A method for the selective hydrogenation of alkenyl halides to alkyl halides

Article abstract of DOI:10.1021/ja502885c

A general method for the selective hydrogenation of alkenyl halides to alkyl halides is described. Fluoro, chloro, bromo, iodo, and gem-dihaloalkenes are viable substrates for the transformation. The selectivity of the hydrogenation is consistent with reduction by a hydrogen atom transfer pathway.

Full text of DOI:10.1021/ja502885c

Communication
pubs.acs.org/JACS
A Method for the Selective Hydrogenation of Alkenyl Halides to Alkyl
Halides
Sandra M. King, Xiaoshen Ma, and Seth B. Herzon*
Department of Chemistry, Yale University, New Haven, Connecticut 06520, United States
S
* Supporting Information
place the metal atom distal to the halogen, elimination of the
ABSTRACT: A general method for the selective hydro-
genation of alkenyl halides to alkyl halides is described.
Fluoro, chloro, bromo, iodo, and gem-dihaloalkenes are
viable substrates for the transformation. The selectivity of
the hydrogenation is consistent with reduction by a
hydrogen atom transfer pathway.
resulting β-haloalkylmetal intermediate (4), and reduction
(Figure 2, top).8 Other manifolds, such as C−X oxidative
addition, can be envisioned, and the pathway operative is likely
to depend on the nature of the substituents (X and R).
he selective hydrogenation of alkenyl halides to alkyl
T
halides is a capricious transformation that is often
complicated by the formation of hydrodehalogenation products
(Figure 1, top). A literature survey reveals a small number of
Figure 2. The formation of hydrodehalogenation products in metal-
catalyzed hydrogenation of alkenyl halides may arise from generation
of unstable β-haloalkylmetal intermediates (e.g., 4, top). Postulated
hydrogen atom addition to an alkenyl halide to provide an α-
haloradical 5 (bottom).
We reasoned that metal-mediated hydrogen atom transfer
may provide a viable approach to alkenyl halide hydrogenation.
Halogen atoms decrease adjacent (α) C−H bond dissociation
energies by 3.7, 4.9, 3.0, and 1.5 kcal/mol for F, Cl, Br and I,
respectively (for H−CH2−X; compare to 4.5 kcal/mol for a
methyl group).9 Thus, in the absence of substituents that might
override the halogen bias, hydrogen atom addition to form a
stable α-haloradical intermediate (5) should be favored (Figure
2, bottom). Trapping of 5 with a hydrogen atom donor would
provide the desired product.
The reduction of alkenes by metal-mediated hydrogen atom
transfer has been extensively studied since the 1970s.10 Many of
these investigations employed early metal hydrides and 1,3-
dienes or styrenes as substrates.11 In detailed measurements of
hydrogen atom transfer from (η5-C5H5)Cr(CO)3H, Norton has
shown that the steric environment of the alkene and the
stability of the incipient alkyl radical significantly influence the
rate of hydrogen atom transfer.12 For example, hydrogen atom
addition to 1,1-diphenylethylene is ∼780 times faster than
addition to 1,1-diphenyl-2-methylethylene and is seven orders
of magnitude faster than addition to 1-octene. Hydrogen atom
addition to 2-methyloctene was reported to occur ∼50-fold
faster than addition to 1-octene, indicating that electronic
activation by the internal methyl group overrides the additional
Figure 1. Reduction of alkenyl halides often results in mixtures of
hydrogenation and hydrodehalogenation products (top). Reduction of
dehydroacutumine (1) to form (−)-acutumine (2) and (−)-dechlor-
oacutumine (3, bottom).3
focused studies of this reaction that proceed in variable yields
and selectivities.1 Although other isolated reductions of alkenyl
halides have been reported,2 a general process that is broadly
amenable to simple alkenyl halides does not exist, to our
knowledge. In a recent synthetic project in our laboratory3
selective hydrogenation of the complex alkenyl chloride 1 was
required to access the natural product (−)-acutumine (2,
Figure 1, bottom).4 Among a broad range of catalysts surveyed,
5
[Rh(nbd)(dppb)]BF4 was uniquely, although modestly,
effective (17%; 56% based on unreacted 1).6 Nearly all other
catalysts investigated formed the hydrodechlorination product
(−)-dechloroacutumine (3)7 exclusively or provided only trace
amounts of 2, in accord with our expectations based on the
literature.
