Stereochemische Aspekte bei metallacyclischen Neunring‐Zirconoxycarbenkomplexen aus (Butadien)zirconocen, Hexacarbonylwolfram und Ketonen oder Aldehyden

Stereochemical Aspects with Metallacyclic Nine‐Membered Zirconoxycarbene Complexes from Coupling (Butadiene)‐zirconocene, Hexacarbonyltungsten, and Ketones or Aldehydes Reaction of (butadiene)zirconocene complexes with M(CO) 6 (M =Cr, W) and subsequently with ketones or aldehydes O=CR 1 R 2 (R 1 , R 2 = H, alkyl, alkenyl, or aryl) produces the metallacyclic nine‐membered zirconoxycarbene complexes ( 7a‐r . The complexes trans ‐7 contain chiral metallacyclic ring systems. Starting from many ketones two diastereomeric compounds trans ‐7 and trans ‐7′ are obtained. From the dynamic 1 H‐NMR spectra an activation barrier of ΔG(317K)=16.5 ± 0.4 kcal/mol has been estimated fro the trans ‐7k [(2R*)(4,5,6‐pS*)] → trans ‐7k′ [(2R*)(4,5,6‐pR*)] rearrangement. For the tert ‐alkyl‐substituted nine‐membered metalacycles trans ‐ 7f‐h only the respective (2R*)(4,5,6‐pS*) isomers have been found Starting from aldehydes with less sterically demanding alkyl groups mixtures of the trans ‐7 [(2R*)(4,5,6‐pS*)] compounds and their cis ‐ 7 isomers are obtained. Using the more bulky bis( tert ‐butylcyclopentadienyl)zirconium moiety for coupling butadiene and W(CO) 6 with simple aldehydes leads to a large excess of the trans ‐isomers (e. g. R 1 = H, R 2 = i‐C 3 H 7 : trans ‐ 7q : Cis 7q = 90: 10).