Oxidative addition of silyl cyanides to rhodium porphyrin radical: Isocyanide or cyanide transfer mechanism

Article abstract of DOI:10.1021/om049253h

Rhodium porphyrin radical, coordinated with pyridine, activated the carbon - silicon bond of silyl cyanides to yield rhodium porphyrin silyls and cyanide. The reaction with Me₃SiCN exhibited second-order kinetics, rate - k_absd[Rh(tmp)][Me₃SiCN], at a sufficiently high concentration of pyridine, and the mechanism was interpreted to involve a cyanide or isocyanide group transfer to rhodium radical.

Full text of DOI:10.1021/om049253h

Organometallics 2004, 23, 6097-6098
6097
Oxidative Addition of Silyl Cyanides to Rhodium
Porphyrin Radical: Isocyanide or Cyanide Transfer
Mechanism
Kin Shing Chan,* Lirong Zhang, and Chun Wah Fung
Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories,
Hong Kong, People’s Republic of China
Received September 26, 2004
Summary: Rhodium porphyrin radical, coordinated with
pyridine, activated the carbon-silicon bond of silyl
cyanides to yield rhodium porphyrin silyls and cyanide.
The reaction with Me3SiCN exhibited second-order
kinetics, rate ) kobsd[Rh(tmp)][Me3SiCN], at a suffici-
ently high concentration of pyridine, and the mechanism
was interpreted to involve a cyanide or isocyanide group
transfer to rhodium radical.
Activation of silicon-carbon bonds by transition- and
lanthanide-metal complexes is fundamentally interest-
ing and industrially important with potential applica-
tions in the catalytic synthesis and modification of new
organosilicon polymers (eq 1).1 The application of this
Figure 1.
studies in identifying a novel radical type isocyanide or
cyanide transfer process.
2M + SiCN f MSi + MCN
(1)
A solution of Rh(tmp)7,8 in benzene reacted with 10
equiv of Me3SiCN at 110 °C for 10 h to give Rh(tmp)-
method in organic synthesis has been reported in
insertion reactions with alkynes.2 Most reported ex-
amples of carbon-silicon bond activation involve clas-
sical oxidative addition at a metal center by a two-
electron process,3,4 electrophilic aromatic substitution,5
and σ-bond metathesis.6 We have recently reported that
(tetramesitylporphyrinato)rhodium(II) [Rh(tmp)] (Fig-
ure 1),7 a metal-centered radical, activates nonstrained
aliphatic carbon-carbon bonds of nitroxides to yield
rhodium porphyrin alkyls.8,9 Expanding the scope of
substrates, we have found that Rh(tmp) undergoes a
formal carbon-silicon bond activation10 with silyl cya-
nides11,12 and now report the results of our kinetic
13
SiMe3 and Rh(tmp)CN14 in 16 and 14% 1H NMR
yields, respectively (eq 1). The carbon-silicon bond has
been activated, but the yields were rather poor.
When pyridine was added to Rh(tmp) to form the
electron-rich five-coordinate complex pyRh(tmp),15,16 the
rate of carbon-silicon bond activation (CSA) was much
