The Sila-Wittig rearrangement

Full text of DOI:10.1021/ja9630957

J. Am. Chem. Soc. 1997, 119, 233-234
233
Communications to the Editor
Scheme 1
The Sila-Wittig Rearrangement
Atsushi Kawachi, Noriyuki Doi, and Kohei Tamao*
Institute for Chemical Research, Kyoto UniVersity
Uji, Kyoto 611, Japan
Scheme 2
ReceiVed September 3, 1996
The [2,3]-Wittig rearrangement of R-alkoxy carbanions has
been extensively studied because the rearrangement offers useful
methodologies for regio- and stereoselective C-C bond forma-
tion (Scheme 1).1,2 Recently, the aza-[2,3]-Wittig rearrangement
of R-amino carbanions has been also studied due to its potential
utility.3 In contrast to these carbanions, little attention has been
paid to the analogs of other group 14 elements. On the basis
of our recent sudies of the R-heteroatom-substituted silyl anions,4
we now report the first examples of silicon analogs to the [2,3]-
Wittig rearrangements, that is, [2,3]-sila-Wittig and aza-sila-
Wittig rearrangements, which involve intramolecular migration
of an allyl group from an oxygen or nitrogen to silicon in
[(allyloxy)silyl]lithium or [(allylamino)silyl]lithium (Scheme 1).
The term “sila-Wittig” rearrangement is used to differentiate
the present reaction from the “silyl-Wittig” rearrangement
(reversed Brook rearrangement).5
Scheme 3a
A typical example is shown in Scheme 2. [(Allyloxy)silyl]-
stannane 1, a precursor of [(allyloxy)silyl]lithium 2, was readily
prepared from (chlorosilyl)stannane 3 and tertiary allyl6 alcohol
4. A solution of 1 in THF was treated with n-butyllithium (2.0
equiv) at -78 °C for 3 h. The reaction mixture was stirred at
room temperature for 2 h, and the reaction was quenched with
Me3SiCl to give the rearrangement product, allylsilane-contain-
ing disiloxane 5, in 68% isolated yield. No [1,2]-rearrangement
product was detected. The intermediate [(allyloxy)silyl]lithium
2 could be trapped with Me3SiCl at -78 °C to afford the
corresponding disilane 6 in 51% yield, together with the
rearrangement product 5 in 21% yield.7,8 Thus, 2 undergoes
the [2,3]-sila-Wittig rearrangement to form lithium allylsilanolate
(1) For reviews, see: (a) Marshall, J. A. In ComprehensiVe Organic
Synthesis; Trost, B. M., Fleming, I., Pattenden, G., Eds.; Pergamon Press:
Oxford, 1991; Vol. 3, pp 975-1014. (b) Nakai, T.; Mikami, K. Chem. ReV.
1986, 86, 885. (c) Hoffmann, R. W. Angew. Chem., Int. Ed. Engl. 1979,
18, 563.
(2) For recent reports, see: (a) Tomooka, K.; Keong, P-H.; Nakai, T.
Tetrahedron Lett. 1995, 36, 2789. (b) Katritzky, A. R.; Wu, H.; Xie, L. J.
Org. Chem. 1996, 61, 4035.
a For clarity, -SiPh2SnMe3 and -SiPh2OSiMe3 groups are abbrevi-
ated to -SiSn and -SiOSi, respectively.
(3) (a) Durst, T.; Elzen, R. V. D.; LeBelle, M. J. Am. Chem. Soc. 1972,
94, 9261. (b) Broka, C.; Shen, T. J. Am. Chem. Soc. 1989, 111, 2981. (c)
Coldham, I. J. Chem. Soc., Perkin Trans. 1 1993, 1275. (d) A˙ hman, J.;
Somfai, P. J. Am. Chem. Soc. 1994, 116, 9781. (e) A˙ hman, J.; Somfai, P.
Tetrahedron Lett. 1995, 36, 303. (f) Anderson, J. C.; Siddons, D. C.; Smith,
S. C.; Swarbrick, M. E. J. Chem. Soc., Chem. Commun. 1995, 1835. (g)
Coldham, I.; Collis, A. J.; Mould, R. J.; Rathmell, R. E. J. Chem. Soc.,
Perkin Trans. 1 1995, 2739. (h) Gawley, R. E.; Zhang, Q.; Campagna, S.
J. Am. Chem. Soc. 1995, 117, 11817. (i) Anderson, J. C.; Siddons, D. C.;
Smith, S. C.; Swarbrick, M. E. J. Org. Chem. 1996, 61, 4820.
