A soft vinyl enolization approach to α-acylvinyl anions: Direct aldol/aldol condensation reactions of (E)-β-chlorovinyl ketones

Article abstract of DOI:10.1002/anie.201209876

Synthesizing the synthons: The development of α-acylvinyl anion synthons has been achieved using direct α-vinyl enolization of α,β-unsaturated ketones under mild reaction conditions. The synthetic utility of such synthons has been demonstrated in intermolecular aldol and aldol condensation reactions, which provide synthetically useful allenyl ketone and enyne derivatives. Copyright

Full text of DOI:10.1002/anie.201209876

Angewandte
Chemie
DOI: 10.1002/anie.201209876
Synthetic Methods
A Soft Vinyl Enolization Approach to a-Acylvinyl Anions: Direct
Aldol/Aldol Condensation Reactions of (E)-b-Chlorovinyl Ketones**
Hun Young Kim, Jian-Yuan Li, and Kyungsoo Oh*
Keto–enol tautomerization is one of the fundamental chem-
ical phenomena that empowers our ability to modulate the
reactivity of carbonyl compounds. The enol and metal enolate
forms of carbonyl compounds constitute one of the most
compounds are highly desirable for the development of
a-acylvinyl nucleophile equivalents.
During our investigation of the elimination pathway of
b-halovinyl ketones,[10] we observed a facile a-vinyl enoliza-
tion of (E)-b-chlorovinyl ketones by the action of a weak
base, NEt3. The non-planar conformational preference of (E)-
b-chlorovinyl ketones, as opposed to the planar conforma-
tional preference of (Z)-b-chlorovinyl ketones, was believed
À
widely used nucleophiles in the C C bond forming strategies
that readily accommodate various electrophiles as reaction
partners.[1] Whereas regio- and stereoselective enol(ate)
3
À
formation of carbonyl compounds with a-C(sp ) H bonds
has been achieved with an increasing degree of sophistica-
to result in better orbital overlap between the pC–O bonding
tion,[2] the generation of corresponding enol and enolate
orbital and the s
anti-bonding orbital, thus allowing for
*
aÀCÀH
2
À
forms of carbonyl compounds with a-C(sp ) H bonds (such
a mild a-vinyl enolization of (E)-b-chlorovinyl ketones
as allenol and allenolates) has been limited to a few excep-
tional cases of a,b-unsaturated carbonyl compounds
(Scheme 1).[3]
(Scheme 2). Motivated by our earlier studies combined with
Scheme 1. Molecular orbital overlap requirement for a-deprotonation.
The unfavorable orbital overlap between the pCÀO bond-
*
ing orbital and the s
anti-bonding orbital in a conjugate
aÀCÀH
system makes the a-vinyl deprotonation extremely challeng-
ing under mild reaction conditions.[4] Moreover, the presence
2
3
À
of other C(sp /sp ) H bonds in conjugated carbonyl com-
pounds under strong basic conditions adds more complexity
to the degree of chemocontrol.[5] Although alternative
synthetic methods to form allenolates from alkynyl carbonyl
compounds have been developed to avoid such regio- and
chemoselectivity issues,[6–9] new methods for the direct gen-
eration of allenol/allenolates from a,b-unsaturated carbonyl
Scheme 2. Proposed mechanism for mild a-vinyl enolization of
(E)-b-chlorovinyl ketones.
the lack of access to a-vinyl nucleophilic synthons from a,b-
unsaturated carbonyl compounds under mild reaction con-
ditions,[5] we examined the a-vinyl enolization of (E)-b-
À
chlorovinyl ketones for intermolecular C C bond formations.
Herein, we describe the realization of such direct and mild
À
[*] Dr. H. Y. Kim, J.-Y. Li, Prof. Dr. K. Oh
Department of Chemistry and Chemical Biology, Indiana University
Purdue University Indianapolis (IUPUI)
Indianapolis, IN 46202 (USA)
C C bond formations in the context of aldol and aldol
condensation reactions through a facile vinyl enolization
method.
