Catalytic generation of zinc carbenes from alkynes: Zinc-catalyzed cyclopropanation and Si-H bond insertion reactions

Article abstract of DOI:10.1002/anie.201203914

Think zinc: Synthetically relevant zinc(II) carbenes are catalytically generated from alkynes (see scheme). This new approach allows zinc-catalyzed cyclopropanation and Si-H bond insertion reactions to generate the corresponding cyclopropylfurans or silan

Full text of DOI:10.1002/anie.201203914

Angewandte
Chemie
DOI: 10.1002/anie.201203914
Zinc Catalysis
Catalytic Generation of Zinc Carbenes from Alkynes: Zinc-Catalyzed
À
Cyclopropanation and Si H Bond Insertion Reactions**
Rubꢀn Vicente,* Jesffls Gonzµlez, Lorena Riesgo, Javier Gonzµlez, and Luis A. López*
The preparation of zinc carbenoids and their applications in
alkene cyclopropanations were reported by Simmons and
Smith for the first time in 1958.[1] Even today, this synthetic
methodology remains one of the most important ways to
access relevant cyclopropane rings.[2] Despite the extensive
studies on this process, the number of available precursors for
the generation of zinc carbenoids still remains very limited
(Scheme 1a). In fact, more than fifty years after the seminal
work of Simmons and Smith, diiodoalkanes remain the most
common precursors of zinc carbenoids. Regarding the zinc
source, subsequent developments enabled the replacement of
the originally used Zn/Cu couple by alternative stoichiomet-
ric zinc precursors, which in a number of cases improved the
efficiency or selectivity of the cyclopropanation reaction.[3]
Elegant studies performed by Denmark et al. and Charette
et al. support a halomethylzinc structure for these reagents.[4]
Alternatively, zinc carbenoids can be generated by making
use of diazo compounds and zinc salts as disclosed by Wittig
and Schwarzenbach.[5] Moreover, Motherwell et al. described
that carbonyl groups and acetals, upon treatment with zinc
and chlorotrialkylsilanes, afford zinc carbenoids which can be
additionally trapped.[6] It is noteworthy that all these
approaches to zinc carbenoids are stoichiometric processes,
with, to our knowledge, only one remarkable example of
a catalytic transformation using phenyldiazomethane as the
sole carbenoid source.[7] Consequently, the development of
broadly applicable methodologies for the catalytic generation
of zinc carbenoids is highly desirable.
Additionally, zinc complexes proved capable of activating
alkynes, both in a stoichiometric or catalytic fashion.[8]
Interestingly, as early as 1976, Ohloff and co-workers
reported a ZnCl2-mediated rearrangement of propargyl
acetates which afforded cyclopropane derivatives, albeit in
a very low yield.[9] Despite these precedents, the catalytic
generation of efficient cyclopropanating intermediates from
alkynes and zinc salts has, to date, not evolved into a syntheti-
cally useful methodology. To achieve this goal, we hypothe-
sized that a sequence involving initial coordination of the zinc
to the alkyne and a subsequent nucleophilic attack, similar to
that reported for other transition metals,[10] could afford
intermediates such as A or B, which could be active in
cyclopropanation reactions (Scheme 1b).
Herein, we report the catalytic generation of zinc(II)
furylcarbenes derived from carbonyl ene-yne compounds and
zinc salts as well as their reactivity in addition and insertion
processes. A theoretical study of the nature of the involved
intermediates is also disclosed.
We started our investigation by using compound 1a as the
model substrate in the presence of styrene (2a), the carbenoid
trapping reagent. Initial reactions were run at room temper-
ature in dichloromethane by employing a series of zinc
catalysts (10 mol%; Scheme 2; see the Supporting Informa-
tion for a summary of the screening). To our delight, we found
that ZnCl2 produced the desired cyclopropane 3a in nearly
quantitative yield (99%) as a 3.5:1 mixture of diastereoiso-
Scheme 1. Different approaches for the generation of zinc carbenoid
species.
