Novel substitutions of 1-alkoxy-and 1-arylsulfonyloxy-η3- allylmolybdenum complexes. A case for η1-alkenyl carbene complexes as intermediates

Article abstract of DOI:10.1021/om100305f

A series of acyclic and cyclic 1-alkoxy-and 1-arylsulfonyloxy-substituted TpMo(CO)₂(η³-allyl) complexes was synthesized and characterized, and exchange of the oxygenated substituent was investigated under a variety of reaction condit

Full text of DOI:10.1021/om100305f

Organometallics 2010, 29, 5083–5097 5083
DOI: 10.1021/om100305f
Novel Substitutions of 1-Alkoxy- and 1-Arylsulfonyloxy-
η3-allylmolybdenum Complexes. A Case for η1-Alkenyl Carbene
Complexes as Intermediates†
Thomas C. Coombs,‡ Wenwei Huang,§ Ethel C. Garnier-Amblard, and Lanny S. Liebeskind*
Sanford S. Atwood Chemistry Center, Emory University, 1515 Dickey Drive, Atlanta, Georgia 30322.
Current address: Department of Medicinal Chemistry, University of Kansas, 1251 Wescoe Hall Dr.,
‡
Lawrence, KS 66045-7582. §Current address: National Institutes of Health, NIH Chemical Genomics
Center, 9800 Medical Center Dr. MSC: 3370, Bethesda, MD 20892-3370
Received April 15, 2010
A series of acyclic and cyclic 1-alkoxy- and 1-arylsulfonyloxy-substituted TpMo(CO)2(η3-allyl) com-
plexes was synthesized and characterized, and exchange of the oxygenated substituent was investigated
under a variety of reaction conditions. 1-Alkoxy-substituted η3-allyl and η3-butenyl complexes partici-
pated in direct, uncatalyzed exchange of the alkoxy substituent with benzylamine, but required a Lewis
acid for exchange with alcohols. The 1-alkoxy-substituted η3-cyclohexenyl complex was unreactive toward
exchange under all conditions investigated. The corresponding acyclic arylsulfonyloxy-substituted com-
plexes underwent direct, uncatalyzed exchange with both benzylamine and alcohols, while the arylsulfo-
nyloxy-substituted cyclohexenyl compounds participated in direct substitution with benzylamine, but not
alcohols. High enantiopurity acyclic and cyclic alkoxy- and arylsulfonyloxy-substituted complexes
provided exchange products with predominant, but incomplete, losses in enantiomeric excess in all cases
examined. Mechanisms accounting for the observed reactivity trends and for the losses in enantiomeric
excess are discussed. Reactions of alkoxy-substituted complexes through an associative mechanism and of
arylsulfonyloxy-substituted compounds through a dissociative mechanism are suggested.
Introduction
these readily available and easily synthesized organometallic
π-complexes function as scaffolds from which widely diffe-
ring families of complex and biologically significant oxa- and
aza-heterocyclic organic structures can be elaborated in an
enantiospecific fashion.
TpMo(CO)2(η3-pyranyl) and TpMo(CO)2(η3-pyridinyl)
complexes are versatile organometallic enantiomeric scaffolds
(Tp=hydridotris(pyrazolyl)borate).1 Single enantiomers of
An earlier study described the rapid enantiocontrolled
construction of substituted oxabicyclo[3.2.1]octenes via a
stepwise [5þ2] cycloaddition pathway through the reaction
of a highly enantioenriched TpMo(CO)2(η3-pyranyl) com-
plex with a number of electron-deficient alkenes in the pre-
sence of EtAlCl2 (Scheme 1).1r For most olefinic substrates
the cycloaddition reaction was rapid at room temperature in
the presence of catalytic quantities of EtAlCl2 and delivered
cycloadducts of high enantiopurity. However, less reactive
alkenes such as acrylonitrile required larger amounts of EtAlCl2
and led to an unexpected slight racemization of the product.1r
Control experiments traced the lower enantiopurity to a slow
racemization of the high enantiopurity TpMo(CO)2(η3-pyranyl)
complex in the presence of EtAlCl2. Racemization was easily
eliminated for the less reactive alkenes, as long as the alkene
was present in greater amount than the Lewis acid. An ana-
logous racemization of TpMo(CO)2(η3-pyridinyl) complexes
was also observed under similar reaction conditions.1p,q
Racemization requires a π-face migration of the TpMo-
(CO)2 moiety. In appropriate acyclic systems this can occur
by a sequential slippage from η3 to η1, rotation about the
newly generated Csp3-Csp2 bond, and re-formation of a
η3-complex. However, in cyclic π-systems this cannot occur:
slippage of the allyl from η3 to η1 is feasible, but full rotation
about the newly generated Csp3-Csp2 bond is restricted by
† Part of the Dietmar Seyferth Festschrift. Dedicated to colleague Dietmar
Seyferth to honor his many years of excellent science and his outstanding
service to the community as Founding Editor of the journal Organometallics.
*To whom correspondence should be addressed. E-mail: chemLL1@
emory.edu.
(1) (a) Coombs, T. C.; Zhang, Y.; Garnier-Amblard, E. C.; Liebeskind,
L. S. J. Am. Chem. Soc. 2009, 131, 876–877. (b) Cheng, B.; Liebeskind, L. S.
Org. Lett. 2009, 11, 3682–3685. (c) Chen, W.; Liebeskind, L. S. J. Am. Chem.
Soc. 2009, 131, 12546–12547. (d) Garnier, E. C.; Liebeskind, L. S. J. Am.
Chem. Soc. 2008, 130, 7449–7458. (e) Coombs, T. C.; Lee, M. D., IV; Wong, H.;
Armstrong, M.; Cheng, B.; Chen, W.; Moretto, A. F.; Liebeskind, L. S. J. Org.
ꢀ
Chem. 2008, 73, 882–888. (f) Arrayas, R. G.; Yin, J.; Liebeskind, L. S. J. Am.
Chem. Soc. 2007, 129, 1816–1825. (g) Zhang, Y.; Liebeskind, L. S. J. Am.
Chem. Soc. 2006, 128, 465–472. (h) Zhang, Y.; Liebeskind, L. S. J. Am. Chem.
Soc. 2005, 127, 11258–11259. (i) Shu, C.; Liebeskind, L. S. J. Am. Chem. Soc.
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2003, 125, 2878–2879. (j) Arrayas, R. G.; Liebeskind, L. S. J. Am. Chem. Soc.
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2003, 125, 9026–9027. (k) Alcudia, A.; Arrayas, R. G.; Liebeskind, L. S. J. Org.
Chem. 2002, 67, 5773–5778. (l) Shu, C.; Alcudia, A.; Yin, J.; Liebeskind, L. S.
ꢀ
J. Am. Chem. Soc. 2001, 123, 12477–12487. (m) Arrayas, R. G.; Liebeskind,
L. S. J. Am. Chem. Soc. 2001, 123, 6185–6186. (n) Yin, J.; Llorente, I.;
Villanueva, L. A.; Liebeskind, L. S. J. Am. Chem. Soc. 2000, 122, 10458–
10459. (o)Moretto, A. F.;Liebeskind, L. S.J. Org. Chem. 2000, 65, 7445–7455.
(p) Malinakova, H. C.; Liebeskind, L. S. Org. Lett. 2000, 2, 4083–4086.
(q) Malinakova, H. C.; Liebeskind, L. S. Org. Lett. 2000, 2, 3909–3911. (r) Yin,
J.; Liebeskind, L. S. J. Am. Chem. Soc. 1999, 121, 5811–5812. (s) Ward, Y. D.;
Villanueva, L. A.; Allred, G. D.; Liebeskind, L. S. Organometallics 1996, 15,
4201–4210. (t) Ward, Y. D.; Villanueva, L. A.; Allred, G. D.; Liebeskind, L. S.
J. Am. Chem. Soc. 1996, 118, 897–898. (u) Villanueva, L. A.; Ward, Y. D.;
Lachicotte, R.; Liebeskind, L. S. Organometallics 1996, 15, 4190–4200.
(v) Ward, Y. D.; Villanueva, L. A.; Allred, G. D.; Payne, S. C.; Semones, M. A.;
Liebeskind, L. S. Organometallics 1995, 14, 4132–4156.
r
2010 American Chemical Society
Published on Web 07/01/2010
pubs.acs.org/Organometallics

5084 Organometallics, Vol. 29, No. 21, 2010
Coombs et al.
Scheme 1. Stereocontrol during [5þ2] Cycloaddition Reactions
Scheme 2. η3-Pyranylmolybdenum Complex Racemization
of Pyranylmolybdenum Complexes
Table 1. Synthesis of 1-Alkoxy and 1-Sulfonyloxy Substrates
its inclusion within a ring. By what mechanism then do the
η3-pyranyl- and η3-pyridinylmolybdenum complexes racemize?
Given the generally very robust nature of the molybdenum
π-complexes, it seems unlikely that π-face migration of the
TpMo(CO)2 moiety takes place by homo- or heterolytic clea-
vage of the Mo-allyl bond or even via a bimolecular ex-
change process. If those assumptions are true, then π-face
migration (racemization) must occur by a reversible opening
of the pyranyl ring proceeding through an achiral intermediate.
In the original disclosure of the [5þ2] cycloaddition chemis-
try of TpMo(CO)2(η3-pyranyl) complexes,1r it was proposed that
the TpMo(CO)2 fragment slipped from η3 to η1 at the carbon
atom adjacent to the pyran ring oxygen. This would allow rever-
sible, Lewis acid-assisted opening of the pyran ring (Scheme 2).
Cleavage of the ring carbon-oxygen bond would reversibly
generate the acyclic TpMo(CO)2 carbene complex A, fromwhich
racemization would ensue. As a related precedent, Green and
co-workers have described a ring-opening of η3-γ-lactonyl-
molybdenum complexes.2
entry cmpd
R1
R2 cmpd, yield (%)
R3
yield (%)
1
2
3
4
5
6
7
8
1a
1a
1a
1b
1b
1b
1b
1b
1c
1c
1c
H
H
H
H
H
H
H
H
H
H
H
2a, 54%
2a, 54%
2a, 54%
2b, 83%
2b, 83%
2b, 83%
2b, 83%
2b, 83%
2c, 65%
2c, 65%
2c, 65%
Me
iPr
80, 3a
76, 3b
92, 4a
72, 3c
-b, 4b
70, 4c
81, 3d
80, 3e
83, 3f
32, 4d
65, 4e
SO2Ara
Me
tosyl
E-CH3
E-CH3
E-CH3
E-CH3
E-CH3
-(CH2)3-
-(CH2)3-
-(CH2)3-
SO2Ara
allyl
cinnamyl
Me
tosyl
9
10
11
SO2Ara
a Ar = 2,4,6-triisopropylphenyl. b The 2-butenyl tosylate could not be
isolated due to its instability.
The postulated reversible opening of the η3-pyranyl ring
shown in Scheme 2 should represent a specific case of a more
general family of exchange reactions of 1-alkoxy-η3-allyl-
metals. Little is known about the chemical reactivity of seem-
ingly simple 1-alkoxy-η3-allylmetal or related complexes.2
Synthesis and structural studies have been carried out on a
variety of 1-alkoxy-η3-allylmetal complexes of nickel,3 palla-
dium,4 platinum,4b,d,5 iron,6 cobalt,7 and molybdenum,1s but
no fundamental studies of the exchange chemistry of the
alkoxy substituent have been described. Herein we describe
a study of the synthesis, characterization, and exchange reac-
tions of acyclic and cyclic enantiomerically enriched 1-alkoxy-
and 1-sulfonyloxy-η3-allylmolybdenum complexes. Through
this study we have explored the properties and reaction
chemistry of 1-alkoxy-η3-allylmetals and have probed for
the intermediacy of vinylcarbene complexes along the reaction
pathways.
Table 2. Exchange Reactions of Alkoxy-Substituted Complexes
with Benzylamine
entry cmpd
R1
R2
R3
T (oC) time cmpd yield (%)
1
2
3
4
3a
3c
3e
3f
H
CH3
CH3
H
H
H
Me
Me
23 20 h 5a
reflux 20 h 5b
95
74
58
NRa
cinnamyl reflux 24 h 5b
Me reflux 48 h 5c
-(CH2)3-
a NR: no reaction took place.
Results and Discussion
(2) Butters, C.; Carr, N.; Deeth, R. J.; Green, M.; Green, S. M.;
Mahon, M. F. J. Chem. Soc., Dalton Trans. 1996, 2299–2308.
(3) (a) Krysan, D.; Mackenzie, P. J. Am. Chem. Soc. 1988, 110, 6273–
6274. (b) Johnson, J. R.; Tully, P. S.; Mackenzie, P. B.; Sabat, M. J. Am.
Chem. Soc. 1991, 113, 6172–6177.
A series of acyclic and cyclic 1-alkoxy and 1-arylsulfonyl-
oxyallylmolybdenum complexes was prepared, and the substi-
tution chemistry of these materials was studied. The substitu-
tion investigation fell into three categories: (1) direct nucleophilic
exchange of alkoxy substituents with external amines (Table 2),
(2) Lewis acid-catalyzed exchange of alkoxy substituents
with external alcohols (Table 3), and (3) direct nucleophilic
exchange of arylsulfonate ester substituents with both alco-
hols and amines (Table 4).
ꢀ
(4) (a) Kjellgren, J.; Kritikos, M.; Szabo,K.J.J. Organomet. Chem. 2006,
691, 3640–3645. (b) Morita, M.; Inoue, K.; Ogoshi, S.; Kurosawa, H. Organo-
metallics 2003, 22, 5468–5472. (c) Ogoshi, S.; Yoshida, T.; Nishida, T.; Morita,
M.; Kurosawa, H. J. Am. Chem. Soc. 2001, 123, 1944–1950. (d) Ogoshi, S.;
Morita, M.; Kurosawa, H. J. Am. Chem. Soc. 2003, 125, 9020–9021. (e) Vicart,
ꢀ
N.; Cazes, B.; Gore, J. Tetrahedron Lett. 1995, 36, 535–538.
(5) Morita, M.; Inoue, K.; Yoshida, T.; Ogoshi, S.; Kurosawa, H.
J. Organomet. Chem. 2004, 689, 894–898.
(6) Itoh, K.; Nakanishi, S.; Otsuji, Y. Bull. Chem. Soc. Jpn. 1991, 64,
2965–2977.
(7) Chatani, N.; Yamasaki, Y.; Murai, S.; Sonoda, N. Tetrahedron
Racemic Substrate Synthesis. η3-Allylmolybdenum com-
plexes bearing oxygenated substituents at the 1-position were
synthesized according to previously published protocols or
Lett. 1983, 24, 5649–5652.

