Wittig reagents as metallocarbene precursors: In situ generated monocarbonyl iodonium ylides

Article abstract of DOI:10.1002/ejoc.201300954

A proof of concept study was undertaken to determine the suitability of monocarbonyl iodonium ylides (MCIYs) as metallocarbene precursors. Exposing Wittig reagents to iodosylbenzene results in a pseudo-Wittig reaction that generates MCIYs in situ. These ylides are intercepted by transition-metal catalysts to generate metallocarbenes, which then undergo either dimerization or cyclopropanation reactions with a variety of alkenes. Additionally, the reaction between diazoester-derived metallocarbenes and Wittig reagents afforded cross-coupling products, illustrating a new type of olefination reaction for phosphonium ylides. Monocarbonyl iodonium ylides (MCIYs) represent a possible alternative to the use diazoketones and -esters as metallocarbene precursors. Upon treatment with iodosylbenzene, a Wittig reagent will undergo ylide transfer to generate a MCIY in situ. In the presence of transition-metal catalysts, MCIYs serve as precursors to metallocarbenes, which undergo dimerization or cyclopropanation of alkenes. tfacac = trifluoroacetylacetonate. Copyright

Full text of DOI:10.1002/ejoc.201300954

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201300954
Wittig Reagents as Metallocarbene Precursors: In Situ Generated
Monocarbonyl Iodonium Ylides
[a]
[a]
[a]
Phyllis E. Ho, Jason Tao, and Graham K. Murphy*
Keywords: Diazo compounds / Carbenoids / Wittig reactions / Hypervalent compounds / Iodonium ylides
A proof of concept study was undertaken to determine the
suitability of monocarbonyl iodonium ylides (MCIYs) as
metallocarbene precursors. Exposing Wittig reagents to
iodosylbenzene results in a pseudo-Wittig reaction that gen-
erates MCIYs in situ. These ylides are intercepted by transi-
tion-metal catalysts to generate metallocarbenes, which then
undergo either dimerization or cyclopropanation reactions
with a variety of alkenes. Additionally, the reaction between
diazoester-derived metallocarbenes and Wittig reagents af-
forded cross-coupling products, illustrating a new type of ole-
fination reaction for phosphonium ylides.
Introduction
Transition-metal-catalyzed decomposition of diazo com-
pounds to generate metallocarbenes allows entry into their
diverse portfolio of associated transformations. Metallo-
carbenes readily undergo cyclopropanation, C–H insertion,
and ylide formation, providing general methods for intro-
ducing molecular complexity.[1] One drawback of this meth-
odology is that the initial diazo compounds are reported to
be toxic/carcinogenic and explosive.[ As such, their wide-
2]
[
3]
spread use outside the academic laboratory is limited. To
circumvent the use of the diazo functional group, numerous
reports of surrogates have appeared over the years. For β-
distabilized substrates, iodonium ylides have proven to be Figure 1. (a) β-Distabilized iodonium ylide synthesis and transmet-
highly effective as diazo surrogates, and the efficacy of this alation. (b) Monostabilized iodonium ylide synthesis typically af-
fords α-functionalization products.
method results from the acidity of the α-protons (Fig-
ure 1a). In this case, initially formed iodonium salt 2a is
readily deprotonated to afford iodonium ylide 3, which acts
as a metallocarbene precursor. A variety of electron-with-
acidity for easy deprotonation, and given the high nucleofu-
drawing groups (EWGs) are tolerated, and this methodol-
gality of the phenyliodonio group, it instead undergoes nu-
ogy has now evolved into the one-pot generation and metal-
cleophilic displacement to give α-functionalized products 5.