[*] Dr. R. Vicente, J. Gonzꢀlez, Dr. L. Riesgo, Dr. J. Gonzꢀlez,
Dr. L. A. Lꢁpez
Departamento de Quꢂmica Orgꢀnica e Inorgꢀnica e Instituto
Universitario de Quꢂmica Organometꢀlica “Enrique Moles”
Universidad de Oviedo, c/Juliꢀn Claverꢂa 8, 33007 Oviedo (Spain)
E-mail: vicenteruben@uniovi.es
lalg@uniovi.es
[**] We are grateful to the Ministerio de Economꢂa y Competitividad of
Spain (Grant CTQ2010-20517-C02-01) for funding. J.G. and L.R
thank the MICINN and European Union (Fondo Social Europeo) for
predoctoral grants. The calculations were carried out at the “Cluster
de Modelizaciꢁn Cientꢂfica” (Universidad de Oviedo). R.V. is
a Ramꢁn y Cajal Fellow. We thank Prof. J. Barluenga and Prof. M.
Tomꢀs for helpful discussions.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201203914.
Scheme 2. Zinc-catalyzed reaction of 1a and styrene (2a): initial
finding.
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mers after chromatographic purification ,[11] thus confirming
that a zinc carbene was indeed generated.
(see 3n–s) alkenes. Electron-rich olefins, such as n-butyl-
vinylether or dihydropyran smoothly underwent the zinc-
catalyzed cyclopropanation to afford the corresponding
products 3t–v. Interestingly, the reaction with 1-(4-methoxy-
phenyl)-3-buten-1-yne occurred with complete chemoselec-
tivity, thus affording the corresponding alkynylcyclopropane
3w in a respectable yield (62%). With regard to the substrate
1, a variety of carbonyl ene-yne compounds derived either
from 1,3-diketones or from b-ketoesters undergo this zinc-
catalyzed process. Moreover, the reaction was found to
tolerate a broad range of groups (R3) on the alkyne terminus,
including aryl, alkyl, and alkenyl groups. Interestingly, even
R3 = cyclopropyl is compatible with this catalytic process, thus
affording the 1,1’-bi(cyclopropane) derivative 3x in excellent
yield. The diastereoselectivity of the cyclopropanation reac-
tion was dependent upon the nature of both the alkyne and
the olefin counterpart. In general, alkyl-substituted alkynes
(R3 = alkyl) produced cyclopropanes with higher diastereo-
selectivities.
By using these reaction conditions [ZnCl2 (10 mol%),
CH2Cl2, room temperature] we explored the scope of this
remarkable transformation (Scheme 3). We were pleased to
find that the zinc(II)-catalyzed cyclization/cyclopropanation
sequence tolerated a wide range of olefin substitutions
including monosubstituted (see 3a–m) and disubstituted
Considering the feasibility of generating zinc carbenoid
intermediates in this catalytic manner, we next decided to
exploit this strategy in other processes with the aim to
increase their synthetic potential. Thus, we turned our
À
attention to the study of the insertion into a Si H bond.
À
Although metal carbenoid insertion into X H bonds is a well-
established process,[12] surprisingly, this methodology remains
virtually unexplored in the case of zinc carbenoids.[13] More-
À
over, to the best of our knowledge zinc-catalyzed Si H
insertion reactions have not been yet reported. Thus, we
decided to explore the viability of our system for this
significant reaction class.[14] The results of our study involve
À
a cyclization/Si H bond insertion sequence as shown in
Scheme 4. Again, different substitution patterns in com-
pounds 1 proved to work satisfactorily (R1 = Me, Et; R2 = Me,
Et, EtO; R3 = aryl, alkenyl or alkyl), thus affording the
corresponding silane derivatives 5a–g in moderate to good
yields. Regarding the silane, the reaction proceeded effi-
ciently with various silanes (4), including trialkyl-, dialkylaryl-
, and dialkylsilanes as well as 1,1,1,3,5,5,5-heptamethyltrisil-
oxane. Moreover, when using a silyl-substituted alkyne,
a derivative bearing two different silyl groups, trimethylsilyl
(TMS) and triethylsilyl (TES), at the same carbon atom was
obtained (5 f).
