Kinetic isotope effects in cycloreversion of rhenium (V) diolates

Article abstract of DOI:10.1021/ja017736w

Cycloreversion of 4-methoxystyrene from the corresponding Tp′Re(O)(diolato) complex (Tp′ = hydrido-tris-(3,5-dimethylpyrazolyl)borate) was measured competitively for various isotopomers at 103 °C. Primary (¹²C/¹³C) and secondary (¹H/²H) kinetic isotope effects were determined. The primary KIEs were k_12C/k_13C = 1.041 ± 0.005 at the α position and 1.013 ± 0.006 at the β position. Secondary KIEs were k_H/k_D = 1.076 ± 0.005 at the α position and 1.017 ± 0.005 at the β position. Computational modeling (B3LYP/LACVP*+) located a transition state for concerted cycloreversion of styrene from TpRe(O)(OCH₂-CHPh) exhibiting dramatically different C-O bond lengths. A Hammett study on cycloreversions of substituted styrenes from a series of Tp′Re(O)(diolato) showed dichotomous behavior for electron donors and electron-withdrawing groups as substituents: ρ = -0.65 for electron donors, but ρ = +1.13 for electron-withdrawing groups. The data are considered in light of various mechanistic proposals. While the extrusion of 4-methoxystyrene is concluded to be a highly asynchronous concerted reaction, the Hammett study reflects a likelihood that multiple reaction mechanisms are involved.

Full text of DOI:10.1021/ja017736w

Published on Web 03/21/2002
Kinetic Isotope Effects in Cycloreversion of Rhenium (V)
Diolates
Kevin P. Gable* and Fedor A. Zhuravlev
Contribution from the Department of Chemistry, Oregon State UniVersity,
CorVallis, Oregon 97331-4003
Received December 11, 2001
Abstract: Cycloreversion of 4-methoxystyrene from the corresponding Tp′Re(O)(diolato) complex (Tp′ )
hydrido-tris-(3,5-dimethylpyrazolyl)borate) was measured competitively for various isotopomers at 103 °C.
Primary (12C/13C) and secondary (1H/2H) kinetic isotope effects were determined. The primary KIEs were
k12C/k13C ) 1.041 ( 0.005 at the R position and 1.013 ( 0.006 at the â position. Secondary KIEs were
kH/kD ) 1.076 ( 0.005 at the R position and 1.017 ( 0.005 at the â position. Computational modeling
(B3LYP/LACVP*+) located a transition state for concerted cycloreversion of styrene from TpRe(O)(OCH2-
CHPh) exhibiting dramatically different C-O bond lengths. A Hammett study on cycloreversions of
substituted styrenes from a series of Tp′Re(O)(diolato) showed dichotomous behavior for electron donors
and electron-withdrawing groups as substituents: F ) -0.65 for electron donors, but F ) +1.13 for electron-
withdrawing groups. The data are considered in light of various mechanistic proposals. While the extrusion
of 4-methoxystyrene is concluded to be a highly asynchronous concerted reaction, the Hammett study
reflects a likelihood that multiple reaction mechanisms are involved.
Introduction
tion of osmylation and related reactions illustrates the chal-
lenges.3 For many years, kinetic and stereochemical investigation
Cycloaddition chemistry has from its discovery invited
mechanistic controversy over whether any particular reaction
is concerted or proceeds via an intermediate.1 The issue is
complicated by the fact that the primary probe for concertedness,
stereospecificity with regard to substituents, presumes that an
intermediate capable of single-bond rotation will undergo that
low-energy process (barriers usually <5 kcal/mol)2 faster than
the subsequent bond-forming reaction of the intermediate.
However, if this presumption is not valid, a reaction that is
actually stepwise will appear to be concerted. Likewise, if one
probes symmetry properties of the transition state, the structural
modifications needed to make the test may themselves create
aspects of asymmetry that would make a concerted reaction
appear as if the bond reorganization at two sites was proceeding
in an asynchronous fashion.
The general problem is compounded if transition metals are
involved. Classical organic cycloadditions are mechanistically
constrained in terms of the types of intermediates that can serve
as candidates for stepwise mechanistic proposals; in most
situations the question is limited to whether a process is
concerted or whether a biradical has properties that would mimic
concertedness. However, the ability of transition metals to serve
as templates for bond-forming processes opens a number of new
mechanistic possibilities. The history of mechanistic investiga-
was unable to distinguish two major proposals. Both a concerted
[3 + 2] process4 and a stepwise mechanism proceeding via a
metallaoxetane5 were capable of rationalizing reaction outcomes.
The advent of enantioselective asymmetric dihydroxylation6
raised the importance of the distinction in that very different
processes for molecular recognition and enantiotopic facial
discrimination were required by the two proposals. The issue
was apparently resolved in 1997 by the observation7 that
oxidation of tert-butylethylene showed, within experimental
error, identical primary and secondary kinetic isotope effects
(KIEs) at the two olefinic positions when high level calculations
predicted significantly different KIEs arising from the involve-
ment of a metallaoxetane.
Our work with rhenium diolate chemistry has attempted to
probe the issue from a different perspective. The general
approach has been to examine diolate cycloreversion and tie
the observations to C-O bond-forming processes via the
principle of microscopic reversibility. This assertion is supported
by the observation that cycloreversions are nearly thermoneutral,
(3) (a) Ponec, R. Top. Curr. Chem. 1995, 174, 1-26. (b) Williams, A. Chem.
Soc. ReV. 1994, 23, 93-100.
(4) (a) Criegee, R. Liebigs Ann Chem. 1936, 522, 75-98. (b) Criegee, R.;
Marchand, B. Wannowias, H. Liebigs Ann Chem. 1942, 550, 99-133. (c)
Corey, E. J.; Noe, M. C. J. Am. Chem. Soc. 1996, 118, 11038-11053.
(5) (a) Norrby, P.-O.; Becker, H.; Sharpless, K. B. J. Am. Chem. Soc. 1996,
118, 35-47. (b) Nelson, D. W.; Gypser, A.; Ho, P. T.; Kolb, H. C.; Kondo,
T.; Kwong, H.-L.; McGrath, D. V.; Rubin, A. E.; Norrby, P.-O.; Gable, K.
P.; Sharpless, K. B. J. Am. Chem. Soc. 1997, 119, 1840.
(6) Kolb, H. C.; Van Nieuwenzhe, M. S.; Sharpless, K. B. Chem. ReV. 1994,
94, 2483-2547.
* To whom correspondence should be addressed. E-mail: gablek@
chem.orst.edu.
(1) Woodward, R. B.; Hoffmann, R. The ConserVation of Orbital Symmetry;
Verlag Chemie: Weinstein, 1970.
