Metalloradical-catalyzed rearrangement of cycloheptatrienyl to benzyl

Article abstract of DOI:10.1039/c0cc04655h

Rh(ttp)(C₇H₇) rearranged to give Rh(ttp)(CH ₂Ph) quantitatively at 120°C in 12 d (ttp = 5,10,15,20- tetratolylporphyrinato dianion). This process is 10¹⁰ faster than for the organic analogue. Mechanistic investi

Full text of DOI:10.1039/c0cc04655h

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COMMUNICATION
Metalloradical-catalyzed rearrangement of cycloheptatrienyl to benzylw
Yun Wai Chan and Kin Shing Chan*
Received 28th October 2010, Accepted 22nd February 2011
DOI: 10.1039/c0cc04655h
Rh(ttp)(C H ) rearranged to give Rh(ttp)(CH Ph) quantitatively
7
7
2
at 120 1C in 12 d (ttp = 5,10,15,20-tetratolylporphyrinato
dianion). This process is 10 faster than for the organic analogue.
10
II
Mechanistic investigation suggests that a Rh (ttp)-catalyzed
pathway is operating.
Cycloheptatrienyl transition metal complexes have been known for
over five decades since the first example [Mo(Z-C H )(CO) ][BF ]
7
7
3
4
1
was discovered in 1958. However, the chemistry of cyclo-
heptatrienyl ligand has been studied little in comparison with
that of cyclopentadienyl and arene ligands. Cycloheptatrienyl
transition metal complexes have very rich chemistry and
2
undergo various reactions including lithiation reactions, nucleo-
3
ring, Lewis-base addition and
philic attack on the Z-C
7
H
7
7 7
Fig. 1 ORTEP presentation of Rh(ttp)(C H ) (30% probability
4
ring-slippage reactions. To the best of our knowledge, the
rearrangement of a cycloheptatrienyl transition metal complex
to a benzyl transition metal complex is still unprecedented.
We have been interested in the carbon–hydrogen bond
activation (CHA) and carbon–carbon bond activation (CCA)
displacement ellipsoids). Hydrogen atoms are omitted for clarity:
˚
˚
˚
Rh–C = 2.10 A, C–C = 1.44 to 1.49 A, CQC = 1.43 to 1.45 A;
R = 0.036.
The CQC bond lengths of the cycloheptatrienyl group range
˚
from 1.43 to 1.45 A and that of C–C bonds range from 1.44 to
5
,6
of cycloalkanes with rhodium porphyrin (por) complexes.
II
We have recently discovered the Rh (por)–catalyzed aliphatic
˚
˚
1.49 A (typical CQC and C–C bonds are 1.33 and 1.54 A,
respectively). Therefore, the carbon–carbon bonds of the
6
II
7
CCA of cyclooctane. We now report the Rh (ttp)-catalyzed
ttp = 5,10,15,20-tetratolylporphyrinato dianion) rearrangement
of Rh(ttp)(C ) to Rh(ttp)Bn and the mechanistic studies
identifying the unique catalytic role of Rh (ttp).
(
cycloheptatrienyl are likely conjugated. The cycloheptatrienyl
group does not cause a large distortion of the mean porphyrin
˚
plane from planarity in 1 (0.477 A). The porphyrin structure
7 7
H
II
of Rh(ttp)(C
Fig. S3, ESIw).
The cycloheptatrienyl group is fluxional as evidenced by its
7 7
H ) is slightly distorted to adopt a saddle form
(
1
variable-temperature H NMR spectra (Fig. 2). At room
temperature, the signal of cycloheptatrienyl protons overlapped
with that of methyl groups of porphyrin (d 2.44 ppm in C D ).
