Evidence for formation of a Co-H bond from (H2O) 2Co(dmgBF2)2 under H2: Application to radical cyclizations

Article abstract of DOI:10.1021/ja306037w

Under H₂, the radical cyclization of appropriate dienes can be catalyzed by cobaloximes. H can be abstracted from an intermediate (presumably a cobalt hydride) by trityl radicals (Ar₃C) or by TEMPO. The rate-determining step in these reactions is the uptake of H₂, which is second order in cobalt and first order in hydrogen; the third-order rate constant is 106(3) M^-2As^-1.

Full text of DOI:10.1021/ja306037w

Communication
pubs.acs.org/JACS
Evidence for Formation of a Co−H Bond from (H2O)2Co(dmgBF2)2
under H2: Application to Radical Cyclizations
Gang Li, Arthur Han, Mary E. Pulling, Deven P. Estes, and Jack R. Norton*
Department of Chemistry, Columbia University, 3000 Broadway, New York, New York 10027, United States
S
* Supporting Information
“cobaloximes” are air- and moisture-stable solids that are widely
ABSTRACT: Under H2, the radical cyclization of
appropriate dienes can be catalyzed by cobaloximes. H•
can be abstracted from an intermediate (presumably a
cobalt hydride) by trityl radicals (Ar3C•) or by TEMPO.
The rate-determining step in these reactions is the uptake
of H2, which is second order in cobalt and first order in
hydrogen; the third-order rate constant is 106(3) M−2·s−1.
accepted as models for vitamin B12. Indeed, van der Donk has
shown that vitamin B12 itself can catalyze radical cyclizations
with TiIII as the stoichiometric reductant,6 and Carreira has
shown that photolysis of cobaloxime complexes generates a
catalyst for the cyclization of unsaturated alkyl iodides.7 A CoIII
hydride is probably formed in all three of these reactions: in eq
2 from H2, in the van der Donk chemistry by protonation of the
CoI vitamin B12 anion, and in the Carreira chemistry from an
alkyl−CoIII intermediate.
Cobaloxime hydrides 4, and the hydrides of similar
tetraazamacrocycle complexes, have been proposed as inter-
mediates in the operation of Co catalysts for H2 evolution.
Important recent work in this area has come from Gray,8
Peters,8d,9 Eisenberg,10 Fontecave,11 Sun,11b,12 Alberto,13
Bakac,14 and Tiede.15
ydrogen atom (H•) transfer (HAT) from transition-
H
metal hydrides to unsaturated organic molecules is a key
step in many reactions, in particular hydrogenation and
hydroformylation.1 Our group has measured the rate constants
for HAT from CpCr(CO)3H and HV(CO)4(P−P) to various
olefins2 and used these reactions to carry out radical
cyclizations (eq 1).2c,3 With CpCr(CO)3H, the reaction is
Often cobaloxime hydrides are thermodynamically unstable,
such as 4b (L = py).16 There have been suggestions17 that
hydride 4b (L = PBu3), originally reported to be formed from
ClCo(dmgH)2(PBu3) and NaBH4,18 was mischaracterized and
that the original procedure actually gives a CoII monomer in
equilibrium with its dimer;17 however, a report of the
preparation of the PBu3 hydride has just appeared,19 albeit
catalytic, as CpCr(CO)3H is regenerated from CpCr(CO)3•
with H2,4 but HAT is relatively slow. With HV(CO)4(P−P),
HAT is faster, but cyclizations can be effected only stoichio-
metrically because the V−H bond is too weak for •V(CO)4(P−
P) to cleave H2. Both the Cr and V hydrides are air-sensitive
and thermally unstable.
