Functionalization of acetalic C(sp3)-H bonds by scandium(III) triflate-catalyzed intramolecular redox reactions: Tandem 1,4-hydride transfer/1,5-cyclization processes leading to protected 1-indanones

Article abstract of DOI:10.1002/adsc.201000812

A new C-C bond forming reaction leading to adjacent quaternary carbons is reported. It is a one-pot hydride shift/cyclization process facilitated by the hydricity of the acetalic C-H bonds, with benzylidenemalonate fragments as electrophilic hydride accep

Full text of DOI:10.1002/adsc.201000812

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DOI: 10.1002/adsc.201000812
Functionalization of Acetalic C
N
U
Triflate-Catalyzed Intramolecular Redox Reactions: Tandem 1,4-
Hydride Transfer/1,5-Cyclization Processes Leading to Protected
1-Indanones
Mateo Alajarin,a,* Marta Marin-Luna,a and Angel Vidala,*
a
Departamento de Quꢀmica Orgꢁnica, Facultad de Quꢀmica, Universidad de Murcia, Campus de Espinardo, 30100 Murcia,
Spain
Fax : (+34)-868-884149; phone: (+34)-868-887497; e-mail: alajarin@um.es or vidal@um.es
Received: October 28, 2010; Published online: March 2, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000812.
À
Abstract: A new C C bond forming reaction lead-
ing to adjacent quaternary carbons is reported. It is
a one-pot hydride shift/cyclization process facilitat-
sumed dipolar intermediates.[4] Most of these tandem
H-shift/cyclization processes are related to the long
studied tert-amino effect,[5] whereas the rest of the ex-
À
À
ed by the hydricity of the acetalic C H bonds, with
benzylidenemalonate fragments as electrophilic hy-
dride acceptors, and the catalysis of scandiumACTHNUGTRNEUNG(III)
amples are based on the special activation of C H
bonds adjacent to oxygen atoms[6] and arene groups[7]
(Scheme 1, a).
triflate. The reaction products are 1,2-dihydroin-
dane derivatives. Alkoxy and alkanethiolate groups
can be also intramolecularly transferred from the
acetalic carbon to the electrophilic benzylidenemal-
onate C=C bond.
We recently became engaged in the study of ther-
mally-activated [1,5]-H shift/cyclization tandem reac-
tions in which the hydrogen atom migrating in the
first step is initially placed at the acetalic carbon atom
of 1,3-dioxolane, -dithiolane, and -oxathiolane frag-
ments, whereas the migration terminus is the central
C(sp) carbon atom of heterocumulenic functions such
as ketenimines and carbodiimides.[8] The initial [1,5]-
H shift of these reactions was characterized as an in-
tramolecular hydride transfer by means of computa-
tional DFT studies. In fact, those calculations showed
Keywords: acetals; cyclization; ACTHNUTRGNEUNG[1,4]-H shift; intra-
molecular hydride transfer; scandium triflate
À
the weakening and polarization of the acetalic C H
À
À
C H activation has long been considered a research bond by hyperconjugative interaction of its s* C H
theme of major interest.[1] The growing number of orbital with the lone-pair electrons of the vicinal O
À
methods to functionalize C H bonds is unlocking and S heteroatoms of the (thio)acetalic functions, thus
new perspectives in the synthesis of complex organic facilitating the hydride transfer to the electrophilic
molecules and profoundly affecting the process of central heterocumulenic carbon. These results demon-
À
molecular design. In this context, the C H bond func- strated for the first time the hydride donor ability (hy-
tionalization is a rapidly growing playing field of new dricity) of such acetalic functions.
À
synthetic strategies and the C H bonds are starting to
We reasoned that related tandem processes are
conceivable by changing the hydride-acceptor unit
be viewed as ubiquitous functional groups.[2]
À
Whereas biological C H activation is widespread in from heterocumulenes to other electrophilic function-
nature, in conventional organic chemistry such activa- al groups, while keeping the acetalic functions as the
tion has been long dominated by the metal-catalyzed hydride-releasing fragment. In this line, here we dis-
insertion reactions and cyclometalations.[3] In the last close a novel C
ACHTUNGTRENNUNG
3
À
À
decade a new class of one-pot transformations for C
based on the formal tandem hydride shift/cyclization
H bond functionalization, known as intramolecular strategy by utilizing benzylidenemalonate fragments
redox reactions, has been disclosed, processes which and related functions as hydride-acceptors, and 1,3-di-
are initiated by an intramolecular hydride transfer oxolanes, 1,3-dithiolanes and 1,3-dithianes as hydride-
and continued by a final cyclization step of the pre- donors, both complementary functional groups built
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Scheme 1. Hydrogen shifts followed by cyclizations.
up on an ortho-substituted benzene scaffold. Interest-
ingly, the putative hydride migration is a rare [1,4]-H
shift, more scarcely reported than its [1,5]-H partner,
and the subsequent cyclization step leads finally to
À
the formation of a new sigma C C bond linking adja-
cent quaternary carbon atoms (Scheme 1, b).
