Carbene complexes

TABLE 13.7 Fischer- and Schrock-Type Carbene Complexes 

Characteristic Fischer-Type Carbene Complex Schrock Type Carbene Complex 

Typical metal Middle to late transition metal Early transition metal 

Substituents attached to Ccarbene At least one highly electronegative heteroatom (such as O, N, or S) H or aUcyl 

Typical other ligands in complex Cjood 77 acceptors Good cr or 77 donors 

Alkylation by a source of CH3 such as [(CH3)30][BF4] or CH3I gives the methoxy-carbene complex  

Evidence for double bonding between chromium and carbon is provided by X-ray crystallography, which measures this distance at 204 pm, compared with a typical Cr—C single bond distance of approximately 220 pm. 

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The (l-ethynyl)-2-propenyl acetate derivative 111 undergoes an interesting PdCl2(PhCN)2-catalyzed cyclization to form the 2-cyclopentenone 112[47], A Pd-carbene complex is assumed to be an intermediate of the formation of 112. 

Organometallic Compounds. The predominant oxidation states of indium in organometalUcs are +1 and +3. Iridium forms mononuclear and polynuclear carbonyl complexes including [IrCl(P(C3H3)3)2(CO)2] [14871-41-1], [Ir2014(00)2] [12703-90-1], [Ir4(CO)22] [18827-81 -1], and the conducting, polymeric [IrCl(CO)3] [32594-40-4]. Isonitnle and carbene complexes are also known. 

The olefins that undergo metathesis include most simple and substituted olefins cycHc olefins give linear high molecular-weight polymers. The mechanism of the reaction is beheved to involve formation of carbene complexes that react via cycHc intermediates, ie, metaHacycles. Industrial olefin metathesis processes are carried out with soHd catalysts (30). 

Many other organometaUic compounds also react with carbonyl groups. Lithium alkyls and aryls add to the ester carbonyl group to give either an alcohol or an olefin. Lithium dimethyl cuprate has been used to prepare ketones from esters (41). Tebbe s reagent, Cp2TiCH2AlCl(CH2)2, where Cp = clyclopentadienyl, and other metal carbene complexes can convert the C=0 of esters to C=CR2 (42,43). 

Highly reactive dipolar bis(tnfluoromethyl)-subsmtuted heterodienes are excellent traps for one-skeleton-atom species , even when these are short lived The hetero-1,3-dienes add electron nch and electron poorcarbenes[/2,, 125,126,127,128 129,130], carbene complexes [131], carbene analogues [SnClj, Sn(C< H, )2, GeCl2, Gey [132, 133], isonitriles [128], etc, to give [4-t-l] cycloadducts (equation 29)... 

Trifluoromethyl-substituted diazonium betaines [176]. Synthetic routes to trifluoromethyl-substituted diazo alkanes, such as 2,2,2-trifluorodiazoethane [ 177, 7 78, 179] and alkyl 3,3,3-trifluoro-2-diazopropionates [24], have been developed Rhodium-catalyzed decomposition of 3,3,3-tnfluoro-2-diazopropionates offers a simple preparative route to highly reactive carbene complexes, which have an enormous synthetic potential [24] [3-1-2] Cycloaddition reactions were observed on reaction with nitnles to give 5-alkoxy-4-tnfluoromethyloxazoles [750] (equation 41)... 

The remaining classes oF monohapto organic ligands listed in Table 19.2 are carbene (=CR2), carbyne (=CR), and carbido (C). Stable carbene complexes were first reported in 1964 by E. O. Fischer and A, Maasbol. Initially they OMe... 

JA5190). Upon deprotonation by bases, 285 (R = H) transforms to 286, and 285 (R = Me) goes to 287 because the C2 position is occupied. Protonation of 286 with triflic acid occurs at position 3 of the heteroring to form the benzothienyl carbene complex 288, and deprotonation reverts it to 286. This kind of process is a rarity for the uncomplexed benzothiophenes (81AHC171). 

Retention of configuration may be used for the synthesis of the intramolecular carbene complexes [87J0M(336)C1]. Methyl- or phenyllithium or di-... 

Annelation of chromium carbene complexes in synthesis of heterocycles 99CSR187. 

Silylketenes in formation of (3-lactones and (3-lactams 98JCS(P1)2105. Syntheses of (3-lactams, (3-lactones, and 1,3- and 1,4-diazetidinediones by pho-tochemically induced cycloaddition reactions of chromium carbene complexes with imines, aldehydes, and azo compounds 97T4105. 

Fischer-type carbene complexes in the synthesis of furan, pyrrole, 5//-furanone, and 5//-pyrrolone derivatives 98YGK413. 

