Organometallics 1996, 14, 317-326
317
Activation of C-C and C-H Bonds of cis- and trans-1-Acetyl-2-methylcyclopropane by Bare Metal(1) Cations in the Gas Phase: Comparative Study of the First-Row Transition-MetalIons Cr+-Cu+ Christoph A. Schalley, Detlef Schroder, and Helmut Schwarz” Institut f i r Organische Chemie der Technischen Universitat Berlin, 10623 Berlin, Germany Received June 13, 1994@
Stereochemical effects on transition-metal-mediated C-H and C-C bond activation are probed by examining unimolecular fragmentation reactions of 1-acetyl-2-methylcyclopropane/ M+ complexes (M = Cr+-Cu+) in the gas phase. Basically, three general reaction types can be distinguished: (i) metal-induced ring cleavage leading to losses of molecular hydrogen, ethene, and acetaldehyde (these C-H and C-C bond activation processes can be ascribed to remote functionalization of an acyclic hexenone/M+ intermediate), (ii) insertion of the metal in a n a-C-C bond and subsequent decarbonylation to yield the corresponding olefin/ M+ complexes, and (iii) geminal C-C bond activation leading t o the formation of metal carbene complexes as intermediates for M = Cr, Mn, Co, and Ni. These carbene complexes eventually give rise to unimolecular losses of ethene or propene. Direct C-H bond activation of the terminal methyl group is observed for none of the metals. Stereochemical differences for cis and trans isomers are only observed when the rate-determining steps for C-C bond activation leading to different products, i.e. remote functionalization and decarbonylation, compete directly with each other, and distinct stereochemical effects are observed for the Mn+, Fe+, and Cu+ complexes. The experimental data and the trends within the late firstrow transition metals are compared with previous findings and discussed in terms of activation barriers associated with C-C and C-H bond activation processes and thermodynamics. Scheme 1
Introduction While there have been numerous reports on the gasphase chemistry of bare and partially ligated transitionmetal ions with organic substrates,l detailed studies of the stereochemistry of C-H bond activation mediated by bare or ligated metal cations in the gas phase are scarce.2 Carbonyl compounds may serve as a case in point, in that both the regio- and the stereochemistry of C-H and C-C bond activation by naked Fe+ cations were examined in great detail.3 In particular, the examination of unimolecular dissociations of metastable ketoneme+ complexes provided detailed insight into the reaction mechanisms, rate-determining steps, and kinetic isotope effects associated with Fe+-mediatedC-H and C-C bond activation in the gas phase.2fv3d-f In general, the studies revealed that these reactions,
H\M/+-CHZ H
It
Abstract published in Advance ACS Abstracts, November 15,1994. (1)For recent reviews see: (a) Eller, K.; Schwarz, H. Chem. Reu. 1991,91,1121.(b) Eller, K.Coord. Chem. Rev. 1993,126,93. (2)(a) Nekrasov, Y.S.; Zagorevskii,D. V. Org. Mass Spectrom. 1991, 26,733.(b) Priisse, T.;Fiedler, A.; Schwarz, H. Helu. Chim. Acta 1991, 74,43.(c) Seemeyer,K; Priisse, T.; Schwarz, H. Helu. Chim.Acta 1993, 76, 1632. (d) Seemeyer, K.;Priisse, T.; Schwarz, H.Helv. Chim. Acta 1993,76,113.(e) Seemeyer, K.;Schwarz, H. Helu. Chim.Acta 1993, 76,2384. (0 SchMder, D.; Schwarz, H. J . Am. Chem. SOC.1993,115, 8818.(g) Raabe, N.;Karrass, S.; Schwarz, H. Chem. Ber. 1994,127, 261.(h) For a recent study of the gas-phase stereoseledivity in metalfree 1,a-elimination reactions, see: Rabasco, J. J.; Gronert, s.; GSS, S. R. J . Am. Chem. Soc. 1994,116,3133. (3)(a) Bumier, R. C.; Byrd, G . D.; Freiser, B. S. J . Am. Chem. SOC. 1981,103,4360. (b) Grady, W.L.; Bursey, M. M. Int. J. Mass Spectrom. (c) Lambarski, M.;Allison, J. Int. J . Mass Ion Processes 1983,52,247. Spectrom. Ion Processes 1986,65,31. (d) Schrbder, D.;Schwarz, H. Chimia 1989,43,317. (e) Schroder,D.; Schwarz, H. J . Am. Chem. SOC. 1990,112,5947. (f) Schroder,D. Ph.D. Thesis, Technische Universitat Berlin D83, 1993.