We first selected diethyl ortho-(1,3-dioxolan-2-yl)-
benzylidenemalonate 2a for our initial tests. Com-
pound 1a was readily prepared starting from ortho-
bromobenzaldehyde in three simple steps: acetal for-
mation with ethylenediol, halogen-lithium exchange
with subsequent formylation to give the known ortho-
phthalaldehyde monoacetal 1a,[9] and further Knoeve-
nagel condensation with diethyl malonate (Scheme 2).
The heating in solution of 2a under different reac-
tion conditions (acetonitrile 828C, benzene 808C, tol-
uene 1108C, toluene 1808C sealed tube, nitrobenzene
2118C, dimethylformamide 1108C) proved to be inef-
ficient for promoting the desired tandem process, in
Scheme 2. Reagents and conditions: i) HOACHTNUGTRNEG(UN CH2)2OH, p-
TsOH, benzene, reflux, 4 h; ii) n-BuLi, THF, À788C, 30 min,
then N-formylpiperidine, THF, room temperature, 3 h; iii)
diethylmalonate, piperidine, AcOH, benzene, reflux, 2 h.
all cases the starting material was recovered unaltered Most of these attempts yielded mixtures of the start-
along with small amounts of the acetal-hydrolyzed ing material 2a and the aldehyde 3a resulting from
product 3a. These discouraging results demonstrated the hydrolysis of the acetalic function (entries 1–5).
that the styryl a-carbon atom of species 2a is appa- Only the use of ScACTHUNRGTNEUNG(OTf)3 as catalyst at 808C in a
rently less electrophilic than the central carbon of ke- sealed tube (entries 6–8) provided some results indi-
tenimines or carbodiimides in the presumed hydride cating the viability of the planned synthetic strategy,
migration event. Activation of the hydride-acceptor these latter reactions giving rise to moderate yields of
properties of the benzylidenemalonate fragment was diethyl indan-1-one-2,2-dicarboxylate 5a. Apparently,
next pursued by the use of Lewis acids able to coordi- the oxophilicity of the catalyst promoted, as desired,
nate at the carbonyl oxygen atoms of the ethoxycar- the planned tandem process but also contributed to
bonyl groups. Thus, we next attempted catalytic pro- the hydrolysis of the acetalic function in the expected
tocols with a few Lewis acids [AlCl3, BF3·Et2O, spirocyclic product 4a (in fact detected in a small pro-
Sc
ACHTUNGTRENNUNG(OTf)3] in dichloromethane solution under a range portion by NMR monitoring in the first stages of the
of temperature conditions (Table 1 and Scheme 3). reactions) as well that of the starting dioxolane 2a,
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Functionalization of Acetalic CACTHNUTRGNEU(GN sp ) H Bonds
Scheme 3. Lewis acid-catalyzed [1,4]-H transfer/1,5-cyclization processes in ortho-(1,3-dioxolan-2-yl)benzylidenemalonates.
Table 1. Indan-1-one 2,2-dicarboxylates 5.[a]
Entry
2
R
R1
Catalyst (equiv.)
Temp. [8C]
5 (yield [%])
1
2
3
4
5
6
7
8
2a
2a
2a
2a
2a
2a
2a
2a
2b
2c
2d
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Me
CO2Et
CO2Me
H
H
H
H
H
H
H
H
AlCl3 (0.1)
BF3·Et2O (0.2)
25
80
25
50
60
80
80
80
80
80
80
Sc
Sc
Sc
Sc
Sc
Sc
Sc
Sc
Sc
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
T
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
5a (30)
5a (58)
5a (55)
5b (54)
5c (41)
5d (35)
9
10
11
H
OMe
OMe
AHCTUNGTRENNUNG
[a]
Reactions were conducted on 0.6 mmol scale of benzylidenemalonate with a catalytic amount of ScACHTNUTGRNEUNG(OTf)3 (0.12 mmol) in
anhydrous dichlorometane (20 mL) at 808C in a sealed tube.
this latter event contributing to lowering the global
yields in 5a.
