Controlling secondary alkyl radicals: Ligand effects in chromium-catalyzed C-P bond formation

Article abstract of DOI:10.1021/om300846u

Chromium cyclopentadienyl β-diketiminate catalysts have been used to form Ph₂PCy from Ph₂PY (Y = Cl, PPh₂, H) and CyX (X = Br, Cl) substrates. Manganese powder activated by PbX₂ or Me₃SiCl was used as the stoichiometric reductant. The Cr(III) cyclohexyl intermediate has been synthesized and structurally characterized. The observed variations in catalytic activity have been correlated with the previously observed reactivity differences imparted by modifying the N-aryl substituents on the β-diketiminate ancillary ligand.

Full text of DOI:10.1021/om300846u

Communication
pubs.acs.org/Organometallics
Controlling Secondary Alkyl Radicals: Ligand Effects in Chromium-
Catalyzed C−P Bond Formation
Wen Zhou,† K. Cory MacLeod,† Brian O. Patrick,‡ and Kevin M. Smith*,†
†Department of Chemistry, University of British Columbia Okanagan, 3333 University Way, Kelowna, British Columbia, Canada V1V
1V7
‡Department of Chemistry, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z1
S
* Supporting Information
ABSTRACT: Chromium cyclopentadienyl β-diketiminate catalysts have been
used to form Ph2PCy from Ph2PY (Y = Cl, PPh2, H) and CyX (X = Br, Cl)
substrates. Manganese powder activated by PbX2 or Me3SiCl was used as the
stoichiometric reductant. The Cr(III) cyclohexyl intermediate has been
synthesized and structurally characterized. The observed variations in catalytic
activity have been correlated with the previously observed reactivity
differences imparted by modifying the N-aryl substituents on the β-
diketiminate ancillary ligand.
The ability of Cr(II) to trap carbon-based radicals is well-
irst-row transition metals have found increasing use in the
Fcatalytic cross-coupling reactions of secondary alkyl established16 and forms the basis for several chromium-
mediated reactions for organic synthesis.17 Furstner pioneered
halides.1 For typical palladium catalysts, secondary R−X
̈
the use of manganese powder as a stoichiometric single-
electron reductant for Cr(III) that is also compatible with
organic halides.18 We have recently been exploring the
synthesis of well-defined paramagnetic CpCr[(ArNCMe)2CH]-
(R) complexes,19 the role of sterics in inducing Cr(III)−R
homolysis, and their utility in controlling radical bond-forming
reactions.20 The Cr(III) CpCr[(XylNCMe)2CH](X) com-
plexes (X = Br (1a), Cl (1b)) are effective catalysts for the
radical cyclization of C(sp3)−Br and C(sp3)−Cl substrates.21
To date, we have been unsuccessful in synthesizing secondary
Cr(III) alkyls suitable for single-crystal X-ray diffraction with
the symmetric xylyl-substituted ligand. The mixed N-aryl
derivatives CpCr[DppNC(Me)CHC(Me)NTol](X) (X = Br
(2a), Cl (2b)) are more suitable precursors for secondary alkyl
complexes, due to the reduced steric demands of the β-
diketiminate ligand.22
substrates are challenging due to slow rates of oxidative
addition and competing β-elimination processes.2 Many of the
first-row metal-catalyzed cross-coupling reactions of secondary
alkyl halides have been demonstrated to involve carbon-based
radicals,3 consistent with the long-known propensity of 3d
organometallic species to engage in single-electron-transfer
reactivity.4 The introduction of reversible metal−alkyl bond
homolysis provides exciting new options for catalysis, such as
combining cross-coupling with radical intramolecular cycliza-
tion5 and converting racemic secondary alkyl halide substrates
to enantiomerically pure products.2,6 Due to their ability to trap
carbon-based radicals7 and their stability with respect to β-H
elimination,8 it is often possible to isolate and characterize
catalytically relevant first-row metal alkyl complexes despite
