Carbene from isonitrile complexes

Table 2.4. Heteroatom-substituted carbene complexes prepared from isonitrile complexes.

The structure of rhodium(II) carboxylate-derived carbene complexes has been assessed both by quantum mechanical calculations [19,20] and by the study of rhodium(II) carboxylate isonitrile complexes [20,21]. Recent investigations [20] suggest, that also in these highly electrophilic carbene complexes there is a significant n backbonding from rhodium to carbon. 

Cr(PF3)e and fac-Cr(CO)3(PF3)3. Cr NMR data have also been reported for 46 chromium-carbene complexes and three isonitrile complexes for whidi the chemical shifts (+20 to +303p.p.m., relative to Cr(CO)6 as external standard) correlate well with the donor-acceptor properties of the ligands and show some sensitivity to steric effects. The spectra, measured in THF at ambient temperatures, show line widths ranging from SO to 3500 Hz (Avj/2 Cr(CO)g = 2-3 Hz). The narrowest lines are observed for the isonitriie compounds. 

The binary gold carbonyl complexes are instable, but the family of inorganic 14-electron complexes [Au (Cl)L] is known for its stability with a variety of L ligands (CO, PR3, RNC, pyridine, etc.). These complexes are also prepared from the precursor complex [Au(Cl)(Me2S)] or [Au(Cl)(tmt)], tmt = tetramethylthiophene. The isonitrile complexes undergo the addition of nucleophiles (alcohols, amines) to yield carbene complexes ... 

It is assumed that compound (37) is produced by an analogous mechanism to that postulated for the synthesis of Fischer carbenes from metal-carbonyl or metal-isonitrile complexes according to Scheme 2.20. 

Merlic developed a new variation of the thermally induced benzannulation reaction. The dienylcarbene complex 132 was reacted with isonitrile to give an orf/zo-alkoxyaniline derivative 135 [76] (Scheme 56). This annulation product is regiocomplementary to those reported from photochemical reaction of chromium dienyl(amino)carbene complexes. The metathesis of the isocyanide with the dienylcarbene complex 132 generates a chromium-complexed di-enylketenimine intermediate 133 which undergoes electrocyclisation. Final tau-tomerisation and demetalation afford the orf/zo-alkoxyaniline 135. 

The reactions of alkenyl carbene complexes with isonitriles can produce a variety of products whose distribution is sensitive to the nature of the isonitrile. Reacting 3 equiv. of aryl isonitrile with the alkenyl complex (383) produces the dihydro- y-carbolinone (385). 74 The ketenimine in (384) is liberated with a second equivalent of the isonitrile, and the the third equivalent undergoes a formal [4 -i- 2] cycloaddition with the free ketenimine. With sterically encumbered isonitriles, the ketenimine is more rapidly liberated from the metal by a second isonitrile and 2 equiv. are required to drive the reaction to completion. Free ketenimines that are generated in this manner can be trapped via their [4 + 2] cycloadditions with maleic anhydride. 7 Primary and secondary alkyl isonitriles will react with alkenyl carbene complexes to give cycloadducts that can be converted to pyrroles or products that contain five-member ring carbo-cycles. 7 ... 

Figure 26 Isomers of gold carbene complexes obtained from coordinated isonitriles.

Ketenimine complexes, such as R(OMe)C=C=N(R ) Cr(CO)5, 3, which can be generated from a carbene complex and an isonitrile, dimerize in the metal template to form the blue dimer complexes 4. The formation of the unexpected dimer is caused by the metal template... 

As a versatile alternative to the generation of free carbenes, Hashmi and co-workers developed a methodology for the synthesis of abnormal NHCs where the carbene is constructed directly at the metal center. Thus, a [3 + 2] dipolar cycloaddition of azomethine ylides and gold isonitriles afforded the abnormal saturated imidazolinylidene gold eomplexes 9 [eqn (3.3)]. This methodology circumvents the regioseleetivity issue of C4 vs. C5 eoordination because the carbenic carbon is pre-coordinated to the metal center in the isonitrile precursor. Moreover, this procedure provided aeeess to a new subclass of abnormal carbenes and yielded the first abnormal earbene complexes featuring a saturated heteroeyele, unlike all previous abnormal imidazolylidene complexes, derived from unsaturated heteroeyeles. 

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