An enantiomerically pure adamantylimido molybdenum alkylidene complex. An effective new catalyst for enantioselective olefin metathesis

Article abstract of DOI:10.1021/ja021204d

An enantiomerically pure Mo-based complex that bears an alkylimido ligand is prepared and characterized through NMR spectroscopy and X-ray analysis. Mo complex 4 is the only reported chiral alkylimido catalyst; all previous chiral complexes are arylimido

Full text of DOI:10.1021/ja021204d

Published on Web 02/06/2003
An Enantiomerically Pure Adamantylimido Molybdenum
Alkylidene Complex. An Effective New Catalyst for
Enantioselective Olefin Metathesis
W. C. Peter Tsang,† Jesper A. Jernelius,‡ G. A. Cortez,‡ Gabriel S. Weatherhead,‡
Richard R. Schrock,*,† and Amir H. Hoveyda*,‡
Contribution from the Department of Chemistry, Merkert Chemistry Center, Boston College,
Chestnut Hill, Massachusetts 02467, and Department of Chemistry, Massachusetts Institute of
Technology, Cambridge, Massachusetts 02139
Received September 23, 2002
Abstract: An enantiomerically pure Mo-based complex that bears an alkylimido ligand is prepared and
characterized through NMR spectroscopy and X-ray analysis. Mo complex 4 is the only reported chiral
alkylimido catalyst; all previous chiral complexes are arylimido systems. These studies show that the chiral
Mo catalyst exists exclusively as the syn isomer and that it offers unique reactivity and selectivity profiles
in asymmetric olefin metathesis.
Chart 1. Representative Chiral Mo Arylimido Alkylidene
Introduction
Complexes
During the past five years, research in these laboratories has
focused on the synthesis, characterization, and development of
optically pure chiral Mo-based arylimido alkylidene complexes
that can be used to promote efficient enantioselective olefin
metathesis.1 Several representative examples are illustrated in
Chart 1.2,3 A collection of catalytic asymmetric ring-closing
(ARCM)4 and ring-opening metathesis (AROM)5 transforma-
tions have thus been designed and developed. One underlying
* Corresponding author. E-mail: amir.hoveyda@bc.edu.
† Massachusetts Institute of Technology.
‡ Boston College.
(1) For an overview of chiral Mo-based olefin metathesis catalysts, see:
Hoveyda, A. H.; Schrock, R. R. Chem. Eur. J. 2001, 7, 945-950.
(2) (a) Alexander, J. B.; La, D. S.; Cefalo, D. R.; Hoveyda, A. H.; Schrock, R.
R. J. Am. Chem. Soc. 1998, 120, 4041-4142. (b) Zhu, S. S.; Cefalo, D.
R.; La, D. S.; Jamieson, J. Y.; Davis, W. M.; Hoveyda, A. H.; Schrock, R.
R. J. Am. Chem. Soc. 1999, 121, 8251-8259. (c) Alexander, J. B.; Schrock,
R. R.; Davis, W. M.; Hultzsch, K. C.; Hoveyda, A. H.; Houser, J. H.
Organometallics 2000, 19, 3700-3715. (d) Aeilts, S. L.; Cefalo, D. R.;
Bonitatebus, P. J.; Houser, J. H.; Hoveyda, A. H.; Schrock, R. R. Angew.
Chem., Int. Ed. 2001, 40, 1452-1456. (e) Hultzsch, K. C.; Bonitatebus, P.
J., Jr.; Jernelius, J.; Schrock, R. R.; Hoveyda, A. H. Organometallics 2001,
20, 4705-4712. (f) Tsang, W. C. P.; Schrock, R. R.; Hoveyda, A. H.
Organometallics 2001, 20, 5658-5669. (g) Schrock, R. R.; Jamieson, J.
Y.; Dolman, S. J.; Miller, S. A.; Bonitatebus, P. J., Jr.; Hoveyda, A. H.
Organometallics 2002, 21, 409-417. (h) Hultzsch, K. C.; Jernelius, J. A.;
Hoveyda, A. H.; Schrock, R. R. Angew. Chem., Int. Ed. 2002, 589-593.
(3) For reports on chiral Ru catalysts for olefin metathesis, see: (a) Seiders,
T. J.; Ward, D. W.; Grubbs, R. H. Org. Lett. 2001, 3, 3225-3228. (b) Van
Veldhuizen, J. J.; Garber, S. B.; Kingsbury, J. S.; Hoveyda, A. H. J. Am.
