Catalytic Asymmetric Synthesis of Cyclohexanes by Hydrogen Borrowing Annulations

Article abstract of DOI:10.1002/anie.201907514

Hydrogen borrowing catalysis serves as a powerful alternative to enolate alkylation, enabling the direct coupling of ketones with unactivated alcohols. However, to date, methods that enable control over the absolute stereochemical outcome of such a process have remained elusive. Here we report a catalytic asymmetric method for the synthesis of enantioenriched cyclohexanes from 1,5-diols via hydrogen borrowing catalysis. This reaction is mediated by the addition of a chiral iridium(I) complex, which is able to impart high levels of enantioselectivity upon the process. A series of enantioenriched cyclohexanes have been prepared and the mode of enantioinduction has been probed by a combination of experimental and DFT studies.

Full text of DOI:10.1002/anie.201907514

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Accepted Article
Title: Catalytic Asymmetric Synthesis of Cyclohexanes by Hydrogen
Borrowing Annulations
Authors: Roly Armstrong, Wasim Akhtar, Tom Young, Fernanda
Duarte, and Timothy James Donohoe
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Catalytic Asymmetric Synthesis of Cyclohexanes by Hydrogen
Borrowing Annulations
Roly J. Armstrong,[a] Wasim M. Akhtar,[a] Tom A. Young,[a] Fernanda Duarte,[a]‡* and Timothy J.
Donohoe[a]*
Abstract: Hydrogen borrowing catalysis serves as a powerful
alternative to enolate alkylation, enabling the direct coupling of
ketones with unactivated alcohols. However, to date, methods that
enable control over the absolute stereochemical outcome of such a
process have remained elusive. Here we report
a catalytic
asymmetric method for the synthesis of enantioenriched
cyclohexanes from 1,5-diols via hydrogen borrowing catalysis. This
reaction is mediated by the addition of a chiral iridium(I) complex
which is able to impart high levels of enantioselectivity upon the
process. A series of enantioenriched cyclohexanes have been
prepared and the mode of enantioinduction has been probed by a
combination of experimental and DFT studies.
Enolate alkylation is a fundamental process in organic chemistry
and is widely used as a strategy for C–C bond formation.1 In this
chemistry a carbonyl substrate is typically deprotonated with a
strong base (e.g. LDA) and the resulting enolate is then trapped
with a reactive electrophile. Alkylation of a substituted enolate
results in the generation of a new α-stereogenic centre and an
abundance of methods (both stoichiometric and catalytic) have
been developed which enable this process to be carried out in
an asymmetric manner (Scheme 1A).2 Whilst this approach is
highly effective for alkylation with primary electrophiles,
alkylation with secondary electrophiles is significantly more
challenging and often results in sluggish reactivity accompanied
by competing elimination processes.1 Moreover, when
unsymmetrical secondary electrophiles are employed, a new
stereogenic centre is formed at the β-position and only a handful
of methods have been reported which allow control over the
stereochemical outcome of such a process.3
Scheme 1. Previous work and strategy for catalytic asymmetric hydrogen
borrowing. LDA = lithium diisopropylamide; THF = tetrahydrofuran.
catalytic cycle. The Ph* group plays a key role in facilitating this
chemistry; the bulky doubly ortho-substituted aromatic group is
oriented orthogonal to the carbonyl and shields against
competing reduction and homodimerization processes.5
Moreover, acyl Ph* derivatives can readily be converted to a
wide range of functional groups via an ipso-substitution process
(>30 examples).5,6 Remarkably, despite numerous recent
advances in the field of enolate hydrogen borrowing catalysis,
no general strategy has been reported allowing the absolute
stereochemical outcome of this process to be controlled.7,8 We
rationalized that the enantiodetermining step in these reactions
involves the return of iridium hydride to an achiral enone. Since
this step bears some resemblance to existing methods for
asymmetric hydrogenation we anticipated that a chiral transition-
metal complex might be able to control the facial selectivity of
this process (Scheme 1C).9 We recognized that the key to
success would lie in identifying a transition metal complex that
can perform three key roles: (i) efficient oxidation of alcohols; (ii)
a challenging reduction of sterically demanding Ph* substituted
enones; (iii) controlling facial selectivity within this reduction
process resulting in high levels of enantioselectivity.
