Synthetic and mechanistic aspects on the competition between C-H insertion and hydride transfer in copper-mediated transformations of α-diazo-β- keto sulfones

Article abstract of DOI:10.1055/s-0033-1340471

Competition between C-H insertion and hydride transfer is reported for the copper-catalysed reactions of a range of phenyl-substituted α-diazo- β-keto sulfones. Control of chemoselectivity is possible by alteration of the electronic properties of the diazo substrate. The production of enantioenriched cyclopentanones (up to 89% ee), formed via C-H insertion, and alkylidene tetrahydrofurans (up to 43% ee), produced via hydride transfer, is described. The isolation of products derived from hydride transfer provides mechanistic insight into the copper-mediated C-H insertion of α-diazoA?carbonyl compounds. Georg Thieme Verlag Stuttgart New York.

Full text of DOI:10.1055/s-0033-1340471

LETTER
▌591
lSetyternthetic and Mechanistic Aspects on the Competition between C–H Insertion
and Hydride Transfer in Copper-Mediated Transformations of α-Diazo-
β-Keto Sulfones
Transformations of α-Diazo-β-Keto Sulfones
Catherine N. Slattery,a Alan Ford,a Kevin S. Eccles,b Simon E. Lawrence,b Anita R. Maguire*b
a
Department of Chemistry, Analytical and Biological Research Facility, University College Cork, Cork, Ireland
b
Department of Chemistry, School of Pharmacy, Synthesis and Solid State Pharmaceutical Centre, Analytical and Biological Research Facility,
University College Cork, Cork, Ireland
Fax +353(21)4274097; E-mail: a.maguire@ucc.ie
Received: 06.11.2013; Accepted: 23.11.2013
mation of a mixture of C–H insertion and aromatic addi-
Abstract: Competition between C–H insertion and hydride transfer
tion or cyclopropanation products.
is reported for the copper-catalysed reactions of a range of phenyl-
substituted α-diazo-β-keto sulfones. Control of chemoselectivity is
Ph
(a)
possible by alteration of the electronic properties of the diazo sub-
strate. The production of enantioenriched cyclopentanones (up to
89% ee), formed via C–H insertion, and alkylidene tetrahydrofurans
(up to 43% ee), produced via hydride transfer, is described. The iso-
lation of products derived from hydride transfer provides mechanis-
tic insight into the copper-mediated C–H insertion of α-diazocarbonyl
compounds.
t-Bu
N2
Rh(II)
+
N
N
N
Ph
H
t-Bu
t-Bu
CH2Cl2
O
O
O
1
2
Rh2(cap)4
Rh2(pfb)4
Rh2(OAc)4
97
5
32
:
:
:
3
95
68
Key words: diazocarbonyl, copper catalysis, bis(oxazoline), C–H
insertion, hydride transfer
(b)
N2
Rh(II)
+
O
Metal carbenes derived from α-diazocarbonyl compounds
are highly reactive species and may undergo a broad spec-
trum of chemical transformations including X–H inser-
tion, cyclopropanation, ylide formation, dimerisation,
aromatic addition, and Wolff rearrangement.1 Of these
processes, intramolecular C–H insertion reactions of α-di-
azocarbonyls have been particularly well-studied, offer-
ing excellent regioselectivity in the formation of
carbocyclic products, typically cyclopentanones.2 Control
of chemoselectivity is also possible for such transforma-
tions by careful choice of catalyst and diazo substrate.3–5
Padwa and coworkers reported that choice of rhodium(II)
ligand has a profound influence on product distribution
for reactions in which competition between C–H insertion
and aromatic addition or cyclopropanation can occur.6,7
C–H insertion to provide γ-lactam 1 was found to be dom-
inant for reactions catalysed by Rh2(cap)4, while near ex-
clusive formation of the competing aromatic addition
product 2 was observed for reactions employing Rh2(pfb)4
(Scheme 1, a). Similar ligand effects were recorded for
decomposition of diazo 3 in which C–H insertion and
cyclopropanation are competitive. Thus, catalysis by
Rh2(cap)4 provided cyclopropane 4 as the sole product
while exclusive formation of 5 via C–H insertion was ob-
served for reaction in the presence of Rh2(pfb)4 (Scheme
1, b). In both cases, use of Rh2(OAc)4 resulted in the for-
CH2Cl2
H
O
O
3
4
5
Rh2(cap)4
Rh2(pfb)4