Chem. Soc. 2002, 124, 4954-4955. For a recent report regarding chiral
W-based catalysts for olefin metathesis, see: Tsang, W. C. P.; Hultzsch,
K. C.; Alexander, J. B.; Bonitatebus, P. J., Jr.; Schrock, R. R.; Hoveyda,
A. H. J. Am. Chem. Soc. 2003, 125, in press.
(4) (a) Refs 1a,c. (b) La, D. S.; Alexander, J. B.; Cefalo, D. R.; Graf, D. D.;
Hoveyda, A. H.; Schrock, R. R. J. Am. Chem. Soc. 1998, 120, 9720-
9721. (c) Weatherhead, G. S.; Houser, J. H.; Ford, J. G.; Jamieson, J. Y.;
Schrock, R. R.; Hoveyda, A. H. Tetrahedron Lett. 2000, 41, 9553-9559.
(d) Cefalo, D. R.; Kiely, A. F.; Wuchrer, M.; Jamieson, J. Y.; Schrock, R.
R.; Hoveyda, A. H. J. Am. Chem. Soc. 2001, 123, 3139-3140. (e) Kiely,
A. F.; Jernelius, J. A.; Schrock, R. R.; Hoveyda, A. H. J. Am. Chem. Soc.
2002, 124, 2868-2869. (f) Dolman, S. J.; Sattely, E. S.; Hoveyda, A. H.;
Schrock, R. R. J. Am. Chem. Soc. 2002, 124, 6991-6997. (g) Teng, X.;
Cefalo, D. R.; Schrock, R. R.; Hoveyda, A. H. J. Am. Chem. Soc. 2002,
124, 10779-10784.
theme that has emerged from these investigations is that subtle
variations in the structure of a chiral catalyst can lead to dramatic
differences in reactivity and selectivity.2b,4d,g,6 Since such
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A R T I C L E S
Tsang et al.
changes are typically based on nonobvious (particularly a priori)
alterations of the energetics of the catalytic cycle that are also
often unpredictable, we have adopted the strategy that involves
the development of a class of potential catalysts that can be
screened to achieve optimal results. This approach becomes
especially attractive when the modular structure of the Mo-based
alkylidenes is noted, an attribute that lends itself to the synthesis
of an assortment of catalyst candidates. Thus far, we have been
exclusively concerned with arylimido complexes, where the
influence of changes in the diolate (e.g., 1a vs 2a) and
substituents of the imido ligand (e.g., 1a vs 1c) on reactivity
and stereoselectivity have been probed. More recently, to expand
the structural diversity of available catalysts, we have set out
to synthesize and examine the catalytic activity of chiral Mo
catalysts in which the substituent in the imido ligand is an alkyl,
not an aryl, group.
At the time we initiated our studies, only two non-aryl-sub-
stituted imido groups had been used to prepare imido alkylidene
molybdenum complexes. These included a tert-butylimido and
a 1-adamantylimido group. Osborn and co-workers disclosed
the synthesis and characterization of Mo(N-t-Bu)(CHR)[OCH-
(CF3)2]2 systems (R ) CMe3 or Ph) capable of initiating the
slow but productive metathesis of internal olefins such as
2-pentene.7 The synthesis route normally employed for the
preparation of arylimido complexes8 was utilized to access Mo-
(N-1-adamantyl)(CHCMe2Ph)(OTf)2(dme), from which Mo(N-
1-adamantyl)(CHCMe2Ph)[OCMe(CF3)2]2 was obtained (Ad )
adamantyl).9 Analogous complexes that contain hexafluoroiso-
propoxide ligands (e.g., Mo(NAd)(CHCMe2Ph)[OCH(CF3)2]2-
(2,4-lutidine)) have been synthesized and shown to promote
living polymerization of o-trimethylsilylphenylacetylene.10
Figure 1. Thermal ellipsoid plot (50% probability level) of the structure
of 4.
Moreover, we present data clearly indicating that members of
this new class of chiral Mo catalysts deliver reactivity and
enantioselectivity levels that are not accessible by the complexes
disclosed previously.
Results and Discussion
1.SynthesisandCharacterizationofMo(NAd)(CHCMe2Ph)-
(Biphen). a. Synthesis. The precursor to adamantylimido cata-
lysts, Mo(NAd)(CHCMe2Ph)(OTf)2(dme) (6), is synthesized
from (NH4)2Mo2O7 by previously reported methods.9 Dipotas-
sium salt 5 is easily prepared by addition of KH to 3,3′-di-tert-
butyl-5,5′,6,6′-tetramethyl-1,1′-biphenyl-2,2′-diol. As depicted
in eq 1, Mo(NAd)(CHCMe2Ph)(biphen) (4) is prepared by
dropwise addition of 5 to a THF solution of 6. Chiral Mo
complex 4 is isolated in 80% yield on a 2 g scale in racemic
and enantiomerically pure forms (recrystallized from Et2O).11
In this paper we report the synthesis and characterization of
a chiral Mo-based complex (4) that bears an alkylimido ligand.
