Catalysis of Radical Cyclizations from Alkyl Iodides under H2: Evidence for Electron Transfer from [CpV(CO)3H]-

Article abstract of DOI:10.1021/jacs.8b02119

Radical cyclizations are most often achieved with Bu₃SnH in the presence of a radical initiator, but environmental considerations demand that alternative reagents be developed - ones that can serve as a synthetic equivalent to the hydrogen atom. We have revisited [CpV(CO)₃H]^-, a known replacement for Bu₃SnH, and found that it can be used catalytically under H₂ in the presence of a base. We have carried out tin-free catalytic radical cyclizations of alkyl iodide substrates. The reactions are atom-efficient, and the conditions are mild, with broad tolerance for functional groups. We have, for example, achieved the first 5-exo radical cyclization involving attack onto a vinyl chloride. We suggest that the radicals are generated by an initial electron transfer.

Full text of DOI:10.1021/jacs.8b02119

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Catalysis of Radical Cyclizations from Alkyl Iodides
2
3
–
Under H: Evidence for Electron Transfer from [CpV(CO)H]
Jonathan L. Kuo, Chris Lorenc, Janine M. Abuyuan, and Jack R. Norton
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b02119 • Publication Date (Web): 15 Mar 2018
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Catalysis of Radical Cyclizations from Alkyl Iodides Under H2:
Evidence for Electron Transfer from [CpV(CO)3H]–
Jonathan L. Kuo, Chris Lorenc, Janine M. Abuyuan,† Jack R. Norton
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Department of Chemistry, Columbia University, 3000 Broadway, New York, New York, 10027
†Department of Chemistry, Barnard College, 3009 Broadway, New York, New York, 10027
Supporting Information Placeholder
generating reactions catalytic, with near-perfect atom econ-
omy.7 H• transfer reactions are finding synthetic applica-
tions;8 the Snyder group recently found that H• transfer from
a metal hydride was the only practical way of hydrogenating
a key enone in the synthesis of exochomine.9
ABSTRACT: Radical cyclizations are most often achieved
with Bu3SnH in the presence of a radical initiator, but envi-
ronmental considerations demand that alternative reagents
be developed – ones that can serve as a synthetic equiva-
lent to the hydrogen atom. We have revisited [CpV(CO)3H]–,
a known replacement for Bu3SnH, and found that it can be
used catalytically under H2 in the presence of a base. We
have carried out the tin-free and catalytic radical cyclization
of alkyl iodide substrates. The reactions are atom-efficient
and the conditions are mild, with broad tolerance for func-
tional groups. We have, for example, achieved the first 5-
exo radical cyclization involving attack onto a vinyl chloride.
We suggest that the radicals are generated by an initial
electron transfer.
Still needed, however, is a replacement for Bu3SnH in the
reduction of R–X. Transition-metal hydrides are known to
reduce alkyl halides,10 but in general these reactions are
stoichiometric, have limited functional group tolerance, or
use harsh reductants to make the reaction catalytic. Hy-
drides that can be regenerated from H2 are a more atom
efficient approach for catalyzing the reduction of R–X.
A hydride known to generate R• stoichiometrically from
R–X (eq 6) is the [CpV(CO)3H]– anion (1), reported by Kin-
ney, Jones, and Bergman by 1978.11 The loss of stereo-
chemistry that is observed when
1 reacts with α-
bromoethylbenzene (eq 7) is consistent with a radical inter-
mediate.
Tin hydrides such as Bu3SnH are frequently used to re-
duce organic halides R–X to the corresponding hydrocar-
bons R–H. These reactions occur by the radical chain
mechanism in eqs 1-5, and thus require stoichiometric
amounts of the tin hydride. Both the resulting Bu3SnX and
any leftover Bu3SnH are toxic and difficult to remove.1 The
few reactions that use tin catalytically rely on a strong hy-
dride donor to reduce the Bu3SnX back to Bu3SnH, reduc-
ing functional-group tolerance.2
A series of observations in the literature imply that 1 can
be regenerated from [CpV(CO)3Br]– (2) under H2. Dissocia-
tion of Br– from 2 should generate the coordinatively unsatu-
rated CpV(CO)3 (3),12 which can be trapped by many two-
initiation
(1)
(2)
initiator
2 init•
init–H
init•
+
Bu3Sn–H
Bu3Sn•
R
+
electron
donors
to
generate
complexes
like
CpV(CO)3(PMe)3 (4a),
CpV(CO)3THF
(4b),13
and
heavy atom abstraction
CpV(CO)3MeCN (4c).11a Similarly, 3 should coordinate H2 to
generate the dihydrogen complex CpV(CO)3(H2) (5),14
which will regenerate the hydride anion [CpV(CO)3H]– (1) if
the H2 ligand were deprotonated. This sequence (Scheme
1) would enable the catalytic hydrodehalogenation of alkyl
halides.
