Journal of the American Chemical Society
whether collapse occurs in the solvent cage40 or not. HAT is
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4
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further supported by the absence of a proximal, empty valence
for hydrometallation or cyclometallation on the putative Co-
salen hydride complex.31 Furthermore, the isomerization ap-
pears to be under kinetic, not thermodynamic control, since
isopentenyl-prenyl malonate 26 cycloisomerizes, whereas
diprenylmalonate 27 does not, even though both species would
give rise to the same radical intermediate subject to the same
unimolecular cyclization rate. Furthermore, terminal alkenes
isomerize one position over and further isomerization is slow,
likely a result of sterics.41 A tentative catalytic cycle based on
these observations is shown in Figure 4.
2012, 134, 10357. (c) Chen, C.; Dugan, T. R.; Brennessel, W. W.;
Weix, D. J.; Holland, P. L. J. Am. Chem. Soc. 2014, 136, 945. (d)
Gauthier, D.; Lindhardt, A. T.; Olsen, E. P. K.; Overgaard, J.;
Skrydstrup, T. J. Am. Chem. Soc. 2010, 132, 7998.
(3) For isomerization of alkenes allylic to heteroatoms, see the
following reviews. Allylic ethers and alcohols: (a) Uma, R.; Crévisy,
C.; Grée, R. Chem. Rev. 2003, 103, 27. Allylic amines: (b) Krom-
piec, S.; Krompiec, M.; Penczek, R.; Ignasiak, H. Coord. Chem.
Rev. 2008, 252, 1819.
(4) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G., In
Principles and Applications of Organotransition Metal Chemistry,
University Science Books: Mill Valley, CA, (1987); p 527.
(5) (a) Trost, B. M. Angew. Chem. Int. Ed. Engl. 1995, 34, 259;
(b) Trost, B. M. Acc. Chem. Res. 2002, 35, 695.
(6) Michelet, V.; Toullec, P. Y.; Genêt, J.-P. Angew. Chem. Int.
Ed. 2008, 47, 4268.
(7) For a comprehensive review of diene cycloisomerizations,
see: Yamamoto, Y. Chem. Rev. 2012, 112, 4736.
(8) Widenhoefer, R. A. Acc. Chem. Res. 2002, 35, 905.
(9) Ojima, I.; Donovan, R. J.; Shay, W. R. J. Am. Chem. Soc.
1992, 114, 6580.
(10) For example see: (a) Okamoto, S.; Livinghouse, T. J. Am.
Chem. Soc. 2000, 122, 1223. For exceptions, see: (b) Takacs, J.
M.; Weidner, J. J.; Newsome, P. W.; Takacs, B. E.; Chidambaram,
R.; Shoemaker, R. J. Org. Chem. 1995, 60, 3473; (c) Piers, W. E.;
Shapiro, P. J.; Bunel, E. E.; Bercaw, J. E. Synlett 1990, 74.
(11) Olefin isomerization using HCo(CO)4: Heck, R. F.; Breslow,
D. S. J. Am. Chem. Soc. 1961, 83, 4023.
9
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12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
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60
(12) For halogen atom-transfer cycloisomerization, see: (a)
Clark, A. J. Chem. Soc. Rev. 2002, 31, 1; (b) Mun
̃oz-Molina, J. M.;
Figure 4. Hypothetical catalytic cycle.
Belderrain, T. R.;Perez, P. J. Eur. J. Inorg. Chem. 2011, 3155.
́
In summary, we have shown that catalytic amounts of
Co(salent-Bu,t-Bu)Cl and organosilanes effect the isomerization
of terminal alkenes, which takes the form of cycloisomeriza-
tion if a pendant alkene is available, and retrocycloisomeriza-
tion if the alkene is adjacent to a strained ring. These reactions
likely proceed by reversible hydrogen atom transfer (HAT) to
generate a transient carbon-centered radical. Given the toler-
ance of these reactions to functional groups, and the number of
permutations available to salen ligand substitution, application
and expansion of these tools are likely.
(13) For alkene isomerization mediated by sulfinyl radical hydro-
gen atom transfer (radical abstraction), see: (a) Vogel, P.; Turks,
M.; Bouchez, L.; Marković, D.; Varela-Álvarez, A.; Sordo, J. A. Acc.
