Acyl carbonyl complexes, structures

The insertions of CO into Pd-Me and of various strained alkenes into Pd-COR supported by the dinitrogen ligand Ar-BIAN have been studied by Vrieze and Elsevier by means of ex situ NMR spectroscopy [22, 51], For example, the sequential insertions of CO and norbomadiene into Pd-alkyl and Pd-acyl bonds allowed the detection and isolation of several intermediates such as acyl(carbonyl) and P-chelate (Scheme 7.24). The Pd complexes isolated after alkene insertion were found to have a structure arising from cis addition of Pd-C(0)R to the exo face of the olefin. 

Treatment of the potentially electrophilic Z-xfi-unsaturated iron-acyl complexes, such as 1, with alkyllithium species or lithium amides generates extended enolate species such as 2 products arising from 1,2- or 1,4-addition to the enone functionality are rarely observed. Subsequent reaction of 2 with electrophiles results in regiocontrolled stereoselective alkylation at the a-position to provide j8,y-unsaturated products 3. The origin of this selective y-deproto-nation is suggested to be precoordination of the base to the acyl carbonyl oxygen (see structures A), followed by proton abstraction while the enone moiety exists in the s-cis conformation23536. 

Palladium-catalyzed ketone synthesis B. The reaction mixture is saturated with carbon monoxide, which intervenes in step 2 by forming a palladium(II) carbonyl complex. Before the transmetalation (above referred to as step 3) takes place a rearrangement is interposed. The ligand Rmisa rji cd undergoes a [l,2]-shift from Pd(II) to the carbon atom of carbon monoxide, leading to the formation of an acylpalladium(II) complex with the structure P lllsa llra cd-(C=0)-Pd(-X) L j. With regard to the above-mentioned steps 3-4 it behaves like the acyl-Pd(II) complex of the ketone synthesis A and, after reductive elimination, i.e. in step 5, yields... 

The nucelophilicity of the activated P-lactam should be high enough to attack the carbonyl carbon of the acyl-enzyme complex and it should yield the dimeric tetrahedral intermediate TI2 (Figure 14.12). We have modeled this structure in a covalent docking step, by connecting the nitrogen of the activated P-lactam... 

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°). 

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