Tri-tert-butylsilyl(triisopropylsilyl)silylene (tBu)3Si-Si-Si(iPr)3 and chemical evidence for its reactions from a triplet electronic state [15]

Full text of DOI:10.1021/ja016325c

8622
J. Am. Chem. Soc. 2001, 123, 8622-8623
(4.1-7.1 kcal/mol).5,8 Thus, 1 seemed to be a likely candidate
for a silylene with a triplet ground state that would undergo
reactions from that state. Synthesis of the key starting material
for 1, (tBu)3Si-SiBr2-Si(iPr)3, 2, was straightforward after
recognition of a useful generalization for the formation of
sterically congested Si-Si bonds by salt-elimination reactions:
Y3SiM + XSiZ3 f Y3Si-SiZ3 + MX. If one reaction partner is
more sterically hindered than the other, it should be the nucleo-
phile Y3SiM.9,10
The most useful precursor of 1, 3-phenyl-1-tri-tert-butylsilyl-
1-triisopropylsilyl-1-silacyclopent-3-ene, 3, was synthesized by
condensation of 2 with a reagent prepared by reduction of
2-phenylbutadiene with activated magnesium (Scheme 1).
Previous reports of selectivity inversions between metal-free
and organometallic reaction systems for the generation of silylenes
and their equivalents,11,12 suggest that insertion product 5 and
addition product 6 are the results of silylenoid reactions (the latter
seemingly the first magnesium-induced addition to be reported).
Room-temperature photolysis of 3 and 6 in methylcyclohexane
solutions containing 2,3-dimethylbutadiene or trimethylsilane
serving as trapping agents gave rise to the products of π-addition,
4, and H-Si insertion, 5, associated with previously investigated
silylenes (Scheme 2).1,13
Tri-tert-butylsilyl(triisopropylsilyl)silylene
(tBu)3Si-Si-Si(iPr)3 and Chemical Evidence for Its
Reactions from a Triplet Electronic State
Ping Jiang and Peter P. Gaspar*
Department of Chemistry
Washington UniVersity
St. Louis, Missouri 63130-4899
ReceiVed May 31, 2001
ReVised Manuscript ReceiVed July 20, 2001
Evidence is presented for the generation of a silylene reacting
from a triplet electronic state. This has proven to be a formidable
task.1-3 A conceptual basis for a successful strategy was provided
in 1991 when it was recognized that the “crossover angle” Z-
Si-Z, beyond which the lowest triplet state lies below the lowest
singlet, decreases with decreasing electronegativity of Z.4 With
Z ) trialkylsilyl the predicted crossover angle of 115-120° should
be accessible, and bis(tri-isopropylsilyl)silylene (iPr3Si)2Si was
generated in the hope that it would possess a triplet ground
electronic state.2 Density functional calculations by Apeloig and
co-workers predict a ∆ES-T ) 1.4-1.7 kcal/mol for (iPr3Si)2Si.5
The EPR experiment that could establish a triplet ground state
has been limited by a scarcity of precursors for the photochemical
generation of (iPr3Si)2Si in an organic glass.6
In the absence of added trapping agents, photolysis of 3 leads
to a mechanistically suggestive product 7 that results formally
from intramolecular insertion of silylene 1 into an H-C bond of
a tert-butyl group (Scheme 3).14
Formation of 7 at room temperature from a species whose
H-Si and π-addition reactions reveal it to be a silylene is
consistent with the reaction of an accessible triplet state. H-C
insertion by a singlet silylene is predicted to require ca. 20 kcal/
mol activation energy and has most commonly been observed as
an intramolecular reaction at high temperatures.15,16 However,
intramolecular H-atom abstraction by triplet 1, followed by radical
coupling, is a feasible pathway to 7 (Scheme 4). Intermolecular
H-atom abstraction from a suitable hydrogen donor should
compete with the intramolecular process. The results (Scheme
5) from photolysis of 3 in the presence of triisopropylsilane lend
support to such a mechanism.
