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Multiple УRemote FunctionalizationФ of Different Sites in Flexible Molecules by УAnchoredФ Bare FeI Ions.

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Multiple “Remote Functionalization”
of Different Sites in Flexible Molecules
by “Anchored” Bare Fe’ Ions**
By Gregor Czekay, Karsten Eller, Detlef Schroder,
and Helmut Schwarz*
Dedicated to Dr. Giinther Ohloff
on the occasion of his 6Sth birthday
The direct functionalization of remote C-H or C-C
bonds, i.e. several carbon atoms away from an activating
group, represents a great challenge. While such processes are
common to enzymes, which “anchor” a functional group
and geometrically select a specific segment of the substrate,
only a few cases are reported“] for relatively rigid molecules
where a similar principle seems to be operative. 5reslow[’”~
has coined the term “remote functionalization” for this
method of coordination of a functional group followed by
selective reactions at sites away from the complexed functionality.
We[’] and later others[’] have demonstrated that a similar
principle holds true in the gas phase for quite a variety of
flexible organic substrates including aliphatic nit rile^,^^*^]
isonitriles, 141 aminesJ5]alcohols,I61ketones,[’] alkynes [*I and
allenes,’’] respectively. For example, the chemistry of the
Fee-complexes of unbranched nitriles e.g. 1 (Scheme 1) is
unique, in that selective C-H activation occurs only at positions remote from the C N group. The results of detailed stud-
,
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Scheme I
[*] Prof. Dr. H. Schwarz, DipLChem. G. Czekay, Dip].-Chem. K. Eller,
Dip].-Chem. D. Schroder
Institut fur Organische Chemie der Technischen Universitat
Strasse des 17. Juni 135, D-1000 Berlin 12
[**I This work was supported by the Volkswagen-Stiftung, the Fonds der
Chemischen Industrie, and the Gesellschaft von Freunden der Technischen
Universitat Berlin.
Angew. Chem. Int. Ed. Engl. 28 (1989) Nr. 9
ies[’] are compatible with a mechanism in which the reaction
commences with the formation of an “end-on” complex 1.
For nitriles containing four to seven carbon atoms, the “anchored” transition-metal ion, Fee, activates exclusively the
C-H bond of the terminal methyl group (R = H) by oxidative
addition (1 + 2). The insertion is followed by competitive
P-hydrogen transfer (2 + 3) or P-cleavage of the C-C bond
(2 + 4) to generate intermediates from which eventually neutral ethylene and molecular hydrogen are eliminated. If the
carbon chain is lengthened, activation of internal C-H bonds
is also observed. While the previous
permitted a
detailed insight into many mechanistic details, no examples
are known for the operation of multiple “remote functionalizations” of different sites in flexible molecules. Here, for the
first time, evidence will be presented for the existence of such
a reaction, which is also without precedence in condensed
phase chemistry.
The metastable ion (MI) mass spectrum of mass-selected
(CH,(CH,),),CHCN/Fe@ (5a -Fee)[”’ contains in addition to signals corresponding to the loss of neutral H, (46%),
C,H4 ( ~ O Y OC,H,
),
(ISYO) and C,H, (3%), two signals
which can be ascribed to the production of C,H, and C,H,
( 3 YOeach). While the genesis of molecular hydrogen and of
the alkenes“ can be accounted for in terms of the traditional concept,[2]there exist two pertinent questions related to
the formation of C,H, and C,H,: (i) Do the neutral compounds correspond to genuinely formed alkanes C,H,,+ or
are the observed mass differences due to the combined production of C,H,,H,/H,? (ii) If the latter is true, are both
species formed from the same alkyl chain, or do the neutral
fragments originate from both alkyl chains, and what sequence prevails?
The experimental findings are unambiguously in favor of
the second alternative. This is evidenced by a study of the
Fee-complex of (CD,(CH,),),CHCN (5 b). If ethane and
propane would be generated from 5b according to the established pathway,[’,’ complex 5b-Fe@should undergo losses
of CD,CH, (Am = 33) and CD,CH,CH, (Am = 47). These
two mass differences are not observed; rather, one finds
Am = 33 and 48. Moreover, if one selects, in a tandem experiment,“’] the [M - 28Ie and [M - 42Ie species unimolecularly generated from Sa-Fee and studies their MI mass spectra, the only product formed corresponds to the production
of H,. While the latter experiment leaves no doubt that
actually no alkanes but rather alkenes and molecular hydrogen are formed,[14]the labeling experiment proves for the C,
neutral (and, by analogy, the same is presumed for the C,neutral) that both alkyl chains are involved in the Fee-mediated C-H and C-C bond activation processes. For the formation of C,H,D,, i.e. sequential losses of CD,CH = CH,
and HD, a mechanism is depicted in Scheme 2.
