Regioselective cycloaddition of 3-azetidinones and 3-oxetanones with alkynes through nickel-catalysed carbon-carbon bond activation

Article abstract of DOI:10.1002/chem.201200167

Get in the ring! The first examples of transition-metal-catalysed C-C bond activation of 3-azetidinones and 3-oxetanones are reported. In the presence of a nickel catalyst and alkynes, a regioselective and high-yielding [4+2] cycloaddition occurs, leading to the formation of pyridinones, pyranones and eventually 4,5-disubstituted 3-hydroxypyridines (see scheme). Copyright

Full text of DOI:10.1002/chem.201200167

DOI: 10.1002/chem.201200167
Regioselective Cycloaddition of 3-Azetidinones and 3-Oxetanones with
Alkynes through Nickel-Catalysed Carbon–Carbon Bond Activation
Kelvin Y. T. Ho and Christophe Aꢀssa*[a]
Table 1. Optimisation of the reaction conditions.
The transition-metal-catalysed activation of single carbon-
À
carbon (C C) bonds has been studied with several classes of
strained carbocycles but not with small heterocycles.[1] In
this context, we have recently reported the first example of
À
a transition metal-catalyzed C C bond activation of azeti-
Entry
L
A
Solvent
T
[8C]
2/3[a]
1:1
1.4:1
5:1
Yield
[%][b]
dine derivatives in the rhodium-catalyzed intramolecular hy-
droacylation of aldehyde-tethered 3-alkylideneazetidines.[2,3]
We hypothesised that the strain energy of azetidines and ox-
1[c]
2[c]
3[c]
4[d]
5[d]
6[d]
P
A
toluene
toluene
dioxane
dioxane
dioxane
dioxane
110
110
100
100
100
90
30
55
79
77
81
90
PPh3 (20)
PPh3 (20)
PPh3 (20)
PPh3 (30)
PPh3 (30)
etanes could promote other processes involving transition-
metal-catalysed C C bond activation. Herein, we report
[4]
À
5:1
that 3-azetidinones and 3-oxetanones can undergo a regiose-
lective cycloaddition in the presence of alkynes and a nickel
catalyst. These results suggest a vast realm of potential
transformations of azetidine and oxetane derivatives in tran-
sition-metal-catalysed heterocyclic chemistry.
10:1
>20:1[e]
[a] Ratio determined by 1H NMR spectroscopy of the crude material.
[b] Yields of isolated 2. [c] 1.5 equiv of alkyne was used. [d] 1.1 equiv of
alkyne was used. [e] Compound
3 could not be detected by using
1H NMR spectroscopy of the crude material.
Murakami and co-workers have shown that cyclobuta-
nones undergo alkyne insertion when treated with 10 mol%
[NiACHTUNGTRENNUNG(cod)2] (COD=cyclooctadiene) and 20 mol% PHCATUNGTERN(NUGN cHex)3
in toluene at 1108C.[5] However, exposing commercially
available N-Boc-3-azetidinone 1 (Boc=tert-butoxycarbamo-
yl) and bis-phenylacetylene to these reaction conditions led
to the formation of the expected product 2 in only modest
yield alongside substituted azetidinone 3 (Table 1, entry 1),
which was presumably obtained through carbon–hydrogen
activation next to the nitrogen atom and alkyne insertion.[6]
Interestingly, the selectivity in favour of 2 was improved by
the transformation. Phenylacetylene did not give the expect-
ed product, presumably due to oligomerisation.
We then examined the scope and regioselectivity of the
insertion of unsymmetrical alkynes (Table 2). In most cases,
the regioselectivity[7] was good and regioisomers 5 and 6
could be separated easily by flash chromatography, giving
excellent yields of the major isolated regioisomer. Alkynes
bearing an alkyl and an aryl substituents gave 1,2-dihydro-
replacing P
(cHex)3 with the smaller and less basic PPh3
pyridin-3ACTHNGUETRNNU(G 6H)-ones 5a–5e as the major regioisomer, which
group (Table 1, entry 2), whereas this ligand was completely
inefficient in reactions with cyclobutanones.[5b] Critical to
the selective formation of 2 was an increase of the loading
of phosphine to 30 mol% (Table 1, entry 4 vs. 5) and a de-
crease of the temperature to 908C (Table 1, entry 5 vs. 6). It
is noteworthy that only a slight excess of alkyne (1.1 equiv)
is necessary without the requirement of slow addition.
