Competition studies of oxidative addition of aryl halides to the (PNP)Rh fragment

Article abstract of DOI:10.1021/om1008956

The (PNP)Rh fragment (2) can be conveniently accessed by dissociation of L from the four-coordinate complexes (PNP)Rh(L) (L = SPrⁱ₂, 3; L = H₂C - CHBu^t, 10), which contain the tridentate PNP pincer ligand (2-ⁱPr₂P-4-Me-C₆H ₄)₂N^-. A new and a more straightforward synthesis of 10 is reported, yielding 65% of 10 based on RhCl₃(H ₂O)_x. A number of new aryl halide oxidation addition products (AHOAP) of the general formula (PNP)Rh(Ar)(Hal) (Hal = Cl, Br, I) have been synthesized through oxidative addition (OA) reactions of meta- and para-substituted aryl halides with 3 or 10. The rotation about the Rh-C _aryl bond is restricted, resulting in rotamers for the meta-substituted aryls that are distinct on the NMR spectroscopy time scale. Reactions of some aryl halides containing a p-NO₂ or p-CO ₂Me with 3 or 10 led to the observation of products of C-H OA that are ostensibly stabilized by coordination to the NO₂ or CO ₂Me group. Analogous C-H OA products were observed for the halide-free nitrobenzene and ethyl benzoate, as well. However, the C-H OA products are thermodynamically unstable with respect to the isomeric AHOAP, to which they convert upon thermolysis. A Hammett-style analysis of the relative electronic effects of the para substituents X in the OA reactions of p-HalC ₆H₄X with 10 was carried out. The positive values of ρ obtained (ρ = 1.51(15) for Ar-Cl, ρ = 0.70(9) for Ar-Br, and ρ = 0.92(9) for Ar-I) illustrate the increase in the OA rate with increasing electron-withdrawing effect of X. Issues relating to the usefulness of the Hammett parameters in comparing various OA reactions are discussed. Comparison with analogous studies on the OA of aryl halides to Pd(0) complexes leads to the notion that the electronic effects have an impact on the rate similar to, but less pronounced than, that of the (PNP)Rh system, possibly indicative of an earlier transition state for the OA of aryl halides with (PNP)Rh.

Full text of DOI:10.1021/om1008956

ARTICLE
pubs.acs.org/Organometallics
Competition Studies of Oxidative Addition of Aryl Halides to the
(PNP)Rh Fragment
Mayank Puri,† Sylvain Gatard,† Dan A. Smith,‡ and Oleg V. Ozerov*,‡
†Department of Chemistry, Brandeis University, MS 015, 415 South Street, Waltham, Massachusetts 02454, United States
‡Department of Chemistry, Texas A&M University, 3255 TAMU, College Station, Texas 77842, United States
S Supporting Information
b
ABSTRACT: The (PNP)Rh fragment (2) can be conveniently
accessed by dissociation of L from the four-coordinate com-
plexes (PNP)Rh(L) (L = SPri2, 3; L = H2CdCHBut, 10), which
contain the tridentate PNP pincer ligand (2-iPr2P-4-Me-
C6H4)2Nꢀ. A new and a more straightforward synthesis of 10 is
reported, yielding 65% of 10 based on RhCl3(H2O)x. A number of
new aryl halide oxidation addition products (AHOAP) of the
general formula (PNP)Rh(Ar)(Hal) (Hal = Cl, Br, I) have been synthesized through oxidative addition (OA) reactions of meta- and para-
substituted aryl halides with 3or 10. The rotation about the RhꢀCaryl bond is restricted, resulting in rotamers for the meta-substituted aryls
that are distinct on the NMR spectroscopy time scale. Reactions of some aryl halides containing a p-NO2 or p-CO2Me with 3 or 10 led to
the observation of products of CꢀH OA that are ostensibly stabilized by coordination to the NO2 or CO2Me group. Analogous CꢀH OA
products were observed for the halide-free nitrobenzene and ethyl benzoate, as well. However, the CꢀH OA products are
thermodynamically unstable with respect to the isomeric AHOAP, to which they convert upon thermolysis. A Hammett-style analysis
of the relative electronic effects of the para substituents X in the OA reactions of p-HalC6H4X with 10 was carried out. The positive values
of F obtained (F = 1.51(15) for ArꢀCl, F = 0.70(9) for ArꢀBr, and F = 0.92(9) for ArꢀI) illustrate the increase in the OA rate with
increasing electron-withdrawing effect of X. Issues relating to the usefulness of the Hammett parameters in comparing various OA
reactions are discussed. Comparison with analogous studies on the OA of aryl halides to Pd(0) complexes leads to the notion that the
electronic effects have an impact on the rate similar to, but less pronounced than, that of the (PNP)Rh system, possibly indicative of an
earlier transition state for the OA of aryl halides with (PNP)Rh.
’ INTRODUCTION
system presents a very convenient framework for the exploration
of competitive OA of various ArꢀHal with Rh. In this paper, we
report our findings, ultimately focused on the influence of the
substituents in ArꢀHal on the rate of OA. Analogous literature
studies of the OA of ArꢀHal to Pd0 complexes11ꢀ14 provide a
backdrop for the comparison with this first study for the OA of
ArꢀHal to RhI.
Oxidative addition (OA) of aryl halides (ArꢀHal) is a critical
step in the versatile chemistry of metal-catalyzed carbonꢀcarbon
and carbonꢀheteroatom coupling.1,2 OA appears to be increas-
ingly difficult for the lighter halides in ArꢀHal. In the case of the
lightest halogen, aryl fluorides rarely undergo OA and, in fact,
explorations of their OA chemistry have historically been treated
somewhat separately3 from the analogous reactions of ArꢀCl,
ArꢀBr, and ArꢀI. OA of aryl halides is most common with
zerovalent group 10 metal complexes, especially those of Pd.1,2
However, the number of examples of the OA of ArꢀHal to RhI
complexes has been steadily rising in the recent years.4 In the
same vein, Rh complexes have also been used to effect catalytic
coupling reactions of aryl halides that likely rely on the OA of
ArꢀHal as one of the key mechanistic steps.5
’ RESULTS AND DISCUSSION
Optimized Synthesis of a Rh(I) Precursor. Our previous
investigations demonstrated the intermediacy of 2 in the OA
reactions of aryl halides. Complex 3 has served as a convenient
masked form of 2 in reactions with aryl halides and with silyl
halides. The synthesis of 3 is depicted in Scheme 2. It consists of
several steps, and while each reaction is intrinsically high-yield-
ing, the losses incurred in isolation of all the intermediates
decrease the overall yield; considerable time is spent on isolation
and purification as well. In addition, the synthesis of 3 requires
the N-methylated PNP ligand 4, which requires one more
Our group has reported on the OA of ArꢀHal to a three-
coordinate Rh transient (PNP)Rh (2),6ꢀ8 in which the RhI
center is supported by a diarylamido/bis(phosphine) PNP
pincer ligand.9,10 We demonstrated that OA indeed requires
the formation of the 14-electron (PNP)Rh fragment 2, accessible
either via dissociation of a placeholder ligand (e.g., from 3) or via
CꢀC reductive elimination (RE) from a five-coordinate RhIII
precursor (e.g., 1, Scheme 1). We surmised that our (PNP)Rh
Received: September 16, 2010
Published: April 06, 2011
r
2011 American Chemical Society
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|

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ARTICLE
synthetic step than its NH relative 7 (Scheme 3). We saw benefit
in developing a new synthesis of a synthon for 2 which minimizes
the time involved and maximizes the yield. To this end, we
envisioned a synthesis of 10, shown in Scheme 3. While it cannot
be carried out in a “one-pot” fashion, it can be performed as a
“single-sequence” synthesis, without isolating or purifying the
intermediates beyond the removal of volatiles under vacuum. We
have previously reported related one-sequence syntheses of the
(PNP)Ir complexes.15
previously described. Complex 8, as is, can then be converted
to the dihydride 9 by using potassium tert-butoxide in PrOH
i
(which should make KOPri) and H2. Both the isopropoxide (via
transfer of β-hydride) and H2 (by formation of an H2 adduct and
deprotonation of the latter with a base) can serve as the source of
the second hydride in 9. We have not investigated whether only
one source of hydride is sufficient. The use of nontertiary
alcohols as sources of H2 or of hydride (with base) is very
common in the synthesis of late-metal hydride complexes.17 We
had anticipated that applying an excess of H2 would help
consume the cyclooctadiene byproduct from the previous step.
