Gold(III) chloride-catalyzed diastereoselective alkylation reactions with chiral benzylic acetates

Article abstract of DOI:10.1002/adsc.200700600

Gold(III) chloride was shown to be an efficient catalyst for the diastereoselective C-C bond formation of various chiral para-methoxybenzylic acetates and different nucleophiles. All electrophilic acetates carried next to the reacting center a stereogenic carbon center bound to a functional group (FG), a methyl substituent and a proton. Selectivities were good (dr > 80/20) in favor of the anti-product for FG = COOMe, NO₂, CN and in favor of the syn-product for FG = SO₂Et, PO(OEt)₂. The reactions proceed most likely via a free carbocation, in which a face differentation is facilitated by a preferred conformation. Several arene nucleophiles were shown to be compatible with the catalysis conditions providing the corresponding substitution products in high yields (13 examples, 62-98%). Moreover, other nucleophiles (allyltrimethylsilane, trimethylsilyl cyanide, 2,2-dimethyl-3- (trimethylsilyloxy)butane, p-toluenesulfonamide, and acetylacetone) reacted with a representative chiral electrophile in a high yielding and diastereoselective fashion.

Full text of DOI:10.1002/adsc.200700600

FULL PAPERS
DOI: 10.1002/adsc.200700600
Gold(III) Chloride-Catalyzed Diastereoselective Alkylation
G
Reactions with Chiral Benzylic Acetates
Philipp Rubenbauera and Thorsten Bacha,*
a
Lehrstuhl für Organische Chemie I, Technische Universität München, Lichtenbergstr. 4, 85747 Garching, Germany
Fax : (+49)-89-289-13315; e-mail: thorsten.bach@ch.tum.de
Received: December 19, 2007; Revised: February 4, 2008; Published online: April 9, 2008
Dedicated to Professor Andreas Pfaltz on the occasion of his 60th birthday.
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: GoldACHTREUNG(III) chloride was shown to be an effi- formation. Several arene nucleophiles were shown to
ꢀ
cient catalyst for the diastereoselective C C bond be compatible with the catalysis conditions providing
formation of various chiral para-methoxybenzylic the corresponding substitution products in high
acetates and different nucleophiles. All electrophilic yields (13 examples, 62–98%). Moreover, other nu-
acetates carried next to the reacting center a stereo- cleophiles (allyltrimethylsilane, trimethylsilyl cya-
genic carbon center bound to a functional group nide, 2,2-dimethyl-3-(trimethylsilyloxy)butane, p-tol-
(FG), a methyl substituent and a proton. Selectivities uenesulfonamide, and acetylacetone) reacted with a
were good (dr>80/20) in favor of the anti-product representative chiral electrophile in a high yielding
for FG=COOMe, NO2, CN and in favor of the syn- and diastereoselective fashion.
product for FG=SO2Et, POACTHRE(UNG OEt)2. The reactions
proceed most likely via a free carbocation, in which Keywords: alkylation; carbocations; diastereoselec-
a face differentation is facilitated by a preferred con- tivity; Friedel–Crafts reactions; gold; Lewis acids
Introduction
of carbonyl compounds with electron-rich arenes.
They established the suitability of AuCl3 to catalyze
The typical choice of electrophilic substrates in Frie- this sequential transformation, which also involves an
del–Crafts alkylation reactions[1] includes alkyl chlor- SN1-type arylation.
ides, alkenes or alkanols and their derivatives. The re-
Recently, we have launched a research program
actions are promoted by a Lewis or Brønsted acid which aims to systematically elucidate the diastereo-
and involve in many instances the formation of free selectivity of reactions occurring at trigonal carbeni-
carbenium ions. While alkylation reactions with alkyl um ions.[10] One of the first reactions we have studied
chlorides and alkenes require only very small quanti- is the Friedel–Crafts alkylation of arenes Ar’H using
ties of the catalyst, reactions with alcohols and related chiral benzylic alcohols
I as cation precursors
alkylating agents (esters, ethers, etc.) have classically (Scheme 1). It was shown that, depending on the
been conducted with stoichiometric quantities of the functional group (FG) and on the alkyl substituent R,
catalyst.[2] In recent times there have been extensive a significant preference for either the anti-product
efforts to find catalytic reaction conditions for the anti-II or for the syn-product syn-II can be expected.
