Hydroaminomethylation beyond Carbonylation: Allene-Imine Reductive Coupling by Ruthenium-Catalyzed Transfer Hydrogenation

Article abstract of DOI:10.1002/anie.201503250

Ruthenium(II)-catalyzed hydrogen transfer from 2-propanol mediates reductive coupling of 1,1-disubstituted allenes with formaldimines with complete branch-regioselectivity, thus representing a new method for hydroaminomethylation beyond classical hydrofor

Full text of DOI:10.1002/anie.201503250

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International Edition: DOI: 10.1002/anie.201503250
German Edition: DOI: 10.1002/ange.201503250
Synthetic Methods
Hot Paper
Hydroaminomethylation Beyond Carbonylation: Allene–Imine
Reductive Coupling by Ruthenium-Catalyzed Transfer
Hydrogenation**
Susumu Oda, Brannon Sam, and Michael J. Krische*
Abstract: Ruthenium(II)-catalyzed hydrogen transfer from 2-
propanol mediates reductive coupling of 1,1-disubstituted
allenes with formaldimines with complete branch-regioselec-
tivity, thus representing a new method for hydroaminomethy-
lation beyond classical hydroformylation/reductive amination.
work of Eilbracht,[4] hydroaminomethylation has been inten-
sively investigated[1] and utilized for the preparation of
diverse pharmaceutical ingredients,[5] including cinacalcet
(Sensipar, Mimpara),[5,6a] ibutilide (Corvert),[5,6b] and fexofe-
nadine (Allegra, Fexidine, Telfast, Fastofen, Tilfur, Vifas,
Telfexo, Allerfexo).[5,6c] Recent advances in hydroaminome-
thylation include the use of ammonia as a reactant,[4c,7]
regioselective reactions of terminal[8a] and internal[8b alkenes
through ligand control[8] or the use of directing groups,[9] the
evolution from rhodium-based to ruthenium-based cata-
lysts,[10] and the emergence of noncarbonylative strategies,
including hydroaminoalkylation[11] [Scheme 1, Eq. (2)] and
photoredox catalysis [Scheme 1, Eq. (3)].[12] The vast majority
of hydroaminomethylation protocols apply to a-olefins. To
our knowledge, the hydroaminomethylation of other p-
unsaturated reactants, such as dienes or allenes, is
unknown.[13] Herein we report a new strategy for hydro-
aminomethylation based on 2-propanol-mediated reductive
coupling of 1,1-disubstituted allenes and formaldimines
derived in situ from 1,3,5-tris(aryl)-hexahydro-1,3,5-triazines
[Scheme 1, Eq. (4)]. These processes deliver branched prod-
ucts of hydroaminomethylation bearing all-carbon quater-
nary centers.[13–20]
H
ydroaminomethylation,[1] the successive one-pot hydro-
formylation/reductive amination, is an important, atom-
efficient method for amine synthesis [Scheme 1, Eq. (1)].
Initial experiments were inspired by our earlier studies on
the ruthenium-catalyzed reductive coupling of formaldehyde
with allenes,[13,14] dienes,[13,15] and alkynes.[13,16] Using the
allene 1a and hexahydro-1,3,5-triazine 2a, a crystalline solid
prepared from paraformaldehyde and p-anisidine,[21] a series
of catalysts derived from the commercial ruthenium complex
[HClRu(CO)(PPh3)3] were evaluated for their ability to
induce reductive coupling by 2-propanol-mediated transfer
hydrogenation (Table 1). In the absence of an exogenous
ligand or upon use of tricyclohexylphosphine as a ligand,
small quantities of the desired homoallylic neopentyl amine
3a were formed as a single regioisomer. Chelating phosphine
ligands were more effective at enforcing higher conversion.
Eventually, it was found that upon use of dCype as a ligand,
the desired homoallylic amine 3a could be obtained in 83%
yield.
Scheme 1. Classical and contemporary strategies for the hydroamino-
methylation of p-unsaturated reactants.
Subsequent to Reppeꢀs discovery of hydroaminomethylation
at BASF in 1949,[2] relatively few studies were disclosed.[3]
However, in the last 15 years, due in large part to the elegant
Under optimized reaction conditions using the ruthenium
catalyst derived in situ from [HClRu(CO)(PPh3)3] and dCype,
the scope of the allene partner was explored in reductive
couplings to 2a (Table 2). The 1,1-dialkylsubstituted allenes
1a–f were each converted to the respective homoallylic
neopentyl amines 3a–f in good to excellent yields with
complete regiocontrol. As illustrated in the formation of
adducts 3c and 3e, this method allows the creation of
congested all-carbon quaternary centers which are vicinal to
tertiary stereocenters. The 1-methyl-1-aryl-disubstituted
[*] S. Oda, B. Sam, Prof. M. J. Krische
University of Texas at Austin, Department of Chemistry
105 E 24th St. (A5300), Austin, TX 78712-1167 (USA)
E-mail: mkrische@mail.utexas.edu
[**] The Robert A. Welch Foundation (F-0038), the NSF (CHE-1265504),
and the University of Texas Center for Green Chemistry and
Catalysis are acknowledged for partial support of this research.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201503250.
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Table 1: Selected ligand-based optimization experiments in the reductive
coupling of the allene 1a with triazine 2a.[a]
Scheme 2. Selected elaborations of hydroaminomethylation product 3a
to form the compounds 9a, 10a, and 11a. Yields are of material
isolated by silica gel chromatography. For 9a: 1) Boc2O, Et3N, DMAP,
CH2Cl2, 258C, 47%. 2) O3, CH2Cl2, À788C, 86%. For 10a: 1) CAN,
MeCN/H2O (1:1), 258C, 79%. 2) Boc2O, Et3N, CH2Cl2, 258C, 92%. For
=
11a: 1) BrCH2CH CH2, K2CO3, DMF, 258C, 88%. 2) Grubbs-II,
CH2Cl2, 258C, 90%. See the Supporting Information for further
