A mild and ligand-free Ni-catalyzed silylation via C-OMe cleavage

Article abstract of DOI:10.1021/jacs.6b10998

Metal-catalyzed transformations that forge carbon-heteroatom bonds are of central importance in organic synthesis. Despite the formidable potential of aryl methyl ethers as coupling partners, the scarcity of metal-catalyzed C-heteroatom bond formations via C-OMe cleavage is striking, with isolated precedents requiring specialized, yet expensive, ligands, high temperatures, and π-extended backbones. We report an unprecedented catalytic ipso-silylation of aryl methyl ethers under mild conditions and without recourse to external ligands. The method is distinguished by its wide scope, which includes the use of benzyl methyl ethers, vinyl methyl ethers, and unbiased anisóle derivatives, thus representing a significant step forward for designing new C-heteroatom bond formations via C-OMe scission. Applications of this transformation in orthogonal silylation techniques as well as in further derivatizations are also described. Preliminary mechanistic experiments suggest the intermediacy of Ni(0)-ate complexes, leaving some doubt that a canonical catalytic cycle consisting of an initial oxidative addition of the C-OMe bond to Ni(0) species comes into play.

Full text of DOI:10.1021/jacs.6b10998

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Article
A Mild & Ligand-Free Ni-catalyzed Silylation via C–OMe Cleavage
Cayetana Zarate, Masaki Nakajima, and Ruben Martin
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 28 Dec 2016
Downloaded from http://pubs.acs.org on December 28, 2016
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Journal of the American Chemical Society
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A Mild & LigandꢀFree NiꢀCatalyzed Silylation via C–OMe Cleavage
Cayetana Zarate,† Masaki Nakajima† and Ruben Martin†§*
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†Institute of Chemical Research of Catalonia (ICIQ), The Barcelona Institute of Science and Technology, Av. Països Cataꢀ
lans 16, 43007 Tarragona, Spain
§ ICREA, Passeig Lluïs Companys, 23, 08010, Barcelona, Spain.
ABSTRACT: Metalꢀcatalyzed transformations that forge C–heteroatom bonds are of central importance in organic
synthesis. Despite the formidable potential of aryl methyl ethers as coupling partners, the scarcity of metalꢀcatalyzed
C–heteroatom bondꢀformations via C–OMe cleavage is striking, with isolated precedents requiring specialized, yet
expensive, ligands, high temperatures and πꢀextended backbones. We report an unprecedented catalytic ipso-silylation
of aryl methyl ethers under mild conditions and without recourse to external ligands. The method is distinguished by its
wide scope, which includes the use of benzyl methyl ethers, vinyl methyl ethers and unbiased anisole derivatives, thus
representing a significant stepꢀforward for designing new C–heteroatom bondꢀformations via C–OMe scission. Appliꢀ
cations of this transformation in orthogonal silylation techniques as well as in further derivatizations are also described.
Preliminary mechanistic experiments suggest the intermediacy of Ni(0)ꢀate complexes, leaving some doubt that a caꢀ
nonical catalytic cycle consisting of an initial oxidative addition of the C–OMe bond to Ni(0) species comes into play
.
requiring the inclusion of, electronꢀrich, and expensive
ligands such as PCy3 or N-heterocyclic carbenes (NHC),
as well as rather elevated temperatures (Scheme 1, path
b).5 Additionally, πꢀextended backbones are essential for
these reactions to occur unless appropriate activating
groups are located at either ortho or para position to the
targeted methoxy group.5 It is worth noting that these
features are recurrent prerequisites found in a myriad of
C–O bondꢀfunctionalization reactions, significantly
limiting the synthetic utility and prospective impact of
these rather appealing processes.6,7
INTRODUCTION
Over the last decade, C–O electrophiles have matured
into robust and reliable alternatives to organic halides
within the crossꢀcoupling arena owing to the lowꢀcost,
readily availability, and benign character of phenol.1
Unlike the use of relatively activated C–O electrophiles
that easily undergo oxidative addition such as aryl sulꢀ
fonates, esters, or carbamates, the employment of aryl
methyl ethers, arguably the simplest and most atomꢀ
economical derivatives in the phenol series, have reꢀ
ceived considerably less attention.2 This observation is
probably attributed to the remarkably high activation
barrier required for effecting C(sp2)–OMe cleavage, and
the lower tendency of methoxy residues to act as leaving
groups.2 Indeed, a close look into the literature data
reveals that the vast majority of crossꢀcoupling reactions
of organic electrophiles such as aryl halides, esters, carꢀ
bamates or sulfonates, among others, perfectly tolerates
the presence of aryl methyl ethers, evidencing the lower
propensity to undergo C–OMe cleavage.3
Scheme 1. CrossꢀCoupling Reactions via C–OMe Cleavage.
