Catalytic use of zinc amide for transmetalation with allylboronates: General and efficient catalytic allylation of carbonyl compounds, imines, and hydrazones

Article abstract of DOI:10.1002/adsc.201201134

The efficient catalytic allylation of ketones, imines, and hydrazones with allylboronates using a catalytic amount of zinc amide is reported. In this reaction, the boron-to-zinc exchange process occurred smoothly to afford the corresponding allylzinc amides, and the desired allylation reactions proceeded in high efficiency (～0.1mol%). A mechanistic study revealed that transmetalation was a rate-determining step in the catalytic cycle, and also that the amide ligand on the zinc center played a key role in preparing reactive allylzinc species. Catalytic asymmetric allylations were also investigated, and high enantioselectivities were obtained using chiral diamine ligands. Copyright

Full text of DOI:10.1002/adsc.201201134

FULL PAPERS
DOI: 10.1002/adsc.201201134
Catalytic Use of Zinc Amide for Transmetalation with
Allylboronates: General and Efficient Catalytic Allylation of
Carbonyl Compounds, Imines, and Hydrazones
Yi Cui,a Wei Li,a Takehiro Sato,a Yasuhiro Yamashita,a and Shu¯ Kobayashia,*
a
Deparment of Chemistry, School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo, Japan
Fax : (+81)-3-5684-0634; phone: (+81)-3-5841-4790; e-mail: shu_kobayashi@chem.s.u-tokyo.ac.jp
Received: December 31, 2012; Revised: March 18, 2013; Published online: April 8, 2013
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201201134.
Abstract: The efficient catalytic allylation of ketones, amide ligand on the zinc center played a key role in
imines, and hydrazones with allylboronates using preparing reactive allylzinc species. Catalytic asym-
a catalytic amount of zinc amide is reported. In this metric allylations were also investigated, and high
reaction, the boron-to-zinc exchange process oc- enantioselectivities were obtained using chiral di-
curred smoothly to afford the corresponding allylzinc
amides, and the desired allylation reactions proceed-
ed in high efficiency (~0.1 mol%). A mechanistic
ACHTUNGTRENaNUNG mine ligands.
study revealed that transmetalation was a rate-deter- Keywords: allylation; catalysts; metal amides; trans-
mining step in the catalytic cycle, and also that the metalation; zinc
Introduction
Our group has recently reported zinc hydroxide-
catalyzed asymmetric allylation of aldehydes and hy-
The development of an efficient and operationally drazono esters with allylboronates.[12a,c,d] In this reac-
À
facile catalytic methodology for C C bond formation tion, zinc nucleophiles were formed through transme-
is an important issue in organic chemistry.[1] The ally- talation from boron to zinc. We envisaged that zinc
lation of carbonyl compounds,[2] imines, and hydra- species with stronger conjugate Lewis bases, such as
ACTHNUTRGNEUNG
zones[3] provides homoallylic alcohol and amine deriv- Zn(HMDS)2,[15] might have advantages in promoting
atives, which are very useful building blocks for fur- turnover of zinc species in a catalytic process.[16,17]
ther transformations. In particular, when an electro- Herein, we report a general catalytic allylation of car-
philic partner is a ketone or a ketimine derivative, bonyl compounds, imines, and hydrazones in the pres-
a crowded carbon center bearing a required function- ence of catalytic amounts of Zn
ACHTUNGTNER(NUNG HMDS)2.
al group can be created.[4] Typical protocols for allyla-
tion of ketones involve the use of allylmetal reagents
prepared in situ under Barbier-type conditions[5] from Results and Discussion
allyl halides and stoichiometric amounts of metals.
Recently, a significant advance has been made in Development of Zinc Amide-Catalyzed Allylation
catalytic allylation using allylboronates.[6] It is notable Reactions
that the use of less toxic allylboronates is environ-
mentally friendly compared with the use of toxic allyl- The evaluation of various zinc catalysts for the allyla-
stannanes.[7] Allylation of carbonyl compounds, tion of acetophenone (2a) with pinacol allylboronate
imines, and hydrazones with allylboronates has been (1a) is summarized in Table 1. While zinc Lewis acids
disclosed in the presence of Cu-,[8] In-,[9] Ir-,[10] Ni-,[11 such as zinc chloride, bromide, fluoride, acetate, and
and Zn[12]-based catalysts, and Brønsted acids;[13] how- even triflate provided poor conversions for the model
ever, in most of those cases, relatively high catalyst system in the presence of 10 mol% catalyst (entries 2–
loadings were required. Zinc catalysts have low toxici- 6), zinc tert-butoxide (1.0 mol%) was also unsatisfac-
ty, are inexpensive, and markedly tolerant toward var- tory, affording poor conversion (entry 7, 10% conver-
ious functional groups.[14]
sion). Zn catalysts prepared from diethylzinc and al-
cohols were also tested, and moderate conversions
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Table 1. Examination of the zinc-catalyzed allylation of ace-
tophenone 2a with allylboronate 1a.[a]
Table 2. Survey of allylboronates and solvents for the zinc
amide-catalyzed allylation of acetophenone 2a.[a]
Entry
Catalyst[b]
Conv. [%][b]
Cat.
