Carbene complexes carbonyl substitution

Another example of a [2s+2sh-1c+1co] cycloaddition reaction was observed by Barluenga et al. in the sequential coupling reaction of a Fischer carbene complex, a ketone enolate and allylmagnesium bromide [120]. This reaction produces cyclopentanol derivatives in a [2S+2SH-1C] cycloaddition process when -substituted lithium enolates are used (see Sect. 3.1). However, the analogous reaction with /J-unsubstituted lithium enolates leads to the diastereoselective synthesis of 1,3,3,5-tetrasubstituted cyclohexane- 1,4-diols. The ring skeleton of these compounds combines the carbene ligand, the enolate framework, two carbons of the allyl unit and a carbonyl ligand. Overall, the process can be considered as a for-... 

Alkenyl Fischer carbene complexes can serve as three-carbon components in the [6 + 3]-reactions of vinylchro-mium carbenes and fulvenes (Equations (23)—(25)), providing rapid access to indanone and indene structures.132 This reaction tolerates substitution of the fulvene, but the carbene complex requires extended conjugation to a carbonyl or aromatic ring. This reaction is proposed to be initiated by 1,2-addition of the electron-rich fulvene to the chromium carbene followed by a 1,2-shift of the chromium with simultaneous ring closure. Reductive elimination of the chromium metal and elimination/isomerization gives the products (Scheme 41). 

Reaction of (butadiene)ZrCp2 (31/32), and substituted Cp variants, with a wide range of metal-carbonyl complexes, generates the chelated metal-carbene complexes 163 (equation 22)163. The crystal structure of a number of these complexes has been determined... 

Because of n-electron donation by the heteroatom, these carbene complexes are generally less electrophilic at C than the corresponding non-heteroatom-substituted complexes (Chapter 3). This effect is even more pronounced in bis-heteroatom-substituted carbenes, which are very weak Tt-acceptors and towards low-valent transition metals show binding properties similar to those of phosphines or pyridine. Alkoxycarbenes, on the other hand, have electronic properties similar to those of carbon monoxide, and stable heteroatom-monosubstituted carbene complexes are also usually formed from metals which form stable carbonyl complexes. 

Table 2.1. Heteroatom-substituted carbene complexes prepared from carbonyl complexes and carbon nucleophiles.

Carbonyl complexes also react with non-carbon nucleophiles. The resulting carbonic acid derivatives can serve as starting material for the preparation of bis-heteroatom-substituted carbene complexes [93]. Heterocyclic carbene complexes can be obtained from nucleophiles with a leaving group in -position (Table 2.2). 

Isonitrile complexes, having a similar electronic structure to carbonyl complexes, can also react with nucleophiles. Amino-substituted carbene complexes can be prepared in this way (Figure 2.6) [109-112]. Complexes of acceptor-substituted isonitriles can undergo 1,3-dipolar cycloaddition reactions with aldehydes, electron-poor olefins [113], isocyanates [114,115], carbon disulfide [115], etc., to yield heterocycloalkylidene complexes (Figure 2.6). 

Donor-substituted alkynes can insert into the C-M double bond of alkoxycarbene complexes, yielding donor-substituted vinylcarbene complexes [191,192]. In addition to this, photolysis or thermolysis of a-alkoxycyclopropyl carbonyl complexes or a-alkoxycyclobutanoyl complexes can lead to rearrangement to metallacyclic carbene complexes (Table 2.11). This methodology has not been used as extensively for the preparation of carbene complexes as the other methods described above. 

The heteroatom-substituted carbene complexes most frequently used in organic synthesis are carbonyl complexes of the type (CO)5M=C(X)R (M Cr, Mo, W X OR, NR2 R H, alkyl, aryl, vinyl, alkynyl, etc.). To some extent such complexes behave as carboxylic esters or amides, the (CO)5M=C group having electronic properties similar to those of a carbonyl group (Figure 2.16). 

Treatment of Fischer-type carbene complexes with different oxidants can lead to the formation of carbonyl compounds [150,253]. Treatment with sulfur leads to the formation of complexed thiocarbonyl compounds [141]. Conversion of the carbene carbon atom into a methylene or acetal group can be achieved by treatment with reducing agents. Treatment of vinylcarbene complexes with diborane can also lead to demetallation and formation of diols [278]. The conversion of heteroatom-substituted carbene complexes to non-heteroatom-substituted carbene complexes... 