V
@
0276-7333f95f2314-0317$09.00/0
involving the activation of ordinary C-H and C-C bonds, can be described in terms of the “remote functionalization” concept4 (Scheme 11, provided one side chain bears at least three carbon atoms. Most interestingly, bare Fe(1) cations were also demonstrated to bring about diastereoselective C-H bond activation of acyclic ketone^.^^,^^
With regard to the topic of stereochemistry in metalmediated bond activation,the investigation of complexes (4)Schwarz, H.ACC.Chem. Res. 1989,22,282.
0 1995 American Chemical Society
Schalley et al.
318 Organometallics, Vol. 14, No. 1, 1995 Chart 1
1
-A( 4 (cispans mixture)
2
3
,dL 5
of cis- and trans-1-acetyl-2-methylcyclopropanes 1 and 2 (Chart 1)with bare transition-metal cations appears t o be particularly promising, as the rigidity of these substrates confers a well-defined stereochemical relationship between the substituents. However, there are other features to be taken into account: the C-C bonds of the cyclopropyl backbone are weakened by the high ring-strain energy, while the cyclopropylic C-H bonds are strengthened and possess high bond dissociation energies in the range of 106 kcal/moL5 Due t o these properties, the stereoisomeric cyclopropyl ketones 1 and 2 are likely not only to provide insight into some stereochemical details of metal-mediated reactions but also to serve as crucial test systems to further probe the concept of remote functionalization of C-H and C-C bonds. The study of unimolecular reactions of metastable ion complexes is ideally suited for this purpose, since they allow us to probe even the small energetic differences which result from stereochemical differences in the precursor molecules (e.g. cis /trans isomers) complexed to a bare metal ~ a t i o n . ~ ~ - g , ~ ~ In a previous paper,e we reported in detail the reactions of metastable 1/Fe+ and 2/Fe+ complexes in the gas phase. By using a combination of tandem mass spectrometric methods and isotopic labeling techniques, we were able to provide evidence for the existence of two competing reaction channels being operative in the unimolecular fragmentation reactions of 1/Fe+ and W e + . Briefly, the comparative study of the isotopologous Fe+ complexes of 1 and 2 (Chart 2) and the isomeric acyclic hexenone complexes 3/Fef-5/Fe+ demonstrated that one reaction channel (Scheme 2, path a) commences with a metal-induced ring cleavage leading eventually to the acyclic 3-hexen-2-one/Fe+ complex 3/M+ (M = Fe). The intermediate 3/Fe+ serves as a branching point for the generation of the three main neutral products, i.e. Hz, CzH4, and CH3CH0, which are formed with similar intensities for the cis and trans isomers l/Fef and 2/Fe+; consequently, path a is associated with a complete loss of stereochemical information. Hydrogen and ethene losses from 3/Fe+ can be described in terms of the concept of remote functionalization, as depicted in Scheme 3 (M = Fe). The metalmediated w-C-H bond activation leads to 7/Fe+,which then, via a P-H shift and reductive elimination of H2, gives rise to 8/Fe+. In competition, ,8-C-C bond cleavage, presumably via 9/Fe+, causes ethene loss to yield (5)(a) Ferguson, K. C.; Whittle, E. Trans. Faraday SOC. 1971,67, 2618. (b) Baghal-Vayjooee, M.H.; Benson, S. W. J.Am. Chem. SOC. 1979,101,2838. (6)Schalley, C.A.;Schroder, D.; Schwarz, H. J.Am. Chem. SOC., in press.
