Facile dichloromethane activation and phosphine methylation. Isolation of unprecedented zwitterionic organozinc and organocobalt intermediates

Article abstract of DOI:10.1039/b817728g

Both C-Cl bonds of CH₂Cl₂ are readily activated by CoCl₂ and metallic Zn allowing quantitative methylation of phosphines; unprecedented zwitterionic Co(ii) and Zn(ii) intermediates have been characterized which display α-m

Full text of DOI:10.1039/b817728g

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Facile dichloromethane activation and phosphine methylation.
Isolation of unprecedented zwitterionic organozinc and organocobalt
intermediatesw
Roberto Pattacini, Suyun Jiez and Pierre Braunstein*
Received (in Cambridge, UK) 8th October 2008, Accepted 24th December 2008
First published as an Advance Article on the web 20th January 2009
DOI: 10.1039/b817728g
Both C–Cl bonds of CH2Cl2 are readily activated by CoCl2 and
metallic Zn allowing quantitative methylation of phosphines;
unprecedented zwitterionic Co(II) and Zn(II) intermediates have
been characterized which display a-metallated phosphoniums.
a rich chemistry,7 anhydrous CoCl2 was reacted with these
ligands and Zn powder in CH2Cl2, in a 1 : 1 : 1 molar ratio.
Unexpectedly, complexes 2ox and 2th, respectively, were isolated
(Scheme 1), in which the zinc ion is coordinated by three
chlorides and an N-bound Ph2P+(Me)CH2CQNCH2CH2O
cation (Fig. 1).y Phosphorus methylation has resulted in a
phosphonium cation associated with a negatively charged
ZnCl3 group in a zwitterionic complex.
The activation of the relatively inert C–Cl bond of chloroalkanes
represents a considerable challenge of chemical and environ-
mental relevance.1 It would be particularly desirable to use the
carbon moiety of a chloroalkane for alkylation chemistry.
Activating both C–Cl bonds of a gem-dichloroalkane represents
an additional difficulty and whereas CH2Br2, CH2I2 or CH2ClI
are used in e.g. Simmons–Smith cyclopropanation,2 the cheaper
CH2Cl2 is far less exploited,3 because of its much lower reacti-
vity. Thus, the recently reported activation of the C–Cl bonds of
CH2Cl2 in the presence of pyridine and Ni(0) nanoparticles
required 100 1C and 10 h reaction time.4 As part of our studies
on the coordination chemistry of P,O- and P,N-functional
phosphines,5 we were interested in isolating Co(I) com-
plexes, inter alia because of their possible involvement in the
cobalt-catalyzed homogeneous carbonylation of methanol,6
and unexpectedly discovered that CH2Cl2 can be used as a
methylation reagent under mild reaction conditions.
Although related chemistry has been observed with cobalt, it
required the use of highly sensitive Co(0) tetraphosphine com-
plexes, produced in situ by reduction of Co(II) with Mg, and
afforded a maximum theoretical yield of 50% based on
phosphine.8 Our results prompted us to attempt to (i) generalize
this reaction to other phosphines and (ii) establish the origin of
the methyl group, CH2Cl2 being the obvious suspect. Phosphines
P(n-Bu)3, PMePh2, PMe2Ph and PPh3 underwent methylation
under the conditions used for 2ox and 2th and quantitative
formation of cobalt metal was observed, according to [eqn (1)]:
2CH2Cl2 + 2PR3 + 2CoCl2 + 4Zn + 2H2O
- (R3PMe)2(ZnCl4) + Zn(OH)2 + 2ZnCl2 + 2Co (1)
In an attempt to synthesize Co(I) complexes bearing the
When the reaction was performed in CD2Cl2, the phosphonium
cations Ph3P+CD3 and Ph3P+CD2H were obtained after
hydrolysis with D2O or H2O, respectively, confirming that
CH2Cl2 is the methylating agent.
phosphinooxazoline Ph2PCH2CQNCH2CH2O (1ox) or its thio
analog Ph2PCH2CQNCH2CH2S (1th) (Scheme 1), which have
With MeCN as co-solvent, an intermediate of [eqn (1)] could
be crystallized, [Zn(m-Cl)Cl(CH2PPh3)]2 (4), in which each
Scheme 1
Laboratoire de Chimie de Coordination, Institut de Chimie
´
(UMR 7177 CNRS), Universite Louis Pasteur, 4 rue Blaise Pascal,
67070 Strasbourg Cedex, France. E-mail: braunstein@chimie.u-strasbg.fr;
Fax: +33 (0)3 9024 1302
w Electronic supplementary information (ESI) available: Supplementary
X-ray structures, crystallographic details, VT NMR spectra, experi-
mental procedures and spectroscopic data. CCDC 697021–697024,
697025 (5ꢀ1/2C6H14), 697026 (2ox), 697027 (2th), 697028 (4), 697029
(4ꢀZnCl2) and 697030. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/b817728g
z Current address: Key Laboratory of Engineering Plastics and
Beijing National Laboratory for Molecular Science, Institute of
Chemistry, Chinese Academy of Sciences, Beijing, 100190, China.
