Iron acyl complexes synthesis

Iron acyl complexes are among the most widely studied of the organometallic iron species, especially as applied to organic synthesis. As previonsly mentioned they are prepared to provide access to iron alkyls (via decarbonylation), and because iron acyls can be deprotonated to form enolates much like any carbonyl they have been utilized as chiral auxiliaries in asymmetric synthesis. Also, iron acyls are an important entry point for the preparation of iron carbenes. 

Following the initial report by Liebeskind et al. last year, using racemic starting materials, Davies and his group have now reported their extensive studies of the asymmetric synthesis of 3-lactams by Michael addition of lithium benzylamide to the optically pure a,3-unsaturated iron acyl complexes (253) and (254) (Scheme 20). 42,143 rp e procjuc-t.s (255) and (256) are formed with excellent optical purity (>100 1) and, given the range of substituents that could potentially be incorporated by this method,... 

This iron anion is a good soft nucleophile for alkyl halides and can be used twice over to produce first a monoanion with one alkyl group and then a neutral complex with two alkyl groups and four CO ligands. Each of these complexes has 18 electrons. If extra CO is added by increasing the pressure, CO inserts into one Fe—C bond to form an iron acyl complex. Finally, reductive elimination couples the acyl group to the other alkyl group in a conceptually simple ketone synthesis. It does not matter which Fe—C bond accepts the CO molecule the same unsymmetrical ketone is produced at the end. 

Preston, S.C. (1989) Asymmetric synthesis via iron acyl complexes. PhD Dissertation. Oxford University Press 1990) Diss. Abstr. Int. B, 51, 2896. 

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

Chiral iron acyl complexes have been applied to the asymmetric synthesis of cyclopropane carboxylic acids, sulfoxides and p-amino acids. Further details and applications may be found in the reviews given in the reference... 

The availability of such procedures coupled with several straightforward methods for synthesis of the diene complexes make these species attractive for use in organic synthesis. Several illustrative examples follow, which include acylation (or diacylation) of iron-diene complexes followed by cleavage and recovery of the free organic ligand. 

Reduction of acid chlorides to aldehydes One of the most useful synthetic transformations in organic synthesis is the conversion of an acid chloride to the corresponding aldehyde without over-reduction to the alcohol. Until recently, this type of selective reduction was difficult to accomplish and was most frequently effected by catalytic hydrogenation (the Rosenmund reduction section 6.4.1). However, in the past few years, several novel reducing agents have been developed to accomplish the desired transformation. Among the reagents that are available for the partial reduction of acyl chlorides to aldehydes are bis(triphenylphosphine)cuprous borohydride , sodium or lithium tri-terf-butoxyaluminium hydride, complex copper cyanotrihydridoborate salts °, anionic iron carbonyl complexes and tri-n-butyltin hydride in the presence of tetrakis(triphenylphosphine)palladium(0). ... 

Synthesis and Reaction Chemistry of a,p-Unsaturated Acyl Complexes Derived from (2). Two methods for the preparation of optically active ( )- and (Z)-a,p-unsaturated iron acyls from (2) have been reported." One method involves aldol condensation of (2) with aldehydes followed by 0-methylation to produce diastereomeric acyls (18). This mixture (18) is then treated with Sodium Hydride to produce predominantly ( )-a,p-unsaturated acyl complexes (19) (eq 13). Alternatively, (2) can be depro-tonated and treated with Chlorotrimethylsilane to produce the C-silylated complex which is subsequently deprotonated and treated with an aldehyde. This Peterson alkenation produced mixtures... 

These conversions can be applied to organic synthesis in various manners. Thus, quadricyclane can be ring opened and carbonylated with iron carbonyls.Decomplexation of the resulting acyl complex leads to a tetracyclic diketone. 

Thomas, S.E., Tustin, G.J., and Ibbotson, A., Synthesis and reactivity of iron carbonyl complexes of a,P-unsaturated acyl silanes. Tetrahedron, 48, 7629, 1992. 

This section covers developments concerning synthesis and reactivity of organometallic iron complexes having a 7 -bound hydrocarbyl ligand, whose a-carbon atom is considered as rp -hybridized, and their formal derived insertion products (acyis, iminoacyis). Iron complexes ligated both by 77 - and 77 - ( = 2 ) hydrocarbyl ligands such as the 77 -alkyl-77 -allyl compounds or the benzene-coordinated acyl complex [Fe PhCH(=0) (C0)3] 4" will be... 

Diels-Alder reactions, 4, 842 flash vapour phase pyrolysis, 4, 846 reactions with 6-dimethylaminofuKenov, 4, 844 reactions with JV,n-diphenylnitrone, 4, 841 reactions with mesitonitrile oxide, 4, 841 structure, 4, 715, 725 synthesis, 4, 725, 767-769, 930 theoretical methods, 4, 3 tricarbonyl iron complexes, 4, 847 dipole moments, 4, 716 n-directing effect, 4, 44 2,5-disubstituted synthesis, 4, 116-117 from l,3-dithiolylium-4-olates, 6, 826 electrocyclization, 4, 748-750 electron bombardment, 4, 739 electronic deformation, 4, 722-723 electronic structure, 4, 715 electrophilic substitution, 4, 43, 44, 717-719, 751 directing effects, 4, 752-753 fluorescence spectra, 4, 735-736 fluorinated derivatives, 4, 679 H NMR, 4, 731 Friedel-Crafts acylation, 4, 777 with fused six-membered heterocyclic rings, 4, 973-1036 fused small rings structure, 4, 720-721 gas phase UV spectrum, 4, 734 H NMR, 4, 7, 728-731, 939 solvent effects, 4, 730 substituent constants, 4, 731 halo... 

The tetra-cA-cycIononatetracne 241 is unstable and easily rearranges at 23 °C (t /2 50 min) to the isomeric d.v-8,9-dihydroindcne 242 (equation 77)89. It is interesting, however, that the iron(III) tricarbonyl complex of tetraene 241 is stable for many days at room temperature and isomerizes to the Fe-complex of 242 only upon heating in octane at 101 °C89. The principle of stabilization of the reactive multiple bonds with metal carbonyl complexes is well-known in modem organic synthesis (e.g. see the acylation of enynes90). 

A viable iron carbonyl-mediated reduction process converts acid chlorides and bromoalkanes into aldehydes [3, 6]. Yields are high, with the exception of nitro-benzoyl chloride, and the procedure is generally applicable for the synthesis of alkyl, aryl and a,(i-unsaturated aldehydes from the acid chlorides. The reduction proceeds via the initial formation of the acyl iron complex, followed by hydride transfer and extrusion of the aldehyde (cf. Chapter 8). 

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