Acyl complexes stereochemistry

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 oxidative addition is quite general with alkyl, allyl, benzyl, vinyl, and aryl halides as well as with acyl halides to afford the palladium (II) complex VII. The frans-bis( triphenylphosphine )alkylpalladium halides can also be carbonylated in an insertion reaction to give the corresponding acyl complexes, the stereochemistry of which (17, 18) proceeds with retention of configuration at the carbon bonded to palladium. The acyl complex also can be formed from the addition of the corresponding acid halide to tetrakis (triphenylphosphine) palladium (0). 

In an attempt to resolve this question of stereochemistry and also to determine whether or not the decarbonylation of an acid chloride containing a f3 hydrogen takes place stereospecifically, erythro- (XI) and fhreo-2,3-diphenylbutanoyl chlorides (XII), obtained by the reaction of the known acids (13, 14) with oxalyl chloride, were synthesized. The reaction of these acid chlorides (see Reaction 8) with chlorotris( triphenyl-phosphine) rhodium gave the corresponding acyl complexes of type lib [R = C6H5CH(CH3)CH(C6H5)]. Decarbonylation of the erythro- cy complex in benzene at 30 °C gave a 90% yield of frans-a-methylstilbene while decarbonylation of the threo-acyl complex under similar reaction... 

The 5-coordinate acyl complex has been the subject of many studies since it is isolable from the reaction mixture (see Table 1). Proton and phosphorus nmr studies show that the acyl complex isomerizes from a cis-to a ran -phosphine stereochemistry which have different chemical shifts and coupling constantsas shown in Table 2. In addition, the infrared spectra of the isomers are different in the Rh-Cl and carbonyl regions (e.g., when R = CH2Ph, Vco = 1720 cm for the cis isomer and Vco = 1770 cm for the trans isomer)... 

Many experiments have shown that CO insertion typically occurs with retention of configuration at the a-carbon. Racemization has been observed in a few cases that occur by radical paths, but inversion has never been observed. The first imequivocal demonstration of the stereochemistry of the a-carbon during inserfiorf is illustrated in Equation 9.21. The threo alkyl complex afforts the threo acyl complex. 

The reactions of some optically active benzyl halides have provided further evidence for the nucleophilicities of the [Pd(PR3)j] species (Lau et al., 1974, 1976 Wong et al., 1974 Stille and Lau, 1976). The salient features of these studies are summarised in Scheme 6. The stereochemistry of the addition is established by reaction of the palladium-alkyl complex with carbon monoxide (this insertion is known to take place by an intramolecular migration process, with retention of configuration in the migrating alkyl group), and subsequent formation of an ester from this acyl complex. 

Manganese.—Elimination of transition-metal hydride from metal alkyls and addition of metal hydrides to alkenes are usually considered to be cA-processes. Since acylmanganese compounds undergo stereospecific reversible decarbonylation, thermal decomposition of (eryrAro-2,3-dimethylpentanoyl)(pentacarbonyl)manga-nese(i) should allow the determination of the stereochemistry of elimination of [MnH(CO)8] (Scheme 4). However, both the erythro and a mixture of the erythro and threo acyl complexes decompose thermally to give the same mixture of cis- and trans-3-methylpent-2-ene and 3-methylpent-l-ene under conditions which do not isomerize these alkenes. It is suggested that the mechanism involves interconversion of... 

Carbonylation of complex 24 where P is (S)-PPh2NCH3CHCH3Ph gives the acyl complex 25 with a stereoselectivity of more than 90%. °° ° This result is not compatible with methyl migration, but rather with CO migration to give the 17 -acetyl intermediate 26. However, the stereochemistry of reaction (41) is consistent with methyl migration. ... 

Reactions of this pseudooctahedral complex have been studied in particular detail by the Davies group at Oxford and the Liebeskind group in the United States because of its potential use as a chiral auxiliary for control of the absolute stereochemistry of various reactions of the acyl enolate. Both R-( — )-l and S-( + )-1 are now available commercially (Fluka), but at a prohibitive cost ( 125.60 per gram). 

In this section, tetramic acids with an acyl group substituent at C-3 are discussed. The simplest of the naturally occurring 3-acyl tetramic acids, tenuazonic acid (6), was first isolated from the culture filtrate of Alter-naria tenuis [18] and, subsequently, from other fungal species (A. alternate, A. longipes, Pyricularia oryzae) [19,20]. Species of Altemaria are known to produce more than 70 secondary metabolites, many of which, particularly those from the Altemaria altemata complex, are mycotoxins [19]. The absolute stereochemistry of 6 (55,65) was deduced from the formation of L-isoleucine on ozonolysis followed by acid hydrolysis [21]. 

Pathways involving alkyl-acyl rearrangements are proposed to explain the carbonylation of a-bonded alkoxy complexes (17). The stereochemistry of the products indicates that the ester group replaces Pd with retention of configuration at the carbon to which Pd is o-bonded. In all these studies with unconjugated dienes the nature of carbonylation products to be expected is clearly influenced by the geometry of the intermediate Pd complexes. 

Reactions of acyclic derivatives with carbon electrophiles have also been examined.33,34 An illustrative reaction involving methylation of the unsubstituted complex [MnCr 4-butadiene)(CO)3], (19), is shown in Scheme 16. Again, the reaction is presumed to occur via a methylmanganese species (20) and after methyl migration the unsaturated metal center is stabilized by formation of a Mn—H—C bridge (isomers 21a and 21b). Deprotonation of equilibrating (21a and 21b) yields the [Mn(l-methylbutadiene)(CO>3]-complex (22), which has exclusively trans stereochemistry.34 This sequence represents alkylation of the terminal carbon of butadiene and complements the iron carbonyl chemistry, where terminal acylation has been achieved as described above. Unpublished results indicate that a second methylation of (22) occurs... 

In order to confirm and to obtain a more detailed knowledge of the above proposed mechanism, precise information is required on the structure and stereochemistry of intermediates such as the enzyme-substrate complex, the tetrahedral intermediate and the acyl-enzyme. Normally, this information cannot be obtained directly because these intermediates are transient species. On the other hand, it can be obtained indirectly from the precise structural information available from high resolution X-ray diffraction studies of several enzyme derivatives or complexes with substrate analogs. 

Several 5-bonded sulfinates of the general type RS(0)2Mn(C0)5 [R = Me (65, 66), Et (66), CHaPh (65, 66), and Ph (66)] have been isolated by treating the appropriate metal alkyl or aryl with liquid SOg. Whereas the reaction of the metal alkyls proceeds readily at - 10°C, that of PhMn(CO)g requires temperatures of 35-4S°C and affords also PhC(0)Mn(C0)5. In the insertion with the R = Me and CHaPh complexes, the intermediate 0-sulfinate was detected by NMR spectroscopy (72, 7J). The last-mentioned reactions were studied kinetically (77, 76). The acyls MeC(0)Mn(C0)5 and CF3C(0)Mn(C0)5, as well as CF3Mn(CO)5, failed to react with neat SOa at reflux (66). The insertion with cw-MeMn(CO)4PPh3 at various temperatures yields m-MeS(0)aMn(C0)4PPhg (68). The stereochemistry of the sulfination of m-MeMn(CO)4 CO (709) was already mentioned in Section III, H. 

Acylation of organozinc reagents. Reaction of acyl chlorides with organozinc compounds catalyzed by palladium-phosphine complexes provides a general synthesis of ketones. The organozincs are readily available by treatment of the corresponding organo-lithium with ZnCh. Alkenylzinc compounds and a,p-unsaturated acyl chlorides react with retention of the stereochemistry. Isolated yields of ketones are 55-90%. 

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