Organometallics 1995,14, 4109-4113
4109
Silylation of gem-Dichlorobicyclo[n~l~O]alkanes and Alkenes with Me3SiCVLiPTHF Reagent: The Dramatic Influence of Lithium Quality? Marie-Catherine Grelier-Marly and Micheline Grignon-Dubois” Laboratoire de Chimie Organique et Organomktallique, Universitk Bordeaux I, 351, cours de la Libkration, 33405 Talence-Cedex, France Received April 28, 1995@ The silylation of gem-dichlorobicyclo[n.l.Olalkanes and alkenes with Me3SiCl/LiPTHF reagent was studied under various experimental conditions. The disilylated derivatives were systematically isolated as the major product when using Li containing 1%of Na, while the chlorosilylated derivatives were isolated when using Li containing only 0.01% of Na. These results show the dramatic influence of lithium quality upon the reaction outcome. Introduction
reaction mixtures were analyzed by gas chromatography, and the results are reported in Table 1. They show Cyclopropylsilanes are of interest due to their original the dramatic influence of the percentage of sodium reactivity.l In this context, silylcyclopropane moieties contained in lithium upon the reaction outcome. Inincluded in a polycyclic structure constitute useful tools deed, using optimum conditions to perform disilylation, to access to functional polycylic derivatives, so it is we isolated as the major product the disilylated derivaimportant t o have an efficient preparation of these tives B with Li (“1% Na”), while the chlorosilylated synthons. We have previously proposed a synthesis of derivatives A were isolated with Li (“0.01%Na”) (Scheme gem-bis(trimethylsilyl)bicyclo[n.1.0lalkanes and alkenes 1,Table 1,entries 1,5,6, and 14). Lithium at 0.1% Na (n = 4 or 6)by silylation of the dichloro derivatives using content led t o intermediate behavior (Table 1, entries Me3SiCVLiPTHF reagent.2 The disilylation product was 3, 4, 12,and 13). In each case, A or B was the major obtained in 60%-65% yield with the saturated series, product, along with small amounts of unreacted starting and in 40%-45% yield along with the chlorosilylated material and monochloro (C)or monosilyl compounds derivatives (10%- 15% yield) with the unsaturated (D). Each of them was isolated by fractional distillation series. For this work, we used lithium ingot from as previously reportedS2The possible reaction pathways Prolabo, which was granulated in mineral oil prior to for all these products are depicted in Scheme 2. They use. More recently, when these compounds were needed involve electron transfer from the metal onto the for synthetic applications, we reproduced these experisubstrate, leading to radical anions and then to radicals4 ments using metal from Aldrich. This change surpris(a o r b ) by losing C1-. Capture of an electron by a o r b ingly resulted in the chlorosilylated derivatives as the followed by the silylation of the corresponding anion major products. This prompted us to reinvestigate this give, respectively, A or B. Hydrogen trapping from reaction in relation t o the experimental conditions and the solvent leads t o the byproducts C and D. Theoretiespecially the lithium used. cally, obtaining A requires only 1 equiv of TMSCl for 2 equiv of Li, while B requires 2 equiv of TMSCl for Results and Discussion 4 equiv of Li. As expected, examination of Table 1 shows that the ratio of reactants was also very signiSilylation of 7,7-dichlorobicyclo[4.l.0lhept-2-ene,1, ficant: and 9,9-dichlorobicyclo[6.l.0lnon-2-ene, 2, with lithium (i)A n increase in the quantity of metal andor TMSCl and trimethylchlorosilane (TMSC1)in THF medium was leads to an increase in the amount of disilylated studied under various conditions. The experiments compounds B obtained, but it is worth noting that 3 were conducted using three kinds of lithium, which contained 0.01%,3a0.1%,3band 1% of Na, respe~tively.~~ equiv of TMSCl (instead of the 2 equiv theoretically required) are necessary to obtain B in good yields (see The influence of the reagent ratio, which was expected Table 1, entries 5, 14,and 15). t o have an influence on the course of the reaction, (ii) An excess of TMSCl is always necessary to limit was also systematically studied, especially in the case side-reactions with the solvent and t o make all of the of compound 2, which was chosen as a model for defining substrate react. the optimum conditions for the respective production of the chlorosilylated or disilylated compounds. The (iii) A n excess of lithium is necessary t o obtain the chlorosilylated derivatives A (see Table 1,entries 8 and .‘ Dedicated to Professor Raymond Calas on the occasion of his 80th 10). Using 2 or 3 equiv led t o the same ratio of B, but birthday. the ratio of A is weaker with 2 equiv, due to an Abstract published in Advance ACS Abstracts, August 1, 1995. (1)For a review see: Paquette, L. A. Chem. Rev. 1986,86, 733. important part of the substrate which was recovered (21 Laguerre, M.; Grignon-Dubois, M.; Dunogubs, J. Tetrahedron unreacted. 1981,37, 1161. Grignon-Dubois, M.; DunoguBs, J.; Ahra, M. R e d . From these results, it appears that the optimum Trav. Chim. Pays-Bas 1988, 107, 216. (3) (a1 Lithium wire, 0.01% sodium content, Aldrich (ref. 22,091-4). experimental conditions are as follows: @
(b) Granulated lithium, 0.1% sodium content, from Prolabo (ref. 2 4996.1501. (c) Lithium wire, 1%sodium content, from Aldrich (ref. 27,832-7).
(4)See, for example: Calas, R. J . Organomet. Chem. 1980,200,11.