While for the nitrile complex Sa-Fe@ the double remote
functionalization constitutes a relatively minor decomposition path, the situation is very different for the Fee-complex
of 5-nonanone (12a-Fee). The MI mass spectrum of this
complex consists of three signals which are due to the losses
of H, (31 YO),C,H, (17%) and C,H, (52%). The study[”]
of the isotopomers 12a-h leaves no doubt that molecular
hydrogen and ethylene are formed in a process analogous to
Scheme 1 (“remote functionalization”). However, the “alkane” generated from 12a-Fee does not correspond to authentic ethane; rather it is due to the combined eliminations of
ethylene and molecular hydrogen which are formed from the
two alkyl chains in an unambiguous manner involving the
o/(w- 1) and o’/(o’- 1) positions. This is evidenced by the
data given in Table
In addition, tandem experiments
with the [M - C,H,]@- and [M - H,]@-species prove
0 VCH Verlagsgesellschaft mbH, 0-6940 Weinheim, 1989
OS70-0833j89jO909-1277 $02.5OjO
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CD3
9
10 (R’ = C3H7)
0
I1
’80
I,
H9C4CC4He
H9C4C(CH2)3CD3
12b
12c
0
I1
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n
H7C$2CCCD2C3H7
H9C4C(CH,),C2D5
12e
12f
F)
12h
Table 1. Unimolecular elimination of molecular hydrogen, ethylene and
“ethane” [a] from the Fee-complexes of 5-nonanone isotopomers 12a-h.
.2
3
4
28
29
30
31
32
33
34
12a
12b
12c
12e
12f
12g
12h
18
6
23
-
IS
-
-
18
-
1s
6
31
25
-
-
-
-
-
17
20
10
16
-
-
-
52
55
-
-
-
26
12
12d
-
-
17
3
20
-
7
23
22
8
27
25
-
-
-
-
-
-
60
-
27
-
-
-
-
-
34
-
40
16
-
-
55
-
-
28
-
23
-
[a] See text; “ethane” actually corresponds to the consecutive losses of ethylene
and molecular hydrogen. [b] Am corresponds to the mass differences with regard to the Fee-complexes of 12a-h; data are given in %, Z fragment
ions = 100%.
once more that the reaction follows the sequence
12a-Fe@+ C,H, --f H, and not the reverse one.
To the best of our knowledge, the present results demonstrate for the first time beyond any doubt that an “anchored” transition-metal ion is capable of consecutively activating different sites of flexible molecules.
Received: May 8, 1989 [Z 3328 IE]
German version: Angew. Chem. 101 (1989) 1306
[l] a) R. Breslow, Chem. SOC.Rev. l(1972) 553; b) R. Breslow, Acc. Chem.
Res. 13 (1980) 170; c) R. Breslow, A. Adams, T. Guo, J. Hunger, Lect.
Hezerocyel. Chem. 9 (1987) 43; d) U. Kerb, M. Stahnke, P. E. Schulze, R.
Wiechert, Angew. Chem. 93 (1981) 89; Angew. Chem. Int. Ed. Engl. 20
(1981) 88.
121 a) C. B. Lebrilla, C . Schulze, H. Schwarz, J. Am. Chem. Soc. 109 (1987) 98;
b) T. Drewello, K. Eckart, C . B. Lebrilla, H. Schwarz, Int. .
I
Mass Spec-
1278
0 VCH
Scheme 2.
trom. Ion Processes 76 (1987) R1; c) C. B. Lebrilla, T. Drewello, H.
Schwarz, ibid. 79 (1987) 287; d) C. B. Lebrilla, T. Drewello, H. Schwarz, J.
Am. Chem. Sor. 109 (1987) 5639; e) T. Priisse, C . B. Lebrilla, T. Drewello,
H. Schwarz, ibid. f10 (1988) 5986; f) T. Priisse, T. Drewello, C. B. Lebrilla,
H. Schwarz, ibid. ffl(1989) 2857; g) G. Czekay, T. Drewello, H. Schwarz,
ibid. I f f(1989) 4561 ;h) For review: H. Schwarz, Acc. Chem. Res., in press.
