Transition metal carbene chemistry. 4. Nucleophilic attachment of DABCO to fischer carbene complexes in MeCN

Article abstract of DOI:10.1021/ic050349t

Rate constants for the attachment of DABCO (1,4-diazabicyclo[2.2.2]octane) to Fischer carbene complexes of the type (CO)₅Cr=C(XR)C ₆H₄Z (X = O and S) in dry MeCN at 25°C are reported. Hammett ρ values are 2.18 ± 0.13 and 0.89 ± 0.07 for DABCO reactions with (CO)₅Cr=C(OMe)C₆H₄Z (10-Cr-Z) and (CO)₅Cr=C(SMe)C₆H₄Z (11-Cr-Z), respectively. The ρ values for the reaction of 10-Cr-Z and 11-Cr-Z with CH(CN)₂^- in 50% MeCN-50% H₂O (v/v) are comparable to the present reactions. The reaction of DABCO with 10-Cr-Z is more closely related to the reaction of (n-Bu)₃P with (CO) ₅W=C(OMe)C₆H₅-Z (23) which also provided a ρ value 2.22. The much higher ρ values and hence much higher reactivity of methoxy carbene complexes over the corresponding thiomethyl derivatives fit a pattern observed previously for alkoxide ion, OH^-, amine, and thiolate ion nucleophiles, and a rational explanation comes from the consideration of the substituent effects not only on the transition state but also on the reactant. A major difference between 10-Cr-Z and 11-Cr-Z is that the π-donor effect of the methoxy group is much stronger than that of thiomethyl group. This leads to a substantial contribution of the zwitterionic form to the structure of 10-Cr-Z with much more localized positive charge on the methoxy group than the negative charge on the (CO)₅Cr moiety. This leads to overall destabilization by an electron-withdrawing phenyl substituents resulting an increase in reactivity. The ethoxycarbene complexes are somewhat less reactive than their methoxy counterparts due to the somewhat more ground state stabilization through its stronger π donor effect and partly due to steric crowding exerted by the slightly larger ethoxy group in the transition state. Higher k₁(W)/k₁(Cr) ratios for (thiomethyl)carbene complexes than methoxy or ethoxycarbene complexes are related to the intrinsic rate constant which is higher for ((thiomethyl)carbene)tungsten complexes than the corresponding Cr ones resulting in an enhanced ratio.

Full text of DOI:10.1021/ic050349t

Inorg. Chem. 2005, 44, 5866−5871
Transition Metal Carbene Chemistry. 4. Nucleophilic Attachment of
DABCO to Fischer Carbene Complexes in MeCN†
Mahammad Ali,* Abhijit Dan, Ambarish Ray, and Kaushik Ghosh
Department of Chemistry, JadaVpur UniVersity, Kolkata 700 032, India
Received March 7, 2005
Rate constants for the attachment of DABCO (1,4-diazabicyclo[2.2.2]octane) to Fischer carbene complexes of the
type (CO)5Cr C(XR)C6H4Z (X O and S) in dry MeCN at 25 C are reported. Hammett values are 2.18 0.13
and 0.89 0.07 for DABCO reactions with (CO)5Cr C(SMe)C6H4Z (11-
Cr-Z), respectively. The 50% H2O
d
)
°
F
±
±
d
C(OMe)C6H4Z (10-Cr-Z) and (CO)5Cr
d
-
F
values for the reaction of 10-Cr-Z and 11-Cr-Z with CH(CN)2 in 50% MeCN
−
(v/v) are comparable to the present reactions. The reaction of DABCO with 10-Cr-Z is more closely related to the
reaction of (n-Bu)3P with (CO)5W C(OMe)C6H5-Z (23) which also provided a value 2.22. The much higher
d
F
F
values and hence much higher reactivity of methoxy carbene complexes over the corresponding thiomethyl derivatives
fit a pattern observed previously for alkoxide ion, OH-, amine, and thiolate ion nucleophiles, and a rational explanation
comes from the consideration of the substituent effects not only on the transition state but also on the reactant. A
major difference between 10-Cr-Z and 11-Cr-Z is that the
π-donor effect of the methoxy group is much stronger
than that of thiomethyl group. This leads to a substantial contribution of the zwitterionic form to the structure of
10-Cr-Z with much more localized positive charge on the methoxy group than the negative charge on the (CO)5Cr
moiety. This leads to overall destabilization by an electron-withdrawing phenyl substituents resulting an increase in
reactivity. The ethoxycarbene complexes are somewhat less reactive than their methoxy counterparts due to the
somewhat more ground state stabilization through its stronger
π donor effect and partly due to steric crowding
exerted by the slightly larger ethoxy group in the transition state. Higher k1(W)/k1(Cr) ratios for (thiomethyl)carbene
complexes than methoxy or ethoxycarbene complexes are related to the intrinsic rate constant which is higher for
((thiomethyl)carbene)tungsten complexes than the corresponding Cr ones resulting in an enhanced ratio.
