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Kinetic Isotope Effects as a Mechanistic Tool for the Elucidation of the Rate-Determining Step(s) in the Transition-Metal-Ion-Mediated Activation of CHCC Bonds. Ethylene Loss from Metastable 4-OctyneM Complexes in the Gas Phase

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C0M M U N I C A T I O N S
Table 1. Ethylene loss from the metastable 4-octyneiM" complexes
l b , CH,CD2CH2C=CC,H,; l c ,
[a, b] l a , CD,CH,CH,CrCC,H,;
CH,CH,CD,C=CC,H,; Id, CD,CD,CH,C=CC,H,.
Substrate C,H,..D,
Cr"
Mn"[c]
Fee
Corn
Ni"
Cum
58
55
55
58
60
66
42
45
45
42
40
34
1.22
55
55
1.50
55
1.94
51
45
1.22
100
49
1.04
100
1.38
50
50
1.00
100
Kinetic Isotope Effects as a Mechanistic Tool for the
Elucidation of the Rate-Determining Step(s) in the
Transition-Metal-Ion-Mediated Activation of
CH/CC Bonds. Ethylene Loss from Metastable
4-Octyne/Me Complexes in the Gas Phase**
By Christian Schulze and Helmut Schwarz *
Dedicated lo Professor Worfgang R. Roth on the occasion
of his 60th birthday
While there exist quite a number of case studies in which
the mechanistic details of CHjCC bond activation by transition-metal complexes in solution were uncovered by using
kinetic isotope effects,"] analogous investigations in the gas
phase are scarce as far as the elucidation of the rate-determining step(s) (RDS) is concerned.r2.31Here we report a
study of the transition-metal-ion-mediated generation of
ethylene from metastable 4-octyne/M@complexes (M = Cr,
Mn, Fe, Co, Ni, Cu) in the gas p h a ~ e . [ ~It- ~will
' be demonstrated that, depending on M@,three distinct cases can be
distinguished: (1) The rate-determining step corresponds to
the activation of a CH bond, (2) the olefin detachment is
rate-determining, and (3) the hydrogen transfer and the loss
of ethylene are associated with kinetic isotope effects.
The analysis of the data displayed in Table 1 is straightforward. For all metal ions, except Mn@,[51the reaction involves exclusively the C1/C2 positions of the substrate. Hydrogen exchange reactions do not take place. In addition, the
reactions 2 + 3 and 4 + 3171are strictly irreversible. Thus, 3
is more likely to dissociate than to revert to the intermediates
involved in its formation (Scheme 1).
A further analysis of the isotope distribution reveals the
following: (1) Comparison of the data for the Cr@and Cu@
complexes 1a and 1b clearly demonstrates that the ligand
detachment 3 + 5 is not associated with a kinetic isotope
effect (C,H, and C,H,D, are eliminated to the same extent
from 1 b). Rather, for these metal-ion complexes it is the CH
bond activation step 2 -+ 3 (or 1-M@+ 4)which constitutes
the RDS. The fact that this primary isotope effect is relatively small (1.38 for Cr@and 1.94 for the Cu@system) points to
bent transition structures,['' which may indicate that the reaction actually proceeds via the route 1-M@+ 2 + 3. (2) In
[*I Prof. Dr. H. Schwarz, Dr. C. Schulze ['I
Institut fur Organische Chemie der Technischen Universitat
Strasse des 17. Juni 135, D-1000 Berlin 12
['I Present address: Department of Chemistry, University of Oslo
P.O. Box 1033 Blindern, N-0315 Oslo 3 (Norway)
["I We are grateful to the Volkswagen-Stiftung, the Fonds der Chemischen
Industrie, and the Deutsche Forschungsgemeinschaft for financial support.