faster. The yields of Rh(tmp)SiMe3 and trans-pyRh-
(tmp)CN also increased significantly (eq 2; Table 1,
C6H6, N2
2Rh(tmp) + RMe2SiCN + py
8
Rh(tmp)SiRMe2 + pyRh(tmp)CN (2)
* To whom correspondence should be addressed. E-mail: ksc@
cuhk.edu.hk.
entries 1-4). The product Rh(tmp)SiMe3 did not coor-
dinate with pyridine either in solution or in the solid
state after isolation, presumably due to the stronger
trans effect of the silyl group.17 The rates and yields
increased slightly from 70 to 130 °C. When the sterically
(1) (a) Suginome, M.; Oike, H.; Ito, Y. J. Am. Chem. Soc. 1995, 117,
1665-1666. (b). Wu, H.-J.; Interrante, L. V. Macromolecules 1992, 25,
1840-1841 and references therein.
(2) Chatani, N.; Hanafusa, T. Tetrahedron Lett. 1986, 27, 4201-
4204.
(3) Hofmann, P.; Heiss, H.; Neiteler, P. Mu¨ller, G.; Lachmann, J.
Angew. Chem., Int. Ed. Engl. 1990, 29, 880-882.
(4) (a) Gilges, H.; Schubert, U. Organometallics 1988, 7, 4760-
47612. (b) Schubert, U. Angew. Chem., Int. Ed. Engl. 1994, 33, 3463-
3465.
(12) The activation of the C(sp)-Si bond of alkynylsilanes has been
reported: (a) Huang, D.; Heyn, R. H.; Bollinger, J. C.; Caulton, K. G.
Organometallics 1997, 16, 292-293. (b) Edelbach, B. L.; Lachicotte,
R. J.; Jones, W. D. Organometallics 1999, 18, 4660-4768.
(13) An authenic sample of Rh(tmp)SiMe3 was prepared by the
reductive silylation of Rh(tmp)I with Na/Hg-Me3SiCl in 15% yield.
See: Tse, A. K.-S.; Wu, B.-M.; Mak, T. C. W.; Chan, K. S. J. Organomet.
Chem. 1998, 568, 257-261.
(5) Steenwinkel, P.; Gossage, R. A.; Maunula, T.; Grove, D. M.; van
Koten, G. Chem Eur. J. 1988, 4, 763-768.
(6) Castillo, I.; Tilley, T. D. J. Am. Chem. Soc. 2001, 123, 10526-
10534.
(7) Wayland, B. B.; Ba, S.; Sherry, A. E. J. Am. Chem. Soc. 1991,
113, 5305-5311.
(14) Poszmik, G.; Carroll, P. J.; Wayland, B. B. Organometallics
1993, 12, 3210-3417.
(8) Tse, M. K.; Chan, K. S. Dalton 2001, 510-511.
(9) Mak, K. W.; Yeung, S. K.; Chan, K. S. Organometallics 2002,
21, 2362-2364.
(15) Wayland, B. B.; Sherry, A. E.; Bunn, A. G. J. Am. Chem. Soc.
1993, 115, 7675-7684.
(10) Sakkai, S.; Ieki, M. J. Am. Chem. Soc. 1993, 115, 2373-2381.
(11) The activation of the C(sp3)-CN bond of nitriles has been
reported: (a) Churchill, D.; Shin, J. H.; Hascall, T.; Hahn, J. M.;
Bridgewater, B. M.; Parkin, G. Organometallics 1999, 18, 2403-2406.
(b) Taw, F. L.; White, P. S.; Bergaman, R. G.; Brookhart, M. J. Am.
Chem. Soc. 2002, 124, 4192-4193.
(16) Collman, J. P.; Boulatov, R. J. Am. Chem. Soc. 2000, 122,
11812-11821.
(17) Rh(tmp)SiMe3 did not show any change of proton chemical shift
in benzene-d6 with excess pyridine added. If there were any pyRh-
(tmp)SiMe3 formed, it would be very low in concentration and the
binding is therefore very weak.
10.1021/om049253h CCC: $27.50 © 2004 American Chemical Society
Publication on Web 11/23/2004

