A tris(phosphine) ligand with a triarylbenzene backbone was employed to support mono-nickel(II) and -palladium(II) complexes. Two phosphine arms coordinated to the metal center, while the third phosphine was found to form a C–P bond with dearomatization of the central arene. Deprotonation effected the rearomatization of the central ring and metal reduction from M(II) to M(0). The overall conversion corresponds to a functionalization of an unactivated arene C–H bond to a C–P bond. This transformation represents a rare type of mechanism of C–H functionalization, facilitated by the interactions of the group 10 metal with the arene π system. This conversion is reminiscent of and expands the scope of recently reported intramolecular rearrangements of biaryl phosphine ligands common in group 10 catalysis

Agapie, Theodor

Suseno, Sandy

Caltech Authors - Main

Intramolecular Arene C-H to C-P Functionalization Mediated by
Ni(II) and Pd(II)
Sandy Suseno and Theodor Agapie*
Division of Chemistry and Chemical Engineering, California Institute of Technology, 1200 East
California Boulevard, MC 127-72, Pasadena, California 91125, United States
Abstract
A trisphosphine ligand with a triarylbenzene backbone was employed to support mono-nickel(II)
and palladium(II) complexes. Two phosphine arms coordinated to the metal center, while the third
phosphine was found to form a C-P bond with dearomatization of the central arene. Deprotonation
effected the rearomatization of the central ring and metal reduction from M(II) to M(0). The
overall conversion corresponds to a functionalization of an unactivated arene C-H bond to a C-P
bond. This transformation represents a rare type of mechanism of C-H functionalization,
facilitated by the interactions of the group 10 metal with the arene π-system. This conversion is
reminiscent of and expands the scope of recently reported intramolecular rearrangements of biaryl
phosphine ligands common in group 10 catalysis.
The functionalization of C-H bonds catalyzed by transition metals has been an active field of
research due to the potential applications in fine and commodity chemical synthesis, and
petrochemical conversion.1 Many metal-facilitated C-H functionalizations involve direct
metal coordination or attack on the C-H moiety. This can take several forms: coordination of
the C-H bond prior to oxidative addition, σ-bond metathesis, 1,2-addition, electrophilic
aromatic metallation, or radical cleavage.1a,1b,2 The oxidative addition mechanism as a
common mode of C-H functionalization is shown in Figure 1a. For the functionalization of
the C-H bonds of aromatic compounds, an alternative, extensively studied mechanism
involves Cr and Mn η6-arene complexes that are susceptible to nucleophilic attack (Figure
1b).3 Subsequent oxidation leads to the rearomatization and release of functionalized arene.
Arene elaboration upon strong binding of metals to part of the aromatic π-system has also
been reported.4 The latter two strategies for C-H functionalization require harsh conditions
for metal removal due to strong metal-arene interactions.
Development of conceptually related chemistry with group 10 metals is appealing given the
weaker metal-arene interactions and potential for metal removal under mild conditions
conducive to catalyst turnover. Intramolecular C-H to C-C conversion has been recently
reported for palladium coordinated biphenyl phosphines.5 C-C bond formation with
dearomatization was also observed for a nickel biphenyl phosphine complex.6 The common
property of these species is interaction of the metal center with arene leading to activation
toward attack of the π-system. Herein, we present a rare example of C-H to C-P conversion
*Corresponding Author: Theodor Agapie (agapie@caltech.edu).
Supporting Information. Detailed experimental procedures, characterization data, and crystallographic details for complexes 3a-Ni,
4b-Ni, 4b-Pd, and 2c-Pd (CIF). This material is available free of charge via the Internet at http://pubs.acs.org.
NIH Public Access
Author Manuscript
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Published in final edited form as:
Organometallics. 2013 June 10; 32(11): . doi:10.1021/om400424a.
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with a mechanism for C-H functionalization unusual for group 10 metals, involving