Moreover, control experiments in which only PPh3 or only
display the alkyl group (R1) in a position to the carbonyl
(Table 2, entries 1–5). This regioselectivity was not influ-
enced by an electron-rich or electron-poor substituent on
the benzene ring of alkynes 4a–4c (Table 2, entries 1–3). A
slight erosion of regioselectivity was observed when the size
of the alkyl group was increased in alkynes 4d and 4e
(Table 2, entries 4 and 5). The weak steric differentiation be-
tween the substituents of the triple carbon–carbon bond of
alkynes 4 f and 4g led to a modest regioselectivity (Table 2,
entries 6 and 7), which was restored completely when
a larger alkyl group was present, as in 4h, giving 5h in 87%
yield (Table 2, entry 8). The reactivity observed with tert-
butyl-substituted internal alkyne 4h is remarkable, because
[NiACHTUNGTRENNUNG(cod)2] were used did not produce 2, supporting the con-
clusion that a nickel–phosphine complex is responsible for
[a] K. Y. T. Ho, Dr. C. Aꢀssa
Department of Chemistry
À
examples of metal-catalysed C C bond formation involving
University of Liverpool
Crown Street, Liverpool L69 7ZD (UK)
Fax : (+44)151-794-3588
such alkynes have been reported only on rare occasions, all
involving aldehydes instead of bulkier ketones.[8] Interesting-
ly, the regioselectivity obtained with 1,3-enyne 4i was far
better than that observed with 4g (Table 2, entry 9 vs. 7), al-
though the steric differentiation between substituents does
E-mail: aissa@liverpool.ac.uk
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201200167.
3486
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 3486 – 3489

COMMUNICATION
Table 2. Regioselective nickel-catalysed cycloaddition of N-Boc-3-azeti-
dinone 1 with unsymmetrical alkynes 4.
Entry
R1
R2
4
5/6[a]
87:13
89:11
89:11
Yield
[%][b]
1
2
3
4
5
6
7
8
Me
nBu
nBu
cHex
iBu
p-CF3C6H4
Me
Me
Ph
4a
4b
4c
4d
4e
4 f
4g
4h
4i
92
89
99
Scheme 1. Proposed rationale for the observed regioselectivity.
p-MeOC6H4
p-MeCOC6H4
p-MeOC6H4
p-MeOC6H4
p-MeOC6H4
iPr
86:14
84:16
88[c]
91[c]
82
complexes.[13] Moreover, only one phosphine ligand would
be present on the metal during the regioselectivity-deter-
mining event[10] whereas the excess of phosphine required
under our optimised reaction conditions would promote the
final reductive elimination.
55:45[d]
60:40[d]
100:0[e]
88:12
90
tBu
87[f]
88
9
Et
C(Me)=CH2
p-PhC6H4
Ph
10
11
12
C(Me)=CH2
SiMe3
SiMe3
4j
4k
4l
60:40[d]
97:3[g]
80:20[d]
86
91[f]
80[h]
Although the regioselectivity observed with silylated al-
kynes 4k and 4l is in agreement with the results obtained in
other intermolecular nickel-catalyzed reactions of silylated
alkynes with electrophiles,[11b,d,g,14] it cannot be explained by
the same steric considerations. The axial strain values of
methyl, isopropyl, trimethylsilyl, and phenyl groups (1.74,
2.21, 2.5 and 2.8 kcalmolÀ1, respectively)[15] suggest that the
steric differentiation between substituents R1 and R2 is
larger in 4g than in 4k. Nevertheless, the regioselectivity ob-
served with 4k was excellent, whereas it was poor with 4g.
