C(sp3)-Cl Bond Activation Promoted by a POP-Pincer Rhodium(I) Complex

Article abstract of DOI:10.1021/acs.organomet.9b00409

The complex [RhCl(κ³P,O,P-{xant(PⁱPr₂)₂})] (1; xant(PⁱPr₂)₂ = 9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene) activates C(sp³)-Cl bonds of mono- and dichloroalkanes and catalyzes the dehalogenation of chloroalkanes and the homocoupling of benzyl chloride. Complex 1 reacts with chlorocyclohexane to give [RhHCl₂(κ³P,O,P-{xant(PⁱPr₂)₂})] (2) and cyclohexene and promotes the dehalogenation of the chlorocycloalkane to cyclohexane using 2-propanol solutions of sodium formate as the reducing agent. The oxidative addition of benzyl chloride to 1 leads to [Rh(CH₂Ph)Cl₂(κ³P,O,P-{xant(PⁱPr₂)₂})] (4). The dehalogenation of this chloroalkane with 2-propanol solutions of sodium formate, in the presence of 1, gives toluene and 1,2-diphenylethane. The latter is selectively formed with KOH instead of sodium formate. Complex 1 also reacts with trans-1,2-dichlorocyclohexane and dichloromethane. The reaction with the former gives [RhCl₃(κ³P,O,P-{xant(PⁱPr₂)₂})] (5) and cyclohexene, whereas complex 1 undergoes oxidative addition of dichloromethane to afford cis-dichloride-[Rh(CH₂Cl)Cl₂(κ³P,O,P-{xant(PⁱPr₂)₂})] (6a), which evolves into its trans-dichloride isomer 6b. The kinetic study of the overall process suggests that the oxidative addition is cis-concerted and the isomerization an intramolecular reaction which takes place through a σ-C-Cl intermediate with two conformations.

Full text of DOI:10.1021/acs.organomet.9b00409

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
3
C(sp )−Cl Bond Activation Promoted by a POP-Pincer Rhodium(I)
Complex
Sheila G. Curto, Laura A. de las Heras, Miguel A. Esteruelas,* Montserrat Olivan
́
,
and Enrique Onate
̃
Departamento de Química Inorganica-Instituto de Síntesis Química y Catalisis Homogenea (ISQCH)-Centro de Innovacion en
́ ́ ́ ́
Química Avanzada (ORFEO-CINQA), Universidad de Zaragoza-CSIC, 50009 Zaragoza, Spain
*
S Supporting Information
3
i
ABSTRACT: The complex [RhCl(κ P,O,P-{xant(P Pr ) })]
2
2
i
(
(
1 ; x a n t ( P P r )
=
9 , 9 - d i m e t h y l - 4 , 5 - b i s -
2
2
3
diisopropylphosphino)xanthene) activates C(sp )−Cl bonds
of mono- and dichloroalkanes and catalyzes the dehalogena-
tion of chloroalkanes and the homocoupling of benzyl
chloride. Complex 1 reacts with chlorocyclohexane to give
3
i
[
RhHCl (κ P,O,P-{xant(P Pr ) })] (2) and cyclohexene and
2 2 2
promotes the dehalogenation of the chlorocycloalkane to cyclohexane using 2-propanol solutions of sodium formate as the
3
i
reducing agent. The oxidative addition of benzyl chloride to 1 leads to [Rh(CH Ph)Cl (κ P,O,P-{xant(P Pr ) })] (4). The
2
2
2 2
dehalogenation of this chloroalkane with 2-propanol solutions of sodium formate, in the presence of 1, gives toluene and 1,2-
diphenylethane. The latter is selectively formed with KOH instead of sodium formate. Complex 1 also reacts with trans-1,2-
3
i
dichlorocyclohexane and dichloromethane. The reaction with the former gives [RhCl (κ P,O,P-{xant(P Pr ) })] (5) and
3
2 2
cyclohexene, whereas complex 1 undergoes oxidative addition of dichloromethane to afford cis-dichloride-[Rh(CH Cl)-
2
3
i
Cl (κ P,O,P-{xant(P Pr ) })] (6a), which evolves into its trans-dichloride isomer 6b. The kinetic study of the overall process
2
2 2
suggests that the oxidative addition is cis-concerted and the isomerization an intramolecular reaction which takes place through
a σ-C−Cl intermediate with two conformations.
3
INTRODUCTION
rhodium-mediated C(sp )−X bond activation reactions is
■
10
awakening notable interest.
Neutral POP diphosphines are hemilabile pincer ligands,
which have the ability to adapt their coordination mode to the
requirements of each particular species. As a consequence of
this flexibility, POP-rhodium derivatives are proving to have
noticeable efficiency in reactions of σ-bond activation with
implication in a wide range of interesting organic reactions,
as well as in the dehydrocoupling and dehydropolymerization
Oxidative addition to transition-metal complexes is one of the
most relevant procedures for the activation of σ bonds. The
oxidative addition of C−X bonds of organic halides to basic
unsaturated metal centers has particular interest by its
connection with the catalytic formation of C−C bonds and
with the metal-catalyzed degradation of these substrates. The
1
11
2
12
3
13
latter is a priority target from an environmental point of view,
since the accumulation of these substrates is a serious health
14
of amine−boranes. In agreement with this, we have recently
shown that the square-planar monohydride [RhH(κ P,O,P-
4
hazard. The C−X bond enthalpy increases as we go down the
3
group in the periodic table. Thus, the C−Cl rupture is more
challenging than the C−Br and C−I bond activations.
However, chlorides are the most interesting to work with
due to their lower cost and wider diversity.
i
i
{
xant(P Pr ) })] (xant(P Pr ) = 9,9-dimethyl-4,5-bis-
2 2 2 2
(
diisopropylphosphino)xanthene) undergoes a sterically gov-
erned C−Cl bond cis-oxidative addition of chlorobenzene,
chlorotoluenes, chlorofluorobenzenes, and di- and trichlor-
obenzenes to afford rhodium(III) derivatives, which experience
dehydrochlorination to give a wide range of [Rh(aryl)-
Most metal-promoted C−C coupling reactions involve aryl
or alkenyl halides. Alkyl halides have been comparatively much
less employed, particularly those bearing β-hydrogen atoms,
5
3
i
15
(κ P,O,P-{xant(P Pr ) })] complexes (Scheme 1).
