTO.
CH3ReO3 + 3AdNCO f
CH3Re(NAd)3 + 3CO2 (8)
CH3Re(NAd)3 + 3ArCHO f
CH3ReO3 + 3ArCHdNAd (9)
AdNCO + ArCHO f ArCHdNAd + CO2 (10)
Rea ction s w ith Im in es. It seems reasonable to
adopt the mechanism for aldehydes, Scheme 1, to the
reactions between CH3Re(NAd)3 and imines. If so, the
imidorhenium/imine reaction would also be regiospe-
cific. Consistent with that, no diazene was formed,
Scheme 2.
Certain examples of imido/imine reactions have
been reported.33-38 Isolation of such a material, Cp2Zr-
(ButNCHPhNPh), from the reaction of Cp2Zr(N-t-Bu)
and PhCHdNPh supports a cycloaddition processes.34
Kinetic studies on Py3Cl2TidN-t-Bu, however, suggest
an alternative mechanism.35
Th er m od yn a m ic Con sid er a tion s. It is useful to
estimate a value for the RedNAd bond enthalpy in CH3-
Re(NAd)3. The first stage of the reaction between it and
an aldehyde, in which CH3Re(NAd)2O is formed, is
much faster than the final stage that forms MTO (i.e.,
k1 . k3). The values of ∆H are roughly the same for
each, implying the difference resides in the values of
∆S. The values of ∆S for the two steps could be much
different, since CH3Re(NAd)3 imposes a much greater
steric demand than CH3Re(NAd)O2. The CdO bond of
the aldehyde is stronger than the CdN bond of the
Although amines (AdNH2 or ArNH2) and water were
not detected in the reactions of CH3Re(NAd)3 with
imines, it is difficult to rule out that trace amounts of
them might be involved:
imine by about 30 kcal mol-1 31
so that the driving force
,
for the imide/aldehyde reaction derives from the strength
of the RedO interaction. The bond dissociation free
energy of the first RedO bond of MTO is 111 kcal mol-1
,
CH3Re(NAd)3 + H2O f
as measured against V2+/VO2+ in aqueous solution.32 If
the entropy change is ignored for the conversion of CH3-
Re(NAd)O2 to MTO, then the bond enthalpy for the Red
CH3Re(NAd)2O + AdNH2 (11)
AdNH2 + ArCHdNAr′ a
ArCHdNAd + Ar′NH2 (12)
NAd bond is less than ca. 80 kcal mol-1
.
Ca ta lytic F or m a tion of Im in es. The compound
CH3Re(NAd)3 can be prepared according to eq 8, and
its overall reaction with aromatic aldehydes is described
by eq 9. Thus reaction 10, the combination of eqs 8 and
9, should be catalyzed by MTO. Indeed, the formation
of 4-NO2C6H4CHdNAd from the reaction of AdNCO and
4-NO2C6H4CHO is catalyzed by MTO or MeRe(NAd)3.
Ar′NH2 + CH3Re(NAd)3 a
CH3Re(NAd)2(NAr′) + AdNH2 (13)
The involvement of AdNH2 was shown to be unimpor-
tant by demonstrating that its reactions with imines are
very slow. Thus the contributions of reactions 11-13
are negligible.
(28) The C6D6 solution of MeRe(NAd)2O was prepared by mixing
MeRe(NAd)3 and MTO in a 2:1 ratio. The imines used here were
4-NO2C6H4CHdNAd and 4-MeOC6H4dNC6H4(4-t-Bu).
(29) Yamataka, H.; Nagareda, K.; Takatsuka, T.; Ando, K.; Ha-
nafusa, T.; Nagase, S. J . Am. Chem. Soc. 1993, 115, 8570.
(30) Wang, W.-D.; Espenson, J . H. J . Am. Chem. Soc. 1998, 120,
11335.
(31) Sandorfy, C. In The Chemistry of the Carbon-Nitrogen Double
Bond; Patai, S., Ed.; Interscience Publishers: New York, 1970; p 1.
(32) Abu-Omar, M. M.; Appelman, E. H.; Espenson, J . H. Inorg.
Chem. 1996, 35, 7751.
(33) Meyer, K. E.; Walsh, P. J .; Bergman, R. G. J . Am. Chem. Soc.
1994, 116, 2669.
(34) Meyer, K. E.; Walsh, P. J .; Bergman, R. G. J . Am. Chem. Soc.
1995, 117, 974.
(35) McInnes, J . M.; Mountford, P. J . Chem. Soc., Chem Commun.
1998, 1669.
(36) Cantrell, G. K.; Meyer, T. Y. J . Chem. Soc., Chem. Commun.
1997, 1551.
(37) Cantrell, G. K.; Meyer, T. Y. Organometallics 1997, 16, 5381.
(38) Cantrell, G. K.; Meyer, T. Y. J . Am. Chem. Soc. 1998, 120, 8035.

Tris(adamantylimido)methylrhenium
Organometallics, Vol. 18, No. 24, 1999 5175
Im in e-Im in e Meta th esis. The chemical shift of
CH3ReO3 changes significantly when pyridine is coor-
dinated.30 In contrast, it barely changes at all when an
imine is added. The weak imine interaction thus indi-
cated appears to be responsible for the slowness of the
MTO-catalyzed metathesis at room temperature. A
metal-mediated mechanism for catalytic imine metath-
esis has been suggested.38 MTO is very likely involved
because the metathesis rate increases with its concen-
tration. It remains unresolved whether MTO simply
catalyzes the hydrolysis of the imines, eqs 14 and 15,
or if it is directly involved in the exchange.
2RCHdCHR′ a RCHdCHR + R′CHdCHR′ (16)
2RCHdNR′ a RCHdCHR + R′NdNR′ (17)
Con clu sion s
Admantylimidorhenium complexes react with alde-
hydes and imines through a metathesis mechanism.
Sterically hindered aliphatic aldehydes react much more
slowly than linear ones. Electronic effects also play an
important role in the imido-aldehyde metathesis reac-
tions. Tris(admantylimido)methylrhenium is much more
reactive toward aldehydes than tris(2,6-diisopropylphe-
nylimido)methylrhenium. MTO catalyzes imine-imine
exchange reactions, as well as the water-free imine
formation from aromatic isocyanates and aldehydes.
PhNdCHPh + H2O {MTO} PhCHO + PhNH2 (14)
ArNdCHAr′ + H2O {MTO} Ar′CHO + ArNH2 (15)
Ack n ow led gm en t. This research was supported by
a grant from the National Science Foundation (CHE-
9625349). Some experiments were conducted with the
use of the facilities of the Ames Laboratory. We are
grateful to Dr. Ilia A. Guzei of the Iowa State University
Molecular Structures Laboratory for the crystallo-
graphic results reported herein. We also thank Dr.
William S. J enks for helpful discussions.
As shown in eq 7, the anilines thus formed (eqs 14
and 15) will rapidly react with imines to produce the
same products as those given from the direct metathesis
reaction. The imine alone is stable in solution for at least
a week under these conditions; however, small amounts
of aldehydes were observed when MTO/Nb2O5 was used
as a catalyst.
Olefin metathesis (eq 16) is catalyzed by MTO, but
imine metathesis (eq 17) is not. An attempt to reverse
the latter process was attempted with MTO, but the
reaction did not occur in that direction either.
Su p p or tin g In for m a tion Ava ila ble: 1H and 13C NMR
chemical shifts for imines. This material is available free of
charge via the Internet at http://pubs.acs.org.
OM990462P