Enantioselective iridium-catalyzed vinylogous reformatsky-aldol reaction from the alcohol oxidation level: Linear regioselectivity by way of carbon-bound enolates

Article abstract of DOI:10.1002/anie.201100646

Reformatsky reinvented: Highly enantioselective vinylogous Reformatsky-type addition has been achieved from the alcohol oxidation level with linear regioselectivity through carbon-bound enolates (see scheme; Boc=tert- butoxycarbonyl). Complete levels of c

Full text of DOI:10.1002/anie.201100646

DOI: 10.1002/anie.201100646
Synthetic Methods
Enantioselective Iridium-Catalyzed Vinylogous Reformatsky-Aldol
Reaction from the Alcohol Oxidation Level: Linear Regioselectivity by
Way of Carbon-Bound Enolates**
Abbas Hassan, Jason R. Zbieg, and Michael J. Krische*
Vinylogues[1] of the aldol reaction represent an important
class of carbonyl addition processes.[2] The vast majority of
enantioselective vinylogous aldol additions employ dienolates
and dienol ethers derived from b-ketoesters[3] in combination
with chiral Lewis acids,[3a–k] chiral Lewis bases,[3k–m] or chiral
hydrogen-bond donors.[3n] Dienolates and dienol ethers
derived from simple a,b-unsaturated carbonyl com-
pounds[3k–m,4] and 2-siloxy furans[5] also participate in enan-
tioselective vinylogous aldol additions. While excellent regio-
and enantioselectivities have been obtained in certain cases,
the formation and tractability of the requisite dienol ethers
pose a barrier to their use. Additionally, chiral Lewis acid
catalyzed reactions of silyl dienol ethers often suffer from
competing racemic silyl cation catalyzed background reac-
tions, thus requiring slow addition of the dienol ether,
cryogenic conditions, and high catalyst loadings. Vinylogous
direct aldol additions of unmodified unsaturated carbonyl
compounds potentially address these limitations,[6] however,
to date, such transformations are restricted to the use of 2-
(5H)-furanones or nitriles as aldol donors.[6,7]
Scheme 1. Catalytic enantioselective vinylogous aldol reactions.
PG=protecting group.
yl allylation,[10a,b,e–h] crotylation,[10c,f] tert-prenylation,[10d,f] and a
host of related carbonyl allylation processes. For such “C C
À
bond-forming transfer hydrogenations”[10,11] secondary alco-
hol dehydrogenation triggers reductive generation of allyliri-
dium nucleophiles, which, in the presence of exogenous
aldehyde, deliver products of carbonyl allylation. Of greater
significance, primary alcohol dehydrogenation can simulta-
neously generate aldehyde–allyliridium pairs, thus enabling
carbonyl addition from the alcohol oxidation level. For all
reactions that occur by way of substituted p-allyl iridium
intermediates, complete branched regioselectivities are
accompanied by good to complete levels of anti-diastereose-
lectivity, thus suggesting carbonyl addition occurs with allylic
inversion from the primary (E)-s-allyl haptomer. To date, the
formation of linear carbonyl addition products from substi-
tuted allyliridium nucleophiles has proven elusive
(Scheme 2).
In a significant departure from prior art, Kanai, Shibasaki,
and co-workers devised a reductive vinylogous aldol reaction
of allenic esters mediated by pinacolborane.[8] A related
reductive process, the vinylogous Reformatsky reaction,
could potentially deliver identical products, however, enan-
tioselective variants are unknown.[9] Herein, under the con-
ditions of C C bond-forming transfer hydrogenation,[10,11] we
À
report the first examples of enantioselective vinylogous
Reformatsky-type additions, and establish catalytic condi-
tions wherein asymmetric carbonyl addition occurs with equal
facility from the alcohol or aldehyde oxidation level
(Scheme 1).
In recent work, our research group has found that chiral
ortho-cyclometalated iridium C,O-benzoates catalyze carbon-
[*] A. Hassan, J. R. Zbieg, Prof. M. J. Krische
University of Texas at Austin
Department of Chemistry and Biochemistry
1 University Station—A5300, Austin, TX 78712-1167 (USA)
Fax: (+1)512-471-8696
E-mail: mkrische@mail.utexas.edu
[**] Acknowledgement is made to the Robert A. Welch Foundation (F-
0038), the NIH-NIGMS (RO1-GM069445), the University of Texas at
Austin, and the Center for Green Chemistry and Catalysis for
generous financial support. The Higher Education Commission of
Pakistan is acknowledged for graduate student fellowship support
(A.H.).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201100646.
Scheme 2. Carbonyl addition by way of primary s-ally haptomers typically
generates branched products as anti-diastereomers.
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Table 1: Enantioselective vinylogous Reformatsky reaction from the
alcohol oxidation level.[a]
Table 2: Enantioselective vinylogous Reformatsky reaction from the
aldehyde oxidation level.[a]
Entry
1
Product
Yield [%]
87
ee [%]
g/a
Entry
1
Product
Yield [%]
65
ee [%]
g/a
83
1:2[b]
6:1
88
1:2
5:1
5:1
2
3
4
5
6
7
74
77
59
88
75
70
94
95
96
96
99
97
2
3
4
5
6
7
80
88
85
70
83
60
98
92
96
99
99
93
6:1
ꢀ20:1
ꢀ20:1
ꢀ20:1
ꢀ20:1
ꢀ20:1
ꢀ20:1
ꢀ20:1
ꢀ20:1
8
9
61
80
90
86
ꢀ20:1[b]
ꢀ20:1
8
9
58
80
93
94
ꢀ20:1
ꢀ20:1
10
80
95
5:1
10
94
96
5:1
[a] Yields are of isolated product. Enantiomeric excess was determined
by HPLC on a chiral stationary phase. See the Supporting Information for
further details. [b] 24 h. Boc=tert-butoxycarbonyl, PMB=para-methoxy-
benzyl, Ts=4-toluenesulfonyl.
[a] As described in Table 1.
nol 2c under identical conditions revealed an increased
proportion of the linear regioisomer (approximately 5:1 r.r. in
each case), again accompanied by high levels of enantiose-
lectivity (94% ee and 95% ee, respectively; Table 1, entries 2
and 3). Primary neopentyl alcohols 2d–2h delivered products
It was reasoned that linear carbonyl addition products
might be availed upon stabilization of the secondary s-allyl
haptomer through introduction of a carbonyl moiety, in which
case the secondary s-allyl haptomer is equivalent to an h1-C-
bound iridium enolate.[12] The veracity of this analysis is borne
out by the following experiments. Upon exposure of the g-
acyloxy crotonate 1 to primary alcohol 2a in the presence of
the chiral ortho-cyclometalated iridium C,O-benzoate com-
plex modified by (R)-Cl,MeO-biphep, (2,2’-bis(diphenylphos-
À
of C C coupling with complete linear regioselectivity and
exceptional enantioselectivity (Table 1, entries 4–8). In all
cases, roughly equivalent yields and selectivities are observed
À
when the C C coupling was performed using aldehydes 3a–
3h (Table 2, entries 1–8). Finally, benzylic alcohols 2i and 2j
À
and the corresponding aldehydes 3i and 3j engage in C C
coupling to furnish adducts 4i and 4j (Table 1, entries 9 and
10 and Table 2, entries 9 and 10, respectively). A detailed
description of the assignment of absolute stereochemistry is
provided in the Supporting Information.
Linear regioselectivity in response to increased steric
demand of the aldehyde suggests a Curtin–Hammett scenario
wherein carbonyl addition occurs selectively from an equili-
brating mixture of primary and secondary s-allyl hapto-
phino)-5,5’-dichloro-6,6’-dimethoxy-1,1’-biphenyl),
desig-
nated (R)-I, the products of carbonyl addition were obtained
in a combined 87% yield as a 1:2 mixture of linear (88% ee)
and branched regioisomers, thus favoring formation of the
branched adduct, which appeared as a roughly equimolar
À
mixture of diastereomers (Table 1, entry 1). Catalytic C C
coupling of cyclopentanemethanol 2b or cyclohexanemetha-
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Angew. Chem. Int. Ed. 2011, 50, 3493 –3496