However, if cyclooctadiene was not removed prior to addition of
base and H2, complexes other than 10 formed and persisted. This
was evidenced by the observation of broad signals in the31P{1H}
NMR spectra around δ 34 and 43 ppm, which we tentatively
ascribe to (PNP)Rh(olefin) complex(es) where the olefin is one
of the isomers of cyclooctadiene or perhaps a cyclooctene.
Apparently, 9 is a poor hydrogenation catalyst. Therefore, we
found it critical to completely remove the C8 volatiles from the
reaction mixture prior to the conversion of 8 into 9. Addition of
H2CdCHCMe3 (TBE) to 9 led to the formation of the desired
complex 10. The two hydrides of 9 may be simply displaced as H2
or consumed in the hydrogenation of TBE. TBE, being present in
excess, may also be able to displace C8 olefins from the residual
(PNP)Rh(olefin) impurities. All in all, we were able to isolate 10
in 65% yield based on Rh in RhCl3(H2O)n.
The formation of [(COD)RhCl]2 is essentially quantitative,16
and it can be used as is in the next step. It is preferable to use a
small excess of (PNP)H (7) in the reaction with [(COD)RhCl]2
to ensure errors in measurement and the non-100% purity do not
lead to a deficiency of 7. Free 7 does not react with any further
(PNP)Rh complexes here, and its relatively high solubility allows
for its facile separation during the purification of the final product
10. The reaction of 7 and [(COD)RhCl]2 leads to 8, as
Scheme 1
Scheme 4
Scheme 2
Scheme 3
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Scheme 5
1
Table 1. H NMR Chemical Shifts (in C6D6 Solvent) of the Resonances of the PNP Ligands in the Various AHOAP of the General
Formula (PNP)Rh(C6H4X-p)(Hal) (11xHal)
chem shift (δ, ppm)
AHOAP Hal
X
CHAr
CHAr
CHAr
CHiPr
CHiPr
MeAr
MeiPr
MeiPr
MeiPr
MeiPr
11aCl
11bCl Cl OMe
11cCl Cl Me
11dCl Cl
Cl NH2
7.98 (d, 9) 6.90
8.00 (d, 8) 6.89
7.98 (d, 9) 6.89
7.93 (d, 9) 6.85
7.97 (d, 8) 6.88
7.91 (d, 9) 6.84
no 1H data
6.81 (d, 9)
6.82 (d, 9)
6.81 (d, 9)
6.79a
2.96 (m) 2.30 (m) 2.13 1.47 (dvt, 8) 1.07 (dvt, 8) 1.00 (dvt, 7) 0.51 (dvt, 7)
2.96 (m) 2.29 (m) 2.13 1.44 (dvt, 8) 1.06 (dvt, 8) 1.00 (dvt, 6) 0.45 (dvt, 8)
2.96 (m) 2.29 (m) 2.13 1.45 (dvt, 8) 1.07 (dvt, 8) 1.00 (dvt, 6) 0.44 (dvt, 8)
F
2.92 (m) 2.24 (m) 2.13 1.38 (dvt, 8) 1.02 (8)
0.98 (7)
0.35 (dvt, 7)
11eCl
11fCl
Cl
H
6.80 (d, 8)
2.96 (m) 2.29 (m) 2.12 1.42 (dvt, 8) 1.07 (dvt, 8) 0.99 (dvt, 6) 0.41 (dvt, 8)
Cl Cl
6.81ꢀ6.77b 2.91 (m) 2.21 (m) 2.12 1.36 (dvt, 8) 1.00
0.95
0.34 (dvt, 8)
11gCl Cl Br
11hCl Cl CF3
7.93 (d, 10) 6.83ꢀ6.79a,c 6.83ꢀ6.79a,c 2.90 (m) 2.19 (m) 2.13 1.32 (dvt, 8) 0.99 (dvt, 8) 0.94 (dvt, 7) 0.26 (dvt, 8)
11iCl
Cl CO2Me 7.92 (d, 9) 6.79
6.82 (d, 8)
6.81c
2.92 (m) 2.20 (m) 2.12 1.36 (dvt, 8) 1.01 (dvt, 8) 0.95 (dvt, 6) 0.32 (dvt, 8)
11kCl Cl NO2
11aBr Br NH2
11bBr Br OMe
11cBr Br Me
7.88 (d, 9) 6.81c
8.00 (d, 9) 6.91
8.00 (d, 9) 6.89
8.00 (d, 8) 6.90
7.95 (d, 8) 6.86
7.98 (d, 8) 6.88
7.92 (d, 8) 6.85
7.95 (d, 9) 6.87aꢀc
2.86 (m) 2.17 (m) 2.13 1.27 (dvt, 8) 0.96
3.13 (m) 2.33 (m) 2.13 1.53 (dvt, 8) 1.01 (6)
0.92 (7)
0.99 (8)
0.98 (8)
0.99 (8)
0.93 (8)
0.98c
0.19 (dvt, 8)
0.47 (dvt, 8)
0.39 (dvt, 8)
0.40 (dvt, 8)
0.30 (dvt, 8)
0.35 (dvt, 8)
0.30 (dvt, 8)
0.21 (dvt, 8)
0.26 (dvt, 8)
6.81 (d, 8)
6.82 (d, 9)
6.82 (d, 8)
6.81 (d, 8)
6.80 (d, 8)
6.81 (d, 9)
6.87aꢀc
3.1a
2.30 (m) 2.12 1.50 (dvt, 8) 1.00 (6)
3.14 (m) 2.32 (m) 2.13 1.52 (dvt, 8) 1.01 (6)
3.09 (m) 2.26 (m) 2.12 1.44 (dvt, 8) 0.97 (6)
3.14 (m) 2.30 (m) 2.10 1.48 (dvt, 8) 0.98c
3.07 (m) 2.25 (m) 2.13 1.42 (dvt, 8) 0.95c
3.07 (m) 2.23 (m) 2.12 1.40 (dvt, 8) 0.94 (6)
3.09 (m) 2.25 (m) 2.12 1.42 (dvt, 8) 0.94aꢀc
11dBr Br
11eBr Br
F
H
11fBr
11hBr Br CF3
11jBr
11kBr Br NO2
Br Cl
0.95c
0.91 (8)
0.94aꢀc
Br CO2Et 7.94 (d, 8) 6.80
6.84 (d, 8)
6.81aꢀc
7.90 (d, 9) 6.81aꢀc
8.03 (d, 8) 6.92
8.04 (d, 9) 6.91
8.02 (d, 9) 6.91
7.97 (d, 9) 6.87
8.01 (d, 8) 6.90
7.95 (d, 9) 6.86
7.94 (d, 8) 6.85
3.03 (m) 2.20 (m) 2.13 1.34 (dvt, 8) 0.93 (dvt, 6) 0.87 (dvt, 8) 0.15 (dvt, 8)
3.39 (m) 2.35 (m) 2.13 1.61 (dvt, 8) 1.00 (dvt, 6) 0.86 (dvt, 8) 0.42 (dvt, 8)
3.39 (m) 2.32 (m) 2.12 1.59 (dvt, 8) 0.99 (dvt, 6) 0.86 (dvt, 8) 0.35 (dvt, 8)
3.40 (m) 2.33 (m) 2.13 1.59 (dvt, 8) 1.00 (dvt, 6) 0.86 (dvt, 8) 0.34 (dvt, 8)