catalyzed decomposition of iodonium ylides to afford prod-
[
4g,8]
[9]
[4]
[4f]
Although sulfonium
and sulfoxonium ylides serve
ucts derived from cyclopropanation, C–H-insertion,
and oxonium ylides.[5]
as metallocarbene precursors in monostabilized substrates,
these have not been adopted by the synthetic community. If
they could be prepared, the potential of MCIYs as diazo
surrogates could be determined. The only example of
carbenoid reactivity resulting from a monostabilized iodon-
In contrast to β-distabilized substrates, the generation of
monocarbonyl iodonium ylides (MCIYs) for use as diazo
surrogates has not been reported.[6] This is because of our
inability to generate MCIYs by the typical onium (i.e., N,
P, S, etc.) ion α-proton abstraction.[7] The initially formed
iodonium salt 2b (Figure 1b) has no proton of sufficient
[
10]
ium ylide employed nitromethane as the substrate. Han-
sen took advantage of its relatively acidic protons and de-
veloped one-pot processes for iodonium ylide generation
and carbene transfer. This proved moderately effective in
one-pot cyclopropanation and ring cleavage reactions
[
a] Department of Chemistry, University of Waterloo,
2
00 University Ave W., Waterloo, Ontario, N2L3G1, Canada
E-mail: graham.murphy@uwaterloo.ca
(Scheme 1a). The only reported syntheses of MCIYs in-
Homepage: https://uwaterloo.ca/murphy-lab/
volve an acyl-transfer reaction to liberate the iodonium
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201300954.
ylide from a preformed iodonium salt (Scheme 1b).[
11]
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Wittig Reagents as Metallocarbene Precursors
diazoacetate (EDA, 12) and because catalyst influence on
[
1]
that transformation is well documented. Reference sam-
ples of the cyclopropanation (i.e., 13) and carbene dimeriza-
tion (i.e., 14) product isomers were prepared by heating 12,
styrene, and Cu(OTf) (Tf = trifluoromethanesulfonyl) in
2
CHCl at reflux (Scheme 3a). The related experiments using
3
Wittig reagent 10, styrene, and Cu(OTf) were monitored
2
by GC analysis, which indicated the presence of dimers 14
but no cyclopropanation product 13 (Scheme 3b).
Scheme 1. (a) Monostabilized iodonium ylide from nitromethane.
(
=
b) An acyl transfer method for in situ generation of MCIYs. esp
α,α,αЈ,αЈ-tetramethyl-1,3-benzenedipropionic acid.
Results and Discussion
As part of a program focusing on the use of hypervalent
iodine reagents in synthesis, we proposed to develop a
method for the generation and use of MCIYs as metallo-
carbene precursors. With all the potential variables that
could be investigated [substrate, aryl iodide substitution,
counterion (X) and base], we assume there is a combination
Scheme 3. (a) Cyclopropanation of styrene with EDA. (b) At-
tempted cyclopropanation of styrene by in situ generation of
that would prove successful for MCIY synthesis MCIY by ylide transfer between Wittig reagent and iodosylbenz-
ene. Carbene dimerization product 14 results.
(Scheme 2a). Before delving too deeply into these studies,
we wanted to first determine whether MCIYs were effective
as metallocarbene precursors. The key to solving this lay in
our ability to generate an MCIY under conditions that are
compatible with the ylide being intercepted by a transition-
metal catalyst. Simplistically, we viewed the iodonium ylide
arising from a combination of a substrate and a hypervalent
iodine reagent that would react to form the MCIY. Aware
of the oxophilicity of phosphorus, we proposed a pseudo-
Wittig reaction between phosphonium ylide 10 and iodosyl-
A series of experiments was carried out to differentiate
between a carbenoid dimerization and an oxidative dimer-
ization. Repeating the reaction without styrene at room
temperature (Table 1, entry 1) and at reflux (Table 1, en-
try 2) showed an increase in the yield of 14 at elevated tem-
perature. Conducting the reaction in the absence of a cata-
lyst (Table 1, entry 3) gave no 14, though at reflux (Table 1,
entry 4) 14 was observed, which suggests a possible back-
ground oxidative mechanism at elevated temperature. An
increased iodosylbenzene concentration also failed to pro-
duce 14 (Table 1, entry 5). Conducting the reaction without
oxidant afforded no 14, owing to the stability of phos-
phorus ylides, which are not suitable as direct precursors to
benzene, which would generate MCIY 11 and tri-
phenylphosphine oxide (Scheme 2b).[
12]
The following dis-
cussion concerns our results from this proof-of-concept
study.
