Organometallics 1995, 14, 4948-4952
4948
Synthesis and Isomerism of Monofunctional Arylated Cyclotetrasilanes Ulrich Poschl and Karl Hassler" Znstitute of Inorganic Chemistry, Graz University of Technology, Stremayrgasse 16, A-8010 Graz, Austria Received May 25, 1995@ By the introduction of the novel educt 1,2,3,4-tetraphenyl-1,2,3,4-tetra-para-tolylcyclotetrasilane (2), which shows significantly higher solubility and reactivity than octaphenylcyclotetrasilane, the first selective monofunctionalization of a perarylated cyclotetrasilane was achieved. Compound 2 was synthesized by ring closure of phenyl-para-tolyldichlorosilane with lithium. The cyclotetrasilanes SLPk-p-Tol& with X = OS02CF3, F, C1, Br, I, H or t-Bu were prepared by dearylation of 2 with trifluoromethanesulfonic acid and subsequent reaction with lithium or potassium halides, sodium boranate, and tert-butyllithium, respectively. The compounds were isolated in high yield and purity. They were characterized by lH, 13C, 19F,and 29SiNMR spectroscopy.
Introduction The perphenylated cyclosilanes Si4Phs, Si5Phl0,and Si6Ph12 synthesized by Kipping in 1921 were the first cyclosilanes t o be kn0wn.l Until a few years ago the reaction with hydrogen halides was the only way to functionalize perarylated cyclosilanes. Catalytic dearylation with gaseous HC1, HBr, or HI using the respective aluminum halides as catalysts allowed the preparation of the perhalogenated derivatives (C12Si),,' (Br~Sil,,~ and ( I ~ s i ) , ,with ~ m = 4 , 5 , or 6 . In liquid HBr and HI, Si~PhgBr6~ and Si5Ph515,6 respectively, were formed. Selective mono- or disubstitution was not achieved by this kind of dearylation. The first selective monofunctionalizationof decaphenylcyclopentasilane was reported by Uhlig in 1989,7Le., the dearylation of SisPh10 by use of trifluoromethanesulfonic acid (CF~SOSH, abbreviated as TfOH, triflic acid). Matyjaszewski et al. studied the reaction of octaphenylcyclotetrasilanewith triflic acid.8 They identified Si4Ph8-n(OTf)n,with n = 1, 2, 3, or 4, and were able to synthesize the tri- and tetratriflates in high yield. However, they did not arrive at the selective formation of (trifluoromethanesulfony1)oxyheptaphenylcyclotetrasilane, Si4Ph7OTf. Our reasons for the synthesis and characterization of monofunctional arylated cyclosilanes are the following. The partially phenylated cyclosilanes can be used for the preparation of novel cyclosilanes such as Si417t-Bu, Si4Br7H, or Si4C17F as well as for the synthesis of polycyclic silanes. Moreover, we are planning t o use the synthesized compounds as targets for the study of @Abstractpublished in Advance ACS Abstracts, August 1, 1995. (1)Kipping, F. S.; Sands, H. E. J . Chem. Soc. 1921,119,830; 848. (2) Hengge, E.;Kovar, D. J . Organomet. Chem. 1977,125,C29; 2. Anorg. Allg. Chem. 1979,458,163. (3)Hengge, E.;Bauer, G. Angew. Chem. 1973,85,304;Monatsh. Chem. 1975,106,503.Hengge, E.;Lunzer, F. Monatsh. Chem. 1976, 107,371. Kovar, D.;Utvary, K.; Hengge, E. Monatsh. Chem. 1979, 110,1295. (4)Hengge, E.; Kovar, D. Angew. Chem. 1981,93,698. (5) Hengge, E.; Lunzer, F. Monatsh. Chem. 1976,107,371. (6) Hengge, E.;Marketz, H. Monatsh. Chem. 1976,107,371. (7)Uhlig, W.;Tzschach, A. J . Organomet. Chem. 1989,378,C1. (8)Chrusciel, J.;Cypryk, M.; Fossum, E.; Matyjaszewski, K. Organometallics 1992,11, 3257.