(3)
R
Bu3Sn•
Bu3Sn–X
Bu3Sn•
•
+
+
+
()n
X
()n
hydrogen atom transfer
R
R
(4)
(5)
Bu3Sn–H +
•
()n
()n
H
net reaction (omitting initiator)
R–X Bu3Sn–H
Bu3Sn–X
+
R–H
+
Scheme 1. Reactions Relevant to Catalysis Under H2
Transition-metal hydrides are also potent H• donors.
Their M–H bonds are often weaker3 than the Sn–H bond of
Bu3SnH (78 kcal/mol),4 allowing them to transfer H• to sub-
strates that do not react with the latter. For example, transi-
tion-metal hydrides can generate radicals by H• transfer to
activated alkenes and alkynes.5 The resulting M• can be
converted back to M–H by H2,6 making these radical-
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With the optimized conditions from Table 2, we examined
1
2
3
4
5
6
7
8
substrate scope (Table 3). The first three entries involve
substrates for which radical cyclizations have previously
been reported; catalysis by 1 with DBU/H2 generates the
products with similar yields and selectivity. The yield is low-
er in the 6-exo case in entry 3, presumably because the
cyclization is slow enough that the H• transfer is competi-
tive. The functional group tolerance is quite good: the free
alcohol in entry 4 is not a problem. Polyhalogenated sub-
strates react selectively at the iodide (for example, the aryl
bromide in entry 5 remains unchanged). Entry 6 is the first
5-exo cyclization of an alkyl radical21 onto a chlorinated
double bond; Sharp and Zard have observed cyclizations
promoted by Bu3SnH result in the dechlorination of such
substrates.22
H2 must, however, compete with other L (including polar
solvents) for the vacant coordination site of 3. We have
therefore studied the equilibrium between CpV(CO)3L and
CpV(CO)3(H2) (eq 10). The THF adduct of 3 (4b) has been
generated by removal of H– from 1,15 and the MeCN adduct
4c has been made by protonating 1 in CH3CN.16 We have,
however, found that 4c can be made more easily by oxidiz-
ing 1 with Cp*2Fe+ (eq 8).17
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Table 2. Radical Cyclization Optimization
Ph
Ph
Ph
Ph
Ph
H
cat. [NEt4][1]
I
I
1
Eq 10 must lie far to the left, as the H NMR of 4c18 does
H2, DBU
Solvent
not change under 1 atm of H2. However, sufficiently strong
base pulls 4c through 5 to the hydride anion 1. There is no
reaction between 4c and H2 in the presence of Et3N, but
DBU gives 1 (by 1H NMR) after several h (eq 9, 10). These
experiments imply that the pKa of 5 lies between 18 and 25
in MeCN.
N
N
N
HN
Ts
10
N
Ts
9
Ts
11
Ts
8
Ts
8
solventa
8 : 9 : 10 : 11c
Entry cat. loading
DBU
H2
2 eq.
2 eq.
2 eq.
2 eq.
2 eq.
1 eq.
0 eq.
2 eq.
2 eq.
2 eq.
1
2
3
4
5
6
7
8
9
0%
THF
THF
THF
THF
THF
THF
THF
MeCN
60 : -- : 40 : --
60 : 24 : -- : 16
58 : 13 : -- : 30
54 : -- : 13 : 34
-- : 10 : -- : 90
37 : -- : -- : 63
86 : -- : -- : 14
24 : 42 : -- : 34
90 psig
0 psig
15%
15%
15%
15%
15%
15%
15%
15%
0%
15 psig
40 psig
90 psig
90 psig
90 psig
90 psig
90 psig
0 psig
Knowing that each step in Scheme 1 was practical, we
tried 119 as a catalyst for the reduction of alkyl bromides
(vide infra) and alkyl iodides in the presence of DBU and H2.