their relatively weak M−R bonds.9 This affords excellent
opportunities to explore metal-mediated radical reactivity using
well-defined complexes with readily modified ancillary
ligands.10
The desired Cr(III) complexes were prepared from the high-
spin Cr(II) compound CpCr[DppNC(Me)CHC(Me)NTol]
(3),23 as shown in Scheme 1. The Cr(III) bromide 2a was
synthesized by single-electron oxidation of 3 with PbBr2.24
Oxidation of 3 with silver p-toluenesulfonate afforded the
Cr(III) tosylate 4, which was structurally characterized by X-ray
diffraction.25 As previously observed for the synthesis of related
sterically demanding Cr(III) alkyl complexes,19,20a the most
effective route to the Cr(III) cyclohexyl complex 5 utilized
R2Mg reagents with the Cr(III) tosylate precursor.25 The X-ray
crystal structure of 5 is shown in Figure 1. Although the Cr−
C(cyclohexyl) bond length of 2.106(3) Å is longer than the
Cr−CH3 bond (2.01608(17) Å) of the corresponding Cr(III)
Transition-metal-catalyzed carbon−phosphorus bond-form-
ing reactions have largely focused on C(sp2)−P processes.11
Recent work by the groups of Oshima and Studer has
demonstrated the utility of radical C−P formation using tin
and silyl reagents.12,13 The titanium-mediated radical synthesis
of Ph2PCy from P2Ph4 and cyclohexyl bromide was recently
developed by Cossairt and Cummins using stoichiometric
Ti[N(3,5-Me2C6H3)tBu]3.14 Although the three-coordinate
Ti(III) reagent is a powerful single-electron reductant,15 the
Ti(IV) bromo product of halogen atom abstraction from CyBr
is only slowly reduced back to the active Ti(III) species even
using Na/Hg amalgam, effectively precluding a catalytic
reaction.14
Received: August 31, 2012
Published: October 18, 2012
© 2012 American Chemical Society
7324
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Organometallics
Communication
Scheme 1. Synthesis of
CpCr[DppNC(Me)CHC(Me)Tol](X) Complexes (X = Br
(2a), OTs (4), Cy (5))
Scheme 2. Stoichiometric Reactions of Cr(III) Cyclohexyl
Complex 5 with Ph2PY Reagents (Y = Cl, PPh2, H)
Scheme 3
complex 3 can trap the radicals generated by the Cr−C bond
homolysis of 5, preventing the bimolecular reaction between
two cyclohexyl radicals (step 1). However, Ph2PCl reacts
rapidly with 3 to form Cr(III) chloride 2b and P2Ph4 (steps 2
and 3).25 The consumption of the Cr(II) radical trap favors the
production of cyclohexyl radicals, which then react with P2Ph4
(step 4).12,13 Similarly, bromine atom abstraction from CyBr by
Cr(II) generates Cr(III) bromide 2a (step 5), also resulting in
the rapid reaction of Cy• with available P2Ph4 (step 4). The
Cr(II) complex 3 does not react with P2Ph4, and attempts to
prepare a Cr(III)−PPh2 complex via salt metathesis reactions of
KPPh2 with 2a or 4 were unsuccessful. The reaction of Cr(II)
complex 3 with substoichiometric Ph2PCl also produced only
the Cr(III) chloride 2b, P2Ph4, and unreacted 3, indicating that
if any Cr(III)−PPh2 species is generated, it is unstable with
respect to 3 and P2Ph4.
Figure 1. Molecular structure (50% thermal ellipsoids) of compound
5. Hydrogen atoms have been removed for the sake of clarity. Selected
bond lengths (Å) and bond angles (deg): Cr01−C020, 2.106(3);
Cr01−N001, 2.057(2); Cr01−N002, 2.024(2); N001−Cr01−N002,
87.65(9); N001−Cr01−C020, 92.82(10); N002−Cr01−C020,
95.19(10).
The key step in the overall reaction of 5 or 3 with CyBr and
Ph2PH is the rapid intermolecular hydrogen atom abstraction
process shown in step 6,26 which generates Ph2P• radical and
cyclohexane. While the steps proposed in Scheme 3 remain
speculative in the absence of mechanistic studies, the range of
Ph2PY substrates that can be employed to produce Ph2PCy
from Cr(III) cyclohexyl complex 5 attests to the flexibility of
metal-mediated radical routes to C(sp3)−P bond formation.