11aI
11bI
11cI
11dI
11eI
11fI
11gI
11iI
I
I
I
I
I
I
I
I
I
NH2
OMe
Me
F
6.83 (d, 9)
6.84 (d, 8)
6.83 (d, 8)
6.82 (d, 8)
6.82 (d, 8)
6.82 (d, 9)
6.82 (d, 9)
6.84 (d, 7)
6.83b,c
3.36 (m) 2.28 (m) 2.12 1.52 (dvt, 8) 0.95 (6)
0.81 (dvt, 8) 0.25 (dvt, 8)
H
3.40 (m) 2.34 (m) 2.11 1.57 (dvt, 8) 1.00 (dvt, 6) 0.87 (dvt, 8) 0.32 (dvt, 8)
3.34 (m) 2.28 (m) 2.12 1.50 (dvt, 8) 0.95 (dvt, 6) 0.79 (dvt, 8) 0.24 (dvt, 7)
3.30 (m) 2.28 (m) 2.12 1.50 (dvt, 8) 0.95 (dvt, 6) 0.78 (dvt, 8) 0.24 (dvt, 8)
Cl
Br
CO2Me 7.97 (d, 9) 6.81
NO2
7.93 (d, 8) 6.83b,c
3.39 (*) 2.26 (m) 2.12 1.50 (dvt, 8) 0.95 (6)
0.80 (dvt, 8) 0.21 (dvt, 8)
11kI
3.29 (m) 2.22 (m) 2.12 1.43 (dvt, 8) 0.91 (dvt, 6) 0.75 (dvt, 8) 0.07 (dvt, 8)
a Fine structure assignment and precise chemical shift determination complicated by overlap with excess free aryl halide. b Fine structure assignment and
precise chemical shift determination complicated by overlap of the PNP resonance with the resonance of the aryl group in the AHOAP. c Fine structure
assignment and precise chemical shift determination complicated by accidental overlap of two resonances of the PNP ligand in the AHOAP.
Oxidative Addition of Various Aryl Halides. We set out to
prepare an array of aryl halide oxidative addition products of the
general formula (PNP)Rh(Ar)(Hal). We will refer to them as
AHOAP for the sake of brevity. The oxidative addition reactions
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were performed by treatment of 3 or 10 with 3 equiv of the
corresponding aryl halide (Schemes 4 and 5). We have pre-
viously established the relative rates of reaction for the parent
phenyl halides to be PhI > PhBr > PhCl, and this appears to hold
true in this study as well. However, we did not attempt to
optimize the reaction conditions, and the reported reaction
times (6 h to 4 days) and temperatures (room temperature to
70 °C) are not necessarily indicative of the relative reaction
rates. The formation of AHOAP is irreversible,6 as indicated by
the lack of observable changes upon thermolysis of select
AHOAP with other aryl halides (details in the Supporting
Information).
We have previously characterized other closely related
AHOAP in detail, both structurally and spectroscopically.6ꢀ8
Here we report a number of new AHOAP. Only a few were
isolated; others were observed in situ. The AHOAP were easily
Figure 1. Illustration of the reliable correlation of the 31P NMR
chemical shift of (PNP)Rh(C6H4X-p)(Hal) (11xHal) on the nature
of both X and Hal. The horizontal axis corresponds to σꢀ values (vide
infra) of various X.
1
identified, because they possess very similar H and 31P NMR
spectroscopic features and presumably also similar near-square-
pyramidal structures with an apical aryl group.18 In the 1H NMR
spectra, all AHOAP display an unusually upfield-shifted reso-
nance (δ 0.2ꢀ0.5 ppm) for one of the four diastereotopic pairs of
Me groups of the PPri2 arms. Similarly anomalous chemical shifts
have been observed in (PNP)Rh(Ph)(SPh)7 and (PNP)Ir(Ph)-
(Hal),15 which all likely possess similar structures with the aryl
group at the apex of the square pyramid. In contrast, this anomaly
is not observed for (PNP)Rh(Me)(Cl) (6),19 (PNP)Rh(H)(Cl)
(8),19 or (PNP)Rh(SiR3)(Hal),8 which are square pyramidal but
do not contain a Rhꢀaryl group. In these compounds, all the Me
groups in the isopropyls resonate in the δ 0.9ꢀ1.6 ppm range
typical for most PNP complexes. The Me chemical shift anomaly
is also not observed in the (PNP)Ir(Ar)(H)15 compounds or in
(PNP)PdPh,20 which do have a metalꢀaryl fragment but are
either non square pyramidal or possess a hydride instead of an
aryl at the apical position cis to N. However, this anomaly is
indeed observed in the (PNP)Ir(Ph)(Hal) complexes, which
possess essentially the same square-pyramidal structure of the Rh
AHOAP.15 We attribute the Me chemical shift anomaly to the
selective ring current influence of an apical aryl group on a pair of
Me groups. In the apical position (cis to NPNP), the aryl ring is
sandwiched between the two PPri2 arms, with a pair of Me groups
positioned to occupy the space above and below the ring plane.
