Carbenes cobalt-complexes

Scheme 4 Synthesis and oxygenation of Tris-carbene cobalt complex...

The preparation of cyclopropanes by intermolecular cyclopropanation with acceptor-substituted carbene complexes is one of the most important C-C-bond-forming reactions. Several reviews [995,1072-1074,1076,1077,1081] and monographs have appeared. In recent decades chemists have focused on stereoselective intermolecular cyclopropanations, and several useful catalyst have been developed for this purpose. Complexes which catalyze intermolecular cyclopropanations with high enantiose-lectivity include copper complexes [1025,1026,1028,1029,1031,1373,1398-1400], cobalt complexes [1033-1035], ruthenium porphyrin complexes [1041,1042,1230], C2-symmetric ruthenium complexes [948,1044,1045], and different types of rhodium complexes [955,998,999,1002-1004,1010,1062,1353,1401-1405], Particularly efficient catalysts for intermolecular cyclopropanation are C2-symmetric cop-per(I) complexes, as those shown in Figure 4.20. These complexes enable the formation of enantiomerically enriched cyclopropanes with enantiomeric excesses greater than 99%. Illustrative examples of intermolecular cyclopropanations are listed in Table 4.24. 

A peculiar reaction occurs when carbene anions are trapped with chlorosilanes the tetracarbonyl cobalt complex is recovered in high yield ... 

Pauson-Khand cyclization3k 143 of tV-allyl (l-alkynyl)carbene complexes 134 (M = Cr, W R = Ph, Et R1 = H, Me) affords cyclopentenone derivatives 136144 via cobalt complexes 135145 (Scheme 53), as well as chromium complexes.146 Cyclopentenones also have been derived from 7V-diallyl(l-alkynyl)carbene complexes.39 Stable cobalt complexes of type 135 are obtained from O-allyl (l-alkynyl)carbene complexes. Interestingly, the last-named compounds do not form a cyclopentenone on heating instead, they form an enyne by elimination of M(CO)6 in a retro-Fischer reaction. 147... 

Ethoxy(l-alkynyl)carbene complex la forms a stable cobalt complex 182, whose structure has been elucidated by X-ray analysis.1453 11 Cobalt complexes of similar type have been derived from 0-allyl-147 and iV-allyl(l-alkynyl)carbene complexes 135 (Scheme 53).145 In contrast to thermolysis of compounds 135 (Scheme 53), thermolysis of compound 182 affords isomers 183 (structure based on X-ray analysis) and 184 (structure based on spectroscopic evidence only) in 48% total yield (Scheme 76). 

Ethoxy(l-alkynyl)carbene complex, cobalt complex, 227-228... 

The chromium carbonyl linkers 1.40 (98) and 1.41 (99) were prepared from commercial triphenylphospine resin and respectively from pre-formed p-arene chromium carbenes and Fischer chromium amino carbenes. Their SP elaboration is followed by cleavage with pyridine at reflux for 2 h (1.40) and with iodine in DCM for 1 h at rt (1.41) both linkers produce the desired compounds in good yields. A similar cobalt carbonyl linker 1.42 (100) was prepared as a mixmre of mono- (1.42a) and bis- (1.42b) phosphine complex, either from pre-formed alkyne complexes on triphenylphosphine resin or by direct alkyne loading on the bisphosphine cobalt complex traceless cleavage was obtained after SP transformations by aerial oxidation (DCM, O2, hp, 72 h, rt) and modified alkynes were released with good yields and... 

Palladium(ii) and rhodium(ii) acetates were introduced by Teyssie s group (Pd Paulissen et al., 1972 Rh Paulissen et al., 1973). They differ from one another in their ability to coordinate with alkenes and have, therefore, a different regio- and substrate specificity (Anciaux et al., 1980). Cobalt complexes are first of all interesting because of their effect on enantioselectivity. We will discuss them in Section 8.8. Here, we emphasize only that enantioselectivity provides the most convincing evidence for the involvement of metal-carbene intermediates in cyclopropana-tions. 

The reactivity of carbene-metal complexes, amongst others the reactivity with respect to alkenes and alkynes, has been reviewed by Dotz Just like free carbenes the coordinated carbenes add to triple bonds to give cyclopropene derivatives. Other reaction products, however, are also possible. For instance, the carbene ligand of chromium complex 23 reacts with diphenylacetylene to a mixture of products, including naphthalene derivative 24 and furan derivative 25 (equation 18). A carbonyl ligand has participated. Molecular orbital calculations by Hofmann and Hammerle " on this system reveal that the reaction would pass through an y-vinylcarbene type of complex (26) instead of through a planar chromacyclobutene 27. The subsequent steps to yield either phenol or furan could involve vinylketene 28, but this still is a matter of debate. Similar, but more selective, furan syntheses have been observed for carbene complexes based on iron and cobalt. ... 