10/Fe+. Loss of acetaldehyde (Am = 44) must not necessarily involve remote functionalization and can be accounted for by a partial isomerization of 3/Fe+ to 4/Fe+ or 5/Fe+; from the last two complexes acetaldehyde can readily be formed via 1,4- or 1,2-elimination processes. On the other hand, Fe+-induced decarbonylation of 1 and 2 (Scheme 2, path b; M = Fe) was shown6 to commence with a-C-C bond insertion, in analogy to the reaction of acetone with Fe+,3a97t o generate the intermediate 6/Fe+; this species serves as a precursor for carbon monoxide loss. Most interestingly, in contrast to the eliminations of Hz, C2H4, and CHsCHO, the expulsion of carbon monoxide exhibits a distinct stereoselective effect, and the process is favored for the trans complex W e + . The observed stereochemical differences can be rationalized via the stereoselective formations of cis- and trans-3-allyl complexes 6/Fe+ (Scheme 4, M = Fe) in the course of the electrocyclic ring-opening process of the cyclopropylbackbone, where the formation of trans-6/Fe+ is favored such that the decarbonylation of W e + is more abundant as compared t o l/Fe+. As a consequence, the subtle stereochemical differences between the cis and trans isomers l/Fe+ and W e f manifest themselves in the competition of the reaction channels in the unimolecular dissociation of the metastable complexes. Here, we will present a more comprehensive, comparative study of the gas-phase reactions of 1and 2 with late first-row transition-metal ions from Cr+ through Cu+. As will be uncovered in the unimolecular fragmentation patterns, in addition to the reactions depicted for M = Fe in Schemes 2-4, some of the transitionmetal ions induce quite remarkable processes, e.g. metal-carbene formations or losses of neutral metal fragments. Furthermore, the origin of distinct stereochemical effects for some metal complexes of the cis/ trans isomers 1/M+and 2/M+ as well as periodic trends are discussed and explained in terms of the internal energy content of the metastable ions in conjunction with the interplay of competing processes for the various metals.
Experimental Section The experiments were performed with a modified VG ZAB/ HF/AMD 604 four-sector mass spectrometer of BEBE configuration (B stands for magnetic and E for electric sectors), which has been described previously.8 In brief, the metal cations were generated from the substrates given in Table 1 and reacted in a chemical ionization (CI) source with the organic precursors of interest (Charts 1 and 2). Unfortunately, the measurements of Ni+ complexes could not be carried out using bis(cyclopentadienyl)nickel(II), bis(acetylacetonato)nickel( II), or bis(cycloocta-l,5-dienyl)nickelas substrates, because of isobaric interferences arising from these compounds. Therefore, the bis(hexafluoroacetylacetonato)nickel(II) complex (Ni(hfac)z) has been used for the generation of Ni+ cations. Similarly, Cu+ had t o be generated from (Cu(hfac)z). In addition, instead of the isotopes 58Ni+and W u + , complexes with the less abundant 6oNi+and 6 5 C ~isotopes + had t o be (7)(a) Halle, L. F.; Crowe, W. E.; Armentrout, P. B.; Beauchamp, J. L. Organometallics 1984,3,1694.(b) Sonnenfroh, D.M.; Farrar, J. M. J.Am. Chem. SOC. 1986,108,3521. (c) Schultz, R.E.; Armentrout, P. B. J.Phys. Chem. 1992,96,1662. (8)(a) Srinivas, R.;Sulzle, D.; Weiske, T.; Schwarz, H. Znt. J.Mass Spectrom. Ion Processes 1991,107, 368.(b) Srinivas, R.;Siilzle, D.; Koch, W.; DePuy, C. H.; Schwarz, H. J.Am. Chem. SOC.1991,113, 5970.
