D
S. Feng, K. S. Chan
Cluster
Synlett
H
[H]
Ph
RhII(tmp) +
(tmp)Rh
Ph
(tmp)Rh
Ph
1a
3
2d
H
Ph
[H] =
or Rh(tmp)H
Scheme 5 Proposed mechanism of RhII(tmp)-catalyzed ring opening of cyclopropane
Rh(tmp)CH2CH2CH2Ph (2d). Initially, RhII(tmp) (1a) cleaves
the C–C bond of the cyclopropyl group to afford the benzyl
carbon centered stabilized radical 3. Then the radical 3 ab-
stracts a hydrogen atom from the substrate15 or from the
weak Rh(tmp)H (1b) bond9 to generate the 1,3-addition
product Rh(tmp)CH2CH2CH2Ph (2d).
C6H6
Ph
(tmp)Rh
Ph
Rh(tmp)H +
1b
N2, dark, 25 °C, 2 d
(100 equiv)
2d, 58%
Equation 3
As Rh(tmp)H is known to contain a small amount of
RhII(tmp) in equilibrium in solution,16 we therefore propose
the reaction mechanism operates with a RhII(tmp)-cata-
lyzed 1,3-addition of Rh(tmp)H into the C–C bond of cyclo-
propane, analogous to the RhII(ttp)-catalyzed 1,2-addition
of Rh(ttp)H into the C–C bond of cyclooctane (Scheme 3).9
The relationship between substrate and the reaction
rate as well as the product yield also validates this proposal.
The attack of RhII(tmp) to cyclopropane generates a
Rh(tmp) propyl carbon centered radical. The radical stabili-
ty is correlated to the corresponding C–H bond-dissociation
energy of substituted propane. Table 3 shows the α-C–H
bond dissociation energy of selected propane analogues.17
The reaction rate and yield of various cyclopropanes indeed
increase with decreasing C–H bond-dissociation energy ex-
cept that of cyclopropyl benzene, possibly due to the larger
steric hindrance of the phenyl group.
In summary, we have discovered the room-temperature,
regioselective, metalloradical-catalyzed ring-opening CCA
of both polar and nonpolar cyclopropanes by
RhII(tmp)/PPh3 to afford 1,3-addition products in moderate
yields.18 The reaction is water tolerant and functional-
group compatible. Further studies are ongoing.
Funding Information
This work was supported by Hong Kong Research Grants Council (GRF
NO. 400113).
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Supporting Information
Supporting information for this article is available online at
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References and Notes
Table 3 α-C–H Bond-Dissociation Energy (BDE) of Selected Propane
Analogues
(1) Wiberg, K. W. Angew. Chem., Int. Ed. Engl. 1986, 25, 312.
(2) Lambert, J. B.; Napoli, J. J.; Johnson, K. K.; Taba, K. N.; Packard, B.
S. J. Org. Chem. 1985, 50, 1291.
Compound
C–H bond BDE (kcal/mol)
(3) Fumagalli, G.; Stanton, S.; Bower, J. F. Chem. Rev. 2017, 117,
9404.
H-CH(CH3)C(O)Ph
H-CH(CH3)C(O) CH3
H-CH(CH3)CN
92.9
92.3
94.0
85.4
95.6
91.5
(4) Bart, S. C.; Chirik, P. J. J. Am. Chem. Soc. 2003, 125, 886.
(5) Bessmertnykh, A. G.; Blinov, K. A.; Grishin, Y. K.; Donskaya, N.
A.; Beletskaya, I. P. Tetrahedron Lett. 1995, 36, 7901.
(6) Pereyre, M.; Godet, J. Y. Tetrahedron Lett. 1970, 3653.
(7) To, C. T.; Chan, K. S. Acc. Chem. Res. 2017, 50, 1702.
(8) Wayland, B. B.; Ba, S.; Sherry, A. E. J. Am. Chem. Soc. 1991, 113,
5305.
H-CH(CH3)Ph
H-CH(CH3)COOEt
H-C(CH2CH3)(COOEt)2
(9) Chan, Y. W.; Chan, K. S. J. Am. Chem. Soc. 2010, 132, 6920.
(10) Lee, S. Y.; Feng, S.; Chan, K. S. Dalton Trans. 2016, 45, 3522.
(11) Nelson, A. P.; DiMagno, S. G. J. Am. Chem. Soc. 2000, 122, 8569.
(12) Ogoshi, H.; Setsune, J. I.; Yoshida, Z. I. J. Organomet. Chem. 1980,
185, 95.
(13) (a) Wayland, B. B.; Sherry, A. E.; Bunn, A. G. J. Am. Chem. Soc.
1993, 115, 7675. (b) Collman, J. P.; Boulvtov, R. J. Am. Chem. Soc.
2000, 122, 11812. (c) Chan, K. S.; Li, X. Z.; Zhang, L.; Fung, C. W.
Organometallics 2007, 26, 2679. (d) Chan, K. S.; Li, X. Z.; Lee, S. Y.
Organometallics 2010, 29, 2850.
The slower CCA of cyclopropyl benzene with Rh(tmp)H
than that of RhII(tmp) further suggests the attack of
RhII(tmp) at the carbon center is the rate-determining step,
followed by the rapid exothermic H atom abstraction.
On the basis of the above results and earlier re-
ports,9,15,16 Scheme 5 shows the proposed reaction mecha-
nism for the ring opening of cyclopropyl benzene
by RhII(tmp) (1a) to afford the 1,3-addition product
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E