Catalytic asymmetric synthesis of branched chiral allylic phenyl ethers from (E)-allylic alcohols

Article abstract of DOI:10.1002/adsc.200900678

The first di-μ-amidate dipalladium complexes and a new di-μ-carboxylate dipalladium complex of the COP (cobalt oxazoline palladacycle) palladium(II) catalyst family are reported and characterized crystallographically. The di-μ-amidate complex 3 and its enantiomer (ent-3) are the first asymmetric catalysts that allow commercially available, or readily accessible, (E)-2-alken-1-ols to be transformed to enantioenriched branched allylic aryl ethers upon reaction of their trichloroacetimidate derivatives with phenols. The 3-aryloxy-1-alkene products are formed in high enantiomeric purity (typically 90-98% ee) and useful yields (61-88%).

Full text of DOI:10.1002/adsc.200900678

FULL PAPERS
DOI: 10.1002/adsc.200900678
Catalytic Asymmetric Synthesis of Branched Chiral Allylic
Phenyl Ethers from (E)-Allylic Alcohols
Angela C. Olson,a Larry E. Overman,a,* Helen F. Sneddon,a and Joseph W. Zillera
a
Department of Chemistry, University of California, Irvine, 1102 Natural Sciences II, Irvine, California, 92697-2025, U.S.A.
Fax : (+1)-949-824-3866; e-mail: leoverma@uci.edu
Received: September 29, 2009; Published online: December 10, 2009
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200900678.
Abstract: The first di-m-amidate dipalladium com- formed to enantioenriched branched allylic aryl
plexes and a new di-m-carboxylate dipalladium com- ethers upon reaction of their trichloroacetimidate de-
plex of the COP (cobalt oxazoline palladacycle) pal- rivatives with phenols. The 3-aryloxy-1-alkene prod-
ladium(II) catalyst family are reported and charac- ucts are formed in high enantiomeric purity (typi-
terized crystallographically. The di-m-amidate com- cally 90–98% ee) and useful yields (61–88%).
plex 3 and its enantiomer (ent-3) are the first asym-
metric catalysts that allow commercially available, or Keywords: asymmetric catalysis; organometallic
readily accessible, (E)-2-alken-1-ols to be trans- compounds; palladium; synthetic methods
Introduction
Chiral allylic aryl ethers are useful intermediates for
construction of a variety of biologically active mole-
cules. In recent years, considerable progress has been
recorded in assembling such structures in high enan-
tiomeric purity. Of particular importance are metal-
catalyzed asymmetric allylation reactions of phenols
with allylic alcohol derivatives. Numerous transforma- linear ratios as large as 800:1 being observed.[6] How-
tions of this type that proceed by symmetrically sub- ever, this transformation is limited to trichloroaceti-
stituted p-allyl intermediates have been developed for midate derivatives of Z-allylic alcohols, because the
various metals and chiral ligands.[1,2] More limited are corresponding E-stereoisomers undergo competitive
methods for the catalytic asymmetric synthesis of 3- [3,3]sigmatropic rearrangements to form allylic tri-
aryloxy-1-alkenes. Asymmetric allylic alkylation chloroacetamides in the presence of palladium(II)
methods to access this important class of ethers have
been reported using Pd(0),[3] Ru(II),[4] or Ir(I)[5] cata-
lysts. Although high levels of enantioselectivity are
often achieved, yields can be compromised by the for-
mation of achiral linear allylic substitution products,
particularly in the case of the Pd- and Ru-catalyzed
processes.
We recently reported that trichloroacetimidate de-
rivatives of (Z)-2-alken-1-ols are transformed in good
yields and high enantioselectivities to branched allylic
aryl ethers upon reaction with phenol nucleophiles in
the presence of the chiral palladium(II) complex 1,
[(Sp,R)-COP-OAc]2, or its enantiomer [Eq. (1) and
Figure 1].[6,7] A signature feature of this reaction is its
exceptionally high regioselectivity, with branched-to- Figure 1. Three palladium(II) complexes of the COP family.