Hydrogen borrowing catalysis represents
a powerful
alternative strategy to classical enolate alkylation, enabling
direct alkylation of enolates with unactivated alcohols.4 Within
this manifold, we recently reported that an achiral iridium(III)
catalyst can promote alkylation of pentamethylphenyl (Ph*)
ketones with alcohols leading to α- and β-branched ketones.5
This was subsequently extended to a (5+1) annulation process
in which racemic cyclohexanes could be accessed from 1,5-diols
(Scheme 1B).6 These reactions proceed by oxidation of the
alcohol by the iridium catalyst to generate the corresponding
carbonyl compound in situ. After aldol condensation with an
enolate and loss of water, the catalyst “returns” the abstracted
hydrogen to provide the C−C coupled product and complete the
[a]
Dr R. J. Armstrong, Dr W. M. Akhtar, T. A. Young, Prof. Dr F.
Duarte, Prof. Dr T. J. Donohoe
Chemistry Research Laboratory, University of Oxford, Oxford, OX1
3TA (UK)
We commenced our study by investigating the reaction
E-Mail: timothy.donohoe@chem.ox.ac.uk,
fernanda.duartegonzalez@chem.ox.ac.uk
Author to whom correspondence regarding the computational
studies should be addressed.
between pentamethylacetophenone
1
and commercially
available hexane-1,5-diol 2a. In line with our previous studies,6
in the presence of an achiral Ir(III) catalyst along with 4 equiv of
KOtBu in toluene at 110 °C we obtained racemic cyclohexane 3a
in 75% yield and 91:9 d.r. (Table 1, Entry 1). We have previously
‡
Supporting information for this article is given via a link at the end of
the document.
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COMMUNICATION
shown that the high trans-diastereoselectivity in this reaction is a
result of reversible deprotonation of the product.6 We were
delighted to find that by switching to an Ir(I) precatalyst along
with 5 mol% (R)-BINAP (4) we obtained cyclohexane 3a in 76%
yield with a modest but promising 68:32 e.r. (Table 1, Entry 2).
At this point we embarked upon an extensive program of
optimization (for full details, see SI). Changing the ligand to (R)-
H8-BINAP (5) resulted in lower enantioselectivity whereas (R)-
MeO-BIPHEP (6) afforded 3a with similar selectivity (Entries 3-4).
We next evaluated a series of MeO-BIPHEP based ligands
(6-10) bearing phosphine groups with different steric and
electronic properties. Difuryl substituted phosphine 7 resulted in
a significant decrease in enantioselectivity, but when a 3,4,5-
trimethoxy substituted ligand 8 was employed, 3a was isolated
in an improved 73:27 e.r. (Table 1, Entries 5-6). Increasing the
steric bulk of the phosphine clearly provided a beneficial effect –
ligands 9 and 10 afforded 3a in improved selectivities of 86:14
and 87:13 e.r. respectively (Table 1, Entries 7-8).
Table 2. Scope of catalytic asymmetric hydrogen borrowing reaction.a
Entry
Diol substrate
Cyclohexane product
Result
Table 1. Optimization of an enantioselective hydrogen borrowing
reaction.[a]
Entry
[Ir] (4 mol%)[b]
[IrCp*Cl2]2
[Ir(cod)Cl]2
[Ir(cod)Cl]2
[Ir(cod)Cl]2
[Ir(cod)Cl]2
[Ir(cod)Cl]2
[Ir(cod)Cl]2
[Ir(cod)Cl]2
[Ir(cod)Cl]2
[Ir(cod)Cl]2
Ir(cod)acac
Ir(cod)acac
Ligand
Yield[c]
75
d.r.[d]
91:9
95:5
93:7
96:4
91:9
91:9
91:9
91:9
92:8
93:7
92:8
91:9
e.r.[e]
–
1
–
4
2
76
68:32
64:36
69:31
55:45
73:27
86:14
87:13
88:12
89:11
90:10
92:8
3
4
5
78
6
79
5
7
74
6
8
76
7
9
75
8
10
11
11
11
11
77
9
81
10[f]
11[f]
12[f],[g]
80
85
88(87)
[a] Reaction conditions: 1 (1 equiv), diol (2 equiv), [Ir] (4 mol%), ligand (5
mol%), KOtBu (4 equiv), PhMe (3M), 110 oC, 24 h. [b] loading refers to
mol% Ir. [c] Determined by reverse phase HPLC analysis vs durene as an
internal standard; values in parentheses indicate the yield of isolated
product. [d] Determined by reverse phase HPLC analysis. [e] Determined
by normal phase HPLC analysis using a chiral stationary phase. [f] With
tBuOH as solvent. [g] With 2 mol% Ir(cod)acac and at [1] = 1M. cod = 1,5-
cyclooctadiene; acac
methoxyphenyl.
= acetylacetonate; DTBM = 3,5-di-tert-butyl-4-
[a] Reaction conditions: 1 (1 equiv), diol (2 equiv), Ir(cod)acac (4 mol%), (R)-
DTBM-SEGPHOS (5 mol%), KOtBu (4 equiv), tBuOH (3M), 110 oC, 24 h.