The stoichiometric reactions in Scheme 2 can be rendered
catalytic by the use of PbBr2-activated Mn powder27 at ambient
temperature and 1 mol % of the mixed N-aryl Cr(II) complex
3, resulting in a 79% yield of Ph2PCy from Ph2PCl after 28 h
(Table 1, entry 1). Comparable yields were obtained with
catalyst 3 when the reaction was protected from ambient light25
or when P2Ph4 and Ph2PH were used as precursors (entries 2
and 3). In all cases, bipyridine was added at the completion of
the reaction in order to displace Ph2PCy from the paramagnetic
MnBr2 byproduct.28−30 The catalytic activity of the symmetric
CpCr[(XylNCMe)2CH] Cr(II) complex 6 was assessed under
the same reaction conditions. Although we have been unable to
characterize the corresponding Cr(III) cyclohexyl complex by
X-ray crystallography, due to its instability, the catalytic use of
the bulkier 2,6-Me2C6H3 disubstituted ligand led to an
unexpected increase in the yields of Ph2PCy from each
phosphorus substrate (entries 4−6). A much lower yield of
methyl complex,23 it is in the range of 2.10−2.13 Å previously
observed for primary alkyl ligands in CpCrIII complexes bearing
symmetric 2,6-disubstituted β-diketiminate ligands.19 The steric
strain imposed by the secondary alkyl ligand in 5 is evident in
the distortions of the N-aryl substituents away from the
cyclohexyl group and the relatively long Cr−N(Dpp) and Cr−
N(Tol) bonds of 2.057(2) and 2.024(2) Å, respectively. The
UV−vis spectrum of 5 in hexanes displays the strong
absorbance band at 553 nm characteristic of CpCr-
[(ArNCMe)2CH](R) complexes.19
The reactivity of Cr(III) cyclohexyl 5 in stoichiometric
reactions with Ph2PY sources at room temperature in benzene
was evaluated by 31P NMR, with PPh3 added as an internal
standard (Scheme 2). Reaction of CpCr[DppNC(Me)CHC-
(Me)NTol](Cy) with Ph2PCl resulted in the formation of
Ph2PCy in 94% yield (31P NMR). Similarly, treatment of 5 with
P2Ph4 and CyBr resulted in generation of Ph2PCy in 96% yield
(31P NMR).25 The corresponding reaction of 5 with CyBr and
Ph2PH resulted in a lower yield (30%) of Ph2PCy. However,
treatment of Ph2PH with 2.2 equiv of CyBr and 2.7 equiv of the
Cr(II) complex 3 produced 69% Ph2PCy by 31P NMR.25
Our working hypothesis for the reaction mechanism of these
various C(sp3)−P bond-forming processes is outlined in
Scheme 3. In the absence of oxidants, small amounts of Cr(II)
7325
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Organometallics
Communication
a
Table 1. Chromium-Catalyzed Synthesis of Ph2PCy
Chart 1
b
c
d
entry
Cr cat.
CyX
Ph2PY
additive
yield, %
1
3
CyBr
CyBr
Ph2PCl
P2Ph4
PbBr2
PbBr2
PbBr2
PbBr2
PbBr2
PbBr2
79
70
70
92
96
83
82
76
80
29
81
2
3
e
3
3
CyBr
CyBr
CyBr
CyBr
CyBr
CyBr
CyCl
CyCl
CyCl
Ph2PH
Ph2PCl
P2Ph4
4
6
5
6
e
6
6
Ph2PH
Ph2PCl
Ph2PCl
P2Ph4
Cr(II) complex 6 only reluctantly reacts with unactivated
C(sp3)−Cl bonds. Due to its decreased steric requirements, 3 is
much more reactive than 6 for the single-electron oxidative
addition of alkyl halides.22 This increased propensity for
oxidative addition accounts for the relatively high activity of 3
with the more challenging cyclohexyl chloride substrate. The
effect of altering the ancillary ligands on the catalytic reaction
with other alkyl halide substrates is an intriguing subject for
subsequent investigations.