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Catalytic Asymmetric Synthesis of Branched Chiral Allylic Phenyl Ethers
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complex 1 or its chloride-bridged analogue [COP- (1), we attempted to characterize this latter complex
Cl]2.[8,9]
also by X-ray crystallography. Single crystals could be
In this paper, we report two new crystallographical- obtained from dichloromethane, allowing the basic
ly characterized complexes, 2 and 3, of the COP structure to be confirmed by X-ray analysis; however,
(cobalt oxazoline palladacycle) catalyst family. More- the X-ray model of complex 1 did not refine satisfac-
over we disclose that (E)-2-alken-1-ols are trans- torily. To obtain a more soluble analogue of [COP-
formed in useful yields and high enantioselectivities OAc]2 and hopefully an analogue with better diffrac-
to branched allylic aryl ethers upon reaction of their tion properties, the corresponding pivalate-bridged di-
trichloroacetimidate derivatives with phenol nucleo- meric complex 2 was prepared in quantitative yield
[10]
philes in the presence of the anti-di-m-amidate dipalla- by allowing [(Sp ,R)-COP-Cl]2
to react with
dium complex 3 or its enantiomer (ent-3).
2.1 equivalents of silver pivalate in CH2Cl2 at room
temperature.[14] X-ray quality single crystals of the di-
cyclohexane-dichloromethane solvate could be grown
from 2% CH2Cl2/cyclohexane, allowing the structure
of the first di-m-carboxylate COP complex to be
solved (Figure 3).[11b]
Results and Discussion
Preparation of New COP Complexes
Compounds 2 and 3 crystallize in the same chiral
Di-m-amidate dipalladium complex 3 was convenient- space group P212121. There are no crystallographic
ly prepared by heating commercially available symmetry equivalent atoms present in either complex.
[(Sp ,R)-COP-OAc]2 (1)[10] with 2 equivalents of 2,2,2- Bond lengths and angles involving the palladium
trichloroacetamide in benzene with slow azeotropic atoms are quite similar between the two structures.[11]
removal of acetic acid, to give crystalline [(Sp ,R)- The molecular geometry about the palladium atoms
COP-NHCOCCl3]2 (3) in 85% yield.[11] The enantio- in 2 and 3 is approximately square planar. The mean
meric complex, ent-3, was similarly prepared from deviation from planarity is 0.040 ꢁ (Pd), 0.044 ꢁ
commercially available [(Rp,S)-COP-OAc]2 (ent-1).[10] (Pd’) for complex 2 and 0.047 ꢁ (Pd), 0.045 ꢁ (Pd’)
1
The simplicity of the H and 13C NMR spectra of 3 for complex 3. Bond lengths and angles involving the
suggest a head-to-tail arrangement of the bridging Pd, the oxazoline nitrogen and the cyclopentadienyl
amidate ligands. Recrystallization from dichlorome- carbon in complexes 2 and 3 are nearly identical to
thane/cyclohexane provided orthorhombic single crys- those of the related hexafluoroacetylacetonate com-
tals of a dichloromethane-cyclohexane solvate, allow- plex (COP-hfacac).[8b] As observed in COP-hfacac,
ing the structure of 3 to be established by X-ray anal- the palladium-oxygen bonds length of the bridging
ysis (Figure 2).[11a,12,13] As expected from the only close carboxylate ligands are longer in the bond trans to
precedent, the oxygens of the bridging amidate li- carbon [2.130(5) and 2.112(4)] than that trans to nitro-
gands are trans to the cyclopentadienyl rings.[13b]
gen [2.030(5) and 2.043(4)], consistent with a larger
As the reactivity of di-m-amidate complex 3 differs trans influence for the metallocene ligand. As in other
in several respects from that of [(Sp ,R)-COP-OAc]2 complexes containing the (h5-Cp)Co(h4-C4Ph4) frag-
AHCTUNGTRENNUNG
ment, the phenyl substituents of the tetraphenylcyclo-
Figure 2. X-ray model of [(Sp,R)-COP-NHCOCCl3]2 (3).
This complex crystallizes as a dichloromethane and cyclo-
hexane solvate; for clarity, these molecules are deleted from
this X-ray model.