2 2
because the resulting alkyl intermediates decompose by means
Our interest in dehalogenation and C−C coupling processes
3
of a β-hydride elimination reaction. Palladium(0) compounds
has prompted us to study now the activation of C(sp )−Cl
6
are the most used catalysts for these reactions. According to
bonds of mono- and dichloroalkanes, with and without β-
3
i
this, there is a significant amount of work centered about the
hydrogens, promoted by [RhCl(κ P,O,P-{xant(P Pr ) })] (1).
2 2
3
10
7
oxidative addition of C(sp )−X bonds to d metal centers. In
In this paper, we report the oxidative addition of these classes
of substrates to 1 and the first exploration of the behavior of
recent years, examples proving the rhodium efficiency for
coupling of alkyl halides have been also reported, whereas
8
other examples have demonstrated their capacity for
dehalogenation reactions. In the same vein, the study of
Received: June 18, 2019
9
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.9b00409
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Article
Scheme 1. C−Cl Bond Activation of Halogenated Arenes
metal center situated in the common vertex and P(1)−Rh−
P(2), P(1)−Rh−O(1), and P(2)−Rh−-O(1) angles of
1
63.49(2), 82.26(5), and 83.04(5)°, respectively. Thus, the
coordination polyhedron around the rhodium atom can be
described as an octahedron with the hydride disposed trans to
the oxygen atom of the diphosphine (O(1)−Rh−H(01) =
1
76.0(9)°) and the chloride ligands disposed mutually trans
1
(
Cl(1)−Rh−Cl(2) = 177.44(2)°. This is also evident in the H
this square-planar rhodium(I) complex in the dehalogenation
of these organic compounds and toward the use of some of
them for C−C coupling reactions.
1
3
1
and C{ H} NMR spectra of the crystals, in benzene-d at
6
room temperature, which display two signals for the methyl
1
groups of the phosphine isopropyl substituents (δ , 1.49 and
H
RESULTS AND DISCUSSION
1.45; δ13C, 21.9 and 19.6) and a signal for the methyl
■
1
13
Monochloroalkanes. Complex 1 activates the C−Cl bond
of chlorocyclohexane. However, the presence of four β-
hydrogen atoms in the substrate destabilizes the resulting
alkyl intermediate, which undergoes a β-hydride elimination
reaction (Scheme 2). Thus, complex 1 affords cyclohexene and
substituents of the central heterocycle (δ ,1.21; δ , 30.9). In
H C
1
agreement with the presence of the hydride, the H NMR
spectrum contains at −19.79 ppm a doublet of triplets with
1
2
J
and J
coupling constants of 13.6 and 11.5 Hz,
H−Rh
H−P
i
respectively. As expected for equivalent P Pr groups, the
2
3
1
1
P{ H} NMR spectrum shows a doublet at 42.2 ppm, which
1
Scheme 2. C−Cl Bond Activation of Chlorocyclohexane
displays a typical J
coupling constant of 98.9 Hz.
P−Rh(III)
Chlorocyclohexane reacts with sodium formate to give
cyclohexane, NaCl, and CO , in the presence of 1 (eq 1). The
2
3
the rhodium(III) monohydride [RhHCl (κ P,O,P-{xant-
2
i
(
P Pr ) })] (2) in chlorocyclohexane as solvent. According
2
2
dehalogenation is catalytic. Using 1.0 mol % of complex 1 and
3
1
1
to the P{ H}NMR spectrum of the solution, the reaction is
quantitative after 24 h, at 100 °C. Attempts to detect and
characterize the alkyl intermediate A were unsuccessful, even at
room temperature. Under these conditions, complex 2 was also
the only detected species, although its formation is excessively
slow.
1
.2 equiv of sodium formate in 2-propanol, the cycloalkane is
formed in 85% yield, after 24 h, under reflux. Acetone is not
observed during the reaction, indicating that the solvent does
not participate in the dehalogenation. The formation of 2 and
cyclohexene, according to Scheme 2, is consistent with this fact
and should constitute the first part of the catalysis. Thus, the
replacement of one of the chloride ligands by the formate
anion could afford the intermediate B, which should release
CO to give the previously described dihydride [RhH Cl-
Complex 2 was isolated as pale yellow crystals and
characterized by X-ray diffraction analysis. Figure 1 shows a
view of the structure. As expected for a pincer coordination of
the diphosphine, the (POP)Rh skeleton is T-shaped with the
2
2
3
i
12a
(
κ P,O,P-{xant(P Pr ) })] (3). The hydrogenation of cyclo-
2 2
hexene by the latter would lead to the cycloalkane and to
regenerate the catalyst (Scheme 3). Formate salts are
16
promising chemical hydrogen carriers. As a consequence,
Scheme 3. Dehalogenation of Chlorocyclohexane
Figure 1. Molecular diagram of complex 2 (ellipsoids shown at 50%
probability). All hydrogen atoms (except the hydride) are omitted for
clarity. Selected bond distances (Å) and angles (deg): Rh−P(1) =
2
=
.3054(7), Rh−P(2) = 2.3063(7), Rh−Cl(1) = 2.3438(6), Rh−Cl(2)
2.3412(6), Rh−O(1) = 2.2591(16); P(1)−Rh−P(2) = 163.49(2),
Cl(1)−Rh−Cl(2) = 177.44(2), P(1)−Rh−O(1) = 82.26(5), P(2)−
Rh−O(1) = 83.04(5), O(1)−Rh−H(01) = 176.0(9).
B
DOI: 10.1021/acs.organomet.9b00409
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Article
they are receiving noticeable attention as reducing agents in
metal-mediated hydrodehalogenation reactions.
Complex 1 also activates the C−Cl bond of benzyl chloride.