[
13]
metallocarbenes (Table 1, entries 6 and 7).
A variety of
oxidants were tested (Table 1, entries 8–11), none of which
produced 14. Various solvents (CHCl , CH Cl , 1,2-di-
3
2
2
chloroethane, benzene, toluene, MeCN, xylenes, 1,2-di-
chlorobenzene) were also investigated, though no signifi-
cant improvement in product formation (by GC) was ob-
served. We concluded we were observing a pseudo-Wittig
reaction between 10 and iodosylbenzene and that we were
observing unprecedented reactivity between metallocarb-
enes and Wittig reagents.
As the cross-coupling of carbenes and Wittig reagents
and reagent variations; (b) Wittig-type reaction with iodosylbenz- was, to the best of our knowledge, unknown, we carried out
Scheme 2. Possible approaches to generating MCIYs: (a) substrate
ene.
a series of experiments to support our proposal (Scheme 4).
Cross-coupling between EDA (12) and Wittig reagent 15
We investigated the cyclopropanation of styrene by Wit- was most effective with Cu(hfacac)2 (hfacac = hexa-
tig reagent 10 as our model system, specifically because it fluoroacetylacetonate), which gave product 16 in 41% yield
is analogous to the cyclopropanation of styrene with ethyl upon isolation. An additional example was inspired by a
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© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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6541

P. E. Ho, J. Tao, G. K. Murphy
SHORT COMMUNICATION
Table 1. Control experiments to probe the dimerization reaction.
most effective, resulting in a 56% yield of metallocarbene-
derived products.
Table 2. Screening catalysts for efficacy in the MCIY-based cyclo-
propanation reaction.
Entry
Oxidant
1.2 equiv.)
Cu(OTf)
[mol-%]
2
T
[°C]
Yield[a]
[%]
(
1
2
3
4
5
6
7
8
9
PhIO
PhIO
PhIO
PhIO
PhIO (2.5 equiv.)
–
–
PhI(OAc)
Oxone
DMSO
10
10
–
–
–
10
10
10
10
10
10
r.t.
reflux
r.t.
reflux
r.t.
r.t.
reflux
r.t.
r.t.
r.t.
r.t.
8
41
–
17
–
–
2
–
–
–
Entry
Catalyst[a]
Yield [%]
[b]
14[c]
13
1[d]
Cu(OTf)
Cu(OTf)
2
58
–
–
3
–
–
–
–
27
14
11
8
18
8
12
26
12
11
29
15
2
3
4
5
6
7
8
2
2
Rh
Rh
Rh
Rh
Cu(BF
Cu(acac)
Cu(tfacac)
Cu(hfacac)
(OAc)
2
2
2
2
4
(tpa)
4
1
1
0
1
(tfa)
(piv)
4
PhI(OH)OTs
–
4
4
)
2
[
a] The yields were determined by GC analysis of the crude reaction
mixtures. Area-% values were compared to a concentration cali-
2
[
e]
9
1
2
bration curve prepared for maleate 14.
0
2
[
5
a] Reactions were performed with 10 mol-% copper catalyst or
mol-% rhodium catalyst. tfa = trifluoroacetate, piv = pivalate,
report from the Davies laboratory, in which they cross-cou- acac = acetylacetonate. [b] The yields were determined by GC
pled diazoesters 17 and 12.[ We carried out the analogous analysis of the crude reaction mixtures. Area-% values were com-
14]
pared to a concentration calibration curve prepared for product 13.
reaction between diazoester 17 and Wittig reagent 10 and
recovered olefination product 18 in 53% yield.
