Michael addition to α,β-unsaturated arene ruthenium(II) cyclopentadiene complexes: Endo nucleophilic addition

Article abstract of DOI:10.1039/a707386k

Certain carbon and sulfur nucleophiles add to the β-terminus of the styrene system in α,β-unsaturated arene ruthenium(II) cyclopentadiene complexes, and unexpected endo addition occurs preferentially in some cases.

Full text of DOI:10.1039/a707386k

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Michael addition to a,b-unsaturated arene ruthenium(ii) cyclopentadiene
complexes: endo nucleophilic addition
Robert M. Moriarty,*a† Livia A. Enache,a Richard Gilardi,b George L. Goulda and Donald J. Winka
a Department of Chemistry, University of Illinois at Chicago, Chicago, IL 60607-7061, USA
b Laboratory for the Structure of Matter, Naval Research Laboratory, Washington, DC 20375-5341, USA
C(2C)
C(3C)
C(5C)
Certain carbon and sulfur nucleophiles add to the
b-terminus of the styrene system in a,b-unsaturated arene
ruthenium(II) cyclopentadiene complexes, and unexpected
endo addition occurs preferentially in some cases.
C(4C)
N(9C)
C(1C)
C(9C)
Ru
Metals in either their neutral (Cr0, Fe0, Mo0) or charged state
(FeII, RuII, PdII, OsII, CoII) activate complexed unsaturated
ligands towards nucleophilic addition.1 This behaviour is due to
net inductive withdrawal of electron density from the ligand and
stabilization of the anionic charge resulting from addition of the
nucleophile. The addition of the nucleophile occurs on the exo
face of the ligand, distal to the metal.2 Within the context of this
direct nucleophilic addition to the complexed ligand, one might
anticipate that conjugate addition could likewise occur in a,b-
unsaturated ligated systems such as styryl or 1,2-dihydrona-
phthalene, and indeed this has been observed; in both cases
C(9)
C(7)
C(11)
C(8)
C(2)
O
C(3)
C(1)
C(4)
C(10)
C(5)
C(12)
C(6)
Fig. 1 X-Ray structure of compound 2c
to endo addition and exo protonation and differs from the result
observed for conjugate addition in two cases in which the
stereochemistry was determined.3b,5b A similar preference for
the endo addition was determined from combined NMR and
X-ray studies of other products in our study (Scheme 1). An
X-ray of the major component of product 2b also shows endo
selectivity. The major component of product 4 was the
‘expected’ exo.
How is one to explain endo nucleophilic addition? In a direct
displacement upon a metal-complexed aromatic ring, basically
an SN2 type mechanism, the nucleophile attacks at the carbon
atom undergoing displacement forming a new bond to that
center (with inversion) with concomitant cleavage of a bond to
the metal, i.e. the ligated metal is the ‘leaving group’.10 This
description requires exo attack. Conjugate addition of the type
studied here is controlled by the stereoelectronic effects of the
SN2A reaction.11 This process occurs with syn stereochemistry,
although it has not been recognized that the carbon–metal bond
could constitute the leaving group [Fig. 2(a)]. Fig. 2(b) shows
the orbital representation which reveals the LUMO of the
complex and the preference for syn addition.
The reactions were generally slower for 1b as compared to 1a
(rate retarding effect of OMe) and slower still for 1c (steric
effect of Cl methyl group). With KCN (5 equiv.) in DMF–H2O
in the presence of NH4Cl (3 equiv.), the conditions required for
the complete disappearance of the starting material for 70 °C
overnight for 1a ? 2a and 90 °C overnight for 1b ? 2b and 1c
? 2c. The yields of endo/exo mixtures of isomers after
chromatographic separation on silica gel (acetone–CH2Cl2 1:4)
were 86 (endo:exo 1:1), 85 (endo:exo 2:1) and 82%
(endo:exo 3:1) respectively. The reactions with thiophenol
6
h -chromium(0) complexes of the aromatic ring were used. In
6
the simplest case, tricarbonyl(h -styrene)chromium,3 this type
of reaction occurred in 7–30% yield, but with a discouraging
mixture of side-products. Better synthetic results were obtained
by Semmelhack and in the case of addition of LiCMe2CN upon
6
tricarbonyl(h -1,2-dihydronaphthalene)chromium.3b,4 The ster-
eochemistry of the product was proposed to correspond to exo
addition, on the basis of a 1H NMR spectrum of the
decomplexed ligand.4a Two other examples of conjugate
addition to arene-Cr(CO)3 complexes were reported.5
Because of our interest in nucleophilic displacement reac-
6
5
tions upon (h -arene)(h -cyclopentadienyl)ruthenium(ii) com-
plexes,6 we studied the Michael addition in this series with a,b-
unsaturated arene ligands. The objectives were to make the
reaction more synthetically useful, partly as the rate of the
nucleophilic displacement, via addition-elimination, on aryl
halides increases in the order Cr0(CO)3 < Mo0(CO)3 << CpFe+
< Mn+(CO)3,7 and also to establish the stereochemistry of the
addition relative to the result observed with the Cr(CO)3 group.