This effect would indeed only be observed with an aryl group
present, and only in the apical position. It is not specific to the
PNP ligand. For example, Milstein et al. reported a [(POCOP)-
hydrogen atoms of a phenyl group in (PNP)Rh(Ph)(Hal)
(11eCl, 11eBr, 11eI).6 Likewise, all AHOAP in this study
displayed inequivalence of the ortho and meta hydrogens in
para-substituted aryls. This is a common phenomenon for
complexes with a phenyl group in a mer arrangement with two
bulky trans phosphines.22 When the aryl group contained a meta
substituent (12 and 13), we observed a mixture of two AHOAP
rotamers (Scheme 5). The pairs of rotamers were distinct by
NMR spectroscopy at ambient temperature, although the differ-
ences in the chemical shifts of the resonances of the PNP ligand
were small to imperceptible.
The 31P{1H} NMR spectra of all aryl halide oxidative addition
products displayed a single doublet resonance (1JPꢀRh ≈ 103ꢀ
109 Hz). Although the differences between various AHOAP are
small, the exact 31P NMR chemical shift reliably identifies a
particular AHOAP. We illustrate this in Figure 1, where the 31P
NMR chemical shifts of the AHOAP are plotted versus the
Hammett σꢀ values (vide infra) of the para substituents in the
aryl groups. Although there is a decent correlation, we do not
seek to analyze the relationship between the chemical shifts and
the electronic properties quantitatively. Here, we merely wish to
point out that, for a given aryl group, the RhꢀCl, RhꢀBr, and
RhꢀI compounds are readily distinguishable and, vice versa, for a
given halide in the AHOAP, the 31P chemical shift is distinctive
for each aryl.
CꢀH Oxidative Addition. In the oxidative addition reactions
of p-ClC6H4NO2, p-BrC6H4NO2, and p-ClC6H4CO2Me, we
initially observed a mixture of the expected AHOAP and another
product that converted to the AHOAP over time (Scheme 6).
We assign these intermediates as CꢀH oxidative addition
products 14, 15, and 17 on the basis of their distinctive NMR
spectroscopic features. In addition to a doublet resonance
(Table 2) in the 31P{1H} NMR spectra, each of these complexes
displayed a telltale hydride resonance in the 1H NMR spectra as a
doublet of triplets (Table 2). Surmising that the CꢀHal func-
tionality is not necessary for the formation of these products of
CꢀH oxidative addition, we performed reactions of 10 and 3
with nitrobenzene and ethyl benzoate (Scheme 6). These reac-
tions cleanly produced 16 and 18, which possessed key NMR
spectroscopic features similar to those of 14, 15, and 17
(Table 2).
Rh(Ar)][BArF ] complex with PPri2 arms and a presumably
4
apical aryl group where one of the pairs of Me groups resonated
at δ 0.35 ppm.21
In fact, all of the H NMR resonances of the PNP ligand in
1
various AHOAP form a fingerprint pattern, especially for the
AHOAP with the same halide. Table 1 presents these data for all
non-meta-substituted AHOAP. Table 1 provides a numbering
scheme for the products of the OA of para-substituted aryl
halides (11 in Scheme 4) of the general form 11xHal, where x
denotes the para substituent and Hal (Cl, Br, or I) stands for the
Rh-bound halide. Not every combination of “x” and “Hal” in
11xHal was prepared in this study. Table 1 includes every
compound 11xHal that has been observed in this study and
excludes compounds 11jCl, 11gBr, 11iBr, 11hI, and 11jI, which
were not.
We previously noted that the rotation about the RhꢀCaryl
bond is restricted in AHOAP, resulting in the observation of five
broadened 1H NMR resonances for the five inequivalent
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We propose that the relative stability of the CꢀH oxidative
addition products with the nitro and carbethoxy substituents that
allows for their observation arises from the stabilizing effect of the
coordination of the O donor. Because of this, we assume that the
observed CꢀH activation products are the ortho isomers. This
assumption is consistent with the observed hydride chemical
shifts. They are less negative than the chemical shift of the
hydride (ca. ꢀ30 ppm) in the five-coordinate, square-pyramidal
(PNP)Rh(H)(Cl) and closely related (R3P)2Rh(H)(Cl)2.23 The
hydride chemical shift is quite sensitive to the nature of the
occupant of the trans coordination site. The chemical shifts of
14ꢀ18, six-coordinate compounds with oxygen donors trans to
the hydride, compare well to the chemical shift (ca. ꢀ 22 ppm) of
the hydride in Meyer and Kaska’s (PNP*)Rh(H)(Cl)(THF),
where the hydride is trans to THF (PNP* is bis(o-(diphenyl-
phosphino)phenyl)amide).24
applies here as well. Precoordination of the “directing” group is
in fact likely counterproductive, as it blocks the coordination site
necessary for the CꢀH OA.
The competition between CꢀH and CꢀHal activation in
pincer-supported Ir and Rh systems has recently been a subject of
intense scrutiny, with complementary findings by the Milstein
group and ours that were also examined computationally by Hall
et al.6,15,26ꢀ28 The observation of 14, 15, and 17 suggests that
CꢀH OA is kinetically competitive with CꢀCl and CꢀBr OA in
the (PNP)Rh system, but the products are long-lived enough for
observation only when the aryl carries an ortho-chelating (and
electron-withdrawing) substituent. In an earlier report,6 we
attempted to generate (PNP)Rh(C6H4Cl-p)(H), only to detect
(PNP)Rh(Ph)(Cl) (11eCl), the apparent product of its rear-
rangement favoring the CꢀCl over CꢀH OA. The lack of the
observation of the CꢀH OA products in reactions with nitro-
phenyl and carbethoxyphenyl iodides suggests that the CꢀHal
OA is faster (i.e., more competitive with the CꢀH OA) for
heavier halides. On the other hand, the isomerization of 14, 15,
and 17 into the corresponding AHOAP (11kCl, 11kBr, 11iCl)
demonstrates that the CꢀHal OA is thermodynamically pre-
ferred to the CꢀH OA even when the CꢀH OA products are
stabilized by the ortho group.
Goldman and co-workers studied the CꢀH oxidative addition
of nitrobenzene to the closely related (PCP)Ir fragment and
concluded that the ortho selectivity in CꢀH activation is of
thermodynamic, not kinetic, origin.25 This conclusion likely
Scheme 6
Competition between Different CꢀHal Bonds. p-Dihalo-
benzenes as substrates provide an opportunity to test the
preference for the OA of the various CꢀHal bonds in the
intramolecular setting. Within the set of para-substituted sub-
strates (Scheme 4, Table 1), there were six possible dissym-
metric permutations of p-C6H4(Hal)2 where Hal = F, Cl, Br, I.