A mechanistic proposal to rationalize the obtained results
is given in Scheme 5a. Initial coordination of the substrate to
ZnCl2 affords the intermediate I, where zinc is coordinated to
a carbonyl group and the alkyne.[15] The complex I would then
undergo an intramolecular 5-exo-dig cyclization by nucleo-
philic attack of the carbonyl oxygen atom onto the C4 carbon
atom, thus affording the zinc complex II. Subsequent reaction
of II with the alkenes 2 or silanes 4 would afford the final
products 3 or 5, respectively. The structure of the final
products clearly suggests the participation of a zinc furyl
carbene II as a key intermediate, which can be considered
a vinylogous heteroatom-stabilized Fischer-type zinc(II) car-
bene complex. However, the participation of the classical
Simmons–Smith carbenoid intermediate III (see Scheme 5c
for structure) could not be ruled out.[16] For a better under-
standing of the nature of this species, a computational study of
Scheme 3. Zinc(II)-catalyzed synthesis of cycloproplylfurans 3 from
carbonyl ene-yne derivatives 1 and alkenes 2. Reaction conditions:
1 (0.2 mmol), 2 (6 equiv), ZnCl2 (10 mol%), CH2Cl2 (0.1m), RT. Values
in parentheses are the yields of the isolated products. The diastereo-
isomeric ratio was determined by H NMR spectroscopy. [a] Reaction
performed at 08C.
1
2
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Scheme 5. Proposed mechanism for the zinc-catalyzed cyclization of
compounds 1. a) Catalytic cycle. b) Energy profile of the cyclopropana-
tion of styrene. Computational study was performed with 1a; DGrel
values in kcalmolÀ1 with respect to Ia are given in parentheses.
c) Fischer-type zinc carbene (IIa) versus Simmons–Smith carbene
(IIIa) intermediates.
Scheme 4. Zinc(II)-catalyzed synthesis of furan derivatives 5 from
carbonyl ene-yne derivatives 1 and silanes 4. Reaction conditions:
1 (0.2 mmol), 4 (6 equiv), ZnCl2 (10 mol%), CH2Cl2 (0.1m), RT. Values
in parentheses are the yields of the isolated products.
Next, considering the known ability of ZnCl2 to catalyze
some Knoevenagel reactions,[21] we decided to explore the
feasibility of accessing compounds 3 and 5 in a straightforward
manner through a multicomponent reaction. Indeed, we were
pleased to find that heating a mixture of dione 6 (1.0 equiv),
the corresponding 3-substituted prop-2-ynal 7 (1 equiv) and
styrene (2a, 6.0 equiv) in the presence of ZnCl2 (10 mol%) at
608C for 2 hours afforded the cyclopropylfurans 3a,g in
reasonable yields and with similar diastereoselectivities to
those described above [Eq. (1); Scheme 6]. It is noteworthy
this transformation was carried out.[17] Thus, the potential
energy surface of the cyclization of the enyne 1a in the
presence of ZnCl2 was explored (Scheme 5b). The calcula-
tions supported the participation of intermediates Ia (0.0 kcal
molÀ1) and IIa (À2.6 kcalmolÀ1) in the cyclization of 1a. The
À
predicted length of the C Zn bond of IIa is 2.011 ꢀ, which is
similar to the one reported for related complexes.[18] Accord-
ing to our calculations, IIa is predicted to be more stable than
IIIa by 16.8 kcalmolÀ1.
The cyclopropanation reaction involving II was studied
next.[19] The reaction with styrene (2a) occurred through
a concerted, but asynchronous, transition structure, with
TS4a-trans being favored over TS4a-cis (not shown, 21.8
versus 24.0 kcalmolÀ1), as experimentally observed. Finally,
the formation of trans-3a was predicted to be exothermic
(À3.8 kcalmolÀ1). Importantly, the potential energy surface
for the cyclopropanation with the carbenoid intermediate
IIIa was also explored. However, we did not find a first-order
saddle point connecting IIIa and the reaction products.
Accordingly, at this stage, we believe that the Fischer-type
zinc carbene intermediate II is likely the active species.
À
Likewise, the Si H bond insertion took place via a concerted
asynchronous transition structure (see the Supporting Infor-
Scheme 6. Zinc(II)-catalyzed multicomponent synthesis of furans 3
and 5 (yields of isolated products given in parentheses).
mation).[20]
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that in this operationally simple protocol, three commercially
available reagents were assembled through a condensation/
Experimental Section
Representative procedure (3a; Scheme 3): ZnCl2 (2.7 mg, 0.02 mmol,
10 mol%) was added to a solution of the enyne 1a (42 mg, 0.2 mmol)
and styrene (2a) (125 mg, 1.2 mmol, 6.0 equiv) in CH2Cl2 (2 mL). The
mixture was stirred at room temperature until the disappearance of
1a (monitored by TLC; 1.5 h). The solvent was removed under
reduced pressure and the resulting residue was purified by flash
chromatography (SiO2, n-hexane/EtOAc 20:1) to yield 3a (63 mg,
99%, d.r. = 3.5:1) as a pale yellow oil.