(2) (a) Allinger, N. L.; Grev, R. S.; Yates, B. F.; Schaefer, H. F., III. J. Am.
Chem. Soc. 1990, 112, 114-118. (b) Herrebout, W. A.; van der Veken, B.
J.; Wang, A.; Durig, J. R. J. Phys. Chem. 1995, 99, 578-585.
(7) DelMonte, A. J.; Haller, J.; Houk, K. N.; Sharpless, K. B.; Singleton, D.
A.; Strassner, T.; Thomas, A. A. J. Am. Chem. Soc. 1997, 119, 9907-
9908.
9
3970
J. AM. CHEM. SOC. 2002, 124, 3970-3979
10.1021/ja017736w CCC: $22.00 © 2002 American Chemical Society

KIEs in Cycloreversion of Rhenium Diolates
A R T I C L E S
Scheme 1
cycloalkene or at the very least isomerize to the cis-cycloal-
kanediolate, which would then fragment. Similar arguments
discount the ionic mechanism, as does the absence of any
significant solvent effect and the relatively small Hammett F
value observed for extrusion of styrenes from Cp*Re(O)(diolato)
complexes (Cp* ) η-pentamethylcyclopentadienyl).11
The remaining possibilities are either the concerted mecha-
nism, or two stepwise processes involving migration of carbon.
One of the latter would form a metallaoxetane by migration to
the metal, or a coordinated epoxide12 via a migration to the
second diolate oxygen. Either would be a frontside migration
of carbon with retention of stereochemistry and would conclude
with a rapid concerted loss of alkene from the intermediate after
the initial rate-determining step.
Experiments to date have suggested that cleavage of only
one of the two C-O bonds occurs to any extent in the transition
state. The effect of strain in the CdC bond is expected to have
a significant impact on relative transition-state energies for either
cycloaddition or cycloreversion, due to the rehybridization of
the reacting carbon. However, this effect was observed only
for cycloaddition; furthermore, the quantitative impact on ∆Hq
implied a complete relief of strain at the cycloaddition transition
state. The invariance of ∆Hq for cycloreversion of strained and
unstrained disubstituted alkenes agrees that strain is not evident
by the cycloreversion transition state. Both observations are
consistent with a transition-state structure that lacks sp2
character. That at least one C-O bond is cleaved at or before
the transition state is confirmed by the observation of a
significant secondary deuterium KIE on extrusion of ethylene;
kH/kD4 ) 1.25 at 100 °C. This value by itself could be
rationalized with either the concerted (KIE per D ) 1.06) or
stepwise (KIE per D ) 1.13) mechanisms; efforts to perform a
Thornton analysis13 by measuring the KIE for extrusion of
ethylene-d2 were stymied by analytical complications arising
from fragmentation during mass spectrometry.14 However, the
combination of the strain effects with the KIE strongly suggested
a highly asynchronous and probably stepwise process in which
reorganization to an intermediate preceded development of the
new CdC π bond.
driven by entropy, and that incorporation of some strain into
the alkene CdC bond can favor cycloaddition to LReO3 in the
same fashion as alkenes add to OsO4.8 A series of mechanisms
considered for these reactions is seen in Scheme 1.
The high degree of stereospecificity9 (to the limit of detection,
taken as >95% by NMR) suggests that the first two of these,
homolytic and heterolytic cleavage of a C-O bond, are unlikely.
However, the standard restriction applies in that if either of these
intermediates were to cleave the second C-O bond faster than
bond rotation occurs, stereospecificity would be observed.
However, several further arguments mitigate against these
proposals. First, substituents such as a phenyl group would be
expected to stabilize the intermediate and lengthen its lifetime
sufficiently for bond rotation to occur; this is not seen. Second,
diolates of trans-cycloalkanediols are thermally stable, while
the isomeric cis-cycloalkanediolates fragment.10,26 Were a radical
to be produced, it would invert readily and collapse to the cis-
Further experiments agreed with this conclusion. Conforma-
tional analysis of a series of diolates suggested that a staggered
form enhanced the reaction rate, whereas a concerted reaction
presumably would be enhanced by an eclipsed geometry that
would allow proper orbital overlap in the incipient CdC π bond.
Such an effect is evidence that orientation of a C-O bond with
an orbital on rhenium is important. While a Hammett study on
styrene extrusion was uninformative, the curved behavior seen
for stilbene extrusion again suggested a competition between
(asynchronous) cleavage of the two different C-O bonds.
However, in the intervening years, application of high-level
computational methods to our system has strongly and consis-
tently suggested that the concerted process should be energeti-
cally favored over formation or fragmentation of a metalla-
(8) Gable, K. P.; Phan, T. N.; J. Am. Chem. Soc. 1993, 115, 3036-3037.
(9) Gable, K. P.; Juliette, J. J. J. J. Am. Chem. Soc. 1995, 117, 955-962.
(10) Gable, K. P.; Phan, T. N. J. Am. Chem. Soc. 1994, 116, 833-839.
(11) Gable, K. P.; Juliette, J. J. J. J. Am. Chem. Soc. 1996, 118, 2625-2633.
(12) Zhu, Z.; Al-Ajlouni, A. M.; Espenson, J. H. Inorg. Chem. 1996, 35, 1408-
1409.
(13) Thornton, E. R.; Taagepara, M. J. Am. Chem. Soc. 1972, 94, 1168-1177.
(14) Phan, T. N. M.S. Thesis, Oregon State University, 1994.
(15) (a) Deubel, D. V.; Frenking, G. J. Am. Chem. Soc. 1999, 121, 2021-2031.
(b) Gisdakis, P.; Ro¨sch, N. J. Am. Chem. Soc. 2001, 123, 697-701. (c)
Ko¨stlmeier, S.; Ha¨berlen, O. D.; Ro¨sch, N.; Herrmann, W. A.; Solouki,
B.; Bock, H. Organometallics 1996, 15, 1872-1878. (d) Pietsch, M. A.;
Russo, T. V.; Murphy, R. B.; Martin, R. L.; Rappe´, A. K. Organometallics
1998, 17, 2716-2719.
(16) (a) Torrent, M.; Deng, L.; Duran, M.; Sola, M.; Ziegler, T. Organometallics
1997, 16, 13-19. (b) Pidun, U.; Frenking, G. Angew. Chem., Int. Ed. Engl.
1996, 35, 2817-2820. (c) Dapprich, S.; Ujaque, G.; Maseras, F.; Lledos,
A.; Musaev, D. G.; Morokuma, K. J. Am. Chem. Soc. 1996, 118, 11660-
11661.
(17) Singleton D. A.; Thomas, A. A. J. Am. Chem. Soc. 1995, 117, 9357-
9358.