Rh2(OAc)4
100
0
44
:
:
:
0
100
56
Scheme 1 (a) C–H insertion vs. aromatic addition; (b) C–H insertion
vs. cyclopropanation.6,7
In diazo systems where competing aromatic addition or
cyclopropanation is not possible, the efficiency of C–H
insertion reactions may still be affected by the potential
formation of side products arising from O–H insertion, di-
merisation, diazo reduction, or hydride transfer. Limited
literature reports exist documenting hydride transfer as a
competitive transformation to C–H insertion.8–13 In these
earlier studies, transfer of the hydride was promoted by
the presence of an adjacent heteroatom (O, N), forming an
intermediate oxonium or iminium ion which undergoes
nucleophilic attack by the oxygen of the rhodium enolate
to provide the hydride transfer product (see Scheme 2 for
a representative example).9 The hydride transfer products
are typically obtained as minor reaction products, howev-
er, in certain instances this pathway has been found to
dominate over C–H insertion.8,10 The alternative hydride
transfer reaction known as the 1,2-hydride shift (or β-hy-
dride elimination) has been more commonly reported in
the literature and is often observed as a competitive side
SYNLETT 2014, 25, 0591–0595
Advanced online publication: 08.01.2014
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
DOI: 10.1055/s-0033-1340471; Art ID: ST-2013-D1034-L
© Georg Thieme Verlag Stuttgart · New York

592
C. N. Slattery et al.
LETTER
reaction in transformations of α-diazocarbonyls possess- The copper-catalysed reactions of α-diazo-β-keto sulfone
ing β-hydrogen atoms.1,14–16
11a were initially explored (Table 1, entries 1–5). Intra-
molecular C–H insertion was found to be the dominant re-
action pathway for this substrate, providing
cyclopentanone 12a in high enantioselectivity (up to 89%
ee) and good yield following chromatographic purifica-
tion.17 Interestingly, in addition to the C–H insertion prod-
uct 12a, a minor amount of the alkylidene tetrahydrofuran
13a was also recovered from this reaction. Tetrahydrofu-
ran 13a was observed in varying amounts (up to 21% rel-
ative to cyclopentanone formation) in the 1H NMR spectra
of the crude reaction mixtures for the reactions of 11a (Ta-
ble 1, entries 1–5), and was isolated as the Z-isomer exclu-
sively following column chromatography, as confirmed
by X-ray crystallography22 (Scheme 3).
RhLn
RhLn
O
O
O
O
H
H
H
Ph
O
O
Ph
Ph
Scheme 2 Oxygen-promoted hydride transfer
We have previously reported that asymmetric copper-cat-
alysed reactions of α-diazo-β-keto phenylsulfones lead to
highly enantioenriched cyclopentanone products via in-
tramolecular C–H insertion.17,18 We report herein that
competing hydride transfer may also occur, resulting in
the formation of 2-alkylidene tetrahydrofuran products.
The influence of substrate modification adjacent to the
target C–H insertion site on both chemoselectivity and en-
antioselectivity is also explored.
The formation of 13a was rationalised to be due to com-
peting hydride transfer to generate an intermediate pos-
sessing a copper enolate and benzylic carbocation. This
intermediate then undergoes cyclisation via enolate trap-
ping to provide the substituted tetrahydrofuran 13a
(Scheme 3). Evidently, stabilisation of the carbocation at
the benzylic position is sufficient to enable hydride trans-
fer, which is more commonly seen when activated by
oxygen or nitrogen.8–10 Supporting this is the observation
that hydride transfer to the carbenoid carbon was not ob-
served for reactions with other α-diazo-β-keto sulfones in
which the C–H insertion site is adjacent to an alkyl or ben-
zyl substituent, examined as part of an earlier study,17 pre-
sumably because the carbocationic intermediate formed
via the hydride transfer pathway is sufficiently stable to
form only when it is benzylic but not with the other diazo
substrates. Significantly, formation of 13a was only ob-
served for copper-catalysed reactions of diazo 11a and not
for reactions employing Rh2(OAc)4.