À
cyclization/cyclopropanation sequence, in which three C C
À
bonds and one C O bond were selectively created. Moreover,
this protocol was accomplished on
a multigram scale
(10 mmol) with the same efficiency, as demonstrated for the
cyclopropane 3g (2.26 g, 73%). Similarly, by replacing the
olefin by triethylsilane (4a), under otherwise identical
reaction conditions, this multicomponent approach proved
also applicable to the synthesis of the furan derivatives 5a and
5e in good yields [Eq. (2); Scheme 6].
Received: May 21, 2012
Published online: && &&, &&&&
Finally, we decided to apply this zinc-catalyzed one-pot
procedure to the preparation of polycyclic compounds
through an intramolecular cyclopropanation. Thus, the treat-
ment of 2,4-pentanedione (6) with an equimolecular amount
of the corresponding enynal 8 (dichloroethane, 608C, 2–4 h)
in the presence of ZnCl2 (10 mol%) gave rise to synthetically
relevant 3-oxa- and 3-azabicyclo[n.1.0]alkane derivatives 9a–
c in good yields (Scheme 7).[22]
Keywords: carbenes · heterocycles · insertion ·
small ring systems · zinc
.
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Scheme 7. Zinc(II)-catalyzed synthesis of polycyclic compounds 9
(yields of isolated products given in parentheses). Ts = 4-toluenesul-
fonyl.
In conclusion, we have disclosed a novel approach to the
catalytic generation of synthetically valuable zinc carbenes by
using alkynes and zinc salts. These new zinc carbenes have
À
been employed in cyclopropanation and Si H bond insertion
reactions, which can now be accomplished in a catalytic
manner. The overall process enabled an efficient synthesis of
highly substituted furans, thus making use of easily available
materials and inexpensive ZnCl2, having low toxicity, as the
catalyst.[23] Moreover, we have combined this methodology
with known Knoevenagel condensations to develop conven-
ient selective multicomponent or intramolecular reactions. A
computational study indicates the participation of a 2-furyl
zinc(II) Fischer-type carbene complex, whose structure is
different to those previously proposed for Simmons–Smith
reactions. Further investigations directed at the development
of other sustainable and environmentally friendly synthetic
protocols based on this unprecedented approach to zinc(II)
carbenes, as well as the isolation of these intermediates are
currently underway in our group.
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4
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[17] This study was performed with the density functional theory,
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J. Org. Chem. 2002, 67, 154.
[11] A lower catalyst loading (5.0 mol%) gave rise to a similar yield
(93%), however, a longer reaction time was required (24 h).
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styrene (2a) afforded the cyclopropane 3a, but with lower
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À
[20] A concerted pathway has also been proposed for Si H bond
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[23] Prices of commonly used transition-metal salts (related to
10 mmol at > 99.9% purity, Sigma–Aldrich, 1.05.2012): ZnCl2:
11.0 E; FeCl3: 29.6 E; CuCl2: 22.1 E; PdCl2: 128.0 E; AuCl: 358.1
E; AuCl3: 609.1 E; PtCl2: 490.4 E; RhCl3: 1045.8 E.
À
[13] Only two examples of stoichiometric Si H bond insertion
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Communications
Zinc Catalysis
R. Vicente,* J. Gonzꢁlez, L. Riesgo,
J. Gonzꢁlez, L. A. Lꢂpez*
&&&& — &&&&
Catalytic Generation of Zinc Carbenes
from Alkynes: Zinc-Catalyzed
À
Cyclopropanation and Si H Bond
Insertion Reactions
Think zinc: Synthetically relevant zinc(II)
carbenes are catalytically generated from
alkynes (see scheme). This new approach
allows zinc-catalyzed cyclopropanation
generate the corresponding cyclopropyl-
furans or silane derivatives. The structure
of the key carbene intermediate was
studied using theoretical methods.
À
and Si H bond insertion reactions to
6
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