Chem. Res. 2007, 40, 931. For thiol-catalyzed radical isomeriza-
tion, see: (b) Fielding, A. J.; Roberts, B. P. Tetrahedron Lett. 2001,
42, 4061.
(14) Norton observed a reversible, initial HAT in hydrogenation
(a) Tang, L.; Papish, E. T.; Abramo, G. P.; Norton, J. R.; Baik, M.-
H.; Friesner, R. A.; Rappé, A. J. Am. Chem. Soc. 2003, 125, 10093.
Isomerization and cycloisomerization were occasional byproducts
in HAT hydrogenation and reductive cyclization studies: (b) Li, G.;
Pulling, M. E.; Estes, D. P.; Norton, J. R. J. Am. Chem. Soc. 2012,
134, 14662; (c) Hartung, J. Pulling, M. E.; Smith, D. M.; Yang, D.
X.; Norton, J. R. Tetrahedron 2008, 64, 11822.
(15) (a) Nakano, T.; Okamoto, Y. ACS Symp. Ser. 1998, 685,
451. (b) Liao, C.-M.; Hsu, C.-C.; Wang, F.-S.; Wayland, B. B. Peng,
C.-H. Polym. Chem. 2013, 4, 3098; (c) Chiang, L.; Allan, L. E. N.;
Alcantara, J.; Wang, M. C. P.; Storr, T.; Shaver, M. P. Dalton Trans.
2014, 43, 4295.
(16) Gridnev, A. A; Ittel, S. D. Chem. Rev. 2001, 101, 3611.
(17) Iwasaki, K.; Wan, K. K.; Oppedisano, A.; Crossley, S. W.
M.; Shenvi R. A. J. Am. Chem. Soc. 2014, 136, 1300.
(18) Boger also proposes hydrogen radical addition as the initiat-
ing step in his Fe-mediated radical hydrofunctionalizations: (a) Ishi-
kawa, H.; Colby, D. A.; Seto, S.; Va, P.; Tam, A.; Kakei, H.; Rayl, T.
J.; Hwang, I.; Boger, D. L. J. Am. Chem. Soc. 2009, 131, 4904; (b)
Leggans, E. K.; Barker, T. J.; Duncan, K. K.; Boger, D. L. Org. Lett.
2012, 14, 1428; (c) Barker, T. J.; Boger, D. L. J. Am. Chem. Soc.
2012, 134, 13588.
(19) Herzon insightfully invokes HAT in his alkenyl halide reduc-
tion: King, S. M.; Ma, X.; Herzon, S. B. J. Am. Chem. Soc. 2014,
136, 6884.
ASSOCIATED CONTENT
Supporting Information. Experimental procedures and spectro-
scopic data. This material is available free of charge via the Internet
AUTHOR INFORMATION
Corresponding Author
Author Contributions
‡These authors contributed equally.
Notes
A provisional patent (U.S. Serial No. 62/078,140) has been filed.
(20) Mukaiyama, T.; Yamada, T. Bull. Chem. Soc. Jpn. 1995, 68,
17.
ACKNOWLEDGMENT
Dedicated to Carlos F. Barbas, III in memoriam.
(21) (a) Gaspar, B.; Waser, J.; Carreira, E. M. Org. Syn. 2010,
87, 88. (b) Gaspar, B.; Carreira, E. M. J. Am. Chem. Soc. 2009,
131, 13214. (c) Gaspar, B.; Carreira, E. M. Angew. Chem., Int. Ed.
2008, 47, 5758. (d) Gaspar, B.; Waser, J.; Carreira, E. M. Synthesis
2007, 3839. (e) Gaspar, B.; Carreira, E. M. Angew.Chem., Int. Ed.
2007, 46, 4519. (f) Waser, J.; Gaspar, B.; Nambu, H.; Carreira, E.
Financial support for this work was provided by the NIH
(GM104180) and FRQNT (fellowship to F.B.). We thank Carla
Obradors for early contributions, Professors Jin-Quan Yu and Phil
Baran for helpful conversations, and Laura Pasternack for help with
NMR. We are grateful to Eli Lilly, Boehringer Ingelheim, Amgen, the
Baxter Foundation and the Sloan Foundation for additional financial
support.
M. J. Am. Chem. Soc. 2006, 128, 11693. (g) Waser,J.; Gonzaĺez-
Gomez, J. C.; Nambu, H.; Huber, P.; Carreira, E. M. Org. Lett.
́
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