Activation of C-C and C-H Bonds by Metal(I) Cations
+
Organometallics, Vol. 14, No. 1, 1995 319
Chart 2 0
0
p A
+ %
D D
la
lb
IC
Id
le
2a
2b
2c
2d
2e
Scheme 2 M-0
H*
3/M'
CH,CHO
M'
,"""
Scheme 3 n I
3/M'
7/M'
+
Scheme 4
M*\
1 IM'
t 1IM' and Z/M'
9lM'
studied, in order to avoid interferences from isobaric ions. In contrast to electron impact conditions, the relatively high pressure that prevails in the ion source during chemical ionization allows efficient cooling of the complexes generated.6 Nevertheless,the metastable complexes must contain a certain amount of excess internal energy in order to undergo unimolecular dissociation within the time scale of the experiment. The ketones were introduced via the heated septum inlet system (70 "C),and the mixture of ketone and metal precursor was subsequently ionized by a beam of electrons having 50100 eV kinetic energy. The ions of interest were accelerated to 8 keV translational energy and mass-selected by means of B(l)/E(l) at a mass resolution of mlAm = 2000-5000. Unimolecular fragmentations of metastable ions (MI) occurring in the field-free region preceding the second magnet were recorded by scanning B(2). For collisional activation (CA) experiments, mass-selected ions were collided with helium
2/M+
transd IM'
(80% transmission). The CA spectrag are in line with the results reported below but are not pursued further in the present discussion. Therefore,the correspondingdata are not given explicitly in this article; they are available from the authors upon request. The error of the relative intensities in MS/MS experiments does not exceed Some MS/MS/ MS experimentslO were performed with the complexes of unlabeled l/M+-S/M+ (M = Fe, C O ) by ~ , selecting ~ the primary fragment ions by means of B(2), and the collision-induced fragmentations occurring in the subsequent field-free region were recorded by scanning E(2); these experiments will be referred to as MVCA spectra. All spectra were accumulated (9) Schalley, C.A.Diploma Thesis, Technische Universitat Berlin, 1994. (10) Busch, K. L.;Glish, G. L.; McLuckey, S. A. Mass Spectrometry/ Mass Spectrometry: Techniques and Applications of Tandem Mass Spectrometry; VCH: Weinheim, Germany, 1988.
Schalley et al.
320 Organometallics, Vol. 14, No. 1, 1995 Table 1. Precursors and Types of Inlet Systems Used for the Generation of Transition-Metal Ions M+ in the CI Source M+
substrate
inlet syst (temp, “C)
Cr+ Mn+ Fe+ co+ Ni+
Cr(CQ6 Cp’Mn(C0)Y Fe(CO)s CO(CO)~NO Ni(hfac)zc Cu(hfac)zc
direct (23) septum (70) septum (70) septum (70) direct (90) direct (45)
cu+
Cp’ = methylcyclopentadienyl. To avoid interference with the hydrogen losses from heavier isotopologues, measurements have been carried out using the 6oNi and 65Cu isotopes, respectively. hfac = hexafluoroacetylacetonate.
and on-line-processed with the AMD-Intectra data system; 5-30 scans were averaged to improve the signal-to-noise ratio. All unlabeled and lSO- and deuterium-labeled compounds (Chart 2) have been synthesized and purified by standard laboratory procedures and fully characterized by spectroscopic means as described in detail e l ~ e w h e r e . ~ , ~
Results and Discussion The relative intensities of the decomposition products arising from unlabeled 1/M+ and 2/M+ are given in Table 2. These data reveal that, despite common features, the metal ions Cr+ through Cu+ give rise to different products. As far as product variety is concerned, Fe+ seems to be the most selective metal ion in that only 5 products are formed; in contrast, the Cu+ complexes give rise to more than 10 different products. Further, inspection of the data in Table 1 reveals the existence of a stereochemical effect for the metal complexes of 1and 2 with M+ = Mn+, Fe+, Cu+, while for M+ = Cr+, Co+, Ni+ intensity differences due to the stereochemistry of 1 and 2 are not observed.ll For comparative purposes, the complexes of the acyclic isomers 3-5 with M = Cr-Cu have also been included. As already noted,6 for the Fe+ complexes of 1 and 2 ring cleavage leading to 3/Fe+ is supported by the similarity of the MI spectra of l/Fe+, 2/Fe+, and 3/Fe+. We obtain similar results for the complexes of the other M+ ions examined here;9for the sake of brevity the corresponding data will not be given. A complete set of data is available from the authors upon request. The unimolecular dissociations of the Fe+ complexes have been discussed in detail elsewhere,6 and a brief summary has also been given in the Introduction. For comparison, the data for both sets of stereoisomeric isotopologues are reproduced in Table 3; however, they will not be pursued further in the discussion. The reactions of Fe+ with 1and 2 (depicted in Schemes 2-4) may serve as a basis for the interpretation of the processes observed for the corresponding complexes of the other metal cations. In this paper, we will focus on the differences for the group VI11 transition metals Co and Ni as well as for Cr, Mn, and Cu in comparison to the Fe+ complexes. Complexes of 1and 2 with Co+ and Ni+.Table 4 summarizes the data for the gas-phase reactions of Co+ with the l8O-and deuterium-labeled isotopologues of 2. Since no significant cis /trans differences” are observed, ~
~~
(11)A slightly different ethene/CO ratio is observed for the I*Olabeled compounds la/Co+ (34/48) and 2a/Co+ (29/54) (values normalized to X = 100%).However, since these differences are within the experimental error limits, we refrain from drawing any conclusions from these data.