For alkyl iodide substrate 6, we observed a catalytic hy-
drodehalogenation20 to generate 7 in MeCN, but the reac-
tion worked better in the less-coordinating solvent THF (Ta-
ble 1). A higher H2 pressure and an increased concentration
of DBU led to a quantitative yield of 7.
THF (dil.)b -- : -- : -- : 100
THF
61 : -- : 39 : --
10
All reactions run overnight at r.t. a Concentration: 50 mM in substrate b Concentration: 10
mM in substrate c Given as percentages; all crude yields were within 5% expected mass,
ratios determined by 1H NMR analysis.
We have encountered few difficulties when cyclizing sub-
strates containing heterocyclic moieties. Both 3-substituted
and 2-substituted thiophenes (entries 7 and 8) generate the
corresponding tetrahydrofuranyl thiophenes. The elimination
product in entry 8 (see Table 3, footnote c) is presumably
the result of its more acidic C–H bond.23 Pyridyl- and
furanyl-substituted alkenes (entries 9 and 10) generate oth-
er polycyclic products in good yield.
This reaction (catalytic cyclization by [CpV(CO)3H]–) com-
pares favorably to the corresponding reactions with
Bu3SnH. We generate only BH+ ([DBU•HI] precipitates from
solution, and can be removed by a filtration through a silica
plug); Bu3SnH generates an equivalent of Bu3Sn• waste,
which is difficult to remove. Our reaction are done at room
temperature, whereas reactions with Bu3SnH must be run at
elevated temperatures to initiate a radical chain.
Table 4 shows substrates that [CpV(CO)3H]– does not
process effectively. Aryl iodides (entry 1), tertiary alkyl io-
dides (entry 3), and alkyl bromides (entry 2) only undergo
partial conversion under catalytic conditions, although these
reactions work when the hydride anion is used stoichiomet-
rically. The catalyst decomposes before the substrate can
be completely converted to product.
Table 1. Optimization of Radical Deiodination Conditions
Since the hydrodehalogenation reactions of 1 proceed by
radical intermediates, appropriate organic substrates should
undergo radical cyclization reactions. Indeed, 1 catalyzes
the cyclization of 8 with DBU under H2 (Table 2). There is
no catalysis if the 1 (entry 1), H2 (entry 2), or base (entry 7)
is omitted. A background elimination reaction between 8
and DBU results in 9 or its hydrolysis product 10 (entry 10).
Optimizing reaction conditions has allowed us to obtain a
quantitative yield of the cyclized product 11 (entry 9).
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Our observations suggest that the mechanism of the re-
the complete conversion of substrate before complete cata-
lyst decomposition. Electron transfers from low valent tran-
sition metal complexes have ample precedent for generat-
ing organic radicals,25 and an analogous electron transfer
mechanism has been reported for the reaction of 1 with
metal carbonyls.11c We thus propose the overall catalytic
cycle depicted in Scheme 2. The isolation of the hy-
drodehalogenation product in Table 2, Entry 3 suggests that
the vanadium hydride may not be as effective a hydrogen
atom donor as is presently believed.
1
2
3
4
5
6
7
8
action between alkyl halides and [CpV(CO)3H]– should be
reexamined. Kinney, Jones, and Bergman believed that the
reactions occurred by a radical chain mechanism like that in
eqs 1-5, and that the vanadium hydride was a better hydro-
gen atom donor than Bu3SnH. However, they also consid-
ered the possibility that the reaction of R–X and 1 began by
an electron transfer (eqs 11-13). Rates of electron transfer
to R–X vary in the order R–I > R–Br > R–Cl,24 and alkyl
halides > aryl halides. These trends match what is observed
in Tables 3 and 4, suggesting that the radicals are generat-
ed by an electron transfer. A fast electron transfer enables
Table 3. Radical Cyclization Substrate Scope
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Table 4. Substrates that work poorly with the catalytic
system
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Notes
The authors declare no competing financial interests.
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ACKNOWLEDGMENT
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We thank Robert G. Bergman and William D. Jones for
helpful discussions (and for their prior studies on
[CpV(CO)3H]–). Scott A. Snyder, Derek A. Pratt, and Andre-
as Gansäuer are acknowledged for helpful discussions.