With one exception, the formation of a product of hydrogen
acquisition (iPr3Si)2SiH2 whose possible origin from a triplet
silylene was speculative,2 the reactions observed for (iPr3Si)2Si
were those addition and insertion processes already well-known
from the study of singlet silylenes.1 This suggested that, even if
(iPr3Si)2Si has a triplet ground state, reactions of the lowest singlet
state might siphon off the silylene, thus preventing study of triplet
silylene chemistry. Therefore, attention has turned to silylenes
more likely to react from a triplet state, that is, silylenes predicted
to have triplet ground states and a larger singlet-triplet splitting
than (iPr3Si)2Si. Attempts by Wiberg to generate bis(tri-tert-butyl-
silyl)silylene (tBu3Si)2Si by a silylenoid route suggested that this
silylene, while predicted to possess a triplet ground state, might
be too sterically hindered to undergo intermolecular reactions.7
For tri-tert-butylsilyl(triisopropylsilyl)silylene (tBu)3Si-Si-Si-
(iPr)3, 1, ∆ES-T is likely to lie between the values predicted by
Apeloig for (iPr3Si)2Si (1.4-1.7 kcal/mol) and for (tBu3Si)2Si
The presence of HSi(iPr)3 leads to a marked decrease in the
yield of 7. With DSi(iPr)3 deuterium is not incorporated in 7.17
(8) ∆ES-T should increase with increasing Si-Si-Si. A reliable prediction
for 1 is not yet available, but the series of X-ray crystallographic angles for
the model compounds R3Si-SiBr2-SiR′3 is encouraging: 129° for R ) R′
) iPr (Winchester, W. R.; Rath, N. P.; Gaspar, P. P. to be published), 136.0°
for R ) tBu, R′ ) iPr (Jiang, P.; Rath, N. P.; Gaspar, P. P. to be published),
141.5° for R ) R′ ) tBu (Wiberg, N. private communication).
(9) Gaspar, P. P.; Autry, M. E.; Beatty, A. M.; Braddock-Wilking, J.; Chen,
T.; Chen, Y.-S.; Chiang, M. Y.; Haile, T.; Jiang, P.; Klooster, W. T.; Koetzle,
T. F.; Lei, D.; Mason, S. A.; Rath, N. P.; Winchester, W. R.; Xiao, M. New
Synthetic Methods in Organosilicon Chemistry, Seoul, Korea, May 22, 1999;
L-13.
(1) Gaspar, P. P.; West, R. In The Chemistry of Organic Silicon Compounds
II; Rappoport, Z., Apeloig, Y. Eds.; Wiley: Chichester, 1998; Chapter 43, pp
2463-2569.
(10) tBu3SiNa + SiHCl2Ph yielded tBu3Si-SiHClPh whose Li salt reacted
with ClSi(iPr)3 forming (tBu)3Si-SiHPh-Si(iPr)3. Dephenylation with HBr/
AlBr3 led to 2.
(11) Pae, D. H.; Xiao, M.; Chiang, M. Y.; Gaspar, P. P. J. Am. Chem. Soc.
1991, 113, 1281-1288.
(12) Boudjouck, P.; Samaraweera, U.; Sooriyakumaran, R.; Chrusciel, J.;
Anderson, K. R. Angew. Chem., Int. Ed. Engl. 1988, 27, 1355-1356.
(13) Gel permeation chromatography indicated the formation of oligomers
with peaks at 1100 D and ca. 32000 D.
(14) Other products include tBu3Si(iPr3Si)SiH2 8 (e0.5%), iPr3SiH (6.8%),
tBu3SiH (6.5%).
(15) Gordon, M. S.; Gano, D. R. J. Am. Chem. Soc. 1985, 106, 5421-
5425; Davidson, I. M. T.; Scampton, R. J. J. Organomet. Chem. 1984, 271,
249-260; Boo, B. H.; Gaspar, P. P. Organometallics 1986, 5, 698-707.
(16) Gaspar, P. P. In ReactiVe Intermediates; Jones, M., Jr., Moss, R. A.,
Eds.; Wiley: New York, 1981; Vol. 2, pp 335-385.
(17) In the presence of triisopropylsilane the yield of tBu3Si(iPr3Si)SiH2 8
rises slightly to 1.7% (HSi(iPr)3, 1.1% DSi(iPr)3), and with DSi(iPr)3 mass
spectroscopy indicates an isotopic composition: 17% RR′SiH2, 58% RR′SiHD,
25% RR′SiD2. While 8 may be due to hydrogen acquisition by silylene 1,
there is insufficient evidence for such an interpretation.
(2) Gaspar, P. P.; Beatty, A. M.; Chen, T.; Haile, T.; Lei, D.; Winchester,
W. R.; Braddock-Wilking, J.; Rath, N. P.; Klooster, W. T.; Koetzle, T. F.;
Mason, S. A.; Albinati, A. Organometallics 1999, 18, 3921-3932.