Entry Solvent
(x)
1
2
3
4
none
0
2
4
2
ZnCl2 (10 mol%)
ZnBr2 (10 mol%)
ZnF2 (10 mol%)
Time NMR
Time NMR
[h]
Yield
[h]
Yield
[%][b]
[%][b]
5
6
Zn
Zn
Zn
ZnEt
ZnEt
N
0
0
10
48
56
97
7[c]
8[d]
9[d]
10
1
2
3
4
5
6
7
8
THF
Et2O
DCM
CH3CN 2.5
pentane 2.5
toluene 2.5
THF
Et2O
pentane 1.0
pentane 0.1
pentane 0.1
2.5
2.5
2.5
3
3
3
3
3
3
1
1
1
6
24
>99
>99
80
57
>99
82
35
28
67
25
–
–
–
–
–
–
1
1
–
–
–
–
–
–
(O-t-Bu)[e] (1 mol%)
(OEt)[f] (1 mol%)
G
ZnACHTUNGTRENNUNG(HMDS)2 (1 mol%)
[a]
The reaction of 2a (0.40 mmol) with 1a (0.44 mmol) was
performed in 0.4M THF at 208C for 3 h in the presence
of a zinc catalyst, unless otherwise noted. The results
shown in entries 1–7 and 10 have been reported in our
communication.[16]
1.0
1.0
>99
93
>99
73
>99
(96[d])
–
9
10
11
1
16
36
[b]
[c]
Determined by 1H NMR analysis using 1,1,2,2-tetra-
chloroethane as an internal standard.
29
The reaction of 2a (1.60 mmol) with 1a (1.76 mmol) was
performed in 0.8M THF at 208C for 3 h in the presence
12
pentane 0.1
pentane 0.1
48
60
30
43
–
–
13[c]
–
of ZnACHTUNGTRENNUNG(O-t-Bu)2 prepared in situ from 1 mol% of ZnCl2
and 2 mol% of KO-t-Bu.
2a (0.80 mmol) and 1a (0.88 mmol) were used.
ZnEtACHTUNGTRENNUNG(O-t-Bu) was prepared from ZnEt2 and an excess
amount of t-BuOH in THF, and used after concentration
and drying.
ZnEtACHTUNGTRENNUNG(OEt) was prepared from ZnEt2 and an excess
amount of EtOH in THF, and used after concentration
and drying.
[a]
In entries 1–9, the reaction of 2a (0.40 mmol) with
1 (0.44 mmol) was performed at 208C in 0.4M solvent in
[d]
[e]
the presence of Zn
tion of 2a (3.0 mmol) with 1 (3.3 mmol) was performed
at 208C in 1.0M solvent in the presence of Zn(HMDS)2
ACHTUNGERTN(NUNG HMDS)2. In entries 10–13, the reac-
AHCTUNGTRENNUNG
[f]
unless otherwise noted. The results shown in entries 7–13
have been reported in our communication.[16]
Determined by 1H NMR analysis using 1,1,2,2-tetra-
chloroethane as an internal standard.
The reaction was run at 08C.
[b]
[c]
[d]
were observed (entries 8 and 9).[12b] To our delight,
Zn(HMDS)2 was found to be much more reactive,
providing 97% conversion within 3 h with 1 mol%
Isolated yield.
ACHTUNGTRENNUNG
catalyst loading (entry 8). Compared with reported cases, 1b showed higher yields. It was found that both
examples of allylation with allylboronates, this reac- pentane and THF were better solvents for this model
tion is operationally facile, and no additional proton system, and an apolar solvent, pentane, was found to
source is required to achieve high reactivity.
Based on this finding, we conducted further optimi- lizing allylboronate 1b, the reaction proceeded
zation with pinacol allylboronate 1a (Table 2). Initial smoothly even using 0.1 mol% Zn(HMDS)2, and that
experiments were focused on solvent evaluation with after 36 h at 208C, the homoallylic alcohol 3a was ob-
2.5 mol% Zn(HMDS)2. During the optimization, we tained in complete conversion and 96% isolated yield.
be the most suitable solvent. It is noteworthy that, uti-
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
found that THF, diethyl ether, and pentane allowed In contrast, the corresponding reaction with 1a result-
this transformation to achieve full conversion (en- ed in low conversion even when the reaction was con-
tries 1–6). In the subsequent screening, allylboronic ducted for a long time. The employment of allylboro-
acid 2,2-dimethyl-1,3-propanediol ester (1b) emerged nate 1b also has an economic advantage because it
with better activity than pinacol allylboronate (1a), al- could be prepared from 2,2-dimethyl-1,3-propanediol,
though allylboronate 1a has been used as a general al- which is much cheaper than pinacol.[18]
lylating reagent for a long time in synthetic organic
chemistry. We then tested the reactions of 1a and 1b lylation of ketones, imines, and hydrazones with allyl-
with 2a in the presence of 1.0 mol% Zn(HMDS)2 in boronate 1b by using 0.1 mol% of Zn(HMDS)2
the optimized solvents (entries 7–9), and in all these (Table 3). Gratifyingly, the reaction proceeded with
Next, we investigated substrate generality of the al-
A
ACHTUNGTRENNUNG
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Catalytic Use of Zinc Amide for Transmetalation with Allylboronates
good generality for a diverse array of functional ketones were shown to react smoothly. Further, the
groups including both aromatic and heteroaromatic methodology performed with excellent generality for
ketones, providing the corresponding tertiary homoal- aldimines and aldehyde-derived hydrazones, as well
lylic alcohols in excellent yields. Moreover, aliphatic as for ketimines and ketone-derived hydrazones,
which were all transformed into the corresponding
homoallylic amines and hydrazides in high yields.
Table 3. Substrate scope for the zinc amide-catalyzed allyla-
tion of 2 with allylboronate 1b.
Note that several functional groups such as methoxy,
bromo, ester, pyridine, and nitro groups were compat-
ible with this mild and operationally facile catalytic
system.