Introduction
because most transition metal carbene complexes of the
Fischer type are highly electrophilic at the carbene carbon.1-3
Nucleophilic substitution of an alkoxy or akylthio group of
carbene complexes such as 1 is one of the prototypical
reactions of Fischer carbene complexes. It has generally been
assumed to proceed via a tetrahedral intermediate 2 in
analogy to the reaction of carboxylic esters with nucleophiles.
The extensive and ever increasing activity in the area of
transition metal carbene complexes of the Fischer type has
mainly been driven by their usefulness in organic synthesis.
The key to the rich chemistry of this class of compounds is
that they can undergo reactions at several sites: (i) modifica-
tion of the carbene fragment; (ii) reactions on the sidearm;
(iii) reactions on metal involving ligand exchange or change
in oxidation state; (iv) insertion of unsaturated organic
molecules into the metal-carbene bonds.1 Of these, the most
extensive studies have been made on the type i reactions,
Equation 1 shows the reaction scheme involving an anionic
nucleophile, while reactions with neutral nucleophiles such
as amines include a proton transfer as an additional step (eq
2).
* Author to whom correspondence should be addressed. E-mail:
mali@chemistry.jdvu.ac.in.
† Part 3: Ali, M. Indian J. Chem., Sect. A, in press.
(1) (a) Do¨tz, K. H.; Fischer, H.; Hofmann, P.; Kreissl, F. R.; Schubert,
U.; Weiss, K. Transition Metal Carbene Complexes; Verlag Chemie:
Deerfield Beach, FL, 1983. (b) Do¨tz, K. H. Angew. Chem., Int. Ed.
Engl. 1984, 23, 587.
(2) Bernasconi, C. F. Chem. Soc. ReV. 1997, 26, 299.
(3) Bernasconi, C. F. AdV. Phys. Org. Chem. 2003, 37, 137.
5866 Inorganic Chemistry, Vol. 44, No. 16, 2005
10.1021/ic050349t CCC: $30.25
© 2005 American Chemical Society
Published on Web 07/14/2005

Transition Metal Carbene Chemistry
Though the postulate of tetrahedral intermediates generated
by nucleophilic attack on the carbene carbon is reasonable,
the studies discussed above provide no direct evidence for
them. A direct support for such intermediates comes at least
from the reaction of 8-Cr and 8-W with DABCO (X ) N)
or quinuclidine (X ) CH) giving isolable tetrahedral adduct
9-Cr and 9-W.27,43,44
Most synthetic applications have involved common nu-
cleophiles such as amines,1,3,4-7 hydrazine,1,8 oximes,1,9
alkoxide ions,1,3,10 thiolate ions,1,3,11-13 and carbanions13,14-18
(mainly aryl- and alkyllithium) as well as others.1,19-21
The first kinetic investigation of nucleophilic substitution
reaction was that of Werner et al., who studied the reaction
of 4 with several primary aliphatic amines (n-BuNH2, C6H11-
NH2, C6H5CH2NH2) in n-decane, dioxane, methanol, and a
dioxane-methanol (1:1) mixture showing complicated de-
pendence on amine concentrations. There have been few
other kinetic studies in the early literature; these investiga-
tions focused mainly on reactions with amines3,22-24 and
phosphines3,25,26 in weakly polar organic solvents. More
recently, a number of kinetic studies with alcohol,27,28
alkoxide ions,27,28 water,29-33 OH-,29-33 thiol,34-36 thiolate
ions,34-36 amines,33,37-41 and carbanion42 in polar (mainly
water-acetonitrile mixtures) solvents were reported.