these data can be considered as being representative for both stereoisomers. The intensities of the main fragmentation processes of the Co+ complexes, i.e. dehydrogenation, ethene and carbon monoxide formation, and loss of acetaldehyde, differ from those observed for the Fe+ complexes. The high amounts of ethene and CO losses, as compared to dehydrogenation, point to a lower activation barrier for the first two processes. Thus, for Co+ (as well as for Ni+, see below) C-C bond activation is clearly favored over C-H bond activation. This observation is in line with previous findings, and similar trends have been reported for many reactions of group VI11 transitionmetal cations with numerous substrates.’ In addition, formation of methane and water and consecutive losses of H2 and H2O are observed as side reactions of low intensities; these will not be discussed further. As compared to Fe+, the Co+ complexes of labeled 1 and 2 reveal a similar distribution of isotopes for each group of neutral products. Therefore, we conclude that the unimolecular fragmentation reactions of the Fe+ and Co+ complexes follow the same mechanistic pathways. Ring cleavage yielding the acyclic 3/co+ isomer is further supported by the fragmentations of authentic 3/co+, which are very similar to those of 1/Co+ and 2/Co+, respectively. In addition, the MUCA spectra of the ionic products due to etheneIC0 losses from l/Co+5/co+ are indistinguishable within experimental error. This indicates that identical products are formed after the expulsion of ethene and carbon monoxide, respectively, from the cyclopropyl ketone complexes as well as the acyclic isomers and, furthermore, strongly supports the operation of similar mechanisms for Fe+ and Co+. Likewise, comparison of the MI/CA spectra of the ethene loss from 2a/Co+ with the CA spectrum12of an authentic methyl vinyl ketone/Co+ complex reveals that the latter is indeed formed from 2/Co+ in the course of ethene loss. In spite of these similarities, there is one striking difference in the reaction patterns of the Fe+ and the Co+ complexes: while expulsion of [Dslethene (Am = 31) from the [Dalmethyl-labeled complex 2e/Fe+ is negligible (Table 31, for 2e/Co+ the eliminations of [D31ethene and [Dslethene are observed with nearly the same intensities, whereas loss of [Dllethene does not occur at all (Table 4). This isotope distribution immediately rules out simple H/D exchange processes with other positions in 2e/Co+, since in the course of H/D exchange formation of [Dslethene would invariably be accompanied by the expulsion of a [Dll isotopologue. Consequently, not only does ethene loss follow the remote functionalizationmechanism (Scheme 3) but also a “carbene mechanism” is operative, as depicted in Scheme 5 (M = Co). The ethylidene complex 11/Co+ rearranges via a hydrogen shift to 12/Co+,from which ethene is extruded. This mechanistic picture agrees well with the low amount of [Dllethene (arising from 2c/Co+ via the remote knctionalization mechanism) and unlabeled ethene (formed via the carbene intermediate 11/Co+). In line with the MI/CA results mentioned above, both mechanisms (Schemes 3 and 5) give rise to methyl vinyl ketone/Co+ complexes, lO/Co+, as ionic (12) The CA spectra of the methyl vinyl ketone/M+.(M = Fe, Co) complexes lO/M+ were recorded at a reduced acceleration voltage, in order to ensure that these ions have the same kinetic energy as those produced by unimolecular ethene losses from l a +and 2a/M+.