Assumed Mechanism
We started the mechanistic studies by evaluating the
reaction of acetophenone with a series of substituted
allylboronates (Scheme 1). The allylation was found
to proceed smoothly with a-methyl-substituted allyl-
boronate (1c) to afford a-adducts in a 2:3 syn/anti
ratio. On the other hand, with terminal methyl-substi-
tuted boronate (1d), this reaction was too sluggish to
give any significant conversion. Both cases indicated
that the boron-to-zinc exchange process was required
to initiate the catalytic cycle, and that the exchange
process occurred via a cyclic transition wherein the
Scheme 1. Allylation with a series of substituted allylboro-
nates. These results have been reported in our communica-
tion.[16]
[a]
The reaction of 2 (3.0 mmol) with 1b (3.3 mmol) was per-
formed in 1.0M of the solvent system at 208C for 36 h in
the presence of ZnACTHNUTRGNEUNG(HMDS)2 (0.1 mol%) unless otherwise
noted. The yield was determined by isolation after
column chromatography. Solvent conditions shown in pa-
rentheses. Conditions A: in pentane; conditions B: in
pentane-THF (5:1); conditions C: in pentane-THF (3:2);
conditions D: in THF. The results of product 3b, 3e, 3g–
o, 3r, 3s, 3u–w have been reported in our communica-
tion.[16]
[b]
[c]
PMP=p-methoxyphenyl.
NMR yield of the reaction without using ZnACTHNURGTNEUNG(HMDS)2.
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allyl group transferred from boron to zinc.[19] The
steric effect around the allyl group would hinder the
exchange rate.
In preliminary studies, we assumed the formation
of a nucleophilic allylic zinc species for allylation of
acetophenone. On the basis of previous studies and
various experiments, the transmetalation would gener-
ate two allylic zinc species, allylic zinc amide and di-
ACHTUNGTRENNUNGallylic zinc, and both species can react with a ketone.
Our next interest was to clarify how zinc amide was
involved in the reaction. For the zinc amide-catalyzed
allylation using allylboronate, we proposed four possi-
ble mechanistic pathways shown below.
The first possible mechanism is a truly zinc amide-
catalyzed mechanism involving an allylic zinc amide
intermediate as shown in Scheme 2 (Pathway I). The
transformation is initiated by a boron-to-zinc ex-
change reaction to generate nucleophilic allylzinc
amide complex 6 and boron amide 7. Next, this allyl-
zinc amide complex 6 reacts with ketone 2 to provide
zinc alkoxide (product) complex 8. A subsequent
transmetalation from boron amide 7 to zinc alkoxide
Scheme 3. Possible catalytic pathway II.
(product) complex 8 regenerates Zn
lylboronated product 9 is obtained.
ACHTUNGTERN(NUNG HMDS)2, and al-
An alternative mechanism is a zinc amide-initiated
mechanism (Scheme 3, Pathway II). In this pathway,
zinc amide is a precatalyst that reacts with allylboro-
nate 1b to afford diallylic zinc 7 as a viable reaction
intermediate, accompanied by the generation of
boron amide. The subsequent allylation provides allyl-
ic zinc alkoxide (product) complex 11. The following
exchange of this complex with allylboronate 1b af-
fords allylboronated product 9 along with regenera-
tion of diallylic zinc 10 to complete the catalytic cycle.
The third possible mechanism is a zinc amide-cata-
lyzed mechanism with no regeneration of
ZnACHTUNGTRENNUNG(HMDS)2 (Scheme 4, Pathway III). In this mecha-
nism, allylzinc amide species 6 is formed after the
Scheme 4. Possible catalytic pathway III.
first transmetalation and reacts with ketone 2 to form
alkoxy
reacts with allylboronate 1b to regenerate allylzinc
amide species 6 without formation of Zn(HMDS)2.
ACHTUNGTRENUN(NG product)zinc amide complex 8, which further
AHCTUNGTRENNUNG
Finally, the last possible mechanism is also a zinc
amide-initiated mechanism. After an initiation step,
allylzinc alkoxide (product) species 8 forms and
works as a key catalytic species (Scheme 5, Path-
way IV). In this mechanism, the key species 11 can be
formed via alkoxy
species 10.
ACHTUNGTRENN(UNG product)zinc amide 8 or diallylzinc
We performed mechanistic experiments to clarify
which was the most plausible reaction mechanism.
Scheme 2. Possible catalytic pathway I.
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Catalytic Use of Zinc Amide for Transmetalation with Allylboronates
Scheme 5. Possible catalytic pathway IV.
Mechanistic Study
a 1:1 stoichiometry of the exchange. The newly
emerged signals were assigned to those of allylic zinc
species 6. The 11B NMR experiment clearly showed
a new single signal (d=20.9 ppm), which appeared in
the spectrum upfield of the signal of allylboronate
(d=28.1 ppm) and was assigned to the boron amide
7. After 30 min, the exchange achieved 54% boronate
NMR Experiments on Allylzinc Species
Transmetalation of Allyl- and a-Methylallylboronates
with a Stoichiometric Amount of Zinc Amide
To verify the boron-to-zinc transmetalation hypothe- conversion, and after 1 h, complete consumption of
sis directly, we monitored a stoichiometric reaction of allylboronate 1b provided the allylic zinc species in
allyboronate 1b with ZnACHTNUTGRNEUNG
(HMDS)2 by 1H, 13C, and 98% NMR yield. We assigned the major zinc species
1
11B NMR analyses at 208C (Scheme 6). When allyl- to allylic zinc amide 6 based on its H NMR analysis.