J.L.K. acknowledges the National Science Foundation and
the Arun Guthikonda Memorial Fellowship for financial sup-
port, and J.M.A. acknowledges the Bernice G. Segal Sum-
mer Internship and the Barnard College Department of
Chemistry for financial support. Research reported in this
publication was supported by National Institute of General
Medical Sciences (NIGMS) of the National Institutes of
Health under award number R01GM124295.
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REFERENCES
1. (a) Breslow, R.; Light, J. Tetrahedron Lett. 1990, 31, 2957-2958;
(b) Baguley, P. A.; Walton, J. C. Angew. Chem., Int. Ed. Engl.
1998, 37, 3072–3082; (c) Studer, A.; Amrein, S. Synthesis. 2002,
835–849; (d) Clive, D. L. J.; Wang, J. J. Org. Chem. 2002, 67,
1192–1198; (e) Stien, D.; Gastaldi, S. J. Org. Chem. 2004, 69,
4464–4470; (f) Le Grognec, E.; Chrétien, J.-M.; Zammattio, F.;
Quintard, J.-P. Chem. Rev. 2015, 115, 10207-10260.
2. (a) Tormo, J.; Hays, D. S.; Fu, G. C. J. Org. Chem. 1998, 63,
5296–5297; (b) Hays, D. S.; Fu, G. C. Tetrahedron. 1999, 55,
8815-8832; (c) Corey, E. J.; Suggs, J. W. J. Org. Chem. 1975, 40,
2554-2555; (d) Stork, G.; Sher, P. M. J. Am. Chem. Soc. 1986, 108,
303–304.
3. (a) Choi, J.; Pulling, M. E.; Smith, D. M.; Norton, J. R. J. Am.
Chem. Soc. 2008, 130, 4250-4252; (b) Li, G.; Han, A.; Pulling, M.
E.; Estes, D. P.; Norton, J. R. J. Am. Chem. Soc. 2012, 134, 14662-
14665; (c) Estes, D. P.; Grills, D. C.; Norton, J. R. J. Am. Chem.
Soc. 2014, 136, 17362-17365.
There have been previous reports — some catalytic — of
the initiation of radical cyclizations by electron transfer (for
example with tetra(dimethylamino)ethylene,26 Ni(0) with
Zn,10a or Fe(CO)5 with phenanthroline,10d). The present sys-
tem is not only catalytic but uses no reducing agent stronger
than H2The [CpV(CO)3H]– system, with H2 as a chemical
fuel, may be seen as an alternative to photoredox cataly-
sis27 for the generation of reducing intermediates
Scheme 2. Proposed Catalytic Cycle with [CpV(CO)3H]–
4. Laarhoven, L. J. J.; Mulder, P.; Wayner, D. D. M. Acc. Chem.
Res. 1999, 32, 342-349.
5. (a) Smith, D. M.; Pulling, M. E.; Norton, J. R. J. Am. Chem. Soc.
2007, 129, 770-771; (b) Kuo, J. L.; Hartung, J.; Han, A.; Norton, J.
R. J. Am. Chem. Soc. 2015, 137, 1036-1039; (c) Li, G.; Kuo, J. L.;
Han, A.; Abuyuan, J. M.; Young, L. C.; Norton, J. R.; Palmer, J. H.
J. Am. Chem. Soc. 2016, 138, 7698-7704; (d) Estes, D. P.; Norton,
J. R.; Jockusch, S.; Sattler, W. J. Am. Chem. Soc. 2012, 134,
15512-15518.
6. Norton, J. R.; Spataru, T.; Camaoni, D.; Lee, S.-J.; Li, G.; Choi,
J.; Franz, J. A. Organometallics. 2014, 33, 2496-2502.
7. (a) Anastas, P. T.; Warner, J. C., Green Chemistry: Theory and
Practice. Oxford University Press: New York, 1998; (b) Laird, T.
Org. Process Res. Dev. 2012, 16, 1-2.
8. (a) Hoffmann, R. W. Chem. Soc. Rev. 2016, 45, 577-583; (b)
Iwasaki, K.; Wan, K. K.; Oppedisano, A.; Crossley, S. W. M.;
Shenvi, R. A. J. Am. Chem. Soc. 2014, 136, 1300-1303; (c)
Crossley, S. W.; Barabé, F.; Shenvi, R. A. J. Am. Chem. Soc. 2014,
136, 16788-16791; (d) Obradors, C.; Martinez, R. M.; Shenvi, R. A.