We used NMR spectroscopy (Figure 1) to determine which
halide was bound to Rh and which remained bound to C in the
resultant AHOAP. In all six cases, the OA of the CꢀHal bond
with the heavier halide was preferred. Only in the case of p-
BrC6H4Cl were both possible AHOAP observed by NMR
spectroscopy: 96% of the CꢀBr OA (11fBr) and 4% of the
Scheme 7
Table 2. Select NMR Spectroscopic Data for Products of CꢀH OA (14ꢀ18) in Reactions with Various Aryl Reagents
aryl reagent
CꢀH OA product
31P{1H} NMR (δ)
1H NMR (δ)
4-chloronitrobenzene
4-bromonitrobenzene
nitrobenzene
14
15
16
17
18
56.6 (d, J = 105 Hz)
56.9 (d, J = 106 Hz)
56.1 (d, J = 107 Hz)
52.7 (d, J = 108 Hz)
52.2 (d, J = 109 Hz)
ꢀ18.19 (dt, JHꢀRh = 33 Hz, JHꢀP = 13 Hz)
ꢀ18.22 (dt, JHꢀRh = 34 Hz, JHꢀP = 12 Hz)
ꢀ18.02 (dt, JHꢀRh = 34 Hz, JHꢀP = 12 Hz)
ꢀ19.71 (dt, JHꢀRh = 35 Hz, JHꢀP = 13 Hz)
ꢀ19.53 (dt, JHꢀRh = 35 Hz, JHꢀP = 13 Hz)
methyl 4-chlorobenzoate
ethyl benzoate
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ARTICLE
Scheme 8
CꢀCl OA (11gCl). In an intermolecular competition
(Scheme 7), reaction of 10 with equal excesses of PhBr and
PhCl also produced a mixture of 96% CꢀBr OA (11eBr) and
4% CꢀCl OA (11eCl). We have previously established quali-
tatively that the relative rates of OA to 3 were in the order PhI >
PhBr > PhCl. Fluorobenzene does not react with 3, even under
forcing thermolysis conditions, and so the lack of CꢀF
activation29 in p-C6H4(Hal)2 was expected. The general pre-
ference for a heavier halide in the ArꢀHal OA reactions is not
surprising and follows the corresponding trend for the OA of
aryl halides to Pd0 complexes.
Intermolecular Competition among Differently Substi-
tuted Aryl Halides. In order to assess the electronic influence
of the substituents in an aryl halide on the rate of oxidative
addition, we performed an array of intermolecular competition
reactions. Here, we assumed that the mechanism of OA is the
same for all aryl halides: that is, that the formation of the three-
coordinate species 2 precedes interaction of Rh with the aryl
halide and 2 is thus a common intermediate. We previously
obtained evidence in support of this mechanism, although we
have not unequivocally established that for each aryl halide under
study.7
For each halide (Cl, Br, and I), we selected a series of para-
substituted aryl halides with substituents ranging in electronic
effect from carbalkoxy and nitro as the most electron-with-
drawing groups to methoxy and amino as the most electron-
donating. For each halide, we carried out 12 experiments in
which 10 was allowed to react to completion with a mixture of
three aryl halides (same halide), each in 10-fold molar excess, in
C6D6 as solvent. The ratios of the AHOAP were measured in situ
by 31P NMR spectroscopy. Each substituent was used in at least
three separate experiments for each halide.
A generic experiment and the corresponding rate equations
are depicted in Scheme 8. The large and equal excess of each aryl
halide ensures that the concentration of each aryl halide is
approximately constant throughout the experiment. The con-
centration of the (PNP)Rh (2) that confronts the three aryl
halides in the same mixture is the same from the point of view of
each aryl halide at any point in time, even though it is not
constant. From this, we can derive that the ratio of the rate
constants for the reaction of an aryl halide with 2 should be equal
to the ratio of the corresponding AHOAP.30
Because we observed some CꢀH OA products 14, 15, and 17
in reactions with p-ClC6H4NO2, p-BrC6H4NO2, and p-
ClC6H4CO2Me, it was necessary to wonder whether CꢀH
OA in effect “directs” the Rh center toward the CꢀHal bond
in the same molecule. To test this, we attempted to examine
whether the selectivity in the formation of the AHOAP from
a mixture of aryl halides was skewed by the CꢀH OA. The
reaction of 10 with a mixture (10 equiv each) of p-ClC6H4NO2
and p-ClC6H4CF3 at complete consumption of 10 produced 14
(57%), 11kCl (30%), and 11hCl (13%). However, heating this
mixture until 14 was completely converted to the AHOAP
resulted in an 87:14 mixture of 11kCl and 11hCl. This result
suggests that, ostensibly, once formed, 14 is converted exclu-
sively to 11kCl. Because of this, we excluded p-ClC6H4NO2 from
our studies.
Analogous reactions of 10 with of p-BrC6H4NO2 and p-
BrC6H4CF3 yielded less emphatic results. For one, the propor-
tion of 15 initially formed was small (<20%) and while thermo-
lysis of 15 appeared to favor 11kBr more so than the initial
reaction with 10, the preference was less pronounced than with
14. Thermolysis of a mixture of p-BrC6H4NO2 and p-
BrC6H4CF3 produced similar ratios with both 3 and 10 as
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Table 3. Logarithmic Values of the Relative Rate Constants
for the Various Aryl Halides
log10(ki/k0)
para substituent
X
p-XC6H4Cl
p-XC6H4Br
p-XC6H4I
a
b
c
d
e
f
NH2
OMe
Me
ꢀ0.72(7)
ꢀ0.52(5)
ꢀ0.17(6)
ꢀ0.15(8)
0
ꢀ0.38(5)
ꢀ0.24(6)
ꢀ0.09(7)
0.00(8)
0
ꢀ0.30(4)
ꢀ0.20(4)
ꢀ0.05(5)
ꢀ0.04(6)
0
F
H
Cl
0.47(7)
0.34(5)
0.46(5)
g
h
i
Br
CF3
CO2Me
CO2Et
NO2
1.04(8)
1.34(9)
0.52(9)
0.87(6)
1.36(8)
j
0.70(8)
Figure 2. Overlaid Hammett plots for the reactions of aryl chlorides
(blue), aryl bromides (green), and aryl iodides (red) of the general
formula p-HalC6H4X. The horizontal axis corresponds to σꢀ values
(from anilinium pKa’s)36 of various X.
k
0.87(10)
sources of (PNP)Rh. All in all, the deviation potentially caused by
the CꢀH OA in the case of bromides was small and was probably
within experimental error. It is possible that the CꢀH OA may
play a role with other substrates (besides nitroaryl halides), but it
is likely to be even smaller.
