Acylation Acyl complexes

I.3.4.2.5. Chiral Enolates of Acyl-Metal Complexes J. S. McCallum and L. S. Liebeskind I.3.4.2.5.I. Chiral Iron-Acyl Complexes... 

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). 

Alkylation of the anion 2 with iodomethane or other haloalkanes provides alkyldicarbonyl(t/5-cyclopentadienyl)iron complexes such as 53,0 (see also Houben-Weyl, Vol. 13/9a, p 209). Migratory insertion of carbon monoxide occurs on treatment with phosphanes or phosphites9 -11 (see also Houben-Weyl, Vol. d3/9a, p257) to provide chiral iron-acyl complexes such as 6. This is the most commonly used preparation of racemic chiral iron-acyl complexes. 

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

Enantiomerically Pure Chiral Iron-Acyl Complexes... 

The reported preparations of enantiomerically pure chiral iron-acyl complexes have relied upon resolutions of diastereomers. One route1415 (see also Houben-Weyl, Vol. 13/9 a, p 421) employs a resolution of the diastereomeric acylmenlhyloxy complexes (Fe/ )-3 and (FeS )-3 prepared via nucleophilic attack of the chiral menlhyloxide ion of 2 at a carbon monoxide of the iron cation of 1. Subsequent nucleophilic displacement of menthyloxide occurs with inversion at iron to generate the enantiomerically pure iron-acyl complexes (i>)-4 and (f )-4. 

Another route to enantiomcrically pure iron-acyl complexes depends on a resolution of diastereomeric substituted iron-alkyl complexes16,17. Reaction of enantiomerically pure chloromethyl menthyl ether (6) with the anion of 5 provides the menthyloxymethyl complex 7. Photolysis of 7 in the presence of triphenylphosphane induces migratory insertion of carbon monoxide to provide a racemic mixture of the diastereomeric phosphane-substituted menthyloxymethyl complexes (-)-(/ )-8 and ( + )-( )-8 which are resolved by fractional crystallization. Treatment of either diastereomer (—)-(/J)-8 or ( I )-(.V)-8 with gaseous hydrogen chloride (see also Houben-Weyl, Vol 13/9a, p437) affords the enantiomeric chloromethyl complexes (-)-(R)-9 or (+ )-(S)-9 without epimerization of the iron center. 

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). 

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 chiral lithium enolate 2 reacts with symmetrical ketones to produce /(,/i-dialkyl-/l-hydroxy-acyl complexes 3 which serve as precursors to oc,/1-unsaturated iron complexes (see Section 1.3.4.2.5.1.1.). 

Table 4. / -Hydroxy-lron Acyl Complexes 2 and 3 by Aldol Addition of Enolates 1 to 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. 

Conducting the aldol reaction at temperatures below —78 "C increases the diastereoselectivity, but at the cost of reduced yields45. Transmetalation of the lithium enolate 2 a by treatment with diethylaluminum chloride generated an enolate species that provided high yields of aldol products, however, the diastereoselectivity was as low as that of the lithium species45. Pre treatment of the lithium enolate 2a with tin(II) chloride, zinc(II) chloride, or boron trifluoridc suppressed the aldol reaction and the starting iron-acyl complex was recovered. 

The a-alkoxy iron-acyl complex 5 may be deprotonated to generate the lithium enolate 6, which undergoes a highly diastereoselective aldol reaction with acetone to generate the adduct 7 as the major product. Deprotonation of acetone by 6 is believed to be a competing reaction 30% of the starting complex 5 is found in the product mixture48 40. 

The lithium enolate of the oc-silyl-substituted iron-acyl complex 19 reacts with aldehydes, however, products of the Peterson elimination process (E)- and (Z)-22 are usually isolat-ed22- 23,36.37 for t[1js anc other preparations of a,/t-unsaturated iron-acyl complexes see Section I.3.4.2.5.I.3.). 

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

Oxidative decomplexation of iron acyl complexes in the presence of alcohols provides the corresponding carboxylates 7. Usual conditions employ ca. 7% alcohol in dichloromethane or dichloromethane/carbon disulfide as the solvent with bromine as the oxidant. 

Amides are produced if iron - acyl complexes are oxidized in the presence of a secondary or primary amine25 52 59 60. This reaction, usually conducted at low temperatures, employs /Y-bromosucciniinide or bromine as the oxidant (see Table 6). 

The oxidation of /(-amino-substituted iron acyl complexes which are prepared via condensation reactions of iron-acyl enolates and imines or iminium ions26,5 -47-54 generates /(-lactams 32,33,61. Brief treatment with bromine in dichloromethane at low temperature is the usual procedure. 

Table 7. 2-Azctidinones 17 by the Oxidation of /i-Amino-lron-Acyl Complexes 16... 

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. 

I.3.4.2.5.2. Other Transition-Metal-Acyl Complexes 1.3.42.5.2.1. Chiral Cobalt-Acyl Complexes... 

The procedures used to prepare and resolve these chiral cobalt-acyl complexes are as follows ... 

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... 

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... 

One of the properties of transition metal acyl complexes is their ability to lose CO, usually on heating or photolysis. This so-called decarbonylation often represents a special case of the reverse of the CO insertion in Eq. (8), where L = CO. 

Additional complications with regard to recognition of an insertion reaction may arise if the acyl produced defies isolation by undergoing a further transformation. Equations (17)-(19) 78, 134, 115) provide diversified examples of such behavior (the postulated CO insertion intermediates are enclosed in brackets). The intermediacy of acyl complexes in these reactions can sometimes be inferred through examination of the infrared or NMR spectra of reaction solutions. In a number of cases, however. 

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... 

---