Generally tertiary amines possess drastically reduced
nucleophilic character as a direct result of steric hindrance
around the nitrogen (i.e. the bulky R groups) combined with
the inability to generate neutral species (i.e. unable to lose a
proton). Consequently, tertiary amines tend to react as bases
rather than nucleophiles. Though DABCO is used as catalysts
in many Baylis-Hillman type reactions,45 to the best of our
knowledge, there is no report on the kinetic studies on
nucleophilic reactions of DABCO, particularly, with Fischer
carbene complexes. We now report a kinetic investigation
of the reaction of DABCO with 10-Cr-Z (Z ) Me2N, MeO,
Me, H, F, Cl, and CF3), 11-Cr-Z (Z ) Me2N, MeO, H, and
F), and 12-14 in dry MeCN.
(4) Klabunde, U.; Fischer, E. O. J. Am. Chem. Soc. 1967, 89, 7141.
(5) Connor, J. A.; Fischer, E. O. J. Chem. Soc. A 1969, 578.
(6) Fischer, E. O.; Leupold, M. Chem. Ber. 1972, 105, 599.
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359.
(8) Fischer, E. O.; Aumann, R. Chem. Ber. 1968, 101, 963.
(9) Fischer, E. O.; Knauss, L. Chem. Ber. 1970, 103, 1262.
(10) Kreiter, C. G. Angew. Chem., Int. Ed. Engl. 1968, 7, 390.
(11) Fischer, E. O.; Leupold, M.; Kreiter, C. G.; Mu¨ller, J. Chem. Ber.
1972, 105, 150.
(12) Lam, C. T.; Senoff, C. F.; Ward, J. E. H. J. Organomet. Chem. 1974,
70, 273.
(13) Aumann, R.; Schro¨der, J. Chem. Ber. 1990, 123, 2053.
(14) Burkhardt, T. J.; Casey, C. P. J. Am. Chem. Soc. 1973, 95, 5833.
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(16) Fischer, E. O.; Held, W.; Kreissl, F. R. Chem. Ber. 1977, 110, 3842.
(17) Fischer, E. O.; Held, W.; Kreissl, F. R.; Frank, A.; Haltner, G. Chem.
Ber. 1977, 110, 656.
(18) Casey, C. P.; Burkhardt, T. J.; Bunnell, C. A.; Calabrese, J. C. J. Am.
Chem. Soc. 1977, 99, 2127.
(19) Fischer, E. O.; Kreis, G.; Kreissl, F. R.; Kreiter, C. G.; Mu¨ller, J.
Chem. Ber. 1973, 106, 3910.
(20) Casey, C. P.; Brunsvold, W. P. Inorg. Chem. 1977, 16, 391.
(21) Bell, R. A.; Chisholm, M. H.; Couch, D. A.; Rankel, L. A. Inorg.
Chem. 1977, 16, 677.
(22) Heckl, B.; Werner, H.; Fischer, E. O. Angew. Chem., Int. Ed. Engl.
1968, 7, 817.
Results
With neutral nucleophiles such as phosphines and amines,
the nucleophilic attack on the carbene carbon leads to
(23) Werner, H.; Fischer, E. O.; Heckl, B.; Kreiter, C. G. J. Organomet.
Chem. 1971, 28, 367.
(24) Steinmetz, A. L.; Hershberger, S. A.; Angelici, R. J. Organometallics
1984, 3, 461.
(25) Choi, H. S.; Sweigart, D. A. J. Organomet. Chem. 1982, 228, 249.
(26) Pickering, R. A.; Angelici, R. J. J. Organomet. Chem. 1982, 225, 253.