The rate constants for the reactions involving the unsubsti-
tuted PhCl, PhBr, and PhI were set to unity. Each group of 12
experiments generated 36 pairwise determinations of rate con-
stant ratios. In practice this number was slightly reduced, because
overlap of 31P{1H} resonances did not allow for signal discrimi-
nation in a few cases. Nonetheless, the amount of original data
was quite sufficient to solve for the 7 or 8 unknowns (relative rate
constants corresponding to various para substituents) using the
LINEST function in Microsoft Excel. The rate constants relative
to the corresponding Ph-Hal are shown in the log10 form in
Table 3.
addition, Hammett plot graphics are much more common in
publications than complete disclosure of raw measurement data,
precise sources of the σ constants used, or error analysis. This
often makes comparisons of different studies quite tentative. All
of these considerations make the Hammett analysis of the
organometallic OA reactions much less quantitative than a
cursory look may suggest. Nonetheless, with appropriate math-
ematical caution, Hammett plots can provide some information
on the relative influence of substituents on the relative rates of
the reaction. If nothing else, they serve as an organized method to
evaluate qualitatively whether the reaction rate is uniformly
accelerated by electron-withdrawing (or electron-donating)
substituents.
As some others before us,13,14 we have found that the
correlation between the OA rates and the σ proper values
deviates significantly from linearity. On the other hand, σꢀ
values provided a much better fit. The σꢀ values based on the
acidity of phenols and of anilinium cations are very similar for all
substituents pertinent to this study, except for the NH2 group.
Strangely, the σꢀ value for p-NH2 from the phenol series
is ꢀ0.15, whereas the values for p-OMe and p-Me are ꢀ0.26
and ꢀ0.17, respectively.33 This implies that the p-NH2 group is
less donating than either p-OMe or p-Me groups, which seems
simply incorrect. This odd value originally stems from a 1928
determination of the pKa of p-H2NC6H4OH in neat water (not
made in the context of Hammett substituent analysis).37 The
uncertainty in that determination is unknown, and it is also
possible that nontrivial solvation effects alter the acidity of p-
aminophenol specifically in water. Notably, in 20% water/80%
ethanol, the acidity of p-H2NC6H4OH is lower than that of either
p-MeOC6H4OH or p-MeC6H4OH.38 In our study, the amino-
substituted aryl halides displayed the lowest rates of OA, grossly
inconsistent with a σꢀ value of ꢀ0.15. Largely for that reason, we
favor the anilinium series of σꢀ values (used in Figures 1 and
2).36 The corresponding Hammett plots for the aryl halide OA
reaction rates are depicted in Figure 2. The F values from these
plots are 1.51(15) for ArꢀCl, 0.70(9) for ArꢀBr, and 0.92(9) for
ArꢀI.39
Hammett Studies. With the relative rate constant values in
hand, we set out to perform a Hammett-type analysis.31ꢀ35
A
Hammett plot analysis is a fairly common exercise for the study
of the electronic effects of substituents, including in the studies of
oxidative addition of aryl halides.11ꢀ14 Plotting logarithmic rate
constants against a known set of σ values appears to be
straightforward; however, closer scrutiny of the literature unveils
a more equivocal picture. First, there exist at least a few different
sets of standard σ values.33 The original σ values proper are
derived from the pKa values of substituted benzoic acids.
Complementary σꢀ values arise from the pKa values of sub-
stituted phenols; they ostensibly better capture the resonance
effects of strongly electron withdrawing groups stabilizing a
developing negative charge.33 A similar set of σꢀ values has been
derived from the pKa values of substituted anilinium cations.36
On the other hand, σþ values have been developed for use with
reactions where resonance stabilization of the developing posi-
tive charge is especially significant.33 The existence of “compet-
ing” sets of standard σ values naturally leads to the selection,
somewhat arbitrary, of the best-matching ones for a particular
comparison or even a mix-and-match combination of σ values
from different sets. Compounding the situation, most of the
σ proper and σꢀ values are based on the pKa determinations in
water, whereas organometallic reactions typically take place in
solvents of low polarity and the solvation differences may well
skew the effects of various substituents. The experimental
uncertainty in the reported σ values is not always known. In
In all cases, the increase in the electron-withdrawing ability of
the substituents clearly correlated with the acceleration of OA
(F > 0). The uncertainty in the determination of the F values does
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’ CONCLUSION
Investigations of the electronic influence of the substituents
on the aromatic ring on the oxidative addition (OA) of aryl
halides to the (PNP)Rh transient (2) have shown that the OA is
accelerated by the presence of electron-withdrawing groups. This
influence was semiquantitatively evaluated using the Hammett
relationships. The electronic influence of the substituents is
greater for the OA of aryl chlorides than for aryl bromides and
aryl iodides. We also conclude that the electronic influence of the
substituent in the OA of aryl chlorides to 2 is less pronounced
than in the analogous studies with OA of aryl chlorides to Pd(0)
complexes with strongly electron donating phosphines. Lesser
electronic influence (smaller positive F value in the Hammett
analysis) may indicate an earlier transition state in a concerted
OA process.
Competitive experiments between different carbonꢀhalogen
bonds to 2 clearly showed the preference for the OA of a
carbonꢀhalogen bond with the heaviest halide. In some in-
stances, relatively long lived CꢀH OA products were observed,
when stabilized by a donating substituent ortho to the RhꢀC
bond. However, in all cases, the CꢀHal OA (Hal = Cl, Br, I) was
thermodynamically preferred to the CꢀH OA.
Figure 3. Schematic representation of the three-centered transition
state for the concerted OA of a phenyl halide.
not allow us to discriminate with confidence between the values
obtained for ArꢀI and ArꢀBr. Nonetheless, the F values for
ArꢀBr and ArꢀI are close and meaningfully smaller than that for
ArꢀCl. The positive sign of F indicates a buildup of partial
negative charge on the aromatic ring in the transition state (TS)
and is generally consistent with a three-center transition state for
the concerted OA that lies on the continuum toward an SNAr-
type reaction.1 In this TS (Figure 3), the degree of rupture of the
ArꢀHal bond should correlate with an increased transfer of
electron density from the metal to ArꢀHal and thus increased
partial negative charge on the Ar unit. It seems reasonable to
propose that a more positive F value corresponds to a “later” TS.
From this perspective, since the cleavage of ArꢀBr and ArꢀI
bonds should be more facile, the higher positive value of F for
ArꢀCl makes sense.