Fig. 1 ORTEP plot of the X-ray structure of 2ox. Ellipsoids at 50%
probability level. Aromatic and oxazoline H atoms not shown for
clarity. Selected bond distances (A): Zn1–N1 2.051(2), Zn1–Cl3
2.2409(8), Zn1–Cl1 2.2562(8), Zn1–Cl2 2.2614(8), N1–C2 1.270(3),
P1–C5 1.786(3).
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Fig. 2 ORTEP plot of the X-ray structure of 4 (top) and structural
diagram of the X-ray structure of 4ꢀZnCl2 (bottom). For 4ꢀZnCl2, the
asymmetric unit is delimited by dashed lines, the polymer repeating
unit by black lines. Ellipsoids at 50% level. Selected distances for
4 (A): Zn1–C1 2.025(2), Zn1–Cl1 2.3505(6), Zn1–Cl2 2.2491(7), C1–P1
1.752(2). Similar values are found in 4ꢀZnCl2.w
metal centre is s bound to the CH2 carbon of a Ph3P+CH2-
group and coordinated by three chlorides, two of them bridging
the tetrahedrally coordinated metal centres. Under anhydrous
conditions, 4 is the only P-containing species detected after
reaction and is isolated in high yields. In pure CH2Cl2, non-
solvated ZnCl2 forms with 4 the unprecedented coordination
polymer [4ꢀZnCl2]n (Fig. 2 and ESIw).y In the di-zwitterionic
complex 4, the positive and negative charges are on the
phosphorus and the Zn2Cl4 core, respectively. Similarly,
4ꢀZnCl2 is a poly-zwitterion. To the best of our knowledge,
only two structures related to 4 have been reported for
zinc.9
Scheme 2 Suggested pathways for CH2Cl2 activation and phosphine
alkylation.
as catalytic in cobalt. A PR3 : Co ratio higher than 3 was found
detrimental for a complete conversion.
In a separate experiment (Scheme 2, path B), a solution of
[CoCl(PPh3)3] in CH2Cl2 became blue within a few minutes
under nitrogen, indicating oxidation to Co(II). The highly
reactive, zwitterionic complex [CoCl2(CH2PPh3)PPh3] (5)y
was crystallized from the mother liquor. Its metal centre is
tetrahedrally coordinated by PPh3, CH2PPh3 and two
chlorides (Fig. 3). [CoCl2(PPh3)2] and PPh3 are by-products
of this reaction. The Co(III) intermediate (Scheme 2) is reduced
by the Co(I) complex (comproportionation), rather than by Zn
metal, which is absent in path B.
Addition of HCl or water to a solution of 4 readily afforded
the phosphonium salts (Ph3PCH3)2(Zn2Cl6) and (Ph3PCH3)2
(ZnCl4), respectively (crystal structures in ESIw). Similar
chemistry was observed for PMePh2, PMe2Ph and P(n-Bu)3.
Although the presence of Co(II) prevents NMR monitoring of
the reaction of [eqn (1)], we suggest that complexes
[CoCl2(PR3)2] are formed first and consistently, [CoCl2(PPh3)2]
reacted with Zn and CH2Cl2 to form 4 and, upon hydrolysis,
Ph3P+CH3. After reduction of [CoCl2(PPh3)2] to
[CoCl(PPh3)3],10 oxidative-addition of CH2Cl2 could afford the
transient species [CoIIICl2(CH2Cl)(PR3)3], by analogy to related
Rh(I) systems (Scheme 2).11 Similarly, oxidative-addition of
aromatic C–Cl bonds to Co(I) results in octahedral Co(III)
complexes.12 Coupling between the ligands PR3 and –CH2Cl
would then afford the metallated –CH2P+R3 group, as observed
with rhodium.13 An intermolecular coupling is less likely since
when the PR3 : Co ratio was reduced to one, no appreciable
change in reactivity was observed. Finally, reduction and trans-
metallation from Co to Zn would yield 4 (Scheme 2, path A) and
restore the Co(II) reagent, which is in turn reduced by Zn to the
active species [CoCl(PPh3)3]. This cyclic reaction may be viewed
Fig. 3 ORTEP plot of the X-ray structure of 5 in 5ꢀ1/2C6H14
.
Ellipsoids at 50% level. Selected distances (A): Co1–C1 2.040(6),
Co1–Cl1 2.227(2), Co1–Cl2 2.235(2), Co1–P2, 2.407(2), C1–P1
1.738(7).