Major diastereoisomer depicted. Yields refer to isolated material after column
chromatography. [b] Conditions from Table 1, Entry 12.
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We found that changing the biaryl backbone of the ligand from
process; (iii) the similar enantioselectivities observed in the
resubjection experiment and annulation process implies that the
initial C–C bond formation between 1 and 2a occurs with
complete regioselectivity at the primary end of the diol (i.e.
reduction of isomeric enones such as 5 do not account for
formation of the minor enantiomer). We have previously shown
that Ph* containing products such as racemic 3a-3n can be
readily cleaved to the corresponding acid bromide in an ipso-
substitution reaction with Br2 and that the resulting acid
bromides can be employed in situ to afford esters, amides,
alcohols, carboxylic acids and aldehydes without erosion of
stereochemical purity.5,6 This procedure gave us a convenient
opportunity to determine the absolute stereochemistry of the
cyclohexane products. To this end, ketone 3a was treated with
Br2 to generate the corresponding acid bromide. Following
addition of LiAlH4, alcohol 6 was isolated in 90% yield with no
stereochemical erosion (Scheme 2C). Correlation of the specific
rotation value of 6 with that previously reported in the literature
allowed us to determine that the absolute configuration of 6 (and
by extension 3a) is (R,R).12 The remaining examples in Table 2
are assigned by analogy.
MeO-BIPHEP to SEGPHOS provided
increase in enantioselectivity to 88:12 e.r. (Table 1, Entry 9).
Conducting the reaction in tert-butanol led to further
a small additional
a
incremental improvement to 80% yield and 89:11 e.r. (Table 1,
Entry 10). Under these conditions we then screened a series of
Ir, Rh and Ru precatalysts (see SI for full details) and found that
the best result was obtained with Ir(cod)(acac), which afforded
3a in 85% yield and 90:10 e.r. (Table 1, Entry 11). Finally, we
found that with a reduced Ir loading (2 mol%) and increased
dilution (0.1 M) we were able to isolate 3a in 87% yield and 92:8
e.r. (Table 1, Entry 12).
With optimal conditions in hand, we set out to evaluate the
generality of the process. Substitution on the diol backbone was
well tolerated with a diol bearing a geminal dimethyl group at the
δ-position cyclizing to afford 3b in 67% yield, 90:10 d.r. and 94:6
e.r (Table 2, Entry 2). With substitution at the γ-position we
isolated cyclohexanes 3c-3e in high yields and with excellent
levels of diastereo- and enantioselectivity (Table 2, Entries 3-5).
A diol bearing a n-butyl group reacted to afford 3f in 87% yield,
89:11 d.r. and 91:9 e.r. (Table 2, Entry 6). Interestingly,
introduction of an isobutyl group resulted in poor conversion to
cyclohexane 3g which was isolated in 24% yield albeit still with
good enantioselectivity (Table 2, Entry 7).10 Aromatic and
heteroaromatic groups were well tolerated and cyclohexanes 3h
and 3i were isolated in good yields with high levels of
enantioselectivity (Table 2, Entries 8-9). Diols bearing ether and
thioether groups also cyclized smoothly to afford products 3j and
3k in excellent yields and high levels of stereoselectivity (Table 2,
Entries 10-11). Even an acetal was tolerated in the chemistry
providing 3l in 80% yield and 86:14 e.r. with no evidence of any
competing side-reactions (Table 2, Entry 12). We also
investigated an enantiopure diol derived from β-thujone which
we had previously found to undergo annulation with very poor
diastereoselectivity (51:7:42 d.r.).6 We hoped that our optimized
conditions might be able to augment this lack of substrate
control and were pleased to find that 3m was isolated as a 90:10
mixture of diastereoisomers.11 Finally, we investigated formation
of a cyclopentane from 1 and pentane-1,4-diol (Table 2, Entry
14). In this case, 3n was isolated in a reduced yield of 43%
albeit still with high levels of diastereo- and enantioselectivity.
A further benefit of the Ph* group is its highly crystalline
nature. All of the products 3a-3n described above are crystalline
solids and this provides an opportunity to enhance the
enantiomeric purity by stereoselective crystallization. As a
representative example, we carried out the reaction of
pentamethylacetophenone with hexane-1,5-diol (2a) on gram
scale, obtaining 3a in 92% yield with 93:7 d.r. and 92:8 e.r.
(Scheme 2A). After a single recrystallization (81% recovery) we
were able to significantly enhance this stereochemical purity to
>95:5 d.r. and 98:2 e.r.
