One-Pot Cannizzaro Cascade Synthesis of ortho-Fused Cycloocta-2,5-dien-1-ones from 2-Bromo(hetero)aryl Aldehydes

Article abstract of DOI:10.1002/anie.201505347

An intramolecular Cannizzaro-type hydride transfer to an in situ prepared allene enables the synthesis of ortho-fused 4-substituted cycloocta-2,5-dien-1-ones with unprecedented technical ease for an eight-ring carboannulation. Various derivatives could be obtained from commercially available (hetero)aryl aldehydes, trimethylsilylacetylene, and simple propargyl chlorides in good yields. A cascade process that is triggered by an intramolecular Cannizzaro-type hydride transfer to an in situ prepared allene leads to the formation of 4-substituted cycloocta-2,5-dien-1-ones. Various derivatives could be obtained from commercially available (hetero)aryl aldehydes, trimethylsilylacetylene, and simple propargyl chlorides in good yields.

Full text of DOI:10.1002/anie.201505347

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Angewandte
Communications
International Edition: DOI: 10.1002/anie.201505347
German Edition: DOI: 10.1002/ange.201505347
Synthetic Methods
One-Pot Cannizzaro Cascade Synthesis of ortho-Fused
Cycloocta-2,5-dien-1-ones from 2-Bromo(hetero)aryl Aldehydes
Laurence Burroughs, Lee Eccleshare, John Ritchie, Omkar Kulkarni, Barry Lygo,
Simon Woodward,* and William Lewis
Abstract: An intramolecular Cannizzaro-type hydride transfer
to an in situ prepared allene enables the synthesis of ortho-
fused 4-substituted cycloocta-2,5-dien-1-ones with unprece-
dented technical ease for an eight-ring carboannulation.
Various derivatives could be obtained from commercially
available (hetero)aryl aldehydes, trimethylsilylacetylene, and
simple propargyl chlorides in good yields.
efficiency in the “age of sustainability”.[4] We speculated that
a new annulation strategy, based on a Cannizzaro-type
reaction via intermediate C, might be possible. As the hydride
transfer simultaneously exposes a powerful Michael acceptor
(ynone or equivalent) and a carbanion in close proximity,
efficient annulation might be expected to occur (especially if
C is directly attained in situ). Herein, we describe the use of
such a strategy for the formation of eight-membered rings.
The paucity of direct single-pot/step procedures to such
medium rings attests to the known issues associated with their
synthesis;[5] they therefore provide a stringent test for
strategy C. Although redox-based Cannizzaro reactions,
R
eadily (often commercially) available 2-bromo(hetero)aryl
aldehydes (1, Scheme 1) constitute powerful and popular
units for the construction of ortho-fused carbocyclic rings.[1]
Two strategies commonly employed in the multitudinous
syntheses described in the literature involve 1) the derivati-
zation of 1 to intermediates of type A, with a p-system
attacking an electrophile that is ultimately derived from the
aldehyde by functional-group manipulations (e.g., an alcohol,
acetal, or even the aldehyde itself); or 2) formyl group
homologation to an anion-stabilizing unit that attacks a suit-
ably activated p-bond (B). Whereas many elegant catalytic
systems that use intermediates A or B have already been
described,[2,3] the preparation of the suitable precursors is
often step-intensive, raising questions on the overall process
À
including asymmetric and triggered C C bond-forming
versions, have recently been developed,[6] to the best of our
knowledge, they have not been employed for carboannulation
reactions to medium rings. Genuine single-pot/step proce-
dures to eight-membered rings are very limited—Reppeꢀs
historical catalytic access to cyclooctatetraene[7] and Mura-
kamiꢀs [4+2+2] approach[8] being notable exceptions. Modern
alternatives frequently describe catalytic cyclizations that
proceed with exquisitely high yields (for the final step).
Unfortunately, their overall synthetic efficiency is often
compromised by the multistep syntheses required to obtain
the cyclization precursors.[9]
Initial investigation of our own proposal (C) centered on
the combination of readily available 2-bromobenzaldehyde
(1a), trimethylsilylacetylene (2a), and propargyl chloride 3a.