(18) Gable, K. P.; AbuBaker, A.; Zientara, K.; Wainwright, A. M. Organome-
tallics 1999, 18, 173-179.
(21) Corey, E. J.; Noe, M. C.; Grogan, M. J. Tetrahedron Lett. 1996, 4899-
4902.
(19) (a) Curtis, M. D.; Shiu, K.-B.; Butler, W. M. J. Am. Chem. Soc. 1986,
108, 1550-1561. (b) Skagestad, V.; Tilset, M. J. Am. Chem. Soc. 1993,
115, 5077-5083. (c) Etienne, M.; White, P. S.; Templeton, J. L.
Organometallics 1993, 12, 4010-4015. (d) Janiak, C. Chem. Ber. 1994,
127, 1379-1385.
(22) Schlosser, M.; Christmann, K. F. Ger. Pat. DE 1279678, 1968; Chem. Abstr.
1969, 70, 37429
(23) Seebach, D.; Ha¨ssig, R.; Gabriel, J. HelV. Chim. Acta 1983, 66, 308-337.
(24) Yoshino, T.; Manabe, Y.; Kikuchi, Y. J. Am. Chem. Soc. 1964, 86, 4670-
4673.
(25) Melander, L.; Saunders, W. H. Reaction Rates of Isotopic Molecules;
Wiley: New York, 1980; pp 95-102.
(26) Ness, S. L. M.S. Thesis, Oregon State University, 1999.
(20) (a) Gajewski, J. J.; Peterson, K. B.; Kagel, J. R. J. Am. Chem. Soc. 1987,
109, 5545-5546. (b) Houk, K. N.; Loncharich, R. J.; Blake, J. F.; Jorgensen,
W. L. J. Am. Chem. Soc. 1989, 111, 9172-9176. (c) Birney, D. M.; Houk,
K. N. J. Am. Chem. Soc. 1990, 112, 4127-4133.
9
J. AM. CHEM. SOC. VOL. 124, NO. 15, 2002 3971

A R T I C L E S
Gable and Zhuravlev
oxetane.15 The methods and conclusions parallel a large amount
of similar work in osmylation.16 Thus, it became necessary to
reconsider the earlier conclusions and revisit the issue. The value
of measuring KIEs lies in presenting a rigorous picture of
symmetry in the transition state and rigorously testing whether
one or both reaction sites undergoes bonding reorganization.
The work on osmylation reflected the advantage of considering
both primary (12C/13C) and secondary (H/D) effects in creating
a picture of the transition state structure. This report describes
measurement of these kinetic isotope effects for a representative
rhenium diolate.
imposes the potential to bias even a concerted transition state
toward an asynchronous structure. Given that extremely sig-
nificant electronic perturbation-cyano group(s)-is required to
induce substantial asynchronicity in Diels-Alder cycloaddi-
tions,20 the synthetic advantages of using R ) 4-methoxyphenyl
outweighed the risks. Further, a similar experiment has been
reported for osmylation,21 and while asymmetry was noted,
substantial primary KIEs were observed at both carbons. While
not a primary design consideration with regard to the KIE study,
use of a phenyl substituent does allow more systematic
exploration of electronic effects than would any other aliphatic
substituent.
Results
Synthesis of labeled diolates. Syntheses of required diolate
complexes is shown in Schemes 2 and 3. Each experiment
required diols labeled at each of the reacting carbons; incorpora-
tion of label at a reference position was performed to provide
a marker for material unlabeled at the reacting carbons. All of
these syntheses were straightforward. The 13C-labeled materials
were generally produced via a Wittig reaction22 to methylenate
4-methoxybenzaldehyde followed by osmylation. The R-labeled
compound involved producing R-13C-4-methoxybenzaldehyde
Experimental Design The experiment involved examining
a known mixture of isotopomers, reacted to a known degree of
conversion. Unreacted diolate was reisolated and analyzed for
the increase in the slower-reacting isotopomer. We felt NMR
provided an adequate means of measuring isotopic distribution,
from which the KIEs would be determined. This methods avoids
complications in mass spectrometric analysis due to fragmenta-
tion of molecular ions that is commonly seen in these diolates.18
The Singleton methodology17 normally relies on using natural
abundance isotopomers as the substrates for investigation, and
incorporates several controls to correct for potential systematic
errors in peak integration. This has the advantage of eliminating
the need for synthesis of isotopically enriched material, but
imposes a significant cost in instrumental time because of low
signal-to-noise ratios, particularly for deuterium. Use of enriched
material would ease analysis while maintaining the advantages
of NMR as a nondestructive means of measuring isotopic
distribution.
We were faced with two fundamental choices in designing
the substrate for this experiment. Most of the mechanistic
investigations on rhenium diolate cycloreversion involved use
of Cp* as an ancillary ligand on rhenium. The pyrazolylborate
complexes were easier to prepare in high yield, and were
marginally easier to handle due to enhanced air-stability. Earlier
work18,19 had shown strong similarities between these systems
and those using Tp′. As a control, a Hammett study was
performed (see below).
from a Grignard reagent and 13CO2 followed by reduction.
23
The â-labeled styrene was formed from labeled Wittig reagent
prepared from 13CH3I. Placing the reference label in the methoxy
group was problematic because of the lability of 4-vinylphe-
noxide; generation from 4-acetoxystyrene followed by immedi-
ate in-situ conversion to the thallium salt gave a much more
tractable species which was easily alkylated. On the basis of
1H NMR spectra, each diol was found to be >98 atom % 13C.
Synthesis of the deuterated compounds was likewise straight-
forward. The reference isotopomer required mere substitution
of deuteriomethyl iodide. The alpha label was incorporated by
LiAlD4 reduction of methoxybenzoyl chloride; reoxidation
allowed isolation of the deuterated aldehyde. The â label was
most easily incorporated via metalation and D2O quench of the
â-bromo-4-methoxystyrene.24 On the basis of 1H NMR spectra,
each diol was found to be >98.5 atom % D.
Rhenium diolate complexes were prepared from the diols via
the published procedure involving in situ reduction of Tp′ReO3
with PPh3 and cyclocondensation with the diol.18 Two isomers
were formed in a 10:1 ratio and were assigned respectively as
the anti- and syn-diastereomers at the benzylic carbon. (Syn or
anti is defined on the basis of the orientation of the aromatic
ring versus the pyrazole trans to the terminal oxo ligand.) This
was confirmed by the observation of nOe between the methyl
signal at 2.47 ppm and the diolate signals at 6.23 and 5.65 ppm.