E-mail: ohk@iupui.edu
Homepage: http://chem.iupui.edu/people/kyungsoo-oh
To examine the aldol reaction of enol intermediates from
the elimination of (E)-b-chlorovinyl ketones, we subjected
equimolar amounts of (E)-b-chlorovinyl ketone 1a and
2-nitrobenzaldehyde 2a to mild basic conditions (NEt3; see
the Supporting Information for details). To our delight, we
observed the formation of a 1:1 diastereomeric mixture of
[**] This research was supported by IUPUI through a Research Support
Fund Grant (2011 RSFG). We thank Dr. Karl Dria for his assistance
with mass spectra analysis (CHE-0821661). The Bruker 500 MHz
NMR was purchased through an NSF-MRI award (CHE-0619254).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201209876.
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the Supporting Information for details). Although the soft
a-alkyl enolization generally requires stoichiometric amounts
of a metal, we found that a catalytic amount of the hard Lewis
[15]
acid Ti(OiPr)4 was sufficient to induce an aldol condensa-
tion, providing synthetically versatile 2-(1-alkynyl)-2-alken-1-
ones 4 (Scheme 4). It is worth noting that whereas enyne
Scheme 3. Scope of the direct aldol reaction of (E)-b-chlorovinyl
ketones. Yields shown are of isolated product 3. [a] Reactions at 238C
in CH2Cl2.
aldol products 3 (Scheme 3). This simple and direct reaction
procedure provided aldol product 3a in 73% yield within 18 h
of reaction time. To investigate the substrate scope, we
employed a number of aryl aldehydes in the aldol reaction. In
general, electron-deficient aryl aldehydes underwent aldol
reaction (3a–3 f), whereas other aryl aldehydes failed to
undergo aldol reaction and instead exclusively formed the
elimination products.[11] Although more studies are needed to
expand the scope of the electrophiles, a salient feature of the
current mild a-vinyl enolization of (E)-b-chlorovinyl ketones
was well demonstrated upon the aldol reactions of diversely
functionalized/substituted (E)-b-chlorovinyl ketones (3g–
3l).[12] The access to such functionalized allenyl ketone
derivatives is not currently possible by other “hard enoliza-
tion” methods such as the direct lithiation of a-allenyl
ketones[13] and strong base-promoted isomerization of alkynyl
esters.[9] In particular, the employment of aliphatic (E)-b-
chlorovinyl ketones in our mild a-vinyl enolization approach
highlights the unusual chemoselectivity in which a-vinyl
enolization was preferentially performed over a-alkyl enoli-
zation (3k–3l). Our results herein are the first a-vinyl
enolization of a,b-unsaturated carbonyl systems under mild
conditions.
Scheme 4. Scope of aldol condensation by soft a-vinyl enolization.
Yields shown are of isolated product 4. [a] 20 mol% of Ti(OiPr)4.
[b] 50 mol% of Ti(OiPr)4. [c] 100 mol% of Ti(OiPr)4.
derivatives 4 have been widely utilized in various transition-
metal-catalyzed synthetic transformations,[16] the preparation
of 4 has not been trivial owing to their functional-group-rich
nature, which routinely requires four to five linear steps.[16a]
Our aldol condensation reaction provides a facile synthetic
route to a range of acyclic enynes 4 in a two-step reaction
sequence: the AlCl3-catalyzed (E)-selective Friedel–Crafts
acylation of alkyne and the Ti(OiPr)4-catalyzed aldol con-
densation of (E)-b-chlorovinyl ketones. Not only aryl alde-
hydes with different electronic and steric characters (4a–4e),
but also an a,b-unsaturated aldehyde such as cinnamaldehyde
(4 f) and aliphatic aldehydes such as cyclohexanecarboxalde-
hyde (4h), were effective electrophiles. The reaction of
heteroaryl aldehydes also afforded enynes (4i and 4j),
although a stoichiometric amount of Ti(OiPr)4 was necessary
in these cases.[17] A distinctive feature of our aldol condensa-
tion of (E)-b-chlorovinyl ketones was that a diverse array of
functional groups, such as esters and halides, were tolerated
Our observation of the facile formation of an allenol
under the action of a mild base prompted us to investigate the
feasibility of generating metal allenolates in the presence of
a Lewis acid and a weak base, a process termed “soft
enolization”.[14] Thus, we briefly screened different Lewis
acids to broaden the scope of our aldol reaction. Among the
Lewis acids we screened, Ti(OiPr)4 was identified as the
Lewis acid that promotes aldol condensation reactions (see
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and led to the formation of functionalized enynes (4k–4n).