(4) For a review, see: (a) Tamao, K.; Kawachi, A. AdV. Organomet.
Chem. 1995, 38, 1. (b) Tamao, K.; Kawachi, A.; Ito, Y. J. Am. Chem. Soc.
1992, 114, 3989. (c) Tamao, K.; Kawachi, A. Angew. Chem., Int. Ed. Engl.
1995, 34, 818. (d) Tamao, K.; Kawachi, A. Organometallics 1995, 14, 3108.
(5) For a review on rearrangements involving silicon, see: (a) Brook,
A. G.; Bassindale, A. R. In Rearrangements in Ground and Excited States;
de Mayo, P., Ed.; Academic Press: New York, 1980; pp 149-227. (b)
Wright, A.; West, R. J. Am. Chem. Soc. 1974, 96, 3214, 3222, 3227.
(6) The term “tertiary allyl” means that the allylic carbon is tertiary.
The terms primary and secondary are used in a similar way in this paper.
(7) For a formation of silylcuprates from silylstannanes by treatment with
higher order cuprates, see: Lipshutz, B. H.; Reuter, D. C.; Ellsworth, E. L.
J. Org. Chem. 1989, 54, 4975.
7 quantitatively at room temperature and to some extent even
at -78 °C.9 Furthermore, the rearrangement was greatly
enhanced by a crown ether. Treatment of 2 with 12-crown-4
afforded only 5 in 55% yield even at -78 °C within 1 h.10
Some other representative results are summarized in Scheme
3, where the starting materials were prepared from 3 and the
corresponding allylic alcohols in 58-87% yields, as previously
stated. Three points deserve comment. (1) An olefinic stereo-
(9) The rearrangements are considered to be thermodynamically favor-
able: the enthalpy for the reaction, (CH2dCHsCH2O-)Ph2SisLi f
LisOsPh2Si(sCH2sCHdCH2), was estimated to be about -40 kcal/mol
by PM3 calculations, SPARTAN Version 4.0.
(10) A similar solvent effect has been well documented in the Wittig
rearrangements of carbanions. (a) Wittig, G.; Stahnecker, E. Liebigs Ann.
Chem. 1957, 605, 69. (b) Mikami, K.; Kasuga, T.; Fujimoto, K.; Nakai, T.
Tetrahedron Lett. 1986, 26, 4185. See also ref 3i.
(8) The intramolecular fashion of the rearrangement was confirmed by
a crossover experiment; see the Supporting Information.
S0002-7863(96)03095-8 CCC: $14.00 © 1997 American Chemical Society

234 J. Am. Chem. Soc., Vol. 119, No. 1, 1997
Communications to the Editor
Scheme 4
Scheme 5
selection11 has been investigated on 8 involving a chiral tertiary
allylic carbon center. The rearranged allylsilanes preferentially
had E-olefin moieties (9) with an increase in the bulkiness of
the allylic alkyl substituent (8a and 8b), but the reversed
Z-selectivity (10) was observed in the phenyl case (8c).12
Whereas the E-selectivity is similar to the general tendencies
observed for the [2,3]-Wittig rearrangement,1,11 the Z-selectivity
implies a mechanistic complexity in the present silicon version,
which must be clarified by further studies. (2) In addition to
the rearrangement from tertiary allyloxy to primary allylsilanes
(abbreviated as tert-to-prim) (all examples mentioned above and
11a), tert-to-sec (11b), and tert-to-tert (11c) rearrangements
proceeded smoothly to give 12b and 12c, respectively. (3)
Exocyclic (allyloxy)silane 13 was converted into endocyclic
allylsilane 14. Further noted is the regioselectivity observed
in 15 which exclusively underwent migration to the exocyclic
olefin with the endocyclic olefin intact (16).