Treatment of toluene solutions of this compound with 2.0
equiv of the chloroalkane, at room temperature, for 7 h
2
PhCH Cl + 2NaHCO + (CH ) CHOH
17
2
2
3 2
→ PhCH2CH2Ph + 2NaCl + 2HCO2H + (CH3)2CO
(2)
2PhCH Cl + 2KOH + (CH ) CHOH
quantitatively leads to the benzyl derivative [Rh(CH Ph)-
2
3 2
2
3
i
Cl (κ P,O,P-{xant(P Pr ) })] (4), as a result of the oxidative
addition of the C(sp )−Cl bond of the organic substrate to the
metal center of 1 (Scheme 4). In contrast to A, complex 4 is
2
2 2
→
PhCH CH Ph + 2KCl + 2H O + (CH ) CO
3
2
2
2
3 2
(
3)
PhCH Cl + NaHCO → PhCH + NaCl + CO
(4)
2
2
3
2
Scheme 4. C−Cl Bond Activation of Benzyl Chloride
PhCH Cl + KOH + (CH ) CHOH
2
3 2
→
PhCH + KCl + (CH ) CO + H O
(5)
3
3 2
2
Scheme 5. Proposed Mechanism for the Dehalogenative
Homocoupling and Simple Dehalogenation of Benzyl
Chloride
stable and was isolated as a beige solid in 70% yield. The
presence of a coordinated benzyl group in the new species is
1
13
1
strongly supported by the H and C{ H} NMR spectra of the
obtained beige solid, in benzene-d , at room temperature,
6
2
which display a doublet of triplets at 5.64 ( J
= 3.4 Hz and
H−Rh
3
1
2
JH−P = 4.1 Hz) and 17.3 ( J
= 21.5 Hz and J
= 4.1
C−Rh
C−P
Hz) ppm, respectively. These spectra also reveal the mutually
trans disposition of the chloride ligands. Thus, in agreement
with the spectra of 2, they contain two signals for the methyl
1
H
groups of the phosphine isopropyl substituents (δ , 1.35 and
1
3
1
.22; δ , 20.0 and 19.9) and a signal for the methyl
C
1
13
substituents of the central heterocycle (δ , 1.19; δ , 29.7).
The P{ H} NMR spectrum shows a doublet ( J
H
C
3
1
1
1
= 105.2
P−Rh
Hz) at 19.1 ppm.
The S 2 mechanism is the most common for the oxidative
N
18
addition of alkyl halides to basic metal centers. It leads to a
mutually trans disposition of the added fragments. Never-
theless, the cis disposition of the benzyl group to both chloride
ligands in 4 is not consistent with this reaction pathway.
Because transitory intermediates were not spectroscopically
detected during the reaction, even at low temperatures, we
assume that the stereochemistry of 4 is the result of a
concerted addition, which takes place along the O−Rh−Cl axis
19
of 1 with the benzyl group above the chloride ligand. The
preference of this orientation is probably steric.
Complex 1 also catalyzes the dehalogenation of benzyl
chloride with 2-propanol solutions of sodium formate, under
reflux. However, there are significant differences from the
dehalogenation of chlorocyclohexane. In contrast to the latter,
the reaction gives acetone and two dehalogenated products,
Once complex 4 is formed, one of its chloride ligands could
be replaced by a formate anion or alternatively by an
isopropoxide group, which should be generated in the basic
reaction media. Both species, C and D, would afford the
hydride−Rh(III)−benzyl intermediate E by release of CO or
2
1
,2-diphenylethane and toluene. With 1.0 mol % of catalyst,
acetone, respectively. Intermediate E could evolve in two
different manners: reductive elimination of toluene (a) or
reductive elimination of HCl (b). The first process, (a), closes
the cycle for the simple dehalogenation of benzyl chloride to
toluene. The reductive elimination of HCl (b) should lead to
the square-planar intermediate F. This Rh(I)−-benzyl species
could undergo the oxidative addition of a second molecule of
benzyl chloride to give G. Thus, the reductive coupling of the
benzyl groups would generate 1,2-diphenylethane, closing the
cycle for the dehalogenative homocoupling of the chloroal-
kane. Strong bases should favor the dehalogenative homocou-
after 6 h, 42% of the chloroalkane was transformed into 1,2-
diphenylethane, whereas another 50% gave toluene. The
formation of acetone suggests that in this case both sodium
formate and sodium isopropoxide are the reducing agents: i.e.
the formate anion acts as a hydrogen carrier and as a base to
generate the isopropoxide. The hydrocarbons are the result of
two competitive reactions, a dehalogenative homocoupling
(
eqs 2 and 3) and a simple dehalogenation (eqs 4 and 5),
which have as a common intermediate in the benzyl derivative
and can be rationalized according to Scheme 5.
4
C
DOI: 10.1021/acs.organomet.9b00409
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Scheme 6. Reaction of 1 with trans-1,2-Dichlorocyclohexane
pling (b) with regard to the simple dechlorination (a), given its
higher ability to trap the HCl generated from E, whereas it
should increase the isopropoxide concentration to facilitate the
formation of D. In order to corroborate this, we replaced
sodium formate by potassium hydroxide and, in effect, two
significant increases take place: the dehalogenation rate and
the amount of 1,2-diphenylethane. After 2 h, the chloroalkane
disappeared; 93% of benzyl chloride was transformed into the
homocoupling product and only 7% into toluene.
The reaction of 1 with dichloromethane affords two
products, one of them of kinetic control and the other of
thermodynamic control. In dihaloalkane as the solvent,
complex 1 initially gives the cis-dichloride-[Rh(CH Cl)-
2
3
i
Cl (κ P,O,P-{xant(P Pr ) })] (6a), which evolves to its trans-
2
2 2
dichloride isomer 6b (Scheme 7).
Scheme 7. C−Cl Bond Activation of Dichloromethane
Ando and co-workers have performed the [RhCl(PPh ) ]-
3
3
mediated homocoupling of benzyl bromides. There are,
however, significant differences in the reaction conditions
with regard to those previously mentioned, not only in the
catalyst but also in the dehalogenation agent and therefore in
the wastes. In contrast to 2-propanol solutions of KOH, they
used Me Zn in tetrahydrofuran, which generates ethane and
2
The disposition of the chloromethyl group in 6a, cis to one
chloride and trans to the other, is revealed by the C{ H}
NMR spectrum of the new species, which shows four
ZnBr instead of NaCl, water, and acetone. Furthermore, their
2
13
1
catalytic system is much less efficient than 1/KOH/2-
propanol, since it only affords 58% yield after 24 h for the
8d
resonances between 24 and 19 ppm for the diasterotopic
homocoupling of benzyl chloride.
i
methyls of the equivalent P Pr groups and two resonances at
2
Dichloroalkanes. The reactions of 1 with dichloroalkanes
also show a marked dependence upon the presence of β-
hydrogen atoms in the chloroalkyl fragment, which is evident
in the reactions with trans-1,2-dichlorocyclohexane and
dichloromethane. Thus, while the former undergoes double
C−Cl rupture, the product of the oxidative addition of the
latter is stable.