Angew. Chem. lnr. Ed. Engl. 29 (1990) No. 5
-
45
10
45
45
1.22
100
-
82
13
1.38
45
1.22
100
5
57
43
1.32
45
2
2
X
43
-
59
61
62
66
41
1.44
39
1.56
38
1.63
34
1.94
[a] Intensities are normalized to ~ C , H , ~ , D=
. 100%: the reproducibility of
the data is better than f 3%. [b] Elimination of ethylene (% total ion fragment
current) from metastable 4-octyne/Mm amounts to: 15% (Cr"). 40% (Mn"),
98% (Fee), 94% (Co"), 98% (Ni"), and 84% (Cum).[c] For Mn", due to the
partial scrambling between C2 and C3, no k,ikD data are given.
distinct contrast, for M@= Fe@it is the loss of C,H,-,D,
which is affected by isotopic substitution. If the C-H bond
activation constituted the RDS, one should observe a
primary kinetic isotope effect discriminating against loss of
C,H,D, versus C,H, from 1 a-Fe@relative to 1b-Fee. This
C,H,-C-C-C,H,
I
MQ!
l-M@
"CC"
II
I
"CH"
2
1
C,H,-CEC-CH,
C,H,-M"-CH,-CH,
\
,M,"
2
H
\
1 :CH,
CH,
3
i
1
2
H,C=CH,
Scheme 1
5
is not the case. Rather, both isotopomers give exactly thesame isotope distribution from which we obtain a value of
k,/k, = 1.22 (or of 1.10 per deuterium). This value is in line
with what one would expect for secondary isotope effects
caused by rehybridization sp" +sp2 (x > 2) of the carbon
Q VCH Verlagsgesellschaft mbH, 0.6940 Weinheim, i 990
OS70-0833/90~0SOS-OS09
$02.SO/O
509
atoms in the step 3 + 5."". b,91 (3) It is obvious from Table 1
that for the Coe and NiO complexes of 4-octyne both steps,
i.e., C-H bond activation andligand detachment, are subject
to a kinetic isotope effect. For the ligand detachment 3 5
the data for 1 b and 1 d permit the estimation of an isotope
effect k H / k D= 1.10 (per deuterium) for either CoO or Nie.
The C-H bond activation step 2 + 3 (or, less likely,
I-Me -+ 4) is associated with slightly differing k H / k Dvalues
of 1.25 and 1.36 for Coo and Nie, respectively.f'O]Obviously, the various transition-metal ions cause a switching in regard
to the bottleneck of the multistep event which is shown in
Scheme 1 in a simplified manner.
As indicated in Figure 3 , a quite remarkable inverse relationship exists between the magnitude of the kinetic isotope
-
":
I
Cr@
I
I
Mn@ Fe"
1
Coo
I
Ni@
Rhodium-Catalyzed Synthesis
of Trisubstituted Olefins from Ethene Derivatives
and Diazoalkanes **
I
Cu@
Fig. 1. Qualitative relationship between D':(Me-H) (-0-0-0-)
and kinetic
for C-H bond activation (2 --* 3 or 1-Me + 4).
isotope effects (+-+--t)
The data for D(Me-H) are taken from [ll]. For Mna no accurate k,/k, value
can be given (see text).
effect of the oxidative addition of a CH bond to a metal-ion
center and the strength of the Mo-H bond, D"(M@-H), to
be formed.["' In a qualitative sense the data suggest that
weaker and stronger Me-H bonds cause larger and smal/er
isotope effects, respectively. The origin of this relationship
remains to be established through further experimental and
theoretical studies.
Received: January 8, 1990 [Z 3725 IE]
German version: Angew. Chem. 102 (1990) 566
CAS Registry numbers:
Cr, 7440-47-3; Mn, 7439-96-5; Fe, 7439-89-6; Co, 7440-48-4; Ni, 7440-02-0;
Cu, 7440-50-8; atomic deuterium, 16873-17-9;4-octyne, 1942-45-6.
111 Selected references: a) W. D. Jones, F. J. Feher, Acc. Chem. Res. 22 (1989)
91; b) P. 0.Stoutland, R. G . Bergman, J. Am. Chem. Sac. fZO(1988) 5732;
c) F. R. Hartley, S . Patai, (Eds.): The Chemistry of the Metal-Carbon Bond,
Vol. 1,2, Wiley, New York 1985; d) R. H. Crabtree, Chem. Rev. 85 (1985)
245.
[2] a) G. Czekay, T. Drewello, H. Schwarz, J. Am. Chem. Sac. 1I 1(1989)4561.
[3] a) N. Steinriick, H. Schwarz, OrganometaNics 8 (1989) 759; h) C. Schulze,
T. Weiske, H. Schwarz, ibid. 7 (1988) 898.
[4] Preliminary results were previously described for Me = Fee, Cre: a) C.
Schulze, T. Weiske. H. Schwarz, Chimia 40 (1986) 362; b) C. Schulze, H.