Figure 3. X-ray model of [(Sp,R)-COP-O2C-t-Bu]2 (2). This
complex crystallizes with two molecules of cyclohexane and
one of CH2Cl2; for clarity, these solvate molecules are delet-
ed from this X-ray model.
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Angela C. Olson et al.
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butadienyl rings of complexes 2 and 3 adopt propellar bridged complex 7 and alkyl carboxylate-bridged cat-
conformations.[8a,b,15]
alysts 1 and 2. In contrast, in the presence of complex
3, branched allylic phenyl ether 6a was the predomi-
nant product (6a:5=92:8). Isolation of 2-aryloxy-1-
alkene 6a showed that the R enantiomer had been
formed in 90% enantiomeric excess. We also surveyed
the iodide-bridged complex 8 and p-fluorobenzoate-
Evaluation of New COP Complexes as
Enantioselective Catalysts for Allylic Etherification
Early studies of the catalytic effectiveness of the di-m- bridged complex 9, which had been described recently
amidate COP complex 3 showed it to be a kinetically by Richards and shown to be kinetically inferior to
poor catalyst for the allylic imidate rearrangement. [COP-Cl]2 or [COP-OAc]2 as catalysts for allylic tri-
For example, exposure of (E)-allylic trichloroacetimi- chloroacetimidate rearrangements.[9c] With these cata-
date 4a to 5 mol% of [COP-NHCOCCl3]2 (3) in lysts, allylic phenyl ether 6a was the major product;
CH2Cl2 at 388C for 48 h, provided allylic trichloroace- however, enantioselectivity in forming 6a was lower
tamide 5 in only 7% yield [Eq. (2)]. By comparison, than that observed with amidate-bridged complex 3,
as was selectivity for forming the allylic phenyl ether
product.
With [COP-NHCOCCl3]2 (3) identified as a partic-
ularly promising catalyst, we turned to optimizing
conditions for forming allylic phenyl ether 6a. Several
solvents were screened under identical conditions
(1 mol% 3, 5 equivalents of PhOH, [4a]=1.0M,
388C), with enantioselectivity in forming ether 6a
the rearrangement of imidate 4a catalyzed by 5 mol% found to be similar (89–92% ee) in CH2Cl2, CHCl3,
of [COP-Cl]2 proceeds in 18 h at 388C to give amide benzene, or chlorobenzene. However, the yield was
5 in 99% yield.[8d]
higher in the first two solvents. Decreasing the
Because the allylic rearrangement of (E)-allylic imi- amount of phenol from 5 to 3 equivalents, resulted, as
date 4a proceeded slowly in the presence of [COP- expected, in lower selectivity for forming the allylic
NHCOCCl3]2 (3), we turned to survey the coupling of ether product (6a:5=85:15). Attempts to increase
this imidate with phenol in the presence of catalyst 3 product yield by increasing the concentration of the
and several other catalysts of the COP family reactants were limited by the solubility of catalyst 3 in
(Table 1). As expected, allylic imidate rearrangement CH2Cl2 and CHCl3. The effects of catalyst loading
was a significant competing process with chloride- were also investigated. Reducing the catalyst loading
from 1 mol% to 0.5 mol% (5 equivalents of PhOH,
[4a]=1.0M, 388C) resulted in 72% conversion of 4a
Table 1. Enantioselective allylation of phenol with imidate
after 24 h; however, the ratio of branched ether 6a to
4a catalyzed by various Pd(II) COP complexes.
amide 5 was decreased to 87:13. When further re-
duced to 0.1 mol%, a diminished 48% conversion of
4a was observed after 24 h. A catalyst loading of
1 mol% was chosen for our further investigation of
the scope of this catalytic asymmetric synthesis of
allyl phenyl ethers.
