Radiolytic silylation of alkenes and alkynes by gaseous R3Si+ ions. Stereochemical evidence for the β-silyl effect

Article abstract of DOI:10.1021/ja9734152

Carbocation intermediate stabilized by a β silyl group have been characterized using the silylation of alkenes by R₃Si⁺ ions as a route of formation. Neutral silylated products have been obtained from the reaction of R₃Si⁺ ions, generated in a gaseous medium at atmospheric pressure by a radiolytic technique, with selected alkenes, alkynes, and allene, thereby indicating the occurrence of electrophilic silylation. Notable fectures of the charged silylated intermediates emerge from the isomeric product distribution. The silylation of cis- and trans-2-butene shows a high degree of retention of configuration, as expected if a bridged species (I) were the reaction intermediate. Alternatively, the intermediacy of an open structure (II), whereby C-C bond rotation is inhibited by the hyperconjugative interaction between the β silyl group and the vacant up orbital, should be inferred. The charged intermediates from the silylation of alkenes and alkynes are found to be unreactive toward conceivable isomerizations to more stable species, such as the ones bearing the positive charge of silicon. Stereoelectronic factors affect the deprotonation of the silylated intermediates, which may involves loss of the proton either the α or the γ position with respect to the silylated carbon. A comparison of the reactivity of alkenes and alkynes in the cationic silylation reaction is presented.

Full text of DOI:10.1021/ja9734152

J. Am. Chem. Soc. 1998, 120, 1523-1527
1523
+
Radiolytic Silylation of Alkenes and Alkynes by Gaseous R Si Ions.
3
Stereochemical Evidence for the â-Silyl Effect
Barbara Chiavarino, Maria Elisa Crestoni, and Simonetta Fornarini*
Contribution from the Dipartimento di Chimica e Tecnologia delle Sostanze Biologicamente AttiVe,
UniVersit a` di Roma “La Sapienza”; P.le A. Moro 5, I-00185 Roma, Italy
ReceiVed September 30, 1997
Abstract: Carbocation intermediates stabilized by a â silyl group have been characterized, using the silylation
+
of alkenes by R3Si ions as a route of formation. Neutral silylated products have been obtained from the
+
reaction of R3Si ions, generated in a gaseous medium at atmospheric pressure by a radiolytic technique, with
selected alkenes, alkynes, and allene, thereby indicating the occurrence of electrophilic silylation. Notable
features of the charged silylated intermediates emerge from the isomeric product distribution. The silylation
of cis- and trans-2-butene shows a high degree of retention of configuration, as expected if a bridged species
(I) were the reaction intermediate. Alternatively, the intermediacy of an open structure (II), whereby C-C
bond rotation is inhibited by the hyperconjugative interaction between the â silyl group and the vacant p
orbital, should be inferred. The charged intermediates from the silylation of alkenes and alkynes are found to
be unreactive toward conceivable isomerizations to more stable species, such as the ones bearing the positive
charge on silicon. Stereoelectronic factors affect the deprotonation of the silylated intermediates, which may
involve loss of the proton from either the R or the γ position with respect to the silylated carbon. A comparison
of the reactivity of alkenes and alkynes in the cationic silylation reaction is presented.