One particular note regarding the reaction outcome is that the
aldol condensation of (E)-b-chlorovinyl ketones with
a’-hydrogen atoms also led to the exclusive formation of
enynes (4o and 4p), although such a’-hydrogen atoms are
expected to have lower pKa values than a-vinyl hydrogen
atoms.[18]
Examples of the NEt3-promoted aldol reaction typically
required the use of electron-deficient aldehydes to effectively
compete against the protic source that promoted the elimi-
nation pathway to allenyl and propargyl ketones. In contrast,
the Ti(OiPr)4-catalyzed aldol condensation reaction worked
well with various aldehydes with different electronic and
steric features. Thus, it is possible to speculate that there are
different intermediate species between the aldol and aldol
condensation reactions. For the NEt3-promoted aldol reac-
tion, we did not observe the formation of aldol products when
a 2:1 mixture of allenyl ketone 5a and propargyl ketone 6a
was treated with 4-nitrobenzaldehyde under our aldol reac-
tion conditions (Scheme 5). Further studies also showed the
in our aldol products suggests that the stereogenic center
created by the aldol reaction exerts little effect on the alkyne-
allene isomerization.[19]
As for the Ti(OiPr)4-catalyzed aldol condensation reac-
tion, whereas no reaction was observed using (Z)-b-chloro-
vinyl ketones, we found that a facile aldol condensation
reaction between 5a and 6a (in a 2:1 mixture) readily
occurred in the absence of NEt3 when a catalytic amount of
Ti(OiPr)4 was used (Scheme 6). Several observations regard-
Scheme 6. Proposed reaction mechanism for the Ti(OiPr)4-catalyzed
aldol condensation reaction.
ing the Ti(OiPr)4-catalyzed aldol condensation are notewor-
thy: 1) no aldol condensation product B was observed using
g,g-disubstituted allenyl ketones (a possible aldol pathway of
Ti–cumulenolate); 2) the formation of a,g-deuterium-
exchanged product 8[10] was observed from allenyl and alkynyl
ketones under Ti(OiPr)4/CD3OD conditions (a possible
isomerization of alkynyl ketones to allenyl ketones); 3) a
catalytic amount of Ti(OiPr)4 readily induced the dehydration
of aldol product 3c. Although our previous study established
that the conformation of (E)-b-chlorovinyl ketones changes
in the presence of hard and soft Lewis acids,[10] it appears that
Ti(OiPr)4 does not strongly interact with the carbonyl group
of (E)-b-chlorovinyl ketones to cause the conformational
change, this is possibly due to the less Lewis basic nature of
the carbonyl group, as well as the less Lewis acidic nature of
Ti(OiPr)4 relative to TMSOTf. Instead, the more Lewis basic
carbonyl groups of allenyl and propargyl ketones preferen-
tially interact with Ti(OiPr)4 and acidify the a-hydrogen for
deprotonation by a proximal alkoxide.[20] Based on our
experimental observations, we propose a mechanism for the
Ti(OiPr)4-catalyzed aldol condensation reaction that involves
a vinylogous aldol-type reaction of the Ti–cumulenolate
Scheme 5. Proposed reaction mechanism for the NEt3-promoted aldol
reaction.
following: 1) the absence of aldol products A from b-
chlorovinyl ketone (a possible aldol pathway of allenol);
2) no aldol reaction product B using g,g-disubstituted-b-
chlorovinyl ketones (a possible aldol pathway of cumulenol);
3) the absence of aldol products C from propargyl ketone
(a possible aldol pathway of alkynyl enol). On the basis of the
above considerations, we propose an aldol reaction mecha-
nism involving the NEt3-promoted vinylogous aldol-type
reaction, which leads to the observed aldol products after
alkyne–allene isomerization. The lack of diastereoselectivity
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followed by Ti-promoted dehydration, as shown in
Scheme 6.[21] This interpretation is consistent with the fact
that TiCl4 does not promote aldol condensation reaction of
allenyl and alkynyl ketones.