Article
Organometallics, Vol. 29, No. 21, 2010 5085
Table 3. Exchange Reactions of Alkoxy-Substituted Complexes
with Alcohols
cyclic methoxy complex 3f failed to react with benzyl amine,
even at elevated temperatures (Table 2, entry 4),
1H NMR analysis of the amino-substituted complex 5a
supports assignment of the syn-configuration to the amino
substituent (Figure 1). In a prior publication,1v it was shown
that syn protons exhibit coupling constants ranging from 5.4
to 8.1 Hz, while anti protons have consistently larger cou-
pling constants ranging from 8.7 to 12.2 Hz. The benzyla-
mino-substituted η3-allylmolybdenum complex 5a shown in
Figure 1 displayed one large coupling constant for Ha (J =
9.6 Hz), supporting assignment of its anti configuration. IR
spectroscopy of 5a also showed a significant shift to lower wave-
entry cmpd R1
R2 R3 R4-OH T (°C) time cmpd yield (%)
1
2
3
4
3a
3b
3c
H
H
CH3
H
H
H
Me iPrOH 23
iPr MeOH 23
Me iPrOH 23
26 h 3b
4 h 3a
days 3g
72
87
a
numbers for the metal carbonyl stretches (1903, 1783 cm-1
)
-
3f -(CH2)3- Me iPrOH reflux 24 h 3h
NRb
relative to the methoxy-substituted starting material 3a
(1930, 1832 cm-1, Figure 1),1s indicating a large contribution
from zwitterionic resonance structure 5a0.
a The reaction took place very slowly. HPLC analysis showed (time/
reactant/product): 30 h/70/30; 52 h/56/44; 6 d/45/54; 11 d/37/62. b NR: no
reaction took place.
For disubstituted complexes, again, 1H NMR spectroscopy
revealed two large coupling constants (J=11.2 and 9.2 Hz)
for Ha and one smaller coupling constant (J=6.8 Hz) for Hb,
thus confirming the anti-Me, syn-NHBn configuration assig-
ned to 5b (Figure 1). IR spectroscopy also showed the same
shift of the metal carbonyls to lower wavenumbers (1899,
1783 cm-1) relative to the methoxy-substituted starting mate-
rial 3c (1925, 1826 cm-1, Figure 1),1s again indicating signi-
ficant zwitterionic character.
by slight modifications thereof.1s Thus, monosubstituted-
(propenyl), 1,3-disubstituted- (2-butenyl), and cyclic (cyclo-
hexenyl) 1-alkoxy and 1-arylsulfonyloxy compounds were
prepared from the corresponding 1-tert-butyldimethylsiloxy
complexes 2a-2c by treatment with tetrabutylammonium
fluoride (TBAF) or tetrabutylammonium triphenyldifluoro-
silicate (TBAT), followed by addition of the appropriate alkyl-
ating agent or ArSO2Cl (Table 1). In turn, the tert-butyldi-
methylsiloxy complexes 2a-2c were synthesized by addition
of the appropriate R,β-unsaturated aldehyde or ketone to a
solution of Mo(DMF)3(CO)38 in CH2Cl2 followed by reac-
tion with TBSCl and then ligand exchange with potassium
hydridotrispyrazolylborate, KTp.9 The alkoxy-substituted
complexes could also be prepared in one pot from the requi-
site R,β-unsaturated aldehyde or ketone without isolating
the intermediate silyl ethers. With the exception of sulfonate
esters 4a-4d, which were found to partly decompose during
routine purification techniques, the other complexes were
obtained as easily manipulated, air- and moisture-stable mate-
rials. The problematic arylsulfonate esters 4a-4d were iso-
lable in low yields, but were typically generated and used
in situ, making isolation unnecessary.
Direct Exchange of Alkoxy Substituents with Amines and
Alcohols. Alkoxy-substituted complexes 3a, 3c, and 3e reac-
ted directly with benzyl amine in the absence of a catalyst,
providing benzylamino-substituted complexes 5a and 5b, in
which the alkoxy group had been replaced by a benzyl amine
substituent (Table 2). For example, stirring monosubstituted
methoxy complex 3a in THF in the presence of 30 equiv of
benzyl amine at room temperature for 20 h produced the
desired amino-substituted complex 5a in 95% yield as a
bright red solid (Table 2, entry 1). The disubstituted 2-butenyl
methoxy complex 3c underwent a similar substitution of
the methoxy substituent by benzylamine when stirred with
excess benzyl amine in THF at room temperature (Table 2,
entry 2). However, this reaction required several days to
reach completion, and over that period of time significant
decomposition occurred. Alternatively, refluxing complex 3c
(and 3e) in THF with 30 equiv of benzyl amine for 20 h pro-
vided the desired benzylamino-substituted complex 5b in 74%
(and 58%) yield as a bright red solid (Table 2, entries 2 and 3).
In contrast to the acyclic η3-allylmolybdenum complexes,
Previous structural studies of methoxy-substituted η3-
allylmolybdenum complexes such as 3a showed that the
methoxy substituent was oriented in the plane of the allyl
moiety and that significant double-bond character existed
between the oxygen atom and the attached allyl carbon.1s,v
These properties were attributed to resonance overlap of the
oxygen lone pairs with the π-system of the allyl. Oxygen lone
pair overlap with the π-system of the η3-allyl was most
efficient when the methoxy substituent occupied a syn posi-
tion on the η3-allyl since substituents occupying anti posi-
tions bend out of the plane of the allyl in order to minimize
unfavorable nonbonded interactions. Resonance delocaliza-
tion as in structure 3a0 is reinforced by the tendency of
TpMo(CO)2 systems to favor six-coordinate over seven-
coordinate structures10 and by the ability of the six-coordinate
canonical structure to delocalize its negative charge buildup
at molybdenum with three good π-back-bonding ligands:
two terminal CO’s and an O-alkylated η2-enone ligand. The
same arguments would apply to the benzylamino-substituted
complexes 5a and 5b.
Lewis Acid-Catalyzed Exchange of Alkoxy Substituents
with Alcohols. In contrast to the direct substitution of the
acyclic methoxy-substituted η3-allylmolybdenum complexes
with benzyl amine, no reaction occurred upon exposure of
the same complexes to 2-propanol in the absence of a cata-
lyst. However, upon stirring monosubstituted methoxy com-
plex 3a at room temperature with 20 equiv of 2-propanol in
the presence of 0.4 equiv of EtAlCl2, a facile exchange took
place, allowing isolation of isopropyloxy-substituted com-
plex 3b in 72% yield after 26 h (Table 3, entry 1). Conversely,
reaction of isopropyloxy-substituted complex 3b with MeOH,
a much smaller nucleophile than 2-propanol, in the presence
of EtAlCl2 produced 3a in 87% yield after just 4 h (Table 3,
entry 2). The disubstituted 2-butenyl methoxy complex 3c
also reacted with 2-propanol under similar Lewis acid cata-
lysis to produce 3g, but the transformation was exceedingly
(8) Pasquali, M.; Leoni, P.; Sabatino, P.; Braga, D. Gazz. Chim. Ital.
1992, 122, 275–277.
(9) (a) Trofimenko, S. J. Am. Chem. Soc. 1967, 89, 3170–3171.
(b) Trofimenko, S. Acc. Chem. Res. 1971, 4, 17–22.
(10) Curtis, M. D.; Shiu, K. B.; Butler, W. M. Organometallics 1983,
2, 1475–1477.

5086 Organometallics, Vol. 29, No. 21, 2010
Table 4. Exchange Reactions of Sulfonate Ester-Substituted Complexes
Coombs et al.
entry cmpd
R1
R2
R3
Ara
p-tolyl
Ara
yield (%), cmpd
additive
conditions
time (h)
XR4
yield (%), cmpd
1
2
3
4
5
6
7
8
2a
2b
2b
2c
2c
2a
2b
2c
H
CH3
CH3
-(CH2)3-
-(CH2)3-
H
CH3
H
H
H
-b, 4a
-b, 4b
--b, 4c
32, 4d
65, 4e
-b, 4a
-b, 4c
65, 4e
BnNH2 (30 equiv) THF/MeOH (1:1), reflux
BnNH2 (30 equiv) THF/MeOH (1:1), reflux
BnNH2 (30 equiv) THF/MeOH (1:1), reflux
BnNH2 (15 equiv) THF reflux
BnNH2 (30 equiv) THF reflux
THF/MeOH (1:1), 23 °C
2
2
2
18
17
17
14
NHBn
NHBn
NHBn
NHBn
NHBn
OMe
78,c 5a
53,c 5b
74,c 5b
45, 5c
32, 5c
73,c 3a
72,c 3c
decd
p-tolyl
Ara
H
H
Ara
Ara
THF/MeOH (1:1), 23 °C
THF/MeOH (1:1), reflux
OMe
OMe
-(CH2)3-
Ara
a Ar = 2,4,6-triisopropylphenyl. b Sulfonate ester was not isolated, but generated and used in situ. c Isolated yield over 2 steps. d Decomposition.
Figure 1. Structural implications from spectroscopic data.
slow (Table 3, entry 3). Heating did not have a significant
effect on the reaction. Again, in contrast to the acyclic η3-
allylmolybdenum complexes, cyclohexenyl complex 3f showed
no evidence of exchange with 2-propanol in the presence of
EtAlCl2, even at elevated temperatures.
was chosen for most purposes in this study. Instabilities
notwithstanding, once purified, both the toluene-substituted
sulfonate esters and 2,4,6-triisopropylbenzene sulfonate es-
ters were reasonably stable in solution and exhibited similar
reactivities.
Direct Exchange of Sulfonate Esters with Amines and Alcohols.
The sulfonate ester complexes analogous to the methoxy-
substituted complexes discussed in Tables 2 and 3 also reac-
ted directly with benzyl amine in the absence of a catalyst,
providing amino-substituted complexes 5a, 5b, and 5c (Table 4,
entries 1-5). As noted above, sulfonate ester-substituted
complexes proved challenging to work with, as most attempts
at isolation and purification led to significant decomposi-
tion. However, when synthesized on a large scale, samples of
pure material could be crystallized from the bulk material
and used for characterization and small-scale reactions. The
tosyloxy esters 4b and 4d led to more severe decomposition
during isolation than did the corresponding 2,4,6-triisopro-
pylbenzenesulfonate-substituted complexes 4a, 4c, and 4e,
and so the 2,4,6-triisopropylbenzene sulfonate ester group
It also proved possible to generate the desired sulfonate
esters in situ in THF by treating silyloxy complexes 2a and 2b
with tetrabutylammonium triphenyldifluorosilicate (TBAT)
followed by the requisite sulfonyl chloride (tolyl- or 2,4,6-tri-
isopropylphenyl). These reactant solutions could be treated
with benzyl amine in methanol, providing a simple method
for accessing amino-substituted complexes 5a and 5b with-
out the need for isolating the sensitive intermediates. In these
cases reactivity trends were such that the methanol appears
to function solely as a solvent and not as a reactant (see below).
For example, the monosubstituted allyl complex 4a (2,4,6-
triisopropylphenylsulfonyloxy) and the 2-butenyl allyl com-
plexes 4b (p-toluenesulfonyloxy) and 4c (2,4,6-triisopropyl-
phenylsulfonyloxy) were generated in situ from the siloxy
η3-allylmolybdenum complexes 2a and 2b in THF. Addition