COP catalyst
Conversion 6a:5[b] ee, 6a
[%][a]
[%][c,d]
We surveyed the reaction of (E)-allylic trichloroa-
cetimidate 4a with a variety of phenol nucleophiles at
388C in CH2Cl2 and CHCl3 (Table 2). With substitut-
ed phenols, enantioselectivities were generally excel-
lent (>90%), although reaction times varied consider-
ably depending upon the position and electronic
nature of the substituent (entries 1–-11). In all cases,
the branched allylic aryl ether was formed with high
branched-to-linear selectivity, as none of the (E)- or
(Z)-1-phenoxy-2-hexene regioisomer was apparent by
1H NMR analysis of unpurified reaction products. 4-
Chloro- and 2-bromophenol gave similar yields of
branched products 6d and 6e in CH2Cl2 and CHCl3,
but enantioselection was somewhat lower in CH2Cl2
(entries 5–8). With phenol, 4-methylphenol, 4-me-
thoxyphenol, and 2-, 3-, and 4-acetoxyphenols, enan-
G
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
>95
>95
>95
31:69
59:41
74:26
92:8
70:30
87:13
85 (S)
91 (S)
93 (R)
90 (R)
82 (S)
88 (S)
ACHTUNGTRENNUNG[(Sp,R)-COP-NHCOCCl3]2 3 63
A
ACHTUNGTRENNUNG
58
94
[a]
[b]
[c]
[d]
[e]
1
Determined by H NMR analysis using anisole as an in-
ternal standard.
Determined by H NMR analysis using anisole as an in-
1
ternal standard.
Determined by GC analysis using a chiral stationary
phase.
Absolute configuration determined by direct comparison
with authentic samples.[6]
Ar=p-fluorophenyl.
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Catalytic Asymmetric Synthesis of Branched Chiral Allylic Phenyl Ethers
FULL PAPERS
Table 2. Investigation of substituted phenol nucleophiles in catalytic asymmetric allylic etherification.
Entry
R
Solvent
Catalyst
t [h]
6
Yield [%][a]
ee [%][b] (absolute configuration)
1
2
3
4
5
6
7
8
H
H
4-Me
4-OMe
4-Cl
4-Cl
2-Br
2-Br
2-OAc
4-OAc
3-OAc
3-CHO
3-NO2
3-NO2
CH2Cl2
CHCl3
CH2Cl2
CH2Cl2
CH2Cl2
CHCl3
CH2Cl2
CHCl3
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CHCl3
ent-3
3
3
3
3
3
3
3
ent-3
ent-3
ent-3
ent-3
ent-3
ent-3
48
60
96
96
16
12
96
100
48
48
48
96
96
100
6a
6a
6b
6c
6d
6d
6e
6e
6f
6g
6h
6i
82
75
62
45
78
83
72
67
57
53
71
51
48
63
91 (S)
91 (R)
93 (R)[c]
92 (R)[c]
86 (R)
92 (R)
85 (R)
94 (R)
92 (S)[c]
92 (S)[c]
93 (S)[c]
90
9
10
11
12[d]
13[e]
14[e]
6j
6j
25
78
[a]
Mean yields after purification on silica gel of reactions performed in duplicate.
[b]
[c]
Determined by GC or SFC analysis using a chiral stationary phase (mean value of duplicate reactions).
The identical reaction performed in CHCl3 provided 6 in similar yield (Æ5%) and enantioselectivity (Æ2%). See Sup-
porting Information.
Because of limited solubility, the concentration of 3-formylphenol was 3.0M.
Because of limited solubility, the concentration of 3-nitrophenol was 2.0M.
[d]
[e]
tioselectivities were nearly identical in both solvents selectivity (90–98% ee). Only with imidate 4e, which
(entries 1–4, 9–11).[16] Reaction times were longer and contains a carbonyl group at C-6, was enantioselectiv-
yields correspondingly lower when the phenol was ity lower than 90% (entries 8 and 9). The absolute
substituted with an electron-donating group, or when configuration of allylic phenyl ether products 6k–r
an ortho substituent of any type was present (en- was assigned by analogy to products 6a–h (Table 1).
tries 4, 7–10). It was notable that base-labile acetoxy Consistent with these assignments, the major enantio-
substituents on the phenol nucleophile were stable to mer of 6k formed from the reaction of (E)-allylic imi-
the reaction conditions, with the corresponding allylic date 4b with phenol in the presence of catalyst 3 was
ethers 6f–h being formed in excellent enantioselectiv- identical to that formed from the reaction of the cor-
ities (92–93%) and moderate yields (entries 9–11). responding (Z)-allylic imidate with ent-1.[6] This result
Because of limited solubility, the reaction of allylic shows that the same alkene stereoface is activated for
À
imidate 4a with 3-formylphenol or 3-nitrophenol had C O bond-formation in both reactions.
