Stable Carbocation Generated via 2,5-Cyclohexadien-1-one Protonation

Article abstract of DOI:10.1021/acs.joc.7b02668

Protonation of a substituted cyclohexadien-1-one (1) leads to the generation of carbocation [3]⁺, capable of effecting hydride abstraction and oxidation reactions. The molecular structure of [3]⁺ shows it to be structurally similar to [(p-MeO-C₆H₄)Ph₂C]⁺. The ability to easily access [3]⁺ from stable and available precursors, such as 1 and commercially available acids, may allow a wider application of the growing number of trityl-based reactions in organic syntheses.

Full text of DOI:10.1021/acs.joc.7b02668

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Note
A stable carbocation generated via 2,5-cyclohexadien-1-one protonation
Craig Fraser, and Rowan D. Young
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.7b02668 • Publication Date (Web): 05 Dec 2017
Downloaded from http://pubs.acs.org on December 5, 2017
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Page 1 of 5
The Journal of Organic Chemistry
A stable carbocation generated via 2,5-cyclohexadien-1-one proto-
nation
1
2
3
4
5
6
7
8
9
Craig Fraser and Rowan D. Young*
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543
+
ABSTRACT: Protonation of a substitututed cyclohexadienꢀ1ꢀone (1) leads to the generation of carbocation [3] , capable of effectꢀ
+
ing hydride abstraction and oxidation reactions. The molecular structure of [3] shows it to be structurally similar to [(pꢀMeOꢀ
+
+
C H )Ph C] . The ability to easily access [3] from stable and available precursors, such as 1 and commercially available acids, may
6
4
2
allow wider application of the growing number of trityl based reactions in organic syntheses.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Introduction: Protonated oxonium salts play a pivotal role in
Table 1. Selected bond lengths and angles from molecular structures
8
a range of catalytic cycles. In particular, the Brønsted acid
catalyzed reduction of ketones and the Diels Alder reaction
utilizing enone substrates invoke protonated keto oxonium
of compounds 1, 2 and [3][B(3,5ꢀ(CF
3 2
) C
6
H
[3]
1.339(3)
1.423(3)
1.457(3)
1.455(3)
3
)
4
].
+
Bond lengths (Å)
O1ꢀC14
1
2
1.233(1)
1.380(2)
1.483(2)
1.488(2)
1.378(3)
1.536(3)
1.528(3)
1.529(3)
1
C1ꢀC11
intermediates. Although considered highly acidic, many exꢀ
3
C1ꢀC21
amples of isolated sp oxonium salts exist, however, structures
C1ꢀC31
2
of protonated sp oxonium salts are exceedingly rare. Indeed,
Bond angles (°)
structures of nonꢀstablised protonated ketones are not reported.
However, stabilised protonated ketones, such as benzopheꢀ
Σ angles around C1
360.0(2)
360.0(4)
337.8(3)
2
none, or cyclopropanones, are known.
Valid resonance structures of protonated ketones can be postuꢀ
2
lated that represent either a protonated sp oxygen atom, or a
2
hydroxyl motif bound to an sp carbocation, this concept is
perhaps most ideally exemplified in zwitterionic structures of
3
sulfonefluorescein derivatives. Herein, we report the generaꢀ
tion of a stable protonated ketone of the latter form, where
aromatic stablisation allowed delocalization of the carbocation
to generate a trityl analogue (Figure 1).
+
Figure 1. Protonation of 1 generates [3] with possible oxonium and
+
carbocation resonance structures. [3] is analogous to paraꢀ
+
methoxyphenyl substituted trityl, [(MeO)Tr] .
Cyclohexadienꢀ1ꢀone 1, is readily prepared by oxidation of
3
,5ꢀdiꢀtertꢀbutylꢀ4ꢀhydroxyphenylꢀdiphenylꢀmethane
(2),
which is in turn generated from cheap base materials (viz. 2,6ꢀ
4
diꢀtertꢀbutylꢀphenol and benzophenone). Many studies conꢀ
cerning 1, or related quinodal ketones (Fuchsones), have been
conducted regarding their reactivity, and their ability to act as
5
dyes and stabilize radicals. 1 can be considered a hybrid of a
Figure 2. Molecular structures of compounds 1, 2 and cation fragꢀ
ment of [3][B(3,5ꢀ(CF ) C H ) ]. Hydrogen atoms except H1 and H11
quinone and Thiele’s hydrocarbon, with aromatic stabilized
resonance forms of diꢀradical and zwitterion possible.
