Dicarbenes

R = /-Pr, n-Bu, octyl). The reaction of l,r-methylenebis(4-methyl-l,2, 4-tria2olium)diiodide with palladium(II) acetate however proceeds differently and gives the cationic dicarbene [L2Pd(02CMe)]I. 

Trimethylsiloxyphenyl isocyanide enters the cyclization reaction with [MCl2(NCPh)2] (M = Pt, Pd) to yield the homoleptic tetracarbenes 77 (M=Pt, Pd) (97JOM(541)51). Complex 77 (M = Pd) enters an interesting reaction with ammonia to yield the species 78 where two of this benzoxazol-2-ylidene ligands are deprotonated and become C-coordinated benzoxazole moieties, while the other two remain intact. Palladium(II) iodide in these conditions behaves differently yielding the di-Mo-cyanide complex, which in the presence of tetra- -butyl ammonium fluoride gives the dicarbene 79. 

When dicarbene complexes of the form 21 were tested for 1-bntene or propene dimerisation, npon activation with AfEt Cl or MAO in tolnene, rapid deactivation took place yielding Ni(0) [25]. It was shown that this decomposition did indeed involve carbene-hydride and carbene-aUcyl rednctive elimination. Some dimerisation was evident at -15°C (TON = 50), however decomposition of the intermediate Ni species seemed too rapid for effective catalysis. In contrast, when the complexes were... 

Several reports in which NHC-Pd complexes have been employed to catalyse the copolymerisation of alkenes with CO have appeared over the years. Herrmann and co-workers reported that the chelating dicarbene complex 38 (Fig. 4.14) is active for CO/ethylene [43], The highest TON [(mol ethylene + mol CO) mol Pd ] was 3 075 after a 4 h run. The modest TONs coupled with a very high molecular weight copolymer led the authors to conclude that only a small fraction of the pre-catalyst goes on to form an active species. Low molecular weight (M = 3 790) CO/norbomene copolymer resulted when complex 39 (Fig. 4.14) was tested by Chen and Lin [44]. The catalyst displayed only a very low activity, yielding 330 turnovers after 3 days. 

Methylation of the dihaptothioacyl complex 22 affords compound 23 containing a bidentate carbene ligand, which on reaction with chloride ion leads to the neutral monodentate carbene complex 24 (50,51). The chelate carbene complex 26 is generated in a novel interligand reaction from the thiocarboxamidothiocarbonyl cation 25. The thiocarbonyl carbon acts as the electrophilic component in this reaction, and 26 is further alkylated to a bidentate dicarbene species (52). 

To explain the observed rupture of sterically-hindered substituted tertiarysecondary C-C bond on these low dispersed Pt catalysts, a third mechanism was proposed that competes with the dicarbene mechanism.55 This third mechanism involves a metallocyclobutane species as an intermediate. In fact, both McVicker et al.11 and Coq et al.56 have cited the metallocyclobutane mechanism as responsible for the observed hydrogenolysis of 1,2,4-trimethylcyclohexane and 2,2,3,3-tetra-methylbutane, respectively. 

Fig. 11 Cetane number of intermediates and products of the reaction pathway of metal-catalyzed ring opening of decalin via dicarbene mechanism. Adapted from ref. 12. 

In summary, in order to reduce the content of toluene in gasoline while keeping a high octane number, toluene must undergo hydrogenation and ring contraction followed by SRO. The RC step can proceed via bifunctional catalysts and the SRO must use a metal catalyst (e.g. Ir/Si02) that is selective towards the dicarbene mechanism to cleave C-C bonds at unsubstituted positions. 

Fig. 19 Ring opening modes dicarbene (a), 7t-adsorbed olefin (b), metallocyclobutane (c). Adapted from ref. 49.

These observations can be explained taking into consideration that in this study the metal particle sizes were relatively large in all three catalysts, so the dicarbene mechanism dominated, even on the Pt-based catalysts. In any case, a somewhat higher selectivity towards substituted C-C cleavage was observed on the Pt catalysts, relative to the monometallic Ir catalyst. However, as we have recently pointed out, unless the naphthenic rings are opened very selectively at the substituted C-C bonds, no considerable gain in CN can be achieved by RO. This was not the case in any of the Pt-Ir catalysts presented in ref. 111. 

Di-fcrt-butylphenol, liquid-phase oxidation, over Cu"+-TSM, 39 322-324 Dicarbenes, isomerization, 30 56-57 Dicarbynes, 30 80-81... 

Encouraged by the experimental finding (Itoh, 1967 Wasserman et al., 1967) that the dicarbene [15 m = 2] ( = [3]) had a ground quintet state, i.e. all four spins were ferromagnetically coupled, Iwamura and Itoh have been engaged in a project directed towards the construction of the higher series of poly(carbenes) [15], [23] and [24] (Iwamura et al., 1985 Teki et al., 1983, 1985, 1986). 

