Fluoride Ion-Catalyzed 1,2-Desilylative Defluorination: Syntheses of 1-Substituted 2,2-Difluorostyrenes
The cleavage of a C-F bond is not easy, in general, due to the large bond energy (ca. 552 kJ mol-1). However, the bond breaking does occur rather easily when the CF3 group is attached to the π-electron system, such as in aryl and carbonyl groups, since easy electron acceptance into the CF3-attached π-electron system and subsequent extrusion of a fluoride ion occur to drive the C-F bond breaking reaction forward. The reactivity and mechanism on electroreductive cleavage of C-F bonds in fluoromethylarenes (benzylic fluorides) have been investigated by several groups.1 Bordeau and co-workers2a,b and Pe´richon and co-workers2c reported the selective synthesis of PhCF2SiMe3 via an electrochemical reduction of PhCF3 in the presence of chlorotrimethylsilane and the reaction of PhCF2SiMe3 as a synthon of the PhCF2 anion. We have reported Mg(0) metal-promoted benzylic C-F bond activation of 1,4-bis(trifluoromethyl)benzene 1 as a useful building block for (1′,1′,4′,4′,4′-pentafluoro-1′-trimethylsilyl)1,4-xylene 2, and subsequent 1,6-desilylative defluorination (1,6version) of 2 by a catalytic amount of fluoride ion gave
1′,1′,4′,4′-tetrafluoro-1, 4-quinodimethane 3, a precursor of the AF4 polymer (Scheme 1).3 Recently, it was reported that 1,4-desilylative defluorination of 1-methoxy-1-trimethylsilyloxy-2-trifluoromethyl-2-trimethylsilyloxyethene 4 proceeds quite smoothly in the presence of a trace amount (0.2 mol %) of fluoride ion, affording 3,3difluoro-2-trimethylsilyloxyacrylate 5 in an excellent yield (Scheme 1).4 Therefore, the desilylative defluorination reaction by a catalytic amount of fluoride ion is an efficient synthetic method for conjugated gem-difluoroalkenes, which are highly reactive, in general, with nucleophiles such as the fluoride ion.5 There has been no report on 1,2-desilylative defluorination (transformation of 6 to 7) as shown in Scheme 1. Activation of either an R-trimethylsilyl group or R-fluorine atom involved in 8 and their replacement with other elements would provide 1,1-disubstituted 1-aryl-2,2,2-trifluoroethanes, which would be promising synthetic precursors for 1-substituted 2,2-difluorostyrenes 9 (Scheme 2). On this basis, we describe here an excellent synthesis of 1-(3′-chlorophenyl)-1-trimethylsilyl-1,2,2,2-tetrafluoroethane 8 by Mg(0)-promoted defluorinative silylation of 3-chlorophenylpentafluoroethane 10 as a model substrate for pentafluoroethylarenes and divergent transformation of 8 to trifluorostyrene 9a and the related 1-substituted 2,2-difluorostyrenes 9b-d based on the concept of fluoride ioncatalyzed desilylative defluorination, the first 1,2-elimination version. As shown in Scheme 3, 1-(3′-chlorophenyl)-1-trimethylsilyltetrafluoroethane 8 was prepared in an excellent yield by the Mg(0)-promoted defluorinative silylation of 3-chloropentafluoroethylbenzene 10,6 in a Mg-TMSCl-DMPU (1,3-dimethyl3,4,5,6-tetrahydro-2(1H)-pyrimidinone) system. As for solvents, DMPU was excellent, either DMF (81%)7 or NMP (N-methyl2-pyrrolidinone) (90%)7 was usable to give 8 in a high yield, and THF was a poor solvent for the purpose. Next, transformation of 8 to a series of 1-substituted 2,2difluorostyrenes was examined. Fluoride ion-catalyzed 1,2desilylative defluorination of 8 (Scheme 4) with a catalytic amount of TBAT ([n-Bu4N][Ph3SiF2]) would provide 3′-chloro1,2,2-trifluorostyrene 9a. The optimization of conditions and the results are summarized in Table 1. Yield of 9a was highly dependent on the combination of solvent and reaction temperature. When CH2Cl2 was used as a solvent, the desired compound 9a was produced in an excellent yield (96%). This result clearly demonstrates an excellent recycling of fluoride ion in the 1,2-desilylative defluorination reaction. n-Hexane was also an excellent solvent for the purpose, even though TBAT did not completely dissolve in n-hexane.
