Acyl complexes, structures

Synthesis of racemic iron acyl complex (Structure 1) (Scheme 4.1) ... 

Insertion of SO2 into the Fe—C bond in FelPorfCHi was first reported in 1982, giving the sulfinato complexes Fe(Por)S02CH2, which are moderately air stable but can be further oxidized by O2 to give the sulfonato complexes FelPorfSOiCH. " Alkyliron(Ill) porphyrins insert CO to give the acyl complexes Fe(Por)C(0)R. For example, Fe(TPP)C(0)-n-Bu was formed either by this method or by the reaction of I Fe(TPP) r with ClC(0)-/ -Bu, and was characterized by an X-ray crystal structure... 

Because of structural similarities to the vitamin B,2 coenzyme, cobalt(III) complexes of the type RCo(L4) or RCo(L2)2 [L4 = bis(salicylaldehyde)-ethylenediiminate and L2 = dimethylglyoximate, inter alia] have been actively investigated 40, 76, 77, 125). Corresponding acyl complexes have been synthesized 40, 76). However, neither the CO insertion into the Co—-R linkage nor the decarbonylation of the Co—COR moiety has been achieved (77, 125). A probable reason for this was presented in Section II. 

Several successful cyclizations of quite complex structures were achieved using polyphosphoric acid trimethylsilyl ester, a viscous material that contains reactive anhydrides of phosphoric acid.58 Presumably the reactive acylating agent is a mixed phosphoric anhydride of the carboxylic acid. 

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°). 

Coenzyme A (see also p. 106) is a nucleotide with a complex structure (see p. 80). It serves to activate residues of carboxylic acids (acyl residues). Bonding of the carboxy group of the carboxylic acid with the thiol group of the coenzyme creates a thioester bond (-S-CO-R see p. 10) in which the acyl residue has a high chemical potential. It can therefore be transferred to other molecules in exergonic reactions. This fact plays an important role in lipid metabolism in particular (see pp. 162ff), as well as in two reactions of the tricarboxylic acid cycle (see p. 136). 

The structure of the two rhodium-acyl complexes was elucidated using COSY90 spectra, selective decoupling of the phosphorus resonances, and HMQC spectra. Spectrum 3d (Figure 6.13) shows the complete simulation of the NMR spectrum obtained at 223 K. The NMR spectrum... 

More complex structures, often related to natural products are prepared by organic synthesis. Among them can be mentioned (f )-3-hydroxytetradecanoic acid (the double-tail hydrophobic moiety of lipid A), sphingosine derivatives related to the ceramides or 1,2- and l,3-dialkyl(acyl)glycerols related to glyco-glycerolipids, glycerophospholipids, and GPI anchors of membrane proteins. The preparations of the above derivatives were reported several years ago but some improvements have been published more recently. 

The complex structure isolated from Scleranthus uncinatus, 5,7-dihydroxy-3 -meth-oxy-4 -acetoxyflavone-8-C-p-D-xylopyranoside-2 -0-glucoside, was determined by using ID NMR ( H, C, DEPT) and 2D NMR (H-COSY, TOCSY, HMQC, HMBC, NOESY) data sequence and linkage of the sugar chain and acylation site were confirmed by observation of inter-residue NOEs in the NOESY spectrum. 

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. 

The origin of the third diastereomer produced, complex 12, is of particular mechanistic interest. The configuration at Ca of 12 is opposite to that of the other two products 10 and 11 indicating that the opposite face of the enolate 6 has been approached by the epoxide. Two possible alterations of the geometry of enolate 6 inay be invoked to account for this, adoption of the 5yn- -conformer or adoption of the anti-Z-conformer. Examination of the different structures shown reveals that the observed minor product 12 could arise from a matched reaction pair of the ivn-E-enolate and epoxide (Newman Projection G) or from a mismatched reaction pair of the anti-Z-enolate and epoxide (Newman projection I). The absence of diastereomer 13 strongly suggests that the minor product 12 arises from reaction of the. ryn- -enolate, underscoring the extreme reluctance of iron-acyl complexes to form Z-enolates on deprotonation (see scheme on p 955). 

Hydridotris(3,5-dimethyl-l-pyrazolyl)borate]molybdenum-(i72-acyl) complexes, such as 1, are deprotonated by butyllithium or potassium hydride to generate enolate species, such as 488.8> jjie overa]] structure of these chiral complexes is similar to that of the iron and rhenium complexes discussed earlier the hydridotris(3,5-dimethyl-l-pyrazolyl)borate ligand is iso valent to the cyclopentadienyl ligand, occupying three metal coordination sites. However, several important differences must be taken into account when a detailed examination of the stereochemical outcome of deprotonation-alkylation processes is undertaken. 

It has been mentioned already that simple acylic enones are not the only class of substrate amenable to poly-amino acid catalyzed epoxidation. More complex structures and their epoxidation products are shown in Scheme 10.9 [71, 82, 83],... 

An example of more complex structural photoisomerization may be the photo-Fries rearrangement leading to a photorearrangement of O-acyl-phenols or X-acyl-anilines to give the [1,3]- or [l,5]-rearranged products, as shown in Figure 6.5. 

---