Addition of iminium salts to the reductively activated benzene in [Mn(4-C6H6)(CO)3]-

Full text of DOI:10.1021/ja970040b

J. Am. Chem. Soc. 1997, 119, 8365-8366
8365
Scheme 1
Addition of Iminium Salts to the Reductively
Activated Benzene in [Mn(η4-C6H6)(CO)3]-
Sang-Hee Ko Park, Steven J. Geib,† and N. John Cooper*
Department of Chemistry, UniVersity of Pittsburgh
Pittsburgh, PennsylVania 15260
ReceiVed January 6, 1997
ReVised Manuscript ReceiVed June 23, 1997
Iminium salts [R2CdNR2]+ are powerful electrophiles and
valuable reagents in organic synthesis,1 and their cationic nature
suggested that they might offer a solution to a problem we faced
in developing the chemistry of the benzene ligand in the Mn-
(-I) complex [Mn(η4-C6H6)(CO)3]- (1-); this is activated with
respect to protonation,2 [2 + 2] dimerization,3 and ketene
addition,4 but we have had surprisingly little success5 in attempts
to add simple carbon-centered electrophiles to give cyclohexa-
dienyl complexes complementary to those prepared (partly
because of interest in their potential applications in organic
synthesis) by addition of nucleophiles to [Mn(η6-C6H6)(CO)3]+
(2+).6,7 Our discovery, however, that partial naphthalenide
reduction of 2+ leads to addition of anionic 1- to cationic 2+
to give the cyclohexadienyl dimer [{Mn(CO)3}2{µ(η5-C6H6-
η5-C6H6)}]8 suggested that cationic electrophiles might add to
1- and led us to examine the addition of iminium salts to the
activated benzene (Scheme 1); this reaction provides the first
examples of the addition of an iminium salt to an unsaturated
ligand in a transition metal complex.
but a diffraction study14 established that in this case the imin-
ium salt had undergone an unprecedented addition to the
benzene ligand to give the exo-aminoalkyl-substituted cyclo-
hexadienyl complex [Mn(CO)3(η5-C6H6CPh2NMe2)] (4) shown
in Figure 1.
A solution of K1 in THF prepared by potassium naphthalenide
(KNap) reduction of a slurry of 2PF6 in THF at -78 °C9 was
10
added to a solution of [Ph2CdNMe2]BF4 (3BF4) in CH3CN
at -20 °C. The red-brown solution immediately turned red and
then yellow as it warmed to room temperature. After solvent
removal, a pentane soluble yellow adduct was isolated in 50%
yield (70% 1H NMR yield) following chromatography on basic
alumina. Iminium salts have been reported to add to the metal
centers of carbonylmetalates and other low-valent complexes
to give η1- or η2-aminomethyl complexes11 or their derivatives12
(or carbene complexes in the case of chloroiminium salts13),
The reaction is sensitive to the precise reaction conditions,
and it is particularly important that only 2.0 equiv of KNap be
used. The reducing capacity of even modest excesses persists
in the system (although the green color of naphthalenide anion
is not observed) and decreases the addition yield. Use of 2.5
equiv of naphthalenide, for example, leads to 4 in ca. 35% yield.
Iminium salts can be readily prepared from most ketones,
but it was not obvious that iminium addition to 1- would be a
general reaction. We have, however, established that the
dialkyliminium salt [(CH2)5CdN(CH2)4]BF4 (5BF4)15 adds to
the activated benzene in 1- on a 1 mmol scale to give the exo-
aminoalkyl complex [Mn(CO)3{η5-C6H6C(CH2)5N(CH2)4}] (6)
in 54% isolated yield. Eschenmoser’s salt, [H2CdNMe2]I, will
add similarly to 1-, although the yield of [Mn(CO)3(η5-C6H6-
CH2NMe2)] (7) is low (20%).
