Versatile Stereoselective Cycloadditions between Heterocumulenes and Phosphagermaallene Tip(tBu)GeCPMes: Experimental and Theoretical Investigations

• Published in: Chemistry : a European journal 2011-11, Vol.17 (45), p.12763-12772
• Main Authors: Ghereg, Dumitru, André, Erwan, Gornitzka, Heinz, Escudié, Jean, Ouhsaine, Fatima, Saffon, Nathalie, Miqueu, Karinne, Sotiropoulos, Jean-Marc
• Format: Article
• Language: English
• Subjects: carbenes; Carbon disulfide; Catalysis; Chemical Sciences; Coordination chemistry; Cycloaddition; density functional calculations; Formations; group 14 elements; heteroallenes; Insertion; Isocyanates; Mathematical analysis; or physical chemistry; Organic chemistry; Phenyls; Polymers; Reaction mechanisms; Theoretical and
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201100281

Phosphagermaallene Tip(tBu)GeCPMes* 1 (Tip=2,4,6‐triisopropylphenyl, Mes*=2,4,6‐tri‐tert‐butylphenyl) reacts with phenyl isocyanate and tert‐butyl isocyanate by a [2+2] cycloaddition that involves the GeC and CO double bonds to afford 1‐oxa‐2‐germacyclobutanes 2 and 3. With N,N′‐dicyclohexylcarbodiimide, a [2+2] cycloaddition is observed between the GeC and CN unsaturations to lead to 1‐aza‐2‐germacyclobutane 6 with exocyclic PC and CN double bonds. In sharp contrast, 1 reacts with phenyl isothiocyanate, ethyl isothiocyanate, and carbon disulfide according to a [3+2] cycloaddition that involves the whole GeCP unit and the CS double bond to give transient phosphagermacarbenes (PGeHCs) 11, 12, and 13. These new PGeHCs undergo CH insertions into one o‐tBu group of Mes* (in the case of 11 and 12) or one o‐iPr group of Tip (in the case of 13) with formation of tricyclic compounds 8, 9, and 10, respectively. The reaction mechanisms that involve 1 and the phenyl isocyanate and the phenyl isothiocyanate are described and their regioselectivity is explained by theoretical calculations. Acting on GeCP: Phosphagermaallene Tip(tBu)GeCPMes* (Tip=2,4,6‐triisopropylphenyl, Mes*=2,4,6‐tri‐tert‐butylphenyl) reacts with PhNCO by a [2+2] cycloaddition that involves the GeC bond to afford germacyclobutanes; in contrast, it reacts with PhNCS by a [3+2] cycloaddition that involves the whole GeCP group to give transient phosphagermacarbenes (see scheme).