Organometallics 1996,14, 584-585
584
A Silica-SupportedMagnesium-Anthracene Complex Tania R. van den Ancker and Colin L. Raston" Faculty of Science and Technology, Grifith University, Nathan, Brisbane, Queensland 41 11, Australia Received October 11, 1994@ Summary: Hydroxyl-depleted silica surfaces derived from treating chloropropylsilyl- (or chloropropylsilyl I trimethylsilyl-) functwnalized silica with H&NMe3 afford the corresponding organolithium reagent when treated with Li+(biphenyl)*-;successive treatment with 9-(chlorodimethylsily1)anthracene and Mg(anthraceneXTHFj3 generates anthracene and silica-supported "magnesiumanthracene", which gives Grignard reagents of benzylic halides in tetrahydrofuran in excellent yield with the spent silica-supported anthracene being readily recycled. The magnesium-anthracene complex [Mg(anthracene)(THF)al (1;THF = tetrahydrofuran) is an important source of magnesium via either reactions from activated magnesium arising from decomposition of 1 to its constituents or reactions involving the complex dire~tly.l-~ For the latter the formation of Grignard reagents of benzylic halides in almost quantitative yield is noteworthy; such reagents can be difficult to prepare using the classical method of Grignard reagent formation, and even using highly activated forms of m a g n e ~ i u m . ~ Generating -~ Grignard reagents using this method, however, yields solutions loaded with anthracene. Earlier work to overcome this potential inconvenience centered on developing a polystyrenesupported magnesium-anthracene complex. This approach suffers from relatively low loading of the active sites which are anthracene radical anion and dianion species, rather than exclusively dianion species as in the model compound [Mg(S-PhCH2(Me)zSiCl~lo)(THF)21 and also 1. In addition, there is a ca. 10% reduction in Abstract published in Aduance ACS Abstracts, January 1, 1995. (1)(a) Bogdanovic, B.; Janke, N.; Kruger, C.; Mynott, R.; Schlichte, IC; Westeppe, U. Angew. Chem., Znt. Ed. Engl. 1985, 24, 960. (b) Lehmkuhl, H.; Skakoor, A.; Mehler, IC; Kruger, C.; Angermund, K; Tsav, Y.-H. Chem. Ber. 1985,118,4239. (c) Bogdanovic, B.; Liao, S.; Mynott, R.; Schlichte, K.; Westeppe, U. Chem. Ber. 1984, 117, 1378. (2) Itsuno, S.; Darling, G. D.; Stover, H. D.; Frechet, J. M. J. J. Org. Chem. 1987,52,4644. (3) (a) Oppolzer, W.; Schneider, P. Tetrahedron Lett. 1984,25,3305. (b) Bonnemann, H.; Bogdanovic, B.; Brinkmann, R.; Spliethoff, B.; He, D.-W. J. Organomet. Chem. 1993,451, 31. (c) Benn, R.; Bogdanovic, B.; Bruning, M.; Grondey, H.; Herrmann, W.; Kinzelmann, H.-G.; Seevogel, K. Chem. Ber. 1993,126, 225. (4) Bogdanovic, B. Acc. Chem. Res. 1988,21, 261. (5) (a) Alonso, T.; Harvey, S.;Junk, P. C.; Raston, C. L.; Skelton, B. W.; White, A. H. Organometallics 1987, 6, 2110. (b) Brooks, W. M.; Lincoln, F. J.; McGinnity, J. J.; Raston, C. L.; Sue, R. S. Organometallics 1991, 10, 2098. (6) (a) Harvey, S.; Junk, P. C.; Raston, C. L.; Salem, G. J.Org. Chem. 1988, 53, 3134. (b) Gallagher, M. J.; Harvey, S.; Raston, C. L.; Sue, R. E. J. Chem. Soc., Chem. Commun. 1988, 289. (7)Nicoletti, T. M.; Raston, C. L.; Sargent, M. V. J. Chem. Soc., Chem. Commun. 1988, 1491; J. Chem. Soc., Perkin Trans. 1 , 1990, @
133. (8) Harvey, S.; Raston, C. L. J. Chem. Soc., Chem. Commun. 1988, 652. (9) (a) Bogdanovic, B.; Janke, N.; Kinzelmann, H.-G.; Westeppe, U. Chem. Ber. 1988, 121, 33. (b) Bartmann, E.; Bogdanovic, B.; Janke,
N.; Liao, S.; Schlichte, K.; Spliethoff, B.; Treber, J.; Westeppe, U.; Wilczok, U.Chem. Ber. 1990, 123, 1517. (c) Bogdanovic, B.; Janke, N.; Kinzelmann, H.-G. Chem. Ber. 1990, 123, 1507. (d) Bogdanovic, B.; Janke, N.; Kinzelmann, H.-G.; Seevogel, K.; Treber, J. Chem. Ber. 1990,123, 1529.
