Catalytic selective cleavage of a strong C-C single bond by rhodium in solution

Article abstract of DOI:10.1039/a800537k

Reaction of [RhCl(C₈H₁₄)₂]₂ with an excess of the diphosphine 1,3-bis(diisopropylphosphinomethylene)mesitylene 1 in dioxane under mild H₂ pressure (25 psi) or with an excess of HSi(OEt)₃ re

Full text of DOI:10.1039/a800537k

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Catalytic selective cleavage of a strong C–C single bond by rhodium in solution
Shyh-Yeon Liou, Milko E. van der Boom and David Milstein*†
Department of Organic Chemistry, The Weizmann Institute of Science, Rehovot 76100, Israel
Reaction of [RhCl(C8H14)2]2 with an excess of the diphos-
phine 1,3-bis(diisopropylphosphinomethylene)mesitylene 1
in dioxane under mild H2 pressure (25 psi) or with an excess
of HSi(OEt)3 results in catalytic selective cleavage of a strong
C–C single bond.
transfer of a CH2 group from an arene to a silane was observed.
Again the reaction is completely selective.
Reaction of 50 equiv. of the ethyl aromatic phosphine 3 with
[RhCl(C8H14)2]2 at 180 °C under mild H2 pressure in dioxane
for 3 days resulted in formation of compound 4 and C2H6,
although only 4 turnovers were obtained (Scheme 2). Com-
pound 4 was fully identified by comparison with an authentic
sample. The expected amount of C2H6 was observed by GC
analysis of the gas phase. Control reactions showed that 1 and 3
were stable under the same reaction conditions in the absence of
rhodium.
Activation and functionalization of strong C–C single bonds by
soluble transition metal complexes is of considerable current
interest.1–6 Most challenging is the search for the underlying
mechanisms and homogeneous catalysis based on such rela-
tively inert bonds. While several examples of homogeneous
catalytic activation of C–H bonds of hydrocarbons are known,2
the few reports of catalytic C–C bond activation in solution are
limited to weak C–C bonds a to a carbonyl group,3 strained
systems,4 or a combination of both.5 Selective transition metal
insertion into a strong, unstrained C–C single bond in solution
and details related to the mechanism were reported by us only
recently.6 We report here on the catalytic hydrogenolysis and
hydrosilylation of a strong sp2–sp3 C–C bond. This rhodium
catalyzed process is unprecedented, occurs under homogeneous
reaction conditions and is highly selective.
Reaction of [RhCl(C8H14)2]2 (C8H14 = cyclooctene) with 50
equiv. of the phosphine substrate 1 under mild H2 pressure (25
psi; 1 psi ≈ 6.894757 3 103 Pa) in dioxane at 180 °C for one
day leads to catalytic formation of the demethylated phosphine
2 and CH4 (Scheme 1; 16 turnovers based on rhodium and 32%
yield). The CH4 was collected by standard vacuum line
techniques and was identified and quantified by GC. Compound
2 was identified spectroscopically by various NMR and MS
techniques and by comparison with an added authentic
sample.6b Reactions were also performed in [2H8]dioxane to
record 1H, 13C{1H}, 13C-DEPT-135 and 31P{1H} NMR spectra.
Formation of product 2 is highly selective, the other two aryl-
methyl groups remaining unaffected and no other organic
products were formed. Addition of another 50 equiv. of 1 to the
reaction mixture and applying the same reaction conditions
resulted in another 15 turnovers, demonstrating that the catalyst
remains active. Similar results were obtained by performing the
reaction for two days with 100 equiv. of 1 (31 turnovers and
31% yield). Using 500 equiv. of substrate 1 leads to 106
turnovers after three days. The hydrogenolysis proceeds also at
lower temperatures (150 °C), but it is much slower, leading to
only 15 turnovers after one week.
PPri2
PPri2
[RhCl(C8H14)2]2
+ C2H6
Et
+ H2
H
PPri2
PPri2
3
4
Scheme 2
A postulated catalytic cycle is outlined in Scheme 3. The high
selectivity of the catalytic processes, only one alkyl group being
affected, provides strong evidence that both phosphines are
coordinated to the metal centre prior to the C–C bond activation.
Intermediates such as A have been observed with platinum and
ruthenium.7 At this stage competitive C–H and C–C oxidative
addition might occur.6f C–H activation would result in the
reversible formation of a benzylic rhodium species.6 Metal
insertion into the strong Ar–C bond was observed in a
stoichiometric reaction of 1 (0.031 mmol) with 0.5 equiv. of
[RhCl(C8H14)2]2 in [2H6]benzene (2 ml) in a sealed tube for 2 h
1 + 0.5 [RhCl(C8H14)2]2
–2 C8H14
Pri2
Me
Me
P
2
Cl
Rh
Me
P
1
Pri2
A
Pri2
P
Pri2
P
Me
Me
Me
Me
PPri2
Me
PPri2
Me
Cl
Rh
[RhCl(C8H14)2]2
Rh
H
Cl
+ MeR
+ H–R
Me
H
Me
P
P
PPri2
Me
PPri2
R = H, Si(OEt)3
Scheme 1
Pri2
Pri2
Me
1
2
5
B
Pri2
H2
Me
Me
P
To probe the possibility of catalytic hydrosilylation of a C–C
single bond, we used an excess of HSi(OEt)3 instead of H2.
Reaction of [RhCl(C8H14)2]2 with 50 equiv. of 1 and an excess
of HSi(OEt)3 in dioxane (or toluene) at 150 °C for two days
resulted in 10 turnovers to form 2 and MeSi(OEt)3 (Scheme 1),
which were unambiguously characterized by 1H, 13C{1H},
13C-DEPT, 29Si{1H} NMR and by GC–MS.6b Thus, catalytic
H
Rh
Cl
CH4
P
Pri2
6
Scheme 3 Proposed catalytic hydrogenolysis cycle for 1 with rhodium
Chem. Commun., 1998
687