experimental details. Boc=tert-butoxycarbonyl, DMAP=4-(N,N-dime-
thylamino)pyridine, DMF=N,N-dimethylformamide.
[a] Yields are of material isolated by silica gel chromatography. See the
Supporting Information for further experimental details. PMP=para-
methoxyphenyl.
compounds 9a–11a, which embody diverse functional group
arrays (Scheme 2).
Table 2: Ruthenium-catalyzed hydroaminomethylation of the allenes
1a–l with the triazine 2a to form homoallylic neopentyl amines 3a–l.[a]
To further evaluate reaction scope, the N-substituted
hexahydro-1,3,5-triazines, 2a–f were exposed to 1a under the
standard conditions (Table 3). The reaction is most efficient
Table 3: Ruthenium-catalyzed hydroaminomethylation of 1a with the
triazines 2a–f to form homoallylic neopentyl amines 3a–8a.[a]
[a] Yields are of material isolated by silica gel chromatography. See the
Supporting Information for further experimental details.
for electron-rich, para-substituted N-phenyl reactants, as
illustrated in the formation of the adducts 3a and 4a.
Nonetheless, ortho-anisidine substituted hexahydro-1,3,5-tri-
azine 2c delivers the adduct 5a in moderate yield. Addition-
ally, the parent N-phenyl triazine 2d and N-para-fluorophenyl
triazine 2e provide the adducts 6a and 7a, respectively, in
good yield. Finally, as demonstrated in the formation of the N-
pyridyl-substituted adduct 8a, heteroaromatic N-substituents
are tolerated. Under these initially developed reaction
conditions, N-alkyl, N-acyl, and N-sulfonyl triazines and
alkyl-substituted N-PMP imines do not participate in catalytic
[a] Yields are of material isolated by silica gel chromatography. See the
Supporting Information for further experimental details. [b] 1e
(400 mol%). TBS=tert-butyldimethylsilyl.
allenes 1g–l react to form the corresponding neopentyl
amines 3g–l, which bear aryl-substituted all-carbon quater-
nary centers, in good yield as single regioisomers. Monosub-
stituted allenes did not engage in efficient coupling under
these reaction conditions. To illustrate the utility of adducts
3a–l, the homoallylic amine 3a was selectively transformed to
À
C C coupling.
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likely stems from b-hydride elimi-
nation of the allylruthenium species
to form dienes, which are detected
in crude reaction mixtures and may
account for the requirement of
a superstoichiometric amount of
allene. Adventitious water also
may diminish the extent of deute-
rium incorporation.[24] The coupling
of 1a to the deuterated triazine
deuterio-2a delivers deuterio-3a,
which completely retains deuterium
adjacent to nitrogen (> 95% 2H),
À
thus indicating that the C N bond
Scheme 3. General catalytic mechanism for allene hydroaminomethylation by ruthenium-catalyzed
of the amine product is not subject
to reversible dehydrogenation
under these reaction conditions.
À
C C bond-forming transfer hydrogenation.
A
general catalytic mechanism has been proposed
In summary, we report a new strategy for the hydro-
aminomethylation of p-unsaturated reactants beyond classi-
cal hydroformylation/reductive amination: the reductive
coupling of allenes and N-aryl formaldimines by ruthenium-
catalyzed transfer hydrogenation.[25] These transformations
exploit highly tractable saturated s-triazines, derived from
paraformaldehyde and aryl amines as latent imines, and
deliver adducts bearing all-carbon quaternary centers with
complete control of regioselectivity. Related hydroaminome-
thylations of alkynes, 1,3-dienes, and 1,3-enynes are currently
under exploration in our laboratory.
(Scheme 3). The ruthenium hydride complex derived from
[HClRu(CO)(PPh3)3] and dCype hydrometallates the allene
to form a nucleophilic allylruthenium complex. The stoichio-
metric reaction of [HXRu(CO)(PPh3)3] (X = Cl, Br) with
allenes (or dienes) to furnish p-allylruthenium complexes has
been reported.[22] Such p-allylruthenium species are highly
fluxional in nature, thus undergoing rapid geometrical
isomerization by way of the s-bound haptomers.[23] Coordi-
nation of the pentacoordinate s-allylruthenium species to the
formaldimine initiates addition through a closed transition
structure to form the homoallylic ruthenium amide. Proto-
nolytic cleavage of the amidoruthenium intermediate by 2-
propanol releases the product and provides the indicated
ruthenium 2-propoxide, which upon b-hydride elimination
forms acetone and regenerates the starting ruthenium hy-
dride.
Keywords: allenes · green chemistry · ruthenium ·
synthetic methods · transfer hydrogenation
How to cite: Angew. Chem. Int. Ed. 2015, 54, 8525–8528
Angew. Chem. 2015, 127, 8645–8648
To gain further insight into the catalytic mechanism,
deuterium-labelling studies were performed (Scheme 4).
Reductive coupling of 1a to 2a using [D8]-2-propanol delivers
deuterio-3a, which incorporates significant quantities of
deuterium at the interior vinylic position (Ha) and, to
a lesser extent, the terminal vinylic positions (Hb and Hc).
This pattern of deuterium incorporation suggests allene
hydrometallation is fast and reversible and occurs with
incomplete regioselectivity. Incomplete deuterium incorpo-
ration at the interior vinylic position of deuterio-3a (71% 2H)
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Received: April 9, 2015
Revised: May 13, 2015
Published online: June 1, 2015
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