molecular dynamics (conformational changes such as the pseudorotation of cyclopentasilanes and the ring flip of cyclotetrasilanes). Our results concerning mono- and difunctional phenylated cyclopentasilanes have been reported in a previous paper.g
Results and Discussion Despite manifold variations of the reaction conditions we could not achieve selective monofunctionalization of octaphenylcyclotetrasilane by dearylation with triflic acid (triflation). In agreement with Matyjaszewski et al. we ascribe this primarily to the very low solubility of the educt, which applies for every common solvent. The triflations were generally performed in toluene, which is known t o be one of the best solvents for Si4Phg. The reaction mixtures consisted of three phases: triflic acid (liquid) and Si4Phg (solid) suspended in toluene, in which very little Si4Ph8 (3 mg/mL a t 20 "C) and TfOH were dissolved. The monotriflate Si4Ph70Tf, which is formed in a first step of reaction, is very well soluble and therefore undergoes further triflation much faster than the suspended educt. The strong electronwithdrawing effect of the triflate group does inhibit further dearylation (ditriflation), but obviously this deactivation of the monotriflate does not compensate for the solubility, and the triflation of dissolved Si4Ph70Tf occurs faster than that of suspended SiaPhg. In contrast, the five-membered cycle SibPh10is soluble enough (28mg/mL in toluene at 20 "C) to perform triflation at low temperatures in a reaction system, which is homogeneous except for suspended triflic acid. Triflation of Si5Phl0 dissolved in toluene a t 0 "C selectively yields Si5PhgOTf.7J0In the absence of the multiphase effect described above, the deactivation arising from the first triflate substituent sufficiently reduces the rate of further dearylation to allow selective monotriflation. To obtain a perarylated cyclotetrasilane with increased solubility we synthesized 1,2,3,4-tetraphenyl1,2,3,4-tetra-para-tolylcyclotetrasilane (2) by the ring (9) Poschl, U.; Siegl, H.; Hassler, K. J . Orgunomet. Chem., in press. (10)Uhlig, T.; Tretner, C. J . Organomet. Chem. 1992,436,C1. (11)Gilman, H.; Peterson, D. J.; Tomasi, R. A.; Harrell, R. L. J . Organomet. Chem. 1985,4,167.
0276-733319512314-4948$09.00/00 1995 American Chemical Society
Monofunctional Arylated Cyclotetrasilanes
-
Scheme 1
Si,Ph,-p-Tol,OTf
2
9
-
-
7
2a
5-8
t-BW
Si,Ph,-p-Tol,Br
MG-74
Si4Ph,-p-To13X
4
NaBH,
Si,Ph,-p-To13H
Chart 1
LiWKX
TfOH
(Ph-p-TolSi),
Organometallics, Vol. 14, No. 10, 1995 4949
Si4Ph,-p-To13-t-Bu
10
X = F,CI, Br, I
closure reaction of Ph-p-TolSiClz (1) with lithium. Depending on the reaction conditions the ring closure yields a mixture of (Ph-p-TolSi)4 (2) and (Ph-p-TolSi)a (3). In analogy to the perphenylated cyclosilanes, a thermodynamical preference for the formation of 3 is assumed due to the high ring strain of the fourmembered cycle.ll Since excess lithium causes ring cleavage, the influences of reaction temperature, solvent and monomer concentration were investigated using stoichiometric amounts of lithium. In accordance with the standard preparation of Si4Ph8,121 was diluted with THF and added to powdered lithium at a high rate. The heat of reaction caused reflux within a few minutes. After the mixture had been refluxed for several hours, the yield of 2 varied between 0% and 5%, depending primarily on the rate of monomer addition and eventual intermediate cooling. The percentages given €or the yield of 2 refer to the educt 1. In general, only minor amounts of products other than 2 or 3 were observed. Addition of 1 to a suspension of Li in THF, which was refluxed already in advance, led to the formation of 3; under these conditions 2 was not observed at all. At 0 "C the desired product, 2, was obtained in 13% yield. Additional experiments in THF at temperatures down to -70 "C did not result in a significant increase of the yield. In toluene, even after several hours of reflux, no reaction of 1 with Li was observed. Experiments with various solvent mixtures of toluene and THF led to the reaction conditions given in the Experimental Section. The addition of THF to a mixture of lithium, toluene, and 1 at 0 "Cyielded 24%of 2, which readily crystallizes and thus can be easily separated from 3,which is by far more soluble and could not yet be obtained in crystalline state. Variation of the concentration of 1 in the range 0.50.05 g/mL did not cause a significant change of the product composition,regardless of reaction temperature and solvent. Higher dilution only caused an inconveniently strong decrease of the reaction rate. The progress of reaction was observed by 29SiNMR spectroscopy. In the beginning of the reaction the formation of 2 seemed to occur faster than the formation of 3. However, for complete conversion of the spectroscopically detected intermediates, the reaction mixture had to be stirred for several days, and finally the amount of 3 prevailed. Despite manifold variations of the reaction conditions the yield of 2 could not be raised above 24%. (12) Jarvie, A. W. P.;Winkler, H. J. S.; Peterson, D. J.; Gilman, H. J . Am. Chem. Soc. 1961, 83,1921.