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The reaction between [CoCl(PPh3)3] and CH2Cl2 (Scheme 2,
path B) can thus be written as follows:
wR2 (I 4 2s(I)) = 0.206 (939 param., 20 257 obsd refl., 14 092 unique).
5ꢀ1/2C6H14: C37H32Cl2CoP2ꢀ1/2C6H14, M = 668.45 (without solvent,
see ESIw), C2/c, a = 24.424(1), b = 12.4340(9), c = 22.853(1) A,
b = 90.800(4), V = 6939.5(7) A3, Z = 8, Dc = 1.279 g cmꢂ3
,
2[CoCl(PPh3)3] + CH2Cl2
m(MoKa)
= , F(000) = 2760, Rint = 0.060,
0.769 mmꢂ1
- [CoCl2(CH2PPh3)(PPh3)]
R1 (I 4 2s(I)) = 0.075, wR2 (I 4 2s(I)) = 0.206 (379 param.,
10 466 obsd refl., 6128 unique).
+ [CoCl2(PPh3)2] + 2 PPh3
(2)
1 S. Kliegman and K. McNeill, Dalton Trans., 2008, 4191, and
references cited.
Since neither ZnCl2 nor Zn are present in [eqn (2)], hypo-
thetical Zn carbenoids, often invoked in the activation of
dihalomethanes,14 are not involved here, although a carbene
intermediate has been suggested in the reaction of Cr(II)
complexes with CH2Cl2.15 In the CH2Cl2-based methylation
of ketones with a Mg–TiCl4-based system,16 no intermediate
has been isolated. Our cobalt-based system thus involves steps
closer to those suggested with the noble metals Rh(I), Ru(II)
or Os(II),13,17 which, however, only led to stoichiometric
reactions.18 In our case, Zn restores the active Co(I) species
and allows the quantitative methylation of all the phosphines
present. Formation of P–C bonds assisted by cobalt com-
plexes has been reported from the attack of dibromo- or
di-iodomethane on dinuclear phosphides.19 Whereas oxidative-
addition of organic chlorides to Co(I) complexes has been
reported,12 only one chloro, chloromethyl complex has been
characterized from the photoassisted, oxidative-addition of
CH2Cl2 to Co(I).20 Although the chemistry of [CoCl(PPh3)3] has
been intensively explored,21 it is surprising that, to the best of our
knowledge, its fast reaction with CH2Cl2 has never been discussed.
We have shown here that CH2Cl2 can be readily activated by a
cheap metal, without the need for an external Cl-abstracting
reagent, and exploited for phosphorus methylation. The reaction
involves easily accessible reagents and allows methylation of PPh3
by CH2Cl2, CoCl2 and Zn to be carried out under air, in contrast
to reactions involving Co(0) precursors.8 The phosphonium salts
produced are well-known precursors to e.g. the very important
class of phosphorus ylids. Our reaction appears general for
aromatic and aliphatic phosphines, since variation of their stereo-
electronic properties did not result in appreciable differences in
reactivity. We have thus shown that challenging transformations
considered limited to noble metals can be observed or even
improved using a much cheaper metal such as cobalt.
2 (a) H. E. Simmons and R. D. Smith, J. Am. Chem. Soc., 1958, 80,
5323; (b) H. E. Simmons and R. D. Smith, J. Am. Chem. Soc.,
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Chem., 1991, 56, 6974; (d) S. E. Denmark, J. P. Edwards and
S. R. Wilson, J. Am. Chem. Soc., 1992, 114, 2592; For recent
examples of asymmetric Simmons–Smith cyclopropanation see:
(e) H. Shitama and T. Katsuki, Angew. Chem., Int. Ed., 2008, 47,
2450; (f) S. G. Davies, K. B. Ling, P. M. Roberts, A. J. Russell and
J. E. Thomson, Chem. Commun., 2007, 4029.
3 For recent examples of cyclopropanation reactions involving
CH2Cl2: (a) X. Xiang, Q. Shen, J. Wang, Z. Zhu, W. Huang and
X. Zhou, Organometallics, 2008, 27, 1959; (b) K.-W. Lin, S. Yan,
I. L. Hsieh and T.-H. Yan, Org. Lett., 2006, 8, 2265; (c) C.-C. Tsai,
I. L. Hsieh, T.-T. Cheng, P.-K. Tsai, K.-W. Lin and T.-H. Yan,
Org. Lett., 2006, 8, 2261.
4 K. H. Park, I. G. Jung, Y. K. Chung and J. W. Han, Adv. Synth.
Catal., 2007, 349, 411.
5 See e.g.: (a) P. Braunstein, Chem. Rev., 2006, 106, 134;
(b) P. Braunstein and F. Naud, Angew. Chem., Int. Ed., 2001, 40, 680.