3
2
6
3 4
omitted, 50% thermal ellipsoids. Selected bond lengths and angles
given in Table 1.
Indeed, the pK of 1 was determined to be significantly lower
b
than that expected for a ketone, with stoichiometric oxonium
F
4
F
4
ꢀ
tatively protonate 1 to generate the resonance stabilized carboꢀ
acid ([H(OEt ) ][BAr ], BAr = [B(C F ) ] ) able to quantiꢀ
+
2
2
6 5 4
cation [3] (See SI).
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Table 2. Optimisation of the transfer dehydrogenation of 4a to generꢀ
ate toluene (5a).
Table 3. Reaction scope of the oxidation of various substrates using 1
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
F
and [H(OEt ) ][BAr ].
2
2
4
Entry
Acid
ꢀ
ꢀ
Solvent
DCE
MeCN
DCE
MeCN
DCE
MeCN
DCE
MeCN
DCE
MeCN
DCE
MeCN
DCE
Yield (%)
0
0
1
2
3
4
5
F
F
[H(OEt
[H(OEt
pꢀTsOH
pꢀTsOH
TfOH
TfOH
MsOH
MsOH
2
)
)
2
][BAr
][BAr
4
4
]
]
100
100
100
37
81
100
100
22
10
10
0
2
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
b
6
7
b
8
9
b
1
1
1
0
1
2
b
b
c
CF
CF
3
COOH
COOH
3
F
4
13
[H(OEt
2
)
2
][BAr
]
General conditions: 1 mmol of 1, 1 mmol of 4a, 0.1 mmol (10 mol%) of acid and
1
mL solvent heated at 90 °C for 11 h. Yields were determined by GCꢀMS (decꢀ
b
c
ane used as internal standard). Heated for 20 h. DCE = 1,2ꢀdichloroethane.
Reaction run in absence of 1.
+
13
In CD Cl , [3] displays a characteristic methanide C NMR
2
2
+
resonance at 199.5 ppm (cf [Ph C] 211.3 ppm). Attempts to
3
F
4
recrystallize [3][BAr ] resulted in biphasic mixtures. Howevꢀ
ꢀ
er, employing the related borate anion [B(3,5ꢀ(CF ) C H ) ] ,
3
2
6
3 4
recrystallisation of [3][B(3,5ꢀ(CF ) C H ) ] was achieved from
3
2
6
3 4
hexane diffusion into a saturated DCM solution, allowing the
+
structural characterization of [3] (Figure 2). Comparison of
+
+
the structures of 1, 2 and [3] in Table 1 shows that [3] mainꢀ
tains a trigonal planar central carbon atom [Σ angles subtendꢀ
ing C1 = 360.0(4)°]. Protonation of the quinoidal ketone reꢀ
sults in lengthening of the C14ꢀO1 bond in 1 from 1.233(1) Å
+
+
to 1.339(3) Å in [3] (cf 1.378(3) Å in 2). Structurally, [3] is
+
6
similar to [(MeO)Tr] (Figure 1), which has been reported as
an active Lewis acid catalyst for a variety of reactions.
7
Compound 1 was found to act as a highly protected base,
proving resistant to methylation and silylation, with only samꢀ
ples of [3][H(OTf) ] (also structurally characterized, see SI)
2
recovered from attempted reactions with MeOTf and TMSOTf
(presumably from low concentrations of adventitious water
8
introduced during analysis/attempted isolation).
+
Addition of an equivalent of PPh to [3] led to a dynamic
equilibrium of [HPPh ] and [3ꢀPPh3] in a 4:3 ratio. [3ꢀ
PPh ] , where the central carbon of [3] is bonded to PPh ,
3
+
+
F
2 2 4
General conditions: 1 mmol of 1, 1 mmol of 4, 10 mol% [H(OEt ) ][BAr ] and 1
3
+
+
mL DCE heated at 90 °C for 12 h. Yields (of oxidation products) were determined
by GCꢀMS (decane as internal standard), yields using DDQ as an oxidant are in
3
3
a
was identified by spectroscopic comparison to [Ph Cꢀ
parentheses for selected substrates. Heated for 48 h.
