Carbenes platinum-carbene complexe

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 ]. 

Carbene/diphosphine platinum(ll) complexes 141 or carbene/monophosphine platinum(ll) complex 142 were shown to be efficient for the cycloisomerisation of 1,6-enynes 140 (Scheme 5.37) [42]. The product 143 is obtained in higher yields and higher enantioselectivity when catalyst 142 is employed rather than catalyst 141. 

Section III.C A Hydrido(methyl)carbene Complex of Platinum(IV) (223) and Methyl(hydrido)platinum(IV) Complexes with Flexible Tridentate Nitrogen-Donor Ligands (224) are structurally related to the system shown in Scheme 13 and give additional information on how steric and electronic factors influence the stability of platinum(IV) methyl hydrides. 

Aqueous two-phase hydrogenations are dominated by platinum group metal catalysts containing water-soluble tertiary phosphine ligands. The extremely stable and versatile N-heterocyclic carbene complexes attracted only limited interest, despite the fact that such complexes were described in the literature [62-65]. Recently, it was reported that the water-soluble [RuXY(l-butyl-3-methylimi-dazol-2-ylidene) ( 76-p-cymene)]n+ (X=Ch, H20 Y = C1-, H20, pta) complexes preferentially hydrogenated cinnamaldehyde and benzylideneacetone at the C = C double bond (Scheme 38.5) with TOF values of 30 to 60 h 1 in water substrate biphasic mixtures (80 °C, lObar H2) [66]. 

Metallic groups as in case (c) lead to electrophilic or even carbocation-like carbene complexes. Typical examples are Fischer-type carbene complexes [e.g. (CO)5Cr=C(Ph)OMe] and the highly reactive carbene complexes resulting from the reaction of rhodium(II) and palladium(II) carboxylates with diazoalkanes. Also platinum ylides [1,2], resulting from the reaction of diazoalkanes with platinum(Il) complexes, have a strong Pt-C o bond but only a weak Pt-C 7t bond. In situation (d) the interaction between the metal and the carbene is very weak, and highly reactive complexes showing carbene-like behavior result. Similar to uncomplexed carbenes. 

The carbene complexes can also be formed by direct oxidative addition of ze-rovalent metal to an ionic liquid. The oxidative addition of a C-H bond has been demonstrated by heating [MMIM]BF4 with Pt(PPh3)4 in THF, resulting in the formation of a stable cationic platinum carbene complex (Scheme 15) (189). An effective method to protect this carbene-metal-alkyl complex from reductive elimination is to perform the reaction with an imidazolium salt as a solvent. 

When coordinated to metal ions in their normal or higher oxidation states, an isocyanide is rendered susceptible to attack at the ligating carbon atom by nucleophilic reagents.1 When alcohols or amines are the nucleophiles, carbene complexes that may be prepared for a variety of metals and substituent groups are obtained. The first fully characterized compounds were of platinum(H),2 and general methods of their preparation, with particular examples, are given below. 

Even multiple exchange in methane could be explained this way. For methane, a carbene complex would be postulated, and such complexes are now known (87). One objection to the involvement of platinum-alkene complexes is that they are very stable and once formed might not be expected to react further, as was found when exchange in chloropro-pane was attempted (Section III,C) (55). Also the compound formed is a platinum(O) complex. A mathematical analysis of multiple exchange similar to that developed by Anderson and Kemball (/) has been carried out (82). 

The most common reaction of platinum(II) isocyanides is their reaction with nucleophiles. This reaction is a useful one for the formation of platinum carbene complexes, and this will be discussed in the next section. 

The chemistry of carbene complexes is a field which has developed in the past 20 years. The current interest is primarily centered on the chemistry of the early transition elements, but much of the early work was carried out at low valent, metal centers. The major complexes formed by platinum are those with the metal in a divalent oxidation state. 