(3) An EPR-active species formed upon irradiation of a dipropylsilicon
porphyrin with visible light was reported to be a long-lived silicon diradical:
Zheng, J. Y.; Konishi, K.; Aida, T. J. Am. Chem. Soc. 1998, 120, 9838-
9843. Its relationship to triplet silylenes remains to be established.
(4) Grev, R.; Schaefer, H. F., III; Gaspar, P. P. J. Am. Chem. Soc. 1991,
113, 5638-5643.
(5) Holthausen, M. C.; Koch, W.; Apeloig, Y. J. Am. Chem. Soc. 1999,
121, 2623-2624.
(6) Only (iPr3Si)3SiBr has been successfully photolyzed to (iPr3Si)2Si at
77 K (in a 3-methylpentane glass), and an EPR signal was observed at 9750
G (x-band) at 8 K that may be due to (iPr3Si)2Si, but that assignment remains
to be confirmed: Gaspar, P. P.; Chen, T.; Haile, T.; Lei, D.; Lin, T.-S.;
Smirnov, A. I.; Winchester, W. R. 31st Organosilicon Symposium, New
Orleans, May 29-30, 1998, C-3.
(7) Wiberg, N. Coord. Chem. ReV. 1997, 163, 217-252.
10.1021/ja016325c CCC: $20.00 © 2001 American Chemical Society
Published on Web 08/09/2001

Communications to the Editor
J. Am. Chem. Soc., Vol. 123, No. 35, 2001 8623
Scheme 1
Scheme 4
Scheme 2
Scheme 5
ground state awaits discovery of a photochemical precursor for
1 that will function in a glass.
In 1997 Wiberg made a suggestion,7 repeated recently,18 that
a product analogous to 7 from the metal-induced dehalogenation
of (tBu3Si)2SiX2 (X ) F, Cl, Br) is formed via the triplet silylene
(tBu3Si)2Si. While the suggestion is not implausible, no evidence
for the intermediacy of a free silylene in the reactions studied by
Wiberg has been presented, and the silylenoid intermediates that
intervene in these reactions can be expected to yield similar
products. Banaszak-Holl has recently activated a stable germylene
toward intermolecular H-C insertion,19 and intramolecular H-C
insertion by a diarylgermylene has been promoted by Lewis
acids.20
Scheme 3
In conclusion, the observation that photolysis of a precursor
to silylene (tBu)3Si-Si-Si(iPr)3, 1, leads to the formation of a
product of formal intramolecular H-C insertion in addition to
products from intermolecular H-Si insertion and diene π-addition
suggests that free silylene 1 reacted from its predicted triplet
ground state. The intramolecular H-C insertion product is
unlikely to arise from a singlet silylene at room temperature.
Acknowledgment. We thank Professor Nils Wiberg for experimental
details of the preparation and reactions of tBu3SiNa. This work received
financial support from the National Science Foundation under Grants
CHE-9632897 and 9981759.
The mechanism of Scheme 4 is consistent with both of the
following observations: (1) Trapping agent HSiMe3 leads to a
high yield of the product of formal intermolecular H-Si insertion
by silylene 1. (2) In the presence of triisopropylsilane no product
of formal H-Si insertion is found, but the yield of a product of
formal intramolecular H-C insertion decreases.
According to the mechanism of Scheme 4, the difference
between HSiMe3 and HSi(iPr)3 as trapping agents is that the steric
encumbrance of the triisopropylsilyl radical prevents coupling of
the radical pair formed upon H-atom abstraction. In the case of
HSiMe3 the radical pair can couple to form the formal insertion
product 5. Experiments are underway to confirm reactions from
a triplet state of 1 by CIDNP, while confirmation of a triplet
Supporting Information Available: Experimental procedures for the
synthesis of compounds 2-6, the photochemical experiments on the
generation of silylene 1, and the characterization of 7 and 8. (PDF). This
material is available free of charge via the Internet at http://pubs.acs.org.
JA016325C
(18) Wiberg, N.; Niedermayer, W. J. Organomet. Chem. 2001, 628, 57-
64.
(19) Miller, K. A.; Watson, T. W.; Bender, J. E., IV.; Banaszak-Holl, M.
M.; Kampf, J. W. J. Am. Chem. Soc. 2001, 123, 982-983.
(20) Jutzi, P.; Schmidt, H.; Neumann, B.; Stammler, H.-G. Organometallics
1996, 15, 741-746.