Nucleophilic substitutions on silane cation radicals: Stepwise or concerted? [13]

Full text of DOI:10.1021/ja011207a

9698
J. Am. Chem. Soc. 2001, 123, 9698-9699
2H).6 This hypothesis is supported by nanosecond transient
absorption experiments, as described below.
Nucleophilic Substitutions on Silane Cation Radicals:
Stepwise or Concerted?
Pulsed laser excitation (343 nm, 15 ns, 3 mJ) of dioxygen-
saturated dichloromethane solutions containing silanes 1-4 (∼20
mM), N-methylquinolinium hexafluorophosphate (NMQ) as a
photosensitizer,1e and toluene (∼1 M) as a co-donor, all resulted
in the formation of similar transient species with λmax ≈ 640 nm,
similar to that of fluorene cation radical.7 We assign the transients
to 1+•-4+• based on this and the fact that they all reacted with
1,2,4,5-tetramethoxybenzene (TMB) with diffusion-controlled
rates to produce TMB+• (λmax ) 460 nm).8 All of the cation
radicals reacted with a variety of nucleophiles with clean second-
order kinetics. As shown in Table 1, the rate constants for reaction
of 1+•-4+• with nucleophiles (k1, monitored at 640 nm) and the
rate constants for growth of the 9-fluorenyl radical (k2, monitored
at 503 nm in argon-saturated solutions) were found to be
indistinguishable within experimental error.
It is especially noteworthy that k1 ) k2 for silacyclobutane
cation radical 2+•. Making the traditional assumptions that
nucleophilic addition and leaving-group departure occur from axial
positions and that steric constraints require the silacyclobutane
ring to bridge one axial and one equatorial position,9 then a
stepwise mechanism would require nucleophilic addition to 2+•
to form a pentacoordinate intermediate, followed by pseudo-
rotation before departure of the fluorenyl radical. Thus, k1 ) k2
for 2+• requires either nucleophilic addition to be reversible, or
that both pseudorotation and leaving-group departure are rapid
relative to nucleophilic attack. At the highest nucleophile
concentrations used, where the lifetime of 2+• is shortest, our
kinetic data require that the lifetime of the hypothetical intermedi-
ate be e50 ns.
H. J. Peter de Lijser, Darren W. Snelgrove, and
Joseph P. Dinnocenzo*
Department of Chemistry
Center for Photoinduced Charge Transfer
UniVersity of Rochester
Rochester, New York 14627-0216
ReceiVed May 16, 2001
A variety of organosilane cation radicals have recently been
shown to react with nucleophiles via a bimolecular substitution
mechanism.1-4 Left unresolved by these studies is whether the
substitution reactions are stepwise or concerted. Herein we present
a combination of kinetic data, structure-reactivity comparisons,
and stereochemical experiments that favor a concerted substitution
mechanism.
If silane cation radical substitutions proceed via a stepwise
mechanism involving pentacoordinate intermediates that haVe
significant lifetimes then, in principle, it should be possible to
kinetically detect their intermediacy, even if they cannot be
directly observed. In this case, the rate of reaction of the cation
radicals with nucleophiles will exceed the rate of leaving-group
departure. Obviously, a concerted substitution mechanism requires
these rates to be identical. Silanes 1-4 were prepared for this
kinetic test. These silanes have an advantage over previously
studied systems in that transient absorption spectroscopy can be
used to follow both reaction of the silane cation radicals with
The reactivity of 2+• relative to that of the other silane cation
radicals is also noteworthy when compared to data for neutral
silanes. The relative rate constants for reaction of 1-methylsila-
cyclobutane, 1-methylsilacyclopentane, triethylsilane, and 1-
methylsilacyclohexane with hydroxide ion are 106, 10, 1, and 0.1,
respectively.9 As the data in Table 1 clearly show, silane cation
radicals 1+•-4+• do not follow this reactivity trend. Indeed,
silacyclobutane cation radical 2+• is slightly less reactive than
the acyclic cation radical 1+• - a > 106 reversal of reactivity
compared to the neutral silanes! This suggests a significant change
in reaction mechanism.