One of the salient features of organosilicon chemistry is the
stabilizing effect exerted on a carbenium ion by a silyl group
in a position â to the carbon bearing the formal charge, the
that include species I/II as possible isomers, have been generated
from different precursors, and their unimolecular and bimo-
lecular reactivity has been investigated by mass spectrometric
techniques. These experimental studies have been accompanied
by MO calculations, providing a fairly comprehensive picture
of the relative stabilities of isomeric structures and their
1
so-called â effect. A remarkable number of reactivity studies
in solution, mainly focused on solvolytic processes, have
reported on the stabilization of a positive charge by â silicon,
which gives rise to low energy transition states and/or inter-
3
interconversion pathways. However, the question of whether
12
mediates, causing rate enhancements of up to 10 with respect
I or II should be favored remains open. We have used the R -
3
2
+
to model systems. An open question regards the structure of
Si reaction with alkenes as an entry to I/II type species,
+
the postulated intermediate species. It may in fact be described
either as a bridged structure (I), where the Si atom exploits the
ability to expand its coordination, or as a â silylated carbenium
ion (II). The magnitude of the developing â-silicon effect
depends in a crucial way on the p-σCSi hyperconjugation, which
is maximized when the 2p orbital developing on the cationic
carbon and the C-Si σ bonding orbital are aligned in the same
plane. However, the actual involvement of structure I vs II is
hard to prove.2
because the reaction of Me Si ions with alkenes was known
3
to yield abundant adduct ions in a high-pressure ion source.4
The equilibrium constants for the association reactions (eq 1)
were studied as a function of temperature, and thermodynamic
data were derived.
+
+
Me Si + R R CdCR R f [Me Si-CR R CR R ] (1)
3
1
2
3
4
3
1
2
3 4
e
(2) Recent examples include: (a) Lambert, J. B.; Wang, G.; Finzel, R.
B.; Teramura, D. H. J. Am. Chem. Soc. 1987, 109, 7838. (b) Lambert, J.
B.; Chelius, E. C. J. Am. Chem. Soc. 1990, 112, 8120. (c) Bausch, M. J.;
Gong, Y. J. Am. Chem. Soc. 1994, 116, 5963. (d) Gabelica, V.; Kresge, A.
J. J. Am. Chem. Soc. 1996, 118, 3838. (e) Lambert, J. B.; Liu, X. J.
Organomet. Chem. 1996, 521, 203. (f) Surya Prakash, G. K.; Reddy, V.
P.; Rasul, G.; Casanova, J.; Olah, G. A. J. Am. Chem. Soc. 1992, 114,
3
076. (g) Siehl, H.-U.; Kaufmann, F. P.; Apeloig, Y.; Braude, V.; Danovich,
D.; Berndt, A.; Stamatis, N. Angew. Chem., Int. Ed. Engl. 1991, 30, 1479.
e) Siehl, H.-U.; Kaufmann, F. P.; Hori, K. J. Am. Chem. Soc. 1992, 114,
343.
3) (a) Hajdasz, D.; Squires, R. J. Chem. Soc., Chem. Commun. 1988,
(
9
In principle, the study of naked ions in the gas phase should
provide the most objective description of ion structures and
energetics. Gaseous ions of the general formula CnH2n+3Si ,
(
+
1212. (b) Drewello, T.; Burgers, P. C.; Zummack, W.; Apeloig, Y.; Schwarz,
H. Organometallics 1990, 9, 1161. (c) Bakhtiar, R.; Holznagel, C. M.;
Jacobson, D. B. J. Am. Chem. Soc. 1992, 114, 3227. (d) Ciommer, B.;
Schwarz, H. J. Organomet. Chem. 1983, 244, 319. (e) Apeloig, Y.; Karni,
M.; Stanger, A.; Schwarz, H.; Drewello, T.; Czekay, G. J. Chem. Soc.,
Chem. Commun. 1987, 989. (f) Maerker, C.; Kapp, J.; Schleyer, P. v. R. In
Organosilicon Chemistry II; Auner, N., Weis, J., Eds.; VCH: Weinheim,
1996. (g) Angelini, G.; Keheyan, Y.; Laguzzi, G.; Lilla, G. Tetrahedron
Lett. 1988, 29, 4159.
(1) (a) Fleming, I. Chem. Soc. ReV. 1981, 10, 83. (b) Lambert, J. B.