metal(II) activation of an arene π-system toward attack by a nucleophile (Figure 1c).
We have developed a family of bis- and trisphosphine ligands with terphenyl or
triphenylbenzene linkers respectively, that support Ni complexes exhibiting metal-arene
interactions.7 To further explore the reactivity profile of such late metal-arene coordination
complexes, we synthesized a modified trisphosphine 1a/b (Scheme 1). Compounds 1a/b
contain a methylene group between the triarylbenzene and the phosphine moieties, which
renders the framework more flexible compared to the related trisphosphine ligand7c with the
phosphorus atoms directly bound to the outer aryl groups. Treatment of 1a/b with one
equivalent of Pd(MeCN)4(BF4)2 (MeCN = acetonitrile) led to the formation of transient
species, 2a/b-Pd, that display two peaks in the 31P NMR spectrum at 13.6 and 39.9 ppm in a
1:2 ratio. Structural characterization by single crystal X-ray diffraction studies (XRD) for
2a/b-Pd were hindered by their instability. For comparison, the reaction of
Pd(MeCN)4(BF4)2 with a bisphosphine (1c) related to 1a/b but missing one of the
phosphine arms (Scheme S2) was investigated. A Pd(II) bisacetonitrile complex, 2c-Pd,
with the two phosphine donors coordinated in trans fashion (XRD, Figure S47) was isolated,
and shows a single 31P NMR signal at 39.7 ppm.8 This is consistent with an assignment of
2a/b-Pd as species with one free phosphine arm and two arms coordinating to Pd. Within 2
hours at room temperature, 2a/b-Pd converted to new complexes 3a/b-Pd, each with three
peaks in the 31P NMR spectrum. The 1H NMR spectra of 3a/b-Pd exhibit peaks in the
olefinic region around 4.2 ppm. Treatment of 1a/b with one equivalent of Ni(OTf)2 (OTf =
trifluoromethanesulfonate) generated complexes 3-Ni that show 1H NMR features similar to
3-Pd. A single crystal X-ray diffraction (XRD) study of 3a-Ni revealed a C1 NiII complex
(Figure 2). The Ni center is coordinated to two phosphine arms (with P(1)–Ni–P(2) wide
bite angle of 253°)9 and presents short contacts (2.029(1)–2.118(1) Å) with three carbons of
the central ring (Figure 2), consistent with an allyl-like bonding.10 Most interestingly, one
phosphine arm is now bonded to a carbon of the central ring. The sum of CCC and PCC
angles (~329°) at C4 suggests an sp3 carbon. NMR spectroscopy experiments (Figures S16–
S18) allowed for identification of the signal at 3.8–4.2 ppm as the hydrogen on the central
ring sp3 carbon. Two of the central ring C-C bonds are longer than 1.55 Å, consistent with
dearomatization. Additionally, the C5–C6 distance is shorter (1.360(1) Å), indicating an
increased double bond character. Overall, these data suggest the formation of a
phosphonium moiety. X-ray quality single crystals of 3-Pd have not been obtained to date,
but the NMR spectra are consistent with the analogous, structurally characterized 3-Ni.
The dearomatization of the ring in compounds 3-Ni/Pd is stabilized by the strong metal-allyl
interaction. Although metal arene interactions were not observed in the precursors 2-Pd, we
propose that they are transient and activate the arene along the reaction pathway for
nucleophilic attack by the phosphine. Interactions of group 10 metal(II) centers with arenes
have been observed by crystallography, in particular for biaryl phosphines.11 Recent reports
of arene activation with C-C bond formation occur in intramolecular fashion as well. A Pd-
facilitated reaction was proposed to occur via the insertion of an arene olefin moiety into a
metal-aryl bond, as supported by the syn stereochemistry of the product.5a,5b In the present
case, the pendant, free phosphine attacks the ring anti with respect to the metal center
indicating that nucleophile coordination to the metal is not a prerequisite for attack on the
ring. Hence, this provides an alternate mechanistic path for ring functionalization with
heteroatom-based reagents, with potential for the development of intermolecular versions.
Treatment of compounds 3-Ni/Pd with sodium acetate led to clean formation of new
species. The 1H NMR spectra suggest deprotonation at the central ring sp3 position and
rearomatization to generate 4-Ni/Pd (Scheme 1). XRD studies of 4b-Ni/Pd revealed a
planar, aromatic central ring, in which the sum of CCC and PCC angles at C4 is 359°. The