Article
Organometallics, Vol. 29, No. 21, 2010 5087
of benzylamine in MeOH/THF (1:1) gave the corresponding
substitution products after 2 h at reflux (Table 4, entries
1-3). These reactions did proceed at room temperature in
the absence of MeOH using THF alone, but the best yields
for these acyclic complexes were obtained at reflux in the
THF/MeOH mixture.
The in situ generated monosubstituted sulfonate ester 4a
reacted very slowly with benzylamine at room temperature
in THF/MeOH (1:1) (after two days significant quantities
of unreacted 4a remained), while the in situ generated 1,3-
disubstituted sulfonate ester 4c was completely consumed
after 19 h under identical conditions and produced 5b in 35%
isolated yield. Similar results were obtained using other sol-
vents at room temperature.
In those reactions that use 1:1 THF/MeOH as cosolvents,
we cannot rule out the intermediacy of a methoxy-substituted
η3-allylmolybdenum complex that then reacts with benzyl-
amine. For example, in THF/methanol, in the absence of
benzylamine, the acyclic mono- and 1,3-disubstituted sulfo-
nate ester complexes 4a and 4c undergo substitution with
methanol to afford the corresponding methoxy-substituted
complexes 3a and 3c (Table 4, entries 6 and 7). However,
various observations suggest that the sulfonate esters react
directly with benzylamine and do not proceed through methoxy
intermediates when MeOH is present. First, the acyclic 1,3-
disubstituted alkoxy complexes required prolonged times for
the reaction with benzyl amine to reach completion (20-24 h
in refluxing THF; see Table 2, entries 2 and 3), while the 1,3-
disubstituted arylsulfonyloxy complexes reacted with benzyl-
amine in just 2 h in refluxing THF/MeOH (Table 4, entries 2
and 3). If the arylsulfonyloxy substituent was first replaced
by OMe, a much slower reaction would result. Also, the methoxy-
substituted cyclohexenyl complex 3f did not react with ben-
zylamine in refluxing THF (Table 2, entry 4), while the
corresponding arylsulfonyloxy-substituted cyclohexenyl com-
plexes did (Table 4, entries 4 and 5). Additionally, the aryl-
sulfonyloxy-substituted cyclohexenyl complexes failed to
react with MeOH in refluxing THF/MeOH (Table 4, entry 8).
Collectively, these observations imply the direct reaction of
the arylsulfonyloxy-substituted complexes by benzyl amine
even when MeOH is used as a cosolvent.
Figure 2. ORTEP view of the arylsulfonyloxy-substituted cyclo-
˚
hexenyl complex (()-4e. Key bond lengths (A): Mo(1)-C(17),
2.407(5); Mo(1)-C(12), 2.213(5); Mo(1)-C(13), 2.361(5); C(13)-
C(12), 1.429(7); C(12)-C(17), 1.405(7); C(17)-O(3), 1.436(5).
Figure 3. Bond length comparisons between arylsulfonyloxy-
and methoxy-substituted complexes 4e and 3f.
the results from a prior X-ray crystallography study carried
out on methoxy-substituted cyclohexenyl complex 3f (see
Figure 3).1s In that study, the Mo-C bond for the carbon bear-
˚
ing the methoxy substituent was significantly longer (2.664(4) A)
than the remaining two molybdenum allyl Mo-C bonds
(2.256(4) and 2.300(4) A). This lengthening was attributed to
˚
resonance donation from the oxygen atom into the metal
allyl (see Figure 1 for related spectroscopic details).
Exchange Reactions with Chiral, Nonracemic Complexes.
By what mechanism(s) do the reactions depicted in Tables 2-4
occur? Do the alkoxy and sulfonate ester substitution reac-
tions take place with retention of π-complex stereochemistry,
or does the mechanism of substitution allow racemization?
To address these questions, enantiomerically enriched ver-
sions of 1-substituted cyclohexenyl- and 2-butenyl molybdenum
complexes were prepared. To accomplish this task, a chiral
auxiliary screen was conducted in order to find a suitable
auxiliary for efficient resolution of both cyclohexenyl and
2-butenyl diastereomeric molybdenum complexes. The re-
sults are listed in Table 5.
The Cyclohexenyl Series. Camphorsulfonyl-substituted
cyclohexenyl complexes 6 proved too reactive to isolate
(Table 5, entry 1), while the analogous diastereomeric mix-
tures of both (S)-(-)-2-acetoxypropionyl-substituted com-
plexes 8 (Table 5, entry 2) and (-)-menthyl-substituted
complexes 10 (Table 5, entry 5) were isolable, but the dia-
stereomers could not be separated. (R)-(-)-Pantolactone
derivatives of the cyclohexenyl system, however, proved quite
useful, allowing both diastereomers of the cyclohexenyl com-
plex to be isolated in good yields (Table 5, entry 7). Once sepa-
rated via a combination of chromatography and recrystallization
In contrast to the acyclic sulfonate-ester-substituted com-
plexes 4a and 4c, which were unstable and required in situ
generation, cyclohexenyl complex 4e, possessing a 2,4,6-
triisopropylbenzenesulfonyloxy substituent, was quite stable
and easily isolated. The corresponding tosyloxy-substituted
cyclohexenyl complex 4d, while still isolable, was much less
stable (Table 4, entry 4). As listed in Table 4 (entries 4 and 5),
the isolable sulfonate ester-substituted cyclohexenyl com-
plexes 4d and 4e underwent reaction with benzyl amine in
refluxing THF (the methoxy-substituted cyclohexenyl com-
plex 3f does not react with benzylamine), producing the
benzylamino-substituted complex 5c in 45% and 32% yields,
respectively. The remainder of the mass in these reactions
was lost to decomposition. Reactions run in THF/MeOH
mixtures did not provide improved results.
The structural features of the sulfonate ester complexes
were assessed through X-ray diffraction analysis of a single
crystal of 4e that was grown from dichloromethane/hexanes
(Figure 2). Very little lengthening of the Mo(1)-C(17) bond
is observed relative to related methoxy-substituted allyl com-
˚
plexes. The Mo(1)-C(17) bond length is 2.407(5) A, while
the Mo(1)-C(12) and Mo(1)-C(13) bond lengths are 2.213(5)
and 2.362 A, respectively. This observation contrasts with
˚

5088 Organometallics, Vol. 29, No. 21, 2010
Coombs et al.
Table 5. Chiral Auxiliary Screen
a NA: not attempted. b The diastereomers are formed in a roughly 1:1 ratio, but one decomposes upon purification of the mixture. Refer to text for
details.
Figure 4. Pantolactone-based resolution of the cyclohexenyl
complex diastereomers 11.
(Figure 4), the products showed excellent diastereomeric
ratios (up to 99.9:0.1). Of the two, only the more polar dia-
stereomer provided crystals of suitable quality for analysis.
A single-crystal X-ray diffraction analysis of the more polar
diastereomer confirmed the relative and absolute configura-
tion as (1S, R)-1111 (Figure 5).
The 2-Butenyl Series. Diastereomeric arylsulfonyloxy and
carboxylic ester complexes 7 (Table 5, entry 2) and 9 (Table 5,
entry 4) could be generated from the acyclic 2-butenyl system
using camphorsulfonyl chloride and (S)-(-)-2-acetoxypro-
pionyl chloride, respectively, but in both cases the diastere-
omers decomposed during attempts at isolation. However,
(R)-(-)-pantolactone could be used to produce a mixture of
stable diastereomeric carbonates 12. Of these, only one of the
diastereomers could be isolated; the other decomposed dur-
ing processing (Figure 6). The decomposition product from
this unstable diastereomer partly overlapped with the sta-
ble diastereomer, making purification particularly tedious.
Figure 5. ORTEP view of the pantolactonecarbonyloxy-substituted
cyclohexenyl complex (1S, R)-11 (more polar diastereomer).
Key bond lengths (A): Mo(1)-C(12), 2.4055(14); Mo(1)-C(13),
2.2144(14); Mo(1)-C(14), 2.3595(15); C(14)-C(13), 1.435(2);
C(13)-C(12), 1.405(2); C12-O3, 1.4290(17).
˚
Figure 6. Accessing diastereomerically pure 2-butenyl complex 12.
However, repeated iterations of silica gel column chromato-
graphy and recrystallization from dichloromethane/hexanes
(11) Sloan, T. E. Top. Stereochem. 1981, 12, 1–36.

Article
Organometallics, Vol. 29, No. 21, 2010 5089
˚
Figure 7. ORTEP view of pantolactonecarbonyloxy-substituted 2-butenyl complex (1R, R)-12. Selected bond lengths (A): Mo(1)-
C(13), 2.364(4); Mo(1)-C(14), 2.241(4); Mo(1)-C(15), 2.395(4); C(13)-C(14), 1.415(5); C(14)-C(15), 1.390(5); C15-O3, 1.416(4).
Crystallized from EtOAc/hexanes. EtOAc solvate molecule not shown.
Scheme 3
Scheme 4. Amino Exchange of Enantioenriched Cyclohexenyl
Sulfonate Ester 4e
reaction mixtures to arylsulfonyl chlorides led only to de-
composition. Instead, the anions B and C generated upon
cleavage of the carbonate with excess MeLi were successfully
trapped as the corresponding tert-butyldimethylsilyl ether
using TBSCl. Unexpectedly, the anionic intermediates B and
C generated upon treatment of the pantolactone-derived com-
plexes with excess MeLi were prone to racemization, but it
was easily minimized either by using low reaction tempera-
tures or by rapid reaction processing.12 For example, when dia-
stereomerically enriched carbonate (1R,R)-11 (98.0:0.5 dr)
was treated with 5 equiv of MeLi at 0 °C and the reaction
mixture immediately quenched with 10 equiv of TBSCl, the
silyl ether (þ)-2c was produced in 47% yield and 97.5% ee
(Scheme 3). Similarly, upon cleaving the carbonate of the
provided (1R, R)-12 in a low, but useful 22% yield. The abso-
lute configuration of 12 was determined to be (1R, R)11 by
1
single-crystal X-ray diffraction (Figure 7). In the H NMR
spectrum of the 12 only one set of peaks was observed, so the
diastereomeric ratio of the purified material was determined
to be >95:5 (1R, R).
With single diastereomers of both the cyclohexenyl (11)
and 2-butenyl (12) complexes in hand, attention was turned
to converting these complexes into simpler high-enantiopurity
alkoxy- and arylsulfonyloxy-substituted variants for an investi-
gation of the exchange reactions. To accomplish this task,
the carbonates were cleaved to oxidic intermediates (B and C
in Scheme 3) with excess MeLi, and the anionic species was
trapped. Unfortunately, direct exposure of the MeLi-treated
(12) Racemization of the anionic intermediate generated by cleavage
of the η3-allylmolybdenum complexes bearing the pantolactone carbo-
nate is intriguing, but its detailed study was beyond the scope of this
initial exploration of the substitution chemistry of 1-alkoxy- and 1-aryl-
sulfonyloxy-substituted η3-allylmolybdenum complexes.

5090 Organometallics, Vol. 29, No. 21, 2010
Coombs et al.
Scheme 5. Asymmetric Amino Exchange of Complexes 4b and 3e
Figure 8
Scheme 6. Slow Exchange of Enantioenriched Cinnamyloxy
Complex (-)-3e
Figure 9
Scheme 7. Carbene Intermediates in Exchange Reactions
diastereomerically enriched 2-butenyl complex 12 (dr >95:5)
under the same reaction conditions used above for 11, the
silyl ether (-)-2b was obtained in 47% yield and 96.2% ee.
Conversion of the tert-butyldimethylsiloxy-substituted
η3-allylmolybdenum complexes to the desired alkoxy- and
arylsulfonyloxy-substituted complexes was achieved through
fluoride activation of the silyloxide and quenching with an
appropriate electrophile. Although yields are modest because
of the inherent reactivity of the arylsulfonyloxy-substituted
complexes, minimal loss of enantiomeric purity was obser-
ved in the conversion of cyclohexenyl carbonate (1R, R)-11
to the corresponding sulfonate ester complex (þ)-4e (Scheme 4).
In this series of transformations, treatment of 2c with TBAT
at room temperature followed by low-temperature (-78 °C)
addition of 2,4,6-triisopropylbenzenesulfonyl chloride yiel-
ded sulfonate ester (þ)-4e in 96.5% ee as measured by chiral
HPLC. The sulfonate ester (þ)-4e was then refluxed in THF
for 16 h in the presence of 30 equiv of benzyl amine, provi-
ding a largely racemized benzylamino-substituted complex
5c (30.6% ee). We could not determine if the sulfonate ester
(þ)-4e racemized during the reaction, because heating the
sulfonate ester complex in THF in the absence of amine led
only to decomposition.
Along similar lines, activation of the silyl ether (-)-2b with
TBAT at room temperature and trapping with 2,4,6-tri-
isopropylbenzenesulfonyl chloride at room temperature pro-
vided the 2-butenyl sulfonate ester 4c. Unfortunately, its
enantiomers could not be separated under any of the condi-
tions explored with available chiral HPLC columns. Never-
theless the enantiomers of the simpler but less stable tosyloxy-
substituted 2-butenyl complex 4b were resolvable via chiral
HPLC. Due to the instability of tosylate 4b to isolation
procedures, it was generated in situ from (-)-2b of 96.2%
ee (Scheme 5), and its ee was determined by directly analy-
zing a sample of the reaction mixture by chiral HPLC. The
reaction mixture containing the enantioenriched tosylate
(91.0% ee) was then heated at reflux in the presence of 30 equiv
of benzylamine in THF/MeOH (1:1) for 3 h, delivering
racemic amino-substituted complex 5b (1.1% ee) in 59%
yield.
The previously prepared methoxy- and isopropyloxy-
substituted 2-butenylmolybdenum complexes were not suitable
to probe the stereochemical outcome of the substitution of an
acyclic alkoxyl-substituted complex, because their enantio-
mers could not be resolved by chiral HPLC. Since the enantio-
mers of the related cinnamyloxy- and allyloxy-substituted
2-butenyl complexes were resolvable by chiral HPLC, these
latter systems were chosen for study.
The silyl ether (-)-2b (91.1% ee) was activated with TBAT
at -78 °C and then treated with cinnamyl bromide at -78 °C
to generate the cinnamyloxy-substituted complex (-)-3e in
53% yield and 95.1% ee13 (Scheme 5). The cinnamyloxy-
substituted complex was then subjected to the exchange
reaction by refluxing in THF with BnNH2 (30 equiv). After
23 h, racemic benzylamino-substituted complex 5b was iso-
lated in 44% yield and 3.5% ee. Thus, the uncatalyzed ex-
change of sulfonate ester and alkoxy groups with benzyl-
amine occurs with almost complete racemization.
A sample of the enantio-enriched cinnamyloxy-substituted
complex (-)-3e (96.0% ee) was also subjected to Lewis acid-
catalyzed alkoxy exchange (Scheme 6). When stirred in the
presence of allyl alcohol (25 equiv) and EtAlCl2 (0.5 equiv),
an exceedingly slow reaction ensued, producing allyloxy-
substituted complex 3d. After six days, analysis of the crude
reaction mixture by chiral HPLC showed a 22:78 ratio of the
desired allyloxy-substituted exchange product 3d (24.6% ee)
to the starting cinnamyloxy complex 3e. While the enantio-
meric excess of the allyloxy-substituted exchange product 3g
haddegradedto24.6%overthecourseofthe6days,theenantio-
purity of the cinnamyloxy-substituted starting material
(13) These enantiomeric excesses were obtained by integrating the
appropriate HPLC peaks for both (-)-2b and (-)-3e. The slight gain in
enantiomeric excess is likely the result of a small impurity coeluting with
the product peak.