Tetrahedron 1990, 46, 2677. (c) Apeloig, Y. In The Chemistry of Organic
Silicon Compounds; Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1989;
chapter 2. (d) Bassindale, A. R.; Taylor, P. G. The Chemistry of Organic
Silicon Compounds; Chapter 14. (e) Eaborn, C.; Bott, R. W. In Organo-
metallic Compounds of the Group IV Elements; MacDiarmid, A. G., Ed.;
Marcel Dekker: New York, 1968; Vol. 1, Part 1. (f) Nguyen, K. A.; Gordon,
M. S.; Wang, G.; Lambert, J. B. Organometallics 1991, 10, 2798.
(4) Li, X.; Stone, J. A. J. Am. Chem. Soc. 1989, 111, 5586.
S0002-7863(97)03415-X CCC: $15.00 © 1998 American Chemical Society
Published on Web 02/06/1998

1524 J. Am. Chem. Soc., Vol. 120, No. 7, 1998
ChiaVarino et al.
Table 1. Trialkylsilylation of Alkenes by Gaseous R
3
Si+ Ions
For example, the binding energy for the association of Me3-
Si+ to ethylene (-∆H°1 for R1 ) R2 ) R3 ) R4 ) H) was
-
1
found to be equal to 23.6 kcal mol , in good agreement with
a “corrected” theoretical value referred to I as the structure for
+
3e
the [Me3Si-C2H4] adduct ion. A high equilibrium concen-
+
tration of the [Me3Si-alkene] adducts is expected at the
conditions prevailing in the radiolytic experiments, where the
5
high bath gas pressure (ca. 700 Torr) allows collisional
relaxation of possibly excited primary species and inhibits the
occurrence of isomerization processes characterized by sub-
stantial activation energies. These features, combined with the
identification of the neutral reaction products, offered to us the
possibility to investigate the regio- and stereochemistry of the
+
R3Si reaction with alkenes and alkynes and to obtain informa-
tion on the relevant ionic intermediates.
Experimental Section
4 2
Materials. CH and O were high-purity (>99.9 mol %) gases from
Matheson Co. The other chemicals to be used as substrates or additives
or as GLC-MS standards were purchased from Aldrich Chemical Co.
The silylated alkenes that were not commercially available were
3
prepared from the Wurtz-Fittig reaction of R SiCl with the appropriate
6
alkenyl bromide and were purified by preparative GLC using a 4-m
long, 4-mm i.d. stainless steel column, packed with 5% Carbowax 20M-
KOH (2%) on Supelcoport. Their identity was verified by NMR
spectrometry and MS.
Radiolytic Experiments. The gaseous mixtures were prepared in
Pyrex vessels (135 mL) that were thoroughly outgassed and filled with
the gaseous components in the selected ratio. Liquid components were
introduced within weighted fragile glass ampules. The vessels were
sealed, and gases and vapors were allowed to mix and then submitted
to γ-radiation at 40 °C in a 220 Gammacell from Nuclear Canada Ltd
for 2 h at the dose rate of 10 Gy h . The radiolytic products were
extracted by freezing the vessel at 77 K and careful washing of its
inner surface with n-hexane. The final solution, containing the
unreacted substrate as major component (conversion into products is
kept to ca. 1%), was analyzed by GLC-MS using a Hewlett-Packard
a
All systems contained CH
4
(500-600 Torr), a radical scavenger
+
(
O
2
at 10 Torr), and the neutral precursor of R
3
Si (20 Torr of Me
4
Si,
b
c
R ) Me, or Et
cal yield, G
3
SiH, R ) Et). Average error ( 1%. The radiochemi-
-1
M
, is given in units of µmol J . In the absence of added
amine, the formation of silylated products is below the detection limit.
The unreacted substrate does not show any evidence of cis-trans
isomerization.
d
4
-1
Me, Et) which can be generated by this route both by mass
spectrometric techniques and by γ-radiolysis.