The exo stereochemistry of the reactions (confirmed for 6
and 7 by the observation of the characteristic 5-6 Hz coupling
between the vicinal H and the endo H) establishes that iminium
additions do not involve metal-mediated pathways (in contrast
with the endo addition of D+ to 1-)2 and raises an intriguing
question as to whether they involve a two-electron process (a
in Scheme 2) or an SET pathway (b in Scheme 2) in which
intermediate organic and organometallic radicals couple; both
pathways have been observed in other organometallic cation-
anion annihilation reactions.16
† Address crystallographic correspondence to this author.
(1) (a) Kleinman, E. F. In ComprehensiVe Organic Synthesis; Trost, B.
M., Fleming, I., Eds.; Pergamon: Oxford, U.K., 1991; Vol. 2, Section 4.1
and succeeding sections. (b) Iminium Salts in Organic Chemistry; Bo¨hme,
H., Viehe, H. G., Eds.; Advances in Organic Chemistry; Wiley: New York,
1976, Vol. 9, Part 1; 1979, Vol. 9, Part 2.
(2) Thompson, R. L.; Lee, S.; Rheingold, A. L. Organometallics 1991,
10, 1657.
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1982; Vol. 4, Chapter 29. (b) Semmelhack, M. F. In ComprehensiVe Organic
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Cooper, N. J. J. Am. Chem. Soc. 1996, 118, 4190.
(9) Reduction at -78 °C minimizes addition of 1- to 2+ during the
reduction.8
The SET option is particularly attractive in the case of the
(10) Hauser, C. R.; Lednicer, D. J. Org. Chem. 1959, 24, 46
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Sepalak, D. J.; Derveer, D. G. Van Organometallics 1985, 4, 1395.
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Organometallics 1995, 14, 1030.
diaryl addend 3+, since 3+ undergoes facile, reversible one-
(14) Crystal data: monoclinic space group P2(1)/c, Z ) 4, a ) 14.253-
(3) Å, b ) 10.320(2) Å, c ) 13.978(3) Å, b ) 90.58(2)°, V ) 2055.9(7)
Å3. Refinement on F2 converged at RF ) 3.98%, wRF (all data) ) 10.10%,
2
with GOF(F2) ) 1.034.
(15) From the procedure of: Leonard, N. J.; Paukstelis, J. V. J. Org.
Chem. 1963, 28, 3021.
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4491.
S0002-7863(97)00040-1 CCC: $14.00 © 1997 American Chemical Society

8366 J. Am. Chem. Soc., Vol. 119, No. 35, 1997
Communications to the Editor
formation of 4. Some other potential hydrogen atom sources
cannot be premixed with 1- because they are too acidic;
thiophenol, for example, protonates 1- to form [Mn(CO)3(η5-
C6H7)], and in this case 1- was added to a mixture of 3BF4 and
PhSH: neither protonation of 1- nor the ability of PhSH to act
as a hydrogen atom source interfered with iminium addition to
1-.
The most sensitive competition experiments use intramo-
lecular competition between a reaction sequence and a charac-
teristic side reaction of the substrate derived radical, such as
opening of a strained hydrocarbon ring,18 and we examined such
a sequence using the cyclopropyl-substituted iminium salt
[CH2CHPhCHCPhdNMe2]BF420-22 (9BF4). This added to 1-
to give an exo-aminoalkyl product [Mn(CO)3{η5-C6H6CPhNMe2-
(CHCH2CHPh)}] (10), obtained in 35% yield as an equal
mixture of two diastereomers. The more polar of these was
isolated, fully characterized, and shown by NMR to contain an
intact cyclopropyl ring.