[
c] The yields were determined by GC analysis of the crude reaction
mixtures. Area-% values were compared to a concentration cali-
bration curve prepared for maleate 14. [d] Reaction was run with
EDA (12) and styrene and its GC yield [%] values are included for
comparative purposes. [e] Products were isolated as a mixture in
30% yield after purification.
To minimize the occurrence of carbene dimerization
products, we added 10 by syringe pump over various times
ranging from 0.5 to 18 h, but no significant increase in
product formation was observed. Ochiai reported MCIYs
[
11d]
to be unstable above –30 °C,
so we repeated the reaction
at lower temperatures to determine the effect. Reactions
conducted at –60, –40, and 0 °C failed, and analysis of their
reaction mixtures by P NMR spectroscopy revealed
Scheme 4. Cross-coupling reactions between Wittig reagents and
diazoesters.
31
mostly starting ylide 10, suggesting the reaction failed to
initiate at lower temperature. We investigated whether
Having determined that we were observing carbenoid re- iodosylbenzene could be depolymerized in solution,[ with
activity from transient iodonium ylides, we investigated the the goal of making the initial Wittig-type reaction more fac-
effect of catalyst on the transformation. A variety of cata- ile. Lewis acids (BF ·OEt , ZnCl , and MgBr ·OEt ) were
15]
3
2
2
2
2
lysts commonly used for metallocarbene formation were added individually, but little improvement in reaction effi-
tested, and in all cases product 14 was generated (Table 2). cacy was observed. Finally, neither increasing the amount
The Rh (tpa) (tpa = triphenylacetate) catalyst generated of PhIO used nor employing styrene as a reaction solvent
2
4
trace amounts of cyclopropanation product 13 (Table 2, en- resulted in any significant improvement in the generation of
try 4), but when we used Cu(tfacac) (tfacac = trifluoro- 13.
2
acetylacetonate) or Cu(hfacac) as the catalyst, a significant
To provide further evidence of carbenoid reactivity, we
2
increase in yield of cyclopropanation product 13 was ob- conducted the cyclopropanation reaction with cyclohexene
[
16]
served (Table 2, entries 9 and 10). Copper catalysts have (19), norbornene (20), and trans-stilbene (21). For all re-
been observed to be less effective than rhodium catalysts in actions, an inseparable mixture of products 13a–c and 14
one-pot iodonium ylide transformations,[
5,8b]
but only these was isolated in moderate yield (Scheme 5). Though the
catalysts were found to intercept the transient MCIY and yields are moderate, these examples provide clear evidence
induce carbenoid reactivity (other than dimerization) to any that MCIYs are suitable targets for continued investigation
reasonable extent. The GC yields indicate Cu(tfacac) to be as metallocarbene precursors.
2
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Eur. J. Org. Chem. 2013, 6540–6544

Wittig Reagents as Metallocarbene Precursors
nesium sulfate. It was filtered and concentrated by rotary evapora-
tion (with added toluene to remove the excess amount of styrene),
resulting in a crude oil. Purification by column chromatography (2,
5, 10 then 20% EtOAc in hexanes) afforded a mixture of products
13 and 14 (13 mg) in 30% yield.
Supporting Information (see footnote on the first page of this arti-
cle): Experimental procedures and gas chromatograph traces for all
experiments.
Acknowledgments
The Department of Chemistry, University of Waterloo, and the
Natural Sciences and Engineering Research Council of Canada
(
NSERC) are thanked for financial support. Professor J. M. Chong
is acknowledged for his very generous gift of a gas chromatograph.
Scheme 5. Cyclopropanation of alkenes by using MCIY-derived
metallocarbenes. Yield corresponds to the combined yields of iso-
lated products 13 and 14.
[1] M. P. Doyle, M. A. Mckervey, T. Ye, Modern Catalytic Meth-
ods for Organic Synthesis with Diazo Compounds: From Cyclo-
propanes to Ylides, Wiley, New York, 1998, p. 652.