3
6.9 and 30.7 ppm corresponding to the inequivalent methyl
substituents of the heterocyclic link of the diphosphine. The
signal due to the chloromethyl ligand appears at 36.9 ppm, as a
doublet of triplets, with C−Rh and C−P coupling constants of
13
1
2
8.6 and 5.5 Hz, respectively. In agreement with the C{ H}
1
NMR spectrum, the H NMR spectrum shows the
choromethyl resonance as an ABX Y spin system, at 5.17
2
Complex 1 reacts with trans-1,2-dichlorocyclohexane to give
31
1
1
ppm. The P{ H} NMR spectrum contains a doublet ( J =
6.9 Hz) at 26.4 ppm, proving the equivalence of the P Pr
3
i
P‑Rh
[
RhCl (κ P,O,P-{xant(P Pr ) })] (5) and cyclohexene, as a
i
3
2 2
9
2
result of a chloride transfer from the organic substrate to the
metal center of 1. At 100 °C, using this dichloroalkane as the
solvent, the reaction is quantitative after 4 h. This is strongly
groups.
The trans-dichloride isomer 6b was isolated as a brown solid
in 83% yield. In agreement with the benzyl complex 4, which
supported by the 31P{ H} NMR spectrum of the resulting
1
1
13
1
bears the same stereochemistry, its H and C{ H} NMR
spectra contain two signals for the methyl groups of the
1
solution, which only displays a doublet ( J
= 86.1 Hz) at
P−Rh
2
4.9 ppm. The formation of 5 and the olefin can be
1
13
phosphine isopropyl substituents (δ , 1.43 and 1.38; δ , 21.6
H
C
rationalized according to Scheme 6. The oxidative addition
and 20.6), whereas the methyl substituents of the heterocyclic
of one of the C−Cl bonds of trans-1,2-dichlorocyclohexane to
1
13
C
linker give rise to a signal (δ , 1.19; δ , 32.5). In both
H
1
should afford intermediate H, which could evolve to 2,
spectra, the resonance corresponding to the chloromethyl
2
releasing 1-chlorocyclohexene. Thus, the insertion of the
chloroolefin into the Rh−H bond of the latter, followed by the
β-chloride elimination on the chloroalkyl ligand of the
resulting intermediate I, would give 5 and the cycloolefin. In
this context, it should be noted that a hydride abstraction in H
is favored with regard to the chloride abstraction due to the
trans disposition of the rhodium and chloride atoms. However,
when there are atoms of hydrogen and chloride in equivalent β
positions, as in I, the chloride abstraction is favored with regard
to the hydride. In addition, it should be mentioned that, in
contrast to the previous monochloroalkanes, trans-1,2-dichlor-
ocyclohexane does not undergo dehalogenation under the
conditions of eqs 1−5.
group is observed as a doublet of triplets at 6.30 ( J
= 3.4
H−Rh
3
1
Hz and J
= 4.1 Hz) ppm in the H NMR spectrum and at
H−P
1
2
34.3 ( J
= 28.6 Hz and J
= 5.5 Hz) ppm in the
C−Rh
C−P
1
3
1
i
C{ H} NMR spectrum. As expected for equivalent P Pr
groups, the P{ H} NMR spectrum shows a doublet ( J
2
3
1
1
1
=
P−Rh
100.1 Hz) at 23.6 ppm. The stereochemistry inferred from the
NMR spectra was confirmed by X-ray diffraction analysis.
Figure 2 shows a view of the structure. In agreement with the
spectroscopic data, the coordination polyhedron around the
rhodium atom can be described as a distorted octahedron, with
the diphosphine mer coordinated (P(1)−Rh−P(2) =
163.45(7)°, P(1)−Rh−O = 81.69(14)°, and P(2)−Rh−O =
81.84(14)°), the chloride ligands disposed mutually trans
D
DOI: 10.1021/acs.organomet.9b00409
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Article
elimination of the dichloroalkane followed by a new concerted
oxidative addition along the O−Rh−Cl axis: now, with the
chloride of the substrate above the oxygen atom. A noticeable
difference between the trans-S 2 and cis-concerted additions is
N
the value of the negative activation entropy of the process:
lower than −40 eu for the former and higher than −25 eu for
21
the latter. To gain insight about what is going on, we
followed the evolution of 1 in dichloromethane as a function of
3
1
1
the time by P{ H} NMR spectroscopy, in the temperature
range 307−282 K. Figure 3 shows the spectra at 303 K. The
dependence of the amounts of 1, 6a, and 6b with time (Figure
4
) is in accordance with two consecutive irreversible reactions
Figure 2. Molecular diagram of complex 6b (ellipsoids shown at 50%
probability). All hydrogen atoms (except those of the CH Cl moiety)
2
are omitted for clarity. Selected bond distances (Å) and angles (deg):
Rh−P(1) = 2.3334(19), Rh−P(2) = 2.354(2), Rh−Cl(1) = 2.341(2),
Rh−Cl(2) = 2.362(2), Rh−O = 2.252(5), Rh−C(28) = 2.018(8);
P(1)−Rh−P(2) = 163.45(7), Cl(1)−Rh−Cl(2) = 177.28(7), P(1)−
Rh−O = 81.69(14), P(2)−Rh−O = 81.84(14), O−Rh−C(28) =
177.4(3).
(
Cl(1)−Rh-Cl(2) = 177.28(7)°), and the chloromethyl group
situated trans to the oxygen atom of the diphosphine (C(28)−
Rh−O = 177.4(3)°). The rhodium−alkyl distance of 2.018(8)
Å (Rh−C(28)) compares well with the expected value for a
Figure 4. Composition of the mixture as a function of time for the
reaction at 303 K (1, blue ▲; 6a, red ■; 6b, green ●).
III
3
20
Rh −C(sp ) bond.