Our failure to obtain evidence for radical intermediates leads
us to prefer a two-electron pathway for iminium additions to
1- (although it is impossible to rule out an SET mechanism
completely), and this is supported by the observation that 3+
and 5+ undergo similar additions to 1- despite the fact that 5+
should be ca. 0.5 V harder to reduce than 3+.17,23
Figure 1. Molecular structure of [Mn(CO)3(η5-C6H6CPh2NMe2)] (30%
probability ellipsoids). Selected bond lengths (Å): Mn-C(2) ) 2.250-
(2), Mn-C(3) ) 2.136(3), Mn-C(4) ) 2.119(3), Mn-C(5) ) 2.131-
(3), Mn-C(6) ) 2.216(2), C(1)-C(10) ) 1.588(3), C(1)-C(2) )
1.507(3), C(2)-C(3) ) 1.379(4), C(3)-C(4) ) 1.403(4), C(4)-C(5)
) 1.400(4), C(5)-C(6) ) 1.392(4), C(6)-C(1) ) 1.499(3). Dihedral
angle between C(2),C(3),C(4),C(5),C(6) plane and C(1),C(2),C(6) plane
) 35.7(2)°.
Scheme 2
The absence of evidence for involvement of the “[Mn(η6-
C6H6)(CO)3]” radical in the reaction of 1- with 3+ emphasizes
the contrast between the chemistry of 2+ and that of [Fe(η5-
C5H5)(η6-C6H6)]+. Both are monocationic arene complexes of
3d metals, but the Mn cation undergoes two-electron reduction
to 1- (containing an electron-rich reactive benzene ligand2-4,8),
while in the Fe system, the corresponding anion decomposes
rapidly by loss of [C5H5]-.24 Conversely, a central feature of
the Fe chemistry is reduction of the cation by one electron to
the 19-electron complex [Fe(η5-C5H5)(η6-C6H6)]25,26 (with
chemistry dominated by its radical character), while the corre-
sponding Mn complex is unstable with respect to dispropor-
tionation; the two systems offer access to complementary and
fascinating benzene reaction manifolds.
Acknowledgment. We thank the National Science Foundation for
financial support and the reviewers of an early version of this manuscript
for helpful comments.
electron reduction to the Ph2CNMe2 radical (8).17 We have
determined that this couple is at -1.25 V (vs internal ferrocene/
ferrocenium) in CH3CN on a hanging mercury drop electrode,
while the partially reversible two-electron reduction of 2+ to
1- is at -1.26 V under these conditions, so that electron transfer
from 1- to 3+ is a thermodynamically accessible mechanistic
step. The distinctive blue violet color of 8 is not observed
during 2+ addition to 1-, establishing that 8 can only be a
transient intermediate if it is present.
We have explored the possibility that 8 and “[Mn(η6-C6H6)-
(CO)3]” radicals are transient intermediates in 3+ addition to
1- by carrying out competition experiments designed to trap
an intermediate organic or organometallic radical.18 The choice
of traps was restricted by the reactivity of many reagents with
one of the starting materials; Bu3SnH,19 for example, reacts with
3+ as a hydride reductant. It does not, however, react with 1-,
and we were able to mix Bu3SnH with 1- and add the mixture
to 3BF4. Under these conditions Bu3SnH did not interfere with
Supporting Information Available: Spectroscopic and analytical
data for 4, 6, 7, and 10 and crystallographic details for 4 (9 pages).
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JA970040B
(20) Johnson, C. R.; Lockard, J. P.; Kennedy, E. R. J. Org. Chem. 1980,
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(21) Chosen because the Ph group on the unsaturated carbon makes 9+
as easy to reduce as possible given the need to introduce the cyclopropyl
substituent, while the Ph group on the ring should accelerate ring opening
of any intermediate one electron reduction product and compensate for the
stabilizing effect of the Ph group on the C bound to N.
(22) The only identifiable reduction isolated after one-electron naphtha-
lenide reduction of 9BF4 and a chromatographic workup is the ketone
PhCH2CH2CH2C(O)Ph: this is the hydrolysis product of the acid labile
enamine PhCH2CH2CHdC(NMe2)Ph, the anticipated product of cyclopropyl
ring opening of the one-electron reduction product of 9+.
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Chem. Soc. Jpn. 1983, 56, 2442.
(24) Arford, D. J.; Cooper, N. J. Unpublished observations.
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