nucleophiles and displacement of the 9-fluorenyl radical (λmax
)
503 nm).5
Further mechanistic insight into the cation radical substitutions
was provided by stereochemical experiments. Cis- and trans-1,3-
dimethyl-1-(4-methoxybenzyl)silacyclobutane (5,6) were prepared
for this purpose.10 Photooxidation of these silanes using 1-cyano-
4-methylnaphthalene as a photosensitizer and 4,4′-dimethylbi-
phenyl as a co-donor in CH3OH:CH2Cl2 (2.5:1 v:v) produced 1,3-
dimethyl-1-methoxysilacyclobutanes (7,8)11b as the silane substi-
Loss of the fluorenyl group in 1-4 was confirmed by
preparative photooxidation experiments with 9,10-dicyano-
anthracene (DCA) as a photooxidant in CH3OH (or CH3O2H). In
all cases, fluorene (or fluorene-9-2H) was formed in quantitative
yield. Following previous precedent,1 the reaction is presumed
to proceed by initial electron transfer from 1-4 to the singlet
excited state of DCA to produce cation radicals 1+•-4+•. The
silane cation radicals then react with methanol to produce a
methoxysilane and the 9-fluorenyl radical. The radical is subse-
quently reduced by DCA anion radical to produce the 9-fluorenyl
anion, which is protonated by CH3OH(2H) to give fluorene(9-
(6) (a) The reduction potentials of fluorenyl radical and DCA are -0.76 V
and -0.91 V vs SCE, respectively (ref 6, b and c), thus reduction of the
fluorenyl radical by DCA radical anion is exothermic by approximately 3
kcal/mol. (b) Wayner, D. D. M.; McPhee, D. J.; Griller, D. J. Am. Chem.
Soc. 1988, 110, 132. (c) Gould, I. R.; Ege, D.; Moser, J. E.; Farid, S. J. Am.
Chem. Soc. 1990, 112, 4290.
(1) (a) Dinnocenzo, J. P.; Farid, S.; Goodman, J. L.; Gould, I. R.; Todd,
W. P.; Mattes, S. L. J. Am. Chem. Soc. 1989, 111, 8973. (b) Dinnocenzo, J.
P.; Farid, S.; Goodman, J. L.; Gould, I. R.; Todd, W. P. Mol. Cryst. Liq.
Cryst. 1991, 194, 151. (c) Todd, W. P.; Dinnocenzo, J. P.; Farid, S.; Goodman,
J. L.; Gould, I. R. J. Am. Chem. Soc. 1991, 113, 3601. (d) Todd, W. P.;
Dinnocenzo, J. P.; Farid, S.; Goodman, J. L.; Gould, I. R. Tetrahedron Lett.
1993, 34, 2863. (e) Dockery, K. P.; Dinnocenzo, J. P.; Farid, S.; Goodman,
J. L.; Gould, I. R.; Todd, W. P. J. Am. Chem. Soc. 1997, 119, 1876.
(2) Zhang, X.; Yeh, S.-R.; Hong, S.; Freccero, M.; Albini, A.; Falvey, D.
E.; Mariano, P. S. J. Am. Chem. Soc. 1994, 116, 4211.
(3) Schmittel, M.; Keller, M.; Burghart, A. J. Chem. Soc., Perkin Trans. 2
1995, 2327.
(4) Kako, M.; Nakadaira, Y. Coord. Chem. ReV. 1998, 176, 87.
(5) Wong, P. C.; Griller, D.; Scaiano, J. C. J. Am. Chem. Soc. 1981, 103,
5934.
(7) Shida, T. Electronic Absorption Spectra of Radical Ions; Elsevier: New
York, 1988; p. 137.