Friedel–Crafts alkylation with alcohols and their de-
rivatives. Highly effective Lewis acids have been
found which catalyze cleanly the reaction of various
arenes with alcohols[3] and acetates.[4] The use of gold-
ACHTREUNG
(III) chloride[5] for the catalytic Friedel–Crafts alkyla-
tion with benzylic acetates as described by Beller
et al.[6] and with benzylic alcohols as described by
Dyker et al.[7] is of particular relevance to the present
work.[8] Hashmi et al.[9] compared the reactivity of
Scheme 1. Diastereoselective reaction of chiral benzylic al-
cohols I and arenes Ar’H promoted by stoichiometric
AuCl3 with other catalysts for use in the condensation amounts of HBF4·OEt2.
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With R=Me, the functional groups methoxycarbonyl
(COOMe), nitro (NO2), cyano (CN), trimethylsilylal-
kynyl (CCTMS) and chloro (Cl) favor the formation
of product anti-II in a decreasing order of selectivity
along this collection. For COOMe, NO2 and CN the
observed diastereomeric ratios (dr) often exceed
90/10. Contrary to that, the functional groups ethyl-
sulfonyl (SO2Et), ethoxysulfonyl (SO2OEt), tert-butyl
ACHTREUNG(t-Bu), and diethoxyphosphonyl [POCAHTRE(UGN OEt)2] exhibit a
preference for syn-II in an increasing order of selec-
tivity along this enumeration.
Standard reaction conditions employed in all previ-
ous experiments included the stoichiometric use of
HBF4·OEt2 as a Brønsted acid and a gradual warming
from ꢀ788C to ambient temperature in CH2Cl2 as the
solvent. While the reason for the observed diastereo-
selectivities awaits further studies, it was desirable
from a synthetic point of view to conduct the reac-
tions at ambient temperature possibly employing an
appropriate Lewis acid in catalytic amounts. We now
disclose that benzylic acetates derived from alcohols
of type I can be successfully converted into Friedel–
Scheme 2. Comparsion between alcohols 1 and 4 and the
corresponding acetates 2 and 5 in the AuCl3-catalyzed aryla-
tion of 2-methylthiophene.
[11]
Crafts alkylation products II using AuCl3 as the cat-
alyst. Despite the fact that the reactions were con- reaction to product syn-3, the rate of which was accel-
ducted at room temperature the observed diastereose- erated significantly by changing the electrophile pre-
lectivities were similar to the previously recorded cursor from alcohol 1 to acetate 2. In the first exam-
values. Moreover, it could be shown that nucleophiles ple the reaction required 18 h for full conversion, in
other than arenes can be employed in these transfor- the latter example the reaction was over in less than
mations. Full details of our experimental study are 2 h with the increased diastereoselectivity being an
given below.
additional benefit.
Results and Discussion
Variation of the Electrophile
Catalyst Screening
With resorcine dimethyl ether as a reference electro-
phile we screened various chiral acetates 5, 7–11 in
For an initial catalyst screening the well-established the Au-catalyzed conversion to products 12–17
reaction of benzylic alcohol 1 with 2-methylthiophene (Table 1). The acetates were prepared from the
was chosen.[10b] As expected, the syn-product syn-3 known alcohols[10d] by simple acetylation (AcCl, pyri-
was predominantly formed (Scheme 2). Several cata- dine, N,N-dimethylaminopyridine in CH2Cl2). This set
[12]
[13]
lysts including FeCl3
(in CH2Cl2) and Bi
A
of alkylation experiments undermined the high func-
(in MeNO2) performed well providing a rapid, clean tional group compatibility of the catalyst. All reac-
and diastereoselective conversion to product syn-3. tions proceeded cleanly and in good to excellent
From this first set of screening results AuCl3 did not yields. What is more, the high diastereoselectivities of
show an outstanding profile. Yields for syn-3 were Friedel–Crafts alkylation reactions earlier observed in
similar compared to FeCl3 (79%) and Bi
with FeCl3 even providing a slightly improved diaste- duced (entries 1, 2, 4–6) or even improved (entry 3).