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C–C distances of the central ring for 4b-Ni/Pd, between 1.363(6) to 1.460(6) Å, depart
slightly from aromaticity (Figure 2). This is due to the metal binding to one of the olefin
moieties of the central arene, leading to partial localization of single and double bonds in the
ring for which short and long C-C distances alternate.7d The Ni–C (1.986(6) and 2.020(6) Å)
and Pd–C (2.183(4) Å) distances are consistent with M0 oxidation state.7a,12 Deprotonation
hence leads to formal metal reduction from MII to M0.
The transition-metal mediated nucleophilic addition to arene double bond followed by
deprotonation is effectively a C-H bond functionalization that leads to C-P bond formation
in this case. To our knowledge, this is the first example of such a mechanism mediated by
NiII and PdII without involving the formation of σ-complex or strong metal arene bonds
(Scheme 1). Previously reported transition-metal-mediated dearomatization reactions via
nucleophilic attack have been limited to d6-complexes such as (η6-arene)Cr(CO)3 and [(η6-
arene)Mn(CO)3]+.3,13 Rearomatization of the arene requires the presence of an anionic
leaving group or stoichiometric chemical oxidation and metal loss, which has hindered the
development of catalytic versions (Figure 1).3a–c,3i Our system is similar to the
aforementioned such that the electrophilic, metal-coordinated arene moiety is susceptible to
nucleophilic attack (Scheme 1). However, rearomatization to give the functionalized arene,
with metal reduction, can be achieved by facile deprotonation due to the accessible M0/MII
two electron couple for group 10 metals. Additionally, group 10 metals display relatively
weak metal-arene interactions that, although strong enough to facilitate nucleophilic attack,
are susceptible to arene exchange (after deprotonation) with potential for the development of
catalytic versions of arene C-H functionalization.14
In summary, we have demonstrated a rare example of arene C-H functionalization mediated
by NiII and PdII in an intramolecular fashion on a tris(phosphinoaryl)benzene framework.
The NiII and PdII centers have been shown to promote nucleophilic attack on the central
arene to yield a phosphonium moiety and dearomatization facilitated by the metal-allyl
interaction. Deprotonation at the sp3 carbon of these MII-allyl complexes with base results in
rearomatization and formal two-electron reduction of the metal center. The mechanism
consistent with the present transformation has several attractive features stemming from the
utilization of group 10 metals: activation of arene by weak metal-arene interactions,
nucleophilic attack without metal coordination to nucleophile, and a facile M0/MII redox
couple. With the current conceptual demonstration, future studies include the development
of intermolecular versions of this transformation using various nucleophiles.
Supplementary Material
Refer to Web version on PubMed Central for supplementary material.
Acknowledgments
This work was supported by the California Institute of Technology and the Dow Chemical Company Graduate
Student Fellowship (S.S.) for funding. We thank Lawrence M. Henling, Emily Y. Tsui, and David E. Herbert for
crystallographic assistance. The Bruker KAPPA APEXII X-ray diffractometer was purchased via an NSF
CRIF:MU award to Caltech, CHE-063-9094. The 400 MHz NMR spectrometer was purchased via an NIH award,
RR027690.
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Figure 1.
Mechanisms of C-H activation
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Figure 2.
Solid-state structures (left) and selected C–C, Ni–C, and M–P bond distances (Å, right) of
3a-Ni (top), 4b-Ni (middle), and 4b-Pd (bottom). Hydrogen atoms and anions are omitted
for clarity.
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Scheme 1.
Synthesis of metal complexes and intramolecular C-H to C-P conversion
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Intramolecular Arene C−H to C−P Functionalization Mediated by

Nickel(II) and Palladium(II)

English

https://authors.library.caltech.edu/39758/7/om400424a_si_001.pdf

Intramolecular Arene C−H to C−P Functionalization Mediated by Nickel(II) and Palladium(II)

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