Potassium-Graphite Laminate

C8K

[12081-88-8]  · C8K  · Potassium-Graphite  · (MW 135.18)

(powerful reducing agent for many functional groups;1 metalating agent;1 Lewis base;1 hydrogenation catalyst;1,2 catalyst for double bond isomerization and for anionic polymerization;2 tool for preparing activated metals1)

Physical Data: paramagnetic bronze-colored powder; d ~0.73 g cm-3; interlayer distance 5.34 Å; all carbon layers are separated by one layer of potassium; space group D62 C222.

Solubility: may be suspended in hydrocarbon and ethereal solvents; reacts violently with water, alcohols, and ammonia; THF, DME, 1,4-dioxane, and many arenes (e.g. benzene, toluene, pyridine, furan) are able to penetrate into the interlayer space, causing considerable swelling.

Preparative Methods: by stirring potassium and graphite powder at &egt;150 °C under argon without any solvent for 5-10 min. Natural as well as synthetic graphite are suited for its preparation.

Handling, Storage, and Precautions: pyrophoric and must be handled under argon in thoroughly dried solvents. Small quantities should be weighed in a glove box; it can be stored under argon without any significant loss in activity for extended periods of time.

Introduction.

Potassium-graphite laminate may be regarded as a polymeric array of naphthalenide anions and effects almost any reaction that the latter would promote. A major advantage in using C8K as a substitute for Lithium Naphthalenide, for example, lies in its increased reactivity and the easy work-up, consisting of filtration of the graphite only instead of the sometimes tedious removal of naphthalene.1 Moreover, C8K-promoted reactions can be readily monitored by the characteristic color change from bronze of the reagent to black of the graphite host.1

Functional Group Reductions.

Alkyl chlorides and aryl halides are generally reduced to the corresponding hydrocarbons, whereas alkyl iodides afford Wurtz-type coupled products. Alkyl bromides show a reactivity pattern between alkyl chlorides and alkyl iodides. Single-electron transfer is an important pathway in these reductions, since characteristic radical rearrangements are observed with hex-5-enyl halides as mechanistic probes (eq 1).3 vic-Dihalides form alkenes in high yield (eq 2).3,4 C8K leads to reductive cleavage of the C-O bond of aryl ethers, whereas in sulfonate esters the S-O rather than the C-O bond is broken selectively, with formation of the parent alcohols (eq 3).3 Aryl ether and C-Cl cleavage can be done simultaneously, thus allowing the ready destruction of toxic polyhalodibenzodioxines or -dibenzofurans at rt.5 Vinyl sulfones and allyl sulfones afford the corresponding alkenes (the latter after initial isomerization of their double bond) (eq 4).6 The (E):(Z) ratio in these desulfuration reactions is low, with the (E)-isomer being slightly favored.

With Hexamethyldisilazane as cosolvent, C8K in THF selectively reduces the double bond of enones at rt without affecting nonconjugated alkenes (eq 5).7 a,b-Unsaturated acids are similarly reduced at 55 °C, while enoates give dimerization products. Imines afford amines in high yields with this reagent (eq 6),7 whereas ketones may afford mixtures of alcohols and pinacols.8 A large excess of the reagent is required to effect Birch-type reductions of arenes, as exemplified by a series of substituted naphthalene derivatives.9 On treatment with C8K in THF, benzil undergoes a unique coupling of the phenyl rings with formation of 9,10-phenanthrenequinone (eq 7).10 This arene coupling can be extended to 2,3-diphenylquinoxaline and related heterocycles, which afford dibenzo[a,c]phenazine derivatives.11

