Communications
recovered DPIBF. Taken together, these results implicate the
generation of intermediates that do not involve Ar–Li
bonding, for example aryl anions.
Gas phase B3LYP/6-31G(d) calculations and single point
B3LYP/6-311 + G(d,p) calculations, including a continuum
description of the THF solvent,[22] were carried out for the
pathways leading to the elimination and the rearrangement of
[C6H4OTf]ꢀ. Although the use of a truncated model without a
counterion and a simplified continuum description of the
solvent prevent us from quantitatively analyzing the parti-
tioning of the anion, both processes are predicted to be
extremely facile (Figure 2).
Scheme 2. Rearrangement of 18O-, 34S-, and 2H-labeled o-1.
a) 18O-scrambling experiment. b) Crossover experiment.
(Scheme 3). Organosulfinites are known to undergo isomer-
ization to the corresponding sulfone by a heterolytic ion-pair
recombination mechanism,[24] and thus, the combination of a
trifluoromethylsulfinite and
a phenolate (7) would be
expected to isomerize readily. However, rearrangement of
6-[2H]-8 gave [2H]-9 as a single [2H]-isomer, (Scheme 3) ruling
out a sulfinite-based mechanism.
Figure 2. Computed relative energies (kcalmolꢀ1) for the elimination
or the rearrangement of the cisoid isomer of [C6H4(OTf)]ꢀ in a THF
continuum.
The loss of the triflate group is predicted to be mildly
exothermic (10.2 kcalmolꢀ1) and the OAr!CAr sulfonyl
migration is predicted to be highly exothermic (62.0 kcal
molꢀ1). For the latter process, a putative 1,2-oxethietane
intermediate could not be located. Instead, the OAr!CAr
migration proceeded by a single and early transition state
Scheme 3. Probing for symmetrical intermediate 6 by way of 6-[2H]-8.
ꢀ
with a C S distance of 2.68 (3.07 in the starting species)
ꢀ
and a breaking S O bond length of 1.69 (1.59 in the
An additional parameter affecting partitioning between
the rearrangement and the elimination reactions is the
concentration of the free HN(iPr)2 (DIPA).[7] Whereas both
processes nominally generate DIPA from the reaction of
LDA with ArOTf, the DIPA remains strongly complexed to
Li in the rearrangement product[7] and is also consumed by
rapid (LDA-catalyzed) reaction with the aryne to generate
ArN(iPr)2.[10a] Addition of DPIBF, which selectively traps the
aryne (cf. 5), bypasses DIPA consumption and leads to a rise
in the DIPA concentration, which can have a profound effect
on the reaction outcome. For example, reaction of 8 with
DIPA-free LDA generates 9 (30%) and naphthyne-derived
anilines (44%). However, in the presence of DPIBF (1equiv)
the same reaction affords no trace of 9 and a 99% yield of the
naphthyne-DPIBF adduct.[7] This sensitivity to DIPA, albeit
extreme in the case of 8, is also observed in readily rearranged
substrates[7] and suggests that DIPA catalyzes aryne gener-
ation. In Huisgen and Sauerꢀs pioneering studies of the
starting species).
Preliminary calculations show that the two barriers
remain close in energy for the chlorine-substituted aromatic
ring. In the gas phase calculations, a stable triflate/benzyne
ion–molecule complex is found. Although this complex is
4.3 kcalmolꢀ1 less stable than the separated benzyne and
triflate pair in the presence of a continuum solvent, it
suggested that reversible formation of a benzyne/triflate
ion–molecule complex might facilitate thermodynamic rather
than kinetic control. However, reaction of 18O-o-1 gave 18O-
phenol 3, without 18O/16O scrambling (Scheme 2a), strongly
weighing against reversible liberation of the triflate group.
The possibility of an intermolecular transfer of the SO2CF3
group by an anionic chain reaction[23] was tested by simulta-
neously reacting 5-[2H]-o-1 and [34S]-o-1 (prepared in five
steps from 34S8) with LDA in THF from ꢀ788C!room
temperature. The complete lack of crossover (Scheme 2b)
demonstrates that the sulfone group transfer is intramolecu-
lar.
ꢀ
kinetics of aryne generation from Ar X, the presence of
HNR2 was found to efficiently catalyze ortho metalation.[25]
As noted above, metalation ortho to a triflate group (Li, Mg,
Zn) consistently results in an elimination.[18,19] Thus, our
overall model for the reaction of aryl triflates (11) with LDA
We next considered intramolecular attack of an ortho-
anionic triflate at the sulfonyl oxygen atom to generate a
trifluoromethylsulfinite (7) through intermediate
6
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2008, 47, 5067 –5070