At present, catalytic crossꢀcoupling reactions of aryl
methyl ethers remain largely confined to C–C bondꢀ
forming endeavors based on stoichiometric, highly reacꢀ
tive, and in most instances, airꢀsensitive organometallic
reagents (Scheme 1, path a).2 Although metalꢀcatalyzed
C–heteroatom bondꢀforming reactions are of central
importance in organic synthesis,4 it comes as a surprise
that these transformations have rarely been designed
with aryl methyl ethers as coupling partners. The existꢀ
ing precedents for tackling this challenge are currently
based on amination or borylation protocols, necessarily
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Table 1. Optimization of the Reaction Conditions.a
Prompted by the perception that C(sp2)–OMe funcꢀ
1
tionalization might occur by pathways other than “clas-
sical” oxidative addition,8 we recently questioned
whether a mild C–heteroatom bondꢀforming reaction via
C(sp2)–OMe cleavage without recourse to any added
dative ligand, harsh conditions or highly reactive stoiꢀ
chiometric organometallic reagents could ever be imꢀ
plemented.9 We anticipated that the successful realizaꢀ
tion of such a catalytic technique could provide a palette
of conceptually new tactics not apparent at first sight
while significantly improving our knowledge when usꢀ
ing simple aryl methyl ethers as coupling partners via
C–OMe scission. As part of our ongoing interest in cataꢀ
lytic C–O functionalization reactions,10 we describe
herein a mild Niꢀcatalyzed ipso-silylation of aryl methyl
ethers en route to aryl silanes, privileged intermediates
of utmost relevance in organic synthesis and materials
science (Scheme 1, right pathway).11 This catalytic proꢀ
tocol is distinguished by its ligandless conditions12 and
wide substrate scope at room temperature with excepꢀ
tional ease, including the coupling of benzyl and vinyl
methyl ethers as well as the alwaysꢀelusive unbiased
anisole derivatives. The unique features of this transforꢀ
mation are illustrated in orthogonal silylation scenarios
as well as in further derivatization techniques. Initial
mechanistic studies reveal that a canonical catalytic
cycle consisting of an initial oxidative addition of the C–
OMe bond to Ni(0)Ln does not come into play, suggestꢀ
ing the involvement of Ni(0)ꢀate complexes.
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a
Reaction conditions: 1a (0.25 mmol), 2a (1.30 equiv)
mmol), Ni(COD)2 (1 mol%), KOtBu (2.20 equiv), PhMe
(0.20 M), 25 °C, 1.5 h. b GC yields using decane as internal
standard. c Isolated yield. d Zn (1.0 equiv).
Another noteworthy observation concerns the nature
of the base and escorting counterion. Specifically, a
comparison of entry 1 vs entries 8 and 9 showcases the
unique competence of K+, suggesting that the counterion
may play a more important role than initially anticipatꢀ
ed. These results are somewhat reminiscent of recent
literature data that demonstrates the nonꢀinnocent role of
countercations in reactions of aryl ethers,8aꢀ8c contribꢀ
uting to the perception that a “non-classical” scenario
might come into play. As shown in entries 10 and 11, no
reactivity was found for KOMe or KHMDS, evidencing
that a subtle balance of nucleophilicity and steric bulk of
the base is required. Although fluoride salts have shown
to be competent in related silylation events,10c no conꢀ
version to 3a was observed (entry 12). Equally striking
was the influence of the solvent utilized, as THF or
DME were equally unsuitable (entries 13 and 14). Simiꢀ
larly, highly polar solvents such as HMPA, previously
shown to effectively promote nucleophilic aromatic
silylation of aryl halides,18 failed to provide the targeted
3a (entry 15). It is worth noting that the reaction can be
efficiently conducted at 0 ºC,15 albeit prolonged reaction
times were required to reach full conversion (entry 16).
