trans-Disubstituted Cyclohexadienes via Sequential Addition of a Carbon Nucleophile and an Electrophile to (η6-Benzene)tricarbonylchromium: Scope of Carbon Electrophiles

Article abstract of DOI:10.1002/hlca.19900730218

A regio- and stereoselective route from benzene to trans-disubstituted cyclohexadienes via complexation of the arene to the tricarbonylchromium group is reported.The key step involves an alkylation/carbonylation sequence of the anionic tricarbonyl (η⁵-cyclohexadienyl)chromium complex (3) which is readily obtained by the addition of 2-lithio-2-methyldithiane (2) to (η⁶-benzene)tricarbonylchromium (1; cf.Scheme 6).In situ reaction of 3 with alkyl halides (in THF, THF/HMPA, THF/DMPU), followed by oxidation (I2, Ce(IV)) or ligand exchange (CO, Ph3P, Et3N) produced, with complete stereo- and regioselectivity, trans-5,6-disubstituted cyclohexadienes (15 examples).The cyclohexadiene substituent originating from the alkyl halide in all cases is an acyl group which shows that CO insertion into the metal-alkyl bond precedes reductive elimination to form the cyclohexadiene product.When, in the reaction of 3 with MeI, NH3 was used to induce carbonylation and decomplexation, the isomerized, conjugated 1,4-cyclohexadiene 13 was obtained almost exclusively.The electrophile selectivity in the reactions with 3 is consistent with a nucleophilic, S_N2-like mechanism with a high preference for primary iodides.Chloride, ketone, and ester functions in the electrophile are unreactive and are tolerated; a primary alkyl iodide reacted selectively in the presence of a secondary iodide.In one case, the trans-configuration in a cyclohexadiene product 7 was demonstrated by the Diels-Alder reaction with maleic anhydride.High facial selectivity was observed in this reaction giving a single diastereoisomer resulting from endo-addition of the dienophile to the diene face carrying the acyl group.The anionic intermediate 3 was trapped with Ph3SnCl, and an X-ray analysis of the resulting cyclohexadienyl complex 15 provides evidence for electrophile addition to the metal.In the solid state, the Ph3Sn group in 15 is trans-configurated to the dithianyl substituent.The ready access to complexes, the high selectivity of the reactions reported here and the mild decomplexation provide rapid access to cyclohexadienes that possess functionality and are attractive for further transformation.