Article
Organometallics, Vol. 29, No. 21, 2010 5091
Scheme 8. Suggested Associative Mechanism for Substitution of 1-Alkoxy η3-Allylmolybdenum Complexes by PhCH2NH2
was unchanged. This latter observation is important. It pro-
vides strong support for the notion that the low ee’s of the sub-
stitution products described within are not caused by race-
mization of the starting materials under the reaction conditions.
Mechanistic Rationale. For the purpose of this descriptive
study of the substitution chemistry of alkoxy and arylsulfo-
nyloxy η3-allylmolybdenum complexes, the discussion of
mechanism is restricted to reasonable suggestions that are
consistent with observations. A more detailed analysis will
require additional experimentation that is beyond the scope
of this disclosure.
However, the isolation of substitution products that are
significantly, but not fully, racemic requires the presence of
a reactive chiral, nonracemic intermediate that can also be
intercepted by the nucleophile. An obvious candidate is a η3-
vinylcarbene14d-f,15 for which the π-facial information of the
reactant would be retained (Figure 9, E and F). However,
only the acyclic 2-butenyl reactants can form η3-vinyl-
carbenes such as E in Figure 9. Significant geometric con-
straints make this pathway highly unlikely for the cyclohexenyl
complexes (F in Figure 9), which implies that nonracemic
products must result by an alternate mechanism.
As described above, varying degrees of racemization are
observed when enantiomerically enriched 1-alkoxy and
1-sulfonyloxy η3-allylmolybdenum complexes are subjected
to substitution reactions. To accommodate these observa-
tions, we suggest that the substitution reactions proceed through
an achiral molybdenum carbene intermediate from which
stereochemical information can be lost. A generic represen-
tation is depicted in Scheme 7.
Support for this mechanistic assumption comes from the
published structures of known TpM(CO)(Z)(carbene) com-
plexes (M=Mo, W; Z=CO, NO, other L:),14 which, in each
case, depict the ModCR2 plane bisecting the plane defined
by the Mo and the two basal plane pyrazole ligands (Figure 8).
If the substitution reactions investigated here pass through a
carbene complex that possesses the same geometry (D, Figure8),
then full racemization must ensue when these achiral cationic
complexes are attacked by the incoming nucleophile from
either equivalent π-face.
Insight into possible mechanistic pathways comes from a
comparison of the reactivity of the alkoxy- and arylsulfonyloxy-
substituted η3-allyl complexes. For the alkoxy-substituted
η3-allylmolybdenum complexes, the less substituted systems
undergo substitution reactions much faster than the more
substituted systems, with the cyclohexenyl complexes being
completely unreactive under the experimental conditions
explored. In contrast, for the arylsulfonyloxy-substituted
η3-allylmolybdenum complexes, the less substituted systems
undergo substitution reactions much slower than the more
substituted systems, with the cyclohexenyl systems bordering
on being too unstable to handle. Therefore, the alkoxy- and
arylsulfonyloxy-substituted η3-allylmolybdenum complexes
react by different mechanisms: the arylsulfonyloxy-bearing
substrates display SN1-like (dissociative) reactivity patterns,
while the alkoxy-bearing substrates show SN2-like (associative)
reactivity patterns.
Alkoxy-Substituted Complexes. In order to accommodate
the different reactivity profiles as well as the isolation of sub-
stitution products that are partly, but not fully, racemic, we
suggest that the observations are consistent with reaction of
the alkoxy-substituted complexes through an associative mecha-
nism. The specific case of reaction of the alkoxy-substituted
η3-allylmolybdenum complexes with benzylamine is depic-
ted in Scheme 8.
(14) (a) Bruce, A. E.; Gamble, A. S.; Tonker, T. L.; Templeton, J. L.
Organometallics 1987, 6, 1350–1352. (b) Feng, S. G.; White, P. S.; Templeton,
J. L. Organometallics 1993, 12, 1765–1774. (c) Feng, S. G.; White, P. S.;
Templeton, J. L. Organometallics 1994, 13, 1214–1223. (d) Gunnoe, T. B.;
White, P. S.; Templeton, J. L. Organometallics 1997, 16, 370–377. (e) Feng,
S. G.; White, P. S.; Templeton, J. L. J. Am. Chem. Soc. 1990, 112, 8192–8193.
(f) Feng, S. G.; White, P. S.; Templeton, J. L. J. Am. Chem. Soc. 1992, 114,
2951–2960. (g) Frohnapfel, D. S.; White, P. S.; Templeton, J. L. Organome-
tallics 2000, 19, 1497–1506.
(15) (a) Trnka, T. M.; Day, M. W.; Grubbs, R. H. Organometallics
2001, 20, 3845–3847. (b) Mistudo, T.-a. Bull. Chem. Soc. Jpn. 1998, 71,
1525–1538. (c) Hofmann, P.; Hammerle, M.; Unfried, G. New J. Chem.
1991, 15, 769–789. (d) Fischer, H.; Hofmann, J.; Mauz, E. Angew. Chem.,
Int. Ed. 1991, 30, 998–999. (e) Harvey, D.; Brown, M. J. Am. Chem. Soc.
1990, 112, 7806. (f) Feng, S. G.; Gamble, A. S.; Templeton, J. L. Organo-
metallics 1989, 8, 2024–2031. (g) Garrett, K. E.; Sheridan, J. B.; Pourreau,
D. B.; Feng, W. C.; Geoffroy, G. L.; Staley, D. L.; Rheingold, A. L. J. Am.
Chem. Soc. 1989, 111, 8383–8391. (h) Mayr, A.; Asaro, M. F.; Glines, T. J.
J. Am. Chem. Soc. 1987, 109, 2215–2216. (i) Stone, K. C.; Jamison, G. M.;
White, P. S.; Templeton, J. L. Organometallics 2003, 22, 3083–3095.
In lieu of generating a η3-vinylcarbene to explain the
residual stereoselectivity, the data are consistent with asso-
ciation of the incoming nucleophile at molybdenum, either
coincident with or subsequent to η3-η1 slippage of the allyl
(complex 3 f intermediate G f intermediate H). This will
generate an intermediate seven-coordinate η1-allyl H that
retains the stereochemical information of its precursor η3-
allyl complex. Intermediate H can subsequently partition
along two pathways. Ionization of the alkoxy substituent
generates an achiral η1-vinylcarbene intermediate I, which

5092 Organometallics, Vol. 29, No. 21, 2010
Scheme 9. Lewis Acid-Catalyzed Exchange of Alkoxy Complexes with External Alcohols
Coombs et al.
Scheme 10. Suggested Dissociative Mechanism for Substitution of 1-Sulfonyloxy η3-Allylmolybdenum Complexes
would react with the coordinate nucleophile to produce fully
racemic product. Alternatively, a concerted displacement of
the alkoxy leaving group from the η1-allyl by the molybdenum-
coordinated nucleophile (intermediate H f intermediate J)
would deliver nonracemic product at η1 f η3 slippage.
The actual degree of racemization will reflect the relative
rates of reaction of the nucleophile through these two diffe-
rent pathways: via the achiral η1-vinylcarbene I would lead
to full racemization, while the internal displacement on the
η1-allyl would proceed with stereocontrol. Whether the resi-
dual enantiomeric excess in the substitution product is due to
retention or inversion has not yet been determined. Enantio-
merically enriched samples of the amino substitution pro-
ducts 5 are not available by the procedures described within,
and the enantiomers of methoxy- and isopropyloxy-substituted
complexes could not be resolved by chiral HPLC. However,
the enantiomers of the allyloxy-substituted complex depicted
in Scheme 6 are resolvable on chiral HPLC columns. With
additional work this system may allow us to determine if the
residual enantiopurity in the product is due to retention or
inversion of the original stereochemistry
generating a (still chiral) η1-allyl. This ligand coordination
will increase the electron density at Mo and promote a metal-
assisted ionization of the alkoxy group. As the alkoxyl group
departs, it can pick up a proton from the amine as the inter-
mediate partitions, whether following the racemization or
the stereospecific substitution pathway. The observed rela-
tive rates of reaction of the alkoxyl-substituted η3-allyl-
molybdenum complexes (the less substituted η3-allyl complexes
displace faster than more substituted complexes) are also
fully consistent with the requirement for association of the
nucleophile at molybdenum prior to ionization and the in-
creased steric demand that accrues in the intermediate.
In contrast to the reaction of the alkoxy-substituted η3-
allylmolybdenum complexes with benzylamine, the neces-
sary requirement of a Lewis acid for exchange of alkoxy sub-
stituents with external alcohols aligns with the decreased
Lewis basicity of alcohols relative to amines. Prior coordina-
tion of the Lewis acid to the alkoxy substituent removes
electron density from the molybdenum atom (see the electron-
donating attribute of the alkoxy-substituted η3-allylmoly-
bdenum complexes described in Figures 1 and 3), which would
facilitate both slippage to the η1-allyl and coordination of an
alcohol at the molybdenum atom (Scheme 9, Lewis acid-
coordinated complex 3 f intermediate K f intermediate L).
The stereochemistry of this process would reflect partitioning
This proposed associative pathway is consistent with the
poor leaving group ability of an alkoxide, unassisted by protic
or Lewis acids. It is therefore likely that a molecule of benzyl
amine must first fill an empty coordination site at molybdenum,