to be carried out with only a slight excess of the
phenol nucleophile, which led to lower yields (en-
tries 12–14). Only with 3-nitrophenol was enantiose- Conclusions
lection unsatisfactory (entries 13 and 14). The abso-
lute configuration of allyl phenyl ether products 6a–h The first di-m-amidate dipalladium complexes (3 and
was determined by direct comparison ([a]D or HPLC ent-3) of the COP palladium(II) catalyst family are
analysis using a chiral stationary phase) with authen- described. Complex 3 and di-m-carboxylate dipalladi-
tic samples.[6]
um complex 2 were characterized by single-crystal X-
The scope of this enantioselective synthesis of 3-ar- ray analysis and shown to have closely related struc-
yloxy-1-alkenes was examined further by the reaction tures. Nonetheless, their catalytic properties are quite
of phenol or p-chlorophenol with (E)-allylic trichloro- different. The di-m-amidate complex is a poor catalyst
ACHTUNGTRENNUNGacetimidates 4b–e containing various terminal sub- for the 3,3-rearrangement of allylic trichloroacetimi-
stituents (Table 3). With imidates 4b–d, the reaction dates, whereas the di-m-carboxylate dipalladium com-
in either CH2Cl2 or CHCl3 provided the allylic phenyl plexes 1 and 2 are competent catalysts.[19] The di-m-
ether product in useful yields (61–88%) and enantio- amidate dipalladium complexes 3 and ent-3 are the
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Angela C. Olson et al.
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Table 3. Investigation of allylic imidates 4b–e in catalytic asymmetric allylic etherification.
Entry
R
R1
Solvent
Catalyst
t [h]
6
Yield [%][a]
ee [%][b]
1
2
3
4
5
6
7
8
9
U
H
4-Cl
H
4-Cl
4-Cl
H
4-Cl
H
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CHCl3
CH2Cl2
CH2Cl2
CHCl3
CHCl3
ent-3
ent-3
ent-3
ent-3
3
ent-3
ent-3
3
48
16
48
16
12
48
16
96
18
6k
6l
77
83
62
79
86
61
88
59
86
95 (S)[c]
90 (S)[c]
90 (S)[d]
85 (S)[d]
95 (R)[d]
98 (S)[c]
96 (S)[c]
78 (R)
6m
6n
6n
6o
6p
6q
6r
4-Cl
3
80 (R)
[a]
Mean yields after purification on silica gel of reactions performed in duplicate.
Determined by GC, SFC or HPLC analysis using a chiral stationary phase (mean value of duplicate reactions).
[b]
[c]
The identical reaction performed in CHCl3 provided 6 in similar yield (Æ5%) and enantioselectivity (Æ2%). See Sup-
porting Information.
[d]
Enantiomeric excess was determined after cleavage of the silyl protecting group.
pentadienyl C), 85.7 (cyclopentadienyl CH), 85.5 (cyclopen-
tadienyl C), 84.0 (cyclopentadienyl CH), 79.7 (cyclopenta-
dienyl CH), 75.7 (cyclobutadienyl C), 71.2 (oxazolinyl
first useful catalysts for the transformation of
trichloroacetimidate derivatives of prochiral (E)-2-
ACHTUNGTRENNUNG
alken-1-ols to branched allylic phenyl ethers.[19] This
catalytic asymmetric transformation is conveniently
accomplished at just above room temperature under
neutral conditions using 1 mol% of 3 (or ent-3) to
give either enantiomer of the 3-aryloxy-1-alkene
product in generally high enantiomeric purity (90–
98% ee) and moderate to good yields (61–88%).[17]
CH2O), 65.3 [oxazolinyl CHCHACTHUNRTGENN(GU CH3)2], 40.5 [CCAHUTNGTRNEN(UGN CH3)3], 29.0
[CHACTHNUTGRNEUGN(CH3)2], 28.7 (CH3), 18.8 (CH3), 13.3 (CH3); IR (thin
film): n=3056, 2956, 1582, 1499, 1411, 1366 cmÀ1; anal.
calcd. for C88H84Co2N2O6Pd2: C 66.21, H 5.30, N 1.75;
found: C 66.19, H 5.28, N 1.80.
