Scheme 3 Computed reaction profile for the gold-catalyzed oxycyclization reactions of allenols 1j (R = Me, plain values) and 1f (R = Ph, values
in parentheses). Free energies (DG298, in kcal molꢀ1) have been computed at the PCM(CH2Cl2)-B3LYP/def2-SVP level.
of HCl which transforms 5 into 6 due to close proximity of the
hydrogen and chlorine atoms in complex 7 (computed Hꢁ ꢁ ꢁCl
distance of 1.822 and 1.830 A for R = Me and R = Ph,
2 (a) B. Alcaide, P. Almendros and C. Aragoncillo, Chem. Soc. Rev.,
2010, 39, 783; (b) M. Brasholz, H.-U. Reissig and R. Zimmer, Acc.
Chem. Res., 2009, 42, 45; (c) B. Alcaide and P. Almendros, Eur. J.
Org. Chem., 2004, 3377; (d) A. S. K. Hashmi, Angew. Chem., Int.
Ed., 2000, 39, 3590.
respectively). Complex
8
isomerizes into 80-bis after
3 For selected examples, see: (a) A. S. K. Hashmi, Angew. Chem., Int.
Ed., 2010, 49, 5232; (b) A. S. K. Hashmi, Chem. Rev., 2007, 107, 3180.
4 (a) B. Lipshutz and Y. Yamamoto (ed.), Chem. Rev., 2008, 108(8);
(b) G. J. Hutchings, M. Brust and H. Schmidbaur (ed.), Chem. Soc.
Rev., 2008, 37(9).
re-coordination of the metal fragment and is converted into
complex 9 through TS5 (a saddle point associated with the
1,3-migration of the metal moiety).11 Finally, a b-hydride
elimination reaction occurs (via TS6)16 to produce the final
oxetene 3 and AuHCl2 (which would regenerate the initial
catalyst upon reaction with HCl). From the data in Scheme 3,
it becomes obvious that this step constitutes the bottle-neck of
the process when R = Ph (DGa298 = +21.1 kcal molꢀ1) but is
kinetically favored over the corresponding Au–C protonolysis
involving TS3 and leading to dihydrofuran 2f.
5 (a) A. Corma, A. Leyva-Perez and M. J. Sabater, Chem. Rev.,
´
2011, 111, 1657; (b) J. Muzart, Tetrahedron, 2008, 6, 5815.
6 (a) N. Krause and C. Winter, Chem. Rev., 2011, 111, 1994;
(b) N. Bongers and N. Krause, Angew. Chem., Int. Ed., 2008, 47, 2178.
7 J. A. Burkhard, G. Wuitschik, M. Rogers-Evans, K. Muller and
¨
E. M. Carreira, Angew. Chem., Int. Ed., 2010, 49, 9052.
8 (a) A. Hoffmann-Roder and N. Krause, Org. Lett., 2001, 3, 2537;
¨
(b) A. S. K. Hashmi, M. C. Blanco, D. Fischer and J. W. Bats, Eur.
J. Org. Chem., 2006, 1387; (c) M. Asikainen and N. Krause, Adv.
Synth. Catal., 2009, 351, 2305; (d) D. Eom, D. Kang and P. H. Lee,
J. Org. Chem., 2010, 75, 7447.
9 See, for instance: (a) B. Alcaide, P. Almendros and R. Carrascosa,
Chem.–Eur. J., 2011, 17, 4968; (b) B. Alcaide, P. Almendros and
Therefore, it can be concluded that whereas the 5-endo-
trig - loss of HCl - protonolysis sequence (which produces
dihydrofurans 2) is followed for methyl-substituted
allenols, the 4-exo-dig - loss of HCl - 1,3-Au-migration -
b-elimination pathway (which leads to oxetenes 3) is preferred
for phenyl-substituted allenols. Both processes share the same
common intermediate 4, formed through distal coordination
of the allenic double bond, and are strongly favored over the
corresponding 4-endo-dig pathway involving the proximal
coordination.
M. T. Quiro
P. Almendros and T. Martı
2007, 46, 6684.
10 B. Alcaide, P. Almendros and T. Martı
Chem., Int. Ed., 2006, 45, 4501.
´
s, Adv. Synth. Catal., 2011, 353, 585; (c) B. Alcaide,
´
nez del Campo, Angew. Chem., Int. Ed.,
´
nez del Campo, Angew.
11 A. S. K. Hashmi, A. M. Schuster, S. Litters, F. Rominger and
M. Pernpointner, Chem.–Eur. J., 2011, 17, 5661.
12 A. S. K. Hashmi and G. J. Hutchings, Angew. Chem., Int. Ed.,
2006, 45, 7896.
Support for this work by the MICINN [CTQ2009-09318,
CTQ2010-20714-C02-01, and Consolider-Ingenio 2010
(CSD2007-00006)], CAM (Projects S2009/PPQ-1752 and
S2009/PPQ-1634), UCM-Santander (Grant GR35/10-A) is
13 B. Alcaide, P. Almendros, T. Martınez del Campo, E. Soriano and
´
J. L. Marco-Contelles, Chem.–Eur. J., 2009, 15, 9127.
14 H. Ito, T. Saito, T. Miyahara, C. Zhong and M. Sawamura,
Organometallics, 2009, 28, 4829.
gratefully acknowledged. I.F. is a Ramon y Cajal fellow.
´
15 Despite that this mechanistic scenario cannot be completely ruled
out, it seems less unlikely taking into account the computed high
activation barrier of this particular step (see Scheme S2, ESIw).
Besides, the preparation of oxetene 3d from allenol 1d wasalso
accomplished using Pt(II) catalysis (see Scheme S3, ESIw), probably
involving the formation of well known platinum hydride species. It
can be suggested that the hydride transfer to HCl occurs from
AuHCl2 to form H2 regenerating AuCl3.
Notes and references
1 For selected reviews, see: (a) Cumulenes and Allenes, , Science of
Synthesis, Houben-Weyl Method of Molecular Transformation, ed.
N. Krause, George Thieme, Sttutgart, 2007, vol. 44; (b) S. Ma,
Chem. Rev., 2005, 105, 2829; (c) Modern Allene Chemistry, ed.
N. Krause and A. S. K. Hashmi, Wiley-VCH, Weinheim, 2004.
16 See computational details in the ESIw.
c
9056 Chem. Commun., 2011, 47, 9054–9056
This journal is The Royal Society of Chemistry 2011