The minor isomer, on the other hand, showed only one nOe
between a diolate signal at 5.42 and the pyrazolyl methyl at
2.23 ppm; enhancement was also observed for the ortho aromatic
protons at 7.63 ppm. DEPT of the major isomer confirmed the
identity of R and â carbons at 95.9 and 94.5 ppm, respectively;
a 2-D HSQC spectrum confirmed assignment of the proton
signals. See Supporting Information for details.
Kinetic Analysis Evaluation of the KIEs via competition
requires as one component an accurate estimation of the extent
of reaction for the parent (unlabeled) molecule. Thus, extrusion
of 4-methoxystyrene from the corresponding diolate in toluene-
d8 was followed at several temperatures to establish the
activation parameters. Like the extrusion of styrene, this was
The second choice was more critical. Ideally, an alkyl
substituent would provide less bias toward any asymmetry,
however, synthesis of an appropriately labeled reference com-
pound for R ) tert-butyl is significantly involved. Use of a
substituted phenyl group provides an easier synthesis, but
1
found to be cleanly first-order to >4 half-lives by H NMR.
9
3972 J. AM. CHEM. SOC. VOL. 124, NO. 15, 2002

KIEs in Cycloreversion of Rhenium Diolates
A R T I C L E S
Scheme 2
Scheme 3
Between 100 and 120 °C, ∆Hq ) 29.1 ( 0.4 kcal/mol and
∆Sq ) 0.46 ( 1.03 cal/(mol K) were found. The Eyring plot is
shown in Figure 1.
The minor diastereomer was evaluated kinetically at 120 °C;
its rate of cycloreversion was only one-fourth that of the anti
isomer (2.15 × 10-4 s-1 for the syn vs 8.00 × 10-4 s-1 for the
anti).
Given the change in ancillary ligand from the earlier work,
an abbreviated Hammett study was performed with five different
substituted phenylethanediolates. The result is shown in Figure
2; two different slopes were observed depending on the
electronic nature of the substituent. Electron donors show F )
-0.65, but F ) +1.13 for electron-withdrawing groups. Plotting
against σ+ yields a similar relationship, but the small number
of data points precludes any quantitative comparison.
Figure 1. Eyring plot for cycloreversion of 4-methoxystyrene. ∆Hq
29.1 ( 0.4 kcal/mol; ∆Sq ) 0.46 ( 1.03 cal/mol K; r2 ) 0.999.
)
9
J. AM. CHEM. SOC. VOL. 124, NO. 15, 2002 3973

A R T I C L E S
Gable and Zhuravlev
Figure 2. Hammett plot for cycloreversion of substituted phenylethane-
diolates.
Measurement of Isotope Effects. All experiments involved
estimating the ratios of isotopic content at each of the reacting
sites (R and â) relative to that at a nonreacting reference position
(the methoxy group); the latter would serve as a marker for the
amount of compound “unlabeled” (12C2 or 1H3) at the reacting
sites. A spectrum of the initial mixture was collected; after
reaction for a known amount of time (and thus a known extent
of reaction) the diolate was isolated from the reaction mixture
and its spectrum recorded again. The isotope effects were all
calculated from these ratios according to the equation25
Figure 3. 13C NMR lines for diolate carbons (95.9; ν1/2 ) 1.35 Hz; 94.5;
ν1/2 ) 4.38 Hz). Inset: methoxy carbon line on a similar scale; ν1/2
)
0.34 Hz.
Table 1. Primary 12C/13C Kinetic Isotope Effects on Extrusion of
4-Methoxystyrene
run
F
R/R (R)
0
R/R (â)
0
KIE, R
KIE, â
1
2
3
0.905
0.836
0.903
1.083
1.083
1.100
1.020
1.023
1.040
1.035
1.046
1.043
1.041
1.008
1.013
1.017
1.013
average
ln(1 - F)
KIE )
(1)
ln((1 - F)R/R0)
Table 2. Secondary 1H/2H Kinetic Isotope Effects on Extrusion of
4-Methoxystyrene
(F ) extent of reaction; R ) isotopic ratio vs reference at extent
of reaction F; R0 ) initial isotopic ratio vs reference).
Uncertainty in the KIE was derived by propagation of error
from that in the ratios and extent of reaction, expressed as 95%
confidence intervals.
run
F
R/R (R)
0
R/R (â)
0
KIE, R
KIE, â
4
5
6
0.888
0.812
0.896
1.160
1.122
1.178
1.036
1.012
1.060
1.073
1.074
1.080
1.076
1.016
1.007
1.027
1.017
average
To measure primary 12C/13C KIEs, integration of 13C spectra
was performed in accord with conditions laid out by Singleton.17
The spectral window was restricted to the signals of interest;
the use of enriched material ameliorated problems arising from
foldover of other signals into this narrow window. Inverse gated
decoupling was used; a relaxation delay of 5 times the longest
13C T1 value was used. The spectra were zero-filled to 128 K;
the narrowest peak had a minimum of 10 data points. Enough
transients were collected to give a minimum signal-to-noise ratio
of 100:1 for the broadest peak when transformed with no line
broadening correction. Care was taken to provide correct phase
correction and consistent baseline correction.
line-widths, and in this case the signals are close enough to
make correct integration impossible. Further, it was impossible
to prevent small amounts of the minor diastereomer from
contaminating some samples; while the diolate signals were
fairly well separated, the reference methoxy carbons were so
close as to be inseparable for the purpose of integration. Recall
that the minor diastereomer reacted more slowly, thus we
experienced the buildup of this impurity as the reaction
proceeded. Since slightly different conversions were used in
each experiment, this provided another variable that was
impossible to factor out. In retrospect, we were fortunate to have
prepared isotopically enriched compounds, as the standard
integration of 13C signals beginning with the natural abundance
distribution would have failed completely.
We investigated several means of line fitting to provide better
estimates of isotopic composition. However, all gave the same
degree of scatter as the numerical integrations, and we concluded
that the quadrupolar interactions were creating enough signifi-
cant difficulties with the raw data that use of the 13C spectra
was inadvisable. Fortunately, the 1H spectrum provided an
alternative, in that the 13C satellites of the enriched isotopomers
provided an indirect but potentially more accurate estimate of
Despite these precautions, multiple runs gave significant
scatter in the results. Several samples appeared to show larger
KIEs at the â position, and some even showed an inverse KIE
at one or both positions. Several aspects of our system are likely
origins of the problem. First, the diolate signals are both
significantly broadened compared to other signals in the
spectrum (see Figure 3). We have observed this behavior in
other rhenium diolates.26 Although conformational flexibility
of the diolate ring is one potential cause, the failure to observe
any sharpening in the line either on heating or cooling the
sample suggests that the nuclear quadrupole of rhenium (for
2
185Re, this is 2.18 and for 187Re, 2.07 barn; compare H at
0.00286 barn)27 is a more likely cause. In any event, all lines
need to be integrated over a region of at least 10 half-height
(27) Lide, D. E., Ed. CRC Handbook of Chemistry and Physics,78th ed.; CRC
Press: New York, 1997; Chapter 9, pp 85-87.
