Gold(I)-catalyzed functionalization of benzhydryl C (sp3)-H bonds

Article abstract of DOI:10.1002/adsc.201300525

The selective activation/functionalization of benzhydryl C (sp3) H bonds is documented. The gold complex XPhosAuNTf2 turned out to be an efficient catalyst (5 mol%) to transform readily available propargylic esters into di or trisubstituted naphthalenes i

Full text of DOI:10.1002/adsc.201300525

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DOI: 10.1002/adsc.201300525
3
À
Gold(I)-Catalyzed Functionalization of Benzhydryl C
Bonds
N
Gianpiero Cera,a Michel Chiarucci,a Federico Dosi,a and Marco Bandinia,*
a
Department of Chemistry “G. Ciamician”, Alma Mater Studiorum – University of Bologna, via Selmi 2, 40126 Bologna,
Italy
Fax : (+39)-(0)51-209-9456; phone: (+39)-(0)51-209-9751; e-mail: marco.bandini@unibo.it
Received: June 14, 2013; Published online: August 9, 2013
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201300525.
Abstract: The selective activation/functionalization
reported by Barluenga and co-workers.[8] On the con-
trary, [Au]-catalyzed hydride transfer from benzyl or
3
À
of benzhydryl C
N
À
gold complex XPhosAuNTf2 turned out to be an ef-
ficient catalyst (5 mol%) to transform readily avail-
able propargylic esters into di- or trisubstituted
naphthalenes in high yield. A 1,5-hydride shift is
postulated as the key step of the cascade reaction
sequence.
benzhydryl C H bonds has been considerably less in-
vestigated.[9]
As part of our ongoing interest toward the develop-
ment of gold-catalyzed manipulations of propargylic
derivatives,[10] we decided to investigate the suitability
of synthetically flexible propargylic esters as valuable
hydride acceptors under gold assistance.[11] As
a matter of fact, the well-recognized and predictable
gold promoted 1,3-OXO migration of propargylic
esters would lead to the corresponding allenoates that
can undergo further manipulations via nucleophilic
attack (Figure 1).[12,13]
À
Keywords: C H functionalization; gold catalysis;
1,5-hydride shift; naphthalenes
3
À
The selective functionalization of unactivated C
G
H bonds is currently a hot topic in modern organic
synthesis.[1] Step, atom and redox economies are posi-
tively affected by introducing the direct manipulation
(sp ) H bonds with implications
also for the total synthesis of structurally complex
3
À
of unfunctionalized C
U
molecular architectures.[2]
Figure 1. Planning a new synthetic tool for the regioselective
functionalization of small acenes.
1,5-Hydride transfer (HT) is a well-consolidated
methodology to accomplish regioselective C H modi-
fications via LA (Lewis acid), BA (Brønsted acid) or
À
amino-catalyzed intramolecular migration of hydride
Accordingly, we envisioned that the readily accessi-
species to pre-installed electrophilic centers.[3] Gener- ble benzhydrylic substrates 1 could open new synthet-
ally, the so formed highly reactive cationic intermedi- ic enterprises towards small acenes 2, via metal-cata-
3
À
ate can be trapped by the newly generated organic or lyzed C
organometallic nucleophiles. When p-systems are uti- actions.
A
lized as hydride acceptors, carbophilic late-transition
In this regard, it should be emphasized that naph-
metal (LTM) species are conventionally employed in thalenes and hetero-substituted analogues continue to
order to ensure an adequate electrophilic activation play a major role both in catalysis[14] and molecular
of the unsaturated hydrocarbon moiety.[4]
organic electronics.[15] Therefore, the current request
In this research segment, cationic gold(I) com- for effective and site-selective synthetic routes to con-
plexes[5] have already proven competence in trigger- densed arenes is not surprising.[16]
ing 1,5-hydrogen sigmatropic shifts of -OR and -NR2
At the outset of the catalyst optimization, a range
of s- as well as p-acids was screened with the model
3
[6,7]
À
stabilized C
N
Moreover, a complementary approach based on compound 1a (R=t-Bu) and the resulting outcomes
a structurally “facilitated” strategy has been recently are listed in Table 1.
Adv. Synth. Catal. 2013, 355, 2227 – 2231
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Gianpiero Cera et al.