The possibility that Co complexes might catalyze the
generation of radicals from H2 was suggested by the 2006
report5 that the macrocyclic CoII complex (H2O)2Co-
(dmgBF2)2 (3a) (dmg = dimethylglyoximato) could initiate
the polymerization of acrylates under H2 gas (eq 2). Such
1
with a H NMR spectrum that differs from that originally
reported.18 The pKa of the PBu3 hydride 4b was estimated in
the original report as 10.5 from “phase-distribution measure-
ments” between MeOH/H2O and hexane.18
Schrauzer and co-workers reported a reaction between
cobaloxime 3b and H2,20 and there were subsequent studies
of the kinetics of H2 uptake by 3b in the presence of various
acceptors, including Schiff bases,21 the dmgH ligand itself,22
and styrene.23 However, no spectroscopic data were reported
for the dmgBF2 hydridocobaloxime 4a with L = H2O until
2010, when Szajna-Fuller and Bakac assigned a peak at 608 nm
to 4a in water.14 (This peak is close to, but less intense than,
the peak at 610 nm assigned to [CoI(dmgBF2)2]−.14)
We therefore set out to trap cobaloxime hydride 4a from the
reaction of cobaloxime 3a with H2. We began by trying tris(p-
Received: June 20, 2012
© XXXX American Chemical Society
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tert-butylphenyl)methyl radical (5), which is monomeric in
solution (head-to-tail dimerization is prevented by its p-tert-
butyl substituents24) and known to be effective at H•
abstraction from transition metals (eq 3).25 The reactions of
5 are easily monitored by watching the disappearance of its
1
UV−vis signal (λmax = 526 nm) or the appearance of the H
NMR signal of the product, tris(p-tert-butylphenyl)methane
(6).
Indeed, 5 was converted to 6 in the presence of 3a and H2
(eq 4). No 6 was observed unless both 3a and H2 were present.
Cobaloxime 3a catalyzes the reaction between trityl radical 5
and H2, presumably via cobalt hydride 4a. When eq 4 was
monitored by NMR spectroscopy under constant H2 pressure,
the rate of formation of 6 did not change with time (Figure 1);
the reaction is thus zeroth order in 5.
Figure 2. (a) Rate d[6]/dt of eq 4 vs [H2]. (b) Rate d[6]/dt of eq 4 vs
[3a]. All rates were measured at 295 K in C6D6.
Scheme 1. Mechanism of Cobaloxime-Mediated Formation
of 6 from 5
3−
•Co(CN)5
(Halpern),27 •Rh(TMP) (Wayland),28
Figure 1. Cobaloxime-mediated formation of 6 from 5 under 2.4 atm
H2 in C6D6 at 295 K (eq 4). [3a] = 7.60 × 10−4 M and [5]0 = 1.02 ×
10−2 M.
•Cr(CO)3Cp* (Hoff),29 •Cr(CO)3Cp (Franz),4b and
•W(CO)2(NHC)Cp (Bullock).30
Of course the value of k1 should be independent of the
nature as well as the concentration of the trapping radical. We
therefore replaced 5 by 2,2,6,6-tetramethylpiperidine N-oxyl
(TEMPO, 7), giving eq 8. We examined the rate of eq 8 with
We confirmed this conclusion by showing that the rate d[6]/
dt was independent of the initial concentration of 5 (Figure S17
in the Supporting Information). We then studied reaction 4 at
various H2 pressures and various 3a concentrations. During a
given reaction, [H2] remained constant because it was present
in substantial excess, and [3a] remained constant because it was
regenerated. The reaction proved to be first order in H2 (Figure
2a) and second order in cobalt (Figure 2b). These results
suggest efficient trapping of hydride 4a by Ar3C• 5 and rate-
limiting activation of H2 by 3a, that is, the rate law in eq 5,
which is the same as the rate law reported earlier for catalysis of
H2 uptake by 3b.16,22b,d,23
[3a] = 7.60 × 10−4 M and PH = 3.0 atm (Figure S16). With [7]
2
= 0.116 M, the rate d[8]/dt was found to be 1.82(2) × 10−3
Mh−1, implying a k1 value of 115(1) M−2s−1, almost the same as
that found with various concentrations of 5 (Figure S18). This
result is consistent with the mechanism in Scheme 1.31
It seemed likely that the apparent cobalt hydride
