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Catalysis of the Addition of Allyltrimethylsilane to Aldehydes by Silylating Agents.

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[2] U. P. Steinbrecher, S. Parthasarathy, D. S. Leake, J. L. Witztum, D. Steinberg, Proc. Natl. Acad. Sci. USA 1984,81,3883-3887.
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Steinbrecher, J. L. Witztum, S. Parthasardthy, D. Steinberg, Arterioscierosis (Dallas) 1987, 7 , 135-143.
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Catalysis of the Addition of Allyltrimethylsilane
to Aldehydes by Silylating Agents.
Me,SiB(OTf), , a New, ‘Supersilylating’ Reagent **
By Anthony P . Davis* and Marcel Jaspars
The addition of allylsilanes such as 1 to aldehydes and
ketones (Scheme 1)“’ can be induced by catalytic amounts of
fluoride ion, stoichiometric amounts of Lewis acids or (as we
ourselves showed recently) catalytic amounts of the superacid H,OTf+B(OTf), (Tf = CF,S0,).r21 However, it is generally understood that it is not promoted by silylating agents,
such as Me,SiOTfr3.41 and Me,SiI.[S-71 In the course
of catalyzing the addition if given sufficient time. The most
reasonable mechanism would appear to be as in Scheme 2;
the initial adducts 3 are desilylated during work-up to give
homoallyl alcohols 2. A comparison of entries 2 and 3 in
Scheme 2.
Table 1 indicates that Me,SiI is considerably more active
than Me,SiOTf, and also gives a cleaner product. However,
neither catalyst could be said to be preparatively useful.
Yields are at best moderate, due to the formation of substantial amounts of nonpolar by-products.
Table 1. The addition of allyltrimethylsilane 1 to aldehydes RCHO, catalyzed
by silylating agents Me,SiX. [a]
Promoter [b]
Reaction time
Me,SiOTf (20)
Me,SiOTf (20)
Me,SiI (20)
Me,Si B(OTf), (1)
Me,Si B(OTf), (0.2)
Me,Si B(OTf), (1)
Me& B(OTf), (1)
21 h
23 h
45 min
25 min
20 min
15 min
61 [c, dl
27 [cl
65 [el
93 [fl
80 [gl
of an investigation into the mechanism of the superacidcatalyzed version of the reaction we have re-examined the
question of catalysis by ‘Me,SiX’, and have made two discoveries which we report herein. The first, perhaps of minor
importance, is that Me,SiOTf and (especially) Me,SiI are
considerably more active than previous reports have suggested. The second, of greater significance, is a new reagent
which may fairly be described as a ’supersilylating’ agent, is
an excellent catalyst for allylations, and promises to have a
variety of other applications in organic synthesis.
The results of some experiments involving Me,SiOTf and
Me,SiI are presented in Entries 1-3 of Table 1. At room
temperature and fairly high concentrations, both are capable
Department of Chemistry,
Trinity College, IRL-Dublin 2 (Ireland)
This work was supported in part by the Science programme of the
European Community. We are grateful to Dr. M. G. Orchard for helpful
0 V C f f l4rlagsgesellschafl m b f f , W-6940 Weinheim, I992
In the reactions 4-7 in Table 1 the new ‘supersilylating’
reagent Me,SiB(OTf), was employed. It can be prepared as
a solution in dichloromethane or chloroform simply by
adding a solution of Me,SiOTf to a freshly-prepared sample
of B(OTf),[’ol (Scheme 3). An exothermic reaction occurs,
leading to a homogeneous, pale yellow solution.[”] The l l B
NMR spectrum (CH,Cl,, standard BF,.OEt,) of one such
solution showed a single resonance at 6 = -3.17 (W,,, =
28 Hz). Both the chemical shift and narrow line width are as
expected for the ‘spherically symmetrical’ anion B(OTf),
+ 3 HOTf
[*I Dr. A. P. Davis, M. Jaspars
[a] Unless otherwise stated, the reactions were performed in CH,CI, at room
temperature. [RCHO] = 0.5 M, [l] = 0.75M. The initial adduct 3 was desilylated with MeOH/HCI (aq) and purified by flash chromatography. [b] Numerical
values in brackets are amounts of the promoter in mol-%. [c] Desilylative
work-up with NaHCO, (aq). [d] This experiment was mentloned briefly in
Ref. (21.[el Desilylative work-up with Na,S,O, (as). [fl Preparative scale experiment (see Experimental). [g] Desilylation with ether/HCl (aq) was only
partially successful. Yield refers to a ca. 4:l mixture of 2 and 3.