1
boronate 1b was treated with Zn
G
(1.00 equiv.) in THF-d8 at 0.40M at 208C, we could substantially consistent with the observed 1H NMR
clearly observe the boron–zinc exchange [Scheme 6, signals obtained in the reaction of allylboronate 1b
Eq. (6-1)]. In this process, a series of signals belong- with excess ZnACTHNURGTNEUNG(HMDS)2 (1.50 equiv.) [Eq. (6-2)]. The
ing to allylboronate 1b gradually faded. Analysis by typical and sharply contoured signals that appeared in
the continuous variation method was consistent with both cases corresponded to those of allylic zinc amide
6, which was considerably more prevalent than diallyl-
1
ic zinc species 10. The H NMR spectrum of diallylic
zinc species 10 in our system was very obscure (see
the Supporting Information). Moreover, this gradual
exchange process and the generation of allylic zinc
amide 6 were also observed in the 13C NMR spectra.
On the other hand, we performed NMR studies by
using allylboronate 1b combined with 0.5 equiv. of
ZnACTHNUTRGEN(UGN HMDS)2. This condition seemed to be more likely
to resemble the actual catalytic status [Eq. (6-3)]. It
was assumed that an excess amount of allylboronate
1b would contribute to the second allyl group transfer
to the zinc center. Nevertheless, it was found that the
first exchange of the allyl group with one amide
group of ZnACTHNUGTRENUNG(HMDS)2 could take precedence over the
second exchange of another allyl group with the re-
maining amide group. Indeed, the transmetalation
proceeded smoothly with nearly 50% boronate con-
version in 30 min to yield the allylzinc amide 6, which
was shown by 1H NMR analysis. Importantly, we
Scheme 6. NMR experiments for transmetalation of boron
to zinc.
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(110 min, 98% boronate conversion). In contrast, the
exchange of zinc amide with pinacolyl allylboronate
1a was very slow (72 h, 95% conversion). Based on
the previous experiments, even if pinacolyl allylboro-
nate 1a was treated with ZnACHTNUGRTENUNG(HMDS)2 (1.00 equiv.) in
a concentrated THF-d8 solution (0.40M) at 208C, the
allylic zinc species was formed with 66% boronate
conversion after 5 h, and a longer time (16 h) was re-
quired to achieve full conversion. Based on these re-
Scheme 7. Transmetalation of 1e to zinc.
found that the second transmetalation was obtuse and sults, we assumed that less-hindered non-substituted
afforded diallylic zinc 10 in 73% boron conversion cyclic allylboronates might facilitate boron-to-zinc
after 1 h. Moreover, the complete consumption of al- transmetalation more rapidly. As we anticipated, the
lylboronate 1b was protracted, and even after 48 h, exchanges with allylboronates 1f and 1g proceeded
a small amount of allylboronate 1b remained.
strikingly promptly, providing access to the corre-
At this stage, we confirmed the boron-to-zinc trans- sponding allylic zinc species in full conversion quite
metalation as an initial step in the catalytic cycle. quickly (1f, 20 min; 1g, 10 min). Based on these reac-
With the aim of confirming the origin of diastereose- tion outcomes, we attributed the significantly different
lectivity (syn/anti=2/3) by using a-methyl-substituted exchange rates of allylboronates (1a, 1b, 1f, 1g) with
allylboronate, we investigated the exchange of a- Zn
methyl-substituted allylboronate 1e with 1 equivalent mands of the boron sites, which suppressed the ex-
of Zn(HMDS)2 in THF-d8 (0.05M) at 208C. Under change of boron to zinc in a cyclic transition struc-
these conditions, a smooth boron-to-zinc exchange ture.
was observed, achieving complete conversion to These results clearly indicated that ZnACTHUNGTRNE(NUG HMDS)2 re-
ACHTUNGTERNN(UNG HMDS)2 to a tendency for increased steric de-
ACHTUNGTRENNUNG
afford crotyl zinc species 13 with Z/E=1/1.4 selectivi- acted with allylboronates to afford allylzinc amide
ty after 2 h (Scheme 7). This result might explain the species preferentially at the initial stage of the trans-
diastereoselectivity (syn/anti=2/3), if kinetically gen- metalation even in the presence of an excess amount
erated Z and E crotyl zinc amide species 13 reacted of allylboronate.
with a ketone stereospecifically.
Allylation of Acetophenone with Stoichiometric
Amounts of Allylzinc Amide and Diallylzinc
Transmetalation of a Series of Allylboronates with
a Stoichiometric Amount of Zinc Amide
Based on the above results, which give direct evi-
To shed light on the substituent effect on allylboro- dence of the boron-to-zinc transmetalation, we moved
nates and also to seek out the origin of the different our attention to allylation (Scheme 9). Upon addition
reactivities of allylboronates 1a and 1b, we investigat-
ed the transmetalation of zinc amide with one equiva-
lent of each of a series of allylboronates by NMR
analysis (Scheme 8).
As shown in Scheme 8, the addition of allylboro-
nates (1a, 1b, 1f, 1g) to a solution of ZnACTHNUGTRENUNG(HMDS)2
(1.00 equiv.) in dry THF-d8 (0.05M) all provided allyl-
ic zinc amide at 208C. Under these typical conditions,
the exchange of Zn
ACHTUNGERTN(NUNG HMDS)2 with allylboronate 1b
provided the allylic zinc species 6 in a short time
Scheme 8. Effect of allylboronate structures.
Scheme 9. Reactivities of allylzinc amide and diallylzinc.