Interestingly, the use of disilanes as silicon sources reꢀ
sulted in no conversion of 1a to 3a (entry 17), showcasꢀ
ing the unique reactivity of 2a. Rigorous control experꢀ
iments in the absence of either Ni(COD)2 or KOtBu
revealed that these reaction parameters were crucial for
the C–OMe silylation to occur (entry 18).19
RESULTS AND DISCUSSION
We started our investigations by studying the reaction
of 1a with 2a, an easily accessible silyl boronate in bulk
quantities in a single step.13,14 After some experimentaꢀ
tion,15 we found that a simple cocktail containing
Ni(COD)2 (1 mol%) and KOtBu in PhMe at room temꢀ
perature provided the best results, affording 3a in 88%
isolated yield after 1.5 h (Table 1, entry 1). This outcome
was unexpected, as the available literature data on cataꢀ
lytic C(sp2)–OMe cleavage,2 particularly with heteroaꢀ
tomꢀbased nucleophiles, unanimously encouraged the
employment of particularly electronꢀrich ligands at eleꢀ
vated temperatures and higher catalyst loadings.5 As
shown in entries 2 and 3, Ni precatalysts other than
Ni(COD)2 resulted in lower conversions to 3a, even at
higher catalyst loadings. Although tentative, these results
suggest that COD may serve as nonꢀinnocent ancillary
ligand within the catalytic cycle.16 As shown in entry 4,
2ꢀnaphthyl ethyl ether provided lower yields of 3a, indiꢀ
cating an intimate interplay of reactivity with steric efꢀ
fects.17 As expected, the basic conditions employed reꢀ
sulted in quantitative formation of 2ꢀnaphthol via Cꢀ
(acyl)–O cleavage when using 2ꢀnaphthyl pivalate as
substrate (entry 5). Although the use of PCy3 or bulky N-
heterocyclic carbenes have shown to be critical in other
C–heteroatom bondꢀformations,5 the inclusion of these
ligands was inconsequential (entries 6 and 7).
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Table 2. Silylation of πꢀExtended Aryl Methyl Ethers.a,b formations.5 Importantly, the reaction of 1a can be conꢀ
1
ducted at 10 mmol scale without significant erosion in
yield (3a, 79%). Substrates bearing more than one methꢀ
oxy group could be subject to monoꢀ or double silylation
by carefully adjusting the stoichiometry of the reaction
(3m, 3o). Electronically unbiased substituents at ortho
position to the reactive site do not impede the silylation
(3i), albeit moderate yields were obtained in these cases.
While no competitive silaboration, silylation or isomeriꢀ
zation was observed at the alkene terminus in 3k,14 conꢀ
jugated terminal alkenes resulted in a formal hydrosiꢀ
lylation (1v). The presence of ketones (1u) or pyrazole
residues do interfere (1x); while silyl anion attack across
the carbonylꢀoxygen double bond accounts for the forꢀ
mer,20 the lack of reactivity of the latter is attributed to
irreversible binding to Ni(0). Unfortunately, the presence
of acidic hydrogens was not tolerated, even with excess
amounts of KOtBu, recovering 1y unaltered. Interestingꢀ
ly, bisꢀsilylation at the C–Br and C–O bond was mainly
observed with 1w, a result that is in contrast with the
borylation of C–halogen bonds with silyl boronates.21 As
shown for 3qꢀ3s, the reaction could be extended to benꢀ
zylic C(sp3)–OMe bonds with similar ease. It is worth
mentioning that significant amounts of C(sp2)–H silylaꢀ
tion were observed when exposing 1q or 1s in the abꢀ
sence of Ni(COD)2, an observation that is consistent
with a recent C–H silylation mediated by KOtBu.22
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Table 3. Silylation of NonꢀπꢀExtended Backbones and
Vinyl Ethers.a,b
a As Table 1 (entry 1). b Isolated yields, average of at least
c
d
two independent runs. 1a (10 mmol). Ni(COD)2 (10
mol%) at 95 ºC. e 40 ºC. f Ni(COD)2 (5 mol%). g Ni(COD)2
h
i
(10 mol%). 1o:2a = 3.3:1 ratio. 2a (2.60 equiv), KOtBu
(4.40 equiv). j 2a (1.75 equiv), KOtBu (4.50 equiv). k Using
dibenzofuran as substrate at 40 ºC.