the ability of the spent polymer t o take up magnesium after successive uses.* We now report the synthesis and characterization of silica-supported magnesium-anthracene materials and their utility in generating Grignard reagents of benzylic halides. Other highlights of this work are the use of the trimethylamine adduct of alane, H3A1.NMe3,10 to effectively deplete surface hydroxyl groups, and the use of a group 1 radical anion arene species to generate a surface-bound organolithium reagent, "-O)3Si(CH2)3Lin. Silica powders were treated with (Me0)3Si(CH2)3Cl, yielding 2, and then ClSiMe3 to "end cap" residual SiOH groups (2') using literature procedures, although with minor variation in the case of 2, which featured anhydrous toluene as the solvent rather than moist xy1ene.l' Treating both powders with alane gave materials further depleted of Si-OH groups and having residual AI-H moieties, 3 and 3 respectively (Scheme 1).12 These were readily converted t o the corresponding lithium reagents 4 and 4, followed by the anthracenesupported species 5 and 6 , and then the target supported "magnesium-anthracene" species 6 and 6 . The last step involved metathetical exchange with compound 1, which is favored by the ability of silicon to stabilize charge by p o l a r i ~ a t i o n .The ~ ~ ~powders were characterized using FTIR and 13C CPflMAs NMR. In the absence of treating 2 or 2 with H&NMe3 a several-fold excess of Li+(biphenyl)'- was required to generate the supported lithium reagent; similar treatment was necessary for 1 to form supported magnesiumanthracene, presumably because of the reactivity of residual hydroxyl groups. Instead, the use of alane dispenses with the conventional trimethylsilyl endcapping step. Methyllithium in tetrahydrofuran (=THF) was also found to be effective in removing interfering (10)Atwood, J. L.; Bennett, F. R.; Elms, F. M.; Jones, C.; Raston, 1991, 113, 8183. (b) Jones, C. L.; Robinson, IC D. J. Am. Chem. SOC. C.; Koutsantonis, G. A.; Raston, C. L. Polyhedron 1993,12, 1829 and references therein. (11) Dudler, V.; Lindoy, L. F.; Sallin, D.; Schlaepfer, C. W. Aust. J. Chem. 1987,40,1557. (12) Powders 2 and 2' were prepared from silica gel (Fluka, 200 mesh) in toluene." To a suspension of 2 (13.5 g) in THF (75 mL) a t 0 "C was slowly added H&NMe3 (6.4 g). After gas evolution ceased, the mixture was stirred overnight at ca. 20 "C, whereupon the solid O 60 "C for was collected, washed with THF, and then dried in U ~ C U at 2 h a s powder 3 (14.1 g): v(N-H) 1875 cm-'; I3C C P W NMR (20.1 MHz) 6 11.8 (CHZSi),26.5 (CHzCHzCHz),47.8 (CHzCl)(3':2.3 (SiCHs), 11.9, 27.7, 47.2). To a suspension of 3 (1.60 g) in THF (50 mL) was added Li(bipheny1) (ca. 30% excess) in THF (ca. 0.3 M), and the deep blue mixture was stirred overnight. 9-(Chlorodimethylsilyl)anthracene (0.47 g) was then added and the yellow mixture stirred overnight. The powder 5 was filtered, washed with THF, and then dried in uucuo a t 60 "C for 6 h (1.96 g): v(Al-H) 1880 cm-'; 13C C P W N M R (20.1 MHz) 6 11.5 (CH2Si), 18.0 (CHzSiCH3), 28.0 (CHZCHZCHZ), 48.0 (small peak, CHzCl), 128.8 (Cm,,J( 5 2.3 (SiCHs) 11.8,17.2,26.0,47.8 (small peak), 127.2). The powder 5 (2.0 g) was dispersed in THF (25 mL) and compound 1 (0.9 g) added slowly, yielding a deep green mixture. The powder 6 was collected, washed with "F,and then dried in umuo as a green powder (2.0 g, v(Al-H) 1880 cm-'). Preparations of 3 - 6 are similar.