Encouragingly, even an initial run returned significant
amounts of benzo[8]annulene derivative 4aaa (Table 1,
entry 1, ca. 7%) whose identity and regiochemistry was
confirmed by X-ray crystallography (see the Supporting
Information) as initial NMR data was not conclusive.[10]
Unfortunately, under the conditions of entry 1, multiple side
products were also produced. Upon changing from an
organocopper to an organocuprate formulation (entry 2),
the reaction became much cleaner, but the yield of 4aaa
remained low. The recovered side products of entry 2
included significant amounts of the alcohols 5a and 6a
together with unreacted 3a. We could confirm that formation
of D was almost quantitative in < 20 min under all conditions
tried (Et2O or THF, À78 to 08C, based on recovered 5a in
independent reactions). We therefore suspected issues asso-
ciated with both nBuLi–halogen exchange from D and the
copper-promoted SN2’ reaction leading to the progenitor for
eight-ring formation. Further experiments were designed to
probe these hypotheses (Table 1).
Scheme 1. Proposed use of 2-bromo(hetero)aryl aldehydes in
Cannizzaro-triggered annulation cascades compared to traditional
approaches. EWG=electron-withdrawing group, LA=Lewis acid, R1–R3
are generic groups, n and m are typically 0–1.
[*] Dr. L. Burroughs, L. Eccleshare, J. Ritchie, O. Kulkarni,
Prof. Dr. B. Lygo, Prof. Dr. S. Woodward, Dr. W. Lewis
School of Chemistry, University of Nottingham
University Park, Nottingham NG7 2RD (UK)
E-mail: simon.woodward@nottingham.ac.uk
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201505347.
ꢀ 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co.
KGaA. This is an open access article under the terms of the Creative
Commons Attribution License, which permits use, distribution and
reproduction in any medium, provided the original work is properly
cited.
Organometallic compounds derived from D were poorly
soluble in Et2O so the reaction solvent was switched to THF
(entry 3). This led to an improved reaction that enabled us to
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Table 1: Optimization of the Cannizzaro-triggered cascade reaction to eight-ring compound 4aaa.[a]
Entry Solvent Halogen exchange of intermediate D Transmetalation with CuBr·SMe2 Addition of 3a and cyclization
Yield 4aaa [%]
1
2
3
4
Et2O
Et2O
THF
THF
tBuLi (2 equiv), À508C, 20 min
nBuLi (1 equiv), 08C, 10 min
nBuLi (1 equiv), À508C, 15 min
nBuLi (1 equiv), À508C, 15 min
1 equiv, À508C, 20 min
0.5 equiv, À508C, 5 min
0.5 equiv, À508C, 10 min
0.5 equiv, À508C, 1 h
1 equiv, À508C to RT over 5 h
0.5 equiv, À508C to +108C over 1.2 h
0.5 equiv, À508C to 108C over 1.2 h
0.5 equiv, À508C to À108C over 1.5 h;
kept at À108C for 1 h
<10
14[b]
53
70 (69)[c]
5
THF
nBuLi (1 equiv), À508C, 15 min
0.5 equiv, À508C, 1 h
0.5 equiv, À508C to À108C over 1.5 h;
35
kept at À108C for 2 h
[a] Reaction conditions: 1a (0.71 mmol), 2a (0.77 mmol)/nBuLi (0.72 mmol), CuBr·SMe2 (0.36–0.71 mmol), 3a (0.35–0.70 mmol) in Et2O or THF
(1.0 mL). The yields of 4aaa were determined by GC analysis against genuine samples and an internal standard (1-methylnaphthalene, 50 mL). [b] The
mass balance of the reaction included 3a, 5a, and 6a in a 2.4:1.0:1.8 ratio. [c] The yield of isolated product for a reaction performed with 1.39 mmol of
3a is shown in parentheses.