Similarly, steric effects appear inconsequential in the case of
4l, the major product displaying the larger silyl substituent
next to the carbonyl group, in contradiction to the trend ob-
served with 4a–4e. Hence, stereoelectronic effects appear
predominant in determining the regioselectivity of alkynes
4k and 4l. In this context, it is interesting to consider the re-
versed and significantly accentuated polarisation of the sily-
lated alkyne ligands in known complexes C and D as com-
pared with E, which has been established by 13C NMR spec-
troscopy.[16] Whereas steric effects would predominate in the
mechanistic pathway involving less polarized alkynes, the re-
gioselectivity of the reactions of 4k and 4l would be dictat-
ed by the polarity of the alkynes when coordinated to nickel
in a nucleophilic attack onto 1.[17,18]
It was also possible to observe reaction with N-Ts-3-azeti-
dinone 7 (Ts=para-tolylsulfonyl), although a greater excess
of alkynes, slightly higher reaction temperature, and
a higher catalyst loading (in the case of 4a) were required
to reach full conversion (Scheme 2). Hence, pyridinone 8
was isolated in 74% yield. The higher reaction temperature
might explain the slightly lower selectivity obtained for re-
gioisomers 9 and 10 in the cycloaddition of 4a with 7 as
compared with the result obtained with N-Boc-3-azetidinone
1 (Table 2, entry 1).
nOct
[a] Unless otherwise noted, this ratio was determined from the isolated
yields of separated products 5 and 6. [b] Combined isolated yields of 5
and 6. [c] Alkyne and azetidinone were premixed. [d] Ratio determined
1
by H NMR spectroscopy on an inseparable mixture of 5 and 6. [e] Com-
pound 6h could not be detected by 1H NMR spectroscopy of the crude
material. [f] Isolated yield of 5. [g] Ratio determined by 1H NMR spec-
troscopy of the crude. [h] 1.5 equiv of alkyne was used.
not vary greatly when one compares those two substrates.
This result is in good accordance with reports by Jamison
and co-workers, which propose that the alkene moiety of
1,3-enynes directs their intermolecular nickel-catalyzed re-
ductive coupling with aldehydes and ketones by coordina-
tion of the alkene to the metal.[8d,9] This is also in agreement
with recent theoretical investigations.[10] Evaluating the di-
recting effects of an alkene and a benzene ring with 1,3-
enyne 4j, we observed a decrease of the regioselectivity nor-
mally induced by aromatic substituents (Table 2, entry 10 vs.
1–5), confirming the influence of the alkenyl substituent. Fi-
nally, reactions of trimethylsilyl-substituted alkynes 4k and
4l gave predominantly regioisomers 5k and 5l, respectively,
which display the silyl group in the a position to the carbon-
yl.
In line with experimental results,[8d,9,11] recent theoretical
investigations suggest that the regioselectivity of nickel-cata-
lyzed reductive coupling of simple alkynes and aldehydes is
dictated by steric effects.[10] A similar model with N-Boc-3-
azetidinone 1 would favour the formation of metallacycle A
by minimisation of steric interactions (Scheme 1) and would
be consistent with the results presented in entries 1–8 of
À
Table 2. Hence, intermediate A would undergo C C bond
activation toward B and reductive elimination would afford
the major product 5. The diminished steric differentiation
between substituents R1 and R2 in alkynes 4 f and 4g would
explain the poor regioselectivity observed with these sub-
strates. The intermediates invoked in this postulated mecha-
nistic sequence are consistent with the isolation of h2,h2-1,5-
enone-nickel complexes[12] and dimeric nickeladihydrofuran
We then examined the regioselectivity of the ring-opening
of a-substituted azetidinone 11, which gave exclusively 12 in
91% yield (Scheme 3), indicating that only the less substi-
tuted bond of 11 is cleaved in the putative rearrangement of
F to G (Scheme 4). Presumably, the minimisation of steric
Chem. Eur. J. 2012, 18, 3486 – 3489
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3487

C. Aꢀssa and K. Y. T. Ho
Table 3. Nickel-catalysed cycloaddition of 3-oxetanone 13 with alkynes.