Silylation of Alkenes. Table 1 summarizes the pattern of
products from the reaction of R3Si with some representative
alkenes: ethylene (1), propene (2), 1-butene (3), iso-butene (4),
cis-2-butene (5), and trans-2-butene (6). The R3Si-substituted
products are indicative of the formation of covalent [R3Si-
alkene] adducts, which are subsequently neutralized by proton
transfer to a base. The ionic origin of the observed products is
ensured by the presence of O2 in the medium, as an effective
8
9
5
890 gas chromatograph in line with a Model 5989B quadrupole mass
spectrometer. The following columns were used: (i) a 100 m long,
.32-mm i.d. Petrocol DH fused silica capillary column and (ii) a 50
+
0
m long, 0.32-mm i.d. PONA fused silica capillary column. The
radiolytic products were identified by comparisons of their retention
times and EI mass spectra with those of authentic samples under the
same operating conditions. Their yields were determined from the areas
of the corresponding elution peaks using the internal standard calibration
method. Blank experiments were performed to exclude any thermal
contribution to the products of interest. To this end, gaseous mixtures,
identical to the radiolytic samples, were submitted to the same
experimental procedure except for the γ-irradiation.
+
10
silyl radical scavenger, and also by the negligible yield of
silylated products recovered in the absence of a strong nitrogen
base, typically triethylamine (TEA). As pointed out previously,9
the base performs the critical role of deprotonating the cationic
silylated intermediates, which are otherwise prone to evolve by
a desilylation process effected by oxygen nucleophiles. Oxygen
nucleophiles such as H2O, MeOH, and Me2CO are present in
trace amounts in the radiolytic systems, as unavoidable impuri-
ties or radiolytic side products. The silylation of alkenes appears
then to conform to a general scheme similar to the one
Results and Discussion
Silyl cations are easily generated in the gas phase from many
7
different neutral precursors. In particular, the reaction of Me4-
+
+
Si or Et3SiH with the CH5 and C2H5 ions from the primary
+
ionization of CH4 is an efficient source of R3Si ions (R )
9
describing the silylation of aromatics (Scheme 1). In particular,
the ionic intermediate from R3Si attack at the alkene double
(5) (a) Cacace, F. Acc. Chem. Res 1988, 21, 215. (b) Speranza, M. Mass
+
Spectrom. ReV. 1992, 11, 73. (c) Cacace, F. In Structure/ReactiVity and
Thermochemistry of Ions; Ausloos P., Lias, S. G., Eds.; Reidel: Dordrecht,
bond undergoes competition between desilylation by oxygen
nucleophiles and deprotonation (possibly accompanied by
desilylation) by an added base. This reaction pattern explains
why no neutral silylation products are obtained in the absence
of an amine. Because the proton affinity (PA) of the silylated
alkene is expected to be high (a PA value of 199 kcal mol
has been derived for Me3SiCHCH2), strong bases have been
selected, namely TEA (PA ) 232 kcal mol ), piperidine
1
987.
(6) Hudrlik, P. F.; Kulkarni, A. K.; Jain, S.; Hudrlik, A. M. Tetrahedron
1
983, 39, 877.
(
7) Schwarz, H. In The Chemistry of Organic Silicon Compounds; Patai,
S., Rappoport, Z., Eds.; Wiley: New York, 1989; Chapter 7.
8) Wojtyniak, A. C. M.; Stone, J. A. Int. J. Mass Spectrom. Ion
Processes 1986, 74, 59.
9) (a) Cacace, F.; Crestoni, M. E.; Fornarini, S.; Gabrielli, R. Int. J.
Mass Spectrom. Ion Processes 1988, 84, 17. (b) Fornarini, S. J. Org. Chem.
988, 53, 1314. (c) Cacace, F.; Attin a` , M.; Fornarini, S. Angew. Chem.,
Int. Ed. Engl. 1995, 34, 4, 654.
(
-1
3
a
(
-
1 11
1
(10) Niiranen, J. T.; Gutman, D. J. Phys. Chem. 1993, 97, 4106.

Radiolytic Silylation of Alkenes and Alkynes
J. Am. Chem. Soc., Vol. 120, No. 7, 1998 1525
Scheme 1
at the C-C double bond. Even though the alternative possibility
of an open structure (II) cannot a priori be excluded, the present
results do exclude that the formerly double C-C bond may
+
12
freely rotate, following the R3Si addition.