3
F
4
9
+
PPh ][BAr ]. The ability of PPh to interact with [3] as a
3
3
+
Table 4. Transfer dehydrogenation of 9g catalysed by H /(1 or 2).
Lewis acid and as a Brønsted acid is also highlighted in this
equilibrium. With this in mind, we tested 1 as a H acceptor in
2
a Brønsted acid catalyzed reaction that likely proceeds via
+
[
3] .
Brønsted acid transfer hydrogenation via carbocation intermeꢀ
Entry
Condition
Yield
45%
44%
< 1%
diates has been well explored using specific organic H donors
2
F
F
1
0
1
2
3
A: 10 mol % 1, 10 mol % [H(OEt
B: 10 mol % 2, 10 mol % [H(OEt
C: No additives
2
2
)
)
2
][BAr
][BAr
4
4
]
]
such as the Hantzsch ester or 1,4ꢀcyclohexadienes. However,
to our knowledge, this is the first exploration of Brønsted acid
catalyzed hydrocarbon dehydrogenation (i.e. using a sacrificial
2
General conditions: 1 mmol of 1, 1 mmol of 4g, and 1 mL DCE heated at 90 °C
for 16 h. Yields of 5g were determined by GCꢀMS (decane as internal standard).
11
H acceptor). In contrast to many organic H donors that gain
2
2
aromatic stabilization upon loss of H (e.g. Hantzsch ester,
2
acceptor, generating 2. Compound 1 was benchmarked against
2,3ꢀdichloroꢀ5,6ꢀdicyanoꢀ1,4ꢀbenzoquinone (DDQ) as an oxiꢀ
dation reagent. DDQ is an effective stoichiometric oxidation
reagent and considered the ‘Gold Standard’ for chemical oxiꢀ
dants, however, polychlorinated phenol byꢀproducts are highly
1
,4ꢀcyclohexadienes), compound 1 gains aromatic stabilizaꢀ
tion upon acceptance of H to form 2.
2
Compound 1 was shown to be an effective stoichiometric deꢀ
hydrogenation reagent with catalytic amounts of Brønsted
acid. A series of Brønsted acid catalysed transfer hydrogenaꢀ
tion reactions exemplified the ability of 1 to act as a hydrogen
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The Journal of Organic Chemistry
toxic, and DDQ is unstable in water, evolving hydrogen cyaꢀ
All manipulations of airꢀsensitive compounds were carried out
under a dry and oxygenꢀfree nitrogen atmosphere using standꢀ
ard Schlenk and glove box techniques. Reactions were perꢀ
formed in a J. Young NMR tube or in a 4mL reaction vial with
septum cap in a nitrogen atmosphere glovebox. Glassware
were flameꢀdried under vacuum prior to use. All solvents,
including deuterated NMR solvents were distilled, degassed
and dried with calcium hydride before use. NMR spectra were
recorded at 25 °C on Bruker Avance 400 MHz or Bruker
1
2
3
4
5
6
7
8
9
nide.
Transfer hydrogenation was optimized for the dehydrogenaꢀ
tion of 1ꢀmethylꢀ1,4ꢀcyclohexadiene (4a) to toluene (Table 2).
Acids with pKa ca ꢀ2 (or lower) were found to be efficient at
catalyzing the transfer hydrogenation, while trifluoroacetic
acid (pKa = 0.23) performed poorly, with only 10% converꢀ
sion after 20 h (Table 2, entries 11 and 12). The ability of
many acids to catalyse the oxidation was found to be solvent
dependent, which is somewhat expected given the solvent
aq
aq
1
AMX 500 MHz spectrometers. The chemical shifts (δ) for H
13
dependent nature of pK . This may account for the poor perꢀ
NMR and C NMR spectra are given in ppm relative to residꢀ
a
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
formance of sulfonates MsOH and pꢀTsOH in MeCN (Table 2,
entries 6 and 10), and of trifluoroacetic acid (Table 2, enꢀ
ual signals of the solvent. All GCꢀMS studies were performed
on an Agilent GC/MS (Agilent 7890A GC/Agilent 5975C MS)
system. HRMS (ESIꢀTOF) spectra were obtained using an
Agilent Technologies 6230 TOF. Commercially available
try12), given it has a relatively high pK in acetonitrile
a
MeCN
MeCN
+
12
{pKa
(TFA) = 12.65, cf pKa
([HPPh ] ) = 8.0}. The
3
F
F
poorer performance of entries 5ꢀ12 in Table 2 may also reflect
the higher nucleophilicty of the conjugate bases for the acids
used in these reactions, which is perhaps exemplified by the
chemicals were used as purchased. [H(OEt ) ][BAr ] ([BAr ]
2 2 4 4
= [B(C F ) ]) was synthesised according to literature proceꢀ
6
5 4
14
dures.