Scheme 1
Yield of 2
- I+
3 HBr
+ B(OTf),
m S i M e ,
+ HOTf
+ Me,SiOTf
Scheme 3
Angew. Chem. Int. Ed. Engl. 31 (1992) N o . 4
The 'H NMR spectrum (CD,Cl,, standard CH,Cl,) contained a single peak at 6 = 0.58 and the I3C NMR spectrum
(CH,Cl,, standard CH,Cl,) a single peak for C-Si at 6 = 0.0
= 54 Hz) and a quartet for C-F at 6 = I 1 8
('J,, = 31 8 Hz). The precise structure of the species in these
solutions is still uncertain. Two possibilities (representing
extremes of a continuum) are (a) an ion pair Me,Si+
B(OTf),['31 and (b) a covalent structure A in which the
Me,Si group is exchanging rapidly between the sulfonyl oxygens (to maintain the apparent symmetry in the B(OTf),
unit). At present we are inclined to prefer the latter because
of the unexceptional nature of the 'H and I3C NMR data.['41
- /
Irrespective of its structure, the relevant entries in Table 1
clearly suggest that Me,SiB(OTf), is a very powerful source
of electrophilic silyl units. At room temperature it is able to
catalyze the addition of allyltrimethylsilane to a variety of
simple aldehydes with remarkable efficiency. The reactions
are relatively clean, resulting in only trace amounts of the
nonpolar by-products referred to above. Particular attention
is drawn to the preparative-scale reaction (Entry 5 and Experimental Procedure) in which a 93 % yield of product was
obtained using only 0.2 mol % of catalyst. Despite its reactivity, this reagent is not especially difficult to handle. The
dichloromethane solutions may be stored at - 10 "C under
argon for several weeks without loss of activity. Considering
the many applications which have been found for
Me,SiOTf["] and Me,SiI,[16]there is a high probability that
Me,SiB(OTf), will prove to be a valuable addition to the
synthetic chemists' armory.
Finally, as mentioned above, the original motivation for
this work was our curiosity concerning the mechanism of our
earlier variant of the addition of allylsilanes to aldehydes,
catalyzed by the superacid H,OTf+ B(0Tf);. Although it is
still possible that our first proposal (catalysis by 'H+')['I is
correct, the present work suggests a plausible alternative involving the reaction of the superacid with allyltrimethylsilane
to form a supersilylating species within the reaction mixture.
Experimental Procedure
Me,Si B(OTf),: Trifluoromethanesulfonic acid (531 pL, 6 mmol) was added to
boron tribromide (distilled from Al powder; 190 pL, 2 mmol) at 0°C. After
evolution of HBr had ceased the flask was evacuated for 1 h to give B(OT0, as
a viscous yellow liquid[lOl. In a separate flask, a solution of Me,SiOTf was
prepared by addition of trifluoromethanesulfonic acid (177 pL, 2 mmol) to a
solution of allyltrimethylsilane (320 pL, 2 mmol) in CH,CI, (5 mL)[17], and
leaving the resulting mixture to stand for ten minutes. Addition of this solution
to the B(OTf), at 0 "C resulted in the evolution of heat and formation of a pale
yellow solution believed to contain Me,SiB(OTf), (for data, see text).
2 : R = Ph(CH,),: The above solution of Me,SiB(OTf), in CH,CI, (0.4 M,
25 pL, 0.01 mmol) was added to a solution of 3-phenylpropanal (665 pL,
5 mmol) and allytrimethylsilane (1 12 mL, 7 mmol) in CH,CI, (10 mL). Analysis by TLC indicated that the aldehyde bad been consumed after 25 min. MeOH
(1 mL) and hydrochloric acid (36%, 12 drops) were added. Analysis by TLC
indicated the desilylation to be complete in 40 min. The mixture was washed
with NaHCO, aq. and extracted with CH,CI,, and the extract was washed with
NaCl aq. and dried (MgSO,). Evaporation of the solvent followed by flash
chromatography gave l-phenyl-S~hexen-3-ol(824 mg, 93 %) [D. Seebach, R.
Imwinkelried, G. Stucky, Helv. Chim. -4cta 1984,70,448], 'H NMR (300 MHz,
CDCI,): 6 =1.70-2.05 (m. 3 H ; O H + PhCH,), 2.10-2.37 (m, 2 H ;
CH,CHOH), 2.62-2.87 (m, 2 H; CH,CH = CH,), 3.60-3.72 (m. 1 H ; HCOH)
5.05-5.20 (m,2H ; CH = CH,), 5.70-5.95 (m, 1 H ; C H = CH,), 7.08-7.29
(m, 5 H ; Ph). 36 mg of a less polar by-product was also isolated.
Received: October 31, 1991 [Z 5002 IE]
German version: Angew. Chem. 1992, 104,475
Angen. Chem. Int. Ed. Engl. 31 (1992) No. 4
CAS Registry numbers:
1, 762-72-1; 2, R = Ph, 936-58-3; 2, R = Ph(CH,),, 60340-28-5; 2, R = nC,H,, 40575-42-6; 2, R = c-C,H,,, 69036-26-6; 3, R = n-C,H,. 139462-61-6;
A, 139462-62-7; PhCHO, 100-52-7; PhJCH,),CHO, 104-53-0; n-C,H,CHO,
110-62-3; c-C,H, ,CHO, 2043-6 1-0.