0276-733319512314-4109$09.00/0 0 1995 American Chemical Society
4110 Organometallics, Vol. 14, No. 9, 1995
Grelier-Marly and Grignon-Dubois
Table 1. Reductive Silylation of 1-5 product ratio (%),” R,R
reactant ratio (equiv) entry no. 1
2 3 4 5 6
substrate
% Na
Li
TMSCl
H,C1
C1,Cl
1
0.01
4
3 1.5 3 1.5
3
11 9
2
3 2
3 0.1
1
2
0.01
7 8 9 10 11 12 13 14 15 16 17 18 19 20
0.1 1
3d
0.01 1 0.01 1 0.01 1
4
5
21
4 3 4 4 4 3 3 2 2 4 3 4 4 3 4 3
4 3 4
5
3 3 2 1.5 1 1.5
2 5 1
2 3 7 24 28
5 5 7
1 3 1.5
H,Si
7 9 5
4
31
2
2
2
5
2
7
8
3
4
5 7
5
10
3 2
8 4 8
1.5 3
1.5 3 1.5 3
3
C1,Si (anti1syn)C
64 (85115) 77 (80120) 20 (90/10) 44 (85115) 13 (90/10) 73 (80120) 69 (85115) 86 (85115) 65 (85115) 59 (80120) 59 (85115) 19 (90110) 31 (90110)
7 (90110) 59 (60140) 10 (53147) 71 (55145) 11 (51149) 77 (55145)
Si,Si 22
14 68 40 82 23
19 6 6 8 3 69 57 100 85 12 95 15 96 8 100
a Product ratios were determined by GC and NMR. R and R are cyclopropyl substituents. CAntilsynstereochemistry is related to the trimethylsilyl group position with respect to the polymethylene bridge. 7-(2’-Tetrahydrofuranyl)norcaranewas also characterized in the reaction mixture.
Scheme 1 Li (0.01% Na) 4Li,3Z;SiCI n=2 : 7 (63%)
1-2
/
Li ( 1% Na)
.p z n=l : 8 (69%) n=2 : 9 (81%)
(i) 3 equiv of Li (“0.01%Na”) and 1.5 equiv of TMSCl are required to obtain the chlorosilylated compounds A (Table 2). (ii) 4 equiv of Li (“1%Na”) and 3 equiv of TMSCl are required to obtain the disilylated compounds B (Table 3). Under these conditions, the saturated substrates 3-5 (Scheme 3,Tables 1-3) respectively led to 10-12 using Li at “0.01% Na” (3 equiv) and 13-15 using Li at “1% Na” (4equiv). Compared t o our previous results,’ all the product yields have been increased of about 20%. These results confirm the dramatic influence of lithium quality on the silylation outcome. It is well-known that lithium, due to its manufacturing p r o c e ~ s ,always ~ contains small amounts of sodium, which was said to play a role in its efficiency in the silylation processes. However, to the best of our knowledge, this is the first time a systematic study of this factor and such an important effect related to a relatively small variation (5) Guntz, M. C.R. Acad. Sci. l983,117C, 732. Ruff, 0.;Johannsen, 0 . Z . Electrochem. 1906,12,186.
in sodium amount are reported. Moreover, it is worth noting that lithium contains other metallic impurities, which can be as abundant as sodium (see Experimental Section). The chlorosilylated compounds were isolated as an anti lsyn6 isomeric mixture, which was separated by distillation. Their stereochemistry has been unambiguously attributed using 29SiNMR on the basis of the 3JP9Si,lH)coupling constant value^.^ It is worth noting that the antilsyn ratio is always close t o 85115 from 1-2, but 55145-60140 from 3-5. The larger predominance of the anti isomer with unsaturated substrate shows the effect of the double bond on the stereochemistry and will be discussed below. Replacement of a halogen of gem-dihalocyclopropanes by a trimethylsilyl group had been previously accomplished. Compound 10 was obtained as an anti/ ( 6 )The anti lsyn stereochemistry is related to the trimethylsilyl group or the negative charge position with respect to the polymethylene bridge. (7) Grignon-Dubois,M.; Ahra, M.; Laguerre, M.; Barbe, B.; PBtraud, M. Spectrochim. Acta 1989,45A, 911.
Dramatic Influence of Lithium Quality
Organometallics, Vol. 14, No. 9, 1995 4111 Table 2. Reductive Silylation of 1-5 Using 3 Euuiv of Li (0.01%Na) and 1.5 Eauiv of TMSCl
Scheme 2
product ratio (%); R,Rb substrate
1 2 3 4 5
THF
CI
___)
(a)
C 2 e' Me3SiCl
A
H,C1
H,Si
C1,Si (antilsyny
7
8
4
7 10
77 (80l20) 86 (85l15) 59 (60140) 71 (55145) 77(55/45)
C1,Cl 9
1 2 3
7 5
C1,Si Si.Si yield (%) 14 6 12 15 8
Product ratios were determined by GC and NMR. R and R are cyclopropyl substituents. Anti lsyn stereochemistry is related to the trimethylsilyl group position with respect to the polymethylene bridge.
Table 3. Reductive Silylation of 1-5 Using 4 Euuiv of Li (1%Na) and 3 Eauiv of TMSCl ~~
product ratio (%I; substrate
H,C1
C1,Cl
H,Si
R,R
C1,Si (antilsynT
SiSi ~~
'I
1
SiMe3
Me3SiCl
e-
D
1
D(SiMej SiMe3 B
-
Scheme 3
0
= 2 : 3
= 3 : 4 = 4
5
13 (90/10)
5 3
10(53/47) 11 (51l49)
2
D