(5) (a) La, D. S.; Ford, J. G.; Sattely, E. S.; Bonitatebus, P. J.; Schrock, R. R.;
Hoveyda, A. H. J. Am. Chem. Soc. 1999, 121, 11603-11604. (b) La, D.
S.; Sattely, E. S.; Ford, J. G.; Schrock, R. R.; Hoveyda, A. H. J. Am. Chem.
Soc. 2001, 123, 7767-7778. (c) Weatherhead, G. S.; Ford, J. G.; Alexanian,
E. J.; Schrock, R. R.; Hoveyda, A. H. J. Am. Chem. Soc. 2000, 122, 1828-
1829.
b. X-ray Crystal Structure. Yellow crystals suitable for
X-ray diffraction were obtained from a concentrated solution
of (S)-4 at 20 °C; the ORTEP diagram is illustrated in Figure
1. Selected bond distances and angles are depicted in Table 1.12
The overall structure is typical of a solvent-free tetrahedral imido
alkylidene complex. The alkylidene has syn stereochemistry,
the same isomer observed in compounds 1a, 1c, Mo(NAd)-
(CHCMe2Ph)[OCH(CF3)2]3[Li(dme)], and Mo(NAd)(CHCMe2-
Ph)[OCH(CF3)2]2(2,4-lutidine). The Mo-C(1) (ModCR) dis-
tance and Mo-C(1)-C(2) (ModCR-Câ) angle (1.867(4) Å and
149.3(3)°) are comparable to the bond distances and angles in
(6) For representative examples, where structurally related substrates require
different optimum chiral catalysts, see: (a) Shimizu, K. D.; Cole, B. M.;
Krueger, C. A.; Kuntz, K. W.; Snapper, M. L.; Hoveyda, A. H. Angew.
Chem., Int. Ed. Engl. 1997, 36, 1704-1707. (b) Ref 1b. (c) Mizutani, H.;
Degrado, S. J.; Hoveyda, A. H. J. Am. Chem. Soc. 2002, 124, 779-781.
For a detailed discussion regarding catalyst diversity versus specificity,
see: Hoveyda, A. H. In Handbook of Combinatorial Chemistry; Nicolaou,
K. C., Hanko, R., Hartwig, W., Eds.; Wiley-VCH: Weinheim, 2002; pp
991-1016.
(7) Schoettel, G.; Kress, J.; Osborn, J. A. J. Chem. Soc., Chem. Commun. 1989,
1062-1063.
(8) Schrock, R. R. Chem. ReV. 2002, 102, 145-179.
(9) Oskam, J. H.; Fox, H. H.; Yap, K. B.; McConville, D. H.; O’Dell, R.;
Lichtenstein, B. J.; Schrock, R. R. J. Organomet. Chem. 1993, 459, 185-
198.
(11) For simplicity we will use the rac or S label for the entire complex, although
the label refers only to the biphenolate ligand in the complex.
(12) See the Supporting Information for crystal data and structure refinement
parameters.
(10) Schrock, R. R.; Luo, S.; Lee, J. C. J.; Zanetti, N. C.; Davis, W. M. J. Am.
Chem. Soc. 1996, 118, 3883-3895.
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Adamantylimido Mo Alkylidene Complex
A R T I C L E S
Table 1. Selected Bond Lengths (Å) and Angles (deg) for Chiral
Complex 4
Mo(1)-N(1)
Mo(1)-C(1)
Mo(1)-O(1)
Mo(1)-O(2)
1.709(3)
1.867(4)
1.986(3)
1.997(2)
Mo-C(1)-C(2)
149.3(3)
N(1)-Mo(1)-C(1)
N(1)-Mo(1)-O(2)
C(1)-Mo(1)-O(1)
C(1)-Mo(1)-O(2)
O(1)-Mo(1)-O(2)
C(21)-C(26)-C(28)-C(27)
105.37(17)
106.66(13)
107.54(14)
100.34(15)
123.54(1)
96.8(5)
1a (1.885(10) Å and 143.8(7)°),2c 1c (1.872(7) Å and 150.3(6)°),2g
Mo(NAd)(CHCMe2Ph)[OCH(CF3)2]3[Li(dme)] (1.87(2) Å and
152(2)°),10 and Mo(NAd)(CHCMe2Ph)[OCH(CF3)2]2(2,4-luti-
dine) (1.880(4) Å and 145.5(3)°). The Mo-N(1) (ModN) bond
distance and angle (1.709(3) Å and 172.8(3)°) are nearly the
same as those observed in Mo(NAd)(CHCMe2Ph)[OCH(CF3)2]3-
[Li(dme)] (1.67(1) Å and 165(1)°) and Mo(NAd)(CHCMe2Ph)-
[OCH(CF3)2]2(2,4-lutidine) (1.729(3) Å and 166.1(3)°). The
shorter ModN distance in 4 compared to that found in arylimido
systems such as 1a (1.738(6) Å) or 1c (1.745(6) Å) may be
attributed to a greater electron-donating ability (both σ and π)
of the adamantylimido ligand.