These data suggest the challenge of alkenyl halide reduction
arises not from poor reactivity but rather in suppression of
hydrodehalogenation. One pathway for the latter process may
comprise insertion of the π-system into a metal hydride, to
Received: March 21, 2014
Published: April 30, 2014
© 2014 American Chemical Society
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steric encumbrance introduced. This latter determination
provides encouraging precedent for the analysis outlined
above. Norton and co-workers have applied these fundamental
studies to the development of catalytic reductive diene
cyclization reactions initiated by hydrogen atom transfer to
unsaturated esters.13 In addition, Nishinaga,14 Mukaiyama−
Isayama,15 Carreira,16 Boger,17 and others18 have reported
efficient cobalt-, manganese-, and iron-catalyzed (or mediated)
alkene hydrofunctionalization reactions. Of particular relevance
to our work, cobalt and manganese-based catalysts have been
employed in the reduction of unactivated alkenes (using
sodium borohydride as reductant)19 and unsaturated ketones
(using phenylsilane as reductant, eq 1),20 and reduction
products have been observed in cobalt-mediated hydration
reactions.15b Evidence suggests these transformations proceed
by hydrogen atom transfer,16d,17,21 although migratory insertion
into metal hydroperoxide intermediates has also been
proposed.15e,22 These catalysts are unique in their ability to
effect hydrogen atom transfer to unactivated alkenes and
provided the starting point for our studies.23 A recent report
from Shenvi and co-workers describing the reduction of
unactivated alkenes (including an alkenyl bromide and
chloride) by manganese tris(dipivaloylmethane) prompts us
to disclose our results.24
yield of product in the absence of DTBMP was diminished
slightly (59%, Table S4). Both triethylsilane and 1,4-cyclo-
hexadiene were necessary; in the absence of either, the yield of
7a was low (<1% in the absence of Et3SiH, 12% in the absence
of 1,4-cyclohexadiene, Table S4). These observations are
consistent with deuterium-labeling experiments (vide infra)
that indicate each reagent contributes a hydrogen atom. n-
Propyl 4-methoxybenzoate was not detected in any of the
experiments above (1H NMR analysis). Reduction of 6a using
manganese tris(dipivaloylmethane)24a provided 7a in 45%
yield, reduction using cobaloxime complexes under dihydro-
gen13c did not proceed, and reduction with palladium on
carbon as catalyst under 1 atm of dihydrogen provided a 4%
yield of 7a and an 87% yield of n-propyl 4-methoxybenzoate.
An alternative and simpler procedure we developed, which
has proven to be amenable to tri- and tetrasubstituted alkenes,
comprises treatment of the substrate with 1 equiv each of cobalt
bis(acetylacetonate) and TBHP, in the presence of triethylsi-
lane and 1,4-cyclohexadiene under air at ambient temperature.
Under these conditions, the product 7a was obtained in 62%
yield after 40 min (eq 2). Among many catalyst precursors
examined, cobalt bis(acetylacetonate) was the only species that
mediated high conversion of 6a (Table S2). The yield of 7a was
only marginally diminished in the absence of TBHP (58%) and
decreased significantly in the absence of triethylsilane (<1%) or
1,4-cyclohexadiene (5%, Table S5). As cobalt bis-
(acetylacetonate) is relatively inexpensive, this procedure may
be advantageous for its simplicity and rate.
TBHP and triethylsilane have been shown to react with
Co(II) complexes to form Co(III) hydride intermediates,21
which may effect hydrogen atom transfer to the substrate, and
we speculated that 1,4-CHD serves as a trap for the alkyl radical
intermediate. Isotopic labeling experiments were conducted to
probe these hypotheses (Figure 3). These experiments were
We began by studying the hydrogenation of 2-chloroallyl 4-
methoxybenzoate (6a) to form 2-chloropropyl 4-methoxyben-
zoate (7a) in the presence of a variety of manganese and cobalt-
based catalyst precursors and terminal reductants (eq 2). These
optimization experiments revealed production of the ketone 8
and the dimer 9 as competitive with the desired reduction
pathway. We speculate that the ketone is produced by
oxygenation of a carbon-centered radical intermediate.
Ultimately, two experimental protocols were developed. The
first, which appears to be effective only for 2,2-disubstituted
alkenes, comprises a mixture of cobalt bis(acetylacetonate) (25
mol %), activated by t-butylhydroperoxide (TBHP),15f,16,21
tricyclohexylphosphine, 2,6-di-t-butyl-4-methylpyridine
(DTBMP), and a combination of 1,4-cyclohexadiene (1,4-
CHD) and triethylsilane as reducing agents, under argon. After
warming to 50 °C for 10.5 h, the desired reduction product 7a
was obtained in 78% yield.
