An unusual equilibrium chlorine atom transfer process and its potential for assessment of steric pressure by bulky aryls

Article abstract of DOI:10.1021/ja028422r

An unusually facile intermolecular chlorine atom transfer process has been discovered between the phospha-Wittig reagents ArPP ₃ and ArPcl₂(Ar is sterically hindered aryl group). The atom transfer process is catalyzed by added PMe ₃.A mechanism is forwarded that is based upon a Me₃P/Me₃PCl₂ couple that allows shuttling of chlorine atoms between ArP groups. This unique transfer process is exploited to evaluate the steric properties of aryl groups via an equilibrium process. Copyright

Full text of DOI:10.1021/ja028422r

Published on Web 12/07/2002
An Unusual Equilibrium Chlorine Atom Transfer Process and Its Potential for
Assessment of Steric Pressure by Bulky Aryls
Rhett C. Smith, Shashin Shah, Eugenijus Urnezius, and John D. Protasiewicz*
Department of Chemistry, Case Western ReserVe UniVersity, 10900 Euclid AVenue, CleVeland, Ohio 44106-7078
Received September 5, 2002; E-mail: jdp5@po.cwru.edu
Chart 1
Sterically demanding aryls and alkyls have enjoyed great success
in a wide spectrum of applications. In particular, the bulky aryl
Mes* (2,4,6-tBu3C6H2) has been extensively employed since its
pioneering use for stabilizing the first diphosphene.1 The ortho-
tert-butyl groups of this aryl with concomitant close CH contacts
can lead to steric pressures and novel reactivity for directly attached
functional groups. For example, zirconium phosphanido complexes
bearing Mes* undergo surprisingly facile P-C bond cleavage.2 The
strain imposed by Mes* has also been identified as a thermodynamic
driving force promoting formation of iminopnictines and an
iodoselenide.3 More recently, there has been much interest in the
Table 1. Thermodynamic Data Obtained for a Series of Reaction
Partnersa
use of sterically encumbered meta-terphenyls (2,6-Ar2C6H3) for
accessing novel element-element multiple bonds, interesting
molecular geometries and coordination environments, and models
of biological systems.4 These aryls place a steric wall at a greater
distance from a reactive center and thus provide steric protection
comparable to the Mes* group without suffering reactions driven
by steric pressure. A fundamental question for these, and other
sterically demanding units, is the issue of quantifying and dissecting
the overall steric properties. While in some cases structural data
may allow qualitative comparisons,5 simple general methods are
lacking. Herein we present details of an unprecedented equilibrium
chlorine atom exchange reaction involving phosphanylidene-σ4-
phosphoranes6 ArPdPR3 bearing sterically encumbered aryls and
the use of this process as a systematic means to assess steric
pressure imposed by bulky aryls.
We have reported the synthesis and characterization of stable
phosphanylidene-σ4-phosphoranes such as Mes*PdPMe3 (1a) and
2,6-Mes2C6H3PdPMe3 (1b).7 These useful materials act as phospha-
Wittig reagents upon reaction with aldehydes and as precursors to
highly reactive phosphinidenes.8 During our studies, it was dis-
covered that these two materials undergo rapid chlorine atom
exchange with ArPCl2 in solution. Thus, mixing Mes*PCl2 (2a)
and 1b in C6D6 at room temperature results in rapid (<5 min)
equilibration to a mixture of 1b, 2a, 2,6-Mes2C6H3PCl2 (2b), and
∆H , eq3
rxn
Ar
Ar′
K , eq1
eq
Ar
Ar′
(kcal/mol)
2a
2a
2b
2d
1b
1d
1c
1c
g190
g140
1, 1b
2a
2a
2b
2d
5b
5d
5c
5c
-13.2
-9.7
0.2
20 (18)b
-3.4
a Numbers in parentheses determined by reaction of 2c and 1d. b Reaction
was catalyzed by added PMe3.
Mes2C6H3), in modest yields.9 Compound 3 does not readily
participate in chlorine atom transfer reactions.
This atom transfer process was then extended to include two
other commonly used terphenyls with varying steric properties
(Chart 1). These studies showed that the atom transfer process was
indeed general. The rates, however, varied greatly, as did the
position of equilibrium shown in eq 1. Rates for the exchange
process were too slow in some cases to compete with the slow
thermal decomposition of ArPdPMe3. Fortunately, our efforts to
delineate the mechanism of the atom transfer resulted in the
discovery that PMe3 acts as a catalyst for eq 1. For example,
addition of 2 equiv of PMe3 to a NMR tube of 1c and 2b results in
a dramatic reduction of the time for equilibration, from 16 h to 10
min (without impacting the final Keq). Thus, for slow reactions,
PMe3 was used to catalyze the atom transfer reaction. The
equilibrium constants obtained for a series of reaction partners are
listed in the left side of Table 1.
ArPdPMe3 Ar′PCl
+