9
3974 J. AM. CHEM. SOC. VOL. 124, NO. 15, 2002

KIEs in Cycloreversion of Rhenium Diolates
A R T I C L E S
Table 3. Energies for Optimized Structures at the B3LYP/LACVP*+ Level
q
quantity
styrene
TpReO
TpRe(O)(OCHPhCH O)
transition state
∆Erxn (kcal/mol)
+15.5
∆E (kcal/mol)
3
2
E, Hartree
ZPE, kcal/mol
ZPE corrected ∆E
-309.6605
-1007.2715
-1316.9567
-1316.8998
35.7
25.8
89.79
149.58
243.05
233.18
+11.8
Table 4. Structural data for calculated structures
(B3LYP/LACVP*+)
the isotopic composition at each position. Note that this method
inadvertently overdetermines these measurements by permitting
independent comparison of the isotopic composition at each
position with the ratio of R-13C or â-13C to methoxy-13C. Results
for each run are presented in Table 1; according to eq 1 the
KIEs are k12C/k13C ) 1.041 ( 0.005 at the R position, and
1.013 ( 0.006 at the â position.
TpReO
TpRe(O)(OCH CHPh)
styrene
transition state
3
2
RedO
Re-O
Re-N
CH2-O
CHPh-O
C-C
1.719
2.300
1.694
1.703
1.955, 1.938
2.335, 2.170, 2.127
1.434
1.423
1.537
1.794, 1.768
2.313, 2.271, 2.240
1.880
2.112
1.342 1.403
Estimation of the deuterium content by direct NMR measure-
ment was similarly complicated by the width of the signals and
the corresponding peak overlap and loss of adequate signal-to-
noise ratio. Again, though, the relatively well-resolved 1H
spectrum contained the necessary information. Each of the three
frequency of the main rhenium oxo stretch (scaled by 0.9614,
based on the 6-31G* correction)30 was computed to be 909
cm-1, compared to the observed value of 913 cm-1 for TpReO3
and 907 for Tp′ReO3. The diolate converged on a structure that
shared several features with Santos’ structures of TkRe(O)-
(OMe)2 and TkRe(O)(OCH2CH2O) (Tk ) tetrakis-pyrazolyl-
borate):31 the rhenium-oxo bond length was 1.694 Å; Re-N
bonds cis to the oxo were shorter (2.173 and 2.131 Å) than
that trans to the oxo ligand (2.341 Å). The computational
methodology is sufficient to replicate the gross structure of the
coordination sphere of rhenium. C-O bonds are similar (1.434,
1.423 Å); thus, the phenyl group perturbs neither the ground-
state structure of the diolate ring nor any other aspect of
coordination sphere of rhenium to any significant extent. Again,
the scaled vibration for the terminal Re-O stretch was computed
to be 931 cm-1, compared to an actual value of 922 cm-1 in
Tp′Re(O)(OCHPhCH2O).18
The reaction energetics were in agreement with our earlier
qualitative observations. ∆E based on electronic energies was
calculated to be +15.5 kcal/mol for styrene cycloreversion;
application of a correction for zero-point energy differences
decreases this to +11.8 kcal/mol. Note that these quantities all
assume gas-phase reaction; in solution we observe cyclorever-
sion for unstrained alkenes, but preferred cycloaddition of
strained alkenes (e.g., norbornene) to Tp′ReO3. The experimental
observations were previously interpreted to suggest a ∆Hrxn near
zero,8,18 and no more positive than about +5 kcal/mol.
Comparison between theory and experiment are further clouded
by the fact that Tp′ReO3 is poorly soluble in most organic
solvents, complicating the thermochemistry due to its unknown
heat of crystallization.
1
signals in the H spectrum is composed of contributions from
different isotopomers. Solution of appropriate simultaneous
equations for the contribution to different signals allowed the
isotopomeric composition to be calculated. (Correction for the
small amount of signal arising from less than 100 atom %
labeling showed no discernible impact.) Results are shown in
Table 2; from these data secondary KIEs were kH/kD
)
1.076 ( 0.005 at the R position, and 1.017 ( 0.005 at the â
position.
Computational Modeling. Quantum mechanical modeling
of the reaction was undertaken to discover whether a transition
state could be located and if so, whether its properties predicted
KIEs in accord with the experimental observations. The com-
mercial package Jaguar was used.28 The hybrid DFT method
B3LYP was chosen on the basis of its previous success in
modeling related rhenium compounds. The LACVP*+ basis
set was selected on the basis of the following considerations.
This uses a fairly standard 6-31G basis set for all first- and
second-row heavy atoms and hydrogen. An effective core
potential is used for rhenium that implements corrections for
relativistic effects; again, this has been shown to be successful
for the third row transition elements. Use of d-type polarization
functions on heavy atoms has been shown to be critical to
achieving good energies in related systems,15c but since the bond
reorganization does not significantly involve hydrogens, p-type
polarization functions were omitted from hydrogens. Finally,
because the modeling of a transition state involves partial
bonding, diffuse orbital functions were added to the heavy
atoms. Two structural simplifications were implemented to
minimize computational cost: an unsubstituted phenyl ring was
used, and the ancillary ligand was abbreviated to the parent Tp
rather than the methylated Tp′.
A transition state search resulted in the location of structure
c in Figure 4. This converged to a proper saddle point with one
imaginary frequency. The transition state energy, +35.7 kcal/
mol relative to the reactant phenylethanediolate (+25.7 kcal/
mol after factoring in ZPE differences), is in reasonable accord
with the observed ∆Hq of 27.6 kcal/mol for the Tp′ complex.
Substantial asymmetry in the cleavage of the C-O bonds is
observed; the difference in bond lengths is almost 0.25 Å. The
To benchmark the system, calculations were performed on
the overall reaction thermodynamics. Both TpReO3 and the
corresponding phenylethanediolate converged to reasonable
structures that were stationary points based on vibrational
analysis, with no imaginary frequencies. Energies and relevant
metrics are in Tables 3 and 4. For the trioxide, RedO bonds in
the optimized structure were 1.719 Å, compared to an experi-
mental value of 1.707-1.720 Å in TpReO3.29 The vibrational
(29) Degnan, I. A.; Herrmann, W. A.; Herdtweck, E. Chem. Ber. 1990, 123,
1347-1349.
(30) Scott, A. P.; Radom, L. J. Phys. Chem. 1996, 100, 16502-16513.
(31) (a) Paulo, A.; Domingos, A.; Pires de Matos, A.; Santos, I.; Carvalho, M.