Article
Organometallics, Vol. 29, No. 21, 2010 5093
of the chiral, seven-coordinate, η1-allylmolybdenum inter-
mediate L to either the chiral η1-allyl intermediate N, result-
ing from concerted internal displacement of the alkoxy leav-
ing group by the molybdenum-coordinated nucleophile, or
the achiral η1-vinylcarbene M, from which racemic product
would result.
Arylsulfonyloxy-Substituted Complexes. Since the more
substituted arylsulfonyloxy-substituted complexes show greater
reactivity toward substitution than the less substituted rela-
tives, a dissociative mechanism is likely (Scheme 10). These
observations are consistent with a solvolysis-like reaction
wherein η3-to-η1 slippage of the TpMo(CO)2 fragment occurs
in concert with departure of the sulfonate leaving group
(4 f O). The degree of racemization would then be deter-
mined by partitioning between complete ionization to the
discrete achiral η1-carbene P or interception of the still chiral,
partially ionized intermediate O by the incoming nucleophile.
Synthetic Protocols and Characterization Data. (()-Dicarbonyl-
[hydridotris(1-pyrazolyl)borato][(η-1,2,3)-1-(2,4,6-triisopropylben-
zenesulfonyloxy)-2-propen-1-yl]molybdenum (syn-OSO2Ar), 4a.
Complex 2a (50.0 mg, 0.093 mmol, 1.0 equiv) was dissolved in THF
(2 mL), and TBAT (55.4 mg, 0.10 mmol, 1.1 equiv) was added in
one portion at room temperature. After 15 min 2,4,6-triisopropyl-
benzenesulfonyl chloride (30.3 mg, 0.10 mmol, 1.10 equiv) was
added in one portion at room temperature. The solution was stirred
for 15 min at room temperature before being passed through a short
pad of silica gel with hexanes-EtOAc (2:1) and concentrated on
a rotary evaporator. Flash chromatography over silica gel with
hexanes-EtOAc (2:1) provided 4a (58.6 mg, 0.085 mmol, 92%) as a
yellow solid. Due to the sensitivity of 4a to purification techniques,
it was usually generated and used in situ. (()-4a: TLC: Rf = 0.60
(hexanes-EtOAc, 2:1). IR (cm-1): 2964 (m), 1949 (s), 1864 (s). 1H
NMR (400 MHz, CDCl3): δ 7.75-8.55 (br m, 3 H), 7.54 (br s, 3 H),
7.16 (s, 2 H), 6.18(brs, 3H), 4.30(d, J=6.8 Hz, 1 H), 4.07 (hep, J=
6.8 Hz, 2 H), 3.70-3.76 (m, 1 H), 3.27 (dd, J=7.6 Hz, 2.8 Hz, 1 H),
2.92 (hep, J=6.8 Hz, 1 H), 1.25 (d, J=6.8 Hz, 6 H), 1.20 (d, J=6.4
Hz, 6 H), 1.08 (dd, J=9.2Hz,3.2Hz,1H),0.98(d,J=6.8 Hz, 6 H).
13C NMR (100 MHz, CDCl3): δ 228.9, 225.9, 154.5, 151.8, 128.9,
124.1, 105.7, 98.7, 69.5, 45.2, 34.5, 25.0, 24.3, 23.8, 23.6. HRMS
(ESI): calcd for C29H38BMoN6O5S ([M þ H]þ) 691.1766, found
691.1796.
(()-Dicarbonyl[hydridotris(1-pyrazolyl)borato][(η-1,2,3)-1-(p-tolu-
enesulfonyloxy)-2-buten-1-yl]molybdenum (anti-methyl/syn-OTs),
4b. Tosyloxy-substituted complex 4b was synthesized in an ana-
logous manner to 2,4,6-triisopropylbenzenesulfonyloxy-substituted
complex 4c, described below. However, instead of performing an
aqueous workup, the crude reaction mixture was filtered through
a short column of basic alumina and concentrated. Spectroscopic
data were then gathered on this crude material. The use of silica gel
and/or CDCl3 for purification and characterization resulted in
rapid decomposition of 4b. Therefore, for synthetic purposes, 4b
was generated and used in situ without isolation. (()-4b: TLC:
Rf = 0.47 (hexanes-EtOAc, 2:1). IR (cm-1): 2964 (m), 1942 (s),
1857 (s), 1594 (m), 1505 (m), 1432 (m). 1HNMR(C6D6, 400MHz):
δ 8.49 (d, J=2.0 Hz, 1 H), 8.41 (d, J=2.0 Hz, 1 H), 7.71 (d, J=
8.4 Hz, 2 H), 7.22 (d, J = 2.0 Hz, 1 H), (1 proton obscured by
C6H6), 7.01 (d, J=1.6 Hz, 1 H), 6.98 (d, J=2.0 Hz, 1 H), 6.64 (d,
J = 8.4 Hz, 2 H), 5.86 (t, J = 2.0 Hz, 1 H), 5.72 (t, J = 2.4 Hz,
1 H), 5.65 (t, J=2.4 Hz, 1 H), 5.36 (d, J=7.2 Hz, 1 H), 3.78 (t,
J=8.4 Hz, 1 H), 3.54-3.61 (m, 1 H), 1.78 (s, 3 H), 1.10 (d, J=
6.4 Hz, 3 H). 13C NMR (C6D6, 100 MHz): δ 230.9, 227.0, 147.8,
147.6, 145.2, 141.1, 136.5, 136.2, 134.5, 133.6, 130.2 (2), 129.1
(2), 106.6, 106.1, 106.0, 99.3, 71.9, 58.6, 30.8, 21.5, 17.5. HPLC:
Daicel Chiralcel OJ-RH column, gradient solvent system was
used (% CH3CN in H2O with 0.1% TFA) 0-20 min (50% to
75%), 1.5 mL/min, λ=254 nm, (S)-4b: t(S)=10.31 min; (R)-4b:
Conclusions
1-Alkoxy- and 1-arylsulfonyloxy-η3-allylmolybdenum com-
plexes participate in exchange reactions with external amine
and alcohol nucleophiles. Acyclic alkoxyl-substituted com-
plexes (η3-allyl and η3-butenyl) react directly with benzyl-
amine to generate benzylamino-substituted η3-allylmolyb-
denum complexes, but a Lewis acid catalyst is required to
facilitate the substitution reaction with alcohols. The corres-
ponding cyclohexenyl system did not participate in substitu-
tion reactions. Both acyclic (η3-allyl and η3-butenyl) and
cyclic (η3-cyclohexenyl) 1-arylsulfonyloxy complexes react
directly with both amines and alcohols in the absence of any
catalyst to provide the substitution products. In all cases
studied, high enantiopurity starting materials provided sub-
stitution products that were significantly racemized. The
reactivity order of each family of complexes suggests that
the alkoxy-substituted η3-allylmolybdenum complexes react
via an associative process, while the 1-arylsulfonyloxy com-
plexes react via a dissociative process. The loss of enantio-
purity in the substitution products suggests a mechanism
proceeding via an achiral η1-vinylcarbene in each case. While
additional studies are required to tease out the mechanistic
details of these interesting processes, the results disclosed
within highlight a new reactivity principle for η3-allylmetal
complexes bearing substituents that can function as leaving
groups. It is suggested that useful stereocontrol emanating
from enantiomerically enriched complexes could result either
(1) by using an internally tethered nucleophile that is attached
to the π-complex via a preexisting stereocenter (cf., earlier
studies of the stereocontrolled functionalization of η3-pyranyl-
and η3-pyridinylmolybdenum complexes)1 or (2) by explor-
ing reaction variables that can inhibit the partitioning of the
substitution reaction pathway through an achiral η1-vinyl-
carbene. Finally, the chemistry described within, when exten-
ded from molybdenum to tungsten, is likely to provide a useful
procedure to generate stable vinylcarbene complexes.14
t
(R) =10.88 min.
(()-Dicarbonyl[hydridotris(1-pyrazolyl)borato][(η-1,2,3)-1-(2,4,6-
triisopropylbenzenesulfonyloxy)-2-buten-1-yl]molybdenum (anti-
methyl/syn-OSO2Ar), 4c. Complex 2b (500 mg, 0.91 mmol, 1.0
equiv) was dissolved in THF (10 mL), and TBAT (540 mg, 1.00
mmol, 1.1 equiv) was added in one portion at room temperature.
After 15 min, 2,4,6-triisopropylbenzenesulfonyl chloride (606 mg,
2.00 mmol, 2.2 equiv) was added in one portion at room tempe-
rature. The solution was stirred for one hour at room tempera-
ture before it was poured into a separatory funnel containing
water (20 mL) and CH2Cl2 (20 mL). The layers were separated
and the aqueous layer was extracted with CH2Cl2 (10 mL). The
combined organic layers were dried over MgSO4, filtered, and
concentrated under reduced pressure. The crude reaction mix-
ture was then subjected to flash chromatography over silica gel
with hexanes-EtOAc (4:1). The bright yellow band was col-
lected and concentrated without heat on a rotary evaporator
(slight decomposition observed). Compound 4c was obtained as
a yellow-brown solid (447 mg, 0.64 mmol, 70%). A sample for
characterization was obtained by crystallization from the mate-
rial isolated after chromatography. Due to the sensitivity of 4c to
Experimental Section
General Methods. Details of the general methods are provided
in the Supporting Information. Compounds 2a-2c, 3a-3c, and
3f were synthesized according to previously published proto-
cols.1s The absorption that appears near 2400 cm-1 in the infra-
red spectra of some of the complexes is due to the B-H stretch of
the hydridotrispyrazolylborate ligand. This absorption is not
always sufficiently strong to allow designation.

5094 Organometallics, Vol. 29, No. 21, 2010
Coombs et al.
purification techniques, it was usually generated and used in situ.
(()-4c: TLC: Rf=0.69 (hexanes-EtOAc, 2:1). IR (cm-1): 1964
(m), 1945 (s), 1861 (s), 1598 (m), 1463 (m). 1H NMR (600 MHz,
CDCl3): δ 8.51 (s, 1 H), 8.10 (s, 1 H), 7.53 (s, 2 H), 7.48 (s, 1 H),
7.36 (s, 1 H), 7.12 (s, 2 H), 6.26 (s, 1 H), 6.14 (s, 1 H), 6.11 (s, 1 H),
5.00 (d, J=7.2 Hz, 1 H), 3.97 (hep, J=6.6 Hz, 2 H), 3.88 (pent,
J=6.6 Hz, 1 H), 3.62 (t, J=7.8 Hz, 1 H), 2.91 (hep, J=6.6 Hz,
1 H), 1.25 (d, J=7.2 Hz, 6 H), 1.17 (d, J=7.2 Hz, 6 H), 1.11 (d,
J=6.6 Hz, 3 H), 0.86 (d, J=6.6 Hz, 6 H). 13C NMR (150 MHz,
CDCl3): δ 231.8, 225.5, 154.4, 151.6 (2), 147.3, 147.0, 139.6,
135.9 (2), 134.6, 129.2, 123.9 (2), 106.0, 105.6, 105.2, 97.7, 73.9,
56.5, 34.5, 29.9, 29.8, 25.1 (2), 23.8 (3), 23.6, 17.5. HRMS (ESI):
calcd for C30H39BMoN6O5S([MþH]þ) 705.1926, found 705.1926.
(()-Dicarbonyl[hydridotris(1-pyrazolyl)borato][(η-1,2,3)-1-(prop-
2-en-1-oxy)-2-buten-1-yl]molybdenum (anti-methyl/syn-allyloxy), 3d.
Silyl ether 2b (1.00 g, 1.82 mmol, 1.0 equiv) was dissolved in THF
(10 mL), and TBAT (1.08 g, 2.00 mmol, 1.1 equiv) was added in
one portion. The solution was stirred for 55 min at room tem-
perature before allyl bromide (1.57 mL, 18.2 mmol, 10.0 equiv)
was added. After 20 h, the solution was concentrated and the
crude product was subjected to flash chromatography over silica
gel with hexanes-EtOAc (4:1), affording 3d as a yellow solid
(699 mg, 1.47 mmol, 81%). (()-3d: TLC: Rf = 0.70 (hexanes-
EtOAc, 2:1). IR (cm-1): 2872 (m), 1918 (s), 1818 (s). 1H NMR
(CDCl3, 400 MHz): δ 8.48 (d, J=2.0 Hz, 1 H), 8.09 (d, J=2.0 Hz,
1 H), 7.72 (d, J=2.0 Hz, 1 H), 7.55 (dd, J=2.0 Hz, 0.4 Hz, 2 H),
7.50 (dd, J=2.0 Hz, 0.4 Hz, 1 H), 6.25 (t, J=2.0 Hz, 1 H), 6.16
(t, J=2.0 Hz, 1 H), 6.12 (t, J=2.4 Hz, 1 H), 5.94-6.04 (m, 1 H),
5.39 (dq, J = 17.2 Hz, 1.6 Hz, 1 H), 5.27-5.31 (m, 2 H), 4.54
(ddt, J=12.8 Hz, 5.6 Hz, 1.6 Hz, 1 H), 4.33 (ddt, J=12.8 Hz, 5.6
Hz, 1.6 Hz, 1 H), 3.86-3.93 (m, 2 H), 1.33 (d, J=6.0 Hz, 3 H).
13C NMR (CDCl3, 150 MHz): δ 232.8, 229.5, 147.2, 145.0, 141.6,
135.9, 135.8, 134.4, 132.7, 118.8, 117.5, 105.8, 105.3, 105.0, 72.8,
66.0, 58.2, 17.9. HRMS (ESI): calcd for C18H22BMoN6O3 ([Mþ
H]þ) 479.0895, found 479.0900.
(()-Dicarbonyl[hydridotris(1-pyrazolyl)borato][(η-1,2,3)-1-((E)-
3-phenylprop-2-en-1-oxy)-2-buten-1-yl]molybdenum (anti-methyl/
syn-cinnamyloxy), (()-3e, and (-)-Dicarbonyl[hydrotris(1-pyra-
zolyl)borato][(1R)-(η-1,2,3)-1-((E)-3-phenylprop-2-en-1-oxy)-2-
buten-1-yl]molybdenum (anti-methyl/syn-cinnamyloxy), (-)-3e.
Complex (()-2b (1.00 g, 1.82 mmol, 1.0 equiv) was dissolved in
THF (10 mL), and TBAT (1.08 g, 2.00 mmol, 1.1 equiv) was
added in one portion. The solution was stirred 55 min at room
temperature before cinnamyl bromide (1.79 g, 9.10 mmol, 5.0
equiv) was added. After 21 h, the solution was concentrated and
the crude product was subjected to flash chromatography over
silica gel with hexanes-EtOAc (4:1), affording (()-3e (809 mg,
1.46 mmol, 80%) as a yellow solid.
Complex (-)-2b (32.4 mg, 0.059 mmol, 1.0 equiv, 91.1% ee)
was dissolved in THF (2 mL), and the solution was cooled to
-78 °C. TBAT (35.0 mg, 0.065 mmol, 1.1 equiv) was added in
one portion, and the solution was stirred for 15 min. Cinnamyl
bromide (11 μL, 0.071 mmol, 1.2 equiv) was added, and the solu-
tion was stirred for 2 h at -78 °C before being concentrated.
Flash chromatography over silica gel with hexanes-EtOAc
(4:1) afforded (-)-3e (17.1 mg, 0.031 mmol, 53%, 95.1% ee)13
([R]25D -212 (c 1.01, CH2Cl2)) as a yellow solid.
C24H25BMoN6O3 (Mþ) 554.1130, found 554.1129. HPLC: Daicel
Chiralcel OJ-RH column, gradient solvent system was used
(% CH3CN in H2O with 0.1% TFA) 0-20 min (50% to 55%),
20-30 min (55% to 60%), 30-40 min (60% to 65%), 1.5 mL/
min, λ=254 nm, (S)-(þ)-3e: t(S)=33.19 min; (R)-(-)-3e: t(R)
=
35.24 min.
(()-Dicarbonyl[hydridotris(1-pyrazolyl)borato][(η-1,2,3)-1-(p-tolu-
enesulfonyloxy)-2-cyclohexen-1-yl]molybdenum, 4d. Complex 2c
(300 mg, 0.52 mmol, 1.0 equiv) was dissolved in THF (10 mL),
and TBAT (338 mg, 0.63 mmol, 1.2 equiv) was added in one
portion. The solution was stirred for 25 min at room tempera-
ture and cooled to 0 °C, and then p-toluenesulfonyl chloride
(120 mg, 0.63 mmol, 1.2 equiv) was added in one portion. The
solution was stirred for 4 h at 0 °C and then concentrated under
reduced pressure. The crude product was subjected to flash chro-
matography over silica gel with hexanes-EtOAc (4:1), afford-
ing 4d (104 mg, 0.17 mmol, 32%) as a yellow-brown solid. (()-
4d: TLC: Rf = 0.56 (hexanes-EtOAc, 4:1). IR (cm-1): 3150-
1
2853 (m), 1934 (s), 1849 (s), 1409 (s), 1505 (m), 1598 (w). H
NMR (CDCl3, 600 MHz): δ 8.49 (d, J = 1.8 Hz, 1 H), 7.90 (d,
J=2.4 Hz, 1 H), 7.57 (d, J=1.8 Hz, 1 H), 7.56 (d, J=1.8 Hz,
1 H), 7.46 (d, J=2.4 Hz, 1 H), 7.39 (d, J=2.4 Hz, 1 H), 7.34 (d,
J =8.4 Hz, 2 H), 7.00 (d, J =8.4 Hz, 2 H), 6.24 (t, J =2.4 Hz,
1 H), 6.13 (t, J=2.4 Hz, 1 H), 6.10 (t, J=2.4 Hz, 1 H), 3.92 (s,
2 H), 2.69-2.64 (m, 1 H), 2.31 (s, 3 H), 2.24-2.21 (m, 1 H), 2.10
(dt, J=13.8 H, 5.4 Hz, 1 H), 1.84-1.81 (m, 1 H), 1.26-1.20 (m,
1 H), 0.58-0.52 (m, 1 H). 13C NMR (150 MHz, CDCl3): δ 229.0,
226.1, 146.9, 146.8, 144.6, 139.0, 136.0, 135.9, 135.2, 134.4, 129.5
(2), 127.6 (2), 114.4, 105.9, 105.4, 105.3, 66.0, 58.6, 30.0, 22.4,
21.7, 20.5. HRMS (ESI): calcd for C24H26BMoN6O5S ([M þ
H]þ) 619.0827, found 619.0829.
(()-Dicarbonyl[hydridotris(1-pyrazolyl)borato][(η-1,2,3)-1-(2,4,
6-triisopropylbenzenesulfonyloxy)-2-cyclohexen-1-yl]molybdenum,
(()-4e, and (þ)-Dicarbonyl[hydrotris(1-pyrazolyl)borato][(1R)-
(η-1,2,3)-1-(2,4,6-triisopropylbenzenesulfonyloxy)-2-cyclohexen-
1-yl]molybdenum, (þ)-4e. Complex 2c (500 mg, 0.87 mmol, 1.0
equiv) was dissolved in THF (10 mL), and TBAT (516 mg, 0.96
mmol, 1.1 equiv) was added in one portion at room temperature.
After 15 min, the solution was cooled to -78 °C, and 2,4,6-tri-
isopropylbenzenesulfonyl chloride (291 mg, 0.96 mmol, 1.1 equiv)
was added in one portion. (Note: It is important that 2,4,6-tri-
isopropylbenzenesulfonyl chloride be added to the solution at -78 oC.
Addition at higher temperatures leads to severe decomposition.)
The reaction mixture was immediately warmed to 0 °C, where it
was maintained for 4.5 h before being passed through a short
pad of silica gel and concentrated. The crude material was
subjected to flash chromatography over silica gel with hexa-
nes-EtOAc (4:1), and the yellow band was collected to afford
(()-4e (411 mg, 0.56 mmol, 65%) as a yellow solid.
Similar treatment of (þ)-2c (199 mg, 0.34 mmol, 1.0 equiv,
97.5% ee) with TBAT (202 mg, 0.37 mmol, 1.1 equiv) and 2,4,6-
triisopropylbenzenesulfonyl chloride (112 mg, 0.37 mmol, 1.1
equiv) in THF (5 mL) afforded (þ)-4e (56.6 mg, 0.078 mmol,
23%, 96.5% ee) [[R]25D þ82 (c 0.18, CH2Cl2)] as a bright yellow
solid.
(()-4e: TLC: Rf = 0.72 (hexanes-EtOAc, 2:1). IR (cm-1):
1
2961 (m), 1938 (s), 1849 (s), 1602 (m), 1505 (m), 1463 (m). H
(()-3e: TLC: Rf = 0.67 (hexanes-EtOAc, 2:1). IR (cm-1):
3030 (m), 1918 (s), 1818 (s), 1505 (s), 1409 (s). 1H NMR (CDCl3,
600 MHz): δ 8.49 (d, J=1.8 Hz, 1 H), 8.15 (d, J=1.8 Hz, 1 H),
7.73 (d, J=1.8 Hz, 1 H), 7.57 (d, J=2.4 Hz, 2 H), 7.51 (d, J=2.4
Hz, 1 H), 7.41 (d, J=7.2 Hz, 2 H), 7.34 (t, J=7.2 Hz, 2 H), 7.28
(t, J=7.2 Hz, 1 H), 6.68 (d, J=16.2 Hz, 1 H), 6.35 (dt, J=16.2
Hz, 6.6 Hz, 1 H), 6.26 (t, J = 2.4 Hz, 1 H), 6.17 (t, J = 2.4 Hz,
1 H), 6.12 (t, J=2.4 Hz, 1 H), 5.38-5.39 (m, 1 H), 4.69 (ddd, J=
12.6 Hz, 6.0 Hz, 1.2 Hz, 1 H), 4.49 (ddd, J=13.2 Hz, 6.6 Hz, 1.8
Hz, 1 H), 3.91-3.95 (m, 2 H), 1.34 (d, J=6.0 Hz, 3 H). 13C NMR
(CDCl3, 100 MHz): δ 232.9, 229.6, 147.2, 145.1, 141.6, 136.4,
136.0, 135.8, 134.4, 134.3, 128.8 (2), 128.3, 126.8 (2), 123.7, 117.5,
105.9, 105.3, 105.1, 72.7, 66.2, 58.2, 18.0. HRMS (ESI): calcd for
NMR (600 MHz, CDCl3): δ 8.53 (d, J = 1.8 Hz, 1 H), 7.69 (d,
J=2.4 Hz, 1 H), 7.62 (d, J=1.8 Hz, 1 H), 7.57 (d, J=2.4 Hz,
1 H), 7.47 (d, J=2.4 Hz, 1 H), 7.42 (d, J=2.4 Hz, 1 H), 7.11 (s,
2 H), 6.25 (t, J=2.4 Hz, 1 H), 6.19 (t, J=2.4 Hz, 1 H), 5.69 (t,
J=2.4 Hz, 1 H), 4.36 (d, J=7.8 Hz, 1 H), 4.10 (hep, J=6.6 Hz,
2 H), 4.03 (br d, J = 8.4 Hz, 1 H), 2.89 (hep, J = 6.6 Hz, 1 H),
2.59-2.64 (m, 1 H), 2.21 (dd, J=15.0 Hz, 6.0 Hz, 1 H), 2.11 (dt,
J=13.2 Hz, 6.0 Hz, 1 H), 1.87 (dt, J=14.4 Hz, 4.8 Hz, 1 H), 1.25
(d, J=6.6 Hz, 6 H), 1.19-1.21 (m, 1 H), 1.17 (d, J=6.6 Hz, 6 H),
1.00 (d, J = 6.6 Hz, 6 H), 0.52-0.59 (m, 1 H). 13C NMR (100
MHz, CDCl3): δ 229.0, 226.5, 154.1, 150.8 (2), 146.9, 146.1,
139.5, 136.2, 135.8, 134.3, 131.4, 123.9 (2), 114.7, 105.9, 105.6, 104.4,
66.9, 59.6, 34.4, 29.8 (3), 24.5 (2), 24.4 (2), 23.7 (2), 22.6, 20.7.