With the acetal-hydrolyzed ortho-formylbenzylide-
nemalonate 3a in our hands as result of our initial at-
The use of 0.2 equivalents of ScACTHNURTGNENG(U OTf)3 gave the tempts, we could prove that 3a did not convert into
best result (entry 7). Three additional reactions were the indanone 5a under the habitual reaction condi-
run under such conditions (entries 9–11) giving access tions, thus ensuring that compounds 4 are intermedi-
to the new indanones 5b–d in medium yields. The ates in the processes leading from 2 to 5.
presence of an MeO substituent in the para position
In order to improve the efficiency of these or simi-
to the alkenylic side-chain lowered the yield of the lar processes we made a substantial strategic varia-
tandem hydride migration/cyclization/hydrolysis pro- tion. In our previous studies on tandem, thermally-ac-
cess (entries 10 and 11), a result probably related with tivated [1,5]-H shift/cyclization reactions of acetalic
the lowering of the electrophilic character of the ben- ketenimines and carbodiimides we discovered that 2-
zylidene a-carbon atom, the hydride migration termi- monosubsituted 1,3-dithiolane and 1,3-dithiane frag-
nus, by the electron-donating para-MeO group. From ments were also effective hydride-donor functions, al-
a synthetic point of view, the conversion of com- though their hydricity was lower than that of their re-
pounds 2 into 5 mimics the intramolecular hydroacy- spective dioxygenated partners, 1,3-dioxolanes and
lation of 2-vinyl benzaldehyde systems,[10] in the pres- 1,3-dioxanes. We reasoned that changing the dioxo-
ent case by using an acetal-protected aldehyde. The lane function of compounds 2 by a dithiolane or di-
original Rh-catalyzed reaction was reported to pro- thiane group could still give a chance for the occur-
ceed in an extremely sluggish way with benzaldehydes rence of the desired tandem 1,4-hydride shift/cycliza-
bearing b-substituted 2-vinyl groups.[11]
tion sequence, although probably requiring slightly
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Table 2. Spirocyclic 1,3-dithiolanes and 1,3-dithianes 7.[a]
Entry
6
R
R1
R2
R3
n
Time [h]
7 (yield [%])
1
2
3
4
5
6
7
8
9
6a
6b
6c
6d
6e
6f
6g
6h
6i
CO2Et
CO2Me
COMe
CO2Et
CO2Me
CO2Et
CO2Me
CO2Et
CO2Et
CO2Me
CO2Et
CO2Me
CO2Me
CO2Et
CO2Me
CO2Et
CO2Me
CO2Et
CO2Et
CO2Me
H
H
H
H
H
H
H
Me
Me
H
H
Cl
H
1
1
1
1
1
1
1
1
2
2
40
40
40
30
30
45
45
40
80
80
7a (73)
7b (75)
7c (46)
7d (89)
7e (92)
7f (57)
7g (62)
7h (81)
7i (57)
7j (63)
H
OMe
OMe
Cl
H
H
10
6j
H
[a]
Reactions were conducted on 0.6 mmol scale of benzylidene derivative with a catalytic amount of ScACHTUNRGTNEUNG(OTf)3 (0.12 mmol)
in anhydrous dichlorometane (20 mL) at 808C in a sealed tube.
These differences in reactivity are in accord with the
results of our previous experiments with dithioacetalic
heterocumulenes, where the 2-substituted 1,3-dithiane
group was shown to be slightly less reactive than its
dithiolane partner. These results confirmed the feasi-
bility of the planned synthetic strategy. Although the
yields of these reactions are not excellent, the spirocy-
clic products 7 are totally free of carbonyl-deprotect-
ed impurities 5 and the formation of a bond between
two aliphatic quaternary carbon atoms is remarkable
from a structural point of view.
Scheme 4. Lewis acid-catalyzed [1,4]-H transfer/1,5-cycliza-
tion processes in benzylidene derivatives 6.
harsher reaction conditions, but this change would
When a similar methodology was applied to benzyl-
hopefully suppress the disturbing hydrolysis of the idene malonates bearing non-cyclic acetalic functions,
acetalic function aided by the oxophilic Lewis acid as the dimethyl acetals 8a and b, the reaction products
catalyst.
resulted to be the trans-1,3-dimethoxyindane 2,2-di-
With this aim in mind, we next approached the carboxylates 9a and b (Scheme 5 and Table 3).
preparation of a series of benzylidene derivatives 6
bearing 1,3-dithiolane and 1,3-dithiane functions at
the ortho position. After some experiments, including
the preparation of ortho-(1,3-dithiolan-2-yl)benzalde-
hyde[12] and its condensation with dialkyl malonates,
we found as the better synthetic route to compounds
6 the trans-thioacetalization of the easily accessible
dioxolane-malonates 2 with 1,2-ethanedithiol and 1,3-
propanedithiol in the presence of bromodimethylsul-
fonium bromide.[13]
Scheme 5. Lewis acid–catalyzed [1,4]-Y transfer/1,5-cycliza-
We were pleased to find out that heating CH2Cl2
solutions of compounds 6 at 808C in a sealed tube for acetalic functions.
