From Acyl Complexes

Other methods for preparing acyl complexes include the acylation of metallates and the treatment of alkyl carbonyl complexes with nucleophiles. 

An early approach to vinylidenes was by the formal dehydration of metal acyls, which is best achieved by treatment with an electrophile, often the proton in the form of a non- or weakly-coordinating strong acid. The reaction appears to proceed stepwise via a hydroxycarbene formed by protonation of the acyl, subsequent dehydration of which affords the vinylidene. Occasionally, mixtures of the two complexes are obtained, again suggesting the intermediacy of the carbene. 

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From Acyl Complexes Generated from Carbonyl Complexes... 

From Acyl Complexes Generated by Other Methods... 

In general, the iminoacyl complexes are thermodynamically more stable than the acyl complexes, so that deinsertion of isocyanides from iminoacyl complexes has not been observed in contrast to the facile a-elimination of CO from acyl complexes. The removal of a halide ligand from a neutral iminoacylpalladium complex by treatment with AgBp4 affords the corresponding cationic iminoacyl complex from which no deinsertion of the isocyanide proceeded (Eq. 7.12) [61b]. 

Elimination reactions can proceed as the direct reverse of insertion reactions. Thus the elimination of CO from acyl complexes (Eq. 2-66) and of CO2 from car-boxylates (Eq. 2-67) can result in the formation of metal-aryl bonds. Such eliminations occur under the influence of heat and light. 

Reductive Eliminations to Form C-X Bonds from Acyl Complexes... 

Reductive elimination to form carbon-heteroatom bonds from acyl complexes occurs more readily than it does from alkyl or aryl complexes. This fast rate is consistent with the trends noted previously—namely, reductive elimination is faster from complexes containing groups that are sp hybridized, that are electrophilic, and that can coordinate to the metal center during tiie reductive elimination. Reductive elimination of acid halides has been proposed as a step in the Monsanto acetic acid process, and experimental evidence has been obtained for this type of reductive elinunation from acetylrhodium halide complexes generated in solution (Equation 8.66). - A closely related incorporation of KZO into add halides catalyzed by Wilkinson s catalyst provides further evidence for reversible addition and elimination of add halides involving Rh(I) and Rh(III), respectively. ... 

The acylpalladium complex formed from acyl halides undergoes intramolecular alkene insertion. 2,5-Hexadienoyl chloride (894) is converted into phenol in its attempted Rosenmund reduction[759]. The reaction is explained by the oxidative addition, intramolecular alkene insertion to generate 895, and / -elimination. Chloroformate will be a useful compound for the preparation of a, /3-unsaturated esters if its oxidative addition and alkene insertion are possible. An intramolecular version is known, namely homoallylic chloroformates are converted into a-methylene-7-butyrolactones in moderate yields[760]. As another example, the homoallylic chloroformamide 896 is converted into the q-methylene- -butyrolactams 897 and 898[761]. An intermolecular version of alkene insertion into acyl chlorides is known only with bridgehead acid chlorides. Adamantanecarbonyl chloride (899) reacts with acrylonitrile to give the unsaturated ketone 900[762],... 

The reaction is initiated by formation of a donor-acceptor complex 4 from acyl chloride 2, which is thereby activated, and the Lewis acid, e.g. aluminum trichloride. Complex 4 can dissociate into the acylium ion 5 and the aluminum tetrachloride anion 4 as well as 5 can act as an electrophile in a reaction with the aromatic substrate ... 

The a-alkoxy-substituted iron-acyl complex 8 is prepared by oxidation of the enolate prepared from iron-acetyl complex 6 and subsequent etherification12. 

Hydroxy-substituted iron-acyl complexes 1, which are derived from aldol reactions of iron-acyl enolates with carbonyl compounds, are readily converted to the corresponding /i-methoxy or /1-acetoxy complexes 2 on deprotonation and reaction of the resulting alkoxide with iodomethane or acetic anhydride (Tabic 1). Further exposure of these materials to base promotes elimination of methoxide or acetate to provide the a,/ -unsaturated complexes (E)-3 and (Z)-3 (Table 2). 