mers.[13,14] Beyond electronic effects, it is likely that carbonyl
addition from the a-C-bound iridium enolate to form the less-
asymmetric carbonyl addition occurs with equal facility
from the alcohol or aldehyde oxidation level. Good to
excellent levels of regioselectivity and uniformly high levels
of enantioselectivity are observed across a range of alcohols
2a–2i and aldehydes 3a–3i. Furthermore, as demonstrated in
the case of a-chiral alcohol 2k, exceptional levels of catalyst-
directed diastereoselectivity may be achieved. Insight into the
structure-interaction features of the catalytic system vis-ꢀ-vis
partitioning of linear and branched adducts suggests a Curtin–
Hammett scenario, wherein carbonyl addition occurs selec-
tively from an equilibrating mixture of primary and secondary
s-allyl haptomers.[13,14] The collective data are consistent with
carbonyl addition from the secondary a-C-bound iridium
À
substituted C C bond is favored owing to the absence of
gauche interactions in the transition state. For sterically
demanding aldehydes, such gauche interactions would be
accentuated, thus increasing the preference for the linear
regioisomer (Scheme 3).
À
enolate to form the less-substituted C C bond owing to the
absence of gauche interactions in the transition state. Notably,
in reactions conducted from the alcohol oxidation level, the
only stoichiometric by-products formed are carbon dioxide
and tert-butanol. Future studies will focus on the development
À
of related C C bond-forming processes that combine oxida-
tion/construction event and thus bypass discrete alcohol
oxidation and stoichiometric organometallic reagents while
gaining access to more tractable synthetic intermediates in the
form of alcohols.
Scheme 3. Partitioning of branched and linear regioisomers.
Received: January 25, 2011
Published online: March 4, 2011
To further explore the scope of the vinylogous Reformat-
sky–aldol addition, catalyst-directed diastereoselectivity was
explored in the coupling of allylic carbonate 1 to a-chiral
alcohol 2k (Scheme 4).[15] Under standard conditions employ-
Keywords: aldol reaction · allylation · homogeneous catalysis ·
.
iridium · transfer hydrogenation
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matsky–aldol addition from the alcohol oxidation level.
ing (R)-I as the precatalyst, the linear adduct 4k was isolated
as a single regio- and stereoisomer. Using the enantiomeric
precatalyst (S)-I, the linear adduct epi-4k was isolated as a
single regio- and stereoisomer. Thus, complete levels of
catalyst-directed diastereoselectivity are observed.[16] In con-
trast, using the analogous achiral iridium complex modified
by biphep, (2,2’-bis(diphenylphosphino)biphenyl), diastereo-
mers 4k and epi-4k were produced in 58% yield in a 1:1.5
ratio, respectively, along with a 9% yield of the branched
regioisomer. It should be noted that although bibhep is
considered an achiral ligand, metal complexes of biphep
adopt chiral racemic conformations that would attenuate
substrate bias.
In summary,
a catalytic enantioselective vinylogous
Reformatsky-type addition has been described in which
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Communications
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À
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