AHCTNUGTREN[GUNN (Sp,R)-COP-NHCOCCl3]2 (3)
In a 100-mL round-bottom flask fitted with a short-path dis-
tillation column, a solution of [(Sp,R)-COP-OAc]2 (1)[10,18]
(400 mg, 0.26 mmol), 2,2,2-trichloroacetamide (86 mg,
0.53 mmol), and benzene (40 mL) was heated to 808C.
Acetic acid was removed from the reaction mixture by slow
azeotropic distillation. After 8 h, the reaction solution had
been concentrated to ca. 50% of its initial volume. This solu-
tion was allowed to cool to room temperature and then was
concentrated under vacuum. The resulting orange solid was
washed with Et2O (2ꢃ30 mL), taken up in CH2Cl2 (40 mL)
and concentrated under vacuum to afford [(Sp,R)-COP-
NHCOCCl3]2 (3) as an orange crystalline solid; yield:
385 mg (0.225 mmol, 85%); samples of this purity were used
in all catalysis experiments. An analytical sample was ob-
tained by recrystallization from CH2Cl2/cyclohexane: mp
Experimental Section
ACHTUNGTRENNUNG[(Sp,R)-COP-OPiv]2 (2)
In a 5-mL round-bottom flask, silver pivalate (12 mg,
0.058 mmol) was added in a single portion to a solution of
[(Sp,R)-COP-Cl]2 (ent-7)[10,18] (40 mg, 0.027 mmol) in CH2Cl2
(0.8 mL) at room temperature. After stirring for 10 h at
room temperature, the reaction mixture was filtered through
Celiteꢂ. The orange filtrate was concentrated under vacuum
to afford complex 2 as a bright orange crystalline solid;
yield: 44 mg (0.027 mmol, 100%). An analytical sample was
obtained by recrystallization from CH2Cl2/cyclohexane: mp
247–2498C; [a]2D4: À843, [a]
:
À953, (c 0.1, CHCl3);
24
577
1H NMR (500 MHz, CDCl3): d=7.70–7.68 (m, 16H, Ar),
7.26–7.21 (m, 24H, Ar), 4.65 (d, J=1.9, 2H, cyclopentadien-
yl H), 4.60 (s, 2H, cyclopentadienyl H), 4.27 (s, 2H, cyclo-
pentadienyl H), 4.04 (dd, J=8.4, 3.9 Hz, 2H, oxazolinyl
CH2O), 3.24 (dd, J=9.2, 8.9 Hz, 2H, oxazolinyl CH2O), 3.11
225–2318C; [a]2D5: À8.3, [a] : À9.2, [a] : À12.7, (c 0.1,
25
25
577
546
CHCl3); 1H NMR (500 MHz, CDCl3): d=7.63–7.61 (m,
16H, Ar), 7.24–7.23 (m, 24H, Ar), 5.03 (s, 2H, NH), 4.61 (d,
J=1.3 Hz, 2H, cyclopentadienyl H), 4.59 (d, J=2.3 Hz, 2H,
cyclopentadienyl H), 4.51 (dd, J=2.6, 2.2 Hz, 2H, cyclopen-
tadienyl H), 4.10 (dd, J=8.4, 8.2 Hz, 2H, oxazolinyl CH2O),
3.30 (dd, J=9.4, 8.7 Hz, 2H, oxazolinyl CH2O), 3.20 [dt, J=
[m, 2H, oxazolinyl CHCHACHTUNTRGENNUG(CH3)2], 1.86 [m, 2H, CHCAHTUNGTERN(NUNG CH3)2],
1.08 (s, 18H, CH3), 0.42 (d, J=7.1 Hz, 6H, CH3), 0.05 (d,
J=6.6 Hz, 6H, CH3); 13C NMR (125 MHz, CDCl3): d=
9.9, 3.3 Hz, 2H, oxazolinyl CHCH
2H, CH
(CH3)2], 0.45 (d, J=7.1 Hz, 6H, CH3), À0.02 (d, J=
6.7 Hz, 6H, CH3); 13C NMR (125 MHz, CDCl3): d=171.0
ACHTUGNTRENUN(GN CH3)2], 1.81–1.77 [m,
188.3 [CO2C
N
ACHTUNGTRENNUNG