[
2] L. Bretherick, Handbook of Reactive Chemical Hazards: An In-
dexed Guide to Published Data, 2nd ed., Butterworths, London,
Boston, 1979, p. 589.
Conclusions
Few reports of MCIY reactivity exist owing to the inabil- [3] An industrial process that employs an industrial-scale N–H in-
ity to generate these ylides by traditional methods. Though
MCIYs represent a potential surrogate for diazoketones
and -esters, there existed no reports of MCIYs acting as
metallocarbene precursors. By conceiving a means to gener-
sertion reaction is Merck’s synthesis of thienamycin, see:
T. M. H. Liu, D. G. Melillo, K. M. Ryan, I. Shinkai, M. Sletz-
inger, Intermediate for synthesis of thienamycin via (3SR,4RS)-
3
4
-[1(SR)-hydroxyethyl]-2-oxo-4-azetidineacetic
349687, 1982.
acid,
US
ate these elusive intermediates from phosphonium ylides [4] a) A. S. Biland, S. Altermann, T. Wirth, ARKIVOC 2003, 6,
164–169; b) P. Dauban, L. Saniere, A. Tarrade, R. H. Dodd, J.
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Zhdankin, Org. Lett. 2012, 14, 3170–3173.
iodonium ylides in situ. We have shown how MCIYs can
be intercepted by a variety of transition-metal catalysts and
how they can serve as metallocarbene precursors. Both rho-
dium and copper catalysts induced carbenoid dimerization,
and we showed a new crossed-olefination reaction between
metallocarbenes and Wittig reagents. Finally, Cu-
(
tfacac) and Cu(hfacac) were found to be effective cata-
2 2
lysts for inducing cyclopropanation reactions on a variety
of substrates. Though we were unable to improve this pro-
cess to be synthetically viable, this study serves as proof of
concept that MCIYs can serve as metallocarbene precur-
sors. If a suitable method for MCIY synthesis were devel-
oped, their potential as diazo surrogates could be realized.
[
[
5] G. K. Murphy, F. G. West, Org. Lett. 2006, 8, 4359–4361.
6] a) M. Ochiai, M. Kunishima, K. Fuji, M. Shiro, Y. Nagao, J.
Chem. Soc., Chem. Commun. 1988, 1076–1077; b) G. A. Olah,
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109, 7235–7236.
[
7] a) B. M. Trost, L. S. Melvin, Sulfur Ylides: Emerging Synthetic
Intermediates, Academic Press, New York, 1975; b) J. I. G. Ca-
dogan, Organophosphorus Reagents in Organic Synthesis Aca-
demic Press, London, 1979, p. 608.
Experimental Section
General Procedure for in Situ Generation of MCIY-Derived Metallo-
carbenes: An oven-dried 10 mL round-bottomed flask was charged
[
8] a) B. Cimetiere, M. Julia, Synlett 1991, 271–272; b) P. Müller,
D. Fernandez, P. Nury, J. C. Rossier, J. Phys. Org. Chem. 1998,
with iodosylbenzene (114 mg, 0.52 mmol, 1.2 equiv.), Cu(tfacac)
2
11, 321–333; c) B. M. Trost, J. Am. Chem. Soc. 1967, 89, 138.
(16 mg, 10 mol-%), styrene (247 μL, 2.15 mmol, 5.0 equiv.), and
[
9] J. E. Baldwin, R. M. Adlington, C. R. A. Godfrey, D. W. Gol-
lins, J. G. Vaughan, J. Chem. Soc., Chem. Commun. 1993, 1434–
1435.
