Regioselectivity of intramolecular rhodium-catalyzed c-h insertion reactions of α-aryl-α-diazocarboxylates: Influence of the aryl substituent

Article abstract of DOI:10.1055/s-0033-1341062

It was found that α-aryl-α-diazocarboxylates react with variable regioselectivity in intramolecular rhodium-catalyzed C-H insertion reactions. 3-(Trialkoxysilyl)propyl esters underwent a clean cis-γ-lactone formation (62-69%) if the aryl rest was a phenyl group while the analogous α-(2-bromophenyl)-α-diazocarboxylates produced the respective β-lactones. In general β-lactone formation was shown to be the only detectable C-H insertion pathway for the latter substrate class irrespective of the ester substituent. The β-lactone products (eight examples) were obtained in yields of 41-68% with a diastereomeric ratio (dr) of 75:25 to 96:4 in favor of the trans diastereoisomer. The 2-bromo substituent could be displaced by an aryl group in a subsequent Suzuki cross-coupling reaction. Georg Thieme Verlag Stuttgart New York.

Full text of DOI:10.1055/s-0033-1341062

LETTER
▌1081
lRetteergioselectivity of Intramolecular Rhodium-Catalyzed C–H Insertion Reac-
tions of α-Aryl-α-diazocarboxylates: Influence of the Aryl Substituent
C–H Insertion Reactions of α-Aryl-α-diazocarboxylates
Maximilian Wamser,a,b Thorsten Bach*a
a
Department Chemie and Catalysis Research Center (CRC), Technische Universität München, Lichtenbergstr. 4, 85747 Garching, Germany
Fax +49(89)28913315; E-mail: thorsten.bach@ch.tum.de
b
WACKER-Institut für Siliciumchemie, Technische Universität München, Lichtenbergstr. 4, 85747 Garching, Germany
Received: 09.01.2014; Accepted after revision: 04.03.2014
a 2-bromophenyl substituent in α-position strongly favors
Abstract: It was found that α-aryl-α-diazocarboxylates react with
the formation of β-lactones upon rhodium-catalyzed dedi-
variable regioselectivity in intramolecular rhodium-catalyzed C–H
azotation/C–H insertion of α-aryl-α-diazocarboxylates in
benzene solution.
insertion reactions. 3-(Trialkoxysilyl)propyl esters underwent a
clean cis-γ-lactone formation (62–69%) if the aryl rest was a phenyl
group while the analogous α-(2-bromophenyl)-α-diazocarboxylates
produced the respective β-lactones. In general β-lactone formation
was shown to be the only detectable C–H insertion pathway for the
latter substrate class irrespective of the ester substituent. The β-lac-
tone products (eight examples) were obtained in yields of 41–68%
with a diastereomeric ratio (dr) of 75:25 to 96:4 in favor of the trans
diastereoisomer. The 2-bromo substituent could be displaced by an
aryl group in a subsequent Suzuki cross-coupling reaction.
O
O
[Rh2(OAc)4]
(CH2Cl2)
O
O
N2
55%
1
2
Key words: carbene complexes, diastereoselectivity, insertion, lac-
tones, regioselectivity, rhodium, ring closure
O
O
[Rh2(OAc)4]
(CH2Cl2)
O
O
N2
79%
Within the last decades, the intramolecular rhodium-cata-
lyzed C–H insertion reaction of α-diazocarbonyl com-
pounds has been established as a versatile and reliable
method to form carbo- and heterocyclic rings.1 Extensive
efforts have been devoted to elucidate the parameters,
3
4
Scheme 1 Examples of previous work on the rhodium-catalyzed C–
H insertion reaction of α-phenyl-α-diazocarboxylates 1 and 35g
which
control
the
regioselectivity,2
the
The initial discovery of this selectivity change was made
in reactions which aimed at the intermolecular functional-
ization of hydrocarbons by C–H insertion.7 While em-
ploying 3-(trialkoxysilyl)propyl esters 5 an intra-
molecular side reaction was observed, which delivered in
the absence of a competing intermolecular substrate in
good yields the respective lactones 6 as single diastereo-
isomers (Scheme 2).
diastereoselectivity3,4 and the enantioselectivity1,5 of this
transformation. Regarding the regioselectivity, it is gener-
ally assumed that insertion occurs into the most electron-
rich C–H bond and that the substitution pattern of the sub-
strate therefore influences and possibly alters the regiose-
lectivity. As an example the reaction of α-phenyl-α-
diazocarboxylates 1 and 3 is depicted in Scheme 1.5g C–H
insertion occurs at the tertiary but not at the secondary car-
bon atom leading in the former case to the formation of β-
lactone 2 and in the latter case to the formation of γ-lac-
tone 4.
The assignment of the relative configuration is based on
the coupling constant J between the two protons at the
3
stereogenic centers in α- and β-position. For cis-config-
ured α-aryl-substituted γ-lactones the coupling constant is
typically between 8.0 Hz and 9.0 Hz whereas it is signifi-
cantly larger for trans-γ-lactones (3J = 10.5–11.0 Hz).8
For products 6a and 6b the relevant coupling constant was
found to be identical at 3J = 8.1 Hz. The exclusive forma-
tion of the γ-lactones 6 can be tentatively explained by the
ability of the silicon–carbon bond to increase the electron
density in β-position (β-silicon effect)9,10 thus rendering
the respective methylene group at position C2 of the 3-
(trialkoxysilyl)propyl group more reactive. Surprisingly,
however, the ortho-bromo-substituted analogues 7 of
compounds 5 exhibited a completely reversed regioselec-
tivity. The only isolable products of an intramolecular re-
action were identified as β-lactones 8, with a clear
preference for the trans-products trans-8 over their diaste-
Other experiments support this observation, indicating
that it is primarily the substitution pattern of the ester bond
in α-phenyl-α-diazocarboxylates, which determines the
regioselectivity.6 Although it has been recognized that the
substituent at the α-position of an α-diazoketone or an α-
diazocarboxylate can increase or decrease the electronic
discrimination between different positions,1,2 an influence
of the substitution pattern at the phenyl ring of α-aryl-α-
diazocarboxylates has to the best of our knowledge not yet
been observed. In this communication we now show, that
SYNLETT 2014, 25, 1081–1084
Advanced online publication: 03.04.2014
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
DOI: 10.1055/s-0033-1341062; Art ID: ST-2014-B0025-L
© Georg Thieme Verlag Stuttgart · New York