reoselectivity (dr=95/5). Preparatively useful levels of diastereoselectivity were
ACHTRE(UNG OTf)3 (83%) the HBF4-promoted reactions could be fully repro-
The choice for AuCl3 as preferred catalyst for the achieved with almost all electrophiles except for the
current study eventually rested on its superior func- chloro-substituted substrate (9, entry 4, FG=Cl). Re-
tional group compatibility. The reaction of phospho- actions were conducted by adding the catalyst AuCl3
nate 4 and 2-methylthiophene to product 6 was not at ambient temperature to a solution of resorcine di-
promoted by either one of the catalysts, which had methyl ether and the respective acetate in nitrome-
performed well in the reaction 1!syn-3. Only AuCl3 thane. The products 12–17 have been earlier studied
gave a significant yield, which could be further im- and the configuration assignment was discussed else-
proved by choosing the acetate 5 as the precursor in where.[10d] It was proven for the Au-catalyzed experi-
this reaction. A similar observation was made in the ments that the reactions proceed stereoconvergently,
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GoldACHTREUNG(III) Chloride-Catalyzed Diastereoselective Alkylation Reactions
FULL PAPERS
Table 1. AuCl3-catalyzed diastereoselective alkylation of resorcine dimethyl ether with chiral acetates 5, 7–11.
Entry
Substrate
FG
PO
CO2Me
CN
Cl
SO2Et
NO2
Major product
Time [h]
dr[a]
Yield[b] [%]
1
2
3
4
5
6
5
7
8
9
10
11
(OEt)2
syn-12
anti-13
anti-14
anti-15
syn-16
anti-17
2
5
3
1
2.5
0.5
5/95
93/7
93/7
65/35
13/87
94/6
86
86
87
97
79
62
[a]
1
The diastereomeric ratio (anti/syn) of the crude product was determined by H NMR spectroscopy.
Yield of isolated product.
[b]
that is, the relative configuration of the substrate did general, a lower catalyst loading in the alkylation re-
not influence the product configuration. Many of the actions is possible but it led expectedly to a decreased
Au-catalyzed reactions display an intense blue or red reaction rate. The reaction of acetate 7 with 2-methyl-
color, which turns at the end of the reactions fre- thiophene (cf. entry 1) was catalyzed by 1 mol% of
quently into brown.
AuCl3. It was complete after 23 h and yielded 92% of
product 18. The ratio of the two product diastereo-
mers anti-18 and syn-18 (dr=90/10) was unchanged as
compared to the reaction catalyzed by 10 mol% of
AuCl3. Due to the relatively small scale on which the
Variation of the Arene Nucleophile
The b-acetoxycarboxylate 7 was employed to assess reactions were conducted, we consistently used 10
the reactivity pattern of various arenes in the Au-cat- mol% of catalyst. The latter result and a more ex-
alyzed Friedel–Crafts alkylation. This electrophile 7 tended study (vide infra) clearly demonstrate, howev-
appears to be one of the synthetically most useful er, that a catalyst loading of 1 mol% is fully sufficient
building blocks tested in the earlier series (5, 7–11), to achieve high yields and reasonable turnovers.