(27) Bernasconi, C. F.; Flores, F. X.; Gandler, J. R.; Leyes, A. E.
Organometallics 1994, 13, 2186.
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1341.
(38) Bernasconi, C. F.; Whitesell, C.; Johnson, R. A. Tetrahedron 2000,
56, 4917.
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(39) Bernasconi, C. F.; Bhattacharya, S. Organometallics 2003, 22, 426.
(40) Bernasconi, C. F.; Bhattacharya, S. Organometallics 2003, 22, 1310.
(41) (a) Ali, M.; Maiti, D. J. Organomet. Chem. 2004, 689, 3520. (b) Ali,
M. New J. Chem. 2003, 27, 349.
(42) Bernasconi, C. F.; Ali, M. Organometallics 2004, 23, 6134.
(43) Kreissl, F. R.; Fischer, E. O.; Kreiter, C. G.; Weiss, K. Angew. Chem.,
Int. Ed. Engl. 1973, 12, 563.
(32) Bernasconi, C. F.; Perez, G. S. J. Am. Chem. Soc. 2000, 122, 12461.
(33) Bernasconi, C. F.; Bhattacharya, S. Organometallics 2004, 23, 1723.
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1998, 120, 8575.
(35) Bernasconi, C. F.; Kittredge, K. W.; Flores, F. X. J. Am. Chem. Soc.
1999, 121, 6630.
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(45) (a) Baylis, A. B.; Hillman, M. E. D. German Patent 2,155,113, 1972;
Chem. Abstr. 1972, 77, 34174q. (b) Stark, L. M.; Pekari, K.; Sorensen,
E. J. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 12064. (c) Ciganek, E.
Org. React. 1997, 51, 201. (d) Basavaiah, D.; Rao, A. J.; Satya-
narayana, T. Chem. ReV. 2003, 103, 811.
Inorganic Chemistry, Vol. 44, No. 16, 2005 5867

Ali et al.
Table 1. Rate Constants for the Reactions of DABCO with 10-Cr-Z,
11-Cr-Z, and (CO)5CrdC(OEt)(Ph), (CO)5WdC(OMe)(Ph),
(CO)5WdC(OEt)(Ph), and (CO)5WdC(SMe)(Ph) in Dry MeCN with I
) 0.10 M (NaClO4) and at 25 °C
102k1
Z
σ
10-Cr-Z
11-Cr-Z
4-CF3
3-Cl
4-Cl
4-F
4-H
4-Me
4-OMe
4-NMe2
0.54
0.37
0.23
0.06
0.00
-0.17
-0.27
-0.83
172 ( 0.90
1.87 ( 0.08
0.37 ( 0.02
1.31 ( 0.03
1.00 ( 0.02
0.65 ( 0.02
0.42 ( 0.01
0.37 ( 0.02
0.13 ( 0.01
23.4 ( 0.50
18.4 ( 0.24
5.14 ( 0.23
0.18 ( 0.01
Figure 1. Representative Plots of kobs (s-1) vs [DABCO] for the reaction
of DABCO with 11-Cr-H and 12.
compd
k1
(CO)5CrdC(OEt)(Ph) (13)
(CO)5WdC(OMe)(Ph) (8-W)
(CO)5WdC(OEt)(Ph) (14)
(CO)5WdC(SMe)(Ph) (12)
(6.45 ( 0.51) × 10-2
(12.1 ( 2.3) × 10-2
(10.5 ( 0.25) × 10-2
(2.40 ( 0.12) × 10-2
Zwitterionic intermediates such 15-17. If there is an acidic
proton on the intermediate as in the case of 17, the reaction
can proceed further to product by elimination of ROH;
otherwise the reaction stops at the adduct stage.