+
Calculations on [H3Si-CH2-CH-CH3] as a model to
evaluate the â silicon effect on secondary carbenium ions predict
the corresponding open structure II to be more stable than the
-
1 13
cyclic species (I) by only 4 kcal mol . The barrier to C-C
-1
bond rotation in this model system is evaluated as 22 kcal mol
and arises from the increase in energy due to the attainment of
an orthogonal relationship between the vacant p and σCSi orbitals.
+
It may be considered that the Me3Si addition to trans-2-butene
-1
is an exothermic process releasing 31 kcal mol in the primary
4
adduct ions; therefore, C-C bond rotation might occur within
+
*
-
1
excited [Me Si-MeCHCHMe] ions, if the related barrier is
(
(
PPD, PA ) 226 kcal mol ), and N,N-diisopropylethylamine
3
-
1
-
1
indeed around 22 kcal mol . Thus, the observed high degree
of retention of stereochemistry at the double bond is remarkable
and suggests that in the high-pressure environment of the
radiolytic systems fast collisional quenching of the excited
intermediates has taken place.
PEA, PA ) 232 kcal mol ). Their basicities are expected to
significantly exceed those of the silylated alkenes. Therefore
the changes in the absolute yield of products (GM) when the
amine is varied and other factors are kept constant are assigned
to structural factors. An increased yield of products is observed
+
Although the R-deprotonation products, namely the silylated
alkenes retaining the double bond position, have provided
valuable mechanistic information, nevertheless they are ac-
companied by noticeable yields of γ deprotonation products.
Because of the scant thermochemical information on silicon
in the reaction of Me3Si with cis-2-butene on going from PPD
to PEA and in the reaction with trans-2-butene on going from
PPD to TEA. In both cases an increasing steric bulk of the
base favors the formation of the neutral silylated products, a
result that may reflect a reduced extent of an unproductive
desilylation process effected by the increasingly hindered
nitrogen base.
A feature that is common to all the silylated products is that
the entering silyl group appears to bind to one of the carbon
atoms formerly involved in the double bond, while the carbon
skeleton of the olefin is retained. Within the framework of
Scheme 1, this finding implies that the primary ionic intermedi-
ate, either I or II, does not undergo rearrangement processes
14
compounds, it cannot be stated whether this finding may reflect
the relative thermodynamic stabilities of the two types of
products. A major structural difference is that the alkenes
formed by R deprotonation bear a silyl group as substituent of
the double bond, at variance with the ones arising from γ
deprotonation. However, the dependence on the structural
features of the base suggests that the product distribution is
governed by kinetic effects. The formation of γ deprotonation
products can be assigned to the contribution of stereoelectronic
factors at the transition state for the deprotonation which can
be viewed in the Newman projections of II, a similar reasoning
applying to I as the ionic intermediate. For example, species
(
e.g., path d in Scheme 1) but is efficiently deprotonated by
the base B. Thus, the unambiguous structural assignment of
the isomeric silylation products definitely confirms the conclu-
sions of mass spectrometric and computational studies, ascribing
a high activation barrier to the isomerization of I/II, especially
if compared to the energy required for the dissociation to R3-
+
IIa, deriving from Me3Si attack to propene, is best suited to
p-σCSi hyperconjugation, allowed by the parallel orbital align-
ment. Deprotonation at the γ position may occur maintaining
the full advantage of the favorable orbital interaction. However,
in the transition state for deprotonation at the R carbon p-σCSi
hyperconjugation is at least partially lost (IIb), which may
account for the typically lower yields of R deprotonation
products.
+
3
Si and alkene (the reverse of step a in Scheme 1).