With robust conditions in hand, we turned our attenꢀ
tion to examine the preparative scope and limitations of
our Niꢀcatalyzed ipso-silylation of aryl methyl ethers.19
As shown in Table 2, a host of differently substituted πꢀ
extended aryl methyl ethers reacted perfectly well, inꢀ
cluding those bearing aliphatic ethers (3f), silyl ethers
(3g), thioethers (3h), amines (3j, 3l), or even heteroaroꢀ
matic rings (3p), delivering the targeted aryl silanes in
good to excellent yields. Extensions to silyl boronates
other than 2a required higher catalyst loadings and eleꢀ
vated temperatures (3bꢀ3d), an observation likely atꢀ
tributed to a combination of optimal steric and electronic
effects. As shown for 3t, the C(sp2)–O bond of dibenzoꢀ
furan posed no problems in the silylation event. Particuꢀ
larly noteworthy was the observation that low catalyst
loadings generally suffice to effect the silylation at room
temperature, suggesting that the reactivity of our active
Ni catalyst might surpass that of related NiLn species (L
= PCy3, NHC) used in previous C–heteroatom bondꢀ
a As Table 1 (entry 1). b Isolated yields, average of at least
two independent runs. c 2a (2.0 equiv), KOtBu (6.50 equiv).
e
f
g
d Ni(COD)2 (10 mol%). Ni(COD)2 (5 mol%). 50 ºC.
Using (Z)ꢀ4i. h E:Z = 20:1, using 4j (E:Z = 1.5:1).
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Scheme 3. Synthetic Applicability
Page 4 of 8
As judged by the wealth of literature data, πꢀextended
1
arenes have routinely been employed in C–OMe funcꢀ
tionalization,2,6 providing consistently higher rates than
regular anisole derivatives. Such difference in reactivity
likely reflects the better ability of πꢀextended arenes for
η2ꢀcoordination to low valent complexes, and/or the
ability to undergo other dearomatization pathways
(Scheme 2).7,23 Despite the greater accessibility of simꢀ
ple anisole derivatives,2 their use in crossꢀcoupling reacꢀ
tions is not as commonly practiced as one might anticiꢀ
pate. Indeed, the crossꢀcoupling of unbiased anisoles
lacking ortho- or para-activating groups to the methoxꢀ
ide function have only been possible within the realm of
C–C bondꢀformations using stoichiometric organometalꢀ
lic species, typically using high temperatures and/or
electronꢀrich ligands.24 Therefore, it is a worthwhile
endeavor to search for alternate catalytic techniques that
would allow to expand our current crossꢀcoupling portꢀ
folio of unbiased anisole derivatives, particularly in C–
heteroatom bondꢀformations, and under mild conditions.
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Scheme 2. Polyarenes vs Regular Arenes.
As shown in Table 3, a variety of regular anisole deꢀ
rivatives could be coupled under otherwise identical
conditions to those shown in Table 2.19 In contrast to the
prevailing perception that ortho- or para-activating
groups generally provide higher yields when coupling
anisole derivatives,2 we found that 5b resulted in lower
yields than regular anisole (5a).25 Particularly intriguing
was that 4m, previously shown to be suited for C–OMe
cleavage,6f remained unaltered, suggesting that the pyraꢀ
zole motif competes with substrate binding. Strikingly,
unactivated anisoles possessing electronꢀrich groups at
the para position could equally be silylated in good
yields (5f, 5g). These results are noteworthy taking into
consideration the higher activation energy required for
C–OMe cleavage in electronꢀrich anisoles,2 as well as
the lack of reactivity of 4n, even under more forcing
conditions.26 These results challenged the conviction
that electronic effects on the arene are the only factor
coming into play when tackling C–OMe cleavage. Unꢀ
fortunately, no reaction took place with substrates posꢀ
sessing terminal alkynes (4l) whereas TMSꢀprotected
acetylenes resulted in competitive silyl attack to the
ethynyl moiety. Although no C(sp2)–O cleavage ocꢀ
curred in dihydrofuran (4n) or furan, we found that our
protocol could be extended to acyclic vinyl ethers (5i,
5j), thus giving access to synthetically versatile vinyl
silanes11 with an exquisite stereoselectivity profile reꢀ
Encouraged by our findings, we anticipated that orꢀ
thogonal silylation strategies could be within reach when
using aryl methyl ethers as substrates. As shown in
Scheme 3 (top), exposure of 6 to [Rh(coe)2OH]2/L1 and