F. N.; Pombeiro, A. J. L. Inorg Chem. 1994, 33, 4729-4737. (b) Nunes,
D.; Domingos, A.; Paulo, A.; Patricio, L.; Santos, I.; Carvalho, M. F. N.;
Pombeiro, A. J. L. Inorg. Chim. Acta 1998, 271, 65-74.
(28) Jaguar 4.1, Schro¨dinger, Inc., Portland, OR, 1991-2000.
9
J. AM. CHEM. SOC. VOL. 124, NO. 15, 2002 3975

A R T I C L E S
Gable and Zhuravlev
Figure 4. Optimized structures at the B3LYP/LACVP*+ level.
C-C bond is substantially shortened to 1.403 Å (from 1.537 Å
in the phenylethanediolate). The â diolate carbon is still
somewhat pyramidalized (sum of bond angles ) 351°), while
the R carbon appears to have substantially rehybridized (sum
of bond angles ) 358°).
Analysis of charge reorganization on going to the transition
state is revealing. A diagram showing atomic charges derived
from electrostatic potentials32 is seen in Figure 5. While the
rhenium unsurprisingly gains some positive character, the
oxygens show remarkably little change in charge density. The
carbons of the alkene, however, adopt substantial polarization;
the R carbon gains substantial positive charge, while the â
carbon becomes significantly more negative.
Quantitative estimation of kinetic isotope effects from
vibrational analysis has become common; however, in most
cases the simplest congener of a reaction type is used to
minimize the impact of inaccuracies in the specific frequencies
used. In this case the size of the molecule and in particular the
number of normal modes that were unrelated to the reaction
coordinate made such quantitative estimation unwieldy and of
dubious value. However, a qualitative estimation based on our
transition state structure is in all facets consistent with observed
KIEs; the bond reorganization is more extensive at the R
Figure 5. Atomic charges derived from electrostatic potentials for the
ground-state diolate (top) and the transition-state (bottom); changes are in
parentheses.
position, predicting substantially larger primary and secondary
KIEs at that position. Perhaps of some surprise is that the
experimental KIEs at the â position are as small as they are
given the still substantial bond cleavage to that site and the
degree of C-C bond contraction.
Discussion
Primary and secondary kinetic isotope effects for production
of 4-methoxystyrene clearly and consistently show asymmetry
in the transition state. However, the observation of KIEs that
(32) (a) Chirlian, L. E.; Francl, M. M. J. Comput. Chem. 1987, 8, 894-905.
(b) Woods, R. J.; Khalil, M.; Pell, W.; Moffat, S. H.; Smith, V. H., Jr. J.
Comput. Chem. 1990, 11, 297-310. (c) Breneman, C. M.; Wiberg, K. B.
J. Comput. Chem. 1990, 11, 361-373.
9
3976 J. AM. CHEM. SOC. VOL. 124, NO. 15, 2002

KIEs in Cycloreversion of Rhenium Diolates
A R T I C L E S
are significantly greater than unity (even if not by much) at the
â position demonstrate some degree of bond cleavage at that
site. The mechanistic conclusions must be drawn from a
consideration of different types of reaction processes.
two different compensatory electronic effects. Initially, we
envisioned this as a “push” from electron donors making the
benzylic oxygen more nucleophilic in a migration of the
â-carbon to form a coordinated epoxide, and a “pull” by
electron-deficient groups making the benzylic carbon more
electrophilic for an R-carbon migration. However, this model
would demand large â KIEs and small R KIEs and therefore
cannot be operating for extrusion of 4-methoxystyrene. Notably,
the KIEs reported here apply only to the substrate with a fairly
electron-donating methoxy group; the consequences for less
electron-rich styrenes will be discussed below.
Stepwise Migrations. From the perspective of KIEs, the
migratory mechanisms to form either a coordinated epoxide or
a metallaoxetane are very similar. One position should show
both large primary and secondary effects; the other should show
a much smaller effect which ought to be very close to unity for
a k12C/k13C. The secondary effect at the â position may be
nonnegligible, depending on the specific changes in zero-point
energies that accompany bond reorganization, but in general
one expects negligible changes for a site that undergoes no
change in bonding by the transition state. It is reasonable to
assume that because the electronic effect of phenyl substitution
is to increase reaction rate, the substituted R carbon will migrate
in the rate-limiting step of either mechanism and show the large
KIEs.
Observation of a significant primary KIE at the â carbon is
inconsistent with the expectations of these two mechanisms. It
is possible that competition between migration of the R and â
carbons could lead to observation of primary KIEs at both sites,
but a quantitative model makes this all but impossible. Consider
that the rate ratio for styrene vs ethylene extrusion at 100 °C is
61.4;18 this should be to a first approximation the rate differential
for migration of substituted and unsubstituted positions. The
expressions for the unlabeled and labeled rate constants are:
One important aspect of the new observations lies in the
magnitude of the secondary KIE. Recall from above that
cycloreversion of Cp*Re(O)(OCD2CD2O) occurred with a d4-
KIE of 1.25, which could be interpreted as either a per-F KIE
of 1.13 on migration of a single carbon, or 1.06 on concerted
fragmentation of the diolate. The magnitude of the R effect from
the current study, 1.076, is not large enough to be consistent
with the demands of the earlier experiment for a stepwise
migration, but fall in line with expectations from a concerted
model. (Of course, this assumes mechanistic congruence
between compounds with different ancillary ligands.)
Concerted Cycloreversion. The most straightforward ex-
planation of the results is that the extrusion of 4-methoxystyrene
is concerted, but that there are different degrees of bond cleavage
for the two C-O bonds at the transition state. The consistency
of the primary and secondary KIEs argues strongly for this
viewpoint. Location of a computationally modeled transition
state structure that agrees with this proposal further supports
the hypothesis, particularly insofar as the predicted activation
barrier and reaction energy are consistent with observation.
unlabeled:
k
obs(F) ) kbenzyl + kCH2 ) 62.4 kCH2
R-labeled:
However, the Hammett study provides curious additional
information regarding this possibility. In a concerted process,
the Hammond postulate leads to the prediction that there should
be a relationship between ∆Gq and ∆G° for reactions of different
substrates.33 A relatively electron-rich alkene such as 4-meth-
oxystyrene will have a less-favorable ∆G° for cycloreversion
than will styrene and more electron-deficient styrenes. Presum-
ably, ∆Gq should become larger as ∆G° gets less favorable.
