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

Cunningham, Allan F.
Paglia, Patrick
Simmons, Dana P.
Published in Helvetica Chimica Acta. 1990, vol. 73, no. 2, p. 386-404
Abstract 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 Mel, NHS3 was used to induce carbonylation und decornplexation, the isomerized, conjugated 1,4-cyclohexadiene 13 was obtained almost exclusively. The electrophile selectivity in the reactions with 3 is consistent with a nucleophilic, SN2-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 [Cr(CO)3(R)] 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 [Cr(arene)(CO)3] 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.
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KUNDIG, Ernst Peter et al. trans-Disubstituted Cyclohexadienes via Sequential Addition of a Carbon Nucleophile and an Electrophile to (η6-Benzene)tricarbonylchromium: Scope of Carbon Electrophiles. In: Helvetica Chimica Acta, 1990, vol. 73, n° 2, p. 386-404. doi: 10.1002/hlca.19900730218 https://archive-ouverte.unige.ch/unige:6222

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