Me(OTMS)2SiH resulted in a regioselective C(sp2)–H
silylation at the lessꢀelectron rich aromatic ring.28 In
contrast, KOtBuꢀmediated C(sp2)–H silylation with
Et3SiH predominantly led to 7b in 25% yield,22 together
with traces amounts of benzylic silylation.15 This result
could be significantly improved via ortho-metalation
with nBuLi followed by Me3SiCl quench (7a).29 To put
these results into perspective, we found that 9 was exꢀ
clusively formed under our silylation conditions. Taken
together, these results stand as a testament to the potenꢀ
tial of our silylation protocol, representing a compleꢀ
mentary reactivity mode to “classical” orthoꢀ
metalation29 or modern catalytic C–H silylation techꢀ
niques.30 The synthetic applicability of our Niꢀcatalyzed
silylation of aryl methyl ethers is further illustrated in
Scheme 3 (bottom). As shown, 1a could be smoothly
converted into 10ꢀ12, hence representing a formal ipso-
halogenation of aryl methyl ethers under mild condiꢀ
tions. Furthermore, we found that the corresponding
triethylaryl silanes could successfully be engaged in a
Pdꢀcatalyzed oxidative C(sp2)–H functionalization of
benzothiophene, giving rise to 14 in good yield and
excellent regioselectivity at the βꢀposition.31
gardless of the configuration of the substrate employed 27
.
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The unprecedented reactivity observed at room temꢀ
was also ruled out. Such assumption is based on the
observed chemoselectivity (3k, Table 2 and 15, Scheme
4) and the lack of inhibition in the presence of exogeꢀ
neous 1ꢀhexenes or cyclooctenes that would otherwise
have resulted in the addition of triethyl silyl radicals to
unsaturated C–C bonds (Scheme 4, bottom).39
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perature in the absence of electronꢀrich ligands left a
reasonable doubt that a mechanism consisting of an
oxidative addition of the C–OMe bond to Ni(0) species
would be operative. This interpretation gains credence
by the stunning dichotomy exerted by the corresponding
countercations (Table 1, entries 8ꢀ12), the excellent reacꢀ
tivity found for particularly electronꢀrich anisole derivaꢀ
tives (Table 3, 5f and 5g), and the formidable activation
barrier known for oxidative addition of the C(sp2)–OMe
bond to electronꢀrich Ni(0) complexes.8b,8d Prompted by
the high reactivity found in the absence of added ligꢀ
ands, we turned our attention to mercury poisoning exꢀ
periments to identify whether heterogeneous systems
participate or not.32 Interestingly, the reaction of 1a with
2a was not inhibited in the presence of Hg(0).15 This
observation, together with the absence of heterogeneous
metal particles found by TEM analysis of different aliꢀ
quots during the course of the reaction and the lack of an
induction period en route to 3a provided compelling
evidence for a homogeneous catalyst.15,33 Atlhough one
might argue that our results could also be interpreted on
the basis of an ortho C(sp2)–H silylation21 followed by
C–OMe bondꢀhydrogenolysis,34,35 or benzyneꢀtype inꢀ
termediates,36 we believe that these particular manifolds
are highly unlikely. This view is supported by the fact
that exclusive ipso-silylation was observed for 3i (Table
2) or 5dꢀ5e (Table 3), among others.
Scheme 5. Intermediacy of Silyl Anionic Species.
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At present, we propose that KOtBu mediates the conꢀ
version of 2a into either Et3SiK or a silylborate of the
formal composition [Et3Si–BPin(OtBu)]K that might act
as silyl anion surrogate.40 The participation of silyl aniꢀ
onic species was indirectly corroborated by observing
exclusively 19 from either 17 or 18 in the presence or
absence of Ni(COD)2, as silyl anions are known to proꢀ
mote nucleophilic attack on the bromine atom, generatꢀ
ing a transient carbon nucleophile that reacts preferenꢀ
tially with the boryl unit (Scheme 5).21 Additional supꢀ
port for silyl anions came from the observation that
PhSiMe2Li (2e) was found to be competent as reaction
intermediate en route to 3a, either in the absence or
presence of KOtBu or even LiCl, albeit in lower yields
(34%).15 The lower reactivity of PhSiMe2Li is tentativeꢀ
ly ascribed to its partial decomposition, the use of THF
as coꢀsolvent (2e consists of a 1M THF solution) or the
lower nucleophilicity of PhSiMe2Li vs Et3SiK.