c. 1H NMR Studies. The 1H NMR spectrum of 4 (in C6D6)
reveals an alkylidene resonance at 10.88 ppm (JCH ) 119 Hz);
the chemical shift and JCH value are consistent with a syn
solvent-free complex. The proton at 10.88 ppm is through-bond
coupled to the alkylidene R carbon at 274 ppm in the 13C NMR
1
spectrum, according to a H-13C HMQC experiment. Spin
saturation transfer (SST) experiments at 22 °C reveal that a weak
anti alkylidene resonance is located at 13.48 ppm. (The two
isomers are interconverting on the SST time scale at this
temperature.) Collection of many transients showed that the syn/
anti equilibrium constant at 22 °C is ∼200 ((20). Upon addition
of pyridine to the NMR sample of 4, two alkylidene resonances
were observed in approximately a 1:1 ratio at 13.6 (1JCH ) 120
Hz) and 12.2 (1JCH ) 115 Hz) ppm. The downfield (δ 13.6)
proton is coupled to a carbon at 302 ppm, while the upfield (δ
12.2) proton is coupled to another carbon at 293 ppm. On the
basis of the 1JCH coupling values, both resonances were assigned
as syn adducts with the pyridine bound complexes occupying
the two diastereotopic CNO faces. Upon heating the sample to
80 °C, the two resonances at δ 13.6 and 12.2 broaden and merge
presumably as a consequence of pyridine dissociating from each
adduct at a rate on the order of the NMR time scale to give 4.
Upon cooling the sample back to 22 °C, the 1:1 diastereomeric
mixture of syn pyridine adducts is regenerated. Unlike most
arylimido alkylidenes, at no point in these studies was there
any evidence for the formation of anti alkylidene pyridine
adducts.
The ModCH resonances obtained from variable-temperature
1H NMR spectra of 4 in toluene-d8 from -80 to 90 °C in the
presence of 10 equiv of THF are shown in Figure 2. The
alkylidene proton resonance remains at 10.9 ppm upon addition
of THF at 22 °C, suggesting that THF does not strongly bind
to the metal center to any significant extent at ambient
temperature. As the sample is chilled to -60 °C, the alkylidene
resonance gradually moves downfield, broadens, and splits into
two signals at ∼12.5 and 11.1 ppm. The resonance at 12.5 ppm
Figure 2. Alkylidene region (Mo-CH) from variable-temperature 1H NMR
experiments with 4.
continues to sharpen as the sample is further cooled to -80
°C, while the peak near 11.1 ppm decreases in intensity. On
the basis of chemical shift, the resonance at 12.5 ppm can be
attributed to syn-4‚THF complex.13 We ascribe the weak
resonance at 11 ppm to that resulting from an equilibrium
between the alternative syn-4‚THF diastereomer and the un-
bound syn-4. The two signals are approximately the same
distance apart as the two syn-pyridine adducts observed previ-
ously. When the sample is warmed from 20 to 50 °C, the singlet
at 10.9 ppm remains sharp and moves slightly upfield. Above
50 °C, the resonance appears to broaden; the source of this
apparent broadening is not clear at the present time.
It is important to note that the behavior of chiral complex 4
in solution is strikingly different from that of related Mo-based
complexes that bear N-2,6-i-Pr2C6H3 and N-2,6-Me2C6H3 imido
ligands (see 1a and 1b, Chart 1),2c where anti alkylidene isomers
are observed spectroscopically, especially in the presence of
THF or some other two-electron-donor ligand. This difference
may be partly due to the smaller steric presence of the adamantyl
unit in the proximity of the alkylidene substituent. Such a
1
suggestion is consistent with the fact that H NMR spectra of
arylimido complexes bearing the smaller Cl atoms at the C2
and C6 position (vs Me and i-Pr; see 1c, Chart 1) also do not
(13) The resonance was too broad for accurate JCH determination.