In summary, we have synthesized a new class of vinyl
anion synthons from (E)-b-chlorovinyl ketones under mild
basic conditions. The synthetic utility of these synthons has
been demonstrated in intermolecular aldol and aldol con-
densation reactions, which lead to functional-group-rich
allenyl ketones and enyne derivatives. Although further
mechanistic studies remain to be conducted, our preliminary
findings suggest the base-promoted vinylogous aldol-type
reaction of cummulenol(ate)s as a possible reaction pathway.
[8] For the 1,3-transposition of propargylic alcohols by a vanadium
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[11] In Scheme 3, the yields of isolated elimination products, allenyl
and propargyl ketones, accounted for the remaining mass
balance in the reaction. No aldol reaction was observed using
(Z)-b-chlorovinyl ketones.
[12] For (E)-selective AlCl3-catalyzed Friedel – Crafts acylations of
alkynes, see: H. Martens, F. Janssens, G. Hoornaert, Tetrahedron
1975, 31, 177.
[13] a) N. A. Petasis, K. A. Teets, J. Am. Chem. Soc. 1992, 114, 10328.
For a-allenyl esters, see: b) S. Tsuboi, H. Kuroda, S. Takatsuka,
T. Fukawa, T. Sakai, M. Utaka, J. Org. Chem. 1993, 58, 5952.
[14] For selected examples, see: a) M. W. Rathke, P. J. Cowan, J. Org.
Chem. 1985, 50, 2622; b) M. W. Rathke, M. Nowak, J. Org.
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Given the widespread applicability of enols and metal
[2]
À
enolates in stereoselective C C bond forming reactions,
we anticipate that the synthetic scope of cummulenol(ate)s
can be further expanded. Accordingly, our current efforts are
directed towards addressing the low diastereoselectivity issue
by using stereoselective isomerization under Brønsted base
catalysis, and further broadening the synthetic utility of
À
cummulenol(ate)s in other C C bond forming reactions.
Received: December 10, 2012
Revised: January 10, 2013
Published online: && &&, &&&&
Keywords: aldols · condensation reactions · enols · Lewis acids ·
.
vinyl ketones
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[17] A longer reaction time is typically needed in the presence of
a catalytic amount of Ti(OiPr)4, but the enynes (4I and 4j) from
heteroaryl aldehydes were not stable over a prolonged reaction
time.
3
À
[7] For selected lithium alkylide additions to carbonyl followed by
a Brook rearrangement, see: a) H. J. Reich, E. K. Eisenhart,
[18] The pKa value of Ca’(sp ) H of a,b-unsaturated ketones is
around 18 – 24 whereas that of Ca(sp2)-H is estimated around 30
4
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in consideration of known uracil analogues, see: F. M. Won, C. C.
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[20] No aldol reaction was observed when allenyl and alkynyl
ketones were treated with aldehydes using a combination of
NEt3 (0–1 equiv) and iPrOH (1–5 equiv).
[21] Whereas the Ti-catalyzed aldol reaction is a redox neutral
process, and thus capable of regenerating TiIV complexes with
isopropoxide ligands as an active catalyst species, the subsequent
dehydration reaction potentially alters the ligand environment
of TiIV complexes. The real reason for how a catalytic amount of
Ti(OiPr)4 induces the adol condensation is not clear at the
present time. However, the fact that TiIV complexes, especially
the alkoxides, typically aggregate through metathetic ligand
exchange, it is our current interpretation that the aggregation of
TiIV complexes may happen in the presence of a small amount of
water during the dehydration reaction, where water molecules
are loosely coordinated to the TiIV aggregates. This process
potentially preserves most of isopropoxide ligands on the metal
center, which can be recycled to induce the Ti-catalyzed aldol
reaction. We thank one of the referees for pointing out this
catalytic aspect of TiIV complexes.
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Communications
Synthetic Methods
Synthesizing the synthons: The develop-
ment of a-acylvinyl anion synthons has
been achieved using direct a-vinyl enoli-
zation of a,b-unsaturated ketones under
mild reaction conditions. The synthetic
utility of such synthons has been dem-
onstrated in intermolecular aldol and
aldol condensation reactions, which pro-
vide synthetically useful allenyl ketone
and enyne derivatives.
H. Y. Kim, J.-Y. Li, K. Oh*
&&&& — &&&&
A Soft Vinyl Enolization Approach to a-
Acylvinyl Anions: Direct Aldol/Aldol
Condensation Reactions of (E)-b-
Chlorovinyl Ketones
6
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