The deprotonation step may occur with different regioselec-
tivity because the base can remove the proton from either the
R or the γ position with respect to the silyl bearing carbon, to
an extent that depends on the branching at the double bond, on
the electrophile, and on the structure of the base. However,
the most relevant result from the stereochemistry is the R
deprotonation pathway, yielding the silylated alkene. In fact,
the isomeric cis- and trans-2-butenes yield silylated butenes
retaining the original stereochemistry to a noticeable extent,
namely ca. 80% for the cis-isomer and ca. 90% for the trans-
isomer when TEA is the base. The lack of stereoconvergence
from the silylation of the two geometric isomers points to the
effective role of the entering silyl group in preventing C-C
bond rotation prior to deprotonation by the base. The most
straightforward way to explain this effect suggests the formation
Silylation of Alkynes. Table 2 summarizes the pattern of
+
products from the reaction of Me3Si with selected alkynes
(acetylene (7), propyne (8), 2-butyne (10), and 1-pentyne (11))
and allene (9). Once again, the formation of neutral silylated
products is in favor of an ionic silylated intermediate which
can be depicted either as a bridged species or as a â-silyl
substitued vinyl cation. The formation of the silylation product
of acetylene itself, the parent alkyne, is remarkable, in view of
+
of a cyclic structure (I) from the electrophilic attack of R3Si
(
11) Unless stated otherwise, all thermodynamic data is taken from: Lias,
S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D; Mallard,
N G. J. Phys. Chem. Ref. Data, Suppl. 1 1988, 17.
(
12) The ambiguity between a symmetrical cyclic intermediate and an
open structure, whereby the electronic interaction of a neighboring group
with the vacant orbital of a â positive site inhibits C-C bond rotation, is
frequently met in solution chemistry and is also found in gas-phase reactions,
for example: (a) Fornarini, S.; Sparapani, C.; Speranza, M. J. Am. Chem.
Soc. 1988, 110, 34. (b) Fornarini, S.; Sparapani, C.; Speranza, M. J. Am.
Chem. Soc. 1988, 110, 42.
(13) Ibrahim, M. R.; Jorgensen, W. L. J. Am. Chem. Soc. 1989, 111,
819.
(14) Walsh, R. In The Chemistry of Organic Silicon Compounds; Patai,
S., Rappoport, Z., Eds.; Wiley: New York, 1989.
(15) Zhang, W.; Stone, J. A.; Brook, M. A.; McGibbon, G. A. J. Am.
Chem. Soc. 1996, 118, 5764.

1526 J. Am. Chem. Soc., Vol. 120, No. 7, 1998
ChiaVarino et al.
Table 2. Trialkylsilylation of Alkynes and Allene by Gaseous
Me Si Ions
3
However, the occurrence of the propenyl ion f allyl ion
rearrangement can be disproved on the following basis: (i) the
direct deprotonation of an allyl type ion should lead exclusively
to an allene, e.g., Me3SiCHdCdCH2 from the propyne reaction,
whereas this product is accompanied by major amounts of
+
1-Me3Si-1-propyne; (ii) the propenyl ion f allyl ion rearrange-
ment involves substantial activation barriers, even when it is
+
+
markedly exothermic, as in the CH3CdCH2 f CH2CHCH2
process;
11,20
(iii) the allyl product ion loses the hyperconjugative
contribution of â silyl stabilization since no formal positive
charge is placed in a position â to the silyl group. A direct
proof that the propenyl ion f allyl ion rearrangement does not
occur, or at least is by no means complete, can be seen from
+
the products formed from the Me3Si reaction with allene. The
+
Me3Si reaction with allene yields an ionic intermediate that
may rearrange to the same allyl ion (eq 4) that can be obtained
from the propyne reaction.
+
Me Si + CH dCdCH f
3
2
2
a
See note (a) of Table 1. See note (b) of Table 1. c See note (c) of
b
+
+
[Me Si-CH CCH ] f [Me Si-CHCHCH ] (4)
3
2
2
3
2
d
Table 1. Other products contain two substrate units, probably deriving
from addition of a primary [substrateSiMe ] ionic intermediate to a
second substrate molecule, followed by deprotonation.