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at room temperature. 1H, 31P{1H} and 13C{1H} NMR analysis
of the reaction solution showed the formation of 5 in 75% yield.
The Rh–Me group is clearly observed in the 13C{1H} NMR at
d 24.3 [dt, 1J(RhC) 30.2, 2J(PC) 6.4 Hz] and the ipso carbon at
d 166.7 [dt, 1J(Rh, C) 33.6, 2J(PC) ≈ 1.0 Hz]. An isostructural
rhodium(iii) complex was recently fully characterized by X-ray
analysis.6f
yield of 35%. Addition of trioctylphosphine oxide to the reaction mixture as
an internal standard indicated 15.3 turnovers and a yield of 31%. The
addition of authentic samples 1, 2 to the reaction mixture resulted in overlap
of resonances in 31P{1H} and 13C{1H} NMR.6b The same reaction
conditions and analysis of the reaction mixture were used for substrate 3.
Similar reaction conditions were applied for the catalytic hydrosilylation,
only an excess of HSi(OEt)3 (91 mg, 0.556 mmol) was used instead of
H2.
§ Spectral data for 6. 1H NMR (C6D6, 400.1 MHz): d 6.56 (s, 1 H, p-H of
C6HRh), 3.24 [m, 3J(HH) 7.2 Hz, 2 H, CHMe2], 3.14 [dvt, left part of ABq,
Subsequently, complex 5 reacts with H2 to yield complex 6.
Indeed, reaction of 5 with H2 (25 psi) at 80 °C for 1 day and
2
2J(HH) 15.7 Hz, J(HP) not resolved, 2 H, CH2P], 2.95 [dvt, right part of
1
analysis by H, 31P{1H}, 13C{1H} NMR, IR and GC analysis
ABq, 2J(HH) 15.7 Hz, 2J(HP) not resolved, 2 H, CH2P], 2.28 [m, 3J(HH) 7.1
Hz, 2 H, CHMe2], 2.14 (s, 6 H, Me2C6HRh), 1.89, 1.72, 1.38, 1.24 [all q,
3J(HH) ≈ 7.0 Hz, 6 H, CHMe2], 219.36 [dt, 1J(RhH) 31.1, 2J(PH) 12.3 Hz,
1 H, HRh]. 31P{1H} NMR (C6D6, 161.9 MHz): d 65.7 [d, 1J(RhP) 113.6
Hz]. 13C NMR (C6D6, 100.1 MHz): d 161.1 [dm, 1J(RhC) 31.9, 2J(PC) ≈
1.0 Hz, Cispo], 142.8 [t, J(PC) 12.3 Hz, Ar], 129.7 [dt, J(PC) 7.4, J(RhC) 1.4
Hz, Ar], 126.5 (s, Ar), 32.8 [dt, J(PC) 12.6 Hz, CH2P], 26.6 [t, J(PC) 10.0
Hz, CHMe2], 23.2 [t, J(PC) 10.2 Hz, CHMe2], 22.6 (s, Me2Ar), 22.1, 19.7,
showed the quantitative formation of complex 6 and CH4.§ This
reaction may proceed through a rhodium(v) intermediate or via
s-bond metathesis. Complex 6 can also be used as catalyst
under the same reaction conditions. Release of the aryl
phosphine 2 from 6 probably proceeds via B, which undergoes
phosphine exchange with substrate 1 giving back A. This is
likely to be the rate-determining step. Such a process was
demonstrated by treating a dioxane solution of complex 6 with
20 equiv. of PEt3 at 80 °C overnight, which resulted in
