Dienes, acylation complexes

Conjugated dienes, upon complexation with metal carbonyl complexes, are activated for Friedel-Crafts acylation reaction at the akyhc position. Such reactions are increasingly being used in the stereoselective synthesis of acylated dienes. Friedel-Crafts acetylation of... 

Chiral diene—iron tricarbonyl complexes were acylated using aluminum chloride to give acylated diene—iron complexes with high enantiomeric purity (>96% ee). For example, /ra/ j -piperjdene—iron tricarbonyl reacted with acyl haUdes under Friedel-Crafts conditions to give l-acyl-l,3-pentadiene—iron tricarbonyl complex without any racemization. These complexes can be converted to a variety of enantiomericaHy pure tertiary alcohols (180). 

Acylation of terminal alkenes by ( j4-2,4-pentadienoyl chloride)Fe(CO)3 proceeds in high yield in the presence of Lewis acid catalysts67. As shown in equation 51, the reaction generally produces a mixture of ( j4-diene)tricarbonyliron complexes of the /J-chloroketone and /Ly-unsaturated ketone. 

Cleavage of acylated diene-iron complexes with LiAlH4 is illustrated by the following reports of Nes-meyanov and Anisimov (Scheme 12).28,29... 

Nucleophilic attack occurs at C(2) of the diene. The 1,3-cyclohexadiene complex 66 is converted to the homoallyl anionic complex 67 by nucleophilic attack, and the 3-alkyl-1-cyclohexene 68 is obtained by protonation. Insertion of CO to 67 generates the acyl complex 69, and its protonation and reductive elimination afford the aldehyde 70 [20]. Reaction of the butadiene complex 56 with an anion derived from ester 71 under CO atmosphere generates the homoallyl complex 72 and then the acyl complex 73 by CO insertion. The cyclopentanone complex 74 is formed by intramolecular insertion of alkene, and the 3-substituted cyclopentanone 75 is obtained by reductive elimination. The intramolecular version, when applied to the 1,3-cyclohexadiene complex 76 bearing an ester chain at C(5), offers a good synthetic route to the bicyclo[3.3.1]nonane system 78 via intermediate 77 [21]. 

A variety of vanadium carbonyl compounds are obtained from [V(CO)6] . Use of l-chloropenta-2,4-diene under photoirradiation leads to V(jj -C5H7)(CO)4. The -bonded acyl complex is formed similarly by photoinduced substitution with benzoyl chloride. The rf--C,0 coordination mode is... 

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 a, -unsatnrated acyl fenates 105 (Scheme 20)". X-ray diffraction analysis of these vinylketene complexes indicates that the structure may be best represented as a hybrid between an -diene type complex (103 ) and an r] -allyl-zy -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 In addition, the C—C—O ketene array... 

With alkyls not capable of elimination, addition of the alkyl group is possible, e.g.,with methylcobalt carbonyl, which undergoes CO insertion prior to reaction with the diene . The product thus is the acyl complex XLIX. 

SC(0)C(PR3)S)] (R=NMe2) , and substitution reactions of [Fe2(CO)j p-SC(CF3)C(CF3))]. An interesting paper details the exo-selective Diels-Alder reactions of a,P-unsaturated acyl complexes [Fe2(CX))6 p-SPr") p-(MeCH=CH)C=0)] with dienes, addition of iodine or cerium... 

An -ray structure determination of a cationic intermediate (8) isolated during the acylation of [Fe(CO)8(rraAw, ra j-hexa-2,4-diene)] establishes that Friedel-Crafts acylation involves stereospecific endo attack. The stereochemistry of reaction parallels protonation of tricarbonyl(diene)iron, cyclopentadienyl(cyclohexa-l,3-diene)rhodium complexes, and ( j -cyclopentadienyl)rhodium complexes of limonene (9), a-phellandrene (10), and carvone (11). ... 

The most used reaction with respect to electrophilic substitution at (diene)iron complexes is the acylation. (Dienone)iron complexes are obtained by reaction of acyclic tricarbonyl(Ti -l,3-diene)iron complexes with carboxylic acid chlorides in the presence of aluminum trichloride in high yields (Scheme 4-11 i).P65-271]... 

Acylated acyclic tricaibonyl( n -l,3-diene)iion complexes can be introduced in a variety of addition reactions at the carbonyl group. The adjacent planar chiral tricarbonyliron moiety allows to perform such reactions stereoselectively. Several natural products, including insect pheromones, (-)-LTA4 methyl ester, proto-... 

Scheme 4-112. Intramolecular acylation of (ii -diene)iron complexes.

Scheme 4-113. Intramolecular acylation of a (T -diene)iron complex including Lewis acid-promoted double bond migration.

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