At ꢀ50 1C, four broad peaks were observed in the upfield
region (d ꢀ1.37, 0.36, 4.15 and 4.61 ppm). At ꢀ70 1C, four
sets of cycloheptatrienyl proton resonances were well frozen
6
6
ð1Þ
7 7
Rh(ttp)(C H ) 1 was prepared quantitatively by the CHA of
cycloheptatriene (CHT) with both Rh(ttp)H and Rh (ttp)
rapidly at room temperature (eqn (1)). The structure of
Rh(ttp)(C ) 1 was further elucidated by X-ray crystallo-
graphy (Fig. 1). The Rh–C bond length of Rh(ttp)(C ) is
.10 A and is similar to the reported Rh–C bond lengths
2
2
(
d ꢀ1.37, 0.36, 4.15 and 4.61 ppm). The fluxional behaviour
suggests that the Rh–C bond is a weak bond at room temperature.
The thermal stability of Rh(ttp)(C ) was then examined and
it was heated to 120 1C in benzene-d . Rh(ttp)(C H ) rearranged
7
H
7
7 7
H
7 7
H
˚
2
6
7
7
8
5
,6
˚
of various Rh(ttp) alkyls which range from 2.03 to 2.13 A.
cleanly at 120 1C to give Rh(ttp)Bn quantitatively in 12 d
eqn (2)). The kinetics of the rearrangement were then measured
and yielded the rate law: rate = k [Rh(ttp)(C H )], where
(
obs
7
7
Department of Chemistry, The Chinese University of Hong Kong,
Hong Kong SAR, The People’s Republic of China.
E-mail: ksc@cuhk.edu.hk; Fax: +852 26095057;
Tel: +852 26096376
w Electronic supplementary information (ESI) available. CCDC 797520.
For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c0cc04655h
ꢀ
6
ꢀ1
kobs(120 1C) = 4.75 ꢁ 10
s
, or t1/2obs(120 1C) = 1.69 d.
For comparison, the organic analogue CHT rearranges at 475 1C
to give toluene, with the rate = k[CHT], where k(475 1C) =
9a
ꢀ ꢀ1
4
1
.12 ꢁ 10
s . The extrapolated rearrangement rate k(120 1C) is
ꢀ
16 ꢀ1
10 9b
estimated to be 4.61 ꢁ 10
s
or t1/2(120 1C) = 1.74 ꢁ 10 d.
4
802 Chem. Commun., 2011, 47, 4802–4804
This journal is c The Royal Society of Chemistry 2011

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II
Scheme 1 Proposed mechanism for Rh –catalyzed CCA.
2 2
Table 1 Effect of Rh (ttp) on rearrangement
1
Fig. 2 Partial variable-temperature H NMR spectra of Rh(ttp)(C
5
10 [Rh
3
10 [Rh
1/2
1/2
/M
6
ꢀ1
7 7
H )
Entry
2
(ttp)
2
]/M
2
2
(ttp) ]
10 kobs/s
in toluene-d . For complete spectra, see the Supporting Information.
8
1
2
3.48
34.8
5.90
18.6
7.34
21.5
1
0
The rearrangement of Rh(ttp)(C H ) to Rh(ttp)Bn is thus 10
7
7
times faster than that of CHT to toluene.
which further undergoes ring opening to yield the intermediate
In order to gain further mechanistic understanding of the
organometallic rearrangement, the rearrangement rates at
various temperatures (120–150 1C) were determined and
16
5. The b-H elimination of 5 regenerates the aromatic skeleton to
form the benzyl radical 6. Finally, the benzyl radical 6 abstracts
a hydrogen atom from Rh(ttp)H (BDE of Rh(por)–H =
15b,c
the activation parameters evaluated from the temperature
z
ꢀ
1
6
0 kcal mol ) to give Rh(ttp)Bn.
dependent rate constants (120–150 1C) are: DH obs
=
ꢀ1
ꢀ1
z
ꢀ1
To validate the proposed mechanism, the rate promoting
2
2.4 ꢂ 2.4 kcal mol , DS obs = ꢀ26.5 ꢂ 6.0 cal mol
K
,
z
ꢀ1
effect of Rh (ttp) was examined (eqn (3), Table 1). The
2
and DG obs = 32.8 ꢂ 2.4 kcal mol . The Arrhenius activation
2
ꢀ
5
ꢀ
1 10,11
,
addition of Rh
2
(ttp)
2
(3.48 ꢁ 10 M) resulted in the rate
barrier of CHT to toluene is 50 kcal mol
which is much
ꢀ6 ꢀ1 17
enhancement with kobs = 7.34 ꢁ 10
s
(Table 1, entry 2).
was obtained by the addition
2
in 10-fold (Table 1, entry 2). The kobs is directly
larger than that of Rh(ttp)(C H ) to Rh(ttp)Bn.