Reactions of α-diazo-β-keto sulfones examined in this
study were conducted in refluxing dichloromethane in the
presence of a copper catalyst generated in situ from 5
mol% CuCl2, 6 mol% bis(oxazoline) ligand 6–10 (Figure
1), and 6 mol% NaBARF {BARF = tetrakis[3,5-bis-
(trifluoromethyl)phenyl]borate}. The diazo compounds
11a–c were synthesised from the corresponding 4-substituted
phenyl(butanoates) using standard procedures.19–21
O
O
O
O
R
N
N
N
N
R
6
7 R = (R)-Bn
8 R = (R)-Ph
Literature research revealed the description of the synthe-
sis of 13a and a number of related tetrahydrofuran prod-
ucts via base-mediated cyclisation of hydroxyl
propargylic sulfones.23 Using this methodology, (E)-exo-
alkylidene tetrahydrofurans were isolated as the sole
product in high yield (80–85%).23 Similar (Z)-2-alkyli-
dene tetrahydrofuran products have also been obtained
from mercury(II)- and palladium(II)-mediated cyclisation
of acetylenic alcohols and used as the key intermediates in
O
O
O
O
Ph
Ph
N
N
N
N
Ph
t-Bu
t-Bu
Ph
9
10
Figure 1 Bis(oxazoline) ligands 6–10
Scheme 3 Proposed hydride-transfer mechanism
Synlett 2014, 25, 591–595
© Georg Thieme Verlag Stuttgart · New York

LETTER
Transformations of α-Diazo-β-Keto Sulfones
593
prostacyclin PGI2 synthesis.24,25 To the best of our knowl- compound 11a (Table 1, entries 1–4). This outcome is in
edge, this is the first reported example of the synthesis of agreement with previous reports by Hashimoto and co-
alkylidene tetrahydrofurans from diazocarbonyl precur- workers who also observed reduced levels of asymmetric
sors and as such offers an additional route to substituted induction for rhodium(II)-catalysed C–H insertion reac-
tetrahydrofurans with high Z selectivity. The production tions of α-diazo-β-keto esters featuring electron-with-
of 2,3,5-trisubstituted tetrahydrofurans by rhodium-medi- drawing (Br, 70% ee) and electron-donating (OMe, 57%
ated cyclisation of α-diazo esters has been previously de- ee) groups on the para position of the phenyl ring, relative
scribed by Taber,26,27 however, this earlier work involved to reactions with the unsubstituted diazo substrate (76%
insertion at electronically activated sites adjacent to oxy- ee).32
gen rather than a hydride-transfer mechanism as described
in this work.
While the introduction of an electron-withdrawing halide
substituent (R = Br) onto the phenyl ring of the diazo
The formation of products derived from hydrogen transfer framework was found to result in a largely similar product
in competition with the C–H insertion pathway provides distribution relative to reactions with 11a (Table 1, entries
interesting mechanistic insight into the nature of the C–H 1–5 vs. 6–10), a dramatic shift in chemoselectivity to-
insertion process. Thus, in intermediate A (Scheme 4) wards intramolecular tetrahydrofuran formation was ob-
concerted C–C and C–H bond formation with concomi- served for reactions of the p-methoxyphenyl-substituted
tant cleavage of the γ-C–H bond is envisaged to lead to in- diazo sulfone 11c. Thus, for each of the copper-catalysed
sertion product 12a, in line with mechanistic proposals by reactions of 11c examined (Table 1, entries 11–15) hy-
Taber28 and Doyle.29 Davies has described the alternative dride transfer to provide 13c was observed to dominate
formation of C–H insertion products through asynchro- over C–H insertion forming cyclopentanone 12c. This
nous bond formation and breaking in the same transition outcome is in agreement with the mechanism earlier pro-
state.30 The observation of hydride transfer product 13a in posed for the formation of 13a (Scheme 3); the increase in
the copper-mediated reaction of 11a is consistent with the electron density contributed by the p-methoxy substituent
asynchronous insertion pathway, leading at the extreme to favours carbocation formation through complete hydride
hydride transfer, in a process which is competitive with transfer relative to the more concerted C–H insertion path-
C–H insertion. It is possible therefore, for the C–H inser- way. Subsequent intramolecular attack of the carbocation-
tion process to ‘tip over’ to hydride transfer for copper-ca- ic centre by the enolic oxygen leads to the formation of the
talysed reactions in which the intermediate carbocation B novel tetrahydrofuran 13c.
is sufficiently stable.
