Premium
Metal‐Mediated [2+3] Cycloaddition of Nitrones to Palladium‐Bound Isonitriles
Author(s) -
Luzyanin Konstantin V.,
Tskhovrebov Alexander G.,
Guedes da Silva M. Fátima C.,
Haukka Matti,
Pombeiro Armando J. L.,
Kukushkin Vadim Yu.
Publication year - 2009
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200802623
Subject(s) - cycloaddition , chemistry , carbene , palladium , medicinal chemistry , metal , stereochemistry , selectivity , catalysis , organic chemistry
[2+3] Cycloadditions! The metal‐mediated [2+3] cycloaddition of acyclic [ − O + N(R 2 )(H)R 3 ] and nonaromatic cyclic nitrones [ − O + N a CHCH 2 CH 2 C b Me 2 (N a C b )] to palladium‐bound isonitriles cis ‐[PdCl 2 (CNR) 2 ] proceeds under mild conditions to furnish novel heterocyclic carbene complexes, which then undergo NO bond rupture to give imino complexes and free isocyanates (see scheme).The reactions between equimolar amounts of cis ‐[PdCl 2 (CNR) 2 ] (R=cyclohexyl (Cy) 1 , t Bu 2 , 2,6‐Me 2 C 6 H 3 (Xyl) 3 ) and the acyclic nitrones O + N(R 2 )C(H)R 3 (R 2 =Me 5 , R 2 =CH 2 Ph 6 , R 3 = 4‐MeC 6 H 4 ) proceeded in C 6 H 6 at 5 °C for around 4 h and then the reaction systems were maintained at 20–25 °C for 20 h to provide the carbene complexes [PdCl 2 {C(ONR 2 C c HR 3 )=N d R}(CNR)(C c N d )] ( 8 – 13 ) in good yields (54–70 %). The latter species originated from the previously unreported metal‐mediated [2+3] cycloaddition of nitrones to coordinated isonitriles. For the reactions of cis ‐[PdCl 2 (CNR) 2 ] ( 1 – 3 ) and 5 or 6 performed in C 6 H 6 at 20–25 °C, the selectivity of the cycloaddition was lost, and the reactions gave mixtures of 8 – 13 and the complexes [PdCl 2 {N(R 2 )CHR 3 }(CNR)] ( 14 – 19 , ca. 75 %) derived from intramolecular deoxygenation of the nitrones. The reactions of equimolar amounts of cis ‐[PdCl 2 (CNR) 2 ] ( 1 – 3 and R=4‐MeOC 6 H 4 4 ) and the nonaromatic cyclic nitrone − O + N a CHCH 2 CH 2 C b Me 2 (N a C b ) ( 7 ) in CHCl 3 at 5 °C for around 2 h led to the carbene species [PdCl 2 {C(ON a CMe 2 CH 2 CH 2 C b H)=N e R}(CNR)(N a C b )(C b N e )] ( 21 – 24 ), isolated in yields of 78–92 %. When the reactions of cis ‐[PdCl 2 (CNR) 2 ] ( 1 – 4 ) and 7 (1:1 molar ratio) were performed in CHCl 3 at 20–25 °C for around 1 h, the carbenes 21 – 24 and the complexes [PdCl 2 {N f CMe 2 CH 2 CH 2 C g H}(CNR)(N f C g )] ( 25 – 28 , ca. 35 %), derived from the deoxygenation of 7 , were identified by ESI + ‐MS and 1 H NMR analysis (after redissolution in CDCl 3 ). The reactions of cis ‐[PdCl 2 (CNR) 2 ] ( 1 – 4 ) and 7 (1:2 molar ratio) in CHCl 3 at 5 °C furnished mixtures of [PdCl 2 {C(ON a CMe 2 CH 2 CH 2 C b H)N e R}(N f CMe 2 CH 2 CH 2 C g H)(N a C b )(C b N e )(N f C g )] ( 29 – 32 ; ca. 25–30 %), 25 – 28 (ca. 10–15 %), and [PdCl 2 (ON a CMe 2 CH 2 CH 2 C b H) 2 (N a C b )]. The same reactions at room temperature afforded the complex [PdCl 2 (N f CMe 2 CH 2 CH 2 C g H) 2 (N f C g )] ( 33 ; ca. 20 %) in a mixture with 29 – 32 (ca. 5–10 %) and 25 – 28 .