J. Am. Chem. Soc. 2016, 138, 4962-4971; (e) Green, S. A.; Matos,
J. L.; Yagi, A.; Shenvi, R. A. J. Am. Chem. Soc. 2016, 138, 12779-
12782; (f) King, S. M.; Ma, X.; Herzon, S. B. J. Am. Chem. Soc.
2014, 136, 6884-6887; (g) Ma, X.; Herzon, S. B. Chem. Sci. 2015,
6, 6250-6255.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information (synthetic procedures for the
substrate/catalyst, a general procedure for the radical cy-
clization, characterization data (1H- and 13C-NMR, tabulated
IR and HRMS data) is available free of charge on the ACS
Publications website.
9. Gao, A. X.; Hamada, T.; Snyder, S. A. Angew. Chem. Int. Ed.
2016, 55, 10301-10306.
10. (a) Kim, H.; Lee, C. Org. Lett. 2011, 13, 2050-2053; (b) Ekomie,
A.; Lefevre, G.; Fensterbank, L.; Lacote, E.; Malacria, M.; Ollivier,
C.; Jutand, A. Angew. Chem., Int. Ed. 2012, 51, 6942-6946; (c)
Rettenmeier, C.; Wadepohl, H.; Gade, L. H. Chem. Eur. J. 2014,
20, 9657-9665; (d) Hwang, J. Y.; Baek, J. H.; Shin, T. I.; Shin, J. H.;
Oh, J. W.; Kim, K. P.; You, Y.; Kang, E. J. Org. Lett. 2016, 18,
4900-4903; (e) Iwamoto, H.; Akiyama, S.; Hayama, K.; Ito, H. Org.
Lett. 2017, 19, 2614-2617; (f) Haibach, M. C.; Stoltz, B. M.; Grubbs,
AUTHOR INFORMATION
Corresponding Author
jrn11@columbia.edu
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R. H. Angew. Chem., Int. Ed. 2017, 56, 15123-15126; (g) Kyne, S.
stable for many hours, and suitable for studying the equilibrium with
H.; Clémancey, M.; Blondin, G.; Derat, E.; Fensterbank, L.; Jutand,
A.; Lefèvre, G.; Ollivier, C. Organometallics. 2017, 37, 761-771; (h)
Wang, Z.; Li, X.; Sun, H.; Fuhr, O.; Fenske, D. Organometallics.
2018, 37, 539-544.
11. (a) Kinney, R. J.; Jones, W. D.; Bergman, R. G. J. Am. Chem.
Soc. 1978, 100, 7902-7915; (b) Kinney, R. J.; Jones, W. D.;
Bergman, R. G. J. Am. Chem. Soc. 1978, 100, 635-637; (c) Jones,
W. D.; Huggins, J. M.; Bergman, R. G. J. Am. Chem. Soc. 1981,
103, 4415-4423.
12. Jones, W. D. Reactivity Patterns of Transition Metal Hydrides
and Alkyls. Ph.D Thesis, California Institute of Technology,
Pasadena, CA, 1979 (from the research group of Prof R. G.
Bergman)
H2.
1
2
3
4
5
6
7
8
19. During this time, we developed an improved protocol for the
synthesis of [NEt4][CpV(CO)3H] (see SI). There were no differences
between the reactions of the [NEt4]+ salt and the [PPN]+ salt, so the
tetraethylammonium salt was used subsequently.
20. The importantance of green hydrodehalogenation methods is
discussed further in ref 10f and 27a.
21. (a) Patel, V. F.; Andis, S. L.; Enkema, J. K.; Johnson, D. A.;
Kennedy, J. H.; Mohamadi, F.; Schultz, R. M.; Soose, D. J.; Spees,
M. M. J. Org. Chem. 1997, 62, 8868-8874; (b) Boger, D. L.; Boyce,
C. W.; Garbaccio, R. M.; Searcey, M. Tetrahedron Lett. 1998, 39,
2227-2230; (c) Stephenson, M. J.; Howell, L. A.; O'Connell, M. A.;
Fox, K. R.; Adcock, C.; Kingston, J.; Sheldrake, H.; Pors, K.;
Collingwood, S. P.; Searcey, M. J. Org. Chem. 2015, 80, 9454-
9467.