The present method, however, is applicable only to tertiary
allyloxy derivatives. Some attempted transmetalations of [prim-
(allyloxy)silyl]- and [sec-(allyloxy)silyl]stannanes were unsuc-
cessful. However, this limitation has been partly overcome by
a reductive lithiation method,3b,4d as shown in Scheme 4. (sec-
Allyloxy)silyl chloride 17 was treated with lithium naphthalenide
(3.0 equiv) in THF at -45 °C for 1 h, followed by being stirred
at 0 °C for 1 h and trapped with Me3SiCl, to afford the
rearrangement product 18 in 38% yield as a mixture of olefinic
stereoisomers.
silyllithium 20 smoothly underwent the rearrangement at -45
°C to form lithium (allylsilyl)amide 22, which was trapped with
Me3SiCl to give the corresponding allylsilane 23 in 86% yield.9
In the absence of the crown ether, 20 underwent the rearrange-
ment in only 10% yield even after warming to -20 °C, and at
higher temperatures, a complex mixture resulted.10
It is noteworthy that the sila-Wittig rearrangements offer a
novel route from allyl alcohols or allylamines to sila-function-
alized allylsilanes in a regiocontrolled fashion.14 Although a
large number of synthetic methods for allylsilanes have been
reported, those for sila-functionalized allylsilanes are limited.14a
One synthetic utility of the silicon functionality was demon-
strated by conversion of the allylsilane15 into the corresponding
allyl alcohol by H2O2 oxidation.16 Thus, the overall result
corresponds to a novel 1,3-skeletal transformation of the allyl
alcohols. Our current study aims at the scope and limitations,
synthetic applications, and reaction mechanism of the rear-
rangements.
Acknowledgment. We thank the Ministry of Education, Science,
and Culture, Japan, for a Grant-in-Aid for Scientific Research (no.
07405043). Thanks are due to Shin-Etsu Chemical Co., Ltd., and Nitto
Kasei Co., Ltd., for gifts of the phenylchlorosilanes and dimethyl-
dichlorostannane, respectively.
We turned our attention to silicon analogs to the aza-Wittig
rearrangements3 in which the oxygen atom is replaced by a
nitrogen atom. An acyclic [(allylamino)silyl]lithium also
undergoes an aza-sila-Wittig rearrangement, as shown in Scheme
5. Transmetalation of [(allylamino)silyl]stannane 1913 with tert-
butyllithium (2.0 equiv) was complete at -45 °C in 2 h, as
indicated by trapping the silyllithium 20 with Me3SiCl to give
the corresponding disilane 21 in 86% yield. The transmetalation
did not occur at -78 °C. When 12-crown-4 was added,
Supporting Information Available: Experimental details (19
pages). See any current masthead page for ordering and Internet access
instructions.
JA9630957
(14) For reviews on syntheses of allylsilanes, see: (a) Sarkar, T. K.
Synthesis 1990, 969, 1101. (b) Fleming, I.; Dunogues, J.; Smithers, R. Org.
React. 1989, 37, 57. For the syntheses from allyl alcohols, see: (c) Hwu,
J. R.; Lin, L. C.; Liaw, B. R. J. Am. Chem. Soc. 1988, 110, 7252. (d)
Suginome, M.; Matsumoto, A.; Ito, Y. J. Am. Chem. Soc. 1996, 118, 3061.
(15) The silanol and not the disiloxane should be used for oxidation;16
the former can be obtained by trapping the lithium allylsilanolate with water
instead of Me3SiCl.
(16) (a) Tamao, K.; Ishida, N.; Tanaka, T.; Kumada, M. Organometallics
1983, 2, 1694. (b) Tamao, K.; Kawachi, A.; Tanaka, Y.; Ohtani, H.; Ito, Y.
Tetrahedron 1996, 52, 5765. (c) Fleming, I. Chemtracts, Org. Chem. 1996,
9, 1. (d) Jones, G.; Landais, Y. Tetrahedron 1996, 37, 7599. (e) Tamao, K.
In AdVances in Silicon Chemistry; Larson, G., Ed.; JAI Press: Greenwich,
CT, 1996; Vol. 3, pp 1-62 (in press).
(11) (a) Still, W. C.; Mitra, A. J. Am. Chem. Soc. 1978, 100, 1927. (b)
Wu, Y.-D.; Houk, K. N.; Marshall, A. J. Org. Chem. 1990, 55, 1421. (c)
Hoffmann, R.; Bru¨ckner, R. Angew. Chem., Int. Ed. Engl. 1992, 31, 647.
(d) Verner, E. J.; Cohen, T. J. Am. Chem. Soc. 1992, 114, 375. (e) Mikami,
K.; Uchida, T.; Hirano, T.; Wu, Y.-D.; Houk, K. N. Tetrahedron 1994, 50,
5917.
(12) The selectivity was determined by 1H NMR analysis of the isomeric
mixtures, and the stereochemistry was assigned by NOE experiments; see
the supporting information.
(13) [(Allylamino)silyl]stannane 19 was prepared by reaction of 3 with
the corresponding lithium amide.