2857
J . A m . Chem. SOC.1989, 1 1 1 , 2857-2861
Chain-Length-Dependent Ligand Effects on the Direction of Fe+-Mediated Activation and Cleavage of CC Bonds in Alkenyl Nitriles in the Gas Phase+ Tilmann Priisse, Thomas Drewello, Carlito B. Lebrilla, and Helmut Schwarz* Contribution f r o m the Institut f u r Organische Chemie, Technische Uniuersitat Berlin, 0 - 1 0 0 0 Berlin 12, West Germany. Received August 9, 1988
Abstract: The study of mass-selected complexes of bare Fe+ with unsaturated nitriles, CH,(CH,),CH=CH(CHZ),CN, in the gas phase permits a detailed understanding of the chain-length-dependent interaction of the two functional groups with the transition metal ion and its implications for the direction of allylic carbon-carbon bond cleavage processes. This fundamental reaction, which proceeds via a metal-containing cyclic intermediate, is operative only for systems with m > 2, and it involves, without exception in all cases studied, the “exocyclic” allylic C C bond (Le., loss of alkenes from the CH3(CH2), part of the nitrile). Double-bond migration constitutes a side reaction. In those cases in which this reaction is observed, the double bond preferentially migrates away from the C N group. The actual nature of bidentate complexation (and, in particular, the question of “side-on” versus “end-on” complexation of the nitrile group) is dependent upon the chain length (CH,),. For m = 2, 3 the data suggest a bidentate complex in which both the C C double bond and the C N triple bond act as 7 donors (21); however, for larger chain lengths ( m 2 4) the bidentate complex may be described by an “end-on” complexation of the C N group to the metal ion which still interacts with the C C double bond (23). For systems in which the two functional groups are separated by one or no CHz unit, the Fe’ does not “feel” the presence of the double bond. The gas-phase chemistry of these systems ( m = 0, 1) is practically identical with that of the previously described saturated alkyl nitriles for which “end-on” complexation (22) induces functionalization of remote C H bonds. The experimental findings are supported by the study of labeled isotopomers.
There exists ample experimental evidence for the highly sitespecific reactions of bare transition metal ions, e.g., Fe(1) ions, with olefins.’ These studies do not only provide a novel approach for the location of double (and even triple)2 bonds, but, more interestingly, they also shed light on the intriguing problem of the activation of C H and CC bonds, a topic of considerable scientific and technological i n t e r e ~ t . ~For . ~ the bare transition metal ion mediated activation and cleavage of C C bonds in olefins, it is now generally agreed upon that the most likely mechanism consists of the following steps (Scheme I). The initially formed complex 1 4 3 5 rearranges by oxidative addition of the allylic C C bond to the metal ion (1 2). This step is followed by 0-hydrogen transfer to generate, via the olefin/allyl complex 3, the bis(o1efin) complexes 4a and/or 4b. Upon collisional activation (CA),6 the bis(o1efin) complexes dissociate by loss of the ligand7 having the smaller metal ion affinity. In practice, it turned out that it is the smaller olefin ligand which is eliminated from 4a,b, and it has been suggested by Allison et aI.* that a correlation may exist between the proton affinity (PA) of a ligand and its metal ion affinity. Depending on the internal energy of 4a,b gained in their formation, the olefin ligand detachment may even occur without collisional activation (metastable ion (MI) decomposition). An obvious, yet unaddressed problem with regard to the site specifty of the oxidative addition is the following one. Let us assume that one can generate a bidentateg complex of type 5 in which the functional group X serves as an “anchor”’o to which the metal ion Mf is strongly bound (Le., the metal ion affinity of M+ to X is larger than that of M+ to the C C double bond). Let us further assume that the reactivity of the ligated metal ion M+ in 5 is still such that activation of CC bonds may occur. If so, the next question to address is whether, in dependence of the chain length separating the olefin unit from the functional group X, the “exocyclic” (Scheme 11: process a) or the “endocyclic” allylic C C bond (process b) will be activated. Depending on the binding energies of the various ligands generated in the course of the oxidative addition of C C bonds to the (complexed) metal ion M+ (and, of course, on the chain length “m”),the detachment of several neutral species’ is conceivable, thus giving rise to the generation of the likely ionic species 6, 7 and 8 (Scheme 11). The situation outlined in Scheme I1 is not as hypothetical as one might assume. On the contrary, in the course of a detailed
Scheme I. Fe+-Induced Cleavage of Allylic CC Bonds According to Reference 1 Fe
1
-
‘Presented in part at the Annual Meeting of the German Mass Spectrometry Society, Wiirzburg (FRG), 25-27 May 1988.
0002-7863/89/1511-2857$01.50/0
‘
oxid. addition
R’
.........Fe+>R2
.:
k. yH,,/‘
R’
. ...... .’ .. .. ’
3
c Rl)
F+e , . . .
.......
R’
3.
p
4a
4b
of the gas-phase chemistry of unbranched, saturated acyclic nitriles ( R C N ) with bare Fe’, it was inter alia demon(1) (a) Larsen, B. s.;Ridge, D. P. J . A m . Chem. SOC.1984, 106, 1912. Peake, D. A.; Gross, M. L.; Ridge, D. P. Ibid. 1984, 106, 4307. (c) Peake, D. A,; Gross, M. L. Anal. Chem. 1985, 57, 115. (b)
0 1989 American Chemical Society
2858
J . A m . Chem. Soc., Vol. 1 1 1 , No. 8, 1989
Priisse et al.
Scheme 11. Possible Neutral Ligand Detachment and Ionic Products, Generated by the Activation of the "Exocyclic" (Path a) or 'Endocyclic" (Path b) Allylic CC Bonds of 5
@
0
@
0
- R'CH-CH,
- CH,=CH(CH,),X
- R'(CH,),CH
=CHCH,
t M '
Scheme 111. Ethane Loss from Saturated RCN/Fet as a Combination of Reductive Elimination of H2 and Ethylene
1"' i"' iH2
-
I
H H
"
path@"
CH
10
9
11 -
Table I. Metastable Ion (MI) Mass Spectra of RCN/Fet Complexes neutral precursor CHI(CH,),CH= CH(CH2),CNb n m H2 CH4
ligand detachment' C4Hs
C2H4
C3H6
m 28
24
22
3
p J i
8
3
12
2
4
13
13 14
3 4
4 4
12 8
2 1