0276-733319512314-0584$09.00/0 0 1995 American Chemical Society
Communications
Organometallics, Vol. 14,No. 2, 1995 585
Scheme 1 SiMe, ClSiMe,
- HCI C1(CH2),Si(OMe)jf MeOH
-
(i) H,AlNMe,
- H,, - NMe,
/
Powder
3
kAlH3.n
(ii) Li(bipheny1r biphenyl, - LiCl
-
iv) Mg(anthracene)(THF),, 1 -anthracene ArylCH2X, X
(iii) 9-C1SiMez(Cl,H9) - LiCl
a ArylCH,MgX(THF), T b
hydroxyl groups,13 although H&NMe3 is preferable because of its ease of synthesis and good solubility in a variety of solvents. Moreover, the presence of three nucleophilic species per metal center may favor removal of more deeply embedded hydroxyl groups after initial formation of -0-AlHz (proximity effect). This aside, there are residual aluminum hydride species (IR)which are not reactive toward the chloropropyl groups. This is consistent with the formation of stable alane adducts of chloroalkyl-functionalizedtertiary amine, 3-chloroquinuclidine. l4 The supported lithium reagents 4 and 4' were quenched with ClSiMes, affording the corresponding (trimethylsily1)propyl derivatives.13 Powders 6 and 6 are diamagnetic, and given the amount of uptake of magnesium by 6 and 5 , and the amount of magnesium delivered in forming Grignard reagents, the anthracene sites are based on dianions as in the parent compound 1. The presence of both radical anion and dianion sites in a related polymer-supported reagent8 presumably arises from space limitations and/or electrostatic restrictions with the polymer. Powders 6 and 6' were effective in generating Grignard reagents from benzylic halides in high yield.l5 Results for selected reactions for powder 6 are given in Table 1. The spent powders can be reloaded with magnesium, as demonstrated for cycles involving the formation of Grignard reagents of benzyl chloride and bromide, with less than 1%reduction in the uptake of magnesium after each cycle. More difficult t o prepare (13)van den Ancker, T. R.; Raston, C. L., unpublished results. (14)Jones, C. Ph.D. Thesis, Griffith University, 1992.
iMe,Cl,H9
Table 1. Yields of Grignard Reagents" (%) Prepared from Selected Benzylic Chlorides using Silica-Supported 6 and Recycled Supported 6 and, for Comparison, Corresponding Yields using Mg(anthracene)(THF)J(ly 6 benzylic halide
6
PhCHzCl 90 PhCHzBr 90 1,2-(ClCHz)zC6H4' 85 I , ~ , ~ - ( B I C H ~ ) ~ C , &85 ~
1st recycle
2nd recycle
3rd recycle
1
90
90
90
95
85
85
85 90
90
Established by quenching an aliquot with 0.1 M HCl, back-titrating with 0.1 M NaOH, and isolating the ClSiMe3/acid-quenched derivatives. * Benzylic chlorides in THF were added to a slurry of 6 or 1 in THF at ca. 20 "C for target concentrations of 0.1 M of the Grignard reagent. Yield of di and tri Grignard reagents.
poly Grignard reagents were also investigated, and in all cases the yields were high and comparable with those for the parent compound 1,without the complication of having the solutions containing anthracene.
Acknowledgment. We thank the Australian Research Council for support of this work. OM9407795 (15)As a typical experiment, benzyl chloride (0.075 g, 0.6mmol) in THF (10mL) was slowly added to a suspension of 6 (0.63g) in THF ( 5 mL), complete consumption of the supported magnesium-anthracene species being characterized by a change in color from deep green to pale yellow on the addition of 1 drop. The mixture was filtered to remove the magnesium-depleted silica, 5 , and the activity of the solution was determined by titration (see Table 1). ClSiMe3 (0.11g, , 1.0mmol) was then added, the volatile5 were removed in ~ ( I C U Oand hexane (10 mL) was added. Filtration followed by removal of the solvent in uucuo yielded ((trimethylsily1)methyl)benzene(0.087 g, 88% yield, punty checked by GC/MS and 'HPC NMR spectroscopy).
