Carbonyls cationic

Lupinetti, A.J., Jonas, V., Thiel, W., Strauss, S.H. and Frenking, G. (1999) Trends in Molecular Geometries and Bond Strengths of the Homoleptic d Metal Carbonyl Cations [M(CO)J ... 

Reactions of the tetrahedral Ni° complex (994) have already been discussed in Section 6.3.5.4.2397 A nickel carbonyl cation (1062) containing a cyclophosphenium ligand has been assembled through a hydride abstraction reaction according to Equation (37).2552... 

Because nitration of alkenes with nitronium salts proceeds via carbocation intermediates, nitration of electron-deficient alkenes with nitronium salts is rare. Only a few cases are reported. The reaction of a,(3-unsaturated esters with nitronium salts affords products via highly reactive ot-carbonyl cations.75... 

Keywords copper, FER, dual sites, carbonyl, cation site... 

The success of this transformation depends upon the oxidation potential of the ESE group (Eox 1.5 V), which is lower than that of the alkyl silyl ether group (Eax 2.5 V). Recently, Schmittel et al.35 showed (by product studies) that the enol derivatives of sterically hindered ketones (e.g., 2,2-dimesityl-1-phenyletha-none) can indeed be readily oxidized to the corresponding cation radicals, radicals and a-carbonyl cations either chemically with standard one-electron oxidants (such as tris(/>-bromophenyl)aminium hexachloroantimonate or ceric ammonium nitrate) or electrochemically (equation 10). 

These observations illustrate that there are two transformations open to metallocarboxylic acid intermediates reversible loss of OH" accompanied by oxygen exchange, and metal-hydride formation with expulsion of C02. Our entry into this area of chemistry was in 1975 when extensive studies of oxygen lability in metal carbonyl cations were initiated (10). These... 

Figure 2. Synthesis of starting metal carbonyl cations...

Two possible mechanisms are proposed. Primarily the enol radical cation is formed. It either undergoes deprotonation because of its intrinsic acidity, producing an a-carbonyl radical, which is oxidized in a further one-electron oxidation step to an a-carbonyl cation. Cyclization leads to an intermediate cyclo-hexadienyl cation. On the other hand, cyclization of the enol radical cation can be faster than deprotonation, producing a distonic radical cation, which, after proton loss and second one-electron oxidation, leads to the same cyclo-hexadienyl cation intermediate as in the first reaction pathway. After a 1,2-methyl shift and further deprotonation, the benzofuran is obtained. Since the oxidation potentials of the enols are about 0.3-0.5 V higher than those of the corresponding a-carbonyl radicals, the author prefers the first reaction pathway via a-carbonyl cations [112]. Under the same reaction conditions, the oxidation of 2-mesityl-2-phenylethenol 74 does not lead to benzofuran but to oxazole 75 in yields of up to 85 %. The oxazole 75 is generated by nucleophilic attack of acetonitrile on the a-carbonyl cation or the proceeding enol radical cation. 

The reverse sequence is also possible as was demonstrated by the attack of a vic-bisamine on a tungsten carbonyl cation [(PhCsCPh)3W(CO)]. ... 

Alkenes and Alkynes. A series of metal carbonyl cations, such as [Au(CO)n]+,287,288 [c-Pd( i-CO)2]2+,289 [Rh(CO)4]+,290 and [(Pt(CO)3)2]2+,291 was found to induce the formation of carboxylic acids from alkenes and CO in the presence of H2SO4 under mild conditions. A novel water-soluble Pd catalyst292 and Pd complexes of calixarene-based phosphine ligands293 showed high activity in the regioselective carboxylation of vinyl arenes to yield 2-arylpropionic acids or... 

With the exception of 46 and 47 [Eqs. (12) and (13), the other neutral formyls in Table II have been prepared by hydride attack on metal carbonyl cation precursors. The most extensively studied of these have been (i7-C5H5)Re(NO)(CO)(CHO) (38) and (T7-C5H5)Re(NO)(PPh3)(CHO) (39). The former was reported approximately simultaneously by Casey (56), Gladysz (57), and Graham (5Conditions employed for its preparation are summarized in Eq. (10). Formyl 38 can be obtained as a solid, but the... 