identify that the onset of the Cannizzaro-type cascade reaction
occurs at À20 to À108C for this substrate. Independent
treatment of THF solutions containing D with nBuLi followed
by quenching with D2O confirmed that complete Br/Li
exchange had occurred within 15 min at À40 to À608C, and
that the resulting organolithium species was rather stable at
these temperatures. However, above approximately À158C,
the same species began to decompose rapidly. Thus, À40 to
À508C was selected as the temperature range for optimizing
the transmetalation to CuBr·SMe2. When the nominal dio-
rganocuprate was prepared under the conditions of entry 3
(À508C, 10 min, from the aryl lithium intermediate with
CuBr·SMe2) and then treated with methyl vinyl ketone, very
poor conversions into the 1,4-addition product were attained.
This strongly suggested that transmetalation to CuI is rather
slow for this system. Gratifyingly, extending the time for Li/Cu
Scheme 2. Variation of the 2-bromo(hetero)aryl aldehyde; yields start-
exchange to one hour (entry 4) significantly increased the
quantity of 4aaa formed, and the GC yields were comparable
to those attained through isolation (70% vs. 69%). Further
ing from the commercial aldehydes are given.[9] [a] The precursor
aldehyde was obtained from isatin (3 steps).
extending the transmetalation time to two hours at À108C had
yields and time efficiencies of previously reported eight-ring
syntheses.[9]
a detrimental effect on the yield of 4aaa (entry 5).
With an optimized procedure for the preparation of
parent compound 4aaa in hand, we studied the scope of the
reaction with respect to the 2-bromo(hetero)aryl aldehyde
component 1. Using the conditions of entry 4 (Table 1) and
aldehydes 1b–1j, the eight-ring compounds that are shown in
Scheme 2 were attained. Substitution at the 3-position was
better tolerated than at the 1- and 4-positions (products 4baa/
4caa vs. 4daa/4 faa). Monosubstitution at the 2-postion was
also tolerated. A comparison of the yields of 4baa–4gaa
indicates that steric factors are more important than elec-
tronic ones; for example, intramolecular OMe steric contacts
increase the ortho steric profile of 4eaa versus 4jaa. The
yields of compounds 4haa–4jaa also support this trend.
Whereas the yields of 4aaa–4jaa are modest, it should be
borne in mind that they represent one-pot processes to
analytically pure crystalline products on up to multigram
scales, using only inexpensive commercial starting materials.
The new process compares very favorably to the consolidated
By varying the acetylene component using the readily
attained starting materials 2b–2 f, five further examples of
eight-ring compounds 4 were synthesized (Scheme 3). The
terminal alkyne substituent could encompass alkyl, vinyl, or
aryl units, and the corresponding products were all obtained
in acceptable yields. Finally, variation of the propargyl
chloride was investigated using simple 3b–3i (Scheme 3).
The aryl moiety could accommodate both electron-donating
and -withdrawing substituents and some functional groups
(4aae and 4aaf).
Whereas the full mechanistic features of our Cannizzaro-
triggered cascade are still under investigation, a working
hypothesis is shown in Scheme 4. Sequential 1,2-acetylide
addition, halogen exchange, and transmetalation to copper
affords intermediate E. This copper(I) species undergoes a
g-selective SN2’ addition to propargyl chloride 3 affording the
non-isolable allene C’, which undergoes the Cannizzaro-type
hydride shift proposed in Scheme 1. The resulting allyl anion
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Scheme 5. The computational models anti-C’ and syn-C’ and indirect
evidence for the formation of G.
having accepted a hydride. For the formation of 4iaa, we
could isolate appreciable amounts of 8, which is the expected
product of a Cannizzaro-type hydride transfer without sub-
sequent annulation. Use of the deuterated aldehyde
2-BrC6H4CDO ([D]-1a) led to the formation of the expected
product [D]-4aaa, confirming the participation of the alde-
hyde in the Cannizzaro process.