Although C8K was considered for a long time to be of limited value in organic synthesis due to its high reactivity,3 it turned out to be the reagent of choice for the synthesis of furanoid glycals in terms of yield, reaction rate, and flexibility. The intermediate potassium alcoholates, formed upon treatment of 2,3-O-alkylidene glycosyl halides with C8K in THF at low temperature, can be protected in situ with a wide variety of electrophiles (eq 8).12 Aryl thioglycosides can also be reduced to the corresponding glycals by means of C8K.12b Comparable reaction sequences consisting of C8K-induced fragmentations followed by trapping of the intermediate alcoholates have also been carried out successfully with a series of carbohydrate-derived primary halides (eq 9).13

Chlorosilanes are rapidly converted by C8K to disilanes in high to quantitative yields in THF at ambient temperature.14 Phenyl-substituted disilanes may be further reduced to the corresponding silyl potassium reagents, which can be transmetalated with various transition metal halides (Copper(I) Iodide, Copper(I) Cyanide, MnI2, VCl3) to highly selective nucleophilic silylating agents. They react cleanly with enoates, enones, and acid chlorides, the last affording acyl silanes in a one-pot procedure (eq 10).14a

C8K as Polymeric Lewis Base.

The reactivity of C8K towards Brønsted acids is substrate size dependent.3 C8K has been used to achieve selective monoalkylation of alkyl nitriles and phenylacetic acid esters (substrate:C8K:alkyl halide = 1:2:2) at -60 °C in 40-70% yield, with only small amounts of dialkylated products (0-7%) interfering.15 Imines and dihydro-1,3-oxazine derivatives can likewise be deprotonated by this reagent at rt, followed by exclusive C-alkylation of the azaallyl anion (eq 11).16 Some examples of selective dehydrohalogenations in the carbohydrate series due to the basisity of C8K have also been reported (eq 12).17

Organometallic Chemistry.

C8K is an effective reducing agent for transition metal compounds.1 Particularly relevant to organic synthesis is the ready reduction of Hexacarbonylchromium to K2Cr(CO)5, which forms chromium carbenes upon reaction with amides or esters in presence of Chlorotrimethylsilane (eq 13). This procedure allows an improved entry into the rich chemistry of chromium carbenes, with a comparative study clearly pointing out the superiority of C8K as reducing agent over naphthalenide anions.18 Moreover, C8K is the only reagent that cleanly reduces [CpNi2(CO)]2 to the highly nucleophilic nickelate [Cp(CO)Ni]- K+, which affords allylnickel complexes on reaction with Allyl Bromide.19

Metal-Graphite Reagents.1

C8K may be used to reduce metal salts in ethereal solvents to metal-graphite reagents (eq 14). Due to the even distribution and small size (usually in the range of 2-10 nm in diameter) of the metal particles adsorped on the surface of the graphite support, these reagents exhibit very high and sometimes unprecedented degrees of reactivity and are easily removed by simple filtration after use.1 Almost any metal may be activated by this technique.

Zinc-Graphite doped with 10 mol % Silver exhibits an exceptionally high reactivity and wide scope. This reagent promotes Reformatsky reactions at temperatures well below 0 °C with different kinds of halo esters, including the rather unreactive chloroalkanoates (see also Ethyl Bromozincacetate).20 Tandem reactions comprising a Reformatsky step followed by glycidate formation or a Peterson elimination, respectively, have been carried out.21 Zn/Ag-graphite transforms glycosyl halides to the corresponding glycals under aprotic conditions, independent of the ring size and the protecting groups employed (eq 15).22 Deoxyhalosugar derivatives are reductively ring opened by Zn/Ag-graphite with formation of enantiomerically pure enal building blocks of wide applicability to natural product synthesis (eq 16).13,17,23 It is the only reagent that affords such fragmentations under essentially neutral conditions in anhydrous ethereal solvents, exhibits excellent tolerance towards a wide range of functional groups in the substrates, and is therefore well suited for selective transformations of polysubstituted molecules.24 This was shown in a total synthesis of 9-dihydro-FK-506 (eq 17).25 Zn/Ag-graphite is also suitable for metalating functionalized aryl halides at rt.31