As shown in Eqs. (17) and (18), the isolated formyls 19 and 24 are capable of reducing aldehydes and ketones (37, 38, 42. 47, 66). Thus there is no doubt that hydride transfer is an intrinsic chemical property of anionic formyl complexes. One reaction of a neutral formyl complex with an aldehyde has been reported addition of benzaldehyde to (i7-C5H5)Re(NO)(CO)(CHO) (38) yields the alkoxycarbonyl complex (i7-C5H5)Re(NO)(CO)(C02CH2C6Hs) (62). This transformation, which appears to require catalysis by adventitious acid, can be viewed as occurring via attack of initially formed benzyl alcohol upon the intermediate carbonyl cation [(i -C5H5)Re(NO)(CO)2]+. 

Formyl transfers" involving neutral formyl complexes, such as shown in Eq. (22), have recently been reported (68). By precipitation of the metal carbonyl cation by-product, quite pure solutions of kinetically labile neutral formyl complexes may be obtained. 

Observations pertinent to Scheme 2 are as follows (61, 64-66) as followed by H NMR, CH3S03F reacts slowly with 39 at - 70°C but rapidly at -40°C. However, under no conditions (including inverse addition, use of other methylating agents, etc.) can the formation of 54 from 39 be directly observed. Rather, H NMR indicates that substantial quantities of methoxymethyl complex 57 build up. The carbonyl cation product 56 is also present. These data are readily explained if, subsequent to initial O-methylation (Scheme 2, step a), rapid hydride transfer from unreacted 39 to 54 occurs (step b). As shown in Eq. (25), this reaction (conducted with independently synthesized samples) is indeed rapid at -70°C. 

Since nitro groups are readily converted to carbonyls by various procedures,4142 nitroalkenes can serve as a-carbonyl cation equivalents in Michael reactions.43 For example, addition of the amide enolate (158 Scheme 22) to the nitroalkene (157) affords, in 60% yield, the Michael adduct (159), which can be con-... 

In spite of the high reactivity of the organolithiums, other compatible acceptors have also shown synthetic utility. For example, select organolithium reagents add exclusively to 2-(A/-methylanilino)acryloni-trile (59), an a-carbonyl cation equivalent, to afford an intermediate a-aminonitrile anion (Section... 

One of the most interesting reactions of cationic vinylidene complexes is that with dioxygen, which affords the carbonyl cation and the corresponding aldehyde (41) ... 

Metal Atom Synthesis of Organometallic Compounds, IS, 53 Metal Carbonyl Cations, 8, 117... 

Reaction intermediates formed in the nitration of a series of a,/i-unsaturatcd esters, such as (47), with N02BF4 have been reported to exhibit the expected behaviour of a-carbonyl cations. Three diagnostic reaction types were observed (1) Ritter reaction (2) cyclopropane formation from propyl cations (3) Wagner-Meerwein migration of alkyl groups. Semi-empirical calculations of the relative gas-phase stabilities of the proposed intermediate cations have also been performed.53... 

CATIONS OF GROUP 6-12 ELEMENTS 4.5.1. Homoleptic Metal Carbonyl Cations... 

The first homoleptic carbonyl cations of Group 7 metals with the general formula [M (CO)6]+ (M = Mn, Tc, Re) were generated by Fischer and co-workers992 993 and Hieber and co-workers994 995 in the 1960s"6,997 applying halide abstraction by Lewis acids [Eq. (4.236)]. 

Versatile synthetic routes are available to prepare metal carbonyl cations in superacidic media. The most successful methods include reductive carbonylation of metal fluorosulfates or fluorides [Eqs. (4.237)1003 and (4.238)1004]-1002 1005 This... 

Figure 4.4. Metals with known homoleptic carbonyl cations (shading indicates structural characterization).

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