Oxazolinyl-carbene

Scheme 41 Synthesis of the imidazolium precursor of Herrmann s oxazolinyl-carbene ligand...

Gade and co-workers reported the synthesis of an oxazolinyl-carbene which is obtained by direct linkage of the two heterocycles. The new ligand system was obtained by reacting the 2-bromooxazoline 65 [122] with an imidazolium precursor in THF (Scheme 46) [123]. N-heterocyclic carbene rhodium complexes could be obtained by reaction of the imidazolium salt 66 with [ Rh(/i.-OfBu)(nbd) 2] generated in situ [124]. 

Although this reaction is still at an early stage of development, a few modifications have been exploited. These modifications include the extension of the Bellus-Claisen rearrangement to ring strain molecules (such as 2-vinyloxetane, 2-vinyloxirane, and 2-vinylaziridines ) and to tertiary allyl amines, the application of chiral Lewis acid derived from Mgl2 and bis(oxazolinyl)aryl ligands, and the implementation of electron-rich ketene equivalents (e.g., alkoxyketenes, aminoketenes) by the photolysis of chromium carbene complexes. 

The first X-ray structure of a ruthenium porphyrin carbene complex was reported by Simonneaux and coworkers (Fig. 1) [133]. To stabilize the ruthenium carbene complex, ethyl diazomalonate was used instead of ethyl diazomethyl acetate, as was previously reported in the bis(oxazolinyl)pyridine (pybox) series [134]. The presence of this complex as an intermediate in cy-clopropanation was also discussed in relation with stoichiometric transfer to alkenes (vide infra) [135]. 

Xu S, Manna K, Ellem A, Sadow AD. Mixed N-heterocyclic carbene-bis(oxazolinyl) borato rhodium and iridium complexes in photochemical and thermal oxidative addition reactions. OrganometaUics. 2014 33 6840-6860. 

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