2b
2c
2d
On the whole the optimum conditions for the synthesis of 2 are in sharp contrast to the usual preparation of Si4Pb. The highest yields of Si4Ph8 (about 30% relative t o PhzSiCl2) generally are obtained at high temperatures in THF. Presumably this differehce is due to the low solubility of SirPh8, which precipitates from the ' I "suspension immediately after being formed and therefore is hardly affected by ring cleavage with suspended lithium. In the synthesis of 2 and 3 such cleavage probably plays a key role prohibiting higher yields of 2. At present the possibilities for a more efficient, electrochemical synthesis of 2 are investigated. A first electrolysis of 1 using HMF'TET4NBF4 solvent/electrolyte systems and aluminum sacrificial electrodes in an undivided cell yielded very promising results: 2 was formed without 3 being observed at all. Further electrochemical investigations are intended, and comprehensive results will be reported subsequently. As expected, the solubility of (Ph-p-TolSi14(2) is much higher than that of si4Ph~. In toluene a t 20 "C a solubility of about 25 mg/mL was measured for 2 (Si4Ph8, 3 mg/mL). Another important aspect concerning the dearylation by triflic acid are the different reactivities of phenyl and para-tolyl substituents. Compared to phenyl groups, the Si-C bond in paru-tolyl-substituted silanes is known to be significantly more reactive toward protonic acids as dearylating reagents.13 Accordingly, by use of triflic acid para-tolyl groups can be selectively cleaved from silanes containing para-tolyl and phenyl substituents in parallel. A manuscript with the results concerning di- and trisilanes is in preparation.14 For para-anisyl- and naphthyl-substituted monosilanes Schmidbauer et al. found a similar gradation of reactivity toward triflic acid.15 The para-anisyl substituents can be selectively cleaved in the presence of naphthyl groups which in turn can be selectively cleaved in the presence of phenyl substituents. However, in contrast to phenyl- and para-tolyl-substituted chlorosilanes, neither anisyl- nor naphthyl-substituted monosilanes can be reductively coupled with alkali metals to obtain the corresponding oligosilanes we are interested in. Dearylation of 2 with 1equiv of triflic acid selectively yields Siah4-p-TohOTf (4). Further reaction of 4 with lithium or potassium halides leads to Si4PL-p-Tol3F(5), Si4Ph4-p-Tol3Cl (61, Si4Ph4-p-Tol3Br (71, and Si4Ph4-pTo131 (8) in high purity and 90%-95%yield. Si4Ph4-pTol3H (9) and Si4Ph-p-Tol3-t-Bu (10)can be obtained by reaction of 7 with sodium boranate or tert-butyllithium, respectively (Scheme 1). (Ph-p-TolSik is supposed to exist in four diastereomeric forms (Chart 1).l6 Nevertheless, 2 crystallizes ~
(13) Eaborn, C. In Organosilicon Chemisty 2; IUPAC; Butterworths: London, 1969. Hengge, E.; Eberhardt, H. Monatsh. Chem. 1979, 110,39. (14) Koll, W.; Hassler, K. To be submitted. (15) Schrkk, R.; Angermaier, K.; Sladek, A,; Schmidbaur, H. Organometallics 1994, 13,3399. (16)In Charts 1 and 2 and in Scheme 2, the dots at the edges of the four-membered cycles stand for PhSi and para-tolyl groups are indicated by simple vertical lines.
Poschl and Hassler
4950 Organometallics, Vol. 14, No. 10,1995
(a)
Table 1. 2sSiNMR Data (Nppm) for SiJ'hpp-Tol& (4-10) compdno. X Si(1) Si(2,4) 51(3)
(b)
n
34
rm
1s
-I I Pe"
-3 0
4
OTf
34.39 34.08 33.86 33.74 33.42 33.18
-20.91 -20.98 -21.07 -21.19 -21.29 -21.41
-29.88 -29.97 -30.03 -30.16 -30.28 -30.38
5
F c1
-20.64 (d) -20.74 (d) -19.07 -19.12 -19.27 -19.37 -19.49 -19.55
-29.45 (m)
6
33.38 (d) 33.22 (d) 10.93 10.77 10.66 10.60
-23.99 -24.11 -24.23 -24.36
7
Br
2.61 2.41 2.35 2.22 2.13
-20.29 -20.43 -20.48 -20.55 -20.68
-23.56 -23.62 -23.70 -23.77 -23.85
8
I
-21.51 -21.66 -21.72 -21.78
-21.94 -22.12 -22.31
-23.50 -23.65 -23.87 -24.05 -24.18 -24.39
9
H
-54.16 -54.23 -54.38 -54.45 -54.60 -54.67
-22.14 -22.24 -22.40 -22.55
-23.23 -23.35
-3.13 -3.52 -3.59 -3.95 -4.02 -4.35
-21.40 -21.56
-23.01 -23.16
-..e
0
Figure 1. 29SiNMR spectra (SiPhXregion, IH decoupled) of (a) Si4Ph4-p-Tol30Tf (4a-0 and (b) Si4Ph4-p-To4-t-Bu (loa-0.
t-Bu
10
,
-192.2
.