with a variety of nucleophiles have been reported in recent
years.3,22-41 HOCH2CH2S- was found to react with 11-Cr-H
or its tungsten analogue 12 reversibly to form the respective
tetrahedral adducts 20,36 which were found to be quite stable
particularly in the time scale of these experiments. The
adducts do not proceed to the substitution product by
expulsion of the MeS- group, but upon addition of acid, they
revert back to starting materials. These experiments allowed
a determination of k1 ) 6.60 × 102 M-1 s-1 as well as of
k-1 ) 1.78 × 10-3 s-1 and K1 ) k1/k-1 ) 3.07 × 105 M-1.36
The reactions of DABCO with 10-Cr-Z and 11-Cr-Z lead
to the respective adducts 18-Cr-Z and 19-Cr-Z, respectively,
as shown in eq 3. As reported earlier, the time-resolved
spectra leads to the loss of M ) C chromophore resulting in
a blue shift.27
For the reactions of DABCO with 10-Cr-Z, 11-Cr-Z, and
12-14 the respective tetrahedral intermediates appears to
be the final and stable (at least on the time scale of kinetics)
products in dry MeCN. Hence only k1, the rate constant for
nucleophilic attack, could be determined for these reactions.
The existence of no or negligible intercept precludes the
determination of k-1 and hence K1. With respect to the
nucleophilic attack on the carbene carbon there are strong
similarities between the reaction of phosphines or phosphites
and that with DABCO. The reaction of phosphines are
exemplified by eq 4.
The presence of isosbestic points indicates that the reaction
is quite clean (spectra not shown). Formation of the addition
product was confirmed for the case of 9-W by isolating the
synthesized product and comparing its UV/vis spectra with
an infinity solution of a kinetic run.
Kinetic experiments were carried out in dry MeCN.
Throughout the study pseudo-first-order conditions were
maintained, with the carbene complexes as the minor
component. All measurements were performed in a Hewlett-
Packard 8453 Agilent diode array UV-vis spectrophoto-
meter; rates were monitored at λmax of the substrate.
For the reactions of DABCO with all the carbene
complexes only one kinetic process was observed. Plots of
k
obsd, the pseudo-first-order rate constant, versus [DABCO]
However, 15 tends to rearrange to 22 by eq 5, a process
were all linear with intercepts that were indistinguishable
from zero (Figure 1). Second-order rate constants, interpreted
as k1 for nucleophilic attack and calculated as the slopes of
plots of kobsd versus [DABCO], are summarized in Table 1.
Discussion
Mechanism. The kinetic studies of the reactions of carbene
complexes such as 10-Cr-Z or 11-Cr-Z along with 12-14
that is not observed with amine and other nucleophiles.
Equilibrium constants determined by NMR techniques for
5868 Inorganic Chemistry, Vol. 44, No. 16, 2005

Transition Metal Carbene Chemistry
(1) MeO vs MeS is an important consideration. With
respect to rate constants (k1) for the nucleophilic attachment
of DABCO, it is the methoxycarbene complexes that are
more reactive than thiomethyl analogues, indicating a higher
intrinsic rate constants ko(O) for the former than for the latter,
ko(S).51 The higher intrinsic rate constants for the methoxy
complexes can be understood in terms of the principle of
nonperfect synchronization (PNS)52 effects that result from
transition state imbalances. Transition state imbalance arises
mainly due to the following. (i) Steric effect: S being a larger
atom, an early development of the steric effect reduces ko-
(S) for thiomethyl carbene complexes more than ko(O), for
methoxycarbene complexes. (ii) Inductive effect: a dispro-
portionately strong transition state stabilization by the
inductive effect of O in methoxycarbene complexes over the
S in the corresponding thiomethyl derivatives results in a
lag in charge delocalization into the (CO)5M moiety and
increases ko(O) more than ko(S). Though k-1 and K1 and