The mechanistic proposal shown in Scheme 4 was also
probed by DFT calculations at the M06-2X/6-31G(d) and
CBS-QB3 levels of theory[11] on the “metal-free” anions anti-
C’ and syn-C’ (Scheme 5; see also the Supporting Informa-
tion). Whereas the relationship of anti-C’ and syn-C’ to the
reaction coordinate of the true organometallic species
involved in the cascade process needs to be treated with
caution, it does provide further evidence for the proposed
mechanism. The calculated barriers for the Cannizzaro-type
hydride transfer and the rearrangement of the allyl anion (the
equivalent of the transformation of C’ into F in Scheme 4)
were approximately 5 and 7 kcalmolÀ1, respectively. The
calculations also indicate that non-classical enolates of type G
are favored in the reaction over the equivalent allenoate
structures. This idea was further reinforced by the observation
that trapping of the reaction intermediates with iodine,
iodomethane, or carbon dioxide, as opposed to a proton,
leads to the formation of derivatives 9–11 (Scheme 5).
To the best of our knowledge, this Cannizzaro-triggered
cascade process is unique. The nearest analogues that we
could find are cyclizations reported by the Oonishi and Sato
groups;[9b,d] however, in these cases, the allene intermediate
must be separately synthesized, and no Cannizzaro-type
process is involved. Similarly, a single eight-ring side product
(formed in 10% yield) was obtained by Alajarin and Vidal
through a 1,5-hydride shift/8p electrocyclization from a neu-
tral 1,2-allenylacetal at 1108C.[12] Again, no Cannizzaro-type
trigger was proposed. Whereas Cannizzaro shifts have been
used for oxidation-state manipulations in ortho-fused systems,
in these processes, no annulation steps were involved.[13] The
most similar ynone eight-ring closure process that could be
identified was developed by Schreiber and Kelly,[14] but this
transformation involves an alkoxide nucleophile.
Scheme 3. Variation of the terminal acetylene and propargyl chloride.
[a] Run at À508C.
Scheme 4. Mechanistic proposal for the formation of eight-ring com-
pounds 4; L is indicative of the presence of a generic organocopper or
-cuprate species.
In conclusion, we believe that this very simple formation
of ortho-fused cycloocta-2,5-dien-1-one units demonstrates
the potential of the developed method for rapid and efficient
annulations. Investigations into extending the scope of both
the present reaction and other variants are ongoing.
is presumably placed in close proximity to the potent ynone
Michael acceptor, which leads to efficient ring closure. The
use of an aryl moiety as the R2 substituent is apparently
sufficient to stabilize the allyl anion and allow the necessary
trans to cis isomerization. Protonation of the non-classical
enolate G results in the formation of 4. Several pieces of
evidence support this chain of events. First, attempts to use
2-thienylcyanocuprates derived from a related aza substrate
led to the isolation of 7 (X-ray structure) where the
intermediate allene has been carbometalated rather than
Acknowledgements
This study was initiated under funding from the European
Unionꢀs Seventh Programme for research, technological
10650 www.angewandte.org
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Protsahan Yojana (KVPJ) Fellowship.
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> 40%. For the preparation of our own starting materials,
costs were in the range of E0.3 – 8 mmolÀ1. Compound 4iaa is
included to demonstrate the need to avoid rather long starting-
material syntheses.
[10] For these derivatives, the proton attached to C4 gives rise to
a characteristic low-field resonance at approximately 5.0 ppm,
presumably owing to its proximity to the aryl ring current.
Compounds 4 are often highly crystalline, and in a number of
cases, they could be isolated by a simple recrystallization of the
crude reaction material on gram scale. The structures of 4haa
and 4jaa were confirmed by X-ray crystallography (see the
Supporting Information).
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[14] S. L. Schreiber, S. E. Kelly, Tetrahedron Lett. 1984, 25, 1757 –
1760. We thank a referee for this suggestion.
Keywords: aldehydes · annulation · carbocycles ·
medium-ring compounds · synthetic methods
How to cite: Angew. Chem. Int. Ed. 2015, 54, 10648–10651
Angew. Chem. 2015, 127, 10794–10797
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Received: June 11, 2015
Published online: July 29, 2015
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