Figure 3. Deuterium labeling experiments.
executed without added TBHP (which has only a marginal
effect on efficiency, vide supra) and in methanol (or methanol-
d4). Using triethylsilane-d1 in methanol, the product d-7a was
formed with 64% deuterium atom incorporation at C-1 (entry
1). Conducting the reduction with triethylsilane-d1 in
methanol-d4 resulted in 65% deuterium atom incorporation at
C-1 (entry 2). When triethylsilane in methanol-d4 was
employed, deuterium was not detected at C-1 (entry 3), and
within the limits of detection, deuterium was not incorporated
at C-2 in any of these experiments. These data suggest that the
internal (C-2) hydrogen atom derives from 1,4-CHD and that
the terminal (C-1) hydrogen atom derives primarily from
silane. The origin of incomplete C-1 deuteration in entries 1
and 2 is not fully understood but may be due to reaction of the
putative cobalt deuteride with 1,4-CHD to form HD, benzene,
and a cobalt hydride, or exchange of the deuterium with the
acac ligand.
A comprehensive tabulation of the results obtained with a
range of catalyst precursors, hydrogen atom donors, ligands,
bases, and solvents are presented in the Tables S1−S5. Several
observations are noted herein. Tricyclohexylphosphine was
found to accelerate the rate, potentially by preventing
formation of stable cobalt−alkyl intermediates,25 and the
In its present form, the scope of the reaction encompasses
fluoro, chloro, bromo, and iodoalkenes, cyclic and acylic alkenyl
halides, and gem-dihaloalkenes (Table 1). Thus, the fluo-
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roalkene 6c was cleanly reduced to provide the fluoroalkane 7c
(entry 3). The alkenyl bromide 6d and iodide 6e were also
reduced in good yield (entries 4, 5, respectively). The
carbamates 6f and 6g (entries 6 and 7) and the piperidine
derivative 6h (entry 8), which contain aryl iodide, aryl ketone,
and benzyloxycarbonyl functional groups that would be
anticipated to be reactive toward traditional hydrogenation
catalysts, were selectively reduced at the alkenyl halide function.
Finally, the gem-dichloro and gem-dibromoalkenes 6j and 6k,
respectively, were also reduced to provide the gem-dihaloalkane
products 7j and 7k (entries 10 and 11, respectively). The
successful reduction of 6j suggests that the gem-dichloro
substituent exerts a stronger directing effect than a gem-dialkyl
substituent, as initial hydrogen atom transfer to the dichloro
terminus of the alkene would be expected to produce an
unstable β,β-dichloroalkyl radical intermediate. This is con-
sistent with the relative C−H bond dissociation energies of
propane (secondary C−H = 98.1 kcal/mol) and dichloro-
methane (95.7 kcal/mol).9 Reaction of the gem-diiodoalkene 6l
did not proceed to completion (41% product 7l + 42% 6l
remaining, entry 12). Qualitatively, alkenyl bromides and
chlorides appear to react faster than iodides or fluorides.
Attempted reduction of 1-phenyl-1-chloropropene led to
unidentified decomposition products, while attempted reduc-
tion of a trialkylchloroalkene resulted in recovery of starting
material. Nitroarenes are incompatible with the reaction
conditions.
Table 1. Preliminary Scope of the Reduction Reaction
The work we have reported builds on prior studies of metal-
mediated hydrogen atom addition and alkene hydrofunction-
alization reactions and constitutes a useful method for the
selective hydrogenation of alkenyl halides. The selectivity of the
reaction (hydrogenation vs hydrodehalogenation) is thought to
arise from the activating effect of the halogen substituent, which
biases the direction of hydrogen atom addition. To a first
approximation, the feasibility of a given substrate can be
assessed by consideration of homolytic C−H BDEs in the
product. A large number of natural products contain alkyl
halides, and this method may prove useful in their synthesis.26
Future studies will focus on functionalizing the putative α-
haloalkyl radical intermediates.
ASSOCIATED CONTENT
■
S
* Supporting Information
Detailed experimental procedures and characterization data for
all new compounds. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
seth.herzon@yale.edu
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Professor Jack Norton for helpful comments.
Financial support from the Department of Defense (NDSEG
fellowship to S.M.K.) and the National Science Foundation
(CHE-1151563 and Graduate Research Fellowship to S.M.K.)
is gratefully acknowledged. S.B.H. acknowledges early stage
investigator awards from the Alfred P. Sloan Foundation, the
David and Lucile Packard Foundation, the Research Corpo-
a
Isolated yield after purification by flash-column chromatography.
Conditions: Co(acac)2 (25 mol %), PCy3 (25 mol %), TBHP (25
b
mol %), DTBMP (50 mol %), 1,4-CHD (5 equiv), Et3SiH (5 equiv),
c
n-PrOH (0.3 M), argon, 50 °C. Conditions: Co(acac)2 (1 equiv),
TBHP (1 equiv), 1,4-CHD (5 equiv), Et3SiH (5 equiv), n-PrOH (0.3
d
1
M), air, 24 °C. Determined by H NMR analysis against an internal
standard.
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Communication
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