Activation of C-C and C-H Bonds by Metal(I) Cations
Organometallics, Vol. 14,No. 1, 1995 321
Table 2. Neutral Products Generated in the Unimolecular Decomposition Reactions of M+ Complexes of 1-Acetyl-2-methylcyclopropanes1 and 2" Cr+
Mn+
Fe+
co+
Ni+
cu+
product
Am, mu
1
2
1
2
1
2
1
2
1
2
1
Hz CH3 CHq Hz0 Hz HzO COIC2H4 CO Hz
2 15 16 18 20 28 30 40 42 44 54 66 80 98
100
100
100 25
100 34
100
100
14
14
59
61
100 10
4 4
4 1
4
4
2
1
8
58
68
43
76
3 2 2 100
4 5 2 100
4 2 1
8
3 4 2 100
100
3
4
3 1 1 2
4 1 1 2
27
34
5
5
3
4
+
+
c3H4 c3H6
CH3CHO
c4&
MH MCH3 ligand loss
1
2
6
2
1
11
41 9 1 7
78 1 86 1
100 1
46 32 3
28 30 2
1
Intensities are given in percentages of the base peak. For the sake of clarity intensities less than 1% are omitted.
Table 3. Mass Differences (Am in amu) Observed in the Unimolecular Decomposition Reactions of the Isotopologues of m e + and m e + a
Scheme 5
Am -2
1/Fe+ la/Fe+ lb/Fe+ l&e+ ld/Fe+ le/Fe+
59 62 57 33 35 4
2/Fe+ 2a/Fe+ 2b/Fe+ We+ 2d/Fe+ 2e/Fe+
49 50 61 34 34 5
-3
-28
17 17 34
25 11 25 23 13 23
-29
-30
-31
17 17 41
-45
-46
-47
16 11
16 18
6 13 12
1
35
11 22 23 20 18
-44
15 21 10 16
23
16 17
7 11 12
1
14 17 10
5 1 13
Intensities are expressed in X(products) = 100%. Intensities less than 1% are omitted. In addition, minor losses of water are observed; see Table 2 (Am = 18).
Table 4. Mass Differences (Am in amu) Observed in the Unimolecular Decomposition Reactions of the 2/Co+ Isotouolormee Am
-2
21co+ 2a/co+ 2b/Co+ 2c/Co+ 2d/Co+ 2e/Co+
-3
-28
4 4 9
84 29 82 78 56 51
12 12 13 4 5 3
-29
-30
-31
-44
-45
-46
-47
4 54
5 5
9 30 17
16
3 5 2
2 2
Intensities are expressed in X(products) = 100%. Intensities less than 1% are omitted. The expulsions of methane and water and the consecutive losses of Hfl20 (Table 2) are not included, as these minor processes will not be discussed.
products. It should be mentioned that rearrangement processes of metal ethylidenes to metal-olefin complexes have been postulated for the gas-phase reactions of methyl-substituted metallacycl~butanes~~ with Co+ and Fe+. Likewise, unimolecular decarbonylation of propionic acid/Fe+ complexes14 leads to a methylcarbene/Fe(OH# complex, which subsequently rearranges to the corresponding ethene-ligated complex. Furthermore, a recent theoretical study16indicates that ~~~
~
ZM'
6 1 16
~
(13)(a) Armentrout, P. B.; Halle, L. F.; Beauchamp, J. L. J. Am. Chem. SOC.1981,103,6624.(b) Peake, D.A.; Gross, M. L.; Ridge, D. P. J. Am. Chem. SOC.1984.106.4307. (14)Schroder, D.;Zummack,'W.; Schwarz, H. J. Am. Chem. SOC. 1994,116,5857.