Scheme 2. Large scale asymmetric annulation and experiments to determine
competency of enone intermediate and absolute stereochemistry. [a] 1 (1
equiv), diol (2 equiv), Ir(cod)acac (2 mol%), (R)-DTBM-SEGPHOS (5 mol%),
KOtBu (4 equiv), tBuOH (1M), 110 oC, 24 h. [b] 23 oC, MeOH, c = 1.00.
To gain insight into the mechanism of the stereochemical
determining step, density functional theory (DFT) modelling
studies were conducted, employing a computationally tractable
[Ir] complex ligated by (R)-BINAP (Table 1, Entry 2). Following
an extensive search for possible binding modes of an enone to a
model Ir(I) complex (for full details, see SI) the most stable was
found to have both the carbonyl and alkene bound to the Ir
centre. The most stable [IrH(R-BINAP)4] complex was then
located (see Figures 1 and S5 and Table S1).13 Si-coordination
of 4 (Si-INT0) is computed to be favoured by 4.8 kcal mol-1 over
its Re counterpart (Re-INT0). 1,4 hydride insertion then
proceeds from the Si-face with a free energy barrier 0.8 kcal
mol-1 lower than that for Re-insertion and accounts for the
experimentally observed e.r. (68:32 = 0.6 kcal mol-1 at 383 K,
Tables 1 and S1). This preference results from the steric clash
between Ph* and (P)Ph observed in the Re-TS (Figure 1).
To probe the mechanism of the asymmetric hydrogen
borrowing annulation, we independently synthesized the
proposed key intermediate, cyclic enone 4 and subjected it to
the optimized conditions with a n-butyl substituted diol (Scheme
2B). After this reaction we isolated 3a in 77% yield and 90:10 e.r.
The major enantiomer was the same as that obtained in the full
hydrogen borrowing sequence and the yield, diastereo- and
enantioselectivity were also very similar (c.f. Table 2, Entry 1).
Based upon this result, we arrived at the following conclusions:
(i) it is likely that cyclic enone 4 is an intermediate in the
asymmetric hydrogen borrowing reaction; (ii) the absence of any
crossover products implies that formation of 4 is an irreversible
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[1]
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Structures were optimized and thermodynamic/ solvent effects
calculated at the PBE0-D3BJ/def2-SVP,def2-TZVP(Ir) level of
theory with the solvent accounted for using the SMD model.
Single-point energetics were evaluated on these stationary
points at the PBE0-D3BJ/def2-TZVPP level of theory.14
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Figure 1. Computational studies to rationalise absolute stereochemical
outcome.
In conclusion, we have developed a highly enantioselective
synthesis of multisubstituted cyclohexanes via hydrogen
borrowing catalysis. This process is mediated by two
commercially available reagents: Ir(cod)(acac) and DTBM-
SEGPHOS and provides enantioenriched cyclohexanes with
control over both diastereo- and enantioselectivity. The origins of
stereoselectivity in this system have been probed by both
experimental studies and DFT calculations. This approach
constitutes the first general catalytic asymmetric strategy within
the rapidly developing field of enolate hydrogen borrowing
catalysis.
[8]
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[10] The corresponding cyclic enone was also isolated in 48% yield (see SI
for details).
[11] The minor diastereoisomer results from epimerization at the α-
stereocentre implying that reduction proceeds with complete
Acknowledgements
stereocontrol.
A mismatched reaction with (S)-DTBM-SEGPHOS
We thank the EPSRC [R.J.A. and T.J.D., Established Career
Fellowship (EP/L023121/1)] and University College [R.J.A] for
funding. We acknowledge the EPSRC Centre for Doctoral
training, Theory and Modelling in Chemical Sciences
(EP/L015722/1) for a studentship to TY generously supported by
AWE and for access to the Dirac cluster at Oxford.
afforded 3m in 83% yield and 58:5:37 d.r. (see SI for details).
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Keywords: catalysis • hydrogen borrowing • asymmetric •
iridium • enantioselective
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Entry for the Table of Contents
COMMUNICATION
A
catalytic asymmetric method is
R. J. Armstrong, W. M. Akhtar, T. A.
Young, F. Duarte,* and T. J. Donohoe*
described for the synthesis of
enantioenriched cyclohexanes from
racemic 1,5-diols via hydrogen
borrowing catalysis. This reaction is
Page No. – Page No.
Catalytic Asymmetric Synthesis of
Cyclohexanes by Hydrogen
Borrowing Annulations
mediated by
a
chiral iridium(I)
complex which is able to impart high
levels of enantioselectivity upon the
process.
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