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Catalytic Use of Zinc Amide for Transmetalation with Allylboronates
of one equivalent of acetophenone to the resultant al- version in 5 min and full conversion in 20 min
lylic zinc amide system (generated from the combina- [Scheme 10, Eq. (10-1)]. In contrast, allylic zinc
tion of allylboronate 1b with one equivalent of amide (1.00 mol%) catalyzed this reaction to obtain
ZnACHTUNGTRENNUNG(HMDS)2 in 0.40M THF-d8 at 208C, 1 h), the full conversion within 5 min [Eq. (10-2)].
1
À
H NMR spectrum showed that the C C bond forma-
To summarize the NMR transmetalation experi-
tion proceeded immediately with full conversion ments, the transfer of the allyl group from allylboro-
[Scheme 9, Eq. (9-1)]. Furthermore, when acetophe- nates to allylic zinc amides was fast, and the allylation
none, allylboronate 1b, and ZnACTHNUTRGENUG(N HMDS)2 (1:1:1, in in the presence of catalytic amounts of allylic zinc
THF-d8 at 0.40M, 208C) were mixed, the 11B NMR amide identified the indispensable role of one amide
spectrum clearly supported the formation of allyl- group on the zinc center for achieving high reactivity
boronated product 9a (d=16.5 ppm) and boron in our system. These mechanistic experiments were
amide (d=20.9 ppm). On the other hand, the subse- consistent with the formation of a nucleophilic allylic
quent allylation of acetophenone (1.00 equiv.) with di- zinc amide complex as a key catalytic intermediate in
allylic zinc (generated from the combination of allyl- the favored pathways I or III rather than pathways II
boronate 1b with 0.5 equiv. ZnACHTNUTRGNEUNG(HMDS)2 in 0.40M or IV.
1
THF-d8 at 208C, 48 h) led to an untidy H NMR spec-
trum. After quenching the reaction with D2O, the de-
sired homoallylic alcohol was obtained in nearly 80% Kinetic Studies of the Allylation Reactions Using
conversion [Eq. (9-2)].
ZnACTHNGUTRENU(NG HMDS)2
In the possible pathways I and III, we perceived that
the allylation might not be a rate-determining step on
the basis of the results shown in Table 2 and the
above NMR experiments. To confirm our hypothesis,
Allylation of Benzophenone with Catalytic Amounts
of Allylic Zinc Amide and Diallylic Zinc Species
Next, we attempted to clarify a critical catalytic inter- we next conducted a kinetic study to investigate the
mediate in our catalytic system: allylic zinc amide or effect of substrates and zinc amide on the rate of the
diallylic zinc species. From another point of view, we
expected to show the significant effect of the amide
group on the high activity of zinc amide towards ally-
lation compared with other zinc sources (Scheme 10).
We therefore prepared allylic zinc amide and diallylic
zinc as catalysts, and conducted the allylation. It was
found that, in these two different systems, a significant
reactivity difference was observed in relative rates for
the subsequent allylation of benzophenone. In the
presence of a catalytic amount of diallylic zinc
(1.00 mol%), the allylation proceeded with 38% con-
Scheme 10. Catalytic reactivities of allylzinc amide and dia-
llylzinc.
Figure 1. Effect of concentration of allylboronate 1b.
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allylation reaction. To determine the kinetic order of
allylboronate 1b and a ketone, we measured the rate
of the reaction with various concentrations of allyl-
boronate 1b and benzophenone (Figure 1). We were
able to observe a first-order dependence of the allyla-
tion rate on the allylboronate concentration. The ob-
served rate of the product formation remained con-
stant until 30% conversion.
Furthermore, the kinetic study indicated that the
addition of increasing equivalents of benzophenone
resulted in no significant change in the observed rate
of the reaction. The plot clearly showed a zero-order
dependence of the rate on the ketone (Figure 2).
On the other hand, the catalyst curve represented
a first-order dependence of the rate on the increasing
concentration of zinc amide (Figure 3). In view of this
accessible catalytic process, wherein no obvious induc-
tion period and side reactions were observed, we
were able to determine the effect of ketone, allylboro-
nate, and the catalyst concentrations on the rate from
the initial stage. The first orders for allylboronate and
zinc amide were consistent with a mechanism involv-
ing transmetalation of boron to zinc to generate allyl-
ic zinc amide at the outset (Step A in Pathway I or
Step A1 or A2 in Pathway III). The zero order for
the ketone suggested that allylation (Step B in Path-
ways I and III) was not the rate-determining step.
Figure 3. Effect of concentration of ZnACTHNURGTNEUNG(HMDS)2.
Next, we prepared boron amide independently to
clarify its effect on the catalytic turnover. In the
11B NMR spectrum, the signal of the prepared boron
amide was consistent with that observed during trans-
metalation. This signal confirmed our catalytic cycle
and showed that the formation of boron amide was
involved as an intermediate.
With this compound in hand, we conducted a kinetic
study to investigate the effect of boron amide on the
rate of the reaction (Figure 4). Compared with our
general system, additional boron amide accelerated
the reaction rates. Whereas, with various concentra-
tions of additional catalytic amounts of boron amide
[0.80 to 2.0 equiv. with respect to ZnACTHNUTRGENUG(N HMDS)2], given
the lack of an observable induction period, the reac-
tion rates were essentially constant until 30% conver-
sion because of the zero-order kinetics of boron
amide. We considered that the existence of boron
amide might play a role as Lewis acid to accelerate
the reaction rate, but it was clear that transboronation
(Step C in Pathway I) was not a rate-determining
step.
In addition, we found that the introduction of
a large amount of boron amide (0.10 and 0.20 equiv.
with respect to benzophenone) resulted in a 60% de-
crease in the observed initial rate of the reaction. This
Figure 2. Effect of concentration of benzophenone (2h).
