Angewandte
Chemie
the Supporting Information). As opposed to the fluoroben-
zene activation by [1][CHB11Cl11], only the xylyl rings or
solvent molecules are phenylated in the case of [2]
[CHB11Cl11]; the carborane anion remains unchanged and
the phosphine base is isolated only as [HP(o-tol)3]+, not
[PhP(o-tol)3]+.
phenylation prcedure using Ph-CHB11Cl11 and a novel
Friedel–Crafts arylation method. With improvements in the
ꢀ
[24]
preparation of CHB11Cl11
,
silylium carboranes can be
expected to become more common reagents in synthetic
chemistry than they are at present.[25]
The reactions of 1+ and 2+ are carried out in an environ-
ment of exceptional low nucleophilicity, where one could
imagine that they exhibit higher SN1 character than previously
studied systems. To address this important question regarding
the dissociative SN1-like nature of silylium-mediated Caryl–F
activations, the decay of 2+ in different fluoroarene solvents
was monitored by 1H NMR spectroscopy at 300 K. In
fluorobenzene, the disappearance of the cation followed
pseudo first-order kinetics with a half-life of t1/2 = 88 min that
Received: June 20, 2010
Published online: September 3, 2010
ꢀ
Keywords: carboranes · C F activation · fluorides ·
fluorobenzene · silyl cations
.
[1] Value for Me3SiF: R. Becerra, R. Walsh in The Chemistry of
Organic Silicon Compounds, Vol. 2 (Eds.: S. Patai, Z. Rappo-
port), Wiley, Chichester, 1998, chap. 4, p. 170. The most recent
collection of silicon BDE has been reported in: “Bond
Dissociation Energies in Organosilicon Compounds”: R. Walsh
ABCR Catalogue, Gelest Inc., Tullytown PA (USA), 2000,
p. 193.
[3] C. Douvris, E. S. Stoyanov, F. S. Tham, C. A. Reed, Chem.
is independent of the amount of phosphine added (0–12
ꢀ
equivalents) and the carborane counterion (CHB11Cl11
,
ꢀ
CHB11H5Br6 ). In 4-fluorotoluene, [2][CHB11Cl11] decayed
with t1/2 = 52 min. In 1,4-difluorobenzene, no reaction took
place; the concentration of the cation remained constant over
10 hours. The reactivity order 4-MeC6H4F > C6H5F > 4-
FC6H4F suggests that fluoride abstraction proceeds via a
transition state that is lowered by electron-donating substitu-
ents; that is, in which the initial fluoroarene possesses at least
some aryl cationic character.[23]
[4] a) C. Douvris, C. M. Nagaraja, C.-H. Chen, B. M. Foxman, O. V.
In the calculated transition state for 2+ + PhF!5, fluoride
ꢀ
recent report on C F activation followed by C C bond
transfer to silicon is accompanied by nucleophilic attack on
C
ipso by the xylyl p system (Figure 2). While the C6H5 moiety
ꢀ
[5] Caryl F bond activation in the gas phase: a) H. H. Cornehl, G.
shows angle distortions typical of aryl cationic species[7]
b) A. Bajarnason, J. W. Taylor, Organometallics 1989, 8, 2020 –
(Cortho-Cipso-Cortho 1398), it can not be regarded as a free
ꢀ
2024. Activation of Caryl F bonds by transition metals: c) R. H.
Young, Jr., V. V. Grushin, Organometallics 1998, 18, 294 – 296;
Reviews: e) T. Braun, R. N. Perutz in Comprehensive Organo-
metallic Chemistry III, Vol. 1 (Eds.: R. H. Crabtree, M. P.
Mingos), Elsevier, Oxford, 2007, pp. 725 – 758; f) H. Torrens,
[6] P. J. Stang in Divalent Carbocations (Eds.: P. J. Stang, Z.
Rappoport), Wiley, Chichester, 1997, p. 451.
[7] Calculations: a) K. K. Laali, G. Rasul, G. K. S. Prakash, G. A.
Hrusꢂk, D. Schrꢃder, S. Iwata, J. Chem. Phys. 1997, 106, 7541 –
7549; d) A. Nicolaides, D. M. Smith, F. Jensen, L. Radom, J. Am.
Schwarz, W. Koch, Theor. Chem. Acc. 1988, 99, 95 – 99.
[8] Singlet Ph+ in an argon matrix: M. Winkler, W. Sander, Angew.
Figure 2. Calculated transition state for 2+ +PhF!5 (B98/DZ-
(2df,pd)). Si–F 1.680 ꢂ, F–Cipso 2.189 ꢂ, Cipso–Cmeta 2.849 ꢂ. Hatoms
omitted for clarity.
[9] a) H. Zollinger, Diazo Chemistry I, VCH, New York, 1995;
b) Phenyl cations from [Ph-He]+ by tritium decay: M. Speranza,
of a cyclic vinyl cation from a l3-bromane and related l3 halogen
chemistry, see: K. Miyamoto, M. Shiro, M. Ochiai, Angew.
[10] a) V. Dichiarante, A. Salvaneschi, S. Protti, D. Dondi, M.
phenyl cation. Computational analysis of this reaction process
at the MP2/DZ(2df,pd)//B98/DZ(2df,pd) level of theory
predicts DH°calc = 85.77 kJmolꢀ1 (100.17 kJmolꢀ1, B98/DZ-
(2df,pd)), which matches well with the experimental values,
DH° = 82 kJmolꢀ1 and DS° = ꢀ24 Jmolꢀ1 Kꢀ1, determined
ꢀ
with an Eyring plot. These numbers reflect the strong Caryl
F bond that has to be broken and are in line with a reaction in
which two or more molecules combine to form a transition
state.
The activation of fluoroarenes by [1][CHB11Cl11] and [2]
[CHB11Cl11] demonstrates the extreme fluorophilicity of silyl
Lewis acids and paves the way for the development of a
b) I. M. Cuccovia, M. A. da Silva, H. M. C. Ferraz, J. R. Plie-
Angew. Chem. Int. Ed. 2010, 49, 7519 –7522
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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