intermediate 4a would also donate H• to a cyclization substrate
such as 1, so we tried the cobaloximes 3a and 3b under H2 as
catalysts for the cyclization of 1b. With substrate 1b, we saw
quantitative cyclization to 2b and 2′b (eq 9), the same result
(with the same diastereomer ratio) obtained earlier with the Cr
and V hydrides.3
d[Ar CH]
3
= k[(H2O)2Co(dmgBF )2]2[H2]
2
(5)
dt
This rate law suggests the mechanism in Scheme 1 with large
k2 and small k1. From Figure 2a and the value of [3a]2 we
obtain a k1 value of 108(7) M−2s−1 (Figure S7), and from
Figure 2b and the value of [H2] we obtain a k1 value of 104(3)
M−2s−1 (Figure S13), implying that k1 is 106(3) M−2s−1 at 295
K.26 The first step, eq 6 in Scheme 1, is analogous to those for
the activation of H2 by other metalloradicals, such as
B
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Communication
Scheme 4. Mechanism for the Formation of 12 and 13 from
11
We then considered what advantages cobaloximes might
offer over our previous catalysts. If the reaction of cobaloxime
3a with H2 is slower than the reaction of the resulting hydride
4a with our cyclization substrates, the resting state of our
catalyst will be CoII, and the concentration of its hydride 4a will
remain low during the cyclization. The increase in [M•]/[M−
H] will change the distribution of byproducts (typically 9 and
10 in Scheme 2) that accompany cyclization. Hydrogenation to
Cobaloxime 3a thus catalyzes the cyclization of 11 without
net hydrogen uptake, although the presence of H2 is required
(3a has no effect on 11 in the absence of H2). Our kinetics
studies show that 3a and H2 generate an intermediate
(presumably hydride 4a) that can transfer H·. Both of our
catalysts for cyclization, CpCr(CO)3H and 3a, avoid the need
for a heavy atom in the substrate as well as the need for a tin
reagent.
Scheme 2. How Byproducts Are Formed during Cyclization
Reactions
ASSOCIATED CONTENT
* Supporting Information
Synthetic details, spectroscopic data, and kinetic procedures.
This material is available free of charge via the Internet at
http://pubs.acs.org.
■
S
9 requires an additional M−H, while isomerization to 10
requires only M• (here CoII). Thus, the cobalt catalyst should
give less hydrogenation to 9 and more isomerization to 10.
Indeed, with substrate 1a and cobalt catalyst 3a, isomerization
to 10a is the prevailing reaction (eq 10). Only traces of the
cyclization products 2a and 2′a and the hydrogenation product
9a are observed.
AUTHOR INFORMATION
Corresponding Author
jrn11@columbia.edu
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by DOE Grant DE-FG02-
97ER14807. We are grateful to Boulder Scientific and OFS
Fitel for additional support. A.H. gratefully acknowledges the
support of the Herbert Deresiewicz Summer Research
Fellowship. M.E.P. thanks Novartis for a graduate fellowship.
D.P.E. is supported by the Department of Energy Office of
Science Graduate Fellowship Program (DOE SCGF), made
possible in part by the American Recovery and Reinvestment
Act of 2009 and administered by ORISE-ORAU under
Contract DE-AC05-06OR23100. The authors thank John
Hartung for helpful discussions.
However, with the enone substrate 11,32 the use of the cobalt
catalyst is advantageous. The structure of 11 eliminates
isomerization as a possibility. The Cr catalyst results almost
exclusively in hydrogenation to give 12, but the Co catalyst gives
substantial cyclization, albeit to the unhydrogenated product 13
(Scheme 3). Presumably 13 results from removal of H• from
the cyclized radical 14 by CoII (Scheme 4). The phenyl
substituent in 11 directs the cyclization to a five-membered
ring33 instead of the six-membered ring reported for the parent
radical.34 The Co catalyst does give some hydrogenation (the
remaining 50% of substrate 11 is converted to 12).
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