NX-Dimethylpyrazolium iodide with [( -cod)Rh(/A-OEt)]2 gives a carbene complex 94 (L2 = cod), a phenomenon not quite typical for the pyrazole series [97JOM(532)261]. 

Cyclization takes place when tert-butylisocyanide, benzaldehyde, anilinium chloride, and carbonyl(dicyano)cyclopentadienyl ferrate are reacted, the carbene complex 42 being the result (95JOM(491)135). /50-butyraldehyde, tert-butyl isocyanide, ammonium hexafluorophosphate, and [(T -Cp)Fe(CO)(CN)2] give the cationic bis-carbene 43. 

Dimesitylimidazolium chloride in the presence of potassium tert-butylate reacts with [RuCl2(PCy3)2(=CHPh)] or [RuCl2(PCy3)2(=CHCH=CMe2)] to yield the mixed phosphine-carbene complexes [RuCL(L)(PCy3)(=CHPh)] or... 

Dimesitylimidazolium chloride with nickelocene gives the carbene complex [(T) -Cp)NiCl(L)] (L= l,3-dimesitylimidazol-2-ylidene), in which the chloride ligand can be substituted by a methyl group by reacting the product with methyl-lithium (OOJOM(596)3). 

Co-condensation reaction of the vapors of l,3-di-rcrt-butylimidazol-2-ylidene and nickel, palladium, or platinum gives the coordinatively unsaturated 14-electron sandwiches [L M] (M=Ni, Pd, Pt) of the carbene type (990M3228). Palladium(O) carbene complexes can also be prepared by the direct interaction of l,3-R2-imidazol-2-ylidenes (R=/-Pr, r-Bu, Cy, Mes) (L) with the palladium(O) compound [Pd(P(o-Tol)3)2] (OOJOM(595)186), and the product at the first stage is [(L)PdP(o-Tol)3l, and then in excess free carbene [PdL ]. 

Diethylzinc l,3-bis(l-adamantyl)imidazol-2-ylidene adduct is known (93JOM(462)13). Bis(l,3-diphenylimidazol-2-ylidene) mercury(II) carbene complexes follow from the corresponding imidazolium perchlorate and mer-cury(II) chloride (68AGE682). 

Deprotonation of l-methyl-3-ferrocenylimidazolium tetrafluoroborate or iodide (98JOM(552)45) by lithium di-Mo-propylamide and subsequent reaction with W(C0)5-THF gives the carbene complex 107 and bis-carbene 108, even when excess W(CO)j THF is applied (99JOM(572)177). Numerous ferrocenyl benzimidazoles are known (97RCR613, 99JOM(580)26). 

Dimethylbenzimidazolium iodide serves as a starting material for free carbenes (99CEJ1931, 99JA10626, 00AGE541) and carbene complexes (99JOM(585)241). Thus, with palladium(II) acetate it gives [L Pdl ] where L is... 

Free carbenes based on 1,2,4-triazole are not as numerous as those based on imidazole (70ZN(B)1421, 95AGE1021, 97JA6668, 98JA9100). The carbene complex 169 (Ar = Ph, p-Tol) is prepared by the [3 + 2] cycloaddition route from [W(CO)j(C+=NC-HCOOEt)]- and aryldiazonium (930M3241). Oxidative decomplexation causes tautomerization of the 1,2,4-triazole ligand, the products being 170 (Ar= Ph, i-Tol). 

Dimethyl-1,2,4-triazolium iodide with nickel(II) acetate gives the carbene complex l2Ni( 1,4-dimethyl-l,2,4-triazol-5-ylidene)2 (97OM2209). 

The dicationic ditriflate salt of 1,2,4-trimethyltriazolium with silver acetate gives the bis-carbene complex 185 (00JOM(600)l 12). In excess silver acetate, the one-dimensional polymeric species with alternating silver ions and 1,2,4-triazol-3,5-diylidene carbenes result, where both carbon atoms of each heteroring are engaged in coordination. 

Imidazole is characterized mainly by the T) (N) coordination mode, where N is the nitrogen atom of the pyridine type. The rare coordination modes are T) - (jt-) realized in the ruthenium complexes, I-ti (C,N)- in organoruthenium and organoosmium chemistry. Imidazolium salts and stable 1,3-disubsti-tuted imidazol-2-ylidenes give a vast group of mono-, bis-, and tris-carbene complexes characterized by stability and prominent catalytic activity. Benzimidazole follows the same trends. Biimidazoles and bibenzimidazoles are ligands as the neutral molecules, mono- and dianions. A variety of the coordination situations is, therefore, broad, but there are practically no deviations from the expected classical trends for the mono-, di-, and polynuclear A -complexes. 

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