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Tsang et al.
Table 2. Catalytic Activity of 4 Relative to 1a
we examined the Mo-catalyzed AROM/CM between norbornyl
ethers 7a-c and allylboronate 8. As illustrated in Scheme 1,
for reactions between 7a and 8, chiral arylimido Mo complexes
either deliver a mixture of cis and trans olefin products 9 and
10 that are formed along with substantial amounts (up to ∼50%)
of achiral 11 or afford highly inefficient transformations (5 mol
% catalyst, C6H6, 22 °C, 6 h). In contrast, under identical
conditions, adamantylimido complex 4 efficiently and selectiVely
deliVers 9a in 90% ee and 78% isolated yield (>13:1 trans:
cis). As also shown in Scheme 1, similar results are obtained
with substrates 7b,c. It should be noted that the resulting
enantioenriched allylboronate can be converted to the derived
alcohols in 72% overall isolated yield (after metathesis and
oxidation). For example, as depicted in eq 2, 9a is transformed
to the corresponding alcohol in 92% yield upon oxidation with
30% aqueous H2O2.
a All reactions performed in C6D6 at 22 °C with 5 mol % 1a or 4. b All
reactions proceeded to >98% conv; % conv determined by analysis of 500
1
MHz H NMR spectra of unpurified reactions mixtures.
show the presence of the anti isomer.2g Both the N-2,6-Cl2C6H3
(1c) and the NAd complex 4, as will be discussed below, behave
differently as chiral olefin metathesis catalysts compared to 1a
and 1b (see Chart 1).
The relative stability and reactivity of the rotational isomers
of Mo(NAd)(CHCMe2Ph)[OCMe(CF3)2]2 (Ad ) adamantyl)
have been studied and reported.14 These investigations show
that although both syn and anti alkylidenes are observed (12:1
syn:anti, toluene at 298 K), both isomers exhibit the same
reactivity toward 2,3-bis(trifluoromethyl)norbornadiene. In con-
trast, the previously reported studies indicate that the anti
alkylidene of Mo(NAr)(CHCMe2Ph)[OCMe(CF3)2]2 (Ar ) 2,6-
(i-Pr)2C6H3) is nearly 100 times more reactive than its the
corresponding syn isomer toward the same substrate. If syn and
anti isomers of 4 and other alkylidenes derived from them also
have approximately equal reactivity toward a range of olefins,
the predominance of the syn form would suggest that the anti
isomers may not play a significant role in the metathesis
reactions described below.
Mo-Catalyzed AROM/RCM Reactions. Another set of
metathesis reactions that have proved to be inefficient with
various arylimido ligands are AROM/RCM4c,5c of norbornyl
trienes such as 13 in Scheme 2. The desired product is the chiral
spirocyclic 14. However, a typical achiral byproduct is meso
15, arising from RCM between the two terminal alkenes. As
the chart in Scheme 2 illustrates, reactions promoted by various
arylimido Mo complexes suffer from formation of significant
amounts of 15, low enantioselectivities, or low overall conver-
sion (5 mol % catalyst, C6H6, 22 °C, 3 h). However, as depicted
in Scheme 2, under identical conditions, but with 5 mol % 4,
the desired product 14 is obtained with significantly superior
levels of reactivity and selectivity (96% ee, 82% isolated yield,
4:1 14:15).
2. Chiral Complex 4 as Olefin Metathesis Catalyst. a.
Catalyst Activity. As the first step toward examination of the
utility of 4 as an olefin metathesis catalysts, several catalytic
RCM transformations were investigated. Representative data are
summarized in Table 2; for comparison, the same set of reactions
were screened with 1a. Although all substrates undergo rapid
RCM and reactions proceed to >98% conversion in less than 2
h (5 mol % loading), 1a proves to be a somewhat more effective
catalyst than 4. Common functional groups such as ether, ester,
amide, sulfonamide, CF3, and siloxy groups are compatible with
the new chiral complex.
b. Utility in Catalytic Asymmetric Olefin Metathesis. Initial
studies clearly indicate that chiral Mo catalyst 4 offers reactivity
and selectivity levels that are not available through any of the
other arylimido-based complexes. The two examples discussed
below are illustrative.