-192.0
-192.4
,
.
-192.8
PPI4
I
.
-193.0
Scheme 2
Figure 2. 19F NMR spectrum of Si4Ph4-p-To13F(Sa-0 with integral plot.
0 - D - 4
Chart 2 b10a
4b-lob
4c-10c
J \
4d-10d
4e-lOe
4FlOl
d o i\ 2a
4b'-10b'
4e'-1 Oe'
readily and its 29SiNMR spectrum shows only one sharp line with a half-bandwidth of 3 Hz (lH decoupled). lH and I3C and NMR spectra confirmed the existence of several isomers but did not provide definite information about their number and quantitative ratio. The 13C spectrum shows eight groups of aromatic carbon signals (two singlets and six multiplets with two t o six lines) and a methyl signal split up in two lines. The lH spectrum consists of six multiplets: three of the aryl-H multiplets with intensities equivalent to 8 H each, one of them equivalent to 12 H, and the methyl-H multiplet equivalent to 12 H. However, the interpretation of the NMR spectra of the products 4-10 lead to a clear picture of the isomeric composition of 2. Compounds 4-10 are supposed to consist of six diastereomers each, two of the diastereomers existing in two enantiomeric forms (Chart 2). Accordingly their NMR spectra show multiplet structures consisting of six
J \
2b
I\ 2b
2c
Jjq I\ 2b
2d
f\ 2b
2c
lines. Partly the measured signals were overlapping, but all the multiplets which could be resolved showed characteristic patterns with intensity ratios such as 1:2: 1:1:2:1 or 1:1:2:2:1:1(see Table 1 and Figures 1 and 2). These are exactly the ratios expected stochastically. The assumption of a random controlled formation of 2a-d in the course of reductive coupling of 1 with lithium is illustrated in Scheme 2. Presuming equal probability for cis and trans positions, referring to the phenyl and para-tolyl substituents of the added Ph-p-TolSi group, a ratio of 1:4:2:1is obtained for 2a:2b:2c:2d. Supposing equal reactivity of every para-tolyl group of 2 toward triflic acid, this educt composition fully explains the spectroscopicallyobserved intensity ratios by the formation of eight stereoisomersof 4-10 in equimolar amounts. As an example Scheme 3 elucidates the stereochemistry of triflation, leading from the quantitative ratio 1:4:2:1 for the isomers of 2 to the ratio 1:2:1:1:2:1 for 4a:4b:4c:4d:4e:4f. For simplicity, a mechanism of dearylation with configurational retention was assumed in this representation; i.e., the triflate group retains the
Monofunctional Arylated Cyclotetrasilanes
-
Scheme 3
Organometallics, Vol. 14,No. 10, 1995 4951
conversion the suspension had to be stirred for 6 days. Then the solvent mixture was replaced by toluene (300 mL). The 21 4s salts were removed by filtration and washed with toluene. 2b 4b + 4b' + 4c + 4t Precipitation from tolueneheptane yielded 2 as a colorless 2c 4e + 42' crystalline solid (19 g, 24% relative to 1): 785.29 g mol-'; mp 228-234 "C. Anal. C52H48Si4 (foundcalcd): C, 79.5479.53; 2d 4d H, 6.1816.16. 29SiNMR (dlppm) -22.09; I3C NMR (dlppm, CDC13) 138.5, 137.6, 137.4, 135.6, 135.5, 135.3, 135.2, 135.1, position of the para-tolyl group it substitutes. But this 135.0, 130.8, 130.7, 129.8, 129.7, 129.1, 129.0, 128.9, 128.8, is not a necessary precondition. A reaction mechanism 128.7, 127.9, 127.8, 127.7, 127.6, 21.5, 21.4; 'H NMR (dlppm, involving either inversion or configurational equilibraCDC13) 7.35 m (8H), 7.25 m (12H), 7.08 (8H), 6.94 m (8H), 2.32 tion (equal probability for retention and inversion) m (12H). IR (