7
7
ꢀ
5
ꢀ1
s
A larger kobs of 2.15 ꢁ 10
of Rh (ttp)
proportional to [Rh
2
1
/2
2
(ttp)
Rh (ttp). So, rate = kobs [Rh(ttp)(C
In conclusion, we have discovered the Rh -catalyzed rearrange-
ment of Rh(ttp)(C ) to Rh(ttp)Bn and the rearrangement rate
2
]
and confirms the catalytic role of
II
1/2
H
7 7
)][Rh
2
(ttp)
2
]
.
II
ð2Þ
7 7
H
10
is 10 times faster than the CHT–toluene rearrangement.
We thank the Research Grants Council of the HKSAR, the
People’s Republic of China (CUHK400307) for financial support.
A lot of effort has been spent on the mechanistic under-
standing of the rearrangement of CHT to toluene since it is of
interest and involves many carbon–carbon and carbon–hydrogen
bond cleavages and formations. Klump et al. elucidated that
the thermal isomerization of CHT to toluene in the gas phase
Notes and references
1
2
1 For a review: M. H. L. Green and D. K. P. Ng, Chem. Rev., 1995,
9
is a first-order reaction. Norcaradiene which forms from the
rapid equilibrium with CHT is likely the reactive intermediate.
Such an unimolecular reaction mechanism is considered to be
a process analogous to the cyclopropane isomerization to
5, 439.
2
(a) M. D. Rausch, M. Ogasa, M. A. Ayers, R. D. Rogers and
A. N. Rollins, Organometallics, 1991, 10, 2481; (b) M. D. Rausch,
M. Ogasa, R. D. Rogers and A. N. Rollins, Organometallics, 1991,
10, 2084; (c) R. Fierro, M. D. Rausch, R. D. Rogers and
M. Herberhold, J. Organomet. Chem., 1994, 472, 87.
1
3
propylene. Moreover, Harrison et al. reported the rearrange-
14
ment of 7-methylcycloheptatriene to ethylbenzene at 650 1C.
3
(a) C. A. Bunton, M. M. Mhala, J. R. Moffatt and W. E. Watts,
J. Organomet. Chem., 1983, 253, C33; (b) D. A. Brown, J. Burns,
I. El-Gamati, W. K. Glass, K. Kreddan, M. Salama,
D. Cunningham, T. Higgins and P. McArdle, J. Chem. Soc., Chem.
Commun., 1992, 701.
The reaction takes place by means of radical chain processes and
involves species such as cycloheptatrienyl and benzyl radicals as
intermediates. However, neither of the above proposed mecha-
10
nisms could be applied to explain the 10 times rate enhance-
ment of the rearrangement of Rh(ttp)(C ) to Rh(ttp)Bn.
To account the rapid formation of Rh(ttp)Bn from
4
5
(a) T. W. Beall and L. W. Houk, Inorg. Chem., 1973, 12, 1979;
(b) R. Breeze, S. Endud and M. W. Whiteley, J. Organomet. Chem.,
7 7
H
1986, 302, 371; (c) R. A. Brown, S. Endud, J. Friend, J. M. Hill and
M. W. Whiteley, J. Organomet. Chem., 1988, 339, 283.
II
Rh(ttp)(C
Rh(ttp)(C
7
7
H
H
7
), we reason a Rh -catalyzed rearrangement of
Y. W. Chan and K. S. Chan, Organometallics, 2008, 27, 4625.
7
) to Rh(ttp)Bn (Scheme 1). First, Rh(ttp)(C
II
7
7
H )
6 Y. W. Chan and K. S. Chan, J. Am. Chem. Soc., 2010, 132, 6920.
7 F. Allen, O. Kennard, D. G. Waston, L. Brammer, A. G. Orpen
and R. Taylor, J. Chem. Soc., Perkin Trans. 2, 1987, S1.
undergoes homolysis to yield Rh (ttp) and cycloheptatrienyl
1
5
II
radical.