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Table 1. Optimization of reaction conditions.[a]
Run
Catalyst
FeCl3
[Ag]
1a (R)
Yield of 2a [%][b]
1
2
3
4
5
6
7
8
–
–
–
–
–
–
–
1a (t-Bu)
1a (t-Bu)
1a (t-Bu)
1a (t-Bu)
1a (t-Bu)
1a (t-Bu)
1a (t-Bu)
1a (t-Bu)
1a (t-Bu)
1a (t-Bu)
a (t-Bu)
1a (t-Bu)
1a (t-Bu)
1a (t-Bu)
1a (t-Bu)
1a (t-Bu)
1a (Me)
15
Nr
27
nr
In
N
PdCl2
AgOTf
TfOH
HNTf2
PtCl2
PtCl2
AuCl3
PPh3AuCl
PPh3AuCl
PPh3AuNTf2
decomp.
[c]
–
66
[d]
AgOTf
–
AgOTf
AgPF6
–
9
56
18
34
61
36
77
54
91
57
10
11
12
13
14
15
16
17
18
19[e]
20[f]
–
(pCF3C6H4)3PAuCl
IPrAuCl
AgNTf2
AgNTf2
JhonPhosAuNTf2
XPhosAuNTf2
XPhosAuNTf2
XPhosAuNTf2
XPhosAuNTf2
XPhosAuNTf2
–
–
–
–
–
–
1a (Bn)
1a (t-Bu)
1a (t-Bu)
50
traces
45
[a]
All the reactions were carried in anhydrous toluene, under a nitrogen atmosphere unless otherwise specified.
Isolated yield after flash chromatography.
Room temperature.
A complex mixture of unknown products was obtained.
Reaction temperature: 608C.
[b]
[c]
[d]
[e]
[f]
Reaction time: 1 h. Decomp.=decomposition, Nr=no reaction, JhonPhos=(2-biphenyl)di-tert-butylphosphine, XPhos=
dicyclohexyl{2’,4’,6’-tris(1-methylethyl)[1,1’-biphenyl]-2-yl]phosphine}, IPr=1,3-bis(diisopropylphenyl)imidazol-2-ylidene.
Notably, while Brønsted acids (i.e., TfOH, HNTf2) (608C, entry 19) or shorten the reaction time (1 h,
and common s-Lewis acids [i.e., FeCl3, In
not provide 1,2-diphenylnaphthalene 2a in syntheti-
(OTf)3] did entry 20) resulted into unsatisfactory outcomes.[19,20]
ACHTUNGTRENNUNG
Interestingly, the high chemoselectivity towards
cally acceptable isolated yields, late-transition metal naphthalene synthesis guaranteed by XPhosAuNTf2
species furnished promising results with particular should be emphasized, showing a net preference of
regard to [Pt(II)], [Pd(II)], [AuACTHNUGRTENUNG(III)] and [Au(I)] salts the 1,5-hydride transfer event vs. alternative hydroar-
(entries 3, 7, 9, and 12). Among them, PtCl2 and ylation processes as elegantly described by Nolan and
PPh3AuNTf2 furnished the higher yields of 2a in re- co-workers for similar substrates.[21]
fluxing toluene (66%, 61%, respectively).
Having established the catalytic system, the gener-
Delightfully, optimization of the [Au(I)] source led ality of the protocol was then assessed by subjecting
to commercially available well-defined silver-free a range of benzhydryl derivatives (1b–1p) to the ring-
[17]
XPhosAuNTf2 as the optimal catalyst providing 2a closing process. A collection of results is summarized
in nearly quantitative yield (91%, entry 16), although in Figure 2.
satisfying results were obtained also with cationic car-
Propargylic esters derived from secondary alcohols
bene-based gold species i-PrAuCl/AgNTf2 (yield= (1b–1k) were firstly examined. Interestingly, a range
77%, entry 14).[6c,18] Different ester derivatives such as of EWG and EDG substituents were adequately tol-
phenylacetate (1a, R=Bn) and acetate (1a, R=Me) erated in different positions of the benzhydrylic unit
were also tested, but naphthalene 2a was always iso- and acetylene terminals.[22]
lated in lower yields (entries 17 and 18). Finally, at-
Also ortho-substituted aromatic rings proved suita-
tempts to perform the reaction at lower temperature ble in the model protocol (i.e., 2f, yield=82%; 2j,
yield=89%).