1200
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Catalytic Use of Zinc Amide for Transmetalation with Allylboronates
Catalytic Asymmetric Allylation Reactions Using
a Chiral Zinc Amide
Next, we considered asymmetric reactions using
a chiral zinc amide. While catalytic allylations of ke-
tones using zinc catalysts have been intensively stud-
ied, there are few successful examples of asymmetric
allylation using a chiral zinc species. In the initial
screening, we investigated some combinations of
chiral ligands and ZnACTHNUTRGNE(NUG HMDS)2, and found that chiral
diamine ligand L1 derived from 1,2-diphenylethylene-
diamine was promising, and high enantioselectivity
was observed in the allylation reaction of a-keto ester
3o with allylboronate 1b after optimization of reac-
tion conditions (Table 4, entry 1, 92% yield with 92%
ee).[20] Those reaction conditions could also be em-
ployed in the reaction of 3q with 1b; however, lower
enantioselectivity was obtained (entry 2). The reac-
tion at lower temperature improved the enantioselec-
tivity (entry 3). In the same reaction conditions, a-
keto ester 3p also reacted with 1b in moderate enan-
tioselectivity (entry 4). We then further examined the
effect of arylmethyl structures of the ligand and found
that a chiral diamine ligand L2 bearing a 2-naphthyl-
methyl substituent was effective, and higher selectivi-
ties were obtained (entries 5–7). On the other hand,
acetophenone (1a) was also employed in the asym-
metric reaction; however, almost no enantioselectivity
was observed. Those results indicated that the second
chelation moiety of the substrate-to-zinc center was
important to form an effective asymmetric environ-
ment around the ketone moiety.
Figure 4. Effect of concentration of boron amide 7.
result presented one aspect of the equilibrium within
The structure of the chiral zinc amide was then in-
transmetalation, wherein boron amide was formed as vestigated. ZnACHTNUGTERN(UNNG HMDS)2 and L2 were combined in tol-
1
a product. This plot demonstrated competitive boron uene-d8 and the mixture was heated at 758C, and H
amide inhibition and supported the suggestion of rela- and 13C NMR analyses were conducted (see the Sup-
tively slow exchange between allylboronate and zinc porting Information). After heating for 1 h, three tri-
amide (Step A in Pathway I or Step A1 or A2 in methylsilyl peaks and eight other aliphatic carbon
Pathway III) in contrast to allylation (Step B in Path- peaks in 13C NMR charts were observed, although the
ways I and III) and transboronation (Step C in Path- 1H NMR chart was complicated. Further heating of
way I). Herein, we observed first-order kinetic de- the complex solution afforded relatively simple NMR
pendences on allylboronate and zinc amide, and zero- charts, in which two major trimethylsilyl peaks and
order kinetics for ketone and boron amide. The rate four other aliphatic carbons were observed in the
law implied by these kinetic orders demonstrated 13C NMR chart. The solution, after heating for 24 h,
a mechanism involving a turnover-limiting boron-to- was then concentrated under vacuum, and 1H and
zinc transmetalation (Step A in Pathway I or Step A1 13C NMR analyses were conducted after dissolving in
or A2 in Pathway III). Therefore, the mechanistic toluene-d8. In the charts obtained, we observed the
proposal summarized in Scheme 2 or Scheme 4 did disappearance of one trimethylsilyl peak of the two
1
not fit the NMR experimental data and the kinetics major peaks in H and 13C NMR, which correspond to
studies. The last unclear point is whether the plausible HN
catalytic pathway is Pathway I or Pathway III. How- one hydrogen of two NH parts of the ligand was de-
ever, it is difficult to evaluate the two pathways. Fur- protonated by Zn(HMDS)2 after heating to form
ther mechanistic study is required to clarify this issue. a chiral zinc amide (Scheme 11).
ACHTUNGTRNEN(UNG SiMe3)2 (H-HMDS). These results indicate that
AHCTUNGTRENNUNG
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Yi Cui et al.
FULL PAPERS
Table 4. Asymmetric allylation of a-keto ester 2o–q with ZnACTHNUTRGNEUNG
(HMDS)2 and chiral ligand.[a]
Entry
L
Conditions
R1
R2
3
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
L1
L1
L1
L1
L2
L2
L2
À208C, 24 h
À208C, 24 h
À408C, 36 h
À408C, 36 h
À408C, 36 h
À408C, 36 h
À408C, 36 h
Ph
Me
Bn
Bn
Bn
Me
Bn
Bn
3o
3q
3q
3p
3o
3p
3q
92
98
75
81
86
78
72
92
69
78
76
97
83
89
Me
Me
Ph
Ph
Ph
Me
[a]
The reaction was conducted using 2 (0.40 mmol) and 1b (0.44 mmol) in toluene. The chiral zinc amide was prepared by
mixing ZnACHTUNGTRENNUNG(HMDS)2 and the ligand and heating at 758C in toluene for 1 h. The result shown in entry 1 has been reported
in our communication.[16]
Isolated yield.
Determined by chiral HPLC analysis.
[b]
[c]
step of the allylation reaction was the formation of
the allylzinc amide species. Asymmetric allylation
using a chiral zinc amide was also achieved. Further
application of the zinc amide for other carbon-carbon
bond-forming reactions is now ongoing.
Scheme 11. Preparation of chiral zinc amide complex.