In contrast, the cyclotrimerization catalyzed by the Grubbs catalyst involves consecutive intramolecular ring metathesis through carbene complexes or formation of a ruthenabicy-clo[3.2.0]heptatriene intermediate similar to that formed in the cobalt complex-catalyzed reaction. This mechanism is illustrated in Scheme 2 using acetylene as the alkyne source and CpRuCl[COD] as catalyst. 

The heptanuclear iron carbonyl cluster [Fe3(CO)u(/u-H)]2-Fe(DMF)4 (178) acted as an efficient catalyst in the reduction of carboxamides by l,2-bis(dimethylsilyl)benzene in toluene to the corresponding amines in high yields. Several tertiary and secondary amides including a sterically crowded amide were also reduced smoothly A review of the development of optically active cobalt complex catalysts for enan-tioselective synthetic reactions has addressed the applications of ketoiminatocobalt(II) complexes such as (5)-MPAC (179) and (5)-AMAC (180), transition-state models for borohydride reduction, halogen-free reduction by cobalt-carbene complexes. 

Deng and coworkers recently developed the first silyl-donor-functionalized Af-heterocyclic carbene (NHC) cobalt complexes for use as catalysts in hydrosilylation reactions. Of the catalysts screened, 144 proved superior in rapidly producing the desired silylated products 145 and 146 in reactions between 1-octene 143 and PhSiHj while minimizing isomerization side products 147 and 148. The... 

Some of the vinylidene complexes include cobalt, rhodium and rhenium in halfsandwich complexes, which are synthesized from acetylene complexes". This reaction involves an intermediate alkinyl(hydrido) complex, which can sometimes be isolated. The bonding between the metal and the a-carbon atom in vinylidene rhodium complexes is shorter than in carbene rhodium complexes, which indicates a high electron density on the center atom. 

Cobalt complexes are also prone to carbene displacement. In an attempted synthesis of [(IPr)Co(Cp)Me2] from [(Ph3P)Co(Cp)Me2], Baird and co-workers found that an equilibrium was set up between free IPr and the phosphine complex." Isolated [(IPr)Co(Cp)Me2] reacted with PMe3 resulting in complete displacement of the carbene ligand. The authors attributed this instability to steric crowding. 

The N-heterocyclic carbene complexes of cobalt have been less studied than their heavier analogues, rhodium and iridium, even though NHC-cobalt complexes have been known for more than 3 decades. Despite the fact that their... 

The cobalt complex [ n4-l,2 3,4-(rrans-l,3,5-hexatriene) ( n5-cyclopentadienyl)cobalt] was found to undergo dimerisation to form a flyover carbene with concomitant elimination of one equivalent of /ra j-l,3,5-hexatiiene333. 

Reaction of [S(CCPh)2] with [Co2(CO)g] afforded 2 jhe crystallographically characterised adduct [(PhCC)S Co2(CO)6(Ti2-CCPh) ] which, on treatment with further [Co2(CO)g] afforded [S Co2(CO)6(Tl2-CCPh) 2l. The reaction of pentacarbonyl allyloxy((4-methylphenyl)ethynyl)carbene) chromium and tungsten complexes with dicobalt octacarbonyl afforded 63 tpe alkyne complexes (62) (M = Cr, W), which failed to undergo intramolecular Pauson-Khand reactions instead, at room temperature, a 1,2-elimination occurs, regenerating the metal hexacarbonyl with predominant formation of dinuclear cobalt complexes (63) - (65). 

While major advances in the area of C-H functionalization have been made with catalysts based on rare and expensive transition metals such as rhodium, palladium, ruthenium, and iridium [7], increasing interest in the sustainability aspect of catalysis has stimulated researchers toward the development of alternative catalysts based on naturally abundant first-row transition metals including cobalt [8]. As such, a growing number of cobalt-catalyzed C-H functionalization reactions, including those for heterocycle synthesis, have been reported over the last several years to date (early 2015) [9]. The purpose of this chapter is to provide an overview of such recent advancements with classification according to the nature of the catalytically active cobalt species involved in the C-H activation event. Besides inner-sphere C-H activation reactions catalyzed by low-valent and high-valent cobalt complexes, nitrene and carbene C-H insertion reactions promoted by cobalt(II)-porphyrin metalloradical catalysts are also discussed. 

---