In contrast to reactions of 11a and 11b in which tetrahy-
drofurans 13a and 13b, respectively, were obtained exclu-
O
O
sively as the Z-isomer, a mixture of (Z)-13c and (E)-13c
was observed to form for reactions of the p-methoxy-sub-
stituted diazo 11c (Table 2, entries 1–5). Interestingly, a
modest level of asymmetric induction was recorded by
HPLC analysis for the formation of (Z)-13c and (E)-13c in
each of the copper-catalysed reactions of 11c examined,
with the benzyl-substituted bis(oxazoline) 7 affording the
highest enantioselectivity for both isomers (Table 2, entry
2). Importantly, this observation provides direct evidence
of involvement of the copper–bis(oxazoline) complex in
the C–O bond formation leading to the tetrahydrofuran
products produced via the hydride-transfer pathway. Giv-
en the synthetic value of the substituted tetrahydrofuran
framework, work is currently under way to explore a
broader range of substrates and catalysts for the produc-
tion of novel enantioenriched alkylidene tetrahydrofuran
products. In addition, the investigation of additional com-
peting side reactions, believed to originate from O–H in-
sertion, is also ongoing.
O
SO2Ph hydride
SHO2Ph
+
SO2Ph
–
–
C–H insertion
transfer
ML
ML
H
H
+
H
H
concerted C–C &
C–H bond formation
Ph
12a (major)
Ph
Ph
B
A
– O
SO2Ph
H
SO2Ph
O
H
Ph
+
H
H
13a (minor)
Ph
Scheme 4 C–H insertion vs. hydride transfer
In order to investigate the effects on chemo- and enantio-
selectivity of modification of electronic properties at the
benzylic position of 11a, two novel α-diazo-β-keto sul-
fone substrates were prepared: 1-diazo-5-(4-bromophe-
nyl)-1-phenylsulfonylpentan-2-one (11b) and 1-diazo-5-
(4-methoxyphenyl)-1-phenylsulfonylpentan-2-one (11c),
featuring electron-withdrawing and electron-donating
groups, respectively, at the para position of the phenyl
ring. In terms of asymmetric induction, with only one ex-
ception noted (Table 1, entry 5 vs. 15), a slight decrease in
enantioselectivity was recorded for reactions of the bro-
mo- and methoxy-substituted α-diazo-β-keto sulfones
11b and 11c (Table 1, entries 6–14), respectively, relative
to cyclisations with the corresponding unsubstituted diazo
In conclusion, isolation of alkylidene tetrahydrofuran
products 13a–c in the copper-catalysed reactions of α-di-
azo-β-keto sulfones 11a–c provides evidence of asyn-
chronous bond forming/breaking in the copper-mediated
C–H insertion process. In systems such as 11 where a rel-
atively stable carbocation can be formed, the asynchro-
nous C–H insertion ‘tips over’ into hydride transfer as a
competitive pathway. The observed increase in the hy-
dride transfer product upon introduction of the p-methoxy
© Georg Thieme Verlag Stuttgart · New York
Synlett 2014, 25, 591–595

