Hydroxymethylation beyond Carbonylation: Enantioselective Iridium-Catalyzed Reductive Coupling of Formaldehyde with Allylic Acetates via Enantiotopic π-Facial Discrimination

Article abstract of DOI:10.1021/jacs.6b01078

Chiral iridium complexes modified by SEGPHOS catalyze the 2-propanol-mediated reductive coupling of branched allylic acetates 1a-1o with formaldehyde to form primary homoallylic alcohols 2a-2o with excellent control of regio- and enantioselectivity. These

Full text of DOI:10.1021/jacs.6b01078

Communication
pubs.acs.org/JACS
Hydroxymethylation beyond Carbonylation: Enantioselective
Iridium-Catalyzed Reductive Coupling of Formaldehyde with Allylic
Acetates via Enantiotopic π‑Facial Discrimination
Victoria J. Garza and Michael J. Krische*
Department of Chemistry, University of Texas at Austin, Austin, Texas 78712, United States
S
* Supporting Information
Scheme 1. Enantiotopic π-Facial Discrimination in
Electrophilic and Nucleophilic Allylation
ABSTRACT: Chiral iridium complexes modified by
SEGPHOS catalyze the 2-propanol-mediated reductive
coupling of branched allylic acetates 1a−1o with form-
aldehyde to form primary homoallylic alcohols 2a−2o with
excellent control of regio- and enantioselectivity. These
processes, which rely on enantiotopic π-facial discrim-
ination of σ-allyliridium intermediates, represent the first
examples of enantioselective formaldehyde C−C coupling
beyond aldol addition.
nantiotopic facial discrimination of electrophilic allylmetal
E
species is well established in asymmetric allylic alkylation
(W,1a Pd,1b Mo,1c Ir1d) (Scheme 1, eq 1). However, despite
nearly four decades of work on enantioselective carbonyl
allylation,2−8 this mode of enantioselection is unknown in the
context of nucleophilic allylmetal species, where work has
focused exclusively on asymmetric additions to aldehydes or
ketones to form chiral α-stereogenic secondary or tertiary
alcohols, respectively (Scheme 1, eq 2). Nucleophilic allylations
of formaldehyde to form chiral β-stereogenic primary homo-
allylic alcohols have not been described. Indeed, the only
catalytic enantioselective C−C couplings of formaldehyde
reported, to date, involve asymmetric aldol addition.9,10
Merging the chemistry of transfer hydrogenation and carbonyl
addition,7 formaldehyde (or methanol)11b mediated hydro-
hydroxymethylations of diverse π-unsaturated reactants (allene-
s,11a−c dienes11d,e and alkynes11f) were developed.11−13 While
these processes are efficient and regioselective in contexts where
carbonylation/hydroformylation is not, enantioselective variants
have not been established nor has the hydroxymethylation of
allylic acetates been explored. Here, we apply a chiral iridium
catalyst to the enantioselective reductive coupling of allylic
acetates with formaldehyde to form chiral β-stereogenic primary
homoallylic alcohols via enantiotopic π-facial discrimination of σ-
allyliridium intermediates (Scheme 1, eq 3). These processes
constitute the first examples of enantioselective formaldehyde
C−C coupling beyond aldol addition.
The use of paraformaldehyde as an electrophilic partner in
asymmetric nucleophilic allylation poses several challenges. First,
high levels of enantioselectivity require intervention of a single
geometrical isomer of the σ-allylmetal species in the carbonyl
addition event. As our collective data are consistent with a
catalytic mechanism wherein carbonyl addition occurs by way of
a closed chair-like transition structure,7,8 competition between
chair-like vs boat-like transition structures, which inverts the
enantiotopic nucleophile π-face undergoing addition, presents an
additional challenge. This issue figures prominently in
mechanistically related aldol additions.14 Finally, in the presence
of group 9 metals, paraformaldehyde will transform to synthesis
gas,15 which can promote a variety of side reactions and act as a
catalyst poison.
Cognizant of these potential obstacles, initial optimization
experiments were undertaken. Gratifyingly, it was found that the
indicated π-allyliridium C,O-benzoate complex modified by (S)-
SEGPHOS promotes reductive coupling of the 4-bromophenyl-
substituted allylic acetate 1a (150 mol %) with paraformaldehyde
(100 mol %) to provide the desired product, the primary
homoallylic alcohol 2a, in 54% yield and 94% ee as a single
regioisomer (Table 1). It was postulated that synthesis gas
Received: January 29, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.6b01078
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Table 1. Selected Optimization Experiments in the
Enantioselective Iridium-Catalyzed Reductive Coupling of
Allylic Acetate 1a with Formaldehyde Illustrating the
Table 2. Enantioselective Iridium-Catalyzed Reductive
Coupling of Branched Allylic Acetates 1a−1o with
Formaldehyde to Form Primary Homoallylic Alcohols 2a−
a
a
Importance of NMO
2o
a
Yields are of material isolated by silica gel chromatography.
Enantioselectivities were determined by chiral stationary phase
HPLC analysis. See Supporting Information (SI) for further
b
experimental details. Microwave heating, 6 h.
generated upon decomposition of paraformaldehyde15 might
contribute to the formation of catalytically inactive iridium
carbonyl complexes. This hypothesis was corroborated by the
observance of absorptions at 2125 cm−1 in IR spectra taken from
aliquots of crude reaction mixtures.16 Hence, N-methyl morpho-
line oxide (NMO), which is commonly used for the oxidative