Titanium on graphite deserves special emphasis as one of the most efficient reagents for all kinds of carbonyl coupling reactions. It promotes both inter- as well as intramolecular McMurry reactions of (di)ketones and (di)aldehydes (eq 18),26-28 cyclizes oxo esters to cyclanones,26 and was successfully applied to polyoxygenated substrates in cases when other titanium reagents failed to effect any conversion (eq 19).28 Recently, it was used to cyclize acyloxycarbonyl and acylamidocarbonyl compounds to furans, benzo[b]furans, and indoles (eq 20), respectively.29 This new entry to aromatic heterocycles by reductive C-C bond formation of easily accessible precursors is compatible with a variety of other reducible sites in the substrates.29

The performance of magnesium-graphite in reductive carbonyl coupling processes compares favorably to all other kinds of pinacol-forming agents described so far (eq 21).26b Metal-graphite combinations of platinum, palladium, and nickel have been used as highly efficient and selective catalysts for hydrogenation reactions as well as for catalytic C-C bond formations.30 Preparative advantages have also been drawn from the use of other graphite reagents in organic and organometallic synthesis.1,27a

Related Reagents.

Nickel-Graphite; Palladium-Graphite; Platinum on Carbon; Zinc-Graphite.


1. (a) Fürstner, A. AG(E) 1993, 32, 164. (b) Csuk, R.; Glänzer, B. I.; Fürstner, A. Adv. Organomet. Chem. 1988, 28, 85. (c) Savoia, D.; Trombini, C.; Umani-Ronchi, A. PAC 1985, 57, 1887. (d) Setton, R. In Preparative Chemistry Using Supported Reagents, Laszlo, P., Ed., Academic: New York, 1987; p 225.
2. Ebert, L. B. J. Mol. Catal. 1982, 15, 275.
3. Bergbreiter, D. E.; Killough, J. M. JACS 1978, 100, 2126.
4. Rabinovitz, M.; Tamarkin, D. SC 1984, 14, 377.
5. (a) Lissel, M.; Kottmann, J.; Lenoir, D. Chemosphere 1989, 19, 1499. (b) Lissel, M.; Kottmann, J.; Tamarkin, D.; Rabinovitz, M. ZN(B) 1988, 43, 1211.
6. (a) Savoia, D.; Trombini, C.; Umani-Ronchi, A. JCS(P1) 1977, 123. (b) Ellingsen, P. O.; Undheim, K. ACS 1979, B33, 528.
7. Contento, M.; Savoia, D.; Trombini, C.; Umani-Ronchi, A. S 1979, 30.
8. (a) Setton, R.; Beguin, F.; Piroelle, S. Synth. Met. 1982, 4, 299. (b) Lalancette, J.-M.; Rollin, G.; Dumas, P. CJC 1972, 50, 3058.
9. Weitz, I. S.; Rabinovitz, M. JCS(P1) 1993, 117.
10. Tamarkin, D.; Benny, D.; Rabinovitz, M. AG(E) 1984, 23, 642.
11. Tamarkin, D.; Cohen, Y.; Rabinovitz, M. S 1987, 196.
12. (a) Fürstner, A.; Weidmann, H. J. Carbohydr. Chem. 1988, 7, 773. (b) Fürstner, A. LA 1993, 1211.
13. (a) Fürstner, A.; Koglbauer, U.; Weidmann, H. J. Carbohydr. Chem. 1990, 9, 561. (b) Fürstner, A. TL 1990, 3735. (c) Fürstner, A.; Praly, J. P. AG(E) 1994, 33, 751.