hence intrinsic rate constants cannot be determined to support
our proposition, it is based mainly on the detailed analysis
of the reactions of 8-Cr,35 8-W,35 11-Cr-H,36 and 1236 with
HOCH2CH2S- and other thiolate ions. It has been revealed
that the larger rate constants for 8-Cr and 8-W compared to
that of 11-Cr-H and 12 contrast with lower equilibrium
constants for methoxy derivatives compared with thiomethyl
derivatives. This means that the intrinsic barrier for the
reaction of the thiomethyl derivative is higher, which was
shown to result mainly from the large steric effect due to
the bulky size of sulfur compared to that of oxygen that
develops ahead of bond formation at the transition state.36
(2) As is the case for the reactions with OH-,32,42
n-BuNH2,38,40 piperidine,39CH(CN)2-,42 and HOCH2CH2S-,34,36
nucleophilic attachment of DABCO to 8-Cr is considerably
faster than attachment to the corresponding thiomethyl
derivative; the k1(OMe)/k1(SMe) ratio for DABCO (28.4) is
virtually identical with the k1(OMe)/k1(SMe) ratio for
CH(CN)2- (k1(OMe) ) 4.79 × 103 M-1 s-1; k1(SMe) ) 1.46
× 102 M-1 s-1) (32.8) and for HOCH2CH2S- (k1(OMe) )
2.25 × 104 M-1 s-1; k1(SMe) ) 6.60 × 102) (34.1) but
smaller than for OH- (k1(OMe) ) 26.6 M-1 s-1; k1(SMe) )
0.127 M-1 s-1) (209) and n-BuNH2 (k1(OMe) ) 1.60 × 103
M-1 s-1; k1(SMe) ) 9.56 M-1 s-1) (167). In stark contrast,
tertiary amines possess drastically reduced nucleophilic
character as a direct result of steric hindrance around the
nitrogen (i.e. the bulky R groups) combined with the inability
to generate neutral species (i.e. unable to lose a proton).
Consequently, tertiary amines tend to react as bases rather
Figure 2. Hammett plots for the reaction of DABCO with 10-Cr-Z and
11-Cr-Z in dry MeCN.
the addition of several phosphines (n-Bu)3P, (Ph-CH2)3P,
(iPr)3P to (CO)5CrdC(OMe)CH3 and to (CO)5WdC(OMe)-
CH3 in toluene-d8 were reported by Fischer et al.46-49 In the
follow up study, Fischer investigated the temperature and
substitutent effects on the equilibrium constant for the
reaction of 23 (Z ) MeO, Me, H, Br, and CF3) in toluene
with (n-Bu)3P.50 A Hammett plot for log K1 vs σ yielded F
) 2.22; the point corresponding to Z ) OMe deviates
negatively by 0.90 log unit from the line.50
Kinetic studies for the reaction of PR3 with (CO)5M )
C(OMe)R′ (M ) Cr, Mo, and W, R ) n-Bu and n-OBu,
and R′ ) Me and Ph groups) in acetone at 25 °C lead to the
determination of rate constants both for the forward (k1) and
reverse (k-1) reactions and hence equilibrium constants(K1).25
It was demonstrated that the sterric effect affects the k-1
values, while k1 remains mostly unaffected, thereby leading
to the change in K1 values.
Hammett plots for the reactions of 10-Cr-Z and 11-Cr-Z
with DABCO provide excellent linear fit with F values 2.18
( 0.13 and 0.89 ( 0.07, respectively (Figure 2). The point
corresponding to the reaction of 11-Cr-Cl, where chlorine
atom is at the meta position of the phenyl ring, deviates
negatively by ∼0.63 log unit. This departure mainly arises
due to severe steric crowding at the carbene carbon in 24.
than nucleophiles. On the basis of the hard-soft acid-base
53,54
concept
DABCO should behave as a moderately hard
base and this ratio should correspond to the latter values, at
least higher than the former values. The discrepancy in
With respect to structure-reactivity relationships the
following points are noteworthy.
(51) The intrinsic rate constant is the rate constant in the absence of a
thermodynamic driving force.
(46) Fischer, H.; Fischer, E. O.; Kreiter, C. G.; Werner, H. Chem. Ber.
1974, 107, 2459.
(47) Kreissl, F. R.; Kreiter, C. G.; Fischer, E. O. Angew. Chem., Int. Ed.
Engl. 1972, 11, 643.
(48) Kreissl, F. R.; Held, W. J. Organomet. Chem., 1975, 86, C10.
(49) Kreissl, F. R.; Fischer, E. O.; Kreiter, C. G.; Fischer, H. Chem. Ber.
1973, 106, 1262.