129.4 (CH, Ph), 128.0 (CH, Ph), 126.0 (CH, Ph), 99.2 (cyclo-
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Catalytic Asymmetric Synthesis of Branched Chiral Allylic Phenyl Ethers
FULL PAPERS
(C), 169.0 (C), 135.8 (C, Ph), 129.1 (CH, Ph), 128.2 (CH,
Ph), 126.3 (CH, Ph), 98.7 (cyclopentadienyl C), 95.3 (CCl3),
87.6 (cyclopentadienyl CH), 86.7 (cyclopentadienyl C), 84.2
(cyclopentadienyl CH), 79.3 (cyclopentadienyl CH), 75.3
(cyclobutadienyl C), 71.4 (oxazolinyl CH2O), 65.2 [oxazo-
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Commun. 2004, 1870–1871.
linyl CHCHACHTUNGTRENNUNG(CH3)2], 29.1 [CHCAHTUNGTRENN(GUN CH3)2], 18.7 (CH3), 13.3
(CH3); IR (thin film): n=3371, 3058, 2960, 1652, 1609,
1499 cmÀ1; anal. calcd. for C82H68Cl6Co2N4O4Pd2: C 57.36, H
3.99, N 3.26; found: C 57.13, H 3.82, N 3.18.
[5] a) M. Kimura, Y. Uozumi, J. Org. Chem. 2007, 72, 707–
714; b) S. Shekhar, B. Trantow, A. Leitner, J. F. Hart-
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General Procedure for Preparation of Chiral Allylic
Ethers
Di-m-amidate dipalladium complex 3 or its enantiomer (ent-
3) (2 mg, 0.001 mmol) was added in one portion to a solu-
tion of the allylic trichloroacetimidate (0.13 mmol), phenol
1
(0.63 mmol), and CH2Cl2 (0.13 mL) in a sealable = -dram
2
glass vial. The reaction vial was sealed under an Ar atmos-
phere and then heated in a block heater at 388C. After the
reported reaction time, the solution was concentrated under
vacuum and the residue was purified by flash chromatogra-
phy on silica gel to afford the 3-aryloxy-1-alkene product as
a colorless oil. Reactions carried out in CHCl3 were con-
ducted identically at twice the scale.
Other experimental details, characterization data for new
products, and copies of chromatographic traces used to de-
termine enantiomeric purity can be found in the Supporting
Information.
[6] S. F. Kirsch, L. E. Overman, N. S. White, Org. Lett.
2007, 9, 911–913.
[7] For the related reaction to form 3-acyloxy-1-alkenes in
high enantiomeric purity, see: S. F. Kirsch, L. E. Over-
man, J. Am. Chem. Soc. 2005, 127, 2866–2867.
[8] a) M. P. Watson, L. E. Overman, R. G. Bergman, J.
Am. Chem. Soc. 2007, 129, 5031–5044; b) S. F. Kirsch,
L. E. Overman, M. P. Watson, J. Org. Chem. 2004, 69,
8101–8104; c) L. E. Overman, C. E. Owen, M. M.
Pavan, C. J. Richards, Org. Lett. 2003, 5, 1809–1812;
d) C. E. Anderson, L. E. Overman, J. Am. Chem. Soc.
2003, 125, 12412–12413.
[9] For selected examples of the catalysis of allylic imidate
rearrangements with other chiral palladium(II) com-
plexes, see: a) D. F. Fischer, Z.-Q. Xin, R. Peters,
Angew. Chem. 2007, 119, 7848–7851; Angew. Chem.
Int. Ed. 2007, 46, 7704–7707; b) J. Dugal-Tessier, G. R.