1082
M. Wamser, T. Bach
LETTER
O
Table 1 Preparation of α-(2-Bromophenyl)-α-diazocarboxylates 7
from Potassium 2-Bromophenylacetate (9)
O
[Rh2(OAc)4]
α
O
O
(PhH)
O
RCl (10), [KI]
O
N2
β
Si(OR)3
2
[Bu4PBr] (DMF)
R
K
O
O
(RO)3Si
Br
5a R = i-Pr
5b R = t-Bu
62%
69%
6a (cis/trans >90:10)
6b (cis/trans >90:10)
X
X
Br
9
11 X,X = H,H
TsN3, DBU (MeCN)
7
X,X = N2
O
O
O
Alkyl chloride
(10)
Yield of 11 Yield of 7
[Rh2(OAc)4]
(PhH)
(%)a
(%)b
O
N2
Br
1
10a
62
75
(i-PrO)3SiO
(t-BuO)3SiO
(RO)3Si
Br
(RO)3Si
10b
10c
78
66
77
86
7a R = i-Pr
7b R = t-Bu
68%
48%
8a (trans/cis = 85:15)
8b (trans/cis = 85:15)
Scheme 2 Regioselectivity switch upon using a different aryl group
in the rhodium-catalyzed C–H insertion reaction of α-aryl-α-diazo-
carboxylates
10d
60
68
87
Ph
10e
10f
10g
10h
63
–
HO
TBSO
Cl
c
–
reoisomers cis-8. Again, the configuration assignment
was based on the 3J coupling constants of the hydrogen at-
oms at the two vicinal stereogenic centers. This value is
typically around 4.0 Hz for trans-substituted and around
7.0 Hz for cis-substituted β-lactones.11 In our case, the
major diastereoisomers obtained from the C–H insertion
reaction showed in both cases (8a and 8b) identical cou-
35
12d
69
72
a The alkylation reaction was performed at 120 °C in DMF using po-
tassium acetate 9 (1.2 equiv). KI (10 mol%) and tetrabutylphosphoni-
um bromide (2 mol%) were employed in catalytic amounts. The given
yield refers to isolated product.
3
b The diazotransfer was performed with stoichiometric amounts of
TsN3 (1.0 equiv) and an excess of DBU (1.5 equiv). The given yield
refers to isolated product.
pling constants of J = 4.2 Hz. The coupling constant of
the minor diastereoisomers was found to be 3J = 6.5 Hz.
In order to explore the scope of the intramolecular C–H
insertion of α-(2-bromophenyl)-α-diazocarboxylates, sev-
eral other members of this compound class were prepared
c The compound was obtained by desilylation of silyl ether 10g with
tetrabutylammonium fluoride (TBAF) in THF (55% yield).
d Two-fold substitution was observed as a competing side reaction
(Table 1). Starting material was in all cases the commer- (23%). In addition, 39% of unreacted starting material was recovered.
cially available 2-bromophenylacetic acid, which was
quantitatively converted (KOH, EtOH) into the respective
was identified as a by-product. The respective γ-lactone
potassium salt 9. Nucleophilic substitution with an appro-
could neither be detected nor isolated. The same holds
priate alkyl chloride 10 delivered the respective esters
true for the other products 8, which were obtained as mix-
11,12 which were converted into the desired products by
tures of trans- and cis-diastereoisomers and which
diazotransfer employing p-toluenesulfonyl azide (TsN3)
showed no indication of γ-lactone formation. In some of
as the reagent and 1,8-diazabicyclo[5.4.0]undec-7-ene
the reactions impurities resulting from non-converted
(DBU) as the base.13
starting material could not be completely removed.
Yields for the combined steps were satisfactory, except
In order to possibly distinguish between the electronic and
for the alkylation reactions with 1,8-dichlorooctane (10h),
steric influence of the phenyl substitution pattern in α-
for which an extensive amount of double substitution was
aryl-α-diazocarboxylates, other members of this com-
observed. The reaction was not further optimized. Alco-
pound class were prepared. If the aryl group was 2-meth-
hol 10f was prepared from silyl ether 10g (see footnote c
ylphenyl (Figure 1, compound 12) a sluggish reaction was
of Table 1).
observed under the typical reaction conditions given in
The rhodium-catalyzed reaction of α-diazocarboxylates 7
delivered under standard conditions (Scheme 3) the re-
spective β-lactones in moderate to good yields (41–
66%).14 Even with primary alkyl groups a clear preference
for the formation of the trans product was observed,
which varied from 75:25 to 93:7. The isopropyl-substitut-
ed product 8d could not be isolated in pure form. The re-
action remained incomplete and the starting material (7d)
was not fully separable from the product. In addition, re-
duced diazo compound, i.e. 2-bromophenylacetate 11d,
Scheme 3 [c = 0.5 M; 80 °C in benzene; 0.5 mol%
Rh2(OAc)4]. It was possible to identify β-lactone 13 in the
product mixture but the compound could not be isolated in
pure form. The yield was determined by integration of ap-
propriate 1H NMR signals in a purified substrate/product
mixture. The result with the 3-bromophenyl derivative
was very similar. Minor amounts of the β-lactone could be
identified but yields remained low (<20%). The 4-bromo-
phenyl derivative 14 did not deliver any β-lactone. The
Synlett 2014, 25, 1081–1084
© Georg Thieme Verlag Stuttgart · New York