30–80 h and in the presence of Sc(OTf)3 (0.2 equiv.)
tion processes in benzylidenemalonates bearing non-cyclic
AHCTUNGTRENNUNG
Table 3. Indane 2,2-dicarboxylates 9.[a]
cleanly provided the spirocyclic products 7 in medium
to good yields (Scheme 4 and Table 2).
Entry
8
R
Y
Time [h] trans-9 (yield [%])
The reaction was applied to a number of benzyli-
dene malonates (entries 1,2 and 4–10) and to one ben-
zylidene acetylacetate (entry 3) as electrophilic func-
tions, generally using 1,3-dithiolanes as hydride-re-
leasing units but also 1,3-dithianes (entries 9 and 10).
In these latter cases the yields of 7 are lower than
those obtained with 1,3-dithiolanes (compare entries 1
and 2 with 9 and 10, respectively) and the reactions
required longer times for completion (see Table 2).
1
2
3
4
8a CO2Et
OMe
4
6
15
15
9a (53)
9b (45)
9c (75)
9d (68)
8b CO2Me OMe
8c CO2Et SEt
8d CO2Me SEt
[a]
Reactions were conducted on 0.6 mmol scale of benzyli-
denemalonate with catalytic amount of Sc(OTf)3
a
ACHTUNGTRENNUNG
(0.12 mmol) in anhydrous dichlorometane (20 mL) at
808C in a sealed tube.
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Functionalization of Acetalic CACTHNUTRGNEU(GN sp ) H Bonds
In this case the reaction course is interpreted in Experimental Section
terms of a tandem 1,4-methoxide shift/cyclization pro-
cess in which the alkoxy substituent proved to be a Typical Experimental Procedure for the Preparation
better migrating group than the acetalic hydrogen of Compounds 5, 7 and 9
atom. This order of migrating ability is not especially
ACHUTNGERN(NUG Thio)acetal 2, 6 or 8 (0.6 mmol), scandiumACHTUNGTERN(NUGN III) triflate
surprising when considering an anionotropic rear-
rangement,[14] and is reflected in the shorter reaction
times required for taking the reactions to completion
(4–6 h). The trans configuration of compounds 9a and
b is established following their solution 13C NMR
spectra, where only one carbonyl carbon is distin-
guishable. This is indicative of the C2 symmetry axis
of structures trans-9, whereas the isomeric cis species
should show two signals for the otherwise non-equiva-
lent carbonyl carbon atoms of the two CO2Et or
CO2Me groups at C-2 (Cs symmetry).[15] The yields in
which trans-9a and b were obtained are rather
modest, most probably reflecting the contribution of
(0.12 mmol) and anhydrous dichloromethane (20 mL) were
loaded into a dry sealed tube and then heated in an oil bath
set to 808C. When all the starting material was consumed,
as indicated by the TLC of reaction aliquots (3–80 h), the
reaction was quenched with saturated sodium bicarbonate
(20 mL) and vigorously shaken. The aqueous layer was ex-
tracted with dichloromethane (2ꢃ20 mL) and the combined
organic phases were dried over magnesium sulfate. Then,
the solvent was removed under reduced pressure and the
residue was purified by silica gel column chromatography.
Acknowledgements
the ScACHTUNGTRENNUNG(OTf)3 catalyst to promote the partial hydroly-
sis of the acetalic function of 8a and b in the course
of the thermal treatment in CH2Cl2 solution, even
when the habitual precautions are taken to ensure dry
reaction conditions. As expected, better yields were
reached with the dimethyl dithioacetals 8c and d,
easily available by transthioacetalization of the corre-
sponding cyclic acetals 2 with EtSH in the presence of
bromodimethylsulfonium bromide, although the
lower reactivity of these dithioacetals when compared
with their oxygenated partners is again apparent,
these reactions requiring somewhat longer reaction
times. The stereochemical outcome of these tandem
1,4-ethanethiolate shift/cyclization processes is also
the exclusive formation of the trans-9c and d diaste-
reoisomers.
This work was supported by the Ministerio de Ciencia e In-
novacion of Spain (Project CTQ2008-05827/BQU), and Fun-
dacion Seneca-CARM (Project 08661/PI/08).
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