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Gold(I)-Catalyzed Functionalization of Benzhydryl CACTHNUTRGNEUG(N sp ) H Bonds
Figure 3. Possible reaction pathways for the initial hydride
transfer event.
thesis of 4,5-diphenylbenzo[b]thiophene 2o in 82%
isolated yield.
Mechanistically, two distinct reaction pathways can
be envisioned. Specifically, the gold-triggered 1,5-hy-
dride migration could directly involve the electrophili-
cally activated alkynes (dashed lines) or the allenoate
A originated from [Au(I)]-assisted [3,3]-sigmatropic
rearrangement of the internal propargylic ester (plain
lines, Figure 3).[12]
Although a conclusive mechanistic rationale is still
not available, the inertness of the propargylic ethyl
ether 1p towards the transformation employing the
best conditions (Figure 3) led us to hypothesize the
initial formation of the allenoate as the more likely.[23]
Additionally, the disappointing outcomes recorded
with Brønsted acids (entries 5 and 6, Table 1) might
rule out the initial formation of a propargylic cationic
species.[24]
Accordingly, we can speculate that the 1,5-hydride
migration would involve the C-2 position of the allen-
yl unit,[25] leading to either alkyl-[Au(I)] species (B)
that could intercept intramolecularly the stabilized
Figure 2. Proving the scope of the reaction. [In bold the in-
teratomic connections formed during the cascade process.
Nr: no reaction].
Interestingly, 1,2,4-trisubstituted naphthalene 2l was benzhydrylic carbocation intermediate (dashed lines,
also readily accessible in regioselective manner and Figure 4) or the formal “triene” C followed by
acceptable yield (84%).
It is worth noting that the 1,5[H]-transfer reaction Figure 4).[13,26]
worked satisfactorily not only with benzhydryl frame- In order to get more insight into the reaction pro-
a
[4+2]cycloaddition
process
(plain
lines,
works but also in the presence of the allylbenzene de- file, some deuterium-labelling experiments were car-
rivative 1k, delivering 2-phenyl-1-vinylnaphthalene 2k ried out. In particular by subjecting [D2]-1a to the
in 43% yield. Contrarily, ortho-tolyl motifs did not gold-catalyzed cascade process we unambiguously
undergo activation of the corresponding benzylic C- demonstrated the intramolecular 1,5-hydride sigma-
3
À
G
ty of the present protocol toward the realization of position to the 3(b’)-position of the naphthalene ring
substituted heteroarenes was ascertained with the syn- (formally the C-2 of the allenoate intermediate A).
Adv. Synth. Catal. 2013, 355, 2227 – 2231
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Gianpiero Cera et al.
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towards the preparation of different heterocyclic scaf-
folds are currently underway in our laboratory.
Experimental Section
General Procedure
The reaction was carried out under anhydrous conditions.
The flask was charged with 1 mL of dry toluene, 0.1 mmol
of the desired propargylic ester
1 and XPhosAuNTf2
(5 mol%). The reaction flask was placed in a pre-warmed
oil-bath at 1108C and the reaction mixture stirred at the
same temperature until complete consumption of the start-
ing material was ascertained by TLC. The crude reaction
mixture was then charged onto a plug of silica for chromato-
graphic purification (see the Supporting Information for de-
tails).
Figure 4. Possible reaction machineries for the C-C forming
ring-closing event.
Moreover, the full deuterium incorporation at the C-3
position of 2a allowed us to measure a kinetic isotopic
effect (kH/kD), by monitoring the reaction course of
a 1:1 mixture of [H2]- and [D2]-labelled 1a. Interest-
ingly a primary isotopic effect kH/kD =1.94[27] was re-
Acknowledgements
Acknowledgement is made to FIRB Project “Futuro in Ricer-
ca” Innovative sustainable synthetic methodologies for C H
activation processes, PRIN Project PRIN 20099PKHH 004,
“Progettazione e Sviluppo di Nuovi Sistemi Catalitic Innova-
tivi” (MIUR, Rome) and Universitꢀ di Bologna for support
of this work.
À
À
corded supporting the C H cleavage as the rate-de-
termining event of the whole process (Scheme 1b).
This evidence is also in agreement with the moderate
reactivity of the thienyl-substrate 1o at room temper-
ature (Scheme 1c, yield=49%), that could be ration-
alized in terms of increased stabilization of the transi-
ent electrophilic species B or triene C.
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