Entry
R1
R2
Alkyne
14/15[a]
Yield
[%][b]
[c]
1
2
3
Ph
nBu
SiMe3
Ph
–
–
82
92
99
p-MeCOC6H4
Ph
4c
4k
79:21
96:4
[a] The ratio was determined from the isolated yields of separated prod-
ucts 14 and 15. [b] Combined isolated yields of 14 and 15. [c] Bis-phenyl-
acetylene.
Scheme 2. Nickel-catalysed cycloaddition of N-Ts-azetidinone 7.
The pyridinones obtained by nickel-catalysed cycloaddi-
tion of unsymmetrical alkynes with 3-azetidinones are useful
intermediates for the preparation of 4,5-disubstituted 3-hy-
droxypyridines. Hence, carbamoyl-protected pyridinone 17
was obtained in 83% yield from 1 and 16 using our opti-
mised conditions.[19] Then, an acidic treatment in the pres-
ence of silica followed by oxidation gave 18 isolated in 68%
yield (Scheme 4). Our approach toward 4,5-disubstituted 3-
hydroxypyridines with two different substituents in positions
4 and 5 necessitates only the preparation of an unsymmetri-
cal alkyne and offers a good alternative to previous strat-
egies that rely on the synthesis of more elaborated unsym-
metrical precursors.[20]
Scheme 3. Regioselective ring-opening of N-Ts-azetidinone 11.
In conclusion, we have demonstrated that transition-
À
metal-catalysed C C bond activation of small rings, tradi-
tionally studied with all-carbon rings,[1] can be generalized
to 3-azetidinones and 3-oxetanones, owing presumably to
the strain energies of these heterocyclic compounds. This ap-
proach represents a shift of strategy as compared with previ-
ous synthetic designs used for ring-opening[21] or ring-en-
largement[22–24] of azetidines, which all involved cleavage of
À
a carbon–nitrogen bond, as opposed to one of the C C
bond of the small heterocyclic ring.
Scheme 4. Rapid synthesis of 4,5-disubstituted 3-hydroxypyridines.
hindrance favours the formation of G over G’. Importantly,
the enantiomeric purity was not significantly eroded during
the cycloaddition. An N-Ts-3-azetidinone with a benzyl sub-
stituent in a-position did not undergo the nickel-catalyzed
cycloaddition.
Acknowledgements
Financial support from Research Councils UK, EPSRC and AstraZeneca
is gratefully acknowledged. We sincerely thank Dr. Chris Halsall (AZ)
for his support and Dr. Jonathan Iggo (Liverpool) for access to the NMR
spectrometers.
Gratifyingly, we also observed the clean cycloaddition of
commercially available 3-oxetanone 13 with alkynes under
our optimized reaction conditions (Table 3). Hence, pyra-
none 14 was isolated in 82% yield (Table 3, entry 1). Excel-
lent yields were also obtained with unsymmetrical alkynes
4c and 4k (Table 3, entries 2 and 3), albeit the 14c/15c ratio
obtained with 4c (Table 3, entry 2) was lower than the 5c/6c
ratio obtained when the same alkyne underwent reaction
with azetidinone 1 (Table 2, entry 3). This decrease of selec-
tivity might be imputed to the diminished steric hindrance
imposed by the oxygen atom of the oxetane ring of 13 as
compared with the large carbamoyl group on the nitrogen
atom of 1.
À
Keywords: 3-azetidinone · 3-oxetanone · C C activation ·
nickel · ring-expansion
À
[1] For recent reviews on transition-metal-catalysed C C bond activa-
tion, see: a) I. Nakamura, Y. Yamamoto, Adv. Synth. Catal. 2002,
344, 111–129; b) M. E. van der Boom, D. Milstein, Chem. Rev. 2003,
103, 1759–1792; c) T. Nishimura, S. Uemura, Synlett 2004, 201–216;
d) C.-H. Jun, Chem. Soc. Rev. 2004, 33, 610–618; e) T. Satoh, M.
Miura, in Topics in Organometallic Chemistry, Vol. 14, (Ed: S.