(52) The PNS states that if the development of a product-stabilizing factor
lags behind changes or charge transfer at the transition state, k0 is
reduced. The same is true if the loss of a reactant-stabilizing factor
runs ahead of bond change or charge transfer. For product-stabilizing
factors that develop early or reactant-stabilizing factors that are lost
late, k0 is enhanced.
(50) Fischer, H. J. Organomet. Chem. 1979, 170, 309.
(53) Pearson, R. G.; Songstad, J. J. Am. Chem. Soc. 1967, 89, 1827.
Inorganic Chemistry, Vol. 44, No. 16, 2005 5869

Ali et al.
Table 2. Hammett F Values in 50% MeCN-50% Water (v/v) at 25 °C
nucleophiles
DABCO
10-Cr-Z
11-Cr-Z
2.18 ( 0.13a
2.88 ( 0.26b
2.20d
0.89 ( 0.07a
0.69 ( 0.05b
0.59c
-
CH(CN)2
n-BuNH2
MeO- (MeOH)
HOCH2CH2S-
(n-Bu)3P
2.03e
0.78f
1.16b
2.20g
2.20e
OH-
(4) Cr vs W is an important factor. The higher k1 values
for the reactions of DABCO with the tungsten carbene
complexes compared to the corresponding chromium carbene
complexes (k1(12)/k1(11-Cr-H) ) 3.98; k1(8-W)/k1(8-Cr) )
1.80; k1(14)/k1(13) ) 1.62) follows previously observed
patterns.27,30,35 Nucleophilic addition to the tungsten carbene
complexes is slightly favored over the addition to chromium
complexes. The effect on equilibrium constant (K1) is small
but somewhat larger on k1 for the reaction of 11-Cr-H and
12 with various thiolate ions (eq 7).
a This work in pure MeCN. b Reference 42. c Reference 40. d Reference
f
39. e Reference38. (CO)5CrdC(SCH2CH2OH)C6H5-Z.36 g (CO)5CrdC(OMe)-
C6H5-Z.36 g Reference 49.
reactivity pattern may be due to the relative importance of
one over the other effects of the following factors: steric
effects; hard-soft behavior; relative ground-state stability
of the parent carbene complexes. Steric as well as hard-
soft effects should increase the k1(OMe)/k1(SMe) ratio while
ground-state stability of parent compounds disfavor the
ncleophilic attack which is more for methoxycarbene com-
plexes due to strong π-donor capacity of the oxygen over
sulfur, which, hence, lowers the ratio. It is reasonable to think
that the effect of ground-state stability predominates over
the other factors leading to the decrease in k1(OMe)/k1(SMe)
ratio.
The K1(W)/K1(Cr) ratios vary between 1.27 and 1.53, while
k1(W)/k1(Cr) ratios vary between 2.56 and 3.54. Similar
results were obtained for the MeO- addition with K1(W)/
K1(Cr) ) 1.58 and k1(W)/k1(Cr) ) 2.41. The rather small
dependence of K1 on metal may be the result of the extensive
dispersion of the negative charge into the CO ligands of the
adducts. Since this leaves little charge on the metal, the
identity of the metal has only a small influence on the
stability of the adduct. An additional factor is related to the
strong contribution of 25 to the structure of 10-Cr-Z and
8-W, because 25 leads to substantial negative charge on the
(CO)5M moiety of the carbene complexes; the presumably
greater stabilization of the negative charge by the (CO)5W
moiety in the adduct is partially offset by a similar effect on
the carbene complex. The higher (k1(W)/k1(Cr) ratio for
thiomethylcarbene complexes compare to the methoxy- or
ethoxycarbene complexes can also be judged from the above
point of view.
(3) Hammett plots for the reactions of DABCO with 10-
Cr-Z and 11-Cr-Z, are shown in Figure 1. The F values are
summarized in Table 2, along with F values for systems
investigated previously. The F value for the reactions of
DABCO with 10-Cr-Z, methoxycarbene complexes (2.18
( 0.13), is significantly larger than with 11-Cr-Z, the
thiomethyl derivatives (0.89 ( 0.07). The positive F values
imply that electron-withdrawing substituents stabilize the
adduct 18 and/or destabilize the carbene complexes. Since
in the adduct there are both positive and negative charges,
the net substitutent effect on this species cannot be very large;
in view of the fact that negative charge is more dispersed
than the positive charge, an argument could be made that
there should actually be a slight net destabilization of 18.