poorer performance of TfOH as compared to [H(OEt )ꢀ
2
F
4
2]
[BAr ] (Table 2, entries 3,4,7 and 8) with alkyl triflates genꢀ
Synthesis of compound 1
1 was synthesised according to the literature procedure. Data
for 1 matched those reported. Yield 0.71 g, 72%. H NMR
15
erally considered more stable than alkyl etherate oxonium
salts.
A control reaction in the absence of acid gave no toluene
product (Table 2, entries 1 and 2), ruling out a radical pathway
as is seen with quinone type oxidants.
1
(CD Cl ): δ 7.43 (d, 4 H, J = 2.0 Hz), 7.41 (d, 2 H, J = 2.0
2
2
1
3
Hz), 7.25ꢀ 7.23 (m, 4 H), 7.19 (s, 2 H), 1.22 (s, 18 H).
C
NMR (CD Cl ): δ 186.6 (s, 1 C), 156.6 (s, 1 C), 147.9 (s, 2 C),
2
2
The optimized transfer hydrogenation protocol using 1 was
extended to other hydrocarbons, alcohols and heterocycles
141.5 (s, 2 C), 132.5 (s, 2 C), 132.4 (s, 4 C), 130.4 (s, 1 C),
129.7 (s, 2 C), 128.6 (s, 4 C), 35.8 (s, 2 C), 29.8 (s, 6 C).
(
Table 3). Generally, 1 was outperformed by DDQ, however,
the oxidation of 4c-e to paraꢀcymene was found to be much
more effective using 1. The combination of and
H(OEt ) ][BAr ] was also able to dehydrogenate alcohols. It
Synthesis of compound 2
2 was synthesised according to the literature procedure. Data
for 2 matched those reported. Yield 1.33 g, 70%. H NMR
2
1
F
4
1
[
2
2
was found that this reaction competed with acid catalyzed
dehydration of alcohols capable of forming stable carboꢀ
cations. As such, in the case of substrate 4l, loss of water led
to 1,2ꢀdihydronaphthalene as the dominant product, whereas
substrate 4k formed small amounts of anhydride products in
addition to benzaldehyde as the major product.
(CD Cl ): δ 7.30ꢀ7.26 (m, 4 H), 7.21ꢀ7.17 (m, 2 H) 7.14ꢀ7.12
2
2
(m, 4 H), 6.95 (s, 2 H), 5.43 (s, 1 H), 5.11 (s, 1 H), 1.36 (s, 18
13
H); C NMR (CDCl ): δ 152.1 (s, 1 C), 144.8 (s, 2 C), 135.4
3
(s, 2 C), 134.1 (s, 1 C), 129.4 (s, 4 C), 128.1 (s, 2 C), 126.1 (s,
4 C), 126.0 (s, 2 C), 56.8 (s, 1 C), 34.3 (s, 2 C), 30.3 (s, 6 C).
Crystal data for C H O , M = 372.55, monoclinic, C 2/c (No.
27
32
1
The dehydrogenation could be performed catalytically in 1 or
15), a = 19.8182(11), b = 5.9878(3), c = 36.1268(17) Å, α =
3
2
when excess manganese oxide was employed (Table 4). No
90, β = 96.953(4), γ = 90°, V = 4255.5(2) Å , Z = 8, δ
=
calc
ꢀ
3
ꢀ1
conversion was observed when 1 or 2 were absent (i.e. MnO2
could not independently oxidise 4g under these conditions).