+
3
However, the two C3H4 isomers yield a different pattern of
silylated products, whose formation is accounted for by retention
of both the carbon and the hydrogen framework in the primary
intermediates.
Relative Reactivity of Alkenes and Alkynes. The absolute
M
values of radiochemical yields (G ) are fairly high, accounting
the inability to observe the silylated adduct ion in high-pressure
mass spectrometry.15 This contrasting behavior is probably the
result of the different pressure range used by the two techniques.
The Me3Si addition to triple bonds is less exothermic than
the reaction with similarly substituted alkenes. For example,
+
+
+
15
5 2
for a large fraction of the primary CH and C H5 ions
produced from the radiolysis of CH4.
2
1
-
1
This finding testifies
a binding energy of only 18 kcal mol was estimated for the
association of Me3Si to acetylene (eq 2).
+
15,16
that the observed reaction products account for the most
important, if not the exclusive, ionic pathways taking place
within the gaseous medium. It is therefore tempting to relate
the yields to the activation of the individual alkynes and alkenes
toward electrophilic attack, though this reasoning is undermined
by the complex character of the silylation reaction (Scheme 1),
+
+
Me Si + CHCH f [Me Si-CHCH]
(2)
3
3
This finding parallels the somewhat smaller intrinsic stability
of vinyl cations compared to alkyl cations,11 whereas the â
stabilization due to a silyl group is large and comparable in
magnitude for both vinyl and alkyl cations. An additional
feature emerging from the R3Si reaction with alkenes and
+
where the R3Si attack is followed by competing routes, one
of them (the desilylation route) leading to the unreacted
substrate. Each step is likely to be affected to an unknown
extent by the alkyl substituents at the multiple bond, by the
features of the base, and by the attacking electrophile. It is
known, for example, that the presence of Et3Si in the place of
Me3Si as â substituent makes the cationic intermediate more
17
+
+
alkynes is the higher tendency of the [R3Si-alkyne] adduct to
attack a second alkyne molecule yielding dimeric products, as
shown both by the radiolytic reactions and by high-pressure
mass spectrometric experiments.15
The silylation of alkynes and allene does not provide the
stereochemical insight on the structure of the ionic intermediates
as in the case of the alkene reactions. Once again, all silylated
products retain the carbon skeleton of the substrate, and the
silyl group is attached to the unsubstituted end of the triple bond
of the products, whenever such a choice is possible. In
particular, no products are found that may derive from a
22
resistant to nucleophilic desilylation. This fact is confirmed
by the reactivity of 1-butene (3), whose reaction with Et3Si in
+
the presence of TEA gives the highest yield of silylated alkenes.
Within these limitations, absolute yields were obtained that
suggest alkynes to be moderately more reactive than alkenes.
This indication is confirmed by competition experiments where
accurately known concentrations of alkyne and alkene were
+
allowed to react with Me3Si in the presence of PEA. The
1
8,19
1
-silaallyl cation.
Having excluded skeletal rearrangements, the possibility also
exists of a propenyl ion f allyl ion isomerization which may
follow the Me3Si attack to propyne and higher homologues.
ensuing product yields gave an apparent reactivity ratio kalkyne/
(
(
18) Mayer, T. M.; Lampe, F. W. J. Phys. Chem. 1974, 78, 2645.
19) Ketvirtis, A. E.; Bohme, D. K.; Hopkinson, A. C. J. Phys. Chem.
+
1994, 98, 13225.
This process is exemplified in the following propyne reaction
(20) (a) McAllister, M.; Tidwell, T. T.; Peterson, M. R.; Czismadia, I.
G. J. Org. Chem. 1991, 56, 575. (b) Raghavachari, K.; Whiteside, R. A.;
Pople, J. A. Schleyer, P. v. R. J. Am. Chem. Soc. 1981, 103, 5649. (c)
Fornarini, S.; Speranza, M.; Attin a` , M.; Cacace, F.; Giacomello, P. J. Am.