formation of the PCP ligand 2 and RhCl(PEt3)3 by phosphine
exchange. Replacement of a cyclometalated terdentate diamino
ligand of a ruthenium(ii) complex by a phosphorus analogue
was reported very recently.7b The liberated arene 2 most
probably strongly competes with substrate 1, slowing down the
catalytic process. It is well known that a,aA-diphosphine-
m-xylenes such as 2 and 4 undergo readily Ar–H oxidative
addition with rhodium forming thermally stable, isolable
complexes such as 6.6c,8
In summary, a novel catalytic process has been presented
using simple diphosphine substrates. For the first time, an
unstrained, strong Ar–C bond is selectively activated by a metal
centre in solution in a catalytic fashion. Moreover, catalytic
transfer of a methylene group to a primary silane has been
observed using a rhodium complex. Stoichiometric reactions
involved in the catalysis were directly demonstrated. Although
the catalytic reactions were not optimized and the reactions are
at present slow, more than one hundred turnovers were
observed.
19.5 (all s, CHMe2). IR (neat): n 2071 cm21
.
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The research was supported by the US-Israel Binational
Science Foundation, Jerusalem, Israel. D. M. is the holder of the
Israel Matz Professorial Chair of Organic Chemistry.
Notes and References
† E-mail: comilst@wiccmail.weizmann.ac.il
‡
Catalytic hydrogenolysis of an unstrained C–C single bond. A
[2H8]dioxane solution (1.5 ml) of substrate 1 (106 mg, 0.278 mmol) was
added dropwise to a [2H8]dioxane solution (1.5 ml) of [RhCl(C8H14)2]2 (2
mg, 0.00278 mmol), loaded into a 90 ml Fischer porter pressure bottle
equipped with a stirring bar and pressurized with H2 (20–25 psi) (toluene
can be used as well). After heating the reaction solution at 180 °C for 1 day,
the gas phase was collected by standard vacuum line techniques and
analyzed by GC using a molecular sieve column. The formed CH4 was
identified and quantified using authentic samples (13.6 turnovers). 31P{1H}
NMR of the reaction mixture shows two signals at d 5.6 (s, 2 P, 1) and 3.20
(s, 2 P, 2). The ratio of the signals (100:54) indicated 17.5 turnovers and a
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F. Frolow, H.-B. Kraatz and D. Milstein, Inorg. Chem., 1996, 35, 7068;
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1996, 35, 1792.
Received in Cambridge, UK, 20th January 1998; 8/00537K
688
Chem. Commun., 1998