(8) (a) The (irreversible) peak potentials for 1 and 2 obtained by cyclic
voltammetry (150 mV/sec) using a platinum disk electrode in acetonitrile with
ca. 0.1 M tetra-n-butylammonium hexafluorophosphate as the supporting
electrolyte were 1.60 and 1.54 V vs SCE, respectively. The oxidation potential
of TMB is 0.88 V; hence, electron transfer from TMB to 1+• and 2+• is
exothermic by approximately 15-17 kcal/mol.
(9) (a) Sommer, L. H. Stereochemistry, Mechanism and Silicon; McGraw-
Hill: New York, 1965. (b) Fleming, I. In ComprehensiVe Organic Chemistry;
Barton, D., Ollis, D. W., Eds.; Pergamon: New York, 1979; Vol. 3, Chapter
13. (c) Corriu, R. J. P.; Gue´rin, C.; Moreau, J. J. E. Top. Stereochem. 1984,
15, 43. (d) Bassindale, A. R.; Taylor, P. G. In The Chemistry of Organic
Silicon Compounds; Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1989,
Part 1, Chapter 13.
10.1021/ja011207a CCC: $20.00 © 2001 American Chemical Society
Published on Web 09/05/2001

Communications to the Editor
J. Am. Chem. Soc., Vol. 123, No. 39, 2001 9699
Table 1. Rate Constants for Disappearance of Silane Cation Radicals 1+•-4+• (k1) and for Formation of the 9-Fluorenyl Radical (k2) in the
Presence of Nucleophiles, and Relative Rate Constants (in bold)
rate constants (× 107 M-1 s-1
)
3+•
1+•
2+•
4+•
nucleophile
MeOH
k1
1.7(3)a
1.0
k2
k1
k2
k1
k2
k1
k2
1.8(2)
1.0(1)
0.6
1.2(1)
0.34(2)
0.2
0.37(3)
0.24(2)
0.1
0.26(2)
i-PrOH
t-BuOH
MeCN
0.89(4)
1.0
0.49(5)
1.0
1.0(1)
1.0
0.90(8)
0.61(7)
1.1(1)
0.40(2)
0.5
0.35(3)
0.7
0.60(2)
0.6
0.43(3)
0.36(2)
0.66(5)
0.21(9)
0.2
0.14(3)
0.3
0.26(4)
0.3
0.22(2)
0.12(2)
0.27(2)
0.12(1)
0.1
0.08(1)
0.2
0.11(1)
0.1
0.16(4)
0.09(1)
0.11(2)
a The standard deviation in the last digit is given in parentheses.
Scheme 1a
tution products. Methoxysilacyclobutanes 7 and 8 could be
conveniently transformed back into the 4-methoxybenzylsila-
cyclobutanes by reaction with 4-methoxybenzyllithium,12 thus
allowing a classic Walden cycle to be constructed.
At early reaction times,13 GC analysis (HP Ultra 1, 50 m ×
0.2 mm) showed that the photooxidations of 5 and 6 were >98%
stereospecific, producing methoxysilacyclobutanes 7 and 8,
respectively. Reaction of 7 with 4-methoxybenzyllithium produced
silacyclobutane 6. Similarly, reaction of 8 with 4-methoxyben-
zyllithium produced silacyclobutane 5. These data unequivocally
demonstrate that the neutral and cation radical nucleophilic
substitution reactions have opposite reaction stereochemistries.
Nucleophilic substitutions on neutral silacyclobutanes are known
to proceed with retention of configuration at silicon.9,11 Conse-
quently, the cation radical substitutions occur with inversion
(Scheme 1). These stereochemical conclusions were independently
supported by assignments of the relative stereochemical configu-
rations of 5-8 from nOe studies.14 Thus, irradiation of the C3-
methyl protons in 5 led to a 6-fold enhancement of the benzylic
methylene protons relative to the Si-methyl protons. The corre-
sponding experiment with 6 led to a 3-fold enhancement of the
Si-methyl protons relative to that of the benzylic methylene
protons. Virtually identical, relative enhancements of the Si-
methyl vs Si-methoxy groups were found in comparable nOe
experiments with 7 and 8.
a (a) 1-cyano-4-methylnaphthalene, 4,4-dimethylbiphenyl, hν, CH3OH:
CH2Cl2 (b) 4-methoxybenzyllithium, Et2O.