The substrates and transformations are shown in Scheme 1.8
Pure endo forms of products 2b and 2c, and the exo form of
4 could be separated via fractional recrystallization from
acetone–water and the X-ray structures of the major isomers
could be determined.‡
The X-ray structure of the major product from reaction of 1c
with KCN is in Fig. 1. The Cl Me and C2 CN are syn and on the
same face as the metal. This highly strained array corresponds
R2
R2
X
(a)
(b)
Nucleophile
R1
R1
–
–
CpRu]+PF6
CpRu]+PF6
Nu
Ru
1a R1 = R2 = H
b R1 = OMe, R2 = H
c R1 = OMe, R2 = Me
2a R1 = R2 = H, X = CN
endo:exo 1:1
endo:exo 2:1
endo:exo 3:1
endo:exo 3:1
endo:exo 5:4
endo only
b R1 = OMe, R2 = H, X = CN
c R1 = OMe, R2 = Me, X = CN
3a R1 = R2 = H, X = SPh
b R1 = OMe, R2 = H, X = SPh
c R1 = OMe, R2 = Me, X = SPh
Fig. 2 (a) The SN2A type mechanism illustrating endo conjugate nucleophilic
addition to styrene-type substrates complexed to RuII. (b) Simple orbital
diagram which represents LUMO calculated from ZINDO for the
Scheme 1
CpRu(C10H10)
+ model.
Chem. Commun., 1998
1155

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O
A. G. Pearson, Metallo-Organic Chemistry, Wiley, New York, 1988,
pp. 163–389.
–
S+ CH2
2 M. R. Churchill and R. Mason, Proc. R. Soc. Lond., A, 1964, 279, 191;
A. N. Nesmeyanov, N. A. Vol’kenau, L. S. Shilovtseva and V. A.
Petrakova, Izv. Akad. Nauk., Ser. Khim., 1975, 1151; M. F. Semmel-
hack, H. T. Hall, Jr., R. Farina, M. Yoshifuji, G. Clark, T. Bargar, K.
Hirotsu and J. Clardy, J. Am. Chem. Soc., 1979, 101, 3535; in a study
involving benzylic substitution and nucleophilic addition to the benzene
DMSO, room temp.
80%
MeO
MeO
–
–
CpRu]+PF6
CpRu]+PF6
endo:exo 1:5
1b
4
Scheme 2
6
ring in cyclopentadienyl(h -tetrahydronaphthalene)iron(ii) complexes,
Grundy and collaborators were led to believe that “the Fe(Cp) group
may be less effective in ‘face blocking’ of arene nuclei than is the
Cr(CO)3 fragment and is unlikely to be able to exert sufficient influence
to direct a first methyl group stereospecifically exo” (S. L. Grundy,
A. R. H. Sam and S. R. Stobart, J. Chem. Soc., Perkin Trans. 1, 1989,
1663).
3 (a) G. R. Knox, D. G. Leppard, P. L. Pauson and W. E. Watts,
J. Organomet. Chem., 1972, 34, 347; (b) M. F. Semmelhack, W. Seufert
and L. Keller, J. Am. Chem. Soc., 1980, 102, 6584.
4 (a) M. F. Semmelhack, Organic Synthesis Today and Tomorrow,
Proceedings of the 3rd IUPAC Symposium on Organic Synthesis,
Madison, Wisconsin, U.S.A., 15–20 June 1980, pp. 63–69; (b) M. F.