A logical route to the synthesis of platinum carbene complexes is by using a carbene precursor. This method has been successfully used with electron-rich alkenes. Reacting (30) with ci -PtCl2(PPh3)2 and similar complexes leads to the formation of carbene complexes (equation 125).378-380 Heterocyclic chloro compounds have also been used to prepare carbene complexes (equation 126).381... 

An impressive series of carbene- and carbyne-bridged complexes of platinum have been formed by reaction of metal carbene complexes with a platinum complex (equation 141). These complexes have been verified by X-ray crystallography, and the synthetic method appears to be one of some generality. Among the complexes of this type formed from carbenes are ones having metal frameworks with Pt—W,409-413 Pt—Mn414-418 and Pt—Cr413,417 bonds. 

The formation of carbene complexes by protonation of platinum(II) acetylides has been discussed. Both acetylide groups in frflns-Pt(C=CR)2(PMe2Ph)2 (R = H, Me) can be cleaved by HC1 (equation 222).616... 

Alkynes coordinated to platinum(O) are susceptible to electrophilic attack. The reaction which has been most fully studied is the protonation of complexes Pt(alkyne)(PPh3)2 to give vinyl platinum(Il) complexes then alkenes. The reaction has been discussed in Section 52. The vinyl complexes formed undergo isomerization in the final step, since the cis vinyl complex yields some tracts-alkene. Carbene intermediates have been proposed in the pathway for this isomerization.848 Platinum(II) alkyne complexes can be converted into carbene complexes, and this reaction has been discussed in Section 52.4,6. This pattern of differential reactivity is apparent in the IR spectra of the two sets of complexes. For alkyne complexes of platinum(O) the C==C stretching frequency is lowered by some 450 cm-1 upon coordination, but with the platinum(II) analogs the difference is only in the region of 200 cm-1. 

The conventional vinylidene complex could be isolated when the hydroxyl group was protected as the tetrahydropyranyl ether derivative reaction of this with acid immediately gave the cyclic carbene complex, even under mild conditions. The reaction is related to the formation of similar nickel(II)- and platinum(IV)-carbene complexes from the [Pg.71]

The reaction with 4-pentyn-l-ol gave only [Fe t/2-CH2=C(CH2)30) (CO)2(t/-C5H5)]+, and 3-hexyn-l-ol afforded (64, R = Et) (84) no evidence for the participation of the vinylidene tautomers was found. With ruthenium (45) and platinum (47) complexes, on the other hand, rearrangement to the vinylidene is faster than internal attack on the >/2-alkyne, and only the cyclic carbene complex is formed. 

Many of the recent efforts in this area of chemistry have been designed to explore the chemistry of the trifluoromethyl species prepared by these reactions. Among the more interesting results have been the construction of carbene and tetramethylcyclobutadiene ligands affixed to trifluoromethyl platinum cations (78), the synthesis of difluoro-carbene complexes like (Cp)Mo(CO)3CF2+ (79), the formation of (CF3)2CS from CF3Re(CO)5 (38), and the insertion reaction of S02 into CF3-metal bonds (80). 

A very interesting synthetic method was published in 1969 by Richards and co-workers (46). They found that, in the reaction of alcohols with certain isocyanide complexes sueh as those of platinum(II) ], an addition of the alkoxy group to the carbon atom as well as of the hydrogen to the nitrogen atom of the isocyanidc ligand occurs, and one thus obtains the corresponding carbene complexes ... 

Reactions of complexes of 1,2-cycloheptadienes have received only cursory attention. 1,2-cycloheptadiene is readily displaced from bisftriphenyl-phosphine)platinum(O)118 [Eq. (54)], but no reagent has been found that will displace the allene from iron.119 Reaction of the iron complex with alcohol in the presence of base (e.g., 312 — 322) is typical of Fp+ complexes of acyclic allenes.131132 The thermal chemistry of 312 is unusual in its decomposition to 324 (Scheme 41). This is probably attributable to the presence of the triflate counterion, since the corresponding fluoroborate salt is stable when warmed to 40°C for 16 h.119 A mechanism to 324 via carbene complex 321 appears likely. 

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