Chem. Soc. 1984, 106, 2498.
(eq 3).
+
+
Me Si + CH CCH f [Me Si-CHCCH ] f
3
3
3
3
(
21) Ausloos, P.; Lias, S. G.; Gorden, R., Jr. J. Chem. Phys. 1963, 93,
+
4
010. It is important to remind that the silylation reaction follows from a
stepwise pattern where both the CH5 /C2H5 reaction with Me4Si to form
the reactant Me3Si ion and the Me3Si reaction with the unsaturated
substrates face competition with an unproductive reaction channell with
the added base. In this view, the observed GM values compare well with
[
Me Si-CHCHCH ] (3)
3 2
+
+
+
+
(
16) McGibbon, G. A.; Brook, M. A.; Terlouw, J. K. J. Chem. Soc.,
Chem. Commun. 1992, 360.
17) Wierschke, S. G.; Chandrasecha, J.; Jorgensen, W. L. J. Am. Chem.
Soc. 1985, 107, 1496.
-
1
+
+
(
the GM value of ca. 0.3 µmol J of the CH5 /C2H5 ions in methane.
(22) Siehl, H.-U. Pure Appl. Chem. 1995, 67, 769.

Radiolytic Silylation of Alkenes and Alkynes
J. Am. Chem. Soc., Vol. 120, No. 7, 1998 1527
kalkene equal to 7 from the 1-pentyne/1-pentene pair and equal
carbon is established, may not be excluded. In any event, the
to 20 from the propyne/propene pair.
lack of stereoconvergence from the silylation of cis- and trans-
2
-butene shows that the rotation about the former C-C double
Conclusions
bond is largely inhibited. In this context, the behavior of silyl
radicals in the addition reaction to double bonds marks a striking
contrast. The (Me3Si)3Si radical is in fact found to promote
the cis-trans interconversion of alkenes by an addition-
elimination sequence, where the key step is the facile rotation
about the C-C bond of the â silyl substituted alkyl radical
intermediate. In the third place, the characterization of the
end silylation products has demonstrated that neither [R3Si-
alkene] nor [R3Si-alkyne] adducts show any tendency to
isomerize in the high-pressure environment, though their depro-
tonation may involve the R or γ site, yielding isomeric neutral
products.
The investigation of the R3Si+ reaction with alkenes and
.
alkynes using a radiolytic approach has yielded valuable
information on the following issues. In the first place, an
electrophilic silylation forming neutral silylated alkenes, alkynes,
and allene has been achieved, taking advantage of the ready
2
3
+
availability of R3Si ions in the gas phase and of selected
reaction conditions. In particular, a stringent requirement for
obtaining silylated products lies in the presence of a strong and
preferably hindered nitrogen base, able to perform fast depro-
tonation of the charged silylated intermediates, which are
otherwise prone to evolve by a desilylation channel. This
behavior resembles the reactivity pattern typical of aromatic
silylation. In the second place, the foremost role of the â silyl
+
+
Acknowledgment. Financial support by the Italian Ministero
dell’ Universit a` e della Ricerca Scientifica e Tecnologica and
by Consiglio Nazionale delle Ricerche is gratefully acknowl-
edged. The authors wish to thank Prof. Fulvio Cacace for
stimulating discussions.
+
group has clearly emerged from the outcome of the R3Si
reaction with cis- and trans-2-butene, which shows a remarkable
degree of retention of stereochemistry at the double bond. This
result is accounted for by a cyclic silyl-bridged species (I) as
reaction intermediate, though the alternative possiblity of an
open structure (II), whereby a strong hyperconjugative interac-
tion between the σCSi bond and the vacant p orbital on the â
JA9734152
(
23) Chatgilialoglu, C.; Ballestri, M.; Ferreri, C.; Vecchi, D. J. Org.
Chem. 1995, 60, 3826.