Thus, the more electron-rich an alkene (the easier it is to
oxidize), the harder it should be to extrude from a diolate
complex. While this behavior is observed for electron-deficient
styrenes, the opposite behavior is observed for electron-rich
styrenes. (The fact that ∆Hq increases slightly for the methoxy
compound might suggest that free energy is not the appropriate
measure in that entropy changes for different substituents can
have a nontrivial impact.)
kobs(R) ) kbenzyl/zR + kCH2 ) {(61.4 + zR)/zR} kCH2
â-labeled:
k
obs(â) ) kbenzyl + kCH2/zâ ) (61.4 + 1/zâ) kCH2
Here kobs is the observed rate constant for any particular
isotopomer, kbenzyl is the rate of migration of a phenyl-substituted
carbon, and kCH2 is the rate of migration of an unsubstituted
carbon. Mechanistic isotope effects zR and zâ ought to be similar
if not identical, but solving the observed phenomenological KIEs
(kobs(F)/kobs(R) and kobs(F)/kobs(â)) for zR and zâ give, respectively
zR ) 1.0367 and zâ ) 7.25. The latter is clearly unreasonable
for a primary 13C effect. Considering that 4-methoxystyrene
extrusion is faster than styrene extrusion requires an even larger
zâ. A similar approach assuming each mechanistic KIE to be
about 1.04 and solving for the rate differential demands that
kbenzyl be only 1.5 times kCH2, inconsistent with the rates of
ethylene vs styrene (or 4-methoxystyrene) extrusion. It is thus
impossible to reconcile the observed kinetic effect of aryl
substitution with the relative magnitudes of the KIEs at the two
positions.
The only part of this study that initially pointed to a stepwise
migration was the highly curved Hammett plot. As with our
earlier study of stilbene extrusion from Cp*Re(O)(diolato)
complexes, the curvature hinted at a balanced competition
between cleavage of the two C-O bonds. However, this model
predicts that the slope of one leg of this plot should be near-
zero reflecting an attenuated electronic impact on CH2 migration.
The increase in rate on both sides of the Hammett plot suggests
The electronic effects for electron donors are consistent with
a transition-state structure that has some positive charge buildup
at the benzylic site, but electron-deficient styrenes must be
extruded via a transition state that has considerably different
charge distribution. This opens the possibility, consistent with
standard arguments, that two different mechanisms are in
competition. As noted previously, it may be the case that
concerted mechanisms with different electron demands could
be in competition.34 However, this would require that HOMO-
(33) Isaacs, N. Physical Organic Chemistry, 2nd. ed.; Addison Wesley Long-
man: Edinburgh, 1995; pp 118-123.
(34) Norrby, P.-O.; Gable, K. P. J. Chem. Soc., Perkin Trans. 2 1996, 171-
178.
9
J. AM. CHEM. SOC. VOL. 124, NO. 15, 2002 3977

A R T I C L E S
Gable and Zhuravlev
electronic interactions. It remains to confirm that electron
acceptors create a complementary perturbation of the electronic
structure in this system; these experiments are underway.
Conclusions
Cycloreversion of 4-methoxystyrene from Tp′Re(O)(OCH-
PhCH2O) proceeds via a significantly asynchronous but con-
2
certed transition state. Primary 13C and secondary H kinetic
isotope effects are consistent with each other in pointing to
significantly more bond breaking to the R carbon than to the â
carbon. Quantitative analysis of the KIEs coupled with the
observed impact of aryl substitution on the rate of cycloreversion
rigorously excludes the possibility of competing single bond-
cleaving (stepwise) processes being responsible for the observed
KIE values. DFT calculations locate a transition-state structure
for styrene extrusion that agrees with this picture. However,
the changes in electron distribution calculated for this transition
state disagree with the observed acceleration of cycloreversion
for electron-rich substituents. The Hammett behavior suggests
that different substituents can induce shifts in the direction of
electron flow during initial bond cleavage; this indicates that
the transition-state structure (physically and electronically) is
quite flexible. Such a description explains earlier work that
strongly suggested stepwise cleavage of the C-O bonds during
diolate cycloreversion.
Figure 6. Possible variation in metal oxo/alkene HOMO-LUMO interac-
tions as a function of alkene electron demand.
LUMO interactions (Figure 6) were perfectly balanced for the
unsubstituted styrene extrusion and that both electron donors
and electron-withdrawing groups distort the balance and encour-
age one HOMO-LUMO interaction in the transition state. This
might explain why the Cp* complexes show this balanced
behavior for stilbene extrusion and not for styrene extrusion,
but the Tp′ complexes are balanced for styrene extrusion.
A more attractive interpretation is to conclude that for
electron-rich styrenes, the transition state is concerted but highly
asynchronous, with a significant degree of polar character. The
electronic structure of this transition state would appear to be
specifically induced by the electron-donating nature of the
4-methoxyphenyl substituent. A competing transition state
structure would then be necessary for electron-deficient styrenes;
this could be a concerted process with different electron demand,
or it could be one of the stepwise reactions. The calculated
transition state is consistent with the experimental behavior of
electron donors in that the benzylic carbon becomes substantially
more electron deficient, and obviously donors on the ring will
accelerate development of this electron distribution (cf. Figure
5). More work will be necessary to test whether an electron-
deficient substituent can induce a different charge reorganization
in the transition state.
Experimental Section
General Procedures. All reactions were performed using either
standard benchtop or inert atmosphere techniques in a nitrogen-filled
glovebox (Vacuum-Atmospheres Co. HE 493) or on a double-manifold
Schlenk line. Solvents were purified by vacuum distillation prior to
use; ether and THF were distilled from Na/benzophenone, hexane and
benzene from K/Na alloy; dimethyl sulfoxide and toluene were dried
with molecular sieves. Molecular sieves (4Å, Fisher) were activated
by heating under vacuum (0.01 mbar) at 300 °C for at least 4 h prior
to use. Infrared spectra were run on a Nicolet Magna-IR560.
A general method for preparation of the Tp′ diolates have been
described previously. Tp′ReO3 was synthesized by a published proce-
dure.36 Triphenylphosphine was recrystallized from ethanol. 13CO2,
13CH3I, and CD3I were purchased from Aldrich. All diols were obtained
from osmylation of the corresponding styrene.11 Unlabeled p-chloro-,
p-trifluoromethyl-, p-methyl-, and p-methoxystyrenes were used as
received from Aldrich. Synthesis of 13C- and D-labeled substituted
styrenes are described in Supporting Information. General procedures
for kinetic studies have been described elsewhere.18 The general method
employed in KIE studies was described in refs 25 and 17.
Synthesis of Hydrido-tris-(3,5-dimethylpyrazolyl)borato(oxo)-
rhenium (V) (4-Methoxyphenyl)-ethane-1,2-diolate. A representative
preparation is described here. The syntheses of labeled compounds were
all accomplished in the same manner; yields and characterization are
below. Yields were not optimized given the need for maximum purity.