Fig. 16 Temperature dependence of the epr signal intensities due to m,rri- ( ) and tn,p - (O) isomers of dicarbene [20]. Only the latter obeys the Curie law.

The isomeric stilbene dicarbenes [20] were generated in 2-MTHF matrices in an epr cavity at 16 K (Murata et ai, 1987 Iwamura, 1988). The spectra obtained by the photolysis of the diazo-compounds [20a], precursors to m,p -[20] and w,m -[20], at 16 K exhibited conspicuous signals at ca. 250 mT, characteristic of quintet species. The signals of /w,w -[20] showed a dramatic temperature dependence (Fig. 16). First, their intensity increased as the temperature was raised, reaching a maximum at SO K, and then decreased somewhat and eventually irreversibly at above 65 K. In contrast, the intensity of the strong signal at ca. 250 mT and some weak signals due to m,p -[20] decreased linearly with the reciprocal of the temperature as dictated... 

Fig. 22 (a) Epr spectrum obtained by photolysis of the didiazo-compound o-[38a] of a [2.2]paracyclophane. (b) The signals due to unoriented dicarbene o-[38] in the quintet state simulated by a perturbation calculation. 

Fig. 23 The observed order of states for the isomeric [2.2]paracyclophane-dicarbenes [38], in good agreement with the McConnell theory.

The significant increase in catalytic performance is shown by a comparison of the bimetallic compounds 55 and 56, NHC/phosphine complex 54, dicarbene complex 53, and diphosphine complex 52 in ROMP of 1,5-cyclooctadiene (Fig. 2). ... 

As a consequence of the higher coordination energy, the dicarbene complexes 53 disfavor a dissociative pathway similar to that of 52. A mixed NHC/phosphine complex of type 54, however, reveals a phosphine dissociation energy in the same order of magnitude as 52. Therefore, 54 is able to populate the dissociative pathway just as readily as 52. In contrast to 52, however, a phosphine-free species 58 is to be considered as the key intermediate in the catalytic cycle. 

In order to vary the electronic situation at the carbene carbon atom a number of carbo- and heterocycle-annulated imidazolin-2-ylidenes like the benzobis(imida-zolin-2-ylidenes) [58-60] and the singly or doubly pyrido-annulated A -heterocyclic carbenes [61-63] have been prepared and studied. Additional carbenes derived from a five-membered heterocycle like triazolin-5-ylidenes 10 [36], which reveals properties similar to the imidazolin-2-ylidenes 5 and thiazolin-2-ylidene 11 [37] exhibiting characteristic properties comparable to the saturated imidazolidin-2ylidenes 7 have also been prepared. Bertrand reported the 1,2,4-triazolium dication 12 [64]. Although all attempts to isolate the free dicarbene species from this dication have failed so far, silver complexes [65] as well as homo- and heterobimetallic iridium and rhodium complexes of the triazolin-3,5-diylidene have been prepared [66]. The 1,2,4-triazolium salts and the thiazolium salts have been used successfully as precatalysts for inter- [67] and intramolecular benzoin condensations [68]. 

Some bidentate bis(imidazolin-2-ylidene)s 18 have been used for the preparation of complexes with chelating dicarbene ligands [75-80]. The synthesis, properties, and coordination chemistry of tripodal tris(imidazolin-2-ylidene) ligands like 19... 

First attempts to isolate monocarbene-hydrido complexes by oxidative addition of A -(2-pyridyl)imidazolium cations to Pd° with utilization of the chelate effect of the donor-functionalized carbene ligand failed and only the dicarbene complexes such as 29 were isolated [112]. The iridium hydrido complex 30 was obtained in the oxidative addition of an W-(2-pyridylmethyl)imidazolium cation to iridium(I) (Fig. 11) [113]. This reaction proceeds most likely via the initial coordination of the nitrogen donor which brings the imidazolium C2-H bond in close proximity to the metal center. No reaction was observed with Rh under these conditions. 

Monocarbene complexes of Pt° [114] as well as dicarbene complexes of Ni° and Pd° [115] activate the C2-H bond of imidazolium salts under formation of the thermally stable bis (31) and triscarbene hydrido complexes 32 (Fig. 11). The superb donor properties of the NHC ligands in the precursor complexes most likely support the oxidative addition of the imidazolium salt. 

Bielawski et al. have developed Janus-head dicarbene ligands which are able to act as a bridge between two metal centers, thereby leading to dinuclear complexes of type 96 [58-60] (Fig. 32). More recently homonuclear bimetallic ruthenium(II) and iron(II) complexes 97 have been synthesized. It was hoped that the dicarbene ligand would interconnect the redox-active metal centers, but the... 

---