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Directed Formation of Allene Complexes upon Reaction of Non-heteroatom-Substituted Manganese Alkynyl Carbene Complexes with Nucleophiles

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posted on 12.05.2008, 00:00 by Stéphane Sentets, Rémi Serres, Yannick Ortin, Noël Lugan, Guy Lavigne
The non-heteroatom-substituted alkynyl carbene Cp′(CO)2MnC(Tol)CCPh (1, Cp′ ≡ (η5-MeC5H4)) is first shown to react at low temperature with lithium diorganophosphide LiPR2 (R = Ph, Cy) to form an anionic species. Subsequent treatment with CF3SO3H affords the η4-vinylketene complex Cp′(CO)2Mn[η4-{R2P(Ph)CCHC(Tol)CO}] (2; 2a: R = Ph (70% yield), 2b: R = Cy (55% yield)) as the major compound, along with trace amounts of the η2-allene complex syn-Cp′(CO)2Mn[η2-{Ph2P(Tol)CCC(Ph)H}] (syn-3a) for R = Ph, or along with the η2-allene complex Cp′(CO)2Mn[η2-{H(Tol)CCC(Ph)PCy2}] (4b, 26% yield, 1:2 mixture of syn/anti isomers) for R = Cy. On the other hand, subsequent treatment with NH4Claq affords only η2-allene complexes, obtained either as a ca. 1:9 mixture of syn-3a and Cp′(CO)2Mn[η2-{H(Tol)CCC(Ph)PPh2}] (4a) (75% yield) for R = Ph or as a 1:2 mixture of syn- and anti-4b for R = Cy (74% yield). Combined NMR and single-crystal X-ray diffraction studies (for 2a, anti-4b, and syn-4b) revealed that both type 2 and type 4 species result from a nucleophilic attack of the diorganophosphide onto the remote alkynyl carbon atom in 1 (Cγ), whereas type 3 species results from a nucleophilic attack of the carbene carbon atom (Cα). Complexes 3a and 4a,b are prone to undergo a thermal rearrangement to give the η1-phosphinoallene complexes Cp′(CO)2Mn[η1-{Ph2P(Tol)CCC(Ph)H}] (5a) and Cp′(CO)2Mn[η1-{R2P(Ph)CCC(Tol)H}] (6; 6a: R = Ph, 6b: R = Cy), respectively. Reaction of 1 with p-toluenethiol in the presence of NEt3 (20%) affords a 1.8:1 mixture of Cp′(CO)2Mn[η2-{TolS(Tol)CCC(Ph)H}] (syn-11), resulting from a nucleophilic attack at Cα in 1, and Cp′(CO)2Mn[η2-{H(Tol)CCC(Ph)STol}] (12), resulting from a nucleophilic attack at Cγ, whereas treatment of 1 with lithium p-toluenethiolate at –80 °C followed by protonation with NH4Claq gave the same syn-11 and 12 complexes now in a 1:2.3 ratio. Finally, 1 was found to react with cyclohexanone lithium enolate to afford, upon protonation, the η2-allene complex Cp′(CO)2Mn[η2-{H(Tol)CCC(Ph)CH(CH2)4C(O)}] (syn-13), resulting from a nucleophilic attack at Cγ in 1. The solid-state structures of syn-11 and syn-13 are also reported.