Conclusions
We report a chiral Mo-based catalyst for enantioselective
olefin metathesis (4) that bears an alkylimido rather than an
arylimido ligand. Spectroscopic analysis of 4 suggests that,
unlike most arylimido systems reported previously, anti metal
alkylidene isomer is present only to the extent of ∼0.5%. Our
studies illustrate that chiral complex 4 can offer catalytic activity
that is not available by any other chiral metathesis catalyst,
including the related arylimido systems (Schemes 1 and 2).
Mechanistic studies aimed to determine whether the unique
reactivity of 4 is related to the predominance of its syn isomer
are underway. Synthesis and development of additional chiral
Mo-based catalysts for enantioselective synthesis continue in
these laboratories as well.
Experimental Section
Mo-Catalyzed Asymmetric Ring-Opening/Cross Metath-
esis (AROM/CM) Reactions Involving Allylboronates. Al-
lylboronates are synthetically useful partners in catalytic AROM/
CM reactions, as the resulting functionalized chiral nonracemic
products may be readily functionalized further to afford a variety
of additional synthetically useful molecules.15 Toward this end,
General Procedures. Infrared (IR) spectra were recorded on a
ThermoNicolet Avatar 360 or Nicolet 210 spectrophotometer, νmax in
cm-1. Bands were characterized as broad (br), strong (s), medium (m),
and weak (w). 1H NMR spectra are recorded on a Varian INOVA 500
(500 MHz) or INOVA 400 (400 MHz). Chemical shifts are reported
in ppm from tetramethylsilane with the solvent resonance as the internal
standard (CDCl3, δ 7.26; C6D6, δ 7.16). Data are reported as follows:
chemical shift, multiplicity (s ) singlet, d ) doublet, t ) triplet, q )
quartet, br ) broad, m ) multiplet), coupling constants (Hz), and
(14) Oskam, J. H.; Schrock, R. R. J. Am. Chem. Soc. 1993, 115, 11831-11845.
(15) Micalizio, G. C.; Schreiber, S. L. Angew. Chem., Int. Ed. 2002, 41, 152-
154, and references therein.
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Adamantylimido Mo Alkylidene Complex
A R T I C L E S
Scheme 1. Mo-Catalyzed AROM/CM with Allylboronate 8
Scheme 2. Mo-Catalyzed AROM/RCM with Triene 13
analysis (Alltech Associates Chiraldex GTA column (30 m × 0.25 mm)
or BETADEX 120 column (30 m × 0.25 mm)) in comparison with
authentic racemic materials. Elemental analyses were performed by
Robertson Microlit Laboratories (Madison, NJ) or by Kolbe Microana-
lytical Laboratories (Mu¨lheim an der Ruhr, Germany). High-resolution
mass spectrometry was performed by the University of Illinois Mass
Spectrometry Laboratories (Urbana, IL). Absolute stereochemistry was
determined by optical rotation on a Rudolph Research Analytical
Autopol IV polarimeter.
All reactions were conducted in oven (135 °C) and flame-dried
glassware under an inert atmosphere of dry nitrogen. Solvents were
purified under a positive pressure of dry Ar by a modified Innovative
Technologies purification system. Benzene and toluene were sparged
with argon and passed through activated copper and alumina columns.
Tetrahydrofuran, diethyl ether, and dichloromethane were purged with
argon and passed through activated alumina columns. Olefin-free
pentane was generated by stirring commercial grade pentane over
concentrated sulfuric acid for 24 h. The pentane was then poured over
fresh concentrated sulfuric acid. This process was repeated until the
acid layer remained colorless for 48 h. The pentane layer was separated,
washed with water, dried over Na2SO4, filtered, purged with argon,
and then passed successively through activated alumina and activated
copper columns. All catalyst preparations and metathesis reactions were
performed in a glovebox under a nitrogen atmosphere. All substrates
for Mo-catalyzed asymmetric metathesis were rigorously dried by
azeotropic distillation with anhydrous benzene (3×) prior to use.
Commercially available materials were obtained from Aldrich Chemical
Co., Strem Chemicals, Inc., or Lancaster Synthesis and purified by
appropriate methods prior to use.
integration. 13C NMR spectra are recorded on a Varian Unity INOVA
500 (125 MHz) or Varian INOVA 400 (100 MHz) with complete proton
decoupling. Chemical shifts are reported in ppm from tetramethylsilane
with the solvent resonance as the internal standard (CDCl3, δ 77.1;
C6D6, δ 128.1). Enantiomer ratios were determined by chiral GLC
Dipotassium Salt of rac- and (S)-3,3′-Di-tert-butyl-5,5′,6,6′-
tetramethyl-2,2′-dihydroxy-1,1′-biphenyl (rac- and (S)-K2(biphen))
(5). A 50 mL round-bottomed flask was charged with 3,3′-di-tert-butyl-
5,5′,6,6′-tetramethyl-2,2′-dihydroxy-1,1′-biphenyl (0.50 g, 1.4 mmol)
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A R T I C L E S
Tsang et al.
and dry THF (25 mL). Potassium hydride (0.17 g, 4.2 mmol) was slowly
added in several portions to a stirring homogeneous biphenol solution
at 22 °C. After 0.5 h, a white suspension formed. The mixture was
allowed to stir for an additional 12 h. Subsequently, excess KH was
removed by filtration through Celite. The filtered dipotassium salt
solution was immediately used in the synthesis of 4.