Semmelhack, Ann. N.Y. Acad. Sci., 1977, 295, 36.
5 (a) M. Uemura, T. Minami and Y. Hayashi, J. Chem. Soc., Chem.
Commun., 1984, 1193. This paper reported the Michael addition of
(1.5 equiv.) were carried out in NMR tubes in CD3CN in the
presence of Hunig’s base (Pri2NEt). Judging by the disappear-
ance of the signals of the starting material in the NMR spectrum,
substrate 1a reacted completely in 30–45 min when 2 equiv. of
base was used at 45 °C, and overnight with only 0.5 equiv. of
Hunig’s base at 25 °C. Substrate 1b reacted completely at 45 °C
overnight in the presence of Hunig’s base (0.5 equiv.), and
substrate 1c reacted in only ca. 80% when treated with Hunig’s
base (0.5 equiv.) at 60 °C for 36 h. The observed endo:exo
ratios were: 3a (3:1), 3b (5:4) and 3c (endo only).
A more advanced example is the Corey–Chaykovsky cyclo-
propanation in which dimethyloxosulfonium methylide9 is
used in a reaction which occurs at room temp. within minutes
(1b ? 4) (complete disappearance of the starting material,
80% isolated yield after chromatography, endo:exo 1:5,
Scheme 2).
LiC(CN)MeCOCHMeOEt
to
tricarbonyl(5-methoxy-1,2-dihy-
dronaphthalene)chromium. Exo addition was assumed. (b) P. Bloem,
D. M. David, L. A. P. Kane-Maguire, S. G. Pyne, B. W. Skelton and
A. H. White, J. Organomet. Chem., 1991, 407, C19 and references cited
therein. These workers studied the addition of MeLi to tricarbonyl(N-
benzylidene-1-methylaniline)chromium. Exo addition was determined
by X-ray analysis.
6 R. M. Moriarty, Y.-Y. Ku and L. Guo, J. Chem. Soc., Chem. Commun.,
1988, 1621; R. M. Moriarty, Y.-Y. Ku and U. S. Gill, J. Chem. Soc.,
Chem. Commun., 1987, 1493; R. M. Moriarty, Y.-Y. Ku and U. S. Gill,
Organometallics, 1988, 7, 660; R. M. Moriarty and U. S. Gill,
organometallics, 1985, 5, 253; U. S. Gill and R. M. Moriarty, Synth.
React. Inorg. Met.-Org. Chem., 1986, 16, 485; U. S. Gill and R. M.
Moriarty, Synth. React. Inorg. Met.-Org. Chem., 1986, 16, 1103; R. M.
Moriarty, Y.-Y. Ku and U. S. Gill, J. Chem. Soc., Chem. Commun.,
1987, 1837.
How does one reconcile endo addition with the observed exo
addition of Semmelhack and others?3b,5a These workers used Li
carbon anions and the conjugate addition in these cases is
reversible.12 Accordingly, the least hindered exo (cis-1,2)
product perdominates. In contrast, our cyanide addition method
involving in situ NH4Cl/H2O insures formation of the kinetic
product, by immediate protonation at the benzylic position.
Kinetic control is also possible in the addition of PhSH in the
presence of less than 1 equiv. of base. In agreement with the
interpretation, cyclopropanation (1b ? 4) yields the exo
product in 5:1 ratio because of the reversibility associated with
carbanion addition in the absence of a protic acid.9,13 Semi-
empirical quantum calculations (ZINDO, INDO1 level basis
7 A. C. Knipe, S. J. McGuinness and W. E. Watts, J. Chem. Soc., Perkin
Trans. 2, 1981, 193.
5
6
set)14 on a (h -cyclopentadienyl)(h -1,2-dihydronaphthalene)-
Ru cation model complex, as well as its Fe substituted analog,
indicate that a nucleophilic attack at the endo face of the
conjugated p-system is slightly preferred over the exo face.
