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Synthesis of Tetrasubstituted Stannacyclohexadienes a,b, Matthias Zeller Gregory C. Fu a a Massachusetts Institute of Technology, Department of Chemistry, Dreyfus Building, Cambridge, MA 02139-4307 USA, e-mail: [email protected] b new address: Youngstown State University, Department of Chemistry, 1 University Plaza, Youngstown, Ohio 44555-3663, USA, e-mail: [email protected]. Introduction: Stannacyclohexadienes are excellent starting materials for a wide range of six membered aromatic heterocycles isolelectronic to benzene. 1 Examples are the higher homologs of pyridine such as phospha-, arsa- and stibabenzenes, which have been long regarded as exotic compounds or laboratory curiosities. Recently, however, a remarkable change took place, when it was shown that some of the species such as phosphabenzenes function as versatile ligands in coordination chemistry. 2 When the tin moiety of stannacyclohexadienes is exchanged by a negatively charged B-R unit anionic boratabenzenes are formed. Those are able to replace cyclopentadienyl ligands in transition metal complexes and the bis(boratabenzene)zirconium dichlorides are as active catalysts for the polymerization as their pure carbacyclic counterparts. 3 Pn The unsubstituted stannacyclohexadiene itself is easily formed by the reaction of 1,4-pentadiyne, 4 but this method provides no general access to substituted derivatives. Pn = P, As, Sb R SnR2 B R B R R + H2SnBu2 R = Alkyl, Aryl, etc SnBu2 R = H, SiMe3, t-Bu B Me3Si Cl B P Me3 H Asymmetric Stannacyclohexadienes: R R' I R' Li With no general synthetic pathway towards asymmetric stannacyclohexadienes known today, we decided to switch from the usual [C5 + Sn1] approach to a new [C4 + C1Sn1] route. In initial tries, we used dilithium salts of several 1,4-butadiene dianions and 1,2-dihalostannaethanes, but none of the desired products formed. Instead five membered symmetric stannapentadiene derivatives formed. Transmetallation from lithium to copper resulted in no improvement, with both the lithium and the copper reagents reacting very fast even at –78°C. R I SnMe2 R' SnMe2 Li R' R R Less reactive zirconacyclopentadienes can be prepared from non terminal alkynes using the Negishi reagent Cp2Zr(butene). In recent reports Takahashi has demonstrated, that this compound can be reacted with both alkyl- and stannylhalides. When the reactions are catalyzed by copper(I)chloride, good yields of the coupled products have been isolated. 5 When the zirconacyclopentadienes derived from non-terminal alkynes are reacted with the appropriate 1,2-dihalostannaethanes stannacyclohexadienes 1 to 4 are isolated: R R' I R' a) n-BuLi, THF, -78°C, 1h ZrCp2 Cp2ZrCl2 b) 2 R R' -78°C to rt I 2 eq CuCl SnMe2 R' SnMe2 THF, -78°C to rt R' R Discussion ++++ • straight forward one pot synthesis R (1) R = R' = Ph: (2) R = R' =Et: (3) R = SiMe3, R' = Me: (4) R = Et, R' = (CH2)4-bridge: 76% 66% 78% 57% R –––– • readily available starting materials for zirconacyclopentadiene (alkynes, Cp2ZrCl2) • zirconacyclopentadienes have to be tetrasubstituted, reaction fails with H or SnR3 substituents • tetrasubstituted zirconacyclopentadienes can be made regioselective • 1,2-dihalostannaethanes are highly toxic and sensitizers • annelated stannacycles easy to make • the synthesis of 1,2-dihalostannaethanes is either tedious (ISnMe2CH2I) or involves diazomethane (ClSnMe2CH2Cl) • first synthesis of a tetraalkyl or tetraaryl stannacyclohexadiene, easy access to electron rich and electron poor heterocycles 1) G. E. Herberich, H. Ohst, Adv. Organometal. Chem. 1986, 25, 199-236. Hoic, D. A.; Wolf, J. R.; Davis, W. M.; Fu, G. C Organometallics 1996, 15, 1315-1318. 2) Weber, L. Angew. Chem. Int. Ed. 2002, 41(4), 563 - 572. 3) Ashe, A. J.; Al-Ahmad, S.; Fang, X. J. Organomet. Chem. 1999, 581, 92–97. 4) Boese, R.; Finke, N.; Henkelmann, J.; Maier, G.; Paetzold, P.; Reisenauer, H. P.; Schmid, G. Chem. Ber. 1985, 118, 1644-1654. See also: Boese, R.; Finke, N.; Keil, T.; Paetzold, P.; Schmid, G. Z. Naturforsch., B 1985, 40, 1327-1332. 5) Ura, Y.; Li, Y.; Xi, Z.; Takahashi, T. Tet. Lett. 1998, 39, 2787-2790.