Redox-triggered Ci-C coupling of diols and alkynes: Synthesis of β,γ-unsaturated α-hydroxyketones and furans by ruthenium-catalyzed hydrohydroxyalkylation

Article abstract of DOI:10.1002/anie.201311130

Direct ruthenium-catalyzed Ci-C coupling of alkynes and vicinal diols to form β,γ-unsaturated ketones occurs with complete levels of regioselectivity and good to complete control over the alkene geometry. Exposure of the reaction products to substoichiometric quantities of p-toluenesulfonic acid induces cyclodehydration to form tetrasubstituted furans. These alkyne-diol hydrohydroxyalkylations contribute to a growing body of merged redox-construction events that bypass the use of premetalated reagents and, hence, stoichiometric quantities of metallic by-products. More alcohol, more fun: Diols and alkynes engage in ruthenium(0)-catalyzed hydrohydroxyalkylation to form β,γ-unsaturated ketones with good to complete levels of regioselectivity and control of alkene geometry. Exposure of the reaction products to p-toluenesulfonic acid results in cyclodehydration to form tetrasubstituted furans.

Full text of DOI:10.1002/anie.201311130

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Angewandte
Communications
DOI: 10.1002/anie.201311130
À
C C Coupling
Hot Paper
À
Redox-Triggered C C Coupling of Diols and Alkynes: Synthesis of b,g-
Unsaturated a-Hydroxyketones and Furans by Ruthenium-Catalyzed
Hydrohydroxyalkylation**
Emma L. McInturff, Khoa D. Nguyen, and Michael J. Krische*
À
Abstract: Direct ruthenium-catalyzed C C coupling of
alkynes and vicinal diols to form b,g-unsaturated ketones
occurs with complete levels of regioselectivity and good to
complete control over the alkene geometry. Exposure of the
reaction products to substoichiometric quantities of p-toluene-
sulfonic acid induces cyclodehydration to form tetrasubstituted
furans. These alkyne-diol hydrohydroxyalkylations contribute
to a growing body of merged redox-construction events that
bypass the use of premetalated reagents and, hence, stoichio-
metric quantities of metallic by-products.
M
etal-catalyzed reductive coupling reactions of p-unsatu-
rated reactants bypass the use of premetalated C-nucleophiles
in a range of carbonyl and imine additions.[1] Despite
significant advances, catalytic intermolecular reductive cou-
pling reactions of simple alkenes or alkynes to ketones
(activated or unactivated) remain uncommon.[2–7] In connec-
tion with efforts aimed at the development of hydrogen-
mediated reductive coupling reactions beyond hydroformy-
lation,[1f,g,h] rhodium- or iridium-catalyzed hydrogenations of
a-ketoesters in the presence of conjugated or nonconjugated
alkynes were found to promote the formation of a-hydrox-
yesters (Scheme 1).[6b,c,7] More recently, under the conditions
of ruthenium(0)-catalyzed transfer hydrogenation, it was
Scheme 1. Hydrogen-mediated reductive coupling of alkynes to acti-
vated ketones and related hydrohydroxyalkylations. cod=1,5-cycloocta-
diene, BIPHEP=2,2’-bis(diphenylphosphino)-1,1’-biphenyl.