594
C. N. Slattery et al.
LETTER
Table 1 Copper–Bis(oxazoline)-Catalysed Reactions of α-Diazo-β-Keto Sulfones 11a–c
O
O
CuCl2 (5 mol%)
L* (6 mol%)
NaBARF (6 mol%)
PhO2S
SO2Ph
SO2Ph
+
O
H
N2
CH2Cl2, reflux
R
a R = H
b R = Br
c R = OMe
R
R
11
12
13
Entry
Diazo
R
L*
Time (h)
12/13a
Yield (%) 12b
ee (%) of 12c,d
1
2
11a
11a
11a
11a
11a
11b
11b
11b
11b
11b
11c
11c
11c
11c
11c
H
6
2
2
1:0.17
1:0.06
1:0.15
1:0.27
1:0.04
1:0.04
1:0.09
1:0.01
1:0.18
1:0.02
1:8.80f
1:8.69f
1:3.71f
1:8.11f
1:4.98f
87
89 (2R,3R)
81 (2S,3S)
57 (2S,3S)
64 (2R,3R)
52 (2S,3S)
87 (2R,3R)
67 (2S,3S)
39 (2S,3S)
59 (2R,3R)
49 (2S,3S)
88 (2R,3R)
71 (2S,3S)
49 (2S,3S)
58 (2R,3R)
59 (2S,3S)
H
7
58
3
H
8
2
69
4
H
9e
19
2
55
5
H
10
54
6
Br
6
7
6
60
7
Br
5
86
8
Br
8
3
69
9
Br
9
4
73
10
11
12
13
14
15
Br
10
6
2
65
OMe
OMe
OMe
OMe
OMe
2
19g
20g
25g
10g
26g
7
2
8
2
9
2
10
2
a Ratio of C–H insertion (12)/THF formation (13) based on integration of the C(2)H doublet of 12 and the δH = 5.59–5.64 ppm singlet of 13,
respectively, in the 1H NMR spectra of the crude reaction mixtures.
b Yield of trans-cyclopentanone 12 after column chromatography.
c Enantiopurity of 12 as determined by chiral stationary-phase HPLC (see Supporting Information for details).
d Absolute stereochemistry of 12a determined by comparison to previously reported data.31 Absolute stereochemistry of 12b and 12c assigned
by analogy to 12a.
e Reaction catalysed by CuCl.
f Mixture of (Z)-13c and (E)-13c observed in the 1H NMR spectrum of the crude reaction mixture.
g Isolated product contains minor amount (<10%) of (E)-13c.
substituent supports this mechanistic interpretation. The
observation of enantioselectivity in the formation of tetra-
hydrofuran 13c is particularly interesting as it indicates
that the copper–bis(oxazoline) complex remains involved
during the final cyclisation step.
Table 2 Copper–Bis(oxazoline)-Catalysed Reactions of 11c
Entry L* (Z)-13c/(E)-13ca ee of (Z)-13c (%)b ee of (E)-13c (%)b
1
2
3
4
5
6
7
8
9
1:0.01
1:0.02
1:0.45
1:0.01
8
32
3
25
43
6
General Procedure for Reactions of α-Diazo-β-Keto Sulfones
11a–c
9
8
The CuCl2–L*–(NaBARF) catalyst was generated in situ from a
mixture of CuCl2 (5 mol%), bis(oxazoline) ligand (6 mol%), and
NaBARF (6 mol%) in CH2Cl2 (15 mL). This catalytic mixture was
stirred under nitrogen at 40 °C for 1.5 h. α-Diazo-β-keto sulfone
11a–c (150 mg, 1 equiv) was then added dropwise in CH2Cl2 (15
mL) over 0.5 h to the refluxing solution. The progress of the reac-
tion was monitored by IR spectroscopy, where reaction completion
was indicated by the disappearance of the characteristic diazo peak
at 2112–2122 cm–1. Upon reaction completion, evaporation of the
10 1:0.02
2
30
a As determined from the 1H NMR spectrum of the crude reaction
mixture of 11c.
b As determined by chiral stationary-phase HPLC analysis (see Sup-
porting Information for details).
Synlett 2014, 25, 591–595
© Georg Thieme Verlag Stuttgart · New York

LETTER
Transformations of α-Diazo-β-Keto Sulfones
595
reaction solvent at reduced pressure gave the crude product mixture.
Purification by flash chromatography on silica gel, employing EtO-
Ac–hexane as eluent, gave the cyclopentanone (12a–c, less polar
fraction) and tetrahydrofuran (13a–c, more polar fraction) products.
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