Hence, main factor that can account for the positive F values
is a destabilization of the carbene complexes, which is best
understood in terms of an increased electron deficiency on
the carbene carbon. Thus, the key to an understanding of
these findings is to consider the substituent effect not only
on the transition state but also on the reactant. A major
difference between 10-Cr-Z (F ) 2.18) and 11-Cr-Z (F )
0.89) is that the π-donor effect of the methoxy group is much
stronger than that of the thiomethyl group55 and leads to a
substantial contribution of the zwitterionic form 25 to the
structure of 10-Cr-Z. Since the positive charge on the
methoxy group is much more localized than the negative
charge on the (CO)5Cr moiety, the overall effect of an
electron-withdrawing phenyl substituent will be destabilizing
and hence result in an increased reactivity of the carbene
complex.
The resonance structures 29 and 30 are of lesser impor-
tance than 25 and 28, and hence, there is less negative charge
on the M(CO)5 moieties of 11-Cr-H and 12 than those of
8-Cr and 8-W. As a result there is less compensation and
higher value of (k1(W)/k1(Cr) ratio.
(5) Finally, we consider MeO vs EtO. The ethoxycarbene
complexes are somewhat less reactive than their methoxy
counterparts. This arises due to the somewhat stronger
π-donor effect of the ethoxy group and hence more ground-
state stabilization over to that of the methoxy group and
partly due to steric crowding exerted by the slightly larger
(54) (a) Pearson, R. G. SurV. Prog. Chem. 1969, 5, 1. (b) Pearson, R. G.
Inorg. Chem. 1988, 27, 734. (c) Hati S.; Datta, D. Proc. Indian Acad.
Sci., Chem. Sci. 1996, 108, 143.
(55) F(OMe) ) -0.42;56 F(SMe) ) -0.15.56
(56) Hansch, C.; Leo, A.; Taft, R. W. Chem. ReV. 1991, 91, 165.
5870 Inorganic Chemistry, Vol. 44, No. 16, 2005

Transition Metal Carbene Chemistry
high vacuum. The solid product was dissolved in a minimum
volume of 50% MeCN-50% CH2Cl2 (v/v) and charged on a silica
gel column prepared with the same solvent mixture. The yellow
band was collected and dried under high vacuum to get the
crystalline solid. 1H NMR [δ (CDCl3)]: 3.31 (s, 3H, CH3O), 3.34
(m, 6, NCH2), 3.65 (m, 6, +NCH2), 7.47 (m, 5, C6H5).
ethoxy group in the transition state, which corresponds to a
pattern observed earlier.30,35
Experimental Section
Materials. DABCO (Aldrich) was analytical grade and was
purified by the sublimation method. Acetonitrile (Fischer Scientific)
was used as received. MeCN was distilled over CaH2 before use.
The carbene complexes used as substrates were synthesized and
characterized as described in the previous studies28,30,42
Instrumentation and Kinetic Experiments. NMR spectra were
recorded on a Bruker 300 MHz instrument. UV-vis spectra were
obtained on an Agilent 8453 diode array spectrophotometer.
Synthesis of 9-W. The ylide products of the reaction of 8-W
with DABCO was synthesized as follows: A 0.222 g (0.5 mmol)
amount of 8-W was dissolved in 20 mL of dry acetonitrile in a
round-bottom flask under Ar atmosphere. To this solution was added
2 mmol of DABCO in 15 mL of acetonitrile. The mixture was
allowed to react for 1 h whereupon the solvent was removed under
Acknowledgment. This work was supported by Grant
SR/S1/IC-19/2002 from the Department of Science and
Technology (DST), New Delhi, India.
IC050349T
Inorganic Chemistry, Vol. 44, No. 16, 2005 5871