2 h Ar′PdPMe3 + ArPCl2 (1)
1
2
Analysis of these data reveals several interesting facts. First, for
systems having a close steric similarity about the phosphorus centers
(b and c), the equilibrium constant Keq is one within experimental
error. For atoms shielded by terphenyl units, the immediate steric
pressure placed upon the attached atom by the aryl is largely defined
by the ortho-substituents on the outer aromatic rings. Second, the
Mes*P group appears to have the greatest desire to rid itself of
two chlorine atoms of any of the aryls studied.
One hypothesis to account for the equilibria portrayed by eq 1
is that the aromatic group that places the greatest steric pressure
on the directly attached phosphorus atom will show a greater
tendency to host fewer attached groups at this center. This important
point (thermodynamic) should not be confused with the more
1a, as indicated in eq 1. An estimate of Keq g 190 was established
using 1H NMR integration and an internal standard. Consistent with
this estimated equilibrium value, a solution prepared from pure 2b
and 1a showed little or no evidence of net exchange on the same
time scale.
While the reaction between 2a and 1b is rapid, the spectra show
no evidence for line broadening, indicating that the exchange
process is slow on the NMR time scale. Monitoring solutions on
much longer time scales (hours) leads to the observation of signals
for complicated mixtures of all three possible dichlorodiphosphines
of the form Ar(Cl)PP(Cl)Ar. The reaction of 1b and 2b over 1-2
h provided one of these materials, ArP(Cl)P(Cl)Ar (3, Ar ) 2,6-
9
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J. AM. CHEM. SOC. 2003, 125, 40-41
10.1021/ja028422r CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1. Proposed Mechanism for the PMe3-Promoted Chlorine
Atom Transfer Process
extreme insolubility of the independently prepared and isolated
reagent thwarted any catalytic behavior. The more soluble material
nBu3PCl2, however, is indeed an efficient catalyst.
The reactions we report here represent interesting main group
versions of related chlorine atom transfer reactions between
transition metal centers that have received some attention. In fact,
some of these systems exchange phosphine ligands and chlorine
atoms and thus suggest the potential catalytic role of phosphines
in other chlorine atom transfer processes.15 Extensions of the method
here to other sterically encumbered aryls and alkyls should allow
a rapid means (31P NMR spectroscopy) to determine the relative
impact of bulky groups at main group centers and to begin the
more challenging task of unraveling presumed electronic effects
that may be overshadowed by dominating steric forces.
commonly discussed ability of such groups to render so-called
kinetic stabilization to reactive multiply bonded units (for which
other factors are also important, such as thermodynamic destabiliza-
tion of cycloaddition products3c). Using this simple assumption,
one can estimate the relative penalties for accommodating a PCl2
functional group onto the series of hindered aryls by calculating
values of ∆Hrxn (eq 2 and Table 1, right) for related fictional H/PCl2
exchanges.10 A rough correlation exists, and, impressively, the Mes*
Acknowledgment. We acknowledge the National Science
Foundation (CHE-9733412 and CHE-0202040) for support.
ArPCl2
+
h ArH + Ar′PCl
(2)
Ar′H
5
2
2
Supporting Information Available: Experimental and computa-
tional details (PDF). This material is available free of charge via the
Internet at http://pubs.acs.org.
unit is maximally effective at destabilizing a three-coordinate
phosphorus center as compared to all other aryls used in this study,
even terphenyl d.
References
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rings in minimized structures of 2-ArC6H4PCl2.10
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to the mechanism proposed in Scheme 1. Consistent with the
mechanism is the fact that ArPdPMe3 can be prepared by action
of excess PMe3 upon ArPCl2, although the process is somewhat
inefficient without Zn dust to drive the equilibrium to the right (eq
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ArPCl2 + xs PMe3 h ArPdPMe3 + Me3PCl2
(3)
for dehalogenation of halophosphines.14 Traces of PMe3 in these
solutions might arise from the known slow thermal degradation of
ArPdPMe3. The catalytic efficacy of the relatively small PMe3
obviates the need for approach of two phosphorus centers bearing
hindered aryls. Scheme 1 predicts that reaction of 1c and 2b should
also be catalyzed by addition of Me3PCl2, but in this case the
JA028422R
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