’ EXPERIMENTAL SECTION
It is also instructive to compare the F values in this study to
the F values reported11ꢀ14 in the literature for the OA of aryl
halides to Pd(0) complexes. Literature F values for the OA of
aryl halides to Pd(0) were recently tabulated by Mayr et al. and
in 2004 by Dupont et al.11 The groups of Milstein in 199313
and Buchwald in 200814 determined the F values for the OA of
ArꢀCl to Pd complexes supported by strongly donating
trialkyl- or aryldialkylphosphines. The Milstein report13 erro-
neously used the natural logarithm of the rates for the
Hammett analysis (instead of the decadic logarithm), and their
reported value of F = 5.2 (vs the σꢀ set) should be treated as F
= 5.2/(ln 10) = 2.3 for purposes of comparison with other
Hammett studies. This matches the value obtained in the
Buchwald communication.14 The F value of þ2.3 for the
ArꢀCl OA in these Pd studies vs þ1.51(15) in our study
suggests that the TS for the OA of ArꢀCl is “earlier” for the
(PNP)Rh system than even for the most electron rich phos-
phine/Pd(0) systems. This may be taken to reflect the greater
facility of the concerted OA with the (PNP)Rh fragment. This
line of reasoning is supported by computational studies. Hall
et al.26 analyzed the OA of PhCl to a truncated (PNP)Rh
system and found the CꢀCl distance of 1.935 Å in the TS,
whereas Norrby et al.40 calculated much longer CꢀCl dis-
tances of 2.15 and or 2.17 Å in the TS for the OA of p-
CHOC6H4Cl and p-MeOC6H4Cl, respectively, to (tBu3P)Pd.
The CꢀCl bond length in free PhCl has been determined by
various experimental methods to be ca. 1.74 Å.41 The literature
F values for the ArꢀBr and ArꢀI OA to Pd(0) vary broadly,
from þ0.6 to þ2.5.11,12 In some of these cases, the nature of
the Pd(0) fragment undergoing the OA step is not at all clear.
With (Ph3P)4Pd as the Pd reagent, F values in the þ2.0 to þ2.5
range have been reported for the ArꢀBr and ArꢀI OA.11,12 It
may be expected that the F values for the ArꢀI and ArꢀBr OA
should be lower than for ArꢀCl, but Ph3P-supported Pd(0) is
much less electron rich (in fact, incapable of the ArꢀCl OA)
and we would predict much lower F values for the ArꢀBr and
ArꢀI OA with trialkyl- or aryldialkylphosphines.
General Considerations. Unless specified otherwise, all manip-
ulations were performed under an argon atmosphere using standard
Schlenk line or glovebox techniques. Toluene, ethyl ether, and pentane
were dried and deoxygenated (by purging) using a solvent purification
system17 by MBraun and stored over molecular sieves in an Ar-filled
glovebox. C6D6 and THF were dried over and distilled from NaK/
Ph2CO/18-crown-6 and stored over molecular sieves in an Ar-filled
glovebox. Fluorobenzene was dried with and then distilled from CaH2
and stored over molecular sieves in an Ar-filled glovebox. The liquid aryl
halides 4-bromoanisole, ethyl 4-bromobenzoate, 1-bromo-4-fluoroben-
zene, 1-bromo-4-chlorobenzene, 4-bromobenzotrifluoride, 4-bromoto-
luene, bromobenzene, 4-chlorobenzotrifluoride, 2,5-dichlorotoluene,
3-chlorotoluene, chlorobenzene, 4-chloroanisole, 1-chloro-4-fluoroben-
zene, 4-chlorotoluene, iodobenzene, 1-fluoro-4-iodobenzene, and ni-
trobenzene were degassed prior to use and stored in an Ar-filled
glovebox. The solid aryl halides 4-bromonitrobenzene, 4-bromoaniline,
1,4-dichlorobenzene, 4-chloroaniline, methyl 4-chlorobenzoate, 4-iodo-
nitrobenzene, methyl 4-iodobenzoate, 1-bromo-4-iodobenzene, 4-iodo-
toluene, 4-iodoanisole, 1-chloro-4-iodobenzene, 4-iodoaniline, and ethyl
benzoate were introduced under vacuum into and stored in an Ar-filled
glovebox as well. (PNP)Rh(SiPr2) (3), (PNP)Rh(Ph)(Cl) (11eCl),
(PNP)Rh(Ph)(Br) (11eBr), and (PNP)Rh(Ph)(I) (11eI) were synthe-
sized as previously described.6,7 Compounds 11fCl, 11hCl, and 11dBr
have been previously reported.6 All other chemicals were used as
received from commercial vendors. NMR spectra were recorded on a
Varian iNova 400 (1H NMR, 399.755 MHz; 13C NMR, 100.518 MHz;
31P NMR, 161.822 MHz; 19F NMR, 376.104 MHz) spectrometer. For
1H and 13C NMR spectra, the residual solvent peak was used as an
internal reference. 31P NMR spectra were referenced externally using
85% H3PO4 at 0 ppm. 19F NMR spectra were referenced externally using
1 M trifluoroacetic acid in CDCl3 at ꢀ78.5 ppm. Elemental analyses
were performed by CALI Laboratories, Parsippany, NJ.
(PNP)Rh(H2CdCHBut) (10). Under a steady flow of Ar, [RhCl3-
(H2O)] (2.51 g, 11.0 mmol) was transferred into a 100 mL Schlenk
flask. A mixture of ethanol (23.8 mL) and water (7.1 mL) was then
added to the flask. This was degassed for 10 min by bubbling Ar into the
solution through a pipet. Cyclooctadiene (4.15 mL, 33.7 mmol) was
added to the flask and similarly degassed for an additional 5 min. The
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solution was then allowed to reflux at 80 °C for 18 h. During this time, an
orange precipitate formed and the solution changed from dark red to
yellow. The volatiles were then removed in vacuo. In an Ar-filled
glovebox, the residue was redissolved in fluorobenzene (40 mL) and
PNP(H) (7; 4.82 g, 11.2 mmol) was added to the solution. This was
then stirred for 24 h, causing the solution to change from orange to dark
green. The volatiles were then removed in vacuo. Toluene (4 mL) was
added to the flask and immediately removed in vacuo; this was repeated
two more times to aid in the removal of free C8 hydrocarbons. The
residue was then redissolved in fluorobenzene (40 mL), and the solution
was transferred to a Teflon-stoppered gastight round-bottom flask.
Potassium tert-butoxide (1.61 g, 14.3 mmol) was dissolved in isoproptl
alcohol and degassed by bubbling Ar into the solution through a pipet for
10 min. This solution was then added to the flask under a steady flow of
Ar. The flask was briefly placed under vacuum. The flask was then filled
with 1 atm of H2 gas and placed in an 80 °C oil bath for 1 h. During this
time, the solution changed from dark green to dark red. The flask was
then refilled with 1 atm of H2 gas and placed back in the 80 °C oil bath,
and the mixture was stirred for 18 h. An aliquot of the solution was
analyzed by 31P{1H} NMR spectroscopy, revealing ca. 75% formation of
(PNP)RhH2 (9), 5% of (PNP)H (7), and 20% of a unknown byproduct.