Transition metal complexes which react with diazoalkanes to yield carbene complexes can be catalysts for diazodecomposition (see Section 4.1). In addition to the requirements mentioned above (free coordination site, electrophi-licity), transition metal complexes can catalyze the decomposition of diazoalkanes if the corresponding carbene complexes are capable of transferring the carbene fragment to a substrate with simultaneous regeneration of the original complex. Metal carbonyls of chromium, iron, cobalt, nickel, molybdenum, and tungsten all catalyze the decomposition of diazomethane [493]. Other related catalysts are (CO)5W=C(OMe)Ph [509], [Cp(CO)2Fe(THF)][BF4] [510,511], and (CO)5Cr(COD) [52,512]. These compounds are sufficiently electrophilic to catalyze the decomposition of weakly nucleophilic, acceptor-substituted diazoalkanes. 

When acceptor-substituted carbene complexes are generated in the presence of carbonyl compounds, carbonyl ylides can be formed. These intermediates can undergo a wide variety of further transformations [38,1079] (Figure 4.13). 

Treatment of aldehydes or ketones with acceptor-substituted carbene complexes leads to formation of enol ethers [1271-1274], oxiranes [1048], or 1,3-dioxolanes [989,1275] by O-alkylation of the carbonyl compound. Carboxylic acid derivatives... 

Fig. 4.16. Atypical reactions of carbonyl ylides generated from carbonyl compounds and acceptor-substituted carbene complexes [1276,1306],...

Interestingly, sulfonium ylides generated from electrophilic carbene complexes and sulfides can react with carbonyl compounds, imines, or acceptor-substituted alkenes to yield oxiranes [1320-1325], aziridines [1321,1326,1327] or cyclopropanes [1328,1329], respectively. In all these transformations the thioether used to form the sulfonium ylide is regenerated and so, catalytic amounts of thioether can be sufficient for complete conversion of a given carbene precursor into the... 

A plausible intermediate of this olefination is the titanium-methylene sjtecies 4, which is formed from 1 by removal of AlMe2Cl with a Lewis base, from 2 by fragmentation with elimination of isobutene, and from 3 by a-elimination and release of methane. However, none of these three routes to titanium-carbene complexes of type 4 proved to be generally applicable. Consequently, the use of these reagents in synthesis is essentially limited to the transfer of a methylene unit 18]. From a synthetic viewpoint, a general and easy route to substituted titanium-alkylidene species and their use in carbonyl olefinations would be more desirable. 

Alkylidenes have been prepared by reduction of alkyli-dynes, by C H oxidative addition from alkyls, and by treatment of unsaturated metal clusters with diazoalkanes. In most instances, the alkylidene adopts a /r2-h coordination mode. However, alkylidenes with heteroatom substituents may also be found in terminal coordination modes. The latter are typically prepared by the Fischer-type carbene route (see Fischer-type Carbene Complexes) (sequential addition of nucleophilic and electrophilic alkylating agents to carbonyl or isocyanide ligands), by condensation of metal fragments with mono- or dimetallic carbene complexes, or by C-H activation of alkylamines. These heteroatom substituted carbenes may also bind in a p3-ri mode, as in (12). 

By reaction of cationic carbonyl complexes with lithium carbanions, neutral acyl complexes are prepared. Whereas treatment of [> -CpFe(CO)3]BF4 with (a) PhLi gives the expected > -CpFe(CO)2 [C(0)Ph] in 80% yield, with (b) MeLi only traces of > -CpFe(CO)2 [C(0)Me] can be detected . This complex and other phosphane-substituted acyl compounds of the type f -CpM(CO)L[C(0)Me] [M = Fe, Ru L = CO, PPh3, P(hex)j], as well as >/ -CpMo(CO)2P(hex)3[C(0)Me] (prepared by different routes), are protonated with and alkylated with [R3 0]BF4 reversibly, yielding cationic hydroxy- and alkoxy(methyl)carbene complexes, respectively . The formation of the ( + )- and ( —)-acetyl complex / -CpFe(C0)(PPh3)[C(0)Me] from the ( + )-and ( —)-conformers of optically active > -CpFe(C0XPPh3)[C(0)0-menthyl] and MeLi occurs with inversion of configuration at the asymmetric iron atom . 

Deviating from the route via nucleophilic attack of the carbanion at the carbon atom of a CO ligand and then reaction of the acylmetallate with an electrophile are those methods which involve (a) addition of the carbanion to the carbon atom of a carbyne ligand, (b) displacement of halides from transition-metal carbonyl halides by cyclohepta-trienyllithium, or derivatives thereof, followed by hydride abstraction or (c) substitution of a coordinated solvent from a metal-carbonyl complex (see also reaction of LiR with carbene complexes). 

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