removal of carbonyl ligands,17 was employed as an additive. As
hoped, the addition of NMO proved to be beneficial, allowing
homoallylic alcohol 2a to be isolated in 85% yield and 97% ee.
However, this yield could not be reproduced on a larger scale due
to solubility issues involving both paraformaldehyde and NMO.
Fortunately, it was found that microwave heating largely restored
the isolated yield of 2a with the advantage of significantly
shortened reaction times.
To assess scope, optimized conditions were applied to the
reductive coupling of paraformaldehyde with diverse branched
allylic acetates 1a−1o (Table 2). As illustrated in the formation
of primary homoallylic alcohols 2a−2e, a variety of substituted
aryl groups are tolerated, including ortho-substituted aryl
moieties (2c). The formation of compounds 2f−2k establishes
the tolerance of these conditions toward N-, S-, and O-bearing
heterocycles. In each case, aryl- and heteroaryl-substituted
products 2a−2k are formed in good yield with uniformly high
levels of enantioselectivity, consistent with good partitioning of
(E)- and (Z)-σ-allyliridium isomers in the carbonyl addition
event (eq 4). Even in the case of branched allylic acetates
a
Yields are of material isolated by silica gel chromatography.
Enantioselectivities were determined by chiral stationary phase
HPLC analysis. 2k, 8 h. 2l, 10 h. See SI for further experimental
details. 80 °C. 70 °C. The catalyst modified by (S)-DM-SEGPHOS
was used. Isolated as the 4-nitrobenzoate due to volatility.
b
c
d
e
determined by single crystal X-ray diffraction and is the basis for
the stereochemical assignments of compounds 2a−2o. Con-
version of alcohol 2a to the corresponding acrylic ester, followed
by ring-closing metathesis, delivers the α,β-unsaturated δ-lactone
4a (Scheme 2, eq 6). Amination of the alcohol moiety of 2a by
way of the p-toluene sulfonate was challenging due to competing
elimination. However, treating the p-toluene sulfonate derived
from 2a with sodium azide in DMF delivered the primary azide in
good yield. Reduction of the azide provided the amine, which was
isolated as the N-Boc carbamate 5a (Scheme 2, eq 7).
A catalytic mechanism and stereochemical model have been
proposed (Chart 1). Entry into the catalytic cycle occurs via
protonolytic cleavage of the π-allyliridium C,O-benzoate
complex mediated by 2-propanol to furnish the indicated iridium
2-propoxide complex. β-Hydride elimination with loss of acetone
substituted by styryl groups (1l) or alkyl chains (1m−1o), high
levels of enantioselectivity are retained. In all cases, complete
levels of branch regioselectivity are observed. Attempted redox-
neutral hydroxymethylations wherein methanol serves dually as
reductant and formaldehyde precursor were inefficient due to
transesterification and transfer hydrogenation of the terminal
olefin of the allylic acetate.
The products of reductive coupling serve as useful building
blocks in chemical synthesis (Scheme 2). For example, rhodium
catalyzed hydroformylation18 of adduct 2a provides a lactol,
which upon treatment with pyridinium chlorochromate is
converted to the enantiomerically enriched δ-lactone 3a
(Scheme 2, eq 5). The absolute stereochemistry of 3a was
B
DOI: 10.1021/jacs.6b01078
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Chart 1. General Catalytic Mechanism and Stereochemical Model
generates an iridium hydride, which upon deprotonation forms
an anionic iridium(I) species. Ionization of the allylic acetate
delivers the monosubstituted π-allyliridium complex. Complete
levels of branch regioselectivity accompanied by high levels of
enantioselectivity suggest formaldehyde addition occurs pre-
dominantly by way of the primary (E)-σ-allyliridium haptomer
through a closed 6-centered transition structure.7,8 Protonolytic
cleavage of the resulting homoallylic alkoxide mediated by 2-
propanol regenerates the iridium 2-propoxide complex to close
the catalytic cycle. The absolute stereochemical course of the
carbonyl addition event is consistent with our previously
proposed stereochemical model.8b Allylic acetate recovered
from the reaction is racemic, indicating kinetic resolution does
not occur in the ionization event.
In summary, we report the first enantioselective formaldehyde
C−C couplings beyond aldol addition. Specifically, chiral iridium
complexes modified by SEGPHOS catalyze the 2-propanol-
mediated reductive coupling of branched allylic acetates 1a−1o
with formaldehyde to form primary homoallylic alcohols 2a−2o
with complete control of regioselectivity and uniformly high
levels of enantioselectivity. This process provides access to
products of hydroxyalkylation that are inaccessible using classical
carbonylative methods11,12 and supports the feasibility of related
transformations, including the noncarbonylative aminomethyla-
tion19 of branched allylic acetates.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.6b01078.
■
S
Single crystal X-ray diffraction data for compound 3a
(CIF)
Experimental details and data (PDF)
AUTHOR INFORMATION
Corresponding Author
*mkrische@mail.utexas.edu
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The Robert A. Welch Foundation (F-0038) and the NIH-
NIGMS (RO1-GM069445) are acknowledged for partial
support of this research. Chinh Ngo, Zhicheng Zhang, and
Nicole Behnke are acknowledged for technical assistance.
Scheme 2. Elaboration of Adduct 2a and Assignment of
a
Absolute Stereochemistry
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