The flask was placed back under 1 atm of H2 gas and the solution stirred
in a 100 °C oil bath for 48 h. An aliquot was taken again, but the ratio of
(PNP)Rh(H)(H) to byproduct did not change in the 31P{1H} NMR
spectra. The volatiles of the solution were then removed in vacuo, and
the residue was redissolved in fluorobenzene (40 mL). 3,3-Dimethyl-1-
butene (7.09 mL, 55 mmol) was then added to the flask and the mixture
stirred for 18 h at ambient temperature. An aliquot of the solution was
then analyzed through 31P{1H} NMR spectroscopy to confirm the
disappearance of 9 and the appearance of 10. The volatiles of the
solution were then removed in vacuo, and the residue was redissolved in
diethyl ether (30 mL). The solution was filtered through a layer of Celite.
The volatiles of the filtrate were removed in vacuo, and the residue was
redissolved in diethyl ether (30 mL). Silica gel was added to the solution,
and the mixture was stirred for 10 min and filtered through a layer of
Celite once more. The volatiles of the filtrate were removed in vacuo
once more. Pentane (20 mL) was then used to dissolve the residue;
however, the product was not entirely soluble. The flask was then placed
in a ꢀ35 °C freezer for 18 h. After this time, the solution was decanted
and the solids were washed with cold pentane and dried in vacuo to yield
3.78 g of orange powder. The decanted solution was combined with the
washings, and the volatiles were removed in vacuo. The residue was then
redissolved in pentane, placed in the freezer, and collected in the same
manner. Combined yield: 4.3 g (63%). NMR spectroscopic data were
identical with those previously reported.7 Anal. Calcd: C, 62.43; H, 8.51.
Found: C, 62.36; H, 8.62.
(20.1 μL, 195 μmol) and C6D6 (0.58 mL). The solution was mixed
and transferred to a J. Young NMR tube, where it was then placed in
a 50 °C oil bath. 31P{1H} NMR and 1H NMR spectroscopy analysis
revealed that 10 converted entirely to 16 after 24 h. The product was
1
characterized in situ through H and 31P{1H} NMR spectroscopy.
1H NMR (C6D6): δ 8.12 (d, 1H, J = 8 Hz, C6H4), 7.87 (d, 1H, J = 7
Hz, C6H4), 7.77 (overlap, 4H, C6H5 of nitrobenzene (2H) þ ArꢀH
of PNP (2H)), 6.88ꢀ6.30 (overlap, 4H, C6H5 of nitrobenzene (1H)
þ ArꢀH of PNP (2H) þ C6H4 (1H)), 6.79 (br s, 2H, ArꢀH of
PNP), 6.70 (overlap, 3H, C6H5 of nitrobenzene (2H) þ C6H4
(1H)), 5.80 (dd, 1H, J = 6 Hz, CH2dCH of 3,3-dimethyl-1-butene),
4.89 (dd, 2H, J = 14 Hz, CH2dCH of 3,3-dimethyl-1-butene), 2.19
(s, 6H, ArꢀCH3 of PNP), 1.99 (br m, 2H, CH(CH3)2), 1.82 (br m,
2H, CH(CH3)2), 1.04 (app quartet (dvt), 6H, JHꢀH = 7 Hz,
CH(CH3)2), 0.95 (s, 9H, C(CH3)3 of 3,3-dimethyl-1-butene),
0.88 (app quartet (dvt), 6H, JHꢀH = 8 Hz, CH(CH3)2), 0.70
(overlap, 12H, CH(CH3)2) þ CH(CH3)2), ꢀ18.0 (dt, 1H, JHꢀRh
34 Hz, JHꢀP = 12 Hz, RhꢀH). 31P{1H} NMR (C6D6): δ 56.1 (d, JPꢀRh
107 Hz, PPri2).
=
=
Observation of (PNP)Rh(H)(C6H4CO2Et) (18). Compound 3
(20 mg, 31 μmol) was dissolved in 0.80 mL of C6D6 and treated with
ethyl benzoate (13.3 μL, 92 μmol) in a J. Young NMR tube. The mixture
was placed into a 60 °C oil bath. After 7 h, the reaction mixture contained
a ca. 63:37 mixture of 18 and 3, as well as the corresponding amounts of
liberated SPri2 and excess ethyl benzoate. Further thermolysis at 60 °C
for another 55 h did not result in a noticeable change in the ratio of 18 to
3. Selected NMR spectroscopic data for 18 are as follows (some
resonances could not be reliably identified, owing to overlaps). 1H
NMR (C6D6): δ 7.89 (t, 1H, 6 Hz, ArꢀH of C6H4CO2Et), 7.83 (d, 1H,
8 Hz, ArꢀH of PNP), 6.92 (br s, 1H, ArꢀH of PNP), 6.84 (d, 1H, 8 Hz,
ArꢀH of PNP), 3.92 (q, 2H, 7 Hz, OCH2CH3), 2.23 (s, 6 H, ArꢀCH3 of
PNP), 2.09 (m, 2 H, CHMe2), 2.03 (m, 2 H, CHMe2), 0.97 (t, 3H, 7 Hz,
OCH2CH3), ꢀ19.53 (dt, JHꢀRh = 35 Hz, JHꢀP = 13 Hz). 31P{1H} NMR
(C6D6): δ 52.2 (d, JPꢀRh = 109 Hz).
’ ASSOCIATED CONTENT
S
Supporting Information. Text, tables, figures, and a
b
spreadsheet giving experimental details and mathematical calcu-
lations. This material is available free of charge via the Internet at
http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: ozerov@chem.tamu.edu.
Syntheses of 11xHal, 12, and 13. AHOAP Compounds 11xHal,
12, and 13 were synthesized through reactions of 3 equiv of the
corresponding aryl halides with either 3 or 10 in C6D6. The details for
each compound are described in the Supporting Information; they were
typically characterized in situ in the presence of excess aryl halide and of
either SPri2 or TBE.
Intermolecular Competition between PhBr and PhCl.
Complex 10 (20.0 mg, 32.5 μmol) was transferred to a glass vial,
followed by bromobenzene (10.3 μL, 97.5 μmol), chlorobenzene (9.3
μL, 97.5 μmol), and C6D6 (0.58 mL). The solution was mixed and
transferred to a J. Young NMR tube, which was then placed in a 75 °C oil
bath. 31P{1H} NMR analysis revealed that 10 converted to the two
distinct products 11eBr and 11eCl after 20 h, in a 96:4 ratio. During this
time, the solution changed from dark red to dark green. 11eBr was
identified in situ by 1H and 31P{1H} NMR spectroscopy, while 11eCl
was only clearly observable by 31P{1H} NMR spectroscopy.
’ ACKNOWLEDGMENT
Support of this research by the NSF (Nos. CHE-0517798,
CHE-0809522, CHE-0944634), the Welch Foundation (Grant
A-1717), the Alfred P. Sloan Foundation (Research Fellowship
to O.V.O.), and the Dreyfus Foundation (Camille Dreyfus
Teacher-Scholar Award to O.V.O.) is gratefully acknowledged.
We thank Yanjun Zhu for helpful discussions of some of the data
and assistance with ref 36. We thank Profs. Mark R. Biscoe and
Stephen L. Buchwald for allowing us access to some of their raw
data from ref 14.
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