Biddle and Reich
SCHEME 1. Proposed Test for Detection of Siliconate Complex Reactivity
the reactive species was a free carbanion intermediate. On the
basis of a high axial selectivity on reaction of a nucleophilic
acetylide formed by fluoride-catalyzed desilylation (we will refer
to these reactions as fluorodesilylations), Kuwajima, Nakamura,
and Hashimoto3a also argued that a free acetylide anion was a
likely intermediate. Majetich and co-workers showed that the
fluoride-catalyzed allyl transfer was, in some cases, superior to
other methods, especially for conjugate addition to R,â-
unsaturated nitriles and esters. They proposed that it was the
intermediate siliconate complex which was the reactive species,
largely on the basis of considerations of the high pKa value of
the allyl anion being generated.7 McDougal and co-workers also
proposed a siliconate complex as the reactive nucleophile in a
stereospecific fluoride desilylation-benzaldehyde trapping ap-
plied to the syn and anti diastereomers of a phenylthioalkylsi-
lane.8 Hydride transfers from hydrido siliconate complexes seem
to be well established.2f,6a,9
FIGURE 1. Proposed catalytic cycle of fluoride-mediated allylation
of benzaldehyde with allyltrimethylsilane.
When the silicon bears multiple electronegative substituents
(F, O, N), dissociation of the C-Si bond to form free anion
becomes more endothermic. The silicon also becomes more
electrophilic, and regiospecific and stereospecific allyl transfer
directly from siliconate complex intermediates has been dem-
onstrated in multiple systems1b,1c,6b,10-12 and has been proposed
for alkynyl13 and other transfers. The question we ask here is
whether siliconate complexes bearing only a single electroneg-
ative group on silicon are effective nucleophiles.
Although it is possible to make arguments for either mech-
anism, there has been no systematic study of the catalytic cycle
of Figure 1, nor have direct tests to detect the reactivity of
intermediate siliconate complexes been reported. In this article,
we describe our efforts to address this issue by comparing the
diastereo- and regioselectivities of the carbanionic species
formed in the fluorodesilylations with that of separated ion pairs
(SIPs) formed by treating lithium reagents with the strongly
coordinating cosolvents HMPA (N,N,N′,N′,N′′,N′′-hexameth-
ylphosphoric triamide), DMPU (N,N′-dimethylpropyleneurea),14a
and crypt[2.1.1] (4,7,13,18-tetraoxa-1,10-diazabicyclo[8.5.5]-
eicosane)15 as outlined in Scheme 1. We have characterized the
solution structure including ion-pair status and reactivity of a
number of lithium reagents16a-c and have also investigated the
reactivity of hypervalent iodine, tellurium, and tin ate complexes
in conjunction with some studies on metal-metalloid exchange
reactions.16d Thus, we felt we were well situated to investigate
the reactive species in the desilylation reactions.
To effectively make such comparisons, several constraints
need to be placed on the carbanionic species. First, the lithium
reagent must demonstrably form separated ion pairs upon
addition of HMPA or other cosolvents, so that it can be
reasonably assumed that the SIP is the reactive species.16a
Second, the fluoride reaction conditions need to resemble those
where the solution structure is known, i.e., low temperature (-78
to -120 °C) in THF or THF/ether solvent. This requirement
(3) (a) Kuwajima, I.; Nakamura, E.; Hashimoto, K. Tetrahedron 1983,
39, 975-982. (b) Urabe, H.; Kuwajima, I. Tetrahedron 1983, 39, 4241-
4244.
(4) Maifeld, S. V.; Lee, D. Org. Lett. 2005, 7, 4995-4998.
(5) Pilcher, A. S.; DeShong, P. J. Org. Chem. 1996, 61, 6901-6905.