Schwarz, D. A. Peake, M. L. Gross, J. Am. Chem. Sac. 109 (1987) 2368;
c) [3bl .
[ 5 ] In the present study Mne is not included in the discussion on the grounds
that, in contrast to the other transition-metal ions, ethylene loss is preceded
by exchange processes involving the CH bonds at positions C2 and C3 of
the substrate (see Table 1). This fact prevents a reliable determination of
the kinetic isotope effects. See also. a) C. Schulze, H. Schwarz, J. Am.
Chem. Sac. 110 (1988) 67; b) [3 b].
[6] The 4-octyne/Me complexes were generated in the ion source of a VG
Instruments ZAB-HF-3F by reacting 4-octyne with Me;the latter ions
were generated by either 100-eV electron impact ionization of a suitable
organometallic compound or by fast atom bombardment of appropriate
transition-metal salts. The complexes thereby formed were accelerated to
510
0 VCH
Verlagsgesellschafl mbH, 0-6940 Weinherm, 1990
8 keV kinetic energy, mass selected by means of B(1)E (B stands for
magnetic and E for electric field) at a resolution sufficient to separate
isobaric species, and the unimolecular dissociations occurring in the fieldfree region between E and E(2) recorded by scanning of B(2). Averaging
techniques were employed to significantly improve the signal-to-noise ratiossuch that theintensitymeasurements were better than 23%. Forafull
description of the apparatus and the techniques used, see: a) [3, 5a]; b) H.
Schwarz, Arc. Chem. Res. 22 (1989) 282; c) C. Schulze, Ph.D. Thesis,
Technische Universitiit Berlin, /989.
171 The description of 3 as a metallacyclopropane derivative is somewhat
arbitrary; of course, the species could also correspond to an qz-olefin
complex.
181 a) F. H. Westheimer, Chem. Rev. 61 (1961) 265; For other examples of
isotope effects in p-hydrogen transfers in organometallic systems, see:
h) [3. 6b]; c) D. S. Bomse, R. L. Woodin, J. L. Beauchamp, J. Am. Chem.
Sac. 101 (1979) 5503.
[9] A. Streitwieser, R. H. Jagow, R. C. Fahey, S . Suzuki, J. An?. Chem. Sor. 80
(1958) 2326
[lo] Since two factors determine the experimental value, the calculations include a deconvolution to separate both effects. For details of the procedure, see [6c].
[I 11 The D"(Me-H) data are taken from: P. B. Armentrout, J. L. Beauchamp,
Acc. Chem. Res. 22 (1989) 315.
By Justin WOKLutz Brandt, Arno Fries,
and Helmut Werner*
During attempts to prepare square-planar carbene metal
complexes of type 1, we discovered a new catalytic reaction
for forming C-C bonds. The vinylidene rhodium and iridium complexes trans-[MCl( = C =CRR)(PiPr,),], which are
structurally related to 1, are known"] and (for M = Ir) have
been used for the synthesis of the cationic carbyne complexes
trans-[MC1(= C-CHRR')(PiPr,),le.[21
trans-[MCl( = CRR)(PiPr,),]
[RhCI(PiPr,),J
1, M = Rh, Ir
2
trans-[RhCI( = CH,)(PiPr,),]
3
The monomeric three-coordinate rhodium compound 2
(Scheme l), used as starting material for the preparation
of the vinylidene complex trans-[RhCl(= C= CH,)-
[RhC ILzI
CPhzNz
CzH,
Ph-H
6a
/L
CI-Rh-NZCPhz
LX
5
Scheme 1. L
=
PiPr,.
[*] Prof. Dr. H. Werner, Dr. J. Wolf, L. Brandt, A. Fries
Institut fur Anorganische Chemie der Universitit
Am Huhland, D-8700 Wurzburg (FRG)
[**I This work was supported by the Deutsche Forschungsgemeinschaft, the
Commission of the European Communities, and the Fonds der Chemischen Industrie.
0570-0833/90/050S-0510S 02.50/0
Angew. Chem. Int. Ed. Engl. 29 (1990) No. 5
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loss, octynem, bond, determinism, metastable, step, ion, chcc, activation, rate, tool, kinetics, gas, transitional, complexes, phase, mediated, isotopes, effect, mechanistic, metali, ethylene, elucidation
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