2H), 5.06-4.95 (m, 2H), 4.58-4.48 (ABq, J ) 11.9 Hz, 2H), 3.71-
3.69 (t, J ) 4.0 Hz, 1H), 2.60-2.50 (br m, 2H), 1.80-1.67 (m, 4H),
1.66-1.64 (d, J ) 6.2 Hz, 2H), 1.21 (s, 12H). 13C NMR (100 MHz,
CDCl3): δ 140.0, 131.7, 128.3, 127.9, 127.7, 127.4, 125.4, 114.5, 87.5,
83.3, 77.4, 73.6, 50.0, 49.2, 29.4, 29.0, 25.0. HRMS of 9a calcd for
derived alcohol C17H22O2: 258.1619, found 258.1617.
Representative Procedure for Tandem Mo-Catalyzed ARCM/
RCM. Triene 13 (25.0 mg, 0.122 mmol) was weighed into a 4 mL
vial containing a stir bar and then diluted with 1.20 mL of benzene.
Chiral complex 4 (4.40 mg, 0.0061 mmol, 5 mol %) was then added,
and the vial was fitted with a Teflon-lined cap. The mixture was allowed
to stir at 22 °C for 3 h. The vessel was then removed from the glovebox,
and the reaction was quenched by exposure to air. Volatiles were
removed in vacuo to afford a viscous black oil. Purification by silica
gel chromatography (40:1 pentane/Et2O) afforded 14 in 82% yield (17.7
mg, 0.100 mmol).
rac- and (S)-Mo(NAd)(CHCMe2Ph)(biphen) (4). A 100 mL round-
bottomed flask was charged with Mo(NAd)(CHCMe2Ph)(OTf)2(DME)
(6)9 (1.1 g, 1.4 mmol) and dry THF (30 mL). The solution was allowed
to stir for 10 min or until it turned homogeneous. The dipotassium salt
solution of the ligand (5) was added dropwise to the stirring bistriflate
solution at 22 °C, and the mixture was allowed to stir for an additional
8 h. All volatiles were removed in vacuo. The resulting residue was
taken up in cold (-30 °C) toluene, and the washings were filtered
through Celite. The filtrate was concentrated by rotary evaporation,
and the resulting residue was recrystallized from diethyl ether at -30
°C. The resulting yellow powder obtained after 12 h was rinsed with
cold pentane (-30 °C) and dried in vacuo to afford 4 in 81% yield
(0.84 g, 1.1 mmol). 1H NMR (C6D6): δ 10.88 (s, 1H, JCH ) 119.24(7)
Hz), 7.52 (d, J ) 7.5 Hz, 2H), 7.42 (s, 1H), 7.36 (s, 1H), 7.23 (t, J )
7.7 Hz, 2H), 7.05 (t, J ) 7.3 Hz, 1H), 2.14 (s, 3H), 2.12 (s, 3H), 1.84
(s, 3H), 1.74 (s, 9H), 1.72 (s, 3H), 1.68 (s, 3H), 1.86-1.68 (m, 9H),
1.66 (s, 9H), 1.35 (s, 3H), 1.26 (m, 6H). 13C NMR (125 MHz, C6D6):
δ 273.8, 154.8, 154.4, 151.7, 139.4, 139.1, 136.4, 136.2, 131.7, 130.8,
130.7, 129.8, 129.0, 128.9, 128.7, 128.7, 128.0, 127.7, 126.3, 77.1,
51.9, 44.7, 36.2, 36.1, 36.0, 33.5, 32.3, 31.0, 30.6, 30.2, 20.9, 20.8,
17.1, 17.0. Anal. Calcd for MoC44H59NO2: C, 72.19; H, 8.13; N, 1.91.
Found: C, 72.06; H, 8.10; N, 1.92. The corresponding diastereotopic
CNO face pyridine adducts were observed by adding 2 equiv of neat
pyridine to a solution of Mo(NAd)(CHCMe2Ph)(Biphen) (4) in C6D6.