Rh (ttp) then adds to the CQC bond of
Rh(ttp)(C H ) to generate the dirhodium complex 3. Sub-
8
K. S. Chan and C. M. Lau, Organometallics, 2006, 25, 260.
7
7
9 (a) W. G. Woods, J. Org. Chem., 1958, 23, 110; (b) The rearrange-
ment rate of CHT to toluene is extrapolated to 120 1C based on the
sequent rearrangement of 3 gives cyclopropylmethyl radical 4
This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 4802–4804 4803

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literature value, see: B. J. Gaynor and R. G. Gilbert, Int. J. Chem.
Kinet., 1976, 8, 695 Details of extrapolation is shown in ESIw.
0 U. Samuni, S. Kahana and Y. Haas, J. Phys. Chem. A, 1998, 102,
see B. B. Wayland, S. Ba and A. E. Sherry, J. Am. Chem. Soc.,
ꢀ1
1991, 113, 5305; (c) The BDE of C–H of ethane is 100.5 kcal mol
,
ꢀ
the BDE of the benzylic C–H of toluene is 88.5 kcal mol and the
1
1
1
1
ꢀ
4
758.
1 (a) R. Atkinson and B. A. Thrush, Proc. R. Soc. London, Ser. A,
970, 316, 123; (b) R. Atkinson and B. A. Thrush, Proc. R. Soc.
methylene C–H of CHT is 76.6 kcal mol : See Y. R. Luo,
Comprehensive Handbook of Chemical Bond Energies, CRC Press,
Boca Raton, FL, 2007
1
London, Ser. A, 1970, 316, 131; (c) R. Atkinson and B. A. Thrush,
Proc. R. Soc. London, Ser. A, 1970, 316, 143.
1
1
2 K. N. Klump and J. P. Chesick, J. Am. Chem. Soc., 1963, 85, 130.
3 T. S. Chambers and G. B. Kistiakowsky, J. Am. Chem. Soc., 1934,
ꢀ1
BDE/kcal mol
56, 399.
R
R–H
Rh(tmp)–R
1
4 A. G. Harrison, L. R. Honnen, H. J. Dauben, Jr. and F. P. Lossing,
J. Am. Chem. Soc., 1960, 82, 5593.
Et
Bn
7 7
C H
100.5
88.5
76.6
48.5
33
1
7 7
5 The BDE of the Rh–C bond of Rh(ttp)(C H ) is estimated as
ꢀ1
B26 (Rh–C of 1)
B26 kcal mol (see ESIw for the details of estimation). (a) The
bond dissociation energies (BDEs) of Rh–C of Rh(tmp)Et
ꢀ
1
and Rh(tmp)Bn are 48.5 and 33 kcal mol
,
tmp = mesitylporphyrinato dianion); see B. B. Wayland, in
respectively
16 F. A. Carey and R. J. Sundberg, Advanced Organic Chemistry Part A:
Structure and Mechanisms, Plenum Press, New York, 1977.
17 Rh (ttp)
(
Energetics of Organometallic Species, ed. J. A. Marinho, NATO
II
equilibrates to give Rh (ttp) via homolysis: see,
2
2
Ser. C: 367, Kluwer Academic Publisher, Dordrecht, The Netherlands,
1992, p. 69; (b) The BDE of Rh–H of Rh(tmp)H is 60 kcal mol :
B. B. Wayland, S. L. van Voorhees and C. Wilker, Inorg. Chem.,
1986, 25, 4039.
ꢀ1
4
804 Chem. Commun., 2011, 47, 4802–4804
This journal is c The Royal Society of Chemistry 2011