Such an approach has been previously reported using catalytic
1.163 Mgm , ꢀ(Cu Kα) = 0.517 mm , T = 100(2) K, colourꢀ
less block, 0.1 x 0.1 x 0.1 mm, 21,810 reflections collected,
3
,766 unique data (2θ ≤ 133.4°), R = 0.0557 [for 2,514 reflecꢀ
1
13
DDQ. Under such conditions, it was found that yields were
tions with I > 2σ(I)], wR = 0.1333 (all data), 253 parameters,
2
+
similar to when stoichiometric 1 was used. Given that [3] is
S = 1.01.
suspected as the active catalyst, a more favorable comparison
can be made when employing DDQ in catalytic amounts, with
similar turnꢀover rates reported for the oxidation of hydrocarꢀ
F
4
Synthesis of compound [3][BAr ]
To a solution of compound 1 (0.020 g) in DCM or MeCN (0.5
13
F
bons as is observed in Table 4.
mL) was added [H(OEt ) ][BAr ] (0.055 g). The orange soluꢀ
2
2
4
1
In conclusion, we have demonstrated the concept of formation
tion turned red immediately. H NMR showed the generation
of [3] to be quantitative. Characterisation was performed diꢀ
+
of a stable, persistent Lewis acid from the combination of 1
+
and suitable Brønsted acids. The resultant Lewis acid [3] was
rectly on the DCM and MeCN solutions. Attempts to crystalꢀ
found to facilitate oxidation reactions with a variety of hydroꢀ
carbons and alcohols.
lise the title compound resulted only in biphasic solutions.
1
H NMR (CD CN): δ 9.23 (s (br), 1 H), 8.01 (t, 2 H, J = 7.0
3
Given the diverse reactions that trityl cations are known to
Hz), 7.74 (t, 4 H, J = 7.0 Hz), 7.63 (s, 2 H), 7.52 (d, 4 H, J =
+
1
partake in, it is hoped that in situ generated [3] may be emꢀ
7
7
7
.0 Hz), 1.40 (s, 18 H); H NMR (CD Cl ): δ 7.59 (t, 2 H, J =
2
2
ployed as an easy to prepare trityl substitute for a range of
catalyzed or stoichiometric reactions. This concept may also
be extended to a range of other highly available precursors that
are bench stable and able to form carbocation Lewis acids
upon protonation (e.g. phenolphthalein, Fluorescein).
.3 Hz), 7.50 (t, 4 H, J = 7.3 Hz), 7.31 (d, 4 H, J = 7.3 Hz),
.30 (s, 2 H), 6.40 (s, (br, 1 H), 1.28 (s, 18 H); C NMR
13
(
CD CN): δ 199.1 (s, 1 C), 174.5 (s, 1 C), 149.1 (d (br), 8 C, J
3
=
237.7 Hz), 145.0 (s, 2 C), 141.8 (s, 1 C), 140.6 (s, 2 C),
139.9 (s, 4 C), 139.3 (d (br), 4 C, J = 241.3 Hz), 139.1 (s, 2 C),
137.3 (d (br), 8 C, J = 244.7 Hz), 134.4 (s, 1 C), 130.3 (s, 4 C),
Experimental
3
5.6 (s, 2 C), 29.7 (s, 6 C); ESIꢀTOFꢀMS (m/z): 371.2362
(
calc. for C H O, 371.2375).
27 31
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To an ovenꢀdried 4 mL glass vial in a glove box was added
1,2ꢀdichloroethane (1 mL), substrate (4g) (1 mmol), MnO (6
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Preparation of [3][B(3,5-(CF ) C H ) ] for crystallographic
3
2
6
3 4
2
study
To a solution of [3][B(3,5ꢀ(CF ) C H ) ] in DCM, prepared as
equiv.), and additives according to conditions AꢀC (see beꢀ
low). The reaction was heated for 16 hours. The reaction mixꢀ
ture was then washed with CH Cl to a 5 mL volumetric flask,
3
2
6
3 4
F
4
above for [3][BAr ], hexane was added slowly to form a
2
2
layered sample. Slow diffusion at room temperature afforded
followed by extraction of a 1 mL aliquot for GCMS analysis.
The conversion of substrate was determined by the integral
ratio of substrate and the product relative to the internal standꢀ
ard.