6098 Organometallics, Vol. 23, No. 26, 2004
Table 1. Reaction of Silyl Cyanides with PyRh(tmp)
Communications
product (% yield)
entry
substratea
temp/°C
time
1
2
3
4
5
6
Me3SiCN
70
90
110
130
110
130
3 h
Rh(tmp)SiMe3 (71)
Rh(tmp)SiMe3 (73)
Rh(tmp)SiMe3 (88)
Rh(tmp)SiMe3 (82)
Rh(tmp)SitBuMe2 (18)
Rh(tmp)SitBuMe2 (22)
pyRh(tmp)CN (72)
pyRh(tmp)CN (70)
pyRh(tmp)CN (81)
pyRh(tmp)CN (84)
pyRh(tmp)CN (85)
pyRh(tmp)CN (80)
Me3SiCN
1 h
Me3SiCN
1 h
1 h
1 day
1 day
Me3SiCN
tBuMe2SiCN
tBuMe2SiCN
a Conditions: 10 equiv of silyl cyanide with 2 equiv of pyridine.
t
more hindered BuMe2SiCN was used, a lower yield of
A few possibilities exist for the the oxidative addition
of the Si-CN bond. (1) An electron-transfer mechanism
via disproportionation of pyRh(py) occurs to yield [pyRhIII-
(tmp)]+ and [pyRhI(tmp)]-,15,16 which may react either
by an electrophilic or a nucleophilic manner, respec-
tively, with Me3SiCN. The pathways involving [pyRhIII-
(tmp)]+ and [pyRhI(tmp)]- existing in preequilibrium or
being involved in the rate-controlling step are ruled out,
since neither [pyRhIII(tmp)]+ nor [pyRhI(tmp)]- reacted
with Me3SiCN to yield Rh(tmp)SiMe3. Therefore, pyRh-
(tmp) radical is the reacting species. (2) A two-electron
oxidative addition process occurs with the involvement
of a formal, neutral cis-silyl Rh(IV) cyanide intermedi-
ate, which then yields Rh(tmp)CN and a silyl radical.
This mechanism is less preferred, as a crowded side-on
cis coordination18 of silyl cyanide to rhodium is neces-
sary. Furthermore, this pathway requires an uncommon
seven-coordinated Rh(IV)19 organometallic species. (3)
A one-electron oxidative addition occurs with the formal
cyanide transfer. The mechanism is consistent with the
Rh(tmp)SitBuMe2 was observed, while pyRh(tmp)CN
still was isolated in high yield. Since Rh(tmp)SitBuMe2
in benzene either with or without excess pyridine added
was thermally stable at 110 °C for at least 1 day, its
poor isolated yield is ascribed to a sterically hindered
reaction.
Kinetic measurements of reaction 2 with Me3SiCN
carried out spectrally at 538 nm under the conditions
70.0 °C, initial concentrations (3.28-9.06) × 10-5 M Rh-
(tmp), (0.61-6.21) × 10-2 M Me3SiCN, and (0.01-4.80)
× 10-3 M pyridine yielded the rate law shown in eq 3.
Under the conditions of the measurements with Me3-
SiCN always in at least 10-fold excess, the rate becomes
pseudo first order, in accord with eq 4. The rate was
independent of pyridine at sufficiently high concentra-
tion (g5.0 × 10-4 M) and exhibited saturation kinetics
in pyridine. The mechanistic scheme shown in eqs 5-7
conforms to the rate law, with reaction 5 attaining a
fast preequilibrium and the formal cyanide group
transfer step to rhodium in reaction 6 being the rate-
determining step. The rates were also measured from
60 to 90 °C.
small ∆H2 ) 13.1 kcal mol-1 and ∆S2 ) -26.1 cal
mol-1 K-1, typical of a bimolecular atom transfer
reaction in cobalt(II) chemistry.20 In this mechanism,
SiCN can coordinate to Rh(tmp) in a linear, kinetically
indistinguishable N- or C-bound form. As Me3SiNC is
also present in Me3SiCN in small amounts and the
proportion increases at elevated temperature,21 the
C-bound (i.e., isonitrile) form may be the more preferred
coordination mode, in view of the reported facile reac-
tions of MeNC and BuNC with Rh(tmp).14,22
In conclusion, we have discovered the novel bond
activation of silyl cyanide by rhodium porphyrin metal-
centered radical. The mechanism has been identified to
be a bimolecular reaction with the isocyanide or cyanide
group transfer to rhodium in the rate-controlling step.
Bond activation by Rh(tmp) and mechanistic studies are
being continued.
q
q
rate ) k′obsd[Rh(tmp)]0[Me3SiCN]
) kobsd[Rh(tmp)]0
(3)
(4)
Rh(tmp) + py yk1z pyRh(tmp)
(5)
k-1
k2 (rds)
pyRh(tmp) + Me3SiCN y
z pyRh(tmp)CN +
k-2
Me3Si (6)
k3
Me3Si + pyRh(tmp)
98 Rh(tmp)SiMe3 + py (7)
-d[Me3Si]/dt ) 0 (under steady-state conditions)
rate ) d[Me3SiRh(tmp)]/dt )
k2K1[py]
Acknowledgment. We thank the Research Grants
Council of Hong Kong of the SAR of China for financial
support (No. 400203).
[Rh(tmp)]0[Me3SiCN] (8)
1 + K1[py]
Supporting Information Available: Text, tables, and
figures giving experimental data for the products, kinetic
studies, and derivation of rate law. This material is available
free of charge via the Internet at http://pubs.acs.org.
Rh(tmp) is known to coordinate with pyridine, and
pyRh(tmp) has been characterized by ESR.15 The stoi-
chiometry and binding constants of reaction 5 were
measured spectrophotometrically at λ ) 536 nm from
20 to 40 °C. Analyses of the data confirmed a 1:1 adduct
and yielded the binding constant at 30.0 °C; K1 ()k1/
k-1) ) 2.95 × 104 M-1, ∆H1 ) -10 ( 1 kcal mol-1, and
∆S1 ) -12 ( 2 cal mol-1 K-1. Substitution of extrapo-
lated values of K1 at 60 to 90 °C in eq 8 allows the
estimation of k2. Evaluation of the activation parameters
concerning k2 from 60 to 90 °C then yielded ∆H2q ) 13.1
OM049253H
(18) Mak, K. W.; Chan, K. S. J. Am. Chem. Soc. 1998, 120, 9686-
9687.
(19) Pestovsky, O.; Bakac, A. Inorg. Chem. 2002, 15, 3975-3982.
(20) Halpern, J. Acc. Chem. Res. 1970, 3, 386-392.
(21) Bither, T. A.; Knoth, W. H.; Lindsey, R. V., Jr.; Sharkey, W. H.
J. Am. Chem. Soc. 1958, 80, 4151-4153.
(22) The sluggish reaction of MeCN with Rh(tmp) at 110 °C supports
the C-bound form of Me3SiCN in coordination.
( 1.3 kcal mol-1 and ∆S2q ) -26.3 ( 3.7 cal mol-1 K-1
.