Article
Organometallics, Vol. 29, No. 21, 2010 5095
HRMS (ESI): calcd for C32H41BMoN6O5S ([M]þ) 731.2079,
found 731.2079. HPLC: Daicel Chiralcel OJ-RH column, gra-
dient solvent system was used (% CH3CN in H2O with 0.1%
TFA) 0-20 min (50% to 75%), 1.5 mL/min, λ = 254 nm,
(S)-(-)-4e: t(S) =18.36 min; (R)-(þ)-4e: t(R) = 17.32 min.
Direct Exchange of Alkoxy Substituents with Amines. (()-Di-
carbonyl[hydridotris(1-pyrazolyl)borato][(η-1,2,3)-1-benzylamino-
2-propen-1-yl]molybdenum (syn-benzylamino), 5a. Methoxy-
substituted complex 3a (100 mg, 0.23 mmol, 1.0 equiv) was
dissolved in THF (3 mL), and benzyl amine (0.75 mL, 6.88
mmol, 30.0 equiv) was added. After 20 h, the reaction mixture
was concentrated and the crude material was subjected to
flash chromatography over silica gel with hexanes-ethyl ace-
tate (4:1). The red band was collected, affording 5a (112 mg,
0.22 mmol, 95%) as a red solid. (()-5a: TLC: Rf=0.45 (hexanes-
EtOAc, 2:1). IR (cm-1): 3397 (m), 3034 (w), 2478 (m), 1903 (s),
1783 (s), 1698 (m), 1571 (s), 1502 (s), 1455 (m). 1H NMR (400
MHz, C6D6): δ 8.41 (d, J = 1.6 Hz, 1 H), 7.51 (d, J = 1.2 Hz,
1 H), 7.46 (d, J=2.0 Hz, 1 H), 7.39 (d, J=2.0 Hz, 1 H), 7.30 (d,
J=2.0 Hz, 1 H), 7.23 (d, J=2.4 Hz, 1 H), 7.03-7.07 (m, 3 H),
6.75-6.77 (m, 2 H), 5.88 (t, J = 2.0 Hz, 1 H), 5.85 (t, J = 2.0
Hz, 1 H), 5.74 (t, J=2.0 Hz, 1 H), 5.27 (app t, J=9.6 Hz, 1 H),
3.36-3.41 (m, 1 H), 3.24-3.32 (m, 3 H), 2.57-2.63 (m, 1 H),
1.48 (dd, J=8.8 Hz, 4.8 Hz, 1 H). 13C NMR (100 MHz, C6D6):
δ 234.5, 231.9, 147.5, 144.7, 140.1, 137.7, 136.2, 135.6, 134.9,
133.1, 129.1 (2), 128.5 (2), 106.3, 105.7, 105.6, 77.9, 57.1, 49.5,
44.9. HRMS (ESI): calcd for C21H23BMoN7O2 ([M þ H]þ)
514.1054, found 514.1060.
(1.0 M in hexanes, 0.092 mL, 0.092 mmol, 0.40 equiv) were added
sequentially. The flask was sealed and the solution was stirred
overnight at room temperature. After 26 h, the solution was con-
centrated and the crude reaction mixture was subjected to flash
chromatography over silica gel with hexanes-EtOAc (9:1). The
yellow band was collected, affording 3b (79.6 mg, 0.17 mmol,
72%) as a yellow solid.
Conversion of 3b to 3a. Isopropyloxy-substituted complex 3b
(43.0 mg, 0.093 mmol, 1.0 equiv) was dissolved in CH2Cl2 (2 mL),
and MeOH (75 μL, 1.85 mmol, 20 equiv) and EtAlCl2 (1.0 M in
hexanes, 0.037 mL, 0.037 mmol, 0.4 equiv) were added sequen-
tially. The solution was stirred 4 h at room temperature and then
concentrated. Flash chromatography over silica gel with hexanes-
EtOAc (4:1) afforded 3a (35.1 mg, 0.080 mmol, 87%) as a yellow
solid.
Direct Exchange of Sulfonate Ester Substituents with Amines
and Alcohols. Conversion of 2a to 5a through Intermediate 4a. Com-
plex 2a (50.0 mg, 0.093 mmol, 1.0 equiv) was dissolved in THF
(2 mL), and TBAT (55.4 mg, 0.10 mmol, 1.1 equiv) was added in one
portion at room temperature. The solution was stirred for 5 min
before 2,4,6-triisopropylbenzenesulfonyl chloride (30.3 mg, 0.10
mmol, 1.1 equiv) was added in one portion at room temperature.
The reaction mixture was stirred for 10 min before benzyl amine
(0.31 mL, 2.79 mmol, 30 equiv) and MeOH (2 mL) were added, and
the solution was refluxed for 2 h. The reaction mixture was con-
centrated on a rotary evaporator and then subjected to flash chro-
matography over silica gel with hexanes-EtOAc (4:1). The red
band was collected, affording 5a (37.2 mg, 0.073 mmol, 78%) as a
red solid.
(()-Dicarbonyl[hydridotris(1-pyrazolyl)borato][(η-1,2,3)-1-ben-
zylamino-2-buten-1-yl]molybdenum (anti-methyl/syn-benzylamino),
5b. Methoxy-substituted molybdenum complex 3c (25.0 mg,
0.056 mmol, 1.0 equiv) was dissolved in THF (2 mL), benzyl amine
(0.18 mL, 1.7 mmol, 30 equiv) was added, and the solution was
heated to reflux. After refluxing for 20.5 h, the solution was
concentrated and the residue was subjected to flash chromato-
graphy over silica gel with hexanes-EtOAc (4:1). The red band
was collected to afford 5b as a red solid (21.8 mg, 0.042 mmol,
74%). (()-5b: TLC: Rf=0.53 (hexanes-EtOAc, 2:1). IR (cm-1):
3401 (br), 2922 (m), 1899 (s), 1783 (s), 1575 (s), 1502 (s), 1455
(m). 1H NMR (400 MHz, C6D6): δ 8.42 (d, J=2.0 Hz, 1 H), 7.48
(d, J=2.0 Hz, 1 H), 7.45 (d, J=2.0 Hz, 1 H), 7.39 (d, J=2.0 Hz,
1 H), 7.23-7.24 (m, 2 H), 7.03-7.08 (m, 3 H), 6.71-6.73 (m, 2
H), 6.01 (dd, J=11.2 Hz, 9.2 Hz, 1 H), 5.87 (q, J=2.4 Hz, 2 H),
5.74 (t, J = 2.4 Hz, 1 H), 3.92 (app pent, J = 6.8 Hz, 1 H),
3.38-3.40 (m, 1 H), 3.27 (dd, J=14.4 Hz, 4.8 Hz, 1 H), 3.04 (dd,
J = 14.4 Hz, 5.2 Hz, 1 H), 2.84 (dd, J = 11.2 Hz, 8.4 Hz, 1 H),
1.73 (d, J=6.4 Hz, 3 H). 13C NMR (100 MHz, C6D6): δ 235.6,
232.1, 147.4, 145.1, 140.3, 137.6, 136.2, 135.5, 135.0, 130.9, 129.0
(2), 128.4 (2), 106.2, 105.7, 105.6, 62.4, 54.3, 48.9, 19.7. HRMS
(ESI): calcd for C22H25BMoN7O2 ([M þ H]þ) 528.1211, found
528.1217. HPLC: Daicel Chiralcel OD-RH column, gradient
solvent system was used (% CH3CN in H2O with 0.1% TFA)
0-20 min (50% to 75%), 1.5 mL/min, λ=254 nm, enantiomer
1: t1 = 13.55 min; enantiomer 2: t2 =15.21 min.
Conversion of 3e to 5b and (-)-3e to 5b. Cinnamyloxy-
substituted molybdenum complex 3e (25.0 mg, 0.045 mmol,
1.0 equiv) was dissolved in THF (2 mL), benzyl amine (0.15 mL,
1.4 mmol, 30 equiv) was added, and the solution was heated to
reflux. After 24 h, the solution was concentrated and the crude
material was subjected to flash chromatography over silica gel
with hexanes-EtOAc (4:1). The red band was collected, afford-
ing 5b (13.7 mg, 0.026 mmol, 58%) as a red solid.
Similar treatment of (-)-3e (17.1 mg, 0.031 mmol, 1.0 equiv,
95.1% ee) with BnNH2 (0.10 mL, 0.93 mmol, 30 equiv) in THF
(1 mL) afforded 5b (7.2 mg, 0.014 mmol, 44%, 3.5% ee) as a red
solid.
Conversion of (()-2b to 5b and (-)-2b to 5b through Inter-
mediate 4b. Silyl ether (()-2b (100 mg, 0.18 mmol, 1.0 equiv) was
dissolved in THF (2 mL), and TBAT (108 mg, 0.20 mmol, 1.1
equiv) was added in one portion. After 5 min, the reaction
mixture was cooled to 0 °C and p-TsCl (38.1 mg, 0.20 mmol, 1.1
equiv) was added. After 10 min, MeOH (2 mL) and benzyl amine
(0.59 mL, 5.40 mmol, 30 equiv) were added, and the solution was
heated to reflux for 2 h and concentrated. Flash chromato-
graphy over silica gel with hexanes-EtOAc (4:1) afforded 5b
(50.2 mg, 0.096 mmol, 53%) as a red solid.
Silyl ether (-)-2b (50.0 mg, 0.091 mmol, 1.0 equiv, 96.2% ee)
was dissolved in THF (1.5 mL), and TBAT (54.0 mg, 0.10 mmol,
1.1 equiv) was added in one portion. After 5 min, the reaction
was cooled to 0 °C and p-TsCl (19.1 mg, 0.10 mmol, 1.1 equiv)
was added. After 10 min, a sample of the crude reaction mixture
was analyzed by chiral HPLC, and tosylate 4b was determined
to have a 91.0% ee. MeOH (1.5 mL) and benzyl amine (0.30 mL,
2.73 mmol, 30 equiv) were added to the reaction, and the solu-
tion was heated to reflux for 3 h and then concentrated. Flash
chromatography over silica gel with hexanes-EtOAc (4:1) affor-
ded 5b (28.3 mg, 0.054 mmol, 59%, 1.1% ee) as a red solid.
Conversion of (()-2b to 5b through Intermediate 4c. Complex
2b (100 mg, 0.18 mmol, 1.0 equiv) was dissolved in THF (2 mL),
and tetrabutylammonium triphenyldifluorosilicate (TBAT)
(108 mg, 0.20 mmol, 1.1 equiv) was added in one portion at room
temperature. The solution was stirred for 5 min before 2,4,6-
triisopropylbenzenesulfonyl chloride (60.6 mg, 0.20 mmol, 1.1
equiv) was added in one portion at room temperature. After
stirring for 10 min, MeOH (2 mL) and benzyl amine (0.59 mL,
5.4 mmol, 30 equiv) were added and the solution was refluxed
for 2 h. The reaction mixture was then concentrated under
reduced pressure and passed through a short pad of silica gel.
The solution was again concentrated and subjected to flash
chromatography over silica gel with hexanes-EtOAc (2:1). The
red band was collected, affording 5b as a red solid (70.1 mg,
0.13 mmol, 74%).
(()-Dicarbonyl[hydridotris(1-pyrazolyl)borato][(η-1,2,3)-1-ben-
zylamino-2-cyclohexen-1-yl]molybdenum, 5c. Complex 4d (52.0 mg,
0.084 mmol, 1.0 equiv) was dissolved in THF (2 mL), benzyl amine
(0.14 mL, 1.27 mmol, 15 equiv) was added, and the solution was
heated to reflux. After 18 h, the solution was concentrated and the
Lewis Acid-Catalyzed Exchange of Alkoxy Substituents with
Alcohols. Conversion of 3a to 3b. Methoxy-substituted com-
plex 3a (100 mg, 0.23 mmol, 1.0 equiv) was dissolved in CH2Cl2
(3 mL), and i-PrOH (0.35 mL, 4.59 mmol, 20 equiv) and EtAlCl2