[3]. R. M. Stepnowski, J. Allison, Or~anometallics7 (1988) 2097.
.
[4] K. Eller, C. B. Lebrilla, T. Drewello, H. Schwarz, J. Am. Chem. SOC.110
(1988) 3068.
151 a) S. KarraO. K. Eller, C. Schulze, H. Schwarz, Angew. Chem. 101 (1989)
634; Angew. Chem. Inl. Ed. Engl. 28 (1989) 607. h) S. KarraD, T. Priisse, K.
Eller, H. Schwarz, J. Am. Chem. Soc., in press.
[6] T. Priisse, H. Schwarz, OrganometaNics, in press.
171 D. Schroder, H. Schwarz, unpublished.
[8] a) C. Schulze, H. Schwarz, D. A. Peake, M. L. Gross, J. Am. Chem. Soc.
f 0 9 (1987) 2368; b) C. Schulze. H. Schwarz, J. Am. Chem. SOC.110 (1988)
67; c) C. Schulze, T. Weiske, H. Schwarz, OrganomeraNics 7 (1988) 898.
[9] N. Steinriick, H. Schwarz, OrganometaNics 8 (1989) 759.
[lo] The experimental set-up has been described in detail in earlier papers [2,
4-91, Briefly, Fee is generated from Fe(CO), by electron impact ionization (100 eV) and reacted with the organic substrate of interest in a chemical ionization source of a VG Instruments ZAB-HF-3F triple sector mass
spectrometer. (For a description of the machine, see: a) T. Weiske, Dissertation D83, Technische Universitat Berlin (1985); b) J. K. Terlouw, T.
Weiske, H. Schwarz, J. L. Holmes, Org. Mass. Spectrom. Zf (1986) 665).
The organometallic complexes corresponding to RCN/Fee and ketoneFee, respectively, and having 8 keV kinetic energy, are mass-selected and
focused with B(1)E (B stands for magnetic and E for electrostatic sector).
Unimolecular reactions occurring in the field-free region between E and
B(2) were recorded by scanning B(2). MS/MS/MS experiments, performed
in order to prove the multi-step decomposition of 5a-Fee and 12a-Fee,
were conducted by mass-selecting 5a-Fee and 12a-Fee by means of B(1);
the products due to metastable losses of H, or alkenes were “isolated” by
using E; fragment ions originating from further dissociations of [M - HJe
or [M -C,H,Je were recorded by scanning B(2). The signal-to-noise ratio
was improved by using signal averaging techniques employing the VG 1l /
250 data system.
[ l l ] A detailed discussion of the mechanisms by which these species are formed,
is given in: G. Czekay, T. Drewello, K. Eller, W. Zummack, H. Schwarz,
OrgnnometaNics, in press.
[12] For the transition metal ion mediated alkane formation in most cases
reported it is, strictly speaking, not possible to distinguish between the two
reaction sequences, i.e. 1) insertion of the metal ion in a C-C bond followed by p-hydrogen transfer, or 2) oxidative addition of a C-H bond
followed by 8-alkyl migration to the metal center. Both variants generate
the same intermediate from which eventually reductive elimination of RH
occurs.
[13] a) F. W. McLafferty (Ed.): Tandem Mass Spectrometry, Wiley Interscience,
New York (1983); b) K. L. Busch, G. L. Glish, S. A. McLuckey: Mass
SpectrometrylMass Spectrometry: Techniquesand Applications of Tandem
Mass Spectrometry, VCH, Weinheim (1988).
[14] The reverse sequence, i.e. loss of H, followed by elimination of
RCH = CH, (R = H, CH,), does not contribute to the formation of “alkanes”. This is evidenced by studying the mass-selected [M - 2Ie species
formed from %Fee; n o signals due to loss of RCH = CH, are present in
this spectrum.
[15] D. Schroder, Dip/oma Thesis, Technische Universitat Berlin (1989).
[16] A full discussion of these and other data, together with a detailed analysis
of the kinetic isotope effects associated with the generation of molecular
hydrogen, ethylene and “ethane”, is given in [7, 151.
~~
H,CHDC(~,C),~(CH,),CHDCH,
Am[b]
11
Verlagsgesellschaft mbH. 0-6940 Weinheim. 1989
0570-0833j89/0909-1278 $02.50/0
Angew. Chem. Int. Ed. Engl. 28 (1989) Nr. 9
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