Murai), Springer: Berlin, 2005, 1–20; f) M. Murakami, M. Makino,
3488
www.chemeurj.org
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 3486 – 3489

Regioselective Cycloaddition of 3-Azetidinones and 3-Oxetanones
COMMUNICATION
S. Ashida, T. Matsuda, Bull. Chem. Soc. Jpn. 2006, 79, 1315–1321;
2009, 131, 13194–13195; thiophthalic anhydrides, e) T. Inami, Y.
Baba, T. Kurahashi, S. Matsubara, Org. Lett. 2011, 13, 1912–1915.
[15] E. L. Eliel, S. H. Wilen, in Stereochemistry of Organic Compounds,
John Wiley & Sons, Inc: New York, 1994, 696–697.
[16] a) T. Bartik, B. Happ, M. Iglewsky, H. Bandmann, R. Boese, P.
Heimbach, T. Hoffmann, E. Wenschuh, Organometallics 1992, 11,
1235–1241; b) U. Rosenthal, C. Nauck, P. Arndt, S. Pulst, W. Bau-
mann, V. V. Burlakov, H. Gçrls, J. Organomet. Chem. 1994, 484, 81–
87.
ˇ
g) D. Necas, M. Kotora, Curr. Org. Chem. 2007, 11, 1566–1591;
h) M. Rubin, M. Rubina, V. Gevorgyan, Chem. Rev. 2007, 107,
3117–3179; i) T. Seiser, N. Cramer, Org. Biomol. Chem. 2009, 7,
2835–2840; j) M. Murakami, T. Matsuda, Chem. Commun. 2011, 47,
1100–1105; k) C. Aꢀssa, Synthesis 2011, 3389–3407.
[2] D. Crꢂpin, J. Dawick, C. Aꢀssa, Angew. Chem. 2010, 122, 630–633;
Angew. Chem. Int. Ed. 2010, 49, 620–623.
[3] An example of rhodium-catalysed cycloisomerisation of an aryl-aze-
tidin-3-ol was reported during the preparation of this manuscript,
see: N. Ishida, S. Sawano, M. Murakami, Chem. Commun. 2012, 48,
1973–1975.
[17] Alkynes are two-electron donors ligands in h2-alkyne-nickel-phos-
phines complexes. See U. Rosenthal, G. Oehme, V. V. Burkalov,
P. V. Petroskii, V. B. Shur, M. E. Volꢃpin, J. Organomet. Chem. 1990,
391, 119–122 and references cited therein.
[18] The Hoffmannꢃs model for the formation of metallacycles by the
maximization of orbital overlap between the p* orbital of azetidi-
none 1 and the in-plane p* orbital of alkynes 4k and 4l would pre-
dict that 6 should be the major regioisomer, see: A. Stockis, R.
Hoffmann, J. Am. Chem. Soc. 1980, 102, 2952–2962.
[4] Different values have been proposed: a) 26.2 kcalmolÀ1 (azetidine)
in S. W. Benson, F. R. Cruickshank, D. M. Golden, G. R. Hangen,
H. E. OꢃNeal, A. S. Rodgers, R. Shaw, R. Walsh, Chem. Rev. 1969,
69, 279–324; b) 25.2 kcalmolÀ1 (azetidine) in T. Dudev, C. Lim, J.
Am. Chem. Soc. 1998, 120, 4450–4458; c) 25.4 kcalmolÀ1 (azetidine)
and 24.7 kcalmolÀ1 (oxetane) in R. D. Bach, O. Dmitrenko, J. Org.
Chem. 2002, 67, 3884–3896; d) 25.2 kcalmolÀ1 (oxetane) in H. K. Ei-
genmann, D. M. Golden, S. W. Benson, J. Phys. Chem. 1973, 77,
1687–1691.
[5] a) M. Murakami, S. Ashida, T. Matsuda, J. Am. Chem. Soc. 2005,
127, 6932–6933; b) M. Murakami, S. Ashida, T. Matsuda, Tetrahe-
dron 2006, 62, 7540–7546.
[6] For leading references, see: a) N. Chatani, T. Asaumi, S. Yorimitsu,
T. Ikeda, F. Kakiuchi, S. Murai, J. Am. Chem. Soc. 2001, 123, 10935–
10941; b) B. DeBoef, S. J. Pastine, D. Sames, J. Am. Chem. Soc.