The trans disposition of the chloromethyl group to a
chloride ligand in 6a is consistent with an S 2 oxidative
N
and fits to eqs 6−8, respectively. Table 1 collects the rate
constants k and k obtained from these expressions.
addition. Nevertheless, this stereochemistry could be also the
result of a cis concerted addition along the O−Rh−Cl axis with
a chloride of the dichloroalkane above the chloride ligand of 1.
The isomerization of 6a into 6b should involve the reductive
1
2
[
1] = [1] e−
k t
1
(6)
0
Figure 3. Stacked 31P{ H} NMR spectra (161.98 MHz, CD Cl , 303 K) showing the evolution of 1 in dichloromethane as a function of time.
1
2
2
E
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Article
kcal mol− and ΔS = −21.8 ± 6.2 eu for the formation of 6a
1
⧧
[
1]0k1
a
[
[
6a] =
[e−k1t − e−k2t
]
⧧
−1
⧧
and ΔH = 21.5 ± 2.5 kcal mol and ΔS = −4.2 ± 8.5 eu
k2 − k
1
(7)
b
b
⧧
for the isomerization from 6a to 6b. The value of ΔS ,
a
significantly far away from that expected for a S 2 mechanism,
N
[
1]0
6b] = [1] +
[k2e−k1t − k1e−k2t
]
is consistent with a cis-concerted addition, whereas the value of
0
⧧
k1 − k
2
(8)
ΔS , close to zero, supports an intramolecular isomerization.
b
The latter could take place via the σ-C−Cl intermediate J,
−
1
which would exist in the conformations J and J shown in
Table 1. Rate Constants for the Formation of 6a (k , s )
and for the Isomerization of 6a into 6b (k , s ) Calculated
a
b
1
−
1
Scheme 8.
2
According to eqs 6−8, as a Function of Temperature (K)
CONCLUDING REMARKS
■
[
temp
k1
k2
This study has revealed that the rhodium(I) complex
−
5
4
−5
−4
2
2
2
3
3
82
86
92
03
07
(2.51 ± 0.07) × 10
(1.47 ± 0.07) × 10
3
i
3
RhCl(κ P,O,P-{xant(P Pr ) })] activates C(sp )−Cl bonds
−5
−5
2 2
(4.8 ± 0.2) × 10
(8.2 ± 0.3) × 10
(3.5 ± 0.2) × 10
(4.8 ± 0.2) × 10
of mono- and dichloroalkanes to give trans-dichloride−
rhodium(III)−alkyl derivatives, which are stable when the
alkyl group does not contain hydrogen atoms in a β position
with regard to the metal center. Species bearing β-hydrogen
atoms release the olefin, resulting in a β-hydride elimination
reaction on the alkyl group. The activation by means of
oxidative addition is a cis-concerted process, which takes place
along the O−Rh−Cl axis. In agreement with its ability to
−5
−5
−
(1.82 ± 0.09) × 10
(1.11 ± 0.06) × 10
−4
−4
(3.7 ± 0.3) × 10
(7.1 ± 0.3) × 10
The activation parameters calculated from the correspond-
ing Eyring analysis (Figures 5 and 6) are ΔHa⧧ = 16.2 ± 1.8
3
activate C(sp )−Cl bonds, this rhodium(I) complex is also an
efficient catalyst for the promotion of the simple dehalogena-
tion of chloroalkanes to the corresponding alkanes, using
sodium formate as reducing agent, and for the dehalogenative
homocoupling of benzyl chloride to 1,2-diphenylethane, with a
2
-propanol solution of KOH as a dehalogenating agent. For
the homocoupling, the new system is more efficient than those
previously reported and generates fewer wastes.
EXPERIMENTAL SECTION
■
General Information. All reactions were carried out with
exclusion of air using Schlenk-tube techniques or in a drybox.
Instrumental methods and X-ray details are given in the Supporting
Information. In the NMR spectra the chemical shifts (in ppm) are
Figure 5. Eyring plot for the formation of 6a.
1
13
1
referenced to residual solvent peaks ( H, C{ H}) or external 85%
3
1
1
H PO ( P{ H}). Coupling constants J and N are given in hertz.
3
4
3
i
12a
[
RhCl(κ P,O,P-{xant(P Pr ) })] (1) was prepared by the published
2 2
methods.
3
i
Reaction of [RhCl(κ P,O,P-{xant(P Pr ) })] (1) with Chlorocy-
clohexane: Preparation of [RhHCl (κ P,O,P-{xant(P Pr ) })] (2).
2
2
3
i
2
2 2
A solution of 1 (100 mg, 0.16 mmol) in chlorocyclohexane (5 mL)
was heated at 100 °C for 24 h. After this time, an analysis of the
resulting solution by gas chromatography showed a peak assigned to
cyclohexene by comparison with the retention time of a pure sample
of this olefin. The solution was cooled at room temperature, filtered
through Celite, and evaporated to dryness, affording a pale yellow
residue. Addition of pentane (4 mL) afforded a pale yellow solid that
was washed with pentane (2 × 2 mL) and dried in vacuo. Yield: 44
mg (30%). The isolated yield is low due to the high solubility of the
complex in pentane. Anal. Calcd for C H Cl OP Rh: C, 52.53; H,
2
7
41
2
2
6
.69. Found: C, 52.24; H, 6.83. HRMS (electrospray, m/z): calcd for
Figure 6. Eyring plot for the isomerization of 6a to 6b.