111 H. Sakurai, Pure Appl. Chem. 1982, 54, 1; I. Fleming, J. Dunogues, R.
Smithers, Organic Reacrions 1989, 37, 57.
[2] A. P. Davis, M. Jaspars, J. Chem. Soc., Chem. Commun. 1990, 1176.
[3] T. Tsunoda, M. Suzuki, R. Noyori, Tetrahedron Lett. 1980, 21, 71.
[4] H. Sakurai, K. Sasaki, A. Hosomi, Tetrahedron Lett. 1981, 22, 745.
(51 A. Mekhalfia, I. E. Marko, Tetrahedron Lett. 1991, 32, 4779.
[6] References [3-51 make specific mention of failed attempts to catalyze the
addition using the relevant silylating agents. These attempts are contrasted
with other reactions which proceed more readily, such as that between
allylsilanes and acetals.
[7] Silylating agents have been shown to be active in catalyzing the reactions
of more nucleophilic species with aldehydes. For example, both
Me,SiOTf [8] and the combination Me,SiCI/SnCI, [9] have been reported
to catalyze the addition of silyl enol ethers to conjugated aldehydes, although in the former case only low conversions were achieved.
[8] S. Murata, M. Suzuki, R. Noyori, Tetruhedron 1988, 44, 4259.
[9] N. Iwasawa, T. Mukaiyama, Chem. L e f t . 1987, 463.
[lo] For the preparation of B(OTf),, see: G. A. Olah, 0. Farooq, S. M. F.
Farnia, J. A. Olah, J. Am. Chem. SOC.1988, 110. 2560. Olah et al. have
studied the interaction of R,SiOTf with BCI, and BBr, by I l B and '9Si
NMR spectroscopy but, as far as we are aware, have not reported any
preparative applications of these mixtures; G. A. Olah, K. Laali, 0. Farooq, Organometallics 1984, 3, 1337.
[ l l ] B(OTf), itself is only slightly soluble in CH,CI, or CHCI,.
[12] Compare 6 = - 3.55 (W,,, 53 Hz) for TfOHlB(0Tf); in SO,CIF at
-35°C; G. A. Olah, K. Laali, 0. Farooq, J. Org. Chem. 1984, 49,4591.
[13] It has been reported that trimethylsilyl perchlorate is 'weakly ionic' in
dichloromethane, according to conductivity measurements; J. B. Lambert,
J. A. McConnell, W. Schilf, W. J. Schultz Jr., J. Chem. Sor. Chem. Commuu.
[14] Compare Me,SiOTf 6(H) = 0.50, 6(C) = 0.3 ('J(C,Si) = 60 Hz); Me,SiI
6(H) = 0.53, 6(C) = 6.5 ('J(C,Si) = 54 Hz). In particular, the fact that
'J(C,Si) for Me,SiB(OTf), is so similar to the foregoing values suggests
that the hybridization at Si is similar to that in Me,SiOTf and Me,Sil;
K. D. Summerhays, D. A. Deprez, J. Organometallic Chem. 1976, 1fR. 19.
[15] H. Emde, D. Domsch, H. Feger, U. Frick, A. Gotz, H. H. Hergott, K.
Hofmann, W. Kober, K. Krageloh, T. Oesterle, W. Steppan, W. West. G.
Simchen, Synthesis 1982, 1.
[16] G. A. Olah, S . C. Narang, Tetrahedron 1982, 38, 2225.
[17] T. Morita, Y.Okamoto, H. Sakurai, Synthesis 1981, 745.
The Electronic Structure of [($-C,H,),Co,] :
Comment on the Existence of a Complex with
an Unsupported Co-Co Double Bond
By Harmon B. Abrahamson, Gerald ,P. Niccolai,
D . Michael Heinekey, Charles P . Casey,
and Bruce E. Bursten*
Schneider et al. have recently reported the synthesis of a
dinuclear cobalt complex 1, formulated as [CpfCo,]
(Cp* = q5-C,Me,), by the cocondensation of Co atoms and
Prof. B. E. Bursten
Department of Chemistry
The Ohio State University
Columbus, OH 43210 (USA)
Prof. H. B. Abrahamson
Department of Chemistry
University of North Dakota
Grand Forks, ND 58202 (USA)
G. P. Niccolai, Prof. C. P. Casey
Department of Chemistry
University of Wisconsin - Madison
Madison, WI 53706 (USA)
Prof. D. M. Heinekey
Department of Chemistry
University of Washington
Seattle, WA 98195 (USA)
Verlagsgesellschaft mbH, W-6940 Weinheim. 1992
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aldehyde, catalysing, silylating, allyltrimethylsilane, agenti, additional
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