Chelating carbenes

Bidentate NHC-Pd complexes have been tested as hydrogenation catalysts of cyclooctene under mild conditions (room temperature, 1 atm, ethanol). The complex 22 (Fig. 2.5), featuring abnormal carbene binding from the O carbon of the imidazole heterocycles, has stronger Pd-C jj, bonds and more nucleophilic metal centre than the bound normal carbene chelate 21. The different ligand properties are reflected in the superior activity of 22 in the hydrogenation of cyclooctene at 1-2 mol% loadings under mild conditions. The exact reasons for the reactivity difference in terms of elementary reaction steps are not clearly understood [19]. 

Fig. 4. Tripodal N-heterocyclic carbene chelators with mesitylene (mes-carbene, left), carbon (TIME, center), and nitrogen (TIMEN, right) anchoring units.

Note The chelate carbene complexes with normal carbene coordination (O) do exist. They have even bulkier wingtip groups (tert-butyl instead of isopropyl) and a more crowded equatorial plane since the abnormally coordinated (C ) carbene chelate ligands are faced with sterically uniquely undemanding hydride ligands. 

Note Isomerism in the ligand sphere ofpalladium(II) carbene complexes with imino functionalised carbene chelate ligands is observed with moderately donating solvents like THF and acetonitrile as the solvent has to be hemilabile itself... 

Similar cts-bis-carbene chelate complexes of palladium(Il) [327,330,331], but without the hydroxy functional groups on the wingtips, were used by the same research group for the copolymerisation of ethylene and CO. Once again, chelating bisphosphane complexes inspired the synthesis and application of their NHC counterparts [332,333]. The actual, defined catalyst precursors were the cationic complexes formed after haUde abstraction with silver salts in acetonitrile as donor solvent. 

Figure 5.6 Conformational flexibility of transition metal carbene chelate complexes.

Catalyst precursors modified with other types of hgands, such as bis(N-methyl-imidazole) 27 [56], N-heterocyclic carbene chelates 28 [57], and cahx[6]arene-derived... 

Difluoroboroxycarbene complexes have been further applied to stereoselective Diels-Alder reaetions. The [4+2]-cycloaddition of alkenyl(difluoroboroxy)-carbene chelate 90 with 2-amino-1,3-dienes 91 afforded the spiroeyclic carbene complex 92 with variable diastereoselectivity (Scheme 40). [92] An extension of this reaction to chiral aminodiene 93 led to the cycloadduct 94 featuring three stereogenic centers including a quaternary carbon in moderate yields but with high asymmetric induction (Scheme 41). [92]... 

The outcome of the reaction from the T.S. should be a cis alkenyl, but it is not rare that trans alkenyls are formed. Interestingly, in the vicinity of the transition state (within a few kcal/mol) was found the skewed structure framed in the Scheme 6.6, which might become more stable than the planar transition state upon change of the alkyne or the complex. From this structure, which can be drawn also as an alkyl/carbene chelated ligand, either cis or trans alkenyl can be formed. This could explain satisfactorily the experimental results. 

A rather unusual confirmation of the fact that, under the conditions of Mizoroki-Heck reactions, only a small part of a robust SRPC is disassembled to afford a catalytically active species was shown in self-coupling transformation of bis-carbene chelate 163 containing styrene residues in the reaction with active substrate 4-bromoacetophenone (7) (Scheme 2.26) [217], A reasonable yield of 78% of doubly arylated complex 164 with conserved chelate structure was isolated. 

Scheme 11.18) [39]. The five-step sequence (cycloaddition, oxidation, ester hydrolysis, carbonyl deprotection, decarboxylation) required for the transformation of chromium carbene chelate 50 into the benzyl-protected rolipram precursor has been performed tvithout isolation of any intermediates after the final debenzyla-tion (-I-)-rolipram 51 is obtained in lotv overall yield. 

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