exhibiting a high diastereoselectivity for an otherwise
To account for the observed diastereoselectivity we
difficult selective b-arylation of a carboxylate. We have suggested a model according to which the free
were delighted to note that the functional group toler- carbenium ion intermediate adopts a preferred con-
ance of the catalyst was perfectly corroborated by formation which, in turn, is dictated by 1,3-allylic
these studies as was the previously observed diaster- strain.[10b,d] The a-proton at the stereogenic center is
eoselectivity.[10d] The method allows access to diaste- positioned syn periplanar to the aryl group with the
reomerically (and if required also enantiomerically) methoxycarbonyl group pointing into one diastereo-
pure b-branched 3,3-diarylpropionates via the easily topic face and the methyl group into the other. With
accessible aldol products. Products anti-18–24 were the latter group being bigger than the former, the in-
obtained in yields of 63–99% in a diastereomeric coming nucleophile prefers the less congested ap-
ratio ranging from 95/15 to 97/3. The difference in proach. Expectedly, the Au-catalyzed reaction with b-
diastereoselectivity seems to parallel the steric re- acetoxycarboxylates, which do not carry a methyl but
quirements of the incoming nucleophile. Arenes a larger alkyl (ethyl, isopropyl) group in the a-posi-
which do not carry a bulky ortho-substituent (en- tion, proceeded with even increased diastereoselectiv-
tries 1, 3, 5, 7) react with lower diastereoselectivity ity. With methylthiophene (entry 1) the selectivitiy
than arenes with an ortho-substituent (entries 2, 6). for substrate 7 with a methyl group at the stereogenic
The reaction of hydroquinone dimethyl ether center was 90/10 (99% yield) but increased with
(entry 4) was somewhat sluggish possibly because of ethyl to 95/5 (98%) and with isopropyl even to 98/2
competing demethylation and oxidation reactions. In (92%).
Adv. Synth. Catal. 2008, 350, 1125 – 1130
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Philipp Rubenbauer and Thorsten Bach
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Other Nucleophiles
with disappointing selectivity, which may be due to
the low steric demand of the nucleophile. The genera-
An extension of the Friedel–Crafts methodology pre- tion of stereogenic centers is reversed as compared to
sented above and in previous papers[10a–d] was con- classical Michael-type addition reactions, in which the
ceived using other nucleophiles either with a proton stereogenic center in b-position is generated prior to
(NuH) or a TMS group (NuTMS) as electrophilic formation of the stereogenic center in a-position, the
ꢀ
leaving group. It was shown that the C C bond form- latter determining the configuration of the former. In
ing reactions with allyltrimethylsilane, trimethylsilyl the bond set presented herein the easily generated a-
cyanide and acetylacetone (Table 2, entries 1, 2, 4) stereogenic center dictates the stereoselectivity of the
occur in high yields (88–94%) and with good diaste- b-stereogenic center. The reaction may therefore
reoselectivity (dr=91/9–97/3). The reaction with the serve in some cases as an alternative useful bond set
heteroatom nucleophile TsNH2 (entry 3) proceeded for the construction of a,b-substituted propionates.
Weaker nucleophiles such as trimethylvinylsilane
did not react with substrate 7 under AuCl3 catalysis.
A slow decomposition was notable with the methoxy-
Table 2. AuCl3-catalyzed diastereoselective alkylation of var-
ious arenes Ar’H with chiral acetate 7.
phenyl group of 7 acting as a nucleophile and leading
to dimeric and oligomeric material. AuCl3 could also
been used to catalyze the reaction of silyl enol ethers
(e.g. Table 3, entry 5) but is slightly inferior to Bi-
[10e]
A
for this specific reaction. The screening of
different catalyst loadings for the reaction of 7 with
acetylacetone (entry 4) showed that these reactions
can be performed with a much lower catalyst loading
than 10 mol% AuCl3. Indeed, the catalyst loading of
10 mol% AuCl3 was – as already stated previously –
chosen for synthetic convenience. The reaction with 5
mol%, 2.5 mol% and 1 mol% AuCl3 varied only
slightly in the reaction times from 2 h to 3.5 h and in
terms of yield (91% to 81%). The diastereomeric
ratio of product 28 was unchanged as compared to
the reaction catalyzed by 10 mol% of AuCl3. Individ-
ual results based on a 0.25-mmol scale, employing
Entry Ar’H
Major
product
Time dr[a] Yield[b]
[h]
[%]
90/
10
1
2
anti-18
anti-19
3
99
97/
3
1
90
78
92/
8
3
4
anti-20
anti-21
0.5
Table 3. AuCl3-catalyzed diastereoselective alkylation of var-
ious nucleophiles NuH or NuTMS with chiral acetate 7.