Scheme 4. Investigations into Radicalꢀtype Pathways.
Scheme 6. Ni(0)ꢀSilyl Ate Species & Mechanistic Rationale.
A priori, the data provided in Tables 2 and 3 do not alꢀ
low us to rigorously rule out whether single electron
transfer (SET) processes intervene or not, as electronꢀ
rich Ni(0) species could also act as electron donors,3
leading to carbonꢀcentered radicals. As shown in Scheꢀ
me 4 (top), 15 was converted exclusively to 16 regardꢀ
less of the concentration of Ni catalyst utilized, thus
leaving a reasonable doubt about a radical scenario that
would have triggered a 5ꢀexoꢀtrig cyclization followed
by ringꢀopening of the cyclopropyl motif. The intermeꢀ
diacy of triethylsilyl radicals generated by either hoꢀ
molysis of Si–B bond37 or from silyl anions via SET38
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Funding Sources
Based on our empirical data, we currently favor a
mechanistic rationale consisting of the intermediacy of a
discrete [Ni(COD)SiEt3]K complex (20) that might be
generated in situ upon exposure of Ni(COD)2 to 2a and
KOtBu (Scheme 6, top pathway). This conclusion is
reminiscent of the structurally wellꢀdefined, yet excepꢀ
tionally sensitive, Ni(0)ꢀate complex 21 reported by
Pörschke that can be obtained by reaction of Ni(COD)2
with MeLi in ethylene atmospheres, the identity of
which was rigorously proven by Xꢀray diffraction analyꢀ
sis.41,42 Unfortunately, and despite extensive experimenꢀ
tation, isolation of 20 or direct spectroscopic evidence
for such putative intermediate was not possible, probaꢀ
bly due to its expected exceptional sensitivity and limꢀ
ited lifetime. In a formal sense, the reaction of 20 with
an aryl methyl ether could be visualized as an internal
metalꢀcatalyzed nucleophilic aromatic substitution asꢀ
sisted by complexation of the K+ counterion with the
lone pair of the ethereal oxygen atom (Scheme 6, path
a).43 Alternatively, it couls also be envisioned a “non-
classical” oxidative addition of the C(sp2)–OMe bond
via Ni(0)ꢀate complexes assisted by the K+ counterion
(Scheme 6, path b).8b,44 These interpretations could exꢀ
plain the strikingly different behavior found for potassiꢀ
um counterions, in which a Lewis acid might aid the
targeted C–O cleavage.8,44,45
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No competing financial interests have been declared.
ACKNOWLEDGMENT
We thank ICIQ, European Research Council (ERCꢀ277883)
and MINECO (CTQ2015ꢀ65496ꢀR & Severo Ochoa Acꢀ
creditation SEVꢀ2013ꢀ0319) for support. Dr. M. N. thanks
Cellex Foundation and Dr. X. Caldentey for the HTEꢀICIQ
unit. Johnson Matthey, Umicore and Nippon Chemical
Industrial are acknowledged for a gift of metal & ligand
sources. C. Z. thanks MINECO for a FPU fellowship.
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CONCLUSIONS
In summary, we have documented a surprisingly facile
Niꢀcatalyzed silylation of aryl methyl ethers that operꢀ
ates without added dative ligands. This protocol is disꢀ
tinguished by its mild conditions and wide scope, inꢀ
cluding the alwaysꢀelusive unbiased anisole derivatives.
Although additional investigations at a molecular level
are warranted to shed light on the intermediacy of shortꢀ
lived and, most likely, exceptionally sensitive Ni(0)ꢀate
entities, we believe that our study constitutes an imꢀ
portant step towards a more prolific use of readily availꢀ
able aryl methyl ethers as coupling partners. The role
exerted by the escorting counterion in the targeted C–
OMe bondꢀcleavage deserves particular attention, as it
may set the basis for systematically examining the influꢀ
ence of a priori innocent additives in related crossꢀ
coupling reactions. That being set, we anticipate that this
strategy will foster further explorations of related transꢀ
formations via C–OMe scission
.
ASSOCIATED CONTENT
Supporting Information. Experimental procedures and
spectral data. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
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2015, 21, 13904. (c) Cornella, J.; Martin, R. Org. Lett. 2013,
* rmartinromo@iciq.es
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