5096 Organometallics, Vol. 29, No. 21, 2010
Coombs et al.
crude reaction mixture was subjected to flash chromatography over
silica gel with hexanes-EtOAc (4:1). The red band was collected,
affording 5c as a red solid (20.9 mg, 0.038 mmol, 45%). (()-5c:
TLC: Rf = 0.53 (hexanes-EtOAc, 4:1). IR (cm-1): 3374 (br, w),
2930 (m), 2478 (m), 1888 (s), 1772 (s). 1H NMR (C6D6, 400 MHz):
δ 8.45 (d, J=1.2 Hz, 1 H), 7.52 (s, 1 H), 7.50 (d, J=2.0 Hz, 1 H),
7.45 (d, J=2.0 Hz, 1 H), 7.37 (d, J=1.2 Hz, 1 H), 7.26 (d, J=2.0
Hz, 1 H), 7.02-7.06 (m, 3 H), 6.70 (d, J=6.4 Hz, 2 H), 5.88-5.90
(m, 2 H), 5.74 (t, J=1.6 Hz, 1 H), 3.93-3.98 (m, 2 H), 3.18-3.44
(m, 2 H), 2.50-2.53 (m, 1 H), 2.35-2.38 (m, 1 H), 2.17-2.22 (m,
1 H), 1.70-1.84 (m, 2 H), 1.23-1.37 (m, 2 H). 13C NMR (C6D6,
100 MHz): δ 233.9 (2), 147.5, 144.2, 140.3, 137.7, 136.2, 135.9,
134.8, 129.1 (2), 128.9 (2), 127.7, 106.2, 105.6, 105.5, 56.7, 53.1, 47.1,
26.3, 25.2, 19.0. HRMS (ESI): calcd for C24H27BMoN7O2 ([M þ
H]þ): 554.1368, found 554.1361. HPLC: Daicel Chiralcel OD-RH
column, gradient solvent system was used (% CH3CN in H2O with
0.1% TFA) 0-20 min (50% to 75%), 1.5 mL/min, λ = 254 nm,
enantiomer 1: t1 =14.37 min; enantiomer 2: t2 =15.84 min.
Conversion of (()-4e to 5c and(þ)-4e to 5c. Complex 4e (50.0 mg,
0.069 mmol, 1.0 equiv) was dissolved in THF (2 mL), benzyl amine
(0.23 mL, 2.06 mmol, 30 equiv) was added, and the solution was
heated to reflux. After 17 h, the solution was concentrated and the
crude reaction mixture was subjected to flash chromatography over
silica gel with hexanes-EtOAc (4:1). The red band was collected,
affording 5c as a red solid (12.1 mg, 0.022 mmol, 32%).
Similar treatment of (þ)-4e (56.0 mg, 0.077 mmol, 1.0 equiv)
with BnNH2 (0.25 mL, 2.31 mmol, 30 equiv) in THF (1 mL)
afforded 5c (16.1 mg, 0.029 mmol, 38%, 30.6% ee) as a red solid.
Conversion of 2a to 3a through Intermediate 4a. Complex 2a
(50.0 mg, 0.093 mmol, 1.0 equiv) was dissolved in THF (2 mL),
and TBAT (55.4 mg, 0.10 mmol, 1.1 equiv) was added in one
portion. After 5 min, 2,4,6-triisopropylbenzenesulfonyl chloride
(30.3 mg, 0.10 mmol, 1.0 equiv) was added in one portion. After
10 min, MeOH (2 mL) was added, and the solution was stirred
17 h at room temperature and concentrated. Flash chromato-
graphy over silica gel with hexanes-EtOAc (4:1) afforded 3a
(29.7 mg, 0.068 mmol, 73%) as a yellow solid.
Conversion of 2b to 3c through Intermediate 4c. Complex 2b
(100 mg, 0.18 mmol, 1.0 equiv) was dissolved in THF (2 mL),
and TBAT (108 mg, 0.20 mmol, 1.10 equiv) was added in one
portion at room temperature. After 5 min, 2,4,6-triisopropyl-
benzenesulfonyl chloride (60.6 mg, 0.20 mmol, 1.1 equiv) was
added in one portion at room temperature. The solution was
stirred for 10 min before MeOH (2 mL) was added. After 14 h,
the reaction mixture was poured into a separatory funnel
containing water (10 mL) and CH2Cl2 (10 mL). The layers were
separated, and the aqueous layer was extracted with CH2Cl2
(10 mL). The combined organic layers were dried over MgSO4,
filtered, and concentrated. Flash chromatography over silica gel
with hexanes-EtOAc (9:1) afforded 3c (59.5 mg, 0.13 mmol,
72%) as a yellow solid.
Dicarbonyl[hydridotris(1-pyrazolyl)borato][η-(1,2,3)-1-((S)-(-)-2-
acetoxypropionyloxy)-2-cyclohexen-1-yl]molybdenum, 8. Com-
plex 2c (1.75 g, 3.05 mmol, 1.0 equiv) was dissolved in CH2Cl2
(10 mL), and TBAT (1.98 g, 3.66 mmol, 1.2 equiv) was added in
one portion. The solution was stirred 30 min at room tempera-
ture before (S)-(-)-acetoxypropionyl chloride (0.42 mL, 3.36
mmol, 1.1 equiv) was added dropwise. After stirring 10 min at
room temperature, the solution was concentrated and the crude
product was subjected to flash chromatography over silica gel
with hexanes-EtOAc (4:1). Recrystallization from dichloro-
methane and hexanes provided 8, a 1:1 mixture of two diaster-
eomers (1.56 g, 2.71 mmol, 89%), as a yellow solid. (()-8: TLC:
Rf=0.51 (hexanes-EtOAc, 4:1). IR (cm-1): 2941 (m), 2849 (w),
2482 (m), 2258 (w), 1934 (s), 1849 (s), 1749 (s). 1H NMR (CDCl3,
400 MHz): δ 8.55 (d, J=2.0 Hz, 1 H), 8.54 (d, J=2.0 Hz, 1 H),
8.04 (d, J=1.6 Hz, 1 H), 8.03 (d, J=2.0 Hz, 1 H), 7.66 (d, J=2.0
Hz, 1 H), 7.63 (d, J=1.6 Hz, 1 H), 7.60 (d, J=2.4 Hz, 2 H), 7.56
(d, J=2.4 Hz, 2 H), 7.50 (dd, J=2.4 Hz, 0.8 Hz, 1 H), 7.48 (d,
J=2.0 Hz, 1 H), 6.25-6.27 (m, 2 H), 6.20-6.21 (dd, J=4.0 Hz,
2.4 Hz, 2 H), 6.13-6.15 (dd, J =4.0 Hz, 2.4 Hz, 2 H), 5.00 (m,
2 H), 4.04-4.12 (m, 3 H), 3.91 (m, 1 H), 2.59-2.41 (m, 2 H),
2.23-2.14 (m, 2 H), 2.12 (s, 3 H), 2.10 (s, 3 H), 2.05-1.85 (m, 4 H),
1.47 (d, J=7.2 Hz, 3 H), 1.20-1.29 (m, 2 H), 1.13 (d, J=6.8 Hz,
3 H), 0.66 (m, 2 H). 13C NMR (CDCl3, 100 MHz): δ 229.0, 228.5,
226.7, 226.4, 170.6, 170.4, 170.3, 169.8, 147.0, 146.8, 146.3, 140.0,
139.6, 136.4, 134.4, 111.5, 111.1, 106.0, 105.6, 105.0, 104.8, 69.3,
69.1, 67.5, 66.6, 60.3, 59.0, 27.8, 22.7, 22.7, 20.8, 20.1, 20.0, 17.1,
16.4. HRMS (ESI): calcd for C22H26BMoN6O6 ([M þ H]þ)
579.1055, found 579.1058.
Dicarbonyl[hydridotris(1-pyrazolyl)borato][η-(1,2,3)-1-((1R,2S,
5R)-2-isopropyl-5-methylcyclohexanoxycarbonyl)-2-cyclohexen-
1-yl]molybdenum, 10. Complex 2c (672 mg, 1.17 mmol, 1.0 equiv)
was dissolved in CH2Cl2 (6 mL), and TBAT (756 mg, 1.40 mmol,
1.2 equiv) was added in one portion at room temperature. The
solution was stirred for 30 min before (-)-menthyl chloroformate
(0.27 mL, 1.29 mmol, 1.1 equiv) was added dropwise. After 3 h, the
solution was concentrated and the crude product was subjected to
flash chromatography over silica gel with hexanes-EtOAc (4:1),
affording 10, a 1:1 mixture of two diastereomers (677 mg, 1.05
mmol, 90%), as a yellow solid. (()-10: TLC: Rf=0.27 (hexanes-
EtOAc, 4:1). IR (cm-1): 2957 (m), 2872 (m), 2478 (m), 1938 (s),
1849 (s), 1749 (s). 1H NMR (CDCl3, 400 MHz): δ 8.56 (br s, 1 H),
8.14 (d, J=2.0 Hz, 0.5 H), 8.12 (d, J=2.0 Hz, 0.5 H), 7.72 (d, J=
2.0 Hz, 0.5 H), 7.70 (d, J=1.6 Hz, 0.5 H), 7.63-7.65 (m, 2 H), 7.61
(br s, 1 H), 7.54 (d, J=2.0 Hz, 0.5 H), 7.53 (d, J=2.0 Hz, 0.5 H),
7.49-7.50 (m, 2 H), 7.37-7.48 (m, 3 H), 6.26-6.28 (m, 2 H),
6.19-6.21 (m, 2 H), 6.09 (t, J=2.4 Hz, 1 H), 6.04 (t, J=2.4 Hz,
1H),4.52(dt, J=10.8 Hz, 4.8 Hz, 1 H), 4.42 (dt, J=10.8 Hz, 4.8 Hz,
1 H), 4.21 (d, J=7.6 Hz, 1 H), 4.16 (d, J=8.0 Hz, 1 H), 4.06-4.12
(m, 2 H), 2.47-2.58 (m, 2 H), 2.11-2.24 (m, 4 H), 1.86-2.02 (m,
4 H), 1.62-1.71 (m, 6 H), 1.22-1.50 (m, 6 H), 0.78-1.13 (m, 6 H),
0.94 (d, J=1.2Hz, 3H), 0.93(s, 3H), 0.85(d, J=6.4 Hz, 3 H), 0.83
(d, J=6.8 Hz, 3 H), 0.76 (d, J=6.8 Hz, 3 H), 0.57-0.68 (m, 2 H),
0.55 (d, J=7.2 Hz, 3 H). 13C NMR (CDCl3, 100 MHz): δ 228.5,
228.4, 226.9, 226.8, 153.7, 153.6, 147.1, 146.5, 140.4, 140.2, 136.3,
136.2, 136.1, 135.2, 134.3, 131.1, 130.3, 128.3, 128.2, 112.6, 112.3,
106.0, 105.5, 104.9, 104.7, 79.3, 79.1, 65.7, 65.6, 61.5, 60.9, 47.4,
47.3, 40.9, 40.8, 34.3 (2), 31.6, 28.2, 28.1, 26.5, 25.9, 23.7, 23.4, 22.7,
22.2, 22.1, 21.0 (2), 20.1 (2), 16.7, 16.3. HRMS (ESI): calcd for
C28H38BMoN6O5 ([Mþ H]þ) 647.2045, found 647.2054.
(þ)-Dicarbonyl[hydridotris(1-pyrazolyl)borato][(1R)-(η-1,2,3)-
1-(((R)-3-hydroxycarbonyl-4,4-dimethyldihydrofuran-2(3H)-one)-
oxy)-2-cyclohexen-1-yl]molybdenum, (þ)-(1R,R)-11, and (-)-Di-
carbonyl[hydrotris(1-pyrazolyl)borato][(1S)-(η-1,2,3)-1-(((R)-3-
hydroxycarbonyl-4,4-dimethyldihydrofuran-2(3H)-one)oxy)-2-cyclo-
hexen-1-yl]molybdenum, (-)-(1S,R)-11. Complex 2c (2.00 g, 3.47
mmol, 1.0 equiv) was dissolved in CH2Cl2 (15 mL), and TBAT
(2.25 g, 4.17 mmol, 1.2 equiv) was added in one portion at room
temperature. After 15 min, (R)-(-)-pantolactone chloroformate
(735 mg, 3.82 mmol, 1.1 equiv) was added dropwise. The solution
was stirred for 45 min, concentrated under reduced pressure, and
subjected to flash chromatography over silica gel with hexanes-
EtOAc (4:1). The unseparated portion of the diastereomeric
mixture was subjected to a second chromatography under the
same conditions. Highly diastereomerically enriched samples of
each diastereomer were thus obtained. Recrystallization of the R,R
diastereomer from CH2Cl2/hexanes improved the dr of the crystals
from 98.5:1.5 to >99.9:0.1 (supernatant dr = 96.2:3.8). Recrys-
tallization of the supernatant from CH2Cl2/hexanes brought the dr
of the crystals to >99.9:0.1. The supernatant (dr = 90.2:9.8) was
set aside. The S,R diastereomer was recrystallized from CH2Cl2/
hexanes, improving the dr of the crystals from 88.8:11.2 to >99.9:0.1.
The supernatant (dr = 80.4:19.6) was combined with the super-
natant from the R,R diastereomer and again chromatographed
with hexanes-EtOAc (4:1). Recrystallization from CH2Cl2/hexanes
provided additional R,R diastereomer in dr >99.9:0.1. Following
this protocol, (1R,R)-11 (850 mg, 1.38 mmol, 40%) {[R]25 þ54
D
(c 1.05, CH2Cl2)} and(1S,R)-11 (741 mg, 1.20 mmol, 35%) {[R]25
D
-36 (c 1.02, CH2Cl2)} were obtained as bright yellow solids.