91/
9
1
63
99
89
92
90/
10
Time [h] dr[a]
Yield[b]
[%]
5
6
anti-22
anti-23
anti-24
0.5
0.5
2
Entry Nucleophile Major
product
1
anti-25
0.1
92/8
91/9
71/29 94
90
77
2
3
anti-26
anti-27
0.5
1
94/
6
4
anti-28
anti-29
1.5
1
97/3
95/5
88
73
85/
15
7
5
[a]
[a]
[b]
The diastereomeric ratio (anti/syn) of the crude product
was determined by H NMR spectroscopy.
Yield of isolated product.
The diastereomeric ratio (anti/syn) of the crude product
was determined by H NMR spectroscopy.
Yield of isolated product.
1
1
[b]
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GoldACHTREUNG(III) Chloride-Catalyzed Diastereoselective Alkylation Reactions
FULL PAPERS
duced pressure. The crude product was purified by flash
chromatography. The products were obtained as diastereo-
meric mixtures.
four equivalents of acetylacetone and a substrate con-
centration of 0.12M were (yield and reaction time)
with 5 mol% AuCl3 91% and 2 h, with 2.5 mol%
AuCl3 84% and 3 h, with 1 mol% AuCl3 81% and
3.5 h. On larger scale (1 mmol of 7) at a concentration
of 0.25M, using 1 mol% AuCl3 it was not only possi-
ble to decrease the amount of acetylacetone to
2 equiv. but also to match the 94% yield achieved
with 10 mol% on a smaller scale (0.25 mmol). The
ratio of the two product diastereomers anti-28 and
syn-28 (dr=97/3) remained unchanged.
Supporting Information
General information, detailed reaction conditions, spectro-
scopic and analytical data of compounds 5, 8–11, anti-21, 25,
26, anti-28 and 29.
Acknowledgements
This work was supported by the Deutsche Forschungsgemein-
schaft (Ba 1372/12–1), by the graduate college NanoCat
(scholarship to P. R.) and by the Fonds der Chemischen In-
dustrie.
Conclusions
In summary, AuCl3 was shown to be a generally appli-
cable catalyst for the diastereoselective conversion of
various benzylic acetates to the corresponding SN1-
type substitution products. Arenes reacted in a Frie-
del–Crafts alkylation to provide the corresponding
products 12–24 in good to very good yields (62–99%)
and with varying selectivities depending on the substi-
tution pattern at the stereogenic center. The b-acet-
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Experimental Section
General Procedure for the Acetylation of Benzylic
Alcohols
The alcohol (1 equiv., 3 mmol), pyridine (6–10 equiv.), the
acetylating agent (Ac2O or AcCl, 2 equiv.) and a catalytic
amount of DMAP were stirred in CH2Cl2 (20 mL) for the
specified time (see Supporting Information). The reaction
was quenched by addition of saturated aqueous NH4Cl
(20 mL) and the aqueous layer was extracted with ethyl ace-
tate (3”20 mL). The combined organic layers were washed
with water (20 mL) and brine (20 mL), dried over Na2SO4
and the solvent was removed under reduced pressure. The
crude product was purified by flash chromatography.
General Procedure for the AuCl3-Catalyzed
Alkylation with Benzylic Acetates
The acetate (1 equiv., 0.25 mmol) and the nucleophile
(4 equiv., 1.0 mmol) were dissolved in nitromethane (2 mL),
AuCl3 (7.6 mg, 25 mmol) was added in one portion and the
solution was stirred until the reaction was complete. The re-
action was quenched by addition of water (10 mL), diluted
with ethyl acetate (10 mL) and the aqueous layer was ex-
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