Dake, D. P. Gates, Organometallics 2007, 26, 6481–
6486; c) H. Nomura, C. J. Richards, Chem. Eur. J. 2007,
13, 10216–10224; d) J. Kang, T. H. Kim, K. H. Yew,
W. K. Lee, Tetrahedron: Asymmetry 2003, 14, 415–418;
e) Y. Donde, L. E. Overman, J. Am. Chem. Soc. 1999,
121, 2933–2934; f) Y. Jiang, J. M. Longmire, X. Zhang,
Tetrahedron Lett. 1999, 40, 1449–1450; g) Y. Uozumi,
K. Kato, T. Hayashi, Tetrahedron: Asymmetry 1998, 9,
1065–1072; h) L. E. Overman, G. G. Zipp, J. Org.
Chem. 1997, 62, 2288–2291; i) M. Calter, T. K. Hollis,
L. E. Overman, J. Ziller, G. G. Zipp, J. Org. Chem.
1997, 62, 1449–1456.
[10] a) C. E. Anderson, S. F. Kirsch, L. E. Overman, C. J.
Richards, M. P. Watson, Org. Synth. 2007, 84, 148–155;
b) C. E. Anderson, L. E. Overman, C. J. Richards, M. P.
Watson, N. White, Org. Synth. 2007, 84, 139–147;
c) A. M. Stevens, C. J. Richards, Organometallics 1999,
18, 1346–1348.
[11] CCDC 748079 (a) and CCDC 748078 (b) contain the
supplementary crystallographic data for this paper.
These data can be obtained free of charge from The
Acknowledgements
We thank the NSF (CHE-0200786) for financial support. The
Royal Commission for the Exhibition of 1851 is acknowl-
edged for postdoctoral fellowship support of H.F.S. The NIH
Predoctoral Fellowship Program (1F31M089137), the U.S.
Department of Education Graduate Assistance in Areas of
National
Need
(GAANN)
Fellowship
Program
(P200A060042), and the UC Irvine Faculty Mentor Program
are acknowledged for financial support of A.C.O. NMR and
mass spectra were obtained at UC Irvine using instrumenta-
tion acquired with the assistance of NSF and NIH Shared In-
strumentation programs.
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[12] Only three di-m-amidate dipalladium complexes are
found in the Cambridge Structural Database.[13] Of
these, only one (BAQSAN) has an unsymmetrical ar-
rangement of the bridging amidate ligands.[13b].
Adv. Synth. Catal. 2009, 351, 3186 – 3192
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
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[17] In favorable cases where the nucleophile is phenol or
an unhindered electron-poor substituted phenol, the
transformation can undoubtedly be carried out in satis-
factory fashion using lower catalyst loadings.
[18] Known COP catalysts employed in this study were pre-
pared according to ref.[10] and ref.[9c] Both enantiomers
of [COP-Cl]2 and [COP-OAc]2 are available from
Aldrich Chemical Co.
[19] The origin of these differences is not yet fully under-
stood. We note that an external anionic ligand has been
shown to be coordinated to palladium during the rate-
determining step of the rearrangement of allylic tri-
chloroacetimidates catalyzed by [COP-Cl2]À,[8a] whereas
both the double bond and the imidate nitrogen have
been proposed to be bound to palladium during the re-
action of allylic trichloroacetimidates with carboxylate
nucleophiles.[7].
[14] Complex 2 was also prepared in 80% yield by heating
[COP-OAc]2 (1) with 2.2 equivalents of pivalic acid in
benzene at 608C in the presence of 4 ꢁ molecular
sieves. Alternatively, this complex can be prepared
analogously to 1 by diastereoselective palladation of
the (h5-Cp)Co(h4-C4Ph4)oxazoline precursor with
ACHTUNGTRENNUNG
PdACHTUNGTRENNUNG(OAc)2 in pivalic acid at 858C; however, removal of
excess pivalic acid is cumbersome.
[15] A. C. Villa, L. Coghi, A. G. Manfredotti, C. Guastini,
Acta Crystallogr. Sec. B 1974, 30, 2101–2112.
[16] See Supporting Information.
3192
asc.wiley-vch.de
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2009, 351, 3186 – 3192