(1) (a) Lung, H. Acta Chem. Scand. 1959, 13, 192-194. (b) Cohen, A. I.; Keeler, B. T.; Coy, N. H.; Yale, H. L. Anal. Chem. 1962, 34, 216-219. (c) Coleman, J. P.; Gilde, H. G.; Utley, J. H. P.; Weedon, B. C. L.; Eberson, L. J. Chem. Soc., Chem. Commun. 1970, 738-739. (d) Coleman, J. P.; Naser-ud-din; Gilde, H. D.; Utley, J. H. P.; Weedon, Basil C. L.; Eberson, L. J. Chem. Soc., Perkin Trans. 2 1973, 1903-1908. (e) Andrieux, C. P.; Combellas, C.; Kanoufi, F.; Saveant, J.-M.; Thiebault, A. J. Am. Chem. Soc. 1997, 119, 9527-9540. (2) (a) Clavel, P.; Leger-Lambert, M. P.; Biran, C.; Serein-Spirau, F.; Bordeau, M.; Roques, N.; Marzouk, H. Synthesis 1999, 829-834. (b) Clavel, P.; Lessene, G.; Biran, C.; Bordeau, M.; Roques, N.; Trevin, S.; de Montauzon, D. J. Fluorine Chem. 2001, 107, 301-310. (c) Saboureau, C.; Troupel, M.; Sibille, S.; Pe´richon, J. J. Chem. Soc., Chem. Commun. 1989, 1138-1139.
(3) (a) Amii, H.; Hatamoto, Y.; Seo, M.; Uneyama, K. J. Org. Chem. 2001, 66, 7216-7218. (b) Preparation of 3 by zinc reduction of 1,4-bis(chlorodifluoromethyl)benzene: Dolbier, W. R., Jr.; Asghar, M. A.; Pan, H. Q.; Celewicz, L. J. Org. Chem. 1993, 58, 1827-1830. (c) Preparation of 3 by PbBr2Al reduction: Zhu, S.-Z.; Mao, Y.-Y.; Jin, G.-F.; Qin, C.-Y.; Chu, Q.-L.; Hu, C.-M. Tetrahedron Lett. 2002, 43, 669-671. (4) Takikawa, G.; Toma, K.; Uneyama, K. Tetrahedron Lett. 2006, 47, 6509-6511. (5) Uneyama, K. Organofluorine Chemistry; Blackwell Publishing: Oxford, UK, 2006. (6) Synthetic method for 10: Carr, G. E.; Chambers, R. D.; Holmes, T. F. J. Chem. Soc., Perkin. Trans. 1 1988, 921-926. (7) Yields were determined by 19F NMR integration of products relative to benzotrifluoride as an internal standard.
Yutaka Nakamura and Kenji Uneyama* Department of Applied Chemistry, Faculty of Engineering, Okayama UniVersity, Okayama 700-8530, Japan
[email protected] ReceiVed April 6, 2007
1-(3′-Chlorophenyl)-1-trimethylsilyl-1,2,2,2-tetrafluoroethane has been prepared in an excellent yield by the Mg-promoted defluorinative silylation of 3-chloropentafluoroethylbenzene and transformed to a series of 1-substituted 2,2-difluorostyrenes by fluoride ion-catalyzed 1,2-desilylative defluorination.
10.1021/jo070721h CCC: $37.00 © 2007 American Chemical Society
5894
J. Org. Chem. 2007, 72, 5894-5897
Published on Web 06/21/2007
SCHEME 1. Fluoride Ion-Catalyzed Desilylative Defluorination
TABLE 1. Fluoride Ion-Catalyzed Desilylative Defluorination of 8 Yield (%)b entry
solvent
1 2c 3 4 5 6
CH2Cl2 n-hexane n-hexane THF THF THF
tempa
(°C)
rt rt 60 -40 0 60
9a
11
12
96 92 98 (90) 16 25 79