ll/M*
12/M'
Table 5. Mass Differences (Am in amu) Observed in the Unimolecular Decomposition Reactions of the 2PNi+ Isotopologuefb Am
-2
2/Ni+ 2a/Ni+ 2b/Ni+ 2c/Ni+ 2d/Ni+ 2e/Ni+
38 40 37 30 27 10
-3
9 8 17
-4
-28
1
62 40 63 52
-29
-31
20 8 21 2
44 2
-30
46
16
7
"Intensities are expressed in X(products) = 100%. Only the main fragmentation reactions (Le. dehydrogenation, ethene formation, and loss of carbon monoxide) are summarized here. Since for the Ni+ complexes no significant cisltruns differences are observed, only the data for the trans isotopomers are given as representative for both stereoisomers.
*
the isomerization barrier for free singlet methylcarbene is as low as 2.0 kcallmol. Thus, we conclude that the rearrangement of the metal ethylidene to an ethene complex, i.e. 11/Co+ lWCo+,is a facile process for the group VI11 metal cations in general. At first sight a comparison of the data given in Table 4 for the Co+ complex cations with those summarized in Table 5 for the Ni+ complexes reveals that the same mechanistic pathways are operative in the decomposition reactions of the complexes of both metal cations, though with different intensities. C-C bond activation is favored over dehydrogenation, but the amount of carbon monoxide generated from l/Ni+ and mi+is much lower as compared to the corresponding Co+ complexes. As indicated by the [Dllethene losses from 2e/Ni+, H/D exchange processes have t o be taken into
-
(15)Ma, B.;Schaefer, H. F. J. Am. Chem. SOC.1994,116,3539.
Schalley et al.
322 Organometallics, Vol. 14, No. 1, 1995 Table 6. Mass Differences (Am in amu) Observed in the Unimolecular Decomposition Reactions of the 2/Cr+ Isotopologue@b
Scheme 7 M’\
0
0
Am -2 2/Cr+ 2a/Cr+ 2b/Cr+ 2c/Cr+ 2d/Cr+ 2e/Cr+
-3
88 89 85 27 55 74 15 11 76
-28 7 6 9 6
-29
-30
-31
-42
-43
-44 -45
3 3 5
I
3 4
5
4
2
5
Cr+
2 2 1 2 2 2
a Intensities are expressed in x(products) = 100%. Intensities less than 1% are omitted. Since the methane and water losses are not discussed in detail, the corresponding data are omitted. Since for the Cr+ complexes no significant cidrrans differences are observed, the data for the trans isotopomers are given as representative for both stereoisomers.
Scheme 6 M’\ 0
2/M‘
account. Nevertheless, the rather high abundance of [Dslethene formation from 2e/Ni+ cannot only be due t o statistical H/D exchange processes. Therefore, the carbene mechanism as discussed for Co+ (Scheme 5) is also operative for the Ni+ complexes. Complexes of 1and 2 with Cr+. The complexes of 1 and 2 with Cr+ exhibit some clear differences as compared to those with the group VI11 transition metals (Table 6). For example, dehydrogenation corresponds t o the by far most intense unimolecular fragmentation of 1/Cr+ and 2/Cr+. Furthermore, in contrast to the group VI11 metal complexes 2c/Cr+ and 2d/Cr+ do not exhibit the same H f l D ratio (Table 3); thus, for Cr+mediated dehydrogenation an alternative mechanism must be operative. Moreover, hydrogen atoms from the methylene group of the cyclopropylring ((33)) contribute significantly to dehydrogenation. These findings can be rationalized by the mechanism depicted in Scheme 6. If path a, which leads from intermediate 13/M+ to ringopened 3/M+, is facile (and largely reversible) the hydrogen atoms attached originally to C(2)/C(3) will equilibrate prior to the remote functionalization. This holds true for the Fe+ complexes, as indicated by the same H f l D ratio for the two isotopomers 2c/Fe+ and 2d/Fe+; both precursors give rise to labeled 3-hexen-2oneme+ intermediates bearing one deuterium atom at the (w - 1)-position of the side chain. In contrast, if the formation of 3/M+ via step a is kinetically impeded, 13/M+ may explore a different route by, for example, bypassing reaction a and following path b. This “direct” activation of the o-Hatom in 13/M+will consequently result in a predominant loss of HD from 2c/Cr+ and likewise in a higher intensity for the H2 loss from 2d/