+
−1
C H OP Rh [M − 2 Cl] , 547.1760; found, 547.1778. IR (cm ):
2
7
41
2
1
ν(Rh−H) 2151 (w), ν(C−O−C) 1099 (m). H NMR (300.13 MHz,
3
4
C D , 298 K): δ 7.28 (m, 2H, CH-arom), 7.10 (dd, J
= 7.6, J
H−H
6
6
H−H
Scheme 8. Mechanism for the Isomerization of 6a into 6b
F
DOI: 10.1021/acs.organomet.9b00409
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
13
1
=
1.3, 2H, CH-arom), 6.91 (t, 3J
= 7.6, 2H, CH-arom), 2.90 (m,
PCH(CH ) ), 1.19 (s, 6H, CH ). C{ H}-apt NMR (75.47 MHz,
3 2 3
H−H
3
4
H, PCH(CH ) ), 1.49 (dvt, JH−H = 5.7, N = 15.6, 12H,
toluene-d , 298 K): δ 154.7 (vt, N = 12.7, C-arom POP), 133.3 (s,
CH-arom POP), 132.5 (s, C-arom POP), 129.1 (s, CH-arom POP),
3
2
8
3
PCH(CH ) ), 1.45 (dvt, J
= 5.7, N = 16.2, 12H, PCH(CH ) ),
3
2
H−H
3 2
1
2
1
.21 (s, 6H, CH ), −19.79 (dt, J
= 13.6, J = 11.5, 1H, Rh−
124.4 (s, CH-arom POP), 123.4 (vt, N = 27.8, C-arom POP), 34.8 (s,
3
H−Rh
H−P
13
1
1
2
H). C{ H}-apt NMR (75.47 MHz, C D , 298 K): δ 155.5 (vt, N =
C(CH ) ), 34.3 (dt, J
= 28.6, J
= 5.5, RhCH Cl), 32.5 (s,
6
6
3
2
C−Rh
C−P 2
1
1.5, C-arom POP), 133.1 (vt, N = 5.1, C-arom POP), 132.4 (s, CH-
arom POP), 128.1 (s, CH-arom POP), 125.5 (vt, N = 24.6, C-arom
POP), 124.3 (s, CH-arom POP), 35.1 (s, C(CH ) ), 30.9 (s,
C(CH ) ), 27.3 (vt, N = 22.3, PCH(CH ) ), 21.6, 20.6 (both s,
3
2
3
2
3
1
1
PCH(CH ) ). P{ H} NMR (121.49 MHz, toluene-d , 298 K): δ
3
2
8
1
23.6 (d, J
= 100.1).
3
2
Rh−P
3
C(CH ) ), 25.3 (vt, N = 24.5, PCH(CH ) ), 21.9, 19.6 (both s,
Spectroscopic Detection of cis-[Rh(CH Cl)Cl (κ P,O,P-{xant-
) })] (6a). A solution of 1 (15 mg, 0.03 mmol) in
2 2
3
2
3
2
2 2
31
1
i
PCH(CH ) ). P{ H} NMR (121.49 MHz, C D , 298 K): δ 42.2 (d,
(P Pr
3
2
6
6
1
JRh−P = 98.9).
dichloromethane (1.5 mL) was placed in an NMR tube, and it was
3
1
1
3
i
periodically checked by P{ H} NMR spectroscopy. After 90 min,
the resulting solution was evaporated to dryness, giving a yellow
Reaction of [RhCl(κ P,O,P-{xant(P Pr ) })] (1) with Benzyl
2
2
3
i
Chloride: Preparation of [Rh(CH Ph)Cl (κ P,O,P-{xant(P Pr ) })]
2
2
2 2
(4). A solution of 1 (100 mg, 0.17 mmol) in toluene (3 mL) was
residue. Addition of toluene-d and subsequent filtration afforded a
8
3
1
1
treated with benzyl chloride (42 μL, 0.34 mmol), and the resulting
solution was stirred at room temperature for 7 h. After this time, the
solution was filtered through Celite and evaporated to dryness to
afford a beige solid. Yield: 92 mg (70%). Anal. Calcd for
C H Cl OP Rh: C, 57.72; H, 6.97. Found: C, 57.31; H, 6.59.
solution whose P{ H} NMR spectrum shows a mixture of 1, 6a, and
6b in a 4:33:63 ratio. The H, P{ H}, and C{ H} NMR spectra
1
31
1
13
1
were recorded at 273 K in order to avoid the progress of the
1
isomerization. Spectroscopic data for 6a are as follows. H NMR
(400.13 MHz, toluene-d , 273 K): δ 7.20 (m, 2H, CH-arom POP),
34
47
2
2
8
+
3
3
HRMS (electrospray, m/z): calcd for C H ClOP Rh [M − Cl] ,
7.04 (d, J
= 7.4, 2H, CH-arom POP), 6.85 (t, J
= 7.7, 2H,
3
4
47
2
H−H
H−H
−
1
1
6
71.1808; found, 671.1840. IR (cm ): ν(C−O−C) 1195 (s). H
CH-arom POP), 5.17 (ABX Y spin system, 2H, RhCH Cl), 3.61 (m,
2
2
3
4
3
NMR (300.13 MHz, C D , 298 K): δ 8.25 (dd, J
= 7.7, J
=
2H, PCH(CH ) ), 2.83 (m, 2H, PCH(CH ) ), 1.85 (dvt, J
= 7.8,
H−H
6
6
H−H
H−H
3
2
3
2
3
1
.6, 2H, CH Ph), 7.11 (m, 7H, CH-arom POP and CH Ph), 6.88 (t,
N = 15.7, 6H, PCH(CH ) ), 1.58 (dvt, J
= 7.4, N = 15.2, 6H,
3
2
H−H
3
2
3
3
JH−H = 7.6, 2H, CH-arom POP), 5.64 (dt, J
= 3.4, J
= 4.1,
= 7.3, N
PCH(CH
3
)
2
), 1.41 (dvt, JH−H = 7.4, N = 15.1, 6H, PCH(CH
3
)
2
),
), 1.15 (s, 6H,
CH ). C{ H}-apt (100.61 MHz, toluene-d , 273 K): δ 155.5 (vt, N
3 8
H−Rh
H−P
3
3
2
H, RhCH Ph), 2.42 (m, 4H, PCH(CH ) ), 1.35 (dvt, J
1.35 (dvt, JH−H = 7.1, N = 14.3, 6H, PCH(CH )
3 2
2
3
2
H−H
3
13
1
=
15.7, 12H, PCH(CH ) ), 1.22 (dvt, J
= 7.12, N = 12.7, 12H,
3
2
H−H
1
3
1
PCH(CH ) ), 1.19 (s, 6H, CH ). C{ H}-apt (75.47 MHz, C D ,
= 13.5, C-arom POP), 134.2 (s, CH-arom POP), 132.6 (vt, N = 6.1,
3
2
3
6
6
2
98 K): δ 155.5 (vt, N = 10.8, C-arom POP), 153.2 (vt, N = 4.9, Cipso
C-arom POP), 129.6 (s, CH-arom POP), 124.7 (vt, N = 5.4, CH-
1
Ph), 133.3 (vt, N = 5.2, C-arom POP), 131.7 (s, CH Ph), 131.1 (s,
arom POP), 123.6 (vt, N = 26.3, C-arom POP), 36.9 (dt, JC−Rh
=
),
2
CH-arom POP), 127.1 (s, CH-arom POP), 125.3 (s, CH Ph), 124.1
28.6, JC−P = 5.5, RhCH
30.7 (s, C(CH ), 27.4 (vt, N = 26.8, PCH(CH
20.8, PCH(CH ), 23.2, 20.5, 20.4, 19.3 (all s, PCH(CH
2
Cl), 36.9 (s, C(CH
)
3
2
), 30.8 (s, C(CH
3
)
2
(
s, CH-arom POP), 123.7 (vt, N = 5.1, C-arom POP), 34.8 (s,
3
)
2
3 2
) ), 25.8 (vt, N =
C(CH ) ), 29.7 (s, C(CH ) ), 26.5 (vt, N = 10.1, PCH(CH ) ), 20.0,
3
)
2
3
)
2
).