2004, 126, 6556–6557. This reaction does not appear to be general.
[19] The minor regioisomer obtained from 1 and 16 was isolated in 8%
yield, see the Supporting Information.
[20] For selected examples of preparation of 4,5-disubstituted 3-hydroxy-
pyridines, see: a) P. Allevi, A. Longo, M. Anastasia, Chem.
Commun. 1999, 559–560; b) T. J. Donohoe, L. P. Fishlock, J. A. Ba-
sutto, J. F. Bower, P. A. Procopiou, A. L. Thompson, Chem.
Commun. 2009, 3008–3010; c) K. Yoshida, F. Kawagoe, K. Hayashi,
S. Horiuchi, T. Imamoto, A. Yanagisawa, Org. Lett. 2009, 11, 515–
518; d) H. Liu, L. Wang, X. Tong, Chem. Commun. 2011, 47, 12206–
12208.
[21] a) M. K. Ghorai, K. Das, A. Kumar, A. Das, Tetrahedron Lett. 2006,
47, 5393–5397; b) M. K. Ghorai, K. Das, A. Kumar, Tetrahedron
Lett. 2007, 48, 4373–4377; c) F. Couty, F. Durrat, G. Evano, Synlett
2005, 1666–1670; d) F. Couty, O. David, F. Durrat, G. Evano, S.
Lakhdar, J. Marrot, M. Vargas-Sanchez, Eur. J. Org. Chem. 2006,
3479–3490; e) F. Couty, O. David, F. Durrat, Tetrahedron Lett. 2007,
48, 1027–1031; f) F. Couty, O. David, B. Drouillat, Tetrahedron Lett.
2007, 48, 9180–9184; g) M. Vargas-Sanchez, S. Lahkdar, F. Couty, G.
Evano, Org. Lett. 2006, 8, 5501–5504.
À
Hence, 4-octyne did not give any product of C H activation.
[7] The structures of 5 and 6 were determined by HMBC and NOESY
experiments, see the Supporting Information.
[8] a) W. R. Jackson, C. G. Lovel, J. Chem. Soc. Chem. Commun. 1982,
1231–1232; b) T. Tsuda, T. Kiyoi, T. Saegusa, J. Org. Chem. 1990,
55, 2554–2558; c) J. R. Johnson, G. D. Cuny, S. L. Buchwald, Angew.
Chem. 1995, 107, 1877–1879; Angew. Chem. Int. Ed. Engl. 1995, 34,
1760–1761; d) K. M. Miller, T. Luanphaisarnnont, C. Molinaro, T. F.
Jamison, J. Am. Chem. Soc. 2004, 126, 4130–4131.
[22] For ring-enlargement of azetidinium intermediates through [1,2]
and [2,3] sigmatropic shifts, see: a) F. Couty, F. Durrat, G. Evano, D.
Prim, Tetrahedron Lett. 2004, 45, 7525–7528; b) F. Couty, F. Durrat,
G. Evano, J. Marrot, Eur. J. Org. Chem. 2006, 4214–4223.
[23] a) F. Couty, F. Durrat, D. Prim, Tetrahedron Lett. 2003, 44, 5209–
5212; b) W. Van Brabandt, R. Van Landeghem, N. De Kimpe, Org.
Lett. 2006, 8, 1105–1108; c) M. Sivaprakasam, F. Couty, O. David, J.
Marrot, R. Sridhar, B. Srinivas, K. Rama Rao, Eur. J. Org. Chem.
2007, 5734–5739; d) B. Drouillat, F. Couty, O. David, G. Evano, J.
Marrot, Synlett 2008, 1345–1348; e) F. Durrat, M. Vargas-Sanchez,
F. Couty, G. Evano, J. Marrot, Eur. J. Org. Chem. 2008, 3286–3297;
f) S. Dekeukeleire, M. Dꢃhooghe, K. W. Tçrnroos, N. De Kimpe, J.
Org. Chem. 2010, 75, 5934–5940; g) K. Mollet, S. Catak, M. Waro-
quier, V. Van Speybroeck, M. Dꢃhooghe, N. De Kimpe, J. Org.