In conclusion, the opposite stereochemistries for nucleophilic
substitutions on neutral silacyclobutanes vs their cation radicals
establishes that the reactions proceed via different mechanisms.
Substitutions on neutral silacyclobutanes have been proposed to
proceed by a stepwise mechanism.9 This rationalizes the retention
of stereochemistry and the high reactivity of neutral silacyclobu-
tanes toward nucleophiles, which can be explained by the release
of bond angle strain during formation of the pentacoordinate
intermediate. Silacyclobutane cation radicals react via an inversion
mechanism, which explains why they do not show enhanced
reactivity. Our kinetic data show that if a pentacoordinate
intermediate is formed in silane cation radical substitutions, it
must be very short-lived (<50 ns), even in the case where it is
most likely be observed, 2+•. On the other hand, a concerted
substitution mechanism fits the data without the need for ad hoc
assumptions. Quantum chemical calculations predict a concerted
mechanism for nucleophilic substitutions on silane cation radi-
cals,15 with a strong preference for an inversion pathway,
consistent with experiment.
(10) Prepared by the reaction of a cis/trans mixture of 1-chloro-1,3-
dimethylsilacyclobutane (Dubac, J.; Mazzerolles, P.; Serres, B. Tetrahedron
1974, 30, 749.) with 4-methoxybenzylmagnesium chloride followed by
separation of 5 and 6 by using preparative gas chromatography on a 18′ ×
3/8′′ OD column packed with 5% SP-1200/1.75% Bentone 34 on 100/120
mesh Supelcoport (Supelco) at 170 °C (100 mL/min).
(11) (a) Dubac, J.; Mazzerolles, P.; Serres, B. Tetrahedron Lett. 1972, 525.
(b) Dubac, J.; Mazzerolles, P.; Serres, B. Tetrahedron 1974, 30, 749. (c)
McKinnie, B. G.; Bhacca, N. S.; Cartledge, F. K.; Fayssoux, J. J. Am. Chem.
Soc. 1974, 96, 2637. (d) McKinnie, B. G.; Bhacca, N. S.; Cartledge, F. K.;
Fayssoux, J. J. Org. Chem. 1976, 41, 1534. (e) Dubac, J.; Mazzerolles, P.;
Fagoaga, P. J. Organomet. Chem. 1977, 139, 271.
(12) Vanermen, G.; Toppet, S.; Van Beylen, M.; Geerlings, P. J. Chem.
Soc., Perkin Trans. 2 1986, 707.
Acknowledgment. Research support was provided by a grant from
the National Science Foundation (Grant No. CHE-9812719). J.P.D. thanks
Professor S. Shaik (Hebrew University) for stimulating discussions during
the course of this work.
(13) Upon prolonged irradiation, silanes 7 and 8 underwent slow inter-
conversion by an as yet unidentified isomerization mechanism. 7 and 8 are
both stable in the unirradiated reaction medium. Neither 7 or 8 react with the
singlet excited state of 1-cyano-4-methylnaphthalene as judged by fluorescence
quenching experiments. We presume that either the anion radical of the
sensitizer or a small amount of methoxide ion produced by reaction of the
anion radical with methanol may act as a catalyst for the isomerization reaction.
(14) Stonehouse, J.; Adell, P.; Keeler, J.; Shaka, A. J. J. Am. Chem. Soc.
1994 16, 6037.
JA011207A
(15) Shaik, S.; Reddy, A. C.; Ioffe, A.; Dinnocenzo, J. P.; Danovich, D.;
Cho, J. K. J. Am. Chem. Soc. 1995, 117, 3205.