9
10
11
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
56
57
58
59
60
13. Hoch, M.; Rehder, D. J. Organomet. Chem. 1985, 288, C25-
C29.
14. George, M. W.; Haward, M. T.; Hamley, P. A.; Hughes, C.;
Johnson, F. P. A.; Popov, V. K.; Poliakoff, M. J. Am. Chem. Soc.
1993, 115, 2286-2299.
22. Sharp, L. A.; Zard, S. Z. Org. Lett. 2006, 8, 831-834.
23. (a) Catalan J. Am. Chem. Soc. 1984, 106, 421-422; (b) Seino J.
Am. Chem. Soc. 1995, 117, 12181-12193.
15. In ref. 13, CpV(CO)3(THF) (4b) was generated by mixing
[Et4N][CpV(CO)3H] with [Ph3C][BF4] in THF, presumably via the
coordinatively unsaturated CpV(CO)3. We tried the analogous
reaction in MeCN to generate CpV(CO)3(MeCN) (4c) using MeOTf
in place of [Ph3C][BF4], but found that the resulting solutions
decompose in a few minutes at room temperature.
24. (a) Ashby, E. C.; DePriest, R.; Goel, A.; Wenderoth, B.; Pham,
T. J. Org. Chem. 1984, 49, 3545-3556; (b) Ashby, E. C.;
Argyropoulos, J. N. J. Org. Chem. 1985, 50, 3274-3283; (c) Ashby,
E. C.; Welder, C. O.; Doctorovich, F. Tetrahedron Lett. 1993, 34,
7235-7238; (d) Ashby, E. C.; Deshpande, A. K. J. Org. Chem.
1994, 59, 3798-3805; (e) Welder, C. O.; Ashby, E. C. J. Org. Chem.
1997, 62, 4829-4833; (f) Ashby, E. C.; Welder, C. O. J. Org. Chem.
1997, 62, 3542-3551.
25. (a) Torii, S.; Inokuchi, T.; Yukawa, T. J. Org. Chem. 1985, 50,
5875-5877; (b) Pattenden, G. Chem. Soc. Rev. 1988, 17, 361-382;
(c) Gansäuer, A.; Fan, C.-A.; Piestert, F. J. Am. Chem. Soc. 2008,
130, 6916-6917; (d) Alonso, F.; Beletskaya, I. P.; Yus, M. Chem.
Rev. 2002, 102, 4009-4091.
26. Murphy, J. A.; Khan, T. A.; Zhou, S. Z.; Thomson, D. W.;
Mahesh, M. Angew. Chem., Int. Ed. 2005, 44, 1356-1360.
27. (a) Nguyen, J. D.; D'Amato, E. M.; Narayanam, J. M. R.;
Stephenson, C. R. J. Nature Chem. 2012, 4, 854-859; (b) Kim, H.;
Lee, C. Angew. Chem., Int. Ed. 2012, 51, 12303-12306; (c) Shen,
Y. Y.; Cornella, J.; Julia-Hernandez, F.; Martin, R. Acs Catal. 2017,
7, 409-412.
16. In ref 11a, Bergman showed that [CpV(CO)3H]– (1) can be
mixed with TfOH to generate CpV(CO)3MeCN (4c). This
presumably first generates the dihydrogen complex CpV(CO)3(H2)
(5), which loses H2. We reacted 1 with the more mild acid
[Et3NH][BF4] to generate 4c, but observed its decomposition over a
few hours at room temperature.
17. In ref 11a, Bergman reported that mixing 1 with AgCl generates
the solvate complex 4c. We speculated that this occurs by an initial
electron transfer, and so we tried the more mild single-electron
oxidant [Cp*2Fe][B(C6F5)4].
18. It has been said that 4b and 4c are thermally unstable at room
temperature (ref 13). However, we have found that solutions of
CpV(CO)4 in MeCN undergo conversion to CpV(CO)3MeCN (4c) in
ambient light (31% conversion over 2977 minutes), there is no
measurable decomposition of 4c at room temperature on this time
scale. We suggest 4b and 4c are, in fact, stable at room
temperature. Solutions of 4c generated from [Cp*2Fe][B(C6F5)4] are
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