Article
Organometallics, Vol. 29, No. 21, 2010 5097
HPLC: Agilent Eclipse XDB-C8 column (% CH3CN in H2O with
0.1% TFA) 0-10 min (50% to 75%), 10-12 min (75% to 90%),
1.5 mL/min, λ = 254 nm, (-)-(1S,R)-11: t(1S,R) = 10.16 min;
(þ)-(1R, R)-11: t(1R,R) = 10.79 min.
1 H), 7.60(d, J=1.6 Hz, 1 H), 7.58 (d, J=2.4 Hz, 1 H), 7.57 (d, J=
2.4 Hz, 1 H), 7.48 (d, J=2.4 Hz, 1 H), 6.27 (t, J=2.0 Hz, 1 H), 6.23
(t, J=2.0 Hz, 1 H), 6.18 (t, J=2.0 Hz, 1 H), 5.99 (d, J=7.6 Hz, 1
H), 5.34 (s, 1 H), 4.18 (t, J=7.6 Hz, 1 H), 4.05-4.07 (m, 1 H), 4.03
(þ)-(1R,R)-11: TLC: Rf = 0.60 (hexanes-EtOAc, 4:1). IR
(cm-1): 3146 (w), 2941 (m), 2482 (m), 2258 (w), 1938 (s), 1849 (s),
1791 (s), 1756 (s). 1H NMR (CDCl3, 600 MHz): δ 8.56 (d, J=1.8
Hz, 1 H), 8.15 (d, J=2.4 Hz, 1 H), 7.70 (d, J=1.8 Hz, 1 H), 7.62
(d, J=1.8 Hz, 1 H), 7.54 (d, J=1.8 Hz, 1 H), 7.50 (d, J=1.8 Hz,
1 H), 6.28 (t, J=2.4 Hz, 1 H), 6.22 (t, J=1.8 Hz, 1 H), 6.07 (t,
J=2.4 Hz, 1 H), 5.30 (s, 1 H), 5.12 (s, 1 H), 4.15 (d, J=7.8 Hz,
1 H), 4.09-4.10 (m, 1 H), 4.01 (dd, J = 9.0 Hz, 2.4 Hz, 2 H),
2.63-2.58 (m, 1 H), 2.20-2.16 (m, 1 H), 1.95-1.91 (dt, J=13.8
Hz, 4.8 Hz, 1 H), 1.32-1.28 (dt, J=14.4 Hz, 5.4 Hz, 1 H), 1.12
(s, 3 H), 1.03 (s, 3 H), 0.65-0.59 (m, 1 H). 13C NMR (CDCl3, 150
MHz): δ 228.2, 225.8, 171.7, 153.2, 147.1, 146.7, 139.9, 136.4,
136.3, 134.4, 111.7, 106.0, 105.5, 104.8, 97.9, 78.7, 76.2, 65.1,
40.2, 27.7, 23.1, 22.6, 20.1, 19.9. HRMS (ESI): calcd for
C24H27BMoN6O7 ([Mþ H]þ) 621.1161, found 621.1163.
(-)-(1S, R)-11: TLC: Rf = 0.62 (hexanes-EtOAc, 4:1). IR
(cm-1): 3146 (w), 2941 (m), 2482 (m), 2258 (w), 1938 (s), 1849 (s),
1791 (s), 1756 (s). 1H NMR (CDCl3, 600 MHz): δ 8.56 (d, J=1.8
Hz, 1 H), 8.15 (d, J=2.4 Hz, 1 H), 7.70 (d, J=1.8 Hz, 1 H), 7.62
(d, J=1.8 Hz, 1 H), 7.54 (d, J=1.8 Hz, 1 H), 7.50 (d, J=1.8 Hz,
1 H), 6.28 (t, J=2.4 Hz, 1 H), 6.22 (t, J=1.8 Hz, 1 H), 6.07 (t,
J=2.4 Hz, 1 H), 5.30 (s, 1 H), 5.12 (s, 1 H), 4.15 (d, J=7.8 Hz,
1 H), 4.09-4.11 (m, 1 H), 4.01 (dd, J = 20.4 Hz, 9.0 Hz, 2 H),
2.63-2.58 (m, 1 H), 2.20-2.16 (m, 1 H), 1.95-1.91 (dt, J=13.8
Hz, 4.8 Hz, 1 H), 1.32-1.28 (dt, J=14.4 Hz, 5.4 Hz, 1 H), 1.12
(s, 3 H), 1.03 (s, 3 H), 0.65-0.59 (m, 1 H). 13C NMR (CDCl3, 150
MHz): δ 228.2, 225.8, 171.7, 153.2, 147.1, 146.7, 139.9, 136.4,
134.4, 111.7, 106.0, 105.5, 104.8, 97.9, 78.7, 76.2, 65.1, 60.5, 40.2,
27.7, 23.1, 22.6, 20.1, 19.9. HRMS (ESI): calcd for C24H27BMo-
N6O7 ([M þH]þ) 621.1161, found 621.1164.
(s, 2 H), 1.28 (d, J=6.4 Hz, 3 H), 1.25 (s, 3 H), 1.11 (s, 3 H). 13
C
NMR (CDCl3, 100 MHz): δ 231.9, 226.1, 171.3, 153.4, 146.9,
146.5, 139.8, 136.1, 135.9, 134.5, 106.1, 105.8, 105.5, 100.6, 79.2,
76.1, 70.4, 55.0, 40.1, 22.8, 19.8, 17.9. HRMS (ESI): calcd for
C22H26BMoN6O7 ([Mþ H]þ) 595.1005, found 595.1005.
Conversion of (1R,R)-11 to (þ)-2c. Complex (1R,R)-11 (448 mg,
0.73 mmol, 1.0 equiv, 97.5% de) was dissolved in THF (10 mL) and
cooled to 0 °C. MeLi (1.6 M in diethyl ether, 2.28 mL, 3.65 mmol,
5.0 equiv) was quickly added dropwise. TBSCl (1.10 g, 7.30 mmol,
10.0 equiv) was added in one portion, and the solution was warmed
to room temperature. After 1.5 h, the reaction mixture was poured
into a separatory funnel containing H2O (20 mL) and CH2Cl2
(20 mL). The layers were separated, and the aqueous layer was
extracted with CH2Cl2 (10 mL). The combined organic layers were
dried over MgSO4, filtered, and concentrated. The crude product
was subjected to flash chromatography over silica gel with hex-
anes-EtOAc (4:1), affording (þ)-2c (199 mg, 0.34 mmol, 47%,
97.5% ee) [[R]25 þ418 (c 0.47, CH2Cl2)] as an orange solid.
D
HPLC: Daicel Chiralcel OD-RH column (% CH3CN in H2O with
0.1% TFA) 0-20 min (50-75%), 20-30 min (75-85%), 1.5 mL/
min, λ = 254 nm, (S)-(-)-2c: t(S) = 19.29 min; (R)-(þ)-2c: t(R)
=
20.19 min.
Conversion of (-)-(1R,R)-12 to (-)-2b. Complex (-)-(1R,R)-
12 (500 mg, 0.84 mmol, 1.0 equiv, dr >95:5) was dissolved in
THF (10 mL) and cooled to 0 °C. MeLi (1.6 M in Et2O, 2.64 mL,
4.22 mmol, 5.0 equiv) was quickly added dropwise, and then
TBSCl (1.27 g, 8.40 mmol, 10.0 equiv) was immediately added.
The reaction was stirred for 2 h at 0 °C and then poured into a
separatory funnel containing H2O (20 mL) and CH2Cl2 (20 mL).
The layers were separated, and the aqueous layer was extracted
with CH2Cl2 (10 mL). The combined organic layers were dried
over MgSO4, filtered, and concentrated. Flash chromatography
over silica gel with hexanes-EtOAc (4:1) afforded (-)-2b (217
mg, 0.39 mmol, 47%, 96.2% ee) {[R]25D -128 (c 1.04, CH2Cl2)}
as a bright yellow solid. HPLC: Daicel Chiralcel OD-RH
column (% CH3CN in H2O with 0.1% TFA) 0-20 min (50%
to 75%), 1.5 mL/min, λ=254 nm, (S)-(þ)-2b: t(S) =18.48 min;
(R)-(-)-2b: t(R) =17.73 min.
(-)-Dicarbonyl[hydridotris(1-pyrazolyl)borato][(1R)-(η-1,2,3)-
1-(((R)-3-hydroxycarbonyl-4,4-dimethyldihydrofuran-2(3H)-one)-
oxy)-2-buten-1-yl]molybdenum (anti-methyl/syn-pantolactonecarbo-
nyloxy), (-)-(1R,R)-12. Complex 2b (2.04 g, 37.0 mmol, 1.0 equiv)
was dissolved in CH2Cl2 (100 mL), and TBAT (2.40 g, 44.4 mmol,
1.2 equiv) was added in one portion. The solution was stirred
20 min at room temperature, and (R)-(-)-pantolactone chloro-
formate (7.84 g, 40.7 mmol, 1.1 equiv) was added dropwise. The
solution was stirred for 1 h and then concentrated under reduced
pressure. The crude product was subjected to flash chromato-
graphy over silica gel with hexanes-EtOAc (9:1 and slowly ramping
to 0:1), followed by a second chromatography under the same
conditions. Iterations of recrystallization from CH2Cl2/hexanes
and further chromatography allowed one diastereomer, (1R,R)-12
(4.76 g, 8.0 mmol, 22%, dr >95:5) {[R]25D -149 (c 1.04, CH2Cl2)},
to be isolated as a yellow solid. The second diastereomer, (1S,R)-
12, decomposed, producing a byproduct that partly overlapped
with the stable diastereomer during chromatography. (-)-(1R, R)-
12: TLC: Rf = 0.34 (hexanes-EtOAc, 2:1). IR (cm-1): 2968 (w),
2930 (w), 2486 (w), 1938 (s), 1849 (s), 1803 (s), 1760 (s). 1H NMR
(CDCl3, 400 MHz): δ 8.50 (d, J=1.6 Hz, 1 H), 7.79 (d, J=2.0 Hz,
Acknowledgment. This work was supported by Grant
GM043107, awarded by National Institute of General
Medical Sciences, DHHS. We thank colleague Dr. Kenneth
Hardcastle for his skilled and efficient assistance with
X-ray crystallography.
Supporting Information Available: General experimental
methods and X-ray crystallographic studies of 4e, (1S,R)-11,
and (1R,R)-12 and copies of 1H and 13C NMR spectra of all new
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.