A round-bottom flask was charged with Tp′ReO3 (2.34 g, 4.40
mmol), 1-(4-methoxyphenyl)ethane-1,2-diol (0.620 g, 3.69 mmol),
triphenylphosphine (1.90 g, 7.30 mmol), p-toluenesulfonic acid (36 mg,
78 mmol). THF (50 mL) was vacuum transferred onto the solids and
the mixture stirred at room temperature for 24 h. The blue mixture
was filtered and the filtrate reduced to a solid in vacuo. The residue
was chromatographed on silica using CH2Cl2/acetone (gradient elution).
The syn-diolate is eluted first followed by the anti-diastereomer.
Removal of the solvent and recrystallization from chloroform/hexane
gave 0.83 g (28%) of the anti- and 0.13 g (4.4%) of the syn-diolates.
The degree of asynchronicity described by the computed
transition state structure is extreme, and on the same order as
noted for Diels-Alder reactions of dicyanoethylene20a,b and
acrolein-BH3 complex.20c (Ozonolysis of styrene, another
reaction accepted to be concerted, shows almost identical per-F
R and â deuterium KIEs.35 The impact of the aryl group by
itself on synchronicity can thus be judged to be minimal.)
Particularly given that very little net charge development is
evident (determined on the basis of the absence of significant
solvent effects),11 this is surprising and suggests that the
concerted transition state is extremely malleable in accom-
modating the demands of the particular system. This to some
extent rationalizes our earlier results in that (for example) in
extruding a strained alkene, the system would avoid developing
sp2 character at both positions until after the transition state.
Thus, the ability of this reaction to easily alter the extent of
synchronous bond cleavage highlights the risk that any probe
of concertedness will create the appearance of a nonconcerted
process. This possibility has been noted previously.15a The
advantage of using isotope effects to probe the system is that
they provide the most minimal perturbation of steric and
(36) Matano, Y.; Northcutt, T. O.; Brugman, J.; Bennett, B. K.; Lovell, S.; Mayer,
(35) Choi, H. S.; Kuczkowski, R. L. J. Org. Chem. 1985, 50, 901-902.
J. M. Organometallics 2000, 19, 2781-2790.
9
3978 J. AM. CHEM. SOC. VOL. 124, NO. 15, 2002

KIEs in Cycloreversion of Rhenium Diolates
A R T I C L E S
1
anti-Diastereomer: H NMR (CDCl3, δ, ppm): 7.37 (d, J ) 8.1 Hz,
NMR Measurements. NMR spectra were obtained on either a
Bruker DP300 (operating at 300.133 MHz for proton or 75.469 MHz
for 13C) or a Bruker DPX400 (operating at 400.134 MHz for proton or
100.614 MHz for 13C). All chemical shifts are referenced either to
residual protons or to carbons in solvent and are expressed in ppm
downfield from tetramethylsilane. A T1 determination was done for
each signal of interest using the inversion-recovery method. For
quantitative NMR work the digital resolution of 19 pts/Hz (1H) and 12
pts/Hz (13C) was used. A relaxation delay of 25 s (>5T1) was used in
both cases. All FIDs and Fourier transformed spectra were baseline
corrected. For quantitative 13C NMR work an inversed gated decoupling
pulse sequence was used. Line shapes were assumed to be Lorenzian,
integrations were determined numerically using a constant integral
region set at 3.08 times the peak width at half-height (80% total peak
area).37 Uncertainties were determined as previously described.38
2H), 6.88 (d, J ) 8.1 Hz, 2H), 6.23 (dd, J ) 6.6, 9.6 Hz, 1H), 5.97 (s,
1H), 5.96 (s, 1H), 5.65 (dd, J ) 6.6, 10.3 Hz, 1H), 5.53 (s, 1H), 4.59
(t, 9.6 Hz, 1H), 3.81 (s, 3H), 2.67 (s, 3H), 2.54 (two unresolved s,
6H), 2.50 (s, 3H), 2.47 (s, 3H), 2.16 (s, 3H). 13C NMR (CDCl3, δ,
ppm): 159.2, 157.5, 156.8, 153.7, 147.7, 146.9, 143.0, 138.7, 128.9,
113.6, 108.2, 107.6, 107.5, 95.9, 94.5, 55.4, 14.5, 14.4, 14.0, 12.7, 12.6,
12.5. MS: (M+ + 1) 667. syn-Diastereomer: 1H NMR (CDCl3, δ,
ppm): 7.63 (d, J ) 8.2 Hz, 2H), 6.98 (d, J ) 8.2 Hz, 2H), 6.02 (s,
1H), 5.99 (s, 1H), 5.87 (dd, J ) 7.7, 10.6 Hz, 1H), 5.78 (t, J ) 9.53
Hz, 1H), 5.50 (s, 1H), 5.42 (t, J ) 9.88 Hz, 1H), 3.85 (s, 3H), 2.67 (s,
3H), 2.65 (3H), 2.57 (s, 3H), 2.55 (s, 3H), 2.23 (s, 3H), 2.17 (s, 3H).
13C NMR (CDCl3, δ, ppm): 158.8, 157.5, 157.0, 154.0, 147.7, 147.1,
143.2, 135.0, 127.5, 113.9, 107.9, 107.8, 107.3, 98.0, 89.1, 55.5, 15.5,
14.2, 13.8, 12.7, 12.6, 12.5. MS: (M+ + 1) 667.
Cycloreversion of 13C- and D-Labeled Hydrido-tris-(3,5-dimeth-
ylpyrazolyl)-borato(oxo)rhenium (V) (4-Methoxyphenyl)-ethane-1,2-
diolates. Typically, a 12 × 75 mm glass vessel was charged with ∼150
mg of an equimolar mixture of the R, â, and p-OMe isotopomers. 3
mL of toluene was vacuum transferred, and the vessel was sealed under
vacuum. This was heated in a thermostated bath to 103 °C; temperature
was measured with calibrated thermometers to (0.1 °C. The percent
conversion was estimated by timing the reaction. Upon completion,
the vessel was opened and the mixture was chromatographed (SiO2,
CH2Cl2) to reisolate unreacted diolate.
Acknowledgment. We thank the NSF for financial support
(CHE0078505).
Supporting Information Available: Additional experimental
details (PDF). This material is available free of charge via the
Internet at http://pubs.acs.org.
JA017736W
(37) Sotak, C. H.; Dumoulin, C. L.; Levy, G. C. Anal. Chem. 1983, 55, 782-
787.
(38) Singleton, D. A. Thomas, A. A. J. Am. Chem. Soc. 1995, 117, 9357-
9358.
9
J. AM. CHEM. SOC. VOL. 124, NO. 15, 2002 3979