Formation of Transient and Long-Lived Cyclopropenyl Anions Tina L. Arrowood and Steven R. Kass* Department of Chemistry UniVersity of Minnesota Minneapolis, Minnesota 55455 ReceiVed April 19, 1999 Cyclobutadiene has been viewed as the Mona Lisa of organic chemistry by Cram et al. 1 This description also applies to cyclopropenyl anion since it too has captured the imagination of chemists, inspires wonder, and has been the subject of consider- able effort. 2-14 However, in contrast to cyclobutadiene, cyclo- propenyl anion remains largely unknown. A more apt analogy, therefore, might be to compare it to the pyramids since it is unclear how they were constructed or how a long-lived cyclopropenyl anion can be prepared in condensed media. In 1994 we reported the first preparation of a stable cyclopro- penyl anion by reacting fluoride ion with 3-carbomethoxy-3- trimethylsilylcyclopropene in the gas phase (eq 1). 15 In this work we provide condensed-phase mechanistic evidence that the fluoride-induced desilylation of a 3-trimethylsilylcyclopropene derivative leads to a transient cyclopropenyl anion intermediate. The first preparation of a long-lived cyclopropenyl anion in solution and its corresponding UV-visible spectrum also are presented. Valuable information regarding the structure and reactivity of cyclopropenyl anions can, in principle, be obtained by studying the fluorodesilylation of 3-trimethylsilylcyclopropenes. In an important experiment, Borden et al. looked at the desilylation of 13 C-labeled 1,2,3-triphenyl-3-trimethylsilylcyclopropene with tetra- n-butylammonium fluoride (TBAF) in tetrahydrofuran. 16 They found that the 13 C label scrambled around the ring in the proton- trapped product. We have explored the regioselectivity in unsym- metrical substrates and have found that carbon electrophiles can be used to give good yields of the trapped cyclopropene (eq 2). 17,18 While these results are consistent with the putative formation of a cyclopropenyl anion intermediate, such a species is not required mechanistically. A hypervalent penta- or hexacoordinate silicon anion cannot be ruled out. To distinguish between these possibilities a chiral cyclopropene (4) was synthesized by photochemical addition of L-menthyl (trimethylsilyl)diazoacetate to tert-butyl-phenylacetylene (eq 3). The 2 diastereomers (4a and 4b) were separated by MPLC to give the optically pure (g98%) esters. These substrates were reacted with TBAF and Schwesinger’s P2-F phosphazenium fluoride source; the latter reagent is a “naked” and extremely reactive source of fluoride. 19,20 If a cyclopropenyl anion inter- mediate is formed, racemization at C-3 is expected although it is not required because of the chiral menthyl group (pathway 1, Scheme 1). However, if a penta- or hexacoordinate silicon intermediate reacts with the electrophile then retention of con- figuration at C-3 should result (pathway 2, Scheme 1). In both cases when the desilylation reaction was carried out in the presence of benzaldehyde four benzylic alcohols were formed in a 1:1: 0.7:0.7 ratio as indicated by 1 H NMR (eq 4). Separation of two of the benzylic alcohols and a mixture of the other two was achieved via HPLC, but subsequent chemical transformations were carried out on equimolar 5a/5b and 5c/5d mixtures. Oxidation of the trapped alcohols with MnO 2 affords the corresponding phenyl ketones in which a chiral center has been eliminated. Both sets of compounds afforded diastereomeric ketones, which indicates that 5a and 5b, as well as 5c and 5d, have different configurations at C-3. In a similar manner, reduction of the menthyl esters with DIBAL at -78 °C afforded enanti- omers. Again, this shows that racemization occurred at C-3. The protodesilylation products (6a and 6b), which are formed with either fluoride source due to the presence of protic impurities, 21 also were produced in a 1:1 ratio. 22 This indicates that complete racemization at C-3 takes place. Pathway 1 in Scheme 1, consequently, appears to be operating, and a cyclopropenyl anion intermediate is formed in these desilylation reactions. This (1) Cram, D. J.; Tanner, M. E.; Thomas, R. Angew. Chem., Int. Ed. Engl. 1991, 30, 1024-1027. (2) Breslow, R. Angew. Chem., Int. Ed. Engl. 1968, 7, 565-570. (3) Breslow, R. Chem. Br. 1968, 4, 100-101. (4) Breslow, R. Pure Appl. Chem. 1971, 28, 111-130. (5) Breslow, R. Acc. Chem. Res. 1973, 6, 393-398. (6) Breslow, R. Pure Appl. Chem. 1982, 54, 927-938. (7) Winkelhofer, G.; Janoschek, R.; Fratev, F.; Spitznagel, G. W.; Chan- drasekhar, J.; Schleyer, P. v. R. J. Am. Chem. Soc. 1985, 107, 332-337. (8) Garratt, P. J. Aromaticity; John Wiley and Sons: New York, 1986. (9) Schleyer, P. v. R.; Kaufmann, E.; Spitznagel, G. W. Organometallics 1986, 5, 79-85. (10) Li, W.-K. J. Chem. Research 1988, 220-221. (11) Minkin, V. I.; Glukhovtsev, M. N.; Simkin, B. Y. Aromaticity and Antiaromaticity: Electronic and Structural Aspects; Wiley-Interscience: New York, 1994. (12) Glukhovtsev, M. N.; Laiter, S.; Pross, A. J. Phys. Chem. 1996, 100, 17801-17806. (13) Klicic, J.; Rubin, Y.; Breslow, R. Tetrahedron 1997, 53, 4129-4136. (14) Merrill, G. N.; Kass, S. R. J. Am. Chem. Soc. 1997, 119, 12322- 12337. (15) Kass, S. R.; Sachs, R. K. J. Am. Chem. Soc. 1994, 116, 783-784. (16) Koser, H. G.; Renzoni, G. E.; Borden, W. T. J. Am. Chem. Soc. 1983, 105, 6359-6360. (17) Han, S.; Kass, S. R. J. Chem. Soc., Perkin Trans 1 1999, 1553- 1558. (18) Han, S. Ph.D. Thesis; University of Minnesota, 1997. (19) The structures of P2-F and P5-5 are as follows: (Me2N)3PdN + d P(NMe2)3 F - and ((Me2N)3PdN)4P + F - , respectively. Schwesinger, R.; Link, R.; Thiele, G.; Rotter, H.; Honert, D.; Limbach, H.; Ma ¨nnle, F. Angew. Chem., Int. Ed. Engl. 1991, 30, 1372-1375. (20) Link, R.; Schwesinger, R. Angew. Chem., Int. Ed. Engl. 1992, 31, 850. (21) When THF-d8 was used as the solvent, no deuterium was incorporated into 6. This strongly suggests that this product does not arise via a radical pathway. (22) Trapping in DMSO also leads to a 1:1 mixture of proton-trapped products. 7272 J. Am. Chem. Soc. 1999, 121, 7272-7273 10.1021/ja991224o CCC: $18.00 © 1999 American Chemical Society Published on Web 07/27/1999