1H NMR (C6D6): δ 13.6 (s, 1, syn ModCH, JCH ) 120 Hz), 12.2 (s,
1, syn ModCH, JCH ) 115 Hz). 13C NMR (125 MHz, C6D6): δ 302.0,
293.0.
Product of Catalytic AROM/CM (14). IR (neat): 3024 (m), 2955
1
(m), 1465 (w), 1440 (w), 1095 (m). H NMR (400 MHz, C6D6): δ
5.88-5.82 (dq, J ) 10.0, 2.4 Hz, 1H), 5.78-5.70 (m, 2H), 5.62-5.56
(dq, J ) 9.2, 2.4 Hz, 1H), 4.36-4.22 (m, 2H), 2.64-2.54 (m, 1H),
2.24-2.06 (m, 3H), 2.02-1.86 (m, 2H), 1.84-1.74 (dq, J ) 18.4, 8.4
Hz), 1.40-1.30 (m, 2H), 1.22-1.12 (dt, J ) 12.8, 5.6 Hz, 1H). 13C
NMR (100 MHz, C6D6): δ 129.4, 126.2, 126.0, 77.3, 63.7, 45.5, 43.3,
29.2, 23.2, 21.9, 19.4. Anal. Calcd for 14 C12H16O: C, 81.77; H, 9.15.
Found: C, 81.84; H, 9.26.
15. IR (neat): 3054 (w), 3024 (w), 2961 (m), 2936 (m), 2861 (w),
2842 (w), 1309 (m). 1H NMR (400 MHz, C6D6): δ 6.02-5.98 (t, J )
2.2 Hz, 2H), 5.80-5.70 (m, 1H), 5.58-5.52 (m, 1H), 4.10-4.07 (dq,
J ) 6.0, 2.0 Hz, 2H), 2.62-2.60 (m, 2H), 2.15-2.05 (m, 4H), 1.98-
1.92 (m, 2H), 1.34-1.20 (m, 2H). 13C NMR (100 MHz, C6D6): δ 135.3,
131.9, 129.2, 97.6, 63.0, 47.4, 29.4, 25.9, 23.7. HRMS of 15 calcd for
C12H16O: 176.1201, found 176.1199.
Acknowledgment. This paper is dedicated to the memory of
the late Professor John A. Osborn, a pioneer in the fields of
catalysis and organometallic chemistry. We thank the National
Institutes of Health (GM-59426 to R.R.S. and A.H.H.) and the
National Science Foundation (CHE-9988766 to R.R.S. and
CHE-0213009 to A.H.H.) for support. We acknowledge the
generous assistance of Dr. Jeffrey H. Simpson in multidimen-
sional NMR experiments.
Representative Procedure for Mo-Catalyzed AROM/CM. Benzyl
ether 7a (40.0 mg, 0.200 mmol) and allylboronate 816 (33.6 mg, 0.200
mmol) were weighed into a 4 mL vial containing a stir bar; the mixture
was then diluted with 1.00 mL of benzene. Chiral complex 4 (7.20
mg, 0.010 mmol) was added, and the vial was fitted with a Teflon-
lined cap. The reaction was allowed to stir at 22 °C for 6 h. The vessel
was removed from the glovebox and the reaction quenched by exposure
to air. Volatiles were removed to give a viscous black oil. Purification
by silica gel chromatography (2% Et2O in pentane) afforded 9a as a
colorless oil in 72% yield (53.0 mg, 0.144 mmol).
2-[3-(2-Benzyloxy-3-vinylcyclopentyl)allyl]-4,4,5,5-tetramethyl-
[1,3,2]dioxaborolane (9a). IR (neat): 3074 (w), 2974 (m), 2930 (m),
2867 (m), 1463 (m), 1319 (s), 1142 (s), 1097 (m). 1H NMR (400 MHz,
CDCl3): δ 7.36-7.20 (m, 1H), 6.02-5.94 (m, 1H), 5.59-5.48 (m,
Supporting Information Available: Fully labeled ORTEP
drawing, crystal data and structure refinement, atomic coordi-
nates, bond lengths and angles, anisotropic displacement
parameters, and hydrogen coordinates for 4 as well as full
characterization data for 9b, 9c, and 12 are available free of
charge via the Internet at http://pubs.acs.org.
(16) Roush, W. R.; Walts, A. E.; Hoong, L. K. J. Am. Chem. Soc. 1985, 107,
8186-8190.
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2596 J. AM. CHEM. SOC. VOL. 125, NO. 9, 2003