À
found that activated secondary alcohols engage in C C
coupling to unsaturated reactants to form products of hydro-
(Scheme 1). The reaction products engage in acid-catalyzed
cyclodehydration to form tetrasubstituted furans.[15]
In a preliminary experiment, racemic trans-1,2-cyclohex-
ane diol (1b) was exposed to 1-phenyl-1-propyne (2a) under
hydroxyalkylation.[8–11] Specifically, the catalytic C C cou-
À
pling of a-hydroxy carbonyl compounds[8] or 1,2-diols[8d,f] with
conjugated dienes,[8a,b,d,e] terminal olefins,[8c] and a,b-unsatu-
rated esters[8f] was achieved. The feasibility of coupling
secondary alcohols to alkynes[6,7,9,12] under the conditions of
ruthenium(0)-catalyzed transfer hydrogenation was unclear,
as ruthenium(0) complexes are efficient catalysts for alkyne
[2+2+2] cycloaddition.[13,14] Here, we report that the ruthe-
nium(0) catalyst formed in situ from [Ru3(CO)12] and tricy-
clohexylphosphine (PCy3) promotes regio- and stereoselec-
tive alkyne-diol hydrohydroxyalkylation to form a-hydroxy-
b,g-unsaturated ketones in good to excellent yields
À
conditions established for the C C coupling of dienes to a-
hydroxy esters or a-hydroxy amides.[8a,b] The anticipated
product of hydrohydroxyalkylation 3b was formed, but in
only 14% yield [Eq. (1)]. In related rhodium- or iridium-
catalyzed alkyne-a-ketoester reductive coupling reactions
using elemental hydrogen as the terminal reductant,[6b,c,7]
carboxylic acid co-catalysts were found to increase the rate
and conversion. As supported by computational studies,[16]
such acid co-catalysts bypass highly energetic four-centered
transition structures for the direct hydrogenolysis of oxame-
tallacyclic intermediates through s-bond metathesis, instead
protonating the metal–oxygen bond to form metal carbox-
ylates, which then react with elemental hydrogen by way of
lower energy six-centered transition structures. This acid co-
[*] E. L. McInturff, K. D. Nguyen, Prof. M. J. Krische
University of Texas at Austin
Department of Chemistry and Biochemistry
1 University Station–A5300, Austin, TX 78712-1167 (USA)
E-mail: mkrische@mail.utexas.edu
[**] Acknowledgement is made to the Robert A. Welch Foundation (F-
0038) and the NIH-NIGMS (RO1-GM069445) for partial financial
support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201311130.
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catalyst effect also is operative in ruthenium(0)-catalyzed
hydrohydroxyalkylations[8c,f] that proceed through oxametal-
lacyclic intermediates that are cleaved through transfer
hydrogenolysis. As borne out experimentally, the use of 1-
adamantanecarboxylic acid (12 mol%) under the aforesaid
conditions led to a 95% yield of the isolated adduct 3b as
a single olefin stereoisomer [Eq. (1)].
Under these conditions, diverse diols 1a–1l react with 1-
phenyl-1-propyne (2a) to form the corresponding a-hydroxy
ketones 3a–3l (Scheme 2). The diol stereochemistry is unim-
portant, as both cis and trans diastereomers engage in
À
efficient C C coupling. Cyclic diols 1a–1d and 1i–1l as well
as acyclic diols 1e–1h deliver coupling products in good to
excellent yields. In all but two cases, adducts 3c and 3d,
complete levels of olefin stereocontrol are observed. For
adducts 3c and 3d, longer reaction times result in a greater
loss of the E/Z stereoselectivity, which suggests these
products initially form with high levels of E/Z stereoselectiv-
ity but isomerize under the reaction conditions. As will be
discussed, the coupling of diol 1l proceeds with inversion of
regioselectivity, as corroborated by single-crystal X-ray
diffraction. By using this specific catalyst system, terminal
vicinal diols that incorporate primary alcohols provide com-
Scheme 3. Ruthenium(0)-catalyzed hydrohydroxyalkylation of alkynes
2a–2h with cyclohexane diol 1b. [a] Yields are of material isolated by
chromatography on silica gel. C10H15CO2H refers to 1-adamantanecar-
boxylic acid. See the Supporting Information for further details.
[b] 20 h. Boc=tert-butoxycarbonyl.