=
3
2
3
2
3 2
1
2
31
1
1
19.9 (both s, PCH(CH ) ), 17.3 (dt, J
= 21.5, J = 4.1,
P{ H} NMR (161.95 MHz, toluene-d
8
, 298 K): δ 26.4 (d, JRh−P
3
2
C−Rh
C−P
31
1
RhCH Ph). P{ H} NMR (121.49 MHz, C D , 298 K): δ 19.1 (d,
96.9, Rh−P).
2
6
6
1
JRh−P = 105.2).
Catalytic Dehalogenations. General Procedure. The dehaloge-
nation reactions were carried out in a two-necked flask fitted with a
condenser and containing a magnetic stirring bar. The second neck
was capped with a Suba-Seal to allow samples to be removed by
syringe without opening the system. Conversions were calculated
from the relative peak area integrations of the reactants and products
in the GC spectra using mesitylene as internal standard. For the
reactions involving benzyl chloride a Hewlett-Packard 4890 gas
chromatograph with a flame ionization detector and a 100% cross-
linked methyl silicone gum column (30 m × 0.32 mm, with 0.25 μm
film thickness) were used (oven conditions: 35 °C (hold 6 min) to
245 °C at 25 °C/min (hold 10 min)). For the reactions involving
chlorocyclohexane as substrate, a Network GC System 6890N gas
chromatograph with a flame ionization detector and a bonded
polyethylene glycol (PEG) phase column (30 m × 0.25 mm, with
0.25 μm film thickness) were employed (oven conditions: 30 °C
(hold 5 min) to 100 °C at 5 °C/min (hold 5 min) and 100 to 250
°C/min (hold 1 min)).
3
i
Reaction of [RhCl(κ P,O,P-{xant(P Pr ) })] (1) with trans-1,2-
2
2
3
Dichlorocyclohexane: Preparation of [RhCl (κ P,O,P-{xant-
3
i
(
P Pr ) })] (5). A solution of 1 (100 mg, 0.17 mmol) in trans-1,2-
2 2
dichlorocyclohexane (3 mL) was heated at 100 °C for 4 h. After this
time, the resulting solution was evaporated to dryness to afford an
orange residue. Addition of pentane (4 mL) afforded an orange solid,
which was washed with further portions of pentane (5 × 4 mL) and
dried in vacuo. Yield: 95 mg (79%). Anal. Calcd for C H Cl OP Rh:
2
7
40
3
2
C, 49.75; H, 6.18. Found: C, 49.93; H, 5.86. HRMS (electrospray, m/
+
z): calcd for C H Cl OP Rh [M − Cl] , 615.0954; found, 615.0981.
27
40
2
2
−
1
1
IR (cm ): ν(C−O−C) 1187 (s). H NMR (300.13 MHz, C D , 298
6
6
3
K): δ 7.29 (m, 2H, CH-arom), 6.93 (dd, J
= 7.6, J
= 1.6, 2H,
= 7.6, 2H, CH-arom), 3.45 (m, 4H,
= 7.5, N = 15.6, 12H, PCH(CH ) ),
= 7.3, N = 14.9, 12H, PCH(CH ) ), 1.05 (s, 6H,
H−H
H−H
3
CH-arom), 6.81 (t,
PCH(CH ) ), 1.69 (dvt, J
1
J
H−H
3
3
2
H−H
3 2
3
.61 (dvt, J
H−H 3 2
13
1
CH3). C{ H}-apt (75.47 MHz, C D , 298 K): δ 155.2 (vt, N = 12.2,
6
6
C-arom), 134.6 (s, CH-arom), 132.4 (vt, N = 5.9, C-arom), 129.9 (s,
CH-arom), 124.6 (vt, N = 5.5, CH-arom), 123.4 (vt, N = 25.4, C-
arom), 34.4 (s, C(CH ) ), 33.2 (s, C(CH ) ), 26.6 (vt, N = 24.4,
Dehalogenation of Chlorocyclohexane Catalyzed by [RhCl-
3
i
−3
(κ P,O,P-{xant(P Pr ) })] (1). In the presence of 1.8 × 10 M 1, the
3
2
3 2
2 2
3
1
1
PCH(CH ) ), 21.7, 19.7 (both s, PCH(CH ) ). P{ H} NMR
treatment of 0.18 M chlorocyclohexane with 0.21 M sodium formate
in 2-propanol at 100 °C under an argon atmosphere led after 24 h to
the transformation of 85% of chlorocyclohexane into cyclohexane.
3
2
3 2
1
(121.49 MHz, C D , 298 K): δ 24.9 (d, J
= 86.1).