LETTER
C–H Insertion Reactions of α-Aryl-α-diazocarboxylates
1083
nyl-di-tert-butylphosphane as ligand and Pd(OAc)2 as
palladium source15 there was a notable conversion at 50
°C (Scheme 4) and bromide 8c delivered the desired prod-
uct 15 in 51% yield. The starting material was used as
88:12 mixture of trans- and cis-isomer. Surprisingly, the
product showed four sets of signals in its NMR spectra,
pairs of which integrated to a ratio of 88:12. It seems like-
ly that the restricted rotation around a C–C single bond is
responsible for the second pair of signals.
O
O
O
O
O
O
[Rh2(OAc)4]
(PhH)
R1
R1
N2
Br
R1
Br
Br
7
trans-8
cis-8
O
O
O
O
O
O
O
O
O
PhB(OH)2, KF
[Pd(OAc)2] (THF)
[(o-biphenyl)P(t-Bu)2]
Br
8c 66%
Br
8d 41%
Br
O
Ph
8e 63%
(trans/cis = 88:12)
(trans/cis = 96:4)
(trans/cis = 93:7)
51%
Br
8c
15
O
O
O
O
O
O
Scheme 4 Palladium-catalyzed Suzuki cross-coupling of aryl bro-
mide 8c with phenylboronic acid
5
Br
Br
Br
Mechanistically, there is no stringent hypothesis, which
explains the influence of the ortho-bromo substituent. As
evident from Scheme 2, the change in regioselectivity is
very pronounced but the effect is neither solely steric nor
solely electronic (Figure 1). Synthetically, the exclusive
formation of β-lactones is useful in particular because fur-
ther bromide substitution reactions are feasible.
HO
TBSO
Cl
8f 58%
8g 50%
8h 58%
(trans/cis = 75:25)
(trans/cis = 82:18)
(trans/cis = 85:15)
Scheme 3 Formationa of β-lactones 8 by rhodium-catalyzed intra-
molecular C–H insertion reactions of α-(2-bromophenyl)-α-diazocar-
boxylates 7. a The reaction was performed by addition of a solution of
substrate 7 in benzene (c = 0.5 M) to a solution of 0.5 mol% catalyst
in benzene at 80 °C. The diastereomeric products 8 were not separable
1
and the ratio of diastereoisomers was determined by H NMR spec-
troscopy.
Acknowledgment
This project was supported by Wacker Chemie AG. Dr. Jürgen
Stohrer (Wacker Chemie AG) is acknowledged for helpful discus-
sions.
Br
O
O
O
O
O
O
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett. Included are
N2
N2
procedures and analytical data for all new compounds.SnoIufirop
m
tgioSrantnugIifoop
r
imaotrtn
12
14
13 20%
(trans/cis = 88:12)
Primary Data for this article are available online at
http://www.thieme-connect.com/ejournals/toc/synthesis and can be
cited using the following DOI: 10.4125/pd0057th. FIDs and asso-
ciated files for the 1H and 13C NMR spectra for compounds 5, 6, 7,
Figure 1 Structures of butyl α-aryl-α-diazocarboxylates 12, 14 and
of β-lactone 13
8, 11, and 15 are provided.mPirDayrmaPitrDayra1t0.4p12d50/05mhP7it.yrarDaZatahle1n01Chemyrst
i
only product derived from the diazo compound was the
respective dimer as a cis-/trans-mixture. In general, apart
from dediazotation of the starting material the formation
of dimers (aryl-substituted fumarates and maleates) was
the major side reaction to be observed in competition with
the desired cyclization.
References and Notes
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© Georg Thieme Verlag Stuttgart · New York
Synlett 2014, 25, 1081–1084