1
crystals suitable for an Xꢀray diffraction study. H NMR
(
CD Cl ): δ 8.03 (t, 2 H, J = 7.5 Hz), 7.76 (t, 4 H, J = 7.5 Hz),
2 2
7
.74 (s, 8 H), 7.63 (s, 2 H), 7.57 (s, 4 H), 7.51 (d, 4 H, J = 7.5
F
Hz), 5.34 (s, 1 H – shoulder on DCM signal), 1.48 (s, 18 H);
Condition A: 10 mol % 1, 10 mol % [H(OEt ) ][BAr ]
2
2
4
13
F
C NMR (CD Cl ): δ 199.5 (s, 1 C), 173.6 (s, 1 C), 162.4 (q, 4
Condition B: 10 mol % 2, 10 mol % [H(OEt ) ][BAr ]
2
2
2 2 4
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
C, JBC = 50.0 Hz), 144.0 (s, 4 C), 141.6 (s, 2 C), 139.9 (s, 2 C),
139.6 (s, 4 C), 139.5 (s, 8 C), 135.4 (s (br), 8 C), 134.1 (s, 1
C), 130.3 (s, 8 C), 129.5 (q, 8 C, JFC = 50.0 Hz), 126.3 (s, 2
C), 124.1 (s, 2 C), 118.1 (s, 4 C), 35.4 (s, 2 C), 30.0 (s, 6 C).
Crystal data for C H B Cl F O , M =1319.68, triclinic, Pꢀ1
Condition C: No additives
Procedure for the reaction of [3][BAr ] with PPh
F
4
3
To an ovenꢀdried J. Young's NMR tube in a glove box was
added dichloromethaneꢀd2 (0.5 mL), 1 (0.03 mmol) and
60 45
1
2
24
1
F
(
1
=
No. 2), a = 13.181(3), b = 13.877(3), c = 16.365(4) Å, α =
[H(OEt
2
)
2
][BAr
4
] (0.03 mmol). The reaction NMR tube was
3
1
01.985(7), β = 96.122(7), γ = 90.289(8)°, V = 2910.5(6) Å , Z
first subjected to screening of the initial H spectrum. One
equivalent of PPh was then added to the NMR tube, the tube
was shaken repeatedly over a few minutes, and then the samꢀ
ples were analysed by H and P NMR spectroscopy. Excess
,6ꢀlutidine was added to the tube to regenerate PPh and 1.
ꢀ
3
ꢀ1
2, δcalc = 1.506 Mgm , ꢀ(Mo Kα) = 0.230 mm , T = 100(2)
3
K, orange block, 0.1 x 0.1 x 0.1 mm, 56,248 reflections colꢀ
1
31
lected, 12,872 unique data (2θ ≤ 55°), R = 0.0543 [for 8,005
1
2
reflections with I > 2σ(I)], wR = 0.1388 (all data), 793 paramꢀ
3
2
eters, S = 0.94.
ASSOCIATED CONTENT
Preparation of [3][H(OTf) ] for crystallographic study
The Supporting Information, including NMR spectra and molecuꢀ
2
F
4
To a solution of 1 (0.10 g) in DCM (3 mL) was added TfOH
(0.2 mL). Hexane (6 mL) was added to the resulting solution
to precipitate the product. Excess solvent was cannula decantꢀ
ed and the resulting red solid was dissolved in DCM (2 mL)
and layered with hexane. Slow diffusion at room temperature
lar structures of 2, [3][BAr
] and [3][H(OTf)
2
], is available free
of charge on the ACS Publications website.
AUTHOR INFORMATION
Corresponding Author
*rowan.young@nus.edu.sg
Note: The authors declare no competing financial interest.
ACKNOWLEDGMENT
We thank the National University of Singapore and the Singapore
Ministry of Education for financial support (WBS Rꢀ143ꢀ000ꢀ
afforded crystals of [3][H(OTf) ] suitable for a Xꢀray diffracꢀ
2
tion study. Yield 0.05 g, 28%. Crystal data for C H F O S ,
2
9
32
6
7 2
M = 670.69, triclinic, Pꢀ1 (No. 2), a = 9.7072(15), b =
9
6
ꢀ
0
5
.9398(16), c = 18.474(3) Å, α = 79.874(5), β = 83.488(5), γ =
3
ꢀ
3
5
86ꢀ112 and Rꢀ143ꢀ000ꢀ666ꢀ114).
1.399(4)°, V = 1539.7(2) Å , Z = 2, δ = 1.447 Mgm ,
calc
ꢀ1
(Mo Kα) = 0.254 mm , T = 100(2) K, red block, 0.1 x 0.1 x
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(
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