In comparison to more traditional 4d metal catalysts, the use
of earth-abundant first-row transition metals can provide not
only a substantial decrease in catalyst cost but also
complementary reactivity. In the case of secondary alkyl halide
substrates,3,33 this distinctive reactivity is often attributable to
the generation of carbon-based radicals as an integral feature of
the catalytic cycle. Despite its long history in organometallic
chemistry,4,7,8 metal-mediated radical reactivity is not central to
the way organometallic chemists currently approach catalysis.34
It is therefore encouraging that the study of well-defined
organometallic complexes with readily modified ancillary
ligands can be used to match catalyst and substrates in these
reactions, as demonstrated in Chart 1.
7
1a
2a
3
Me3SiCl
Me3SiCl
PbCl2
8
9
10
11
6
P2Ph4
PbCl2
6
P2Ph4
PbCl2
a
The reaction temperature/time was 25 °C/28 h for entries 1−8, 55
b
°C/72 h for entries 9 and 10, and 55 °C/312 h for entry 11. 1 mol %
Cr catalyst was used for entries 1−8, and 10 mol % Cr catalyst was
c
used for entries 9−11. Catalytic PbX2 (1−3 mol %) or Me3SiCl (10
d
mol %) was added to activate Mn. The yields were determined by 31
P
e
NMR. 2.4 equiv of CyBr was used with the Ph2PH substrate.
47% was obtained when 1 mol % Cp2Cr was used as the
catalyst, and no Ph2PCy was observed by 31P NMR when the
reaction was conducted without chromium catalyst or when 10
mol % SmI2 or CrCl3 was used.25 Interestingly, 32 mol %
Ph2PCy was obtained without Cr when 10 mol % of both
NaCp and Li[DppNC(Me)CHC(Me)NTol] were added,
suggesting that with the proper ancillary ligands a Mn-based
catalyst may also be viable for this reaction.25 Although related
Ph2PR complexes prepared by radical addition to P2Ph4 have
been amenable to oxidation with H2O2 and purification by
column chromatography,13a our initial attempts to isolate our
product via this procedure were not successful.
While the Cr(II) complex 6 is highly air sensitive, it has been
previously demonstrated that the Cr(III) μ-oxo oxidation
product can be reduced back to the active Cr(II) form with Mn
powder and substoichiometric Me3SiCl.31 As shown in entries 7
and 8, this strategy also allowed the air-stable Cr(III) bromide
complexes 1a and 2a to be used as catalyst precursors in
reactions that are set up in air and purged with N2 prior to
addition of the CyBr, Me3SiCl, and Ph2PCl.25
ASSOCIATED CONTENT
* Supporting Information
■
S
CIF files, tables, text, and figures giving crystallographic data for
4 and 5, complete experimental details, and characterization
data. This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: kevin.m.smith@ubc.ca.
■
Less reactive secondary alkyl chlorides continue to pose a
challenge as substrates for transition-metal-catalyzed reac-
tions.1,32 Although the reaction requires higher catalyst loading
(10 mol % 3), higher temperatures (55 °C), and longer
reaction times (72 h), Ph2PCy can be produced in 80% 31P
NMR yield from P2Ph4 and CyCl (entry 9). Interestingly,
under these reaction conditions, the symmetric Cr(II) complex
6 gives only 29% yield along with 59% unreacted P2Ph4 after 3
days (entry 10), with 81% yield only being achieved after 13
days at 55 °C (entry 11).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the Natural Sciences and Engineering
Research Council of Canada (NSERC), the Canadian
Foundation for Innovation, and the University of British
Columbia for financial support. K.M.S. thanks Prof. Jason
Masuda for helpful discussions.
As summarized in Chart 1, the observed variations in
catalytic activity between 3 and 6 can be attributed to the
reactivity differences imparted by modifying the β-diketiminate
ligand. With the more reactive cyclohexyl bromide substrate,
the higher yields obtained with catalyst 6 are presumably due to
the weaker Cr−Cy bond in the more hindered Cr(III)
secondary alkyl complex, favoring the formation of Ph2PCy as
shown in Scheme 3, step 4. However, as was previously
observed in the catalytic radical cyclization of chloroacetals,21
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