6 A. Bader and E. Lindner, Coord. Chem. Rev., 1991, 108, ;27.
7 (a) R. Pattacini, G. Margraf, A. Messaoudi, N. Oberbeckmann-Winter
and P. Braunstein, Inorg. Chem., 2008, 47, 9886; (b) A. Kermagoret,
R. Pattacini, P. Chavez Vasquez, G. Rogez, R. Welter and
P. Braunstein, Angew. Chem., Int. Ed., 2007, 46, 6438; (c) S. Jie,
M. Agostinho, A. Kermagoret, C. S. J. Cazin and P. Braunstein,
Dalton Trans., 2007, 4472.
8 H. F. Klein and R. Hammer, Angew. Chem., Int. Ed. Engl., 1976, 15, 42.
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¨
Commun., 1998, 285.
10 L. S. Liebeskind, S. L. Baysdon, M. S. South, S. Iyer and
J. P. Leeds, Tetrahedron, 1985, 41, 5839.
11 For recent examples see e.g.: (a) J. A. Wright, A. A. Danopoulos,
W. B. Motherwell, R. J. Carroll, S. Ellwood and
J. Sassmannshausen, Eur. J. Inorg. Chem., 2006, 4857; (b) J. H.
H. Ho, D. S. C. Black, B. A. Messerle, J. K. Clegg and P. Turner,
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J. P. White, Inorg. Chim. Acta, 2006, 359, 3709; (d) J. Y. Zeng,
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The French Embassy in Beijing (doctoral grant to S. J.,
Sino-French Joint Catalysis Laboratory), the CNRS
(post-doctoral position to R.P.) and the Ministere de la
Recherche (Paris) are gratefully acknowledged for support.
We thank Prof. R. Welter for an X-ray diffraction analysis.
12 Y. Chen, H. Sun, U. Florke and X. Li, Organometallics, 2008, 27, 270.
¨
13 T. B. Marder, W. C. Fultz, J. C. Calabrese, R. L. Harlow and
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Notes and references
y X-Ray diffraction data (173 K): 2ox: C17H19Cl3NOPZn, M =
456.02, P21/c, a = 12.8862(3), b = 8.4311(2), c = 18.2833(6) A,
17 J. Gotzig, R. Werner and H. Werner, J. Organomet. Chem., 1985,
290, 99.
b = 98.131(1), V = 2439.7(2) A3, Z = 4, Dc = 1.540 g cmꢂ3
,
0.043,
m(MoKa)
=
1.742 mmꢂ1
,
F(000)
=
928, Rint
=
18 (a) [Mo(CO3)(CO)(PMe3)4] has been found to react slowly with
CH2Cl2 to give an insoluble phosphonium salt, see: E. Carmona,
M. A. Munoz and R. D. Rogers, Inorg. Chem., 1988, 27,
1598; (b) [Fe(CO)4]2ꢂ reacts with PPh3 in CH2Cl2 to give
[Fe(CO)4(CH2PPh3)]: B. Weinberger, G. Tanguy and H. des
Abbayes, J. Organomet. Chem., 1985, 280, C31.
19 H. Werner and R. Zolk, Organometallics, 1985, 4, 601.
20 W. L. Olson, D. A. Nagaki and L. F. Dahl, Organometallics, 1986,
5, 630.
R1 (I 4 2s(I)) = 0.040, wR2 (I 4 2s(I)) = 0.084 (218 param., 3158
obsd refl. 4749 unique); 4: C38H34Cl4P2Zn2, M = 825.13, C2/c,
a = 14.5489(7), b = 13.7296(3), 18.8997(8) A, b = 104.530(2),
V = 3654.5(2) A3, Z = 4, Dc = 1.500 g cmꢂ3, m(MoKa) =
1.720 mmꢂ1, F(000) = 1680, Rint = 0.035, R1 (I 4 2s(I)) = 0.035,
wR2 (I 4 2s(I)) = 0.089 (208 param., 3964 obsd refl., 5318 unique);
ꢀ
4ꢀZnCl2: C76H68Cl12P4Zn6, M = 1922.80, P1, a = 10.261(2), b =
20.439(3), c = 21.995(3) A, a = 63.454(3), b = 86.058(9), g =
83.865(8), V = 4102(1) A3, Z = 2, Dc = 1.557 g cmꢂ3, m(MoKa)
= 1.362 mmꢂ1, F(000) = 5537, Rint = 0.060 R1 (I 4 2s(I)) = 0.075,
21 For a recent example see: J. E. Taylor, M. F. Mahon and
J. S. Fossey, Angew. Chem., Int. Ed., 2007, 46, 2266.
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892 | Chem. Commun., 2009, 890–892