14. (a) Fürstner, A.; Weidmann, H. JOM 1988, 354, 15. (b) Müller, H.; Weinzierl, U.; Seidel, W. Z. Allg. Anorg. Chem. 1991, 603, 15.
15. Savoia, D.; Trombini, C.; Umani-Ronchi, A. TL 1977, 653.
16. Savoia, D.; Trombini, C.; Umani-Ronchi, A. JOC 1978, 43, 2907.
17. Fürstner, A.; Weidmann, H. JOC 1989, 54, 2307.
18. (a) Hegedus, L. S. PAC 1990, 62, 691. (b) Schwindt, M. A.; Lejon, T.; Hegedus, L. S. OM 1990, 9, 2814.
19. Fischer, R. A.; Behm, J.; Herdtweck, E.; Kronseder, C. JOM 1992, 437, C29.
20. (a) Csuk, R.; Fürstner, A.; Weidmann, H. CC 1986, 775. (b) Boldrini, G. P.; Savoia, D.; Tagliavini, E.; Trombini, C.; Umani-Ronchi, A. JOC 1983, 48, 4108.
21. (a) Fürstner, A. JOM 1987, 336, C33. (b) Fürstner, A.; Kollegger, G.; Weidmann, H. JOM 1991, 414, 295.
22. (a) Csuk, R.; Fürstner, A.; Glänzer, B. I.; Weidmann, H. CC 1986, 1149. (b) Pudlo, P.; Thiem, J.; Vill, V. CB 1990, 1129.
23. (a) Fürstner, A.; Jumbam, D. N.; Teslic, J.; Weidmann, H. JOC 1991, 56, 2213. (b) Fürstner, A.; Baumgartner, J.; Jumbam, D. N. JCS(P1) 1993, 131. (c) Fürstner, A.; Baumgartner, J. T 1993, 49, 8541.
24. Ireland, R. E.; Wipf, P.; Miltz, M.; Vanasse, B. JOC 1990, 55, 1423.
25. Ireland, R. E.; Highsmith, T. K.; Gegnas, L. D.; Gleason, J. L. JOC 1992, 57, 5071.
26. (a) Fürstner, A.; Weidmann, H. S 1987, 1071. (b) Fürstner, A.; Csuk, R.; Rohrer, C.; Weidmann, H. JCS(P1) 1988, 1729.
27. (a) Boldrini, G. P.; Savoia, D.; Tagliavini, E.; Trombini, C.; Umani-Ronchi, A. JOM 1985, 280, 307. (b) Clive, D. L. J.; Zhang, C.; Murthy, K. S. K.; Hayward, W. D.; Daigneault, S. JOC 1991, 56, 6447. (c) Burger, P.; Brintzinger, H. H. JOM 1991, 407, 207.
28. Clive, D. L. J.; Murthy, K. S. K.; Wee, A. G. H.; Prasad, J. S.; daSilva, G. V. J.; Majewski, M.; Anderson, P. C.; Evans, C. F.; Haugen, R. D.; Heerze, L. D.; Barrie, J. R. JACS 1990, 112, 3018.
29. (a) Fürstner, A.; Jumbam, D. N. T 1992, 48, 5991. (b) Fürstner, A.; Jumbam, D. N. CC 1993, 211. (c) Fürstner, A.; Hupperts, A.; Ptock, A.; Janssen, E. JOC 1994, 59, 5215. (d) Fürstner, A. T 1995, 51, 773.
30. (a) Savoia, D.; Tagliavini, E.; Trombini, C.; Umani-Ronchi, A. JOC 1981, 46, 5340 and 5344. (b) Savoia, D.; Trombini, C.; Umani-Ronchi, A.; Verardo, G. CC 1981, 540 and 541. (c) Boldrini, G. P.; Savoia, D.; Tagliavini, E.; Trombini, C.; Umani-Ronchi, A. JOM 1984, 268, 97. (d) Fürstner, A.; Hofer, F.; Weidmann, H. J. Catal. 1989, 118, 502.
31. Fürstner, A.; Singer, R.; Knochel, P. TL 1994, 35, 1047.

Alois Fürstner

Max-Planck-Institut für Kohlenforschung, Mülheim, Germany



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