Table 2. x./TUnsaturated Iron-Acyl Complexes 3 from 2 via Elimination Reactions CFe R1 base/THF [FeL R1 [Fe] ... 

Aldol reaction of the a-trimethylsilylated enolate 9 with aldehydes provides nearly equal amounts of chromatographically separable ( )- and (Z)-isomers of iron-acyl complexes 11 via silyloxide elimination from the intermedate aldolate 10 (Table 3). This methodology has been the most commonly employed entry to the (Z)-isomer series. 

Table 3. Iron-Acyl Complexes 11 from CarbonyUr/ -acyclopentadienylHl-oxo -ftriinethylsilyllethyl] (triphenylphosphane)iron (8) by the Aldol Reaction with Aldehydes...

The lithium enolate 2a (M = Li ) prepared from the iron propanoyl complex 1 reacts with symmetrical ketones to produce the diastercomers 3 and 4 with moderate selectivity for diastereomer 3. The yields of the aldol adducts are poor deprotonation of the substrate ketone is reported to be the dominant reaction pathway45. However, transmetalation of the lithium enolate 2a by treatment with one equivalent of copper cyanide at —40 C generates the copper enolate 2b (M = Cu ) which reacts with symmetrical ketones at — 78 °C to selectively produce diastereomer 3 in good yield. Diastereomeric ratios in excess of 92 8 are reported with efficient stereoselection requiring the addition of exactly one equivalent of copper cyanide at the transmetalation step45. Small amounts of triphcnylphosphane, a common trace impurity remaining from the preparation of these iron-acyl complexes, appear to suppress formation of the copper enolate. Thus, the starting iron complex must be carefully purified. 

Table 5. Carboxylic Acids 5 by Removal of the Chiral Auxiliary from Iron-Acyl Complexes 4...

Treatment of a-alkoxy-substituted iron acyl complexes 20 with bromine in the presence of an alcohol produces free acetals 22 with loss of stereochemistry at the center derived from the a-carbon of the starting complexl2,49. Electron donation from the alkoxy group allows formation of the oxonium intermediate 21, which is captured by the alcohol to generate the product acetal. 

The insertion of a carbonyl group into a metal-alkyl or metal-aryl bond, and the reverse reaction involving decarbonylation of an acyl complex, have been studied from both the synthetic and mechanistic points of view. The mechanism proposed for this type of reaction seems well established and is... 

Reductive Elimination from Acyl-Metal-NHC Complexes... 

In studies involving the reaction of a cA-[PdCl(Me)(NHC)]j chloro-bridged dimer with CO it was demonstrated that reductive elimination is also extremely facile for NHC-Pd-acyl complexes, yielding 2-acylimidazolium salts and Pd(0) (Scheme 13.4) [22]. The product distribution was shown to depend on the stracture of the complexes from which reductive coupling took place (pathways A and B, Scheme 13.4). 

Scheme 13.4 Reductive elimination from acyl-Pd-NHC complexes...

Starting from 63, the carbonylation may proceed via coordination and insertion of CO into the vinyl-C-Pd bond to provide an a,P-unsaturated acyl complex. This complex reacts with (ArY) 2, and subsequently the C-Y bond is formed by reductive elimination to give 64 (Scheme 7-14). Because the compound 64 could be directly converted into the corresponding enal 65 by the Pd-catalyzed reduction with BujSnH, this sequence is synthetically equivalent to the regio- and stereoselective thioformy-lation and selenoformylation of alkynes (Eq. 7.49) [53, 54]. 

Nitronate anions react with (jl-allyl)cobalt complexes prepared from acylation of 1,3-dienes by acetylcobalt tetracarbonyl to produce nitro enones (Eq. 5.50).74... 

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