Cyclopentadienyl carbonyl complexes unsaturated

Chiral Metal Atoms in Optically Active Organo-Transition-Metal Compounds, 18, 151 13C NMR Chemical Shifts and Coupling Constants of Organometallic Compounds, 12, 135 Compounds Derived from Alkynes and Carbonyl Complexes of Cobalt, 12, 323 Conjugate Addition of Grignard Reagents to Aromatic Systems, I, 221 Coordination of Unsaturated Molecules to Transition Metals, 14, 33 Cyclobutadiene Metal Complexes, 4, 95 Cyclopentadienyl Metal Compounds, 2, 365... 

Attempted y-deprotonation of the y,y-disubstituted Z-a,/ -unsaturated complex carbonyl(>)5-cyclopentadienyl)(4-methyl-l-oxo-2-pentenyl)(triphenylphosphane)iron, followed by exposure to protonating conditions, leads to a mixture of uncharacterized products no regenerated starting complex was detected37. 

The Diels-Alder reaction is one of the most fundamental means of preparing cyclic compounds. Since discovery of the accelerating effect of Lewis acids on the Diels-Alder reaction of a,)3-unsaturated carbonyl compounds [341-344], its broad and fine application under mild reaction conditions has been amplified. Equations (140) [341] and (141) [345], respectively, illustrate typical dramatic effects from an early reaction and from one reported more recently. Lewis acid-promoted Diels-Alder reactions have been reviewed [7,8,346-353]. In addition to the acceleration of the reaction, other important feature is its alteration of chemo-, regio-, and diastereoselectivity this will be discussed below. The titanium compounds used in Diels-Alder reaction are titanium halides (TiX4), alkoxides (Ti(OR)4), or their mixed salts (TiX (OR)4 n = 1-3). A cyclopentadienyl complex such as Cp2Ti(OTf)2 is also documented as a very effective promoter of a Diels-Alder reaction [354], In addition to these titanium salts, a few compounds such as those in Eq. (142) [355] have recently been reported to effect the Diels-Alder reaction. The third, [(/-PrO)2Ti(bpy)(OTf)(i-PrOH)] (OTf), was estimated to be a more active catalyst than Cp2Ti(OTf)2. 

Johnson and coworkers [1493] have also obtained nonracemic bicyclo-[4.1.0.]-2-heptanones by Simmons-Smith reactions with chiral sulfoximines. The drawback of this methodology is the purification of the starting chiral sulfoximines. Davies and coworkers reacted cyclopentadienyl-iron carbonyl 1.148 (R = E-RCH=CH) with CH2l2/MeLi. After appropriate treatment, nonracemic ethyl frans-cyclopropanecarboxylates are obtained with a high selectivity [522] (Figure 7.83). As usual, the reaction occurs on the most accessible face of the complex ( 7.10.2.1). 

A very different, but similarly effective, auxiliary is the chiral carbonyl(t/5-cyclopentadienyl)(tri-phenylphosphine)iron moiety. When the z./i-unsaturated acyl-iron complex ( -)-(/ )-11 is treated by a modified Simmons Smith reagent, a 91 9 mixture of cyclopropane diastereomers is isolated in good yield73. Precomplexation of the starting iron complex by the Lewis acid zinc(II) chloride seems to be necessary to obtain good selectivity. The chiral iron moiety can then be removed oxidatively by bromine treatment, and the intermediate acyl bromides converted into amides by reaction with (/ )- -phenylethylamine. 

The complex, formally derived from a double oxidative addition of benzene to two Ir centers , is in contrast to the mononuclear product obtained with the pentamethylcyclopentadienyl derivative . In the absence of light there is no reaction presumably photolysis displaces one carbonyl ligand to give a coordinatively unsaturated precursor. An apparent intermediate in the photolysis has H NMR resonances in the aromatic, cyclopentadienyl and hydride regions and may be IrGp-j/ (GO)H(G6H5), but its nature remains uncertain. 

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