1084
M. Wamser, T. Bach
LETTER
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was charged with benzene (12 mL) and Rh2(OAc)4 (0.5
mol%) and the solution was heated to 80 °C. Diazo
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(trans)], 1.32–1.40 [m, 0.24 H, CH2CH3 (cis)], 1.49–1.65
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CHCH2CHH), 2.12–2.21 (m, 1 H, CHCH2CHH), 4.43–4.47
[m, 0.88 H, CHCH2 (trans)], 4.80 [d, 3J = 4.3 Hz, 0.88 H,
CHAr (trans)], 4.89–4.93 [m, 0.12 H, CHCH2 (cis)], 5.26 [d,
3J = 6.6 Hz, 0.12 H, CHAr (cis)], 7.14 [virt. t, 3J  7.4 Hz,
0.12 H, CCHarCHar (cis)], 7.21 [virt. t, 3J  7.4 Hz, 0.88 H,
CCHarCHar (trans)], 7.28 [virt. t, 3J  7.4 Hz, 0.12 H,
CCHarCHarCHar (cis)], 7.36 [virt. t, 3J  7.4 Hz, 0.88 H,
CCHarCHarCHar (trans)], 7.44 [d, 3J = 7.8 Hz, 0.88 H, CCHar
(trans)], 7.48 [d, 3J = 7.8 Hz, 0.12 H, CCHar (cis)], 7.57
[d, 3J = 7.8 Hz, 0.12 H, CHarCBr (cis)], 7.60 [d, 3J = 7.8 Hz,
0.88 H, CHarCBr (trans)]. 13C NMR (90.6 MHz, CDCl3): δ
(major diastereoisomer) = 13.7, 18.4, 30.6, 60.8, 80.1, 123.8,
128.3, 128.7, 129.9, 133.0, 133.2, 168.8. HRMS: m/z calcd
for C9H679BrO2: 224.0195; found: 224.0191. HRMS: m/z
calcd for C9H681BrO2: 226.0175; found: 226.0177.
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