Experimental Section
General Information
Conclusions
NMR spectra were recorded on a JEOL ECX-400, a JEOL
ECA-500, or a JEOL ECX-600 spectrometer. Chemical
shifts were reported downfield from tetramethylsilane
(TMS) or in the scale relative to the corresponding solvent
used as an internal reference. IR spectra were measured
using a JASCO FT/IR-4200 spectrometer. High resolution
In summary, we have developed a highly efficient zinc
amide-catalyzed allylation of carbonyl compounds,
imines, and hydrazones with a more reactive and
more economical allylboronic acid 2,2-dimethyl-1,3-
propanediol ester (1b). Mechanism studies elucidated
that this transformation succeeds via a boron-to-zinc
transmetalation, and this transmetalation successfully
accesses a nucleophilic allylzinc amide species as
mass spectra (HR-MS) were recorded using
a JEOL
JMST100TD (DART) spectrometer. High-performance
liquid chromatography was carried out using following appa-
ratuses; Shimadzu LC-20AB (liquid chromatograph), Shi-
a key catalytic intermediate and has been clearly ob- madzu SPD-M20A (photo diode array detector) and DGU-
20A3. Preparative thin-layer chromatography (PTLC) was
carried out using Wakogel B-5F from Wako Pure Chemical
Industries, Ltd. Flash column chromatography was per-
formed over silica gel 60 N from Kanto Chemical Co., Inc.
All solvents used were commercially available dry solvents
that were further dried and degassed appropriately under an
argon atmosphere, and stored over activated molecular
sieves in an argon box prior to use. Unless otherwise speci-
fied, all ketones, imines and hydrazones (purchased from
commercial sources) used in this work were distilled under
served by NMR spectroscopy analysis. This allylation
presents broad substrate generality with high toler-
ance to various functional groups, and also proceeds
well with a-methyl-substituted boronates to yield the
a-addition product exclusively. It is noteworthy that,
to the best of our knowledge, this report presents the
first example of a synthesis method involving the use
of a catalytic amount of zinc amide. Our mechanistic
study revealed that the actual reactive allylating spe-
cies was allylzinc amide, and that the rate-determining an argon atmosphere or recrystallized prior to use. Pinacol
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Catalytic Use of Zinc Amide for Transmetalation with Allylboronates
allylboronate 1a, allylboronate 1b[21] and a-methylallylboro-
nate 1c[22] were prepared by reported methods; their analy-
ses are in agreement with the reported data. Zinc bis-
2-(4-Nitrophenyl)pent-4-en-2-ol (3d):[9a] Colorless oil;
1H NMR (CDCl3, 600 MHz): d=8.20 (d, J=8.9 Hz,
2H),7.63 (dd, J=6.9, 2.0 Hz, 2H), 5.63–5.56 (m, 1H), 5.17–
5.14 (m, 2H), 2.69 (dd, J=13.7, 6.2 Hz, 1H), 2.56 (dd, J=
(HMDS)2)[17] was prepared ac-
ACHTUNGTRENNUNG[bis(trimethylsilyl)amide] (ZnCAHTUNGTRENNUGN
cording to a reported procedure, and stored in glove box at
À308C. Chiral ligand L23 was prepared according to a litera-
ture procedure. Zinc chloride was purchased from Wako
Pure Chemical Industries, Ltd.; zinc bromide and zinc tri-
flate were purchased from Tokyo Chemical Industry. Co.,
Ltd (TCI); zinc fluoride hydrate and zinc acetate were pur-
chased from Aldrich Co. Inc. All the zinc sources were
stored in glove box at À308C or room temperature, respec-
tively. All reactions were carried out under an argon atmos-
phere in flame-dried glassware.
13.7, 7.6 Hz, 1H), 2.15ACTHNGUTERNNUG
(br, 1H), 1.58 (s, 3H); 13C NMR
(CDCl3, 150 MHz): d=155.1, 146.8, 132.5, 125.9, 123.5,
120.5, 73.7, 48.3, 29.9.
2-(Pyridin-3-yl)pent-4-en-2-ol (3f):[10] Colorless oil;
1H NMR (CDCl3, 500 MHz): d=8.63 (d, J=4.6 Hz, 1H),
8.37 (t, J=5.2 Hz, 1H), 7.84 (dd, J=5.7, 4.0 Hz, 1H), 7.26
(d, J=12.0, 6.9 Hz, 1H), 5.73–5.66 (m, 1H), 5.08–5.04 (m,
2H), 4.68 (brs, 1H), 2.62–2.53 (m, 2H), 1.58 (s, 3H);
13C NMR (CDCl3,125 MHz): d=146.9, 146.4, 143.5, 133.1,
133.0, 122.8, 118.8, 72.0, 48.4, 29.0.