6
6
Rh−P
3
i
Reaction of [RhCl(κ P,O,P-{xant(P Pr ) })] (1) with CH Cl :
2
2
2
2
3
i
3
Preparation of trans-[Rh(CH Cl)Cl (κ P,O,P-{xant(P Pr ) })]
Dehalogenation of Benzyl Chloride Catalyzed by [RhCl(κ P,O,P-
2
2
2 2
i
−3
(6b). Complex 1 (100 mg, 0.17 mmol) was dissolved in dichloro-
{xant(P Pr ) })] (1). In the presence of 1.8 × 10 M 1, the treatment
2 2
methane (3 mL), and this mixture was stirred for 16 h at room
temperature. The resulting solution was filtered through Celite and
was evaporated to dryness to afford a brown solid. Yield: 95 mg
of 0.18 M benzyl chloride with 0.21 M sodium formate in 2-propanol
(5 mL) at 100 °C under an argon atmosphere led after 6 h to the
transformation of 92% of benzyl chloride into a mixture of 1,2-
diphenylethane (42%) and toluene (50%). Under the same
conditions, but using 0.21 M KOH instead of sodium formate,
>99% of benzyl chloride was transformed into a mixture of 1,2-
diphenylethane (93%) and toluene (7%), after 2 h. The presence of
(
83%). Anal. Calcd for C H Cl OP Rh: C, 50.51; H, 6.35. Found:
28 42 3 2
C, 50.79; H, 6.09. HRMS (electrospray, m/z): calcd, for
+
C H Cl OP Rh [M − Cl] , 629.1137; found, 629.1137. IR
2
8
42
2
2
−
1
1
(
cm ): ν(C−O−C) 1086 (m). H NMR (300.13 MHz, toluene-d ,
8
3
1
2
2
98 K): δ 7.12−7.06 (m, 4H, CH-arom POP), 6.90 (t, J
= 7.6,
1,2-diphenylethane was confirmed by H NMR spectroscopy.
H−H
2
3
1
H, CH-arom POP), 6.30 (dt,
J
3
= 3.4,
J
= 4.1, 2H,
= 7.3, N =
Spectroscopic data of 1,2-diphenylethane are as follows. H NMR
H−Rh
H−P
3
RhCH Cl), 3.00 (m, 4H, PCH(CH ) ), 1.43 (dvt, J
5.5, 12H, PCH(CH ) ), 1.38 (dvt, J = 7.2, N = 14.6, 12H,
(300.13 MHz, CDCl , 298 K): δ 7.30 (m, 4H, CH-arom), 7.22 (m,
2
2
H−H
3
3
8d
1
6H, CH-arom), 2.95 (s, 4H, CH2).
3
2
H−H
G
DOI: 10.1021/acs.organomet.9b00409
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
3
NMR Spectroscopic Study of the Reaction of [RhCl(κ P,O,P-
42, 5270−5298. (e) Asay, M.; Morales-Morales, D. Non-symmetric
pincer ligands: complexes and applications in catalysis. Dalton Trans.
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Enantiospecific and Iterative Suzuki-Miyaura Cross-Couplings. J. Am.
i
{
xant(P Pr ) })] (1) with CH Cl . The experimental procedure is
described for a particular case, but the same method was used in all
experiments, which were run in duplicate. In the glovebox, an NMR
tube was charged with a solution of 1 (20 mg, 0.03 mmol) in CD Cl
0.5 mL), and a capillary tube filled with a solution of the internal
2
2
2
2
2
2
Chem. Soc. 2017, 139, 18124−18137. (g) Valdes, H.; García-Eleno,
M. A.; Canseco-Gonzalez, D.; Morales-Morales, D. Recent Advances
in Catalysis with Transition-Metal Pincer Compounds. ChemCatChem
́
(
standard (PCy ) in toluene-d was placed in the NMR tube. The tube
3
8
was immediately introduced into an NMR probe at the desired
temperature, and the reaction was monitored by 31P{ H} NMR at
different intervals of time.
2
018, 10, 3136−3172. (h) Campeau, L.-C.; Hazari, N. Cross-
1
Coupling and Related Reactions: Connecting Past Success to the
Development of New Reactions for the Future. Organometallics 2019,
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1
eq 6) and k (from eq 8, by least-squares adjustment).
2
of recent advances. J. Organomet. Chem. 2019, 893, 39−51.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.9b00409.
(
3) Alonso, F.; Beletskaya, I. P.; Yus, M. Metal-Mediated Reductive
■
*
S
Hydrodehalogenation of Organic Halides. Chem. Rev. 2002, 102,
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(
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spectra (PDF)
1
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CCDC 1925758−1925759 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
(
5) For some relevant examples see: (a) Netherton, M. R.; Dai, C.;
Neuschutz, K.; Fu, G. C. Room-Temperature Alkyl-Alkyl Suzuki
̈
Cross-Coupling of Alkyl Bromides that Possess β Hydrogens. J. Am.
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M. R.; Hills, I. D.; Fu, G. C. Boronic Acids: New Coupling Partners in
Room-Temperature Suzuki Reactions of Alkyl Bromides. Crystallo-
graphic Characterization of an Oxidative-Addition Adduct Generated
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AUTHOR INFORMATION
Corresponding Author
E-mail for M.A.E.: maester@unizar.es.
■
*
1
3662−13663. (c) Menzel, K.; Fu, G. C. Room-Temperature Stille
ORCID
Cross-Couplings of Alkenyltin Reagents and Functionalized Alkyl
Miguel A. Esteruelas: 0000-0002-4829-7590
Bromides that Possess β Hydrogens. J. Am. Chem. Soc. 2003, 125,
3
718−3719. (d) Sumino, S.; Ui, T.; Ryu, I. Synthesis of Alkyl Aryl
Montserrat Olivan
́
: 0000-0003-0381-0917
ate: 0000-0003-2094-719X
Ketones by Pd/Light Induced Carbonylative Cross-Coupling of Alkyl
Enrique On
̃
Iodides and Arylboronic Acids. Org. Lett. 2013, 15, 3142−3145.
Notes
(e) Zhang, Z.-Q.; Yang, C.-T.; Liang, L.-J.; Xiao, B.; Lu, X.; Liu, J.-H.;
The authors declare no competing financial interest.
Sun, Y.-Y.; Marder, T. B.; Fu, Y. Copper-Catalyzed/Promoted Cross-
coupling of gem-Diborylalkanes with Nonactivated Primary Alkyl
Halides: An Alternative Route to Alkylboronic Esters. Org. Lett. 2014,
ACKNOWLEDGMENTS
Financial support from the MINECO of Spain (Projects
CTQ2017-82935-P and Red de Excelencia Consolider
■
1
6, 6342−6345. (f) Wang, G.-Z.; Shang, R.; Cheng, W.-M.; Fu, Y.
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DOI: 10.1021/acs.organomet.9b00409
Organometallics XXXX, XXX, XXX−XXX