À
plex stereoisomeric mixtures of mono- and bis-C C coupling
products. In the absence of diol, alkyne 2a is converted into
products of [2+2+2] cycloaddition.[13,14]
To further evaluate the scope of this process, the coupling
of trans-1,2-cyclohexane diol (1b) to alkynes 2a–2h was
explored (Scheme 3). In all cases, good to excellent yields of
À
the corresponding C C coupling products 3b and 3n–3t were
obtained. With the exception of adduct 3p, complete levels of
olefin E/Z stereoselectivity were observed, and for all non-
symmetric alkynes (2a, 2b, 2d–2h), complete levels of
regioselectivity were observed. Terminal alkynes and di-
alkyl-substituted alkynes do not engage in efficient coupling
under these conditions. As illustrated by the reaction of
racemic ethyl mandelate (1m) to 1-phenyl-1-propyne (2a) to
form adduct 3m, other vicinally deoxygenated reactants
À
participate in C C bond formation under these conditions
and require an acid co-catalyst [Eq. (2)]. Finally, in a prelimi-
nary evaluation of the utility of the coupling products, adducts
3b, 3e, 3j, 3o, 3q, and 3t were exposed to catalytic amounts
of p-toluenesulfonic acid in m-xylene at 808C. Cyclodehydra-
tion to form the corresponding substituted furans 4a–4 f
occurred in good yields (Scheme 4).[15]
A plausible catalytic mechanism is illustrated for the
coupling of trans-1,2-cyclohexane diol (1b) and 1-phenyl-
propyne (2a) to form adduct 3b (Scheme 5). A mononuclear
Scheme 2. Ruthenium(0)-catalyzed hydrohydroxyalkylation of 1-phenyl-
1-propyne (2a) with diols 1a–1l. [a] Yields are of material isolated by
chromatography on silica gel. C10H15CO2H refers to 1-adamantanecar-
boxylic acid. For acyclic 1,2-diols, the terminology “cis” and “trans”
relates to the indicated reactant conformation. See the Supporting
Information for further details. [b] The cis-1,2-diol was employed.
[c] The trans-1,2-diol was employed. [d] 2h. [e] A mixture of cis- and
trans-1,2-diols was employed. [f] 1,1’-bis(diphenylphosphino)ferrecene
(DPPF, 6 mol%) was used instead of PCy3 (12 mol%). [g] 20 h.
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Scheme 6. Model to account for the observed regioselectivity of the
alkyne coupling.
steric demand of the metal center promotes oxidative
coupling at the less bulky carbonyl moiety of the transient
dione, and controls the alkyne orientation (A and C,
Scheme 6). The inversion of the alkyne regioselectivity
evident in the formation of 3l can be explained by the
increase in steric repulsion between the dione and alkyne
substituent (B and D, Scheme 6). The proposed structure of
these complexes is analogous to that of
Scheme 4. Acid-catalyzed cyclodehydration of adducts 3b, 3e, 3j, 3o,
3q, and 3t to form furans 4a–4 f. Yields are of material isolated by
chromatography on silica gel. See the Supporting Information for
further details. TsOH=p-toluenesulfonic acid.
related metallacycles obtained upon ruthe-
nium(0)-mediated ketone-diene oxidative
coupling.[8e]
In summary, we report the direct ruthe-
À
nium-catalyzed C C coupling of alkynes
and vicinal diols to form b,g-unsaturated
ketones with complete levels of regioselec-
tivity and good to complete control over
alkene geometry. Exposure of the reaction
products to substoichiometric quantities of
p-toluenesulfonic acid induces cyclodehy-
dration to form tetrasubstituted furans. The
present alkyne-diol hydrohydroxyalkyla-
tions contribute to a growing body of
merged redox-construction events that
Scheme 5. Postulated mechanism for ruthenium(0)-catalyzed diol-alkyne hydrohydroxyalkyla-
tion as illustrated for the coupling of 1,2-cyclohexane diol (1b) and 1-phenyl-1-propyne (2a).
bypass the use of premetalated reagents
and, hence, stoichiometric quantities of
metallic by-products.
ruthenium(0) complex[17] promotes the oxidative coupling of
dione tetradehydro-1b and 1-phenylpropyne (2a) to form
oxaruthenacycle I.[18,19] As corroborated by low-resolution
MS of crude reaction mixtures, which reveal the presence of
alkene, the requisite dione tetradehydro-1b may be formed
through successive [Ru3(CO)12]-catalyzed transfer of hydro-
gen from cyclohexanediol 1b to the alkyne.[20–22] The proto-
nation of oxaruthenacycle I by cyclohexanediol 1b, or
hydroxyketone didehydro-1b to form the ruthenium alkoxide
III, is postulated to be slow compared to protonolytic
cleavage of oxaruthenacycle I by 1-adamantanecarboxylic
acid to form ruthenium carboxylate II. Exchange of the
ruthenium carboxylate II with 1b or didehydro-1b forms
ruthenium alkoxide III, which upon b-hydride elimination
delivers didehydro-1b or tetradehydro-1b, respectively, and
Received: December 21, 2013
Published online: February 14, 2014
Keywords: alkynes · diols · regioselectivity · ruthenium ·
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transfer hydrogenation
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À
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À
[11] For selected reviews on the direct redox-triggered C C coupling
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