3
CHCl (3.3 mL). A syringe was charged with a freshly prepared
solution of Wittig reagent 10 (150 mg, 0.43 mmol, 1.0 equiv.) and
CHCl (1.0 mL). The solution was added by syringe pump over a
[10] a) H. T. Bonge, T. Hansen, Synlett 2007, 55–58; b) H. T. Bonge,
3
T. Hansen, Tetrahedron Lett. 2008, 49, 57–61.
period of 1 h, and the mixture was stirred for 30 min, by which
time analysis of an aliquot by 31P NMR spectroscopy indicated the
[11] For the preparation and use of MCIYs in various transforma-
tions, see: a) M. Ochiai, Y. Kitagawa, Tetrahedron Lett. 1998,
consumption of 10. At this time, the reaction was analyzed by GC
for determination of product formation. The reaction was
quenched by the addition of 10% aqueous K CO (5 mL) and
2 3
EtOAc (10 mL). The aqueous layer was separated and extracted
with EtOAc (2ϫ 10 mL). The combined organic fraction was
washed with water (10 mL) and brine (20 mL) and dried with mag-
3
9, 5569–5570; b) M. Ochiai, Y. Kitagawa, J. Org. Chem. 1999,
64, 3181–3189; c) M. Ochiai, Y. Kitagawa, J. Synth. Org. Chem.
Jpn. 2000, 58, 1048–1056; d) M. Ochiai, Y. Kitagawa, S. Yama-
moto, J. Am. Chem. Soc. 1997, 119, 11598–11604; e) M. Ochiai,
J. Nishitani, Y. Nishi, J. Org. Chem. 2002, 67, 4407–4413; f) M.
Ochiai, Y. Tuchimoto, N. Higashiura, Org. Lett. 2004, 6, 1505–
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© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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6543

P. E. Ho, J. Tao, G. K. Murphy
SHORT COMMUNICATION
1
508; g) K. Miyamoto, M. Suzuki, T. Suefuji, M. Ochiai, Eur.
Wittig reagent 10, see: b) W. A. Herrmann, M. Wang, Angew.
Chem. 1991, 103, 1709–1711; Angew. Chem. Int. Ed. Engl. 1991,
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T. Honma, Tetrahedron Lett. 1998, 39, 625–626.
J. Org. Chem. 2013, 3662–3666.
[
12] The combination of Wittig reagents and hypervalent iodine
reagents has been shown to produce α-functionalized Wittig
reagents. For examples, see: a) R. M. Moriarty, I. Prakash, O.
Prakash, W. A. Freeman, J. Am. Chem. Soc. 1984, 106, 6082–
6
6
084; b) Z. Z. Huang, X. C. Yu, X. Huang, J. Org. Chem. 2002,
[14] J. H. Hansen, B. T. Parr, P. Pelphrey, Q. H. Jin, J. Autschbach,
H. M. L. Davies, Angew. Chem. 2011, 123, 2592–2596; Angew.
Chem. Int. Ed. 2011, 50, 2544–2548.
[15] R. M. Moriarty, J. W. Kosmeder, V. V. Zhdankin, C. Courillon,
E. Lacôte, M. Malacria, B. Darses, P. Dauban, Iodosylbenzene,
e-EROS, 2012.
7, 8261–8264; c) V. V. Zhdankin, O. Maydanovych, J. Hersch-
bach, J. Bruno, E. D. Matveeva, N. S. Zefirov, J. Org. Chem.
003, 68, 1018–1023; d) E. D. Matveeva, T. A. Podrugina, Y. K.
2
Grishin, V. V. Tkachev, V. V. Zhdankin, S. M. Aldoshin, N. S.
Zefirov, Russ. J. Org. Chem. 2003, 39, 536–541.
[
13] It has been shown that combining metallocarbenes and tri-
phenylphosphane is a means to prepare phosphonium ylides
in situ. For an example of the in situ preparation of methylene-
triphenylphosphorane, see a) H. Lebel, V. Paquet, J. Am.
Chem. Soc. 2004, 126, 320–328; for the in situ preparation of
[16] We conducted the corresponding EDA-based cyclopropanation
reactions to obtain reference materials for product comparison
by GC analysis and NMR spectroscopy.
Received: June 27, 2013
Published Online: September 3, 2013
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Eur. J. Org. Chem. 2013, 6540–6544