Furthermore, the model calculations indicate that an anti
8 Various nitrogen nucleophiles (hydrazine, methylhydrazine, gem-
dimethylhydrazine, 1,2-dimethylhydrazine, 1,2-ethylenediamine, ethy-
lamine, ethanolamine) were also studied. Oxygen nucleophiles did not
undergo addition to 1a–c under our conditions. Since no stereochemical
endo:exo assignments could be reliably made from the available data,
the nitrogen nucleophiles (NMR tube experiments, [2H6]DMSO) were
not extensively included in the present study. Still, they displayed
extremely interesting reactivity patterns worth noting (data from 1H
NMR, 13C NMR, HRMS + FAB analysis). With H2NNH2, H2NEt and
H2NCH2CH2NH2, 1–c reacted completely in 30 min at rt (ca. 80%
conversion of 1a in the first 5 min). H2NNHMe adds slower than
hydrazine itself, and H2NNMe2, adds even slower. H2NCH2CH2OH
reacted very slowly, with < 30% conversion, when heated up to 100 °C.
PriNH2 and MeNHNHMe did not react even after 48 h at rt or 60 °C
overnight.
5
stereochemistry of protonation to the resulting h -pentadienyl
form of the intermediate [Fig. 2(a)] as the a-styryl C would be
expected, consistent with the major selectivity observed in the
Ru system reported here. Interpreting these predictive models in
the context of the stereochemistries reported for SN2A addition11
indicate that a general model in which attack of the nucleophile
syn to the leaving group (Ru–C bond) in p-complexed styryl or
dihydronaphthyl complexes is reasonable. Further theoretical
and experimental studies are in progress to test this hypoth-
esis.
9 E. J. Corey and M. Chaykovsky, J. Am. Chem. Soc., 1965, 87, 1353.
10 B. E. R. Schilling, R. Hoffmann and J. W. Faller, J. Am. Chem. Soc.,
1979, 101, 592.
This work was supported through NSF grant CHE-
9520157.
11 G. Stork and W. N. White, J. Am. Chem. Soc., 1956, 78, 4604; G. Stork
and W. N. White, J. Am. Chem. Soc., 1956, 78, 4609. For a very
comprehensive review of the SN2A reaction, see R. M. Magid,
Tetrahedron Report Number 87, Tetrahedron, 1980, 36, 1901.
12 Semmelhack presents evidence of a mobile equilibrium between I and
II. The example of ref. 5(b) is complicated by the a,b-unsaturation being
–NNCHPh.
Notes and References
† E-mail: moriarty@uic.edu
‡ Crystal data for 2b (major): C17H18F6NOPRu, M = 498.367, monoclinic,
P21/c, a = 7.5734(11), b = 16.771(2), c = 14.6457(12) Å, U = 857.3(4)
Å3, T = 294(2) K, Z = 4, m = 0.996 mm21, 4264 reflections measured
(qmax = 27.50°), 3878 independent (Rint = 0.0124) with 3182 observed [I
> 2s(I)]. For 2c (major): C18H20F6NOPRu, M = 512.394, orthorhombic,
13 An X-ray structure of the major component of product 4 has been
obtained.
P21P21P21, a
= 28.326(3), b = 7.1095(6), c = 9.7180(7) Å, U =
14 Molecular orbital and predictive results were obtained using M. C.
Zerner’s ZINDO program (version 3.0) from CAChe Scientific. Atomic
coordinates were first optimized using molecular mechanics (aug-
mented CAChe MM2 force field), and orbital energies calculated
directly using the INDO 1 basis set. Self-consistent field molecular
orbital energies were obtained in the restricted Hartree–Fock level
within 4 Å self-consistent reaction field (SCRF) cavity with the relative
permittivity and refractive index of water.
1957.1(3) Å3, T = 294(2) K, Z = 4, m = 0.946 mm21, 2917 reflections
measured (qmax = 21.96°), 2395 independent (Rint = 0.0106) with 1727
observed [I > 2s(I)]. For 4 (major): C17H19F6OPRu, M = 485.3681,
monochlinc, P21, a = 8.921(3), b = 25.389(7), c = 9.037(3) Å, U =
1805.0(10) Å3, T = 294(2) K, Z = 4, m = 1.019 mm21, 2675 reflections
measured (qmax = 23.47°), 2413 independent (Rint = 0.0144) with 1921
observed [I > 2s(I)]. CCDC 182/811.
1 S. G. Davies, Organotransition Metal Chemistry: Applications to
Organic Synthesis, Pergamon Press, New York, 1986, pp. 116–155;
Received in Corvallis, OR, USA, 10th October 1997; 7/07386K
1156
Chem. Commun., 1998