Carbonyl acyl complexes

Displacement of the chelate carbonyl from palladium by ethene has never been observed in model studies, which accounts for the virtual absence of double ethene insertions in actual copolymerisation reactions. Indeed, (5-chelate opening is actually brought about by CO to generate a six-membered metallacycle (y-chelate), while p-chelates of catalytically active systems generally react with CO to yield carbonyl acyl complexes, even at very low temperature. For the systems investigated by Bianchini [5e, f], the activation barriers for the conversion of the P-chelates... 

On the basis of the CO dependence of the transformation of the P-chelates into the carbonyl acyl compounds, it was proposed that the rate-limiting step in the overall conversion of the P-chelates to carbonyl acyl complexes is related to the opening of the metallacycle by CO (steps a and b in Scheme 7.13) rather than to the following migratory insertion of the alkyl carbonyl complex that is independent of the CO pressure (step c). 

Decarbonylation is the dominant reaction of metal-carbonyl-acyl complexes under irradiation ... 

Metal carbonyl acyl complexes have been an important class of organometal-lic compound because of the fundamental need to understand the structure and bonding of metal-carbon linkages and the chemistry of organic functional groups that are attached to metal moieties via carbon-donor atoms. These compounds have played a particulariy important role in our understanding of alkyl-migration reactions and other types of liquid transformations. 

The commercially available (tj5-cyclopentadienyl)iron dicarbonyl dimer 1 is the source of the carbonyl(//5-cyclopentadienyl)iron(L) moiety. Reductive or oxidative cleavage of 1 provides reactive monomeric species that may be converted into iron-acyl complexes as described in the following sections (see also Houben-Weyl, Vol. 13/9a, p208). 

Subsequent carbonylation of the alkyl-iron complexes with carbon monoxide provides the desired chiral iron-acyl complexes, with essentially complete inversion of configuration at... 

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). 

Iron(II) alkyl anions fFe(Por)R (R = Me, t-Bu) do not insert CO directly, but do upon one-electron oxidation to Fe(Por)R to give the acyl species Fe(Por)C(0)R, which can in turn be reduced to the iron(II) acyl Fe(Por)C(0)R]. This process competes with homolysis of Fe(Por)R, and the resulting iron(II) porphyrin is stabilized by formation of the carbonyl complex Fe(Por)(CO). Benzyl and phenyl iron(III) complexes do not insert CO, with the former undergoing decomposition and the latter forming a six-coordinate adduct, [Fe(Por)(Ph)(CO) upon reduction to iron(ll). The failure of Fe(Por)Ph to insert CO was attributed to the stronger Fe—C bond in the aryl complexes. The electrochemistry of the iron(lll) acyl complexes Fe(Por)C(0)R was investigated as part of this study, and showed two reversible reductions (to Fe(ll) and Fe(l) acyl complexes, formally) and one irreversible oxidation process."" ... 

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... 

It is believed that the primary step involves excitation of the acyl complex via a metal-to-terminal CO charge transfer (755). The molecule then loses CO and rearranges to the carbonyl alkyl, as shown in Eq. (43). However, it... 

Sparteine 1 was also used in a palladium complex-catalyzed enantios-elective benzoylation of alcohols using monoxide and the organobismuth(V) compound (Scheme 37). The carbonylative acylation of alcohols using carbon monoxide (CO) is known to be an alternative tool for the prepar-... 

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]. 

Carbonyl insertion is preferentially observed in the photoindueed reaction of 22 to give the cyclohexenones 25 and 26 as shown in Scheme 9 [17]. The acyl complex 24 is involved as an intermediate. The eyclohexenone formation appears to be susceptible to conformational effect, as observed in the facile rearrangement of 27 to 28. 

Another peculiarity of the acyl complex is that it prefers solvent coordination, even when the solvent has weakly coordinating ability such as THF, rather than CO coordination, even in the presence of free CO. This also can contribute to the high activity observed for these systems, as MeOH has not to compete with CO for coordination before the product-forming step. It is noteworthy that the carbonylation of the closely related complex [Pd(Me)(Et20)(dippe)]+ in Et20 or [Pd(OTf)(CH3)(dibpp)] results in the complex [Pd(COMe)(CO)(dippe)]+ containing coordinated CO instead of solvent [133]. 

In addition to the ring opening of cyclopropenes noted above, vinylketene complexes 103 have been prepared by (1) ligand initiated carbonyl insertion of vinyl carbene complexes 104 and (2) benzoylation of ,/3-unsaturalcd acyl ferrates 105 (Scheme 20)114. X-ray diffraction analysis of these vinylketene complexes indicates that the structure may be best represented as a hybrid between an /j4-dicnc type complex (103) and an jj3-allyl r/1 acyl complex (106). The Fe-Cl distance (ca 1.92 A) is shorter than the Fe-C2, Fe-C3, or Fe-C4 distances (ca 2.1-2.2 A)113a-C. In addition, the C—C—O ketene array is not linear (bend angle ca 135°). 

Before addition of the benzyl halide, the only carbonyl adsorption peak is found at 1900 cm, indicative of the cobalt tetracarbonyl anion. After addition, this band immediately disappears and peaks at 2000 cm l are observed. These most likely represent the corresponding acyl complex. Reaction with methoxide yields the product and regenerates the cobalt anion. In the absence of sufficient methoxide, the reaction requires attack by the much... 

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

From Acyl Complexes Generated from Carbonyl Complexes... 

In addition to the direct nucleophilic alkylation of carbonyl complexes, the acylation of metallates with, e.g., carboxylic acid chlorides [73,100,102] or anhydrides [79] is a practical way of generating acyl complexes (Figure 2.4). Illustrative examples are given in Table 2.3. 

A further method for preparing acyl complexes consists in the treatment of alkyl complexes containing at least one carbonyl ligand with a strong ligand [44,105,106], Thereby 1,1-insertion of the carbonyl group into the metal-carbon bond can... 

ATR-HP IR spectroscopy has also been used to follow the cobalt-catalysed carbonylation of epoxides to give lactones or polyesters [46]. Addition of excess propylene oxide to [HCo(CO)4] (generated in situ by protonation of [Co(CO)4] ) under 20 bar CO was found to give an acyl complex, [Co(C(0)CH2CH(OH)Me)(CO)4]. Depending... 

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