(6) (a) Boyer, J.; Brelie`re, C.; Corriu, R. J. P.; Kpoton, A.; Poirier, M.;
Royo, G. J. Organomet. Chem. 1986, 311, C39-C43. Becker, B.; Corriu,
R. J. P.; Gue´rin, C.; Henner, B.; Wang, Q. J. Organomet. Chem. 1989,
368, C25-C43. (b) Chuit, C.; Corriu, R. J. P.; Reye, C.; Young, J. C. Chem.
ReV. 1993, 93, 1371-1448.
(14) (a) Mukhopadhyay, T.; Seebach, D. HelV. Chim. Acta 1982, 65,
385-391. (b) Seebach, D.; Amstutz, R.; Dunitz, J. D. HelV. Chim. Acta
1981, 64, 2622-2626. Amstutz, R.; Schweizer, W. B.; Seebach, D.; Dunitz,
J. D. HelV. Chim. Acta, 1981, 64, 2617-2621.
(15) Dietrich, B.; Lehn, J. M.; Sauvage, J. P. Tetrahedron Lett. 1969,
10, 2885-2888.
(7) Majetich, G.; Casares, A.; Chapman, D.; Behnke, M. J. Org. Chem.
1986, 51, 1745-1753.
(16) (a) Reich, H. J.; Borst, J. P.; Dykstra, R. R.; Green, D. P. J. Am.
Chem. Soc. 1993, 115, 8728-8741. (b) Reich, H. J.; Sikorski, W. H. J.
Org. Chem. 1999, 64, 14-15. Sikorski, W. H.; Reich, H. J. J. Am. Chem.
Soc. 2001, 123, 6527-6535. (c) Reich, H. J.; Sikorski, W. H.; Gudmunds-
son, B. O¨ .; Dykstra, R. R. J. Am. Chem. Soc. 1998, 120, 4035-4036. Reich,
H. J.; Kulicke, K. J. J. Am. Chem. Soc. 1995, 117, 6621-6622. Reich, H.
J.; Kulicke, K. J. J. Am. Chem. Soc. 1996, 118, 273-274. (d) Reich, H. J.;
Green, D. P.; Phillips, N. H. J. Am. Chem. Soc. 1989, 111, 3444-3445.
Reich, H. J.; Phillips, N. H. Pure Appl. Chem. 1987, 59, 1021-1026. Reich,
H. J.; Bevan, M. J.; Gudmundsson, B. O¨ .; Puckett, C. L. Angew. Chem.,
Int. Ed. 2002, 41, 3436-3439. (e) We will report these studies separately.
(f) Reich, H. J; Eisenhart, E. K.; Olson, R. E.; Kelly, M. J. J. Am. Chem.
Soc. 1986, 108, 7791-7800. (g) Sikorski, W. H.; Sanders, A. W.; Reich,
H. J. Magn. Reson. Chem. 1998, 36, S118-S124.
(8) (a) McDougal, P. G.; Condon, B. D. Tetrahedron Lett. 1989, 30,
789-790. (b) McDougal, P. G.; Condon, B. D.; Lafosse, M. D.; Lauro, A.
M.; VanDerveer, D. Tetrahedron Lett. 1988, 29, 2547-2550.
(9) Chopra, S. K.; Martin, J. C. J. Am. Chem. Soc. 1990, 112, 5342-
5343.
(10) Hosomi, A.; Kohra, S.; Ogata, K.; Yanagi, T.; Tominaga, Y. J. Org.
Chem. 1990, 55, 2415-2420.
(11) Denmark, S. E.; Coe, D. M.; Pratt, N. E.; Griedel; B. D. J. Org.
Chem. 1994, 59, 6161-6163.
(12) Kinnaird, J. W. A.; Ng, P. Y.; Kubota, K.; Wang, X.; Leighton, J.
L. J. Am. Chem. Soc. 2002, 124, 7920-7921.
(13) Lettan, R. B., II; Scheidt, K. A. Org. Lett. 2005, 7, 3227-3230.
4032 J. Org. Chem., Vol. 71, No. 11, 2006