Benzyl 2-hydroxy-2-phenylpent-4-enoate (3p): Colorless
oil; 1H NMR (CDCl3, 600 MHz): d=7.60–7.59 (m, 2H),
7.34–7.24 (m, 8H), 5.80–5.73 (m, 1H), 5.21 (m, 1H), 5.13–
5.07 (m, 3H), 3.78 (s, 1H),3.00 (dd, J=13.7, 7.6 Hz, 1H),
2.79 (dd, J=14.4, 6.9 Hz, 1H); 13C NMR (CDCl3, 150 MHz):
d=174.4, 141.1, 134.9, 132.1, 128.5, 128.5, 128.2, 128.1, 127.8,
125.5, 119.4, 78.0, 68.0, 43.9.IR (neat): n=3518, 3066, 3033,
2980, 2959, 2918, 1729, 1643, 1497, 1450, 1372, 1262, 1221,
1145, 921, 733, 696 cmÀ1; HR-MS (DART): m/z=265.1230,
Typical Procedure for ZnACHTUNRGTNEUNG(HMDS)2-Catalyzed
Allylation with Allylboronate 1b
To a dried septum-capped 10-mL flask with magnetic stir-
ring bar under an argon atmosphere was added ZnACTHUNRGTNEUNG(HMDS)2
(1.2 mg, 0.0030 mmol, 0.10 mol%). After addition of dry sol-
vent (pentane or THF or co-solvent of both, depending on
substrates) (3.0 mL, 1.0M), allylboronate 1b (508 mg,
560 mL, 3.30 mmol) and the corresponding substrates 2a–w
(3.00 mmol) were added. The mixture was stirred under an
argon atmosphere at 208C for 36 h. After dilution with ethyl
acetate was added a saturated aqueous NH4Cl and the
phases were separated. The aqueous phase was then extract-
ed with ethyl acetate (15 mLꢁ3) and the combined organic
layers were dried on Na2SO4, filtered and concentrated
under vacuum. The residue was purified by silica gel column
chromatography (eluent: n-hexane/ethyl acetate=15:1 to
2:1) to afford the corresponding homoallylic alcohols,
amines and hydrazides 3a–w. Physical data of 3a, 3b, 3e, 3g–
o, 3r, 3s, 3u–w were reported in our communication.[16]
+
calcd. for C18H17O2 [MÀOH]+: m/z=265.1229; HPLC [con-
ditions for 3p: Chiralpak AS-H column, hexane/i.PrOH
(91:9) 0.5 mLminÀ1]: tR =16.0 min (minor), tR =21.5 min
(major); [a]2D0: +5.18 (c 0.42, CHCl3).
Benzyl 2-hydroxy-2-methylpent-4-enoate (3q):[25] Color-
1
less oil; H NMR (CDCl3, 600 MHz): d=7.36–7.35 (m, 5H),
5.75–5.71 (m, 1H), 5.19 (s, 2H), 5.09–5.04 (m, 2H), 3.20 (s,
1H), 2.53 (dd, J=13.8, 7.4 Hz, 1H), 2.42 (dd, J=14.3,
6.9 Hz, 1H), 1.44 (s, 3H); 13C NMR (CDCl3, 150 MHz): d=
176.3, 135.2, 132.1, 128.6, 128.5, 128.2, 119.2, 74.4, 67.4, 44.6,
25.4. HPLC [conditions for 3q: Chiralpak AS-H column,
hexane/i-PrOH (500:1) 0.6 mLminÀ1]: tR =46.9 min (S), tR =
53.1 min (R).
Methyl 2-{2-[4-(dimethylamino)benzoyl]hydrazinyl}-pent-
4-enoate (3t):[12a] Pale yellow oil;1H NMR (CDCl3,
500 MHz): d=8.28 (s, 1H), 7.69–7.67 (m, 2H),6.63–6.62 (m,
2H), 5.91–5.83 (m, 1H), 5.20–5.12 (m, 2H), 4.70 (br,
1H),3.86–3.84 (m, 1H), 3.72 (s, 3H), 2.98 (s, 6H), 2.62 (dd,
J=14.3, 8.0 Hz, 1H), 2.49 (dd, J=14.9, 7.4 Hz, 1H);
13C NMR (CDCl3, 125 MHz): d=173.2, 167.4, 152.7, 133.2,
128.5, 119.1, 118.5, 111.0, 62.5, 51.9, 40.0, 35.3.
Typical Procedure for Asymmetric Allylation of a-
Keto Ester with Allylboronate 1b
To the chiral ligand L (0.024 mmol) in a dry flask was added
ZnACHTUNGTRENNUNG(HMDS)2 (7.7 mg, 0.020 mmol) in dry toluene (0.5 mL)
under an argon atmosphere, and the mixture was stirred at
758C for 1 h, the color of solution gradually changed to red.
Then the solution was cooled to À408C. To the cooled solu-
tion was added allylboronate 1b (68 mg, 75 mL, 0.44 mmol)
and then a-keto ester 1o–q (0.40 mmol) solution in dry tolu-
ene (0.5 mL) was added dropwise, the mixture was stirred at
À408C for 36 h. The reaction was quenched with saturated
aqueous NH4Cl. The resultant mixture was extracted with
ethyl acetate (10 mLꢁ3), and the combined organic layers
were dried over anhydrous Na2SO4. The solvents were
evaporated under vacuum, and the residue was purified by
preparative TLC (hexane/AcOEt=6/1) to give 3o–q. The
enantiomeric excess of the product was determined by
HPLC analysis.
Acknowledgements
This work was partially supported by a Grant-in-Aid for Sci-
ence Research from the Japan Society for the Promotion of
Science (JSPS) and the Global COE Program (Chemistry In-
novation through Cooperation of Science and Engineering),
the University of Tokyo, and MEXT, Japan.
2-(4-Bromophenyl)pent-4-en-2-ol (3c):[24] Colorless oil; References
1H NMR (CDCl3, 500 MHz): d=7.44 (ddd, J=9.2, 5.2,
2.9 Hz, 2H), 7.30 (ddd, J=9.2, 5.2, 2.9 Hz, 2H), 5.62–5.56
(m, 1H), 5.11–5.07 (m, 2H), 2.62 (dd, J=13.7, 6.3 Hz, 1H),
2.47 (dd, J=13.7, 8.0 Hz, 1H), 2.32–2.30 (m, 1H), 1.49 (s,
3H); 13C NMR (CDCl3, 125 MHz): d=146.6, 133.1, 131.0,
126.6, 120.4, 119.5, 73.3, 48.2, 29.5.
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