2/M‘
14/M’
Cr+. This mechanism is also in keeping with the prevailing HD loss from 2e/Cr+. Presumably, the lifetime of the insertion intermediate is smaller for the Cr+ complexes as compared to the group VI11 metal cations and, therefore, the direct route via is preferred for 2/Cr+. As far as the minor fragmentation channels are concerned, the absence of Cl80 loss (Am = 30) from the l*O-labeledcomplex 2a/Cr+ reveals that decarbonylation does not interfere with ethene loss. From the observation of [Dll- and [Dolethene losses from 2c/Cr+ we conclude that at least two mechanisms are operative. One involves remote functionalization (Scheme 3, M = Cr) via the intermediate 3/Cr+, and the other corresponds to the carbene mechanism (Scheme 5, M = Cr). In keeping with this interpretation, only [Dllethene is generated from 2d/Cr+, whereas 2c/Cr+ leads t o losses of [Dol- and [Dllethene and 2e/Cr+ to losses of [Dzl-and [Dslethene, respectively. The formation of carbene intermediates is not limited to ethene expulsion but is also involved in the loss of propene. As clearly shown by the data for the labeled complexes 2c/Cr+-2e/Cr+, the C3H6 moiety stems exclusively from the C(2)/C(3)positions of the cyclopropyl ring and the adjacent 2-methyl group. H/D exchange processes do not occur. These observations are easily rationalized in terms of the mechanism depicted in Scheme 7 (M = Cr). Here, 14/Cr+is formed as a central intermediate, from which propene is lost eventually. The insertion of Cr+ in the C(l)-C(2) bond of the cyclopropyl ring can be rationalized in terms of thermodynamic arguments. The cyclopropylicC(l)-C(2) and C(l)-C(3) bonds are lengthened and weakened by conjugation with the carbonyl group,16 such that C-C insertion of the metal ion is facilitated to some extent. Notably, only the Cr+ and Mn+ complexes give rise to this type of carbene mechanism, whereas the late-transition-metal cations from Fe+ through Cu+ do not mediate propene loss from 2/M+. Methane and water losses are observed as side reactions. Due to the low intensities and some interferences the corresponding data cannot be deconvoluted exactly. Therefore, these reactions are not discussed here.g Last but not least, ligand detachment is observed for the Cr+ complexes of 1and 2. Thus, the activation barriers for the decompositionreactions described above are of the same order of magnitude as the Cr+-ligand bond dissociation energy. Complexes of 1and 2 with Mn+. From the isotope distribution in the neutral fragments (Table 7) we can conclude that dehydrogenation and ethene formation (16)(a) Hoffmann, R. Tetrahedron Lett. 1970,2907.(b) Dill, J. Di Greenberg, A.; Liebman, J. F. J.Am. Chem. SOC.1979,101,6814. (c) Clark, T.;Spitznagel, G. W.; Klose, R.; Schleyer, P. v. R. J . Am. Chem. SOC.1984,106,4413.(d) Cremer, D.; Kraka, E. J. Am. Chem. SOC. 1985, 107,3811. (e) Slee, T. S. In Modern Models $Bonding and Delocalization; Liebman, J. F., Greenberg, A., Eds.;VCH: New York, 1988.
Activation of C-C and C-H Bonds by Metal(I) Cations
Organometallics, Vol. 14,No.1, 1995 323
Table 7. Mass Differences (Am in amu) Observed in the Unimolecular Decomposition Reactions of the 2/Mn+ Isotopologuee hm -2
2/Mn+ 2a/Mn+ 2b/Mn+ 2clMn+ 2d/Mn+ 2e/Mn+
41 41 45 30 30 6
-3
-4
16 16 29