Chem. 2011, 76, 8364–8375.
[24] For an example of deoxofluor-mediated ring-enlargement of azeti-
dines, see: a) G. R. Krow, G. Lin, K. P. Moore, A. M. Thomas, C.
DeBrosse, C. W. Ross III, H. G. Ramjit, Org. Lett. 2004, 6, 1669–
1672. For an example of ring-opening of azetidinium salts with orga-
nolithium derivatives, see: b) M. T. Wills, I. E. Wills, L. Von Dollen,
B. L. Butler, J. Porter, A. G. Anderson Jr. , J. Org. Chem. 1980, 45,
2489–2498. For an example of ring-expansion of vinylazetidines
with Co2(CO)8, see: c) D. Roberto, H. Alper, J. Am. Chem. Soc.
1989, 111, 7539–7543. For an example of ring-expansion of azeti-
dines mediated by trifluoroborate etherate, see: M. Vargas-Sanchez,
F. Couty, G. Evano, D. Prim, J. Marrot, Org. Lett. 2005, 7, 5861–
5864.
[9] K. M. Miller, T. F. Jamison, Org. Lett. 2005, 7, 3077–3080.
[10] a) P. Liu, P. McCarren, P. H.-Y. Cheong, T. F. Jamison, K. N. Houk,
J. Am. Chem. Soc. 2010, 132, 2050–2057; b) P. R. McCarren, P. Liu,
P. H.-Y. Cheong, T. F. Jamison, K. N. Houk, J. Am. Chem. Soc. 2009,
131, 6654–6655.
[11] a) E. Oblinger, J. Montgomery, J. Am. Chem. Soc. 1997, 119, 9065–
9066; b) W.-S. Huang, J. Chan, T. F. Jamison, Org. Lett. 2000, 2,
4221–4223; c) E. A. Colby, T. F. Jamison, J. Org. Chem. 2003, 68,
156–166; d) K. M. Miller, W.-S. Huang, T. F. Jamison, J. Am. Chem.
Soc. 2003, 125, 3442–3443; e) G. M. Mahandru, G. Liu, J. Montgom-
ery, J. Am. Chem. Soc. 2004, 126, 3698–3699; f) T. Luanphaisarn-
nont, C. O. Ndubaku, T. F. Jamison, Org. Lett. 2005, 7, 2937–2940;
g) K. Sa-ei, J. Montgomery, Org. Lett. 2006, 8, 4441–4443; h) M. R.
Chaulagain, G. J. Sormunen, J. Montgomery, J. Am. Chem. Soc.
2007, 129, 9568–9569; i) Y. Yang, S.-F. Zhu, C.-Y. Zhou, Q.-L. Zhou,
J. Am. Chem. Soc. 2008, 130, 14052–14053.
[12] a) S. Ogoshi, M.-a. Oka, H. Kurosawa, J. Am. Chem. Soc. 2004, 126,
11802–11803; b) S. Ogoshi, M. Ueta, T. Arai, H. Kurosawa, J. Am.
Chem. Soc. 2005, 127, 12810–12811.
[13] a) M. Ohashi, H. Saijo, T. Arai, S. Ogoshi, Organometallics 2010, 29,
6534–6540; b) S. Ogoshi, T. Arai, M. Ohashi, H. Kurosawa, Chem.
Commun. 2008, 1347–1349.
[14] For examples with electrophiles other than aldehydes and ketones,
see: CO2, a) K. Shimizu, M. Takimoto, Y. Sato, M. Mori, Org. Lett.
2005, 7, 195–197; 1,4-enals, b) A. Herath, J. Montgomery, J. Am.
Chem. Soc. 2006, 128, 14030–14031; isocyanates, c) H. A. Duong, J.
Louie, Tetrahedron 2006, 62, 7552–7559; salicylic acid ketals, d) A.
Ooguri, K. Nakai, T. Kurahashi, S. Matsubara, J. Am. Chem. Soc.
Received: January 16, 2012
Published online: February 22, 2012
Chem. Eur. J. 2012, 18, 3486 – 3489
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3489