A,ir-Rearrangement

Furthermore, the catalytic allylation of malonate with optically active (S)-( )-3-acetoxy-l-phenyl-1-butene (4) yields the (S)-( )-malonates 7 and 8 in a ratio of 92 8. Thus overall retention is observed in the catalytic reaction[23]. The intermediate complex 6 is formed by inversion. Then in the catalytic reaction of (5 )-(Z)-3-acetoxy-l-phenyl-l-butene (9) with malonate, the oxidative addition generates the complex 10, which has the sterically disfavored anti form. Then the n-a ir rearrangement (rotation) of the complex 10 moves the Pd from front to the rear side to give the favored syn complex 6, which has the same configuration as that from the (5 )-( )-acetate 4. Finally the (S)-( )-mal-onates 7 and 8 are obtained in a ratio of 90 10. Thus the reaction of (Z)-acetate 9 proceeds by inversion, n-a-ir rearrangement and inversion of configuration accompanied by Z to isomerization[24]. 

The term a-ir rearrangement is defined as an intramolecular process in which an organic group that is a bonded (monohapto. h ) to a transition metal becomes n bonded (n-hapto, hn) to the metal. The ir-cr rearrangement is the opposite change (hn-h ). 

Figure 4. a-ir Rearrangement of triphenylchromium tritetrahydrofuranate to diarenechromium Tr-complexes... 

Section 24.13 On being heated, allyl aryl ethers undergo a Claisen rearrangement to form o-allylphenols. A cyclohexadienone, formed by a concerted six-ir-electron reorganization, is an intermediate. 

The nickel(II) dithiocarbamate complexes are neutral, water-insoluble, usually square-planar, species, and they have been studied extensively by a range of physical techniques. The usual methods for the synthesis of dithiocarbamate complexes have been employed in the case of Ni(II), Pd(II), and Pt(II). In addition, McCormick and co-workers (330,332) found that CS2 inserted into the Ni-N bonds of [Ni(aziri-dine)4P+, [Nilaziridinelgf, and [Ni(2-methylaziridine)4] to afford dithiocarbamate complexes. The diamagnetic products are probably planar, but they have properties typical of dithiocarbamate complexes, and IR- and electronic-spectral measurements suggested that they may be examples of N,S-, rather than S,S-, bonded dithiocarbamates. The S,S-bonded complexes are however, obtained, by a slow rearrangement in methanol. The optically active lV-alkyl-iV(a-phenethyl)dithio-carbamates of Ni(II), Pd(II), and Cu(II) (XXIV) have been synthesized, and the optical activity was found to be related to the anisotropy of the charge-transfer transitions (332). 

For the rearrangement 6 —> 7, a ir-coordinate transition state III, similar to -complexes of phosphinomethanides with lanthanide metal centers [5], may be envisaged (see below). 

Also, the Si-C bond formation is not favorable for steric reasons. Therefore, a unique rearrangement takes place to give 18 [9], probably again via a ir-complex IV, comparable to III. The molecular structure of 18 is shown in Figure 2. 

This picture is, however, incomplete because the dynamic nature of the amide bond is not taken into account. We have established through real-time IR spectroscopy that a rapid rearrangement of the hydroxyl-amide to the corresponding ester-amine (see Fig. 4) and vice versa allows a dynamic equilibrium between these two species which is strongly temperature dependent. Such a dynamic equilibrium has also been reported, albeit on a longer time-scale, for 4-hydroxyalkylamides [13]. 

The interaction of metal atoms with monoalkenes has been investigated on both a spectroscopic and preparative scale. It appears that the primary interaction between a metal atom and an alkene at low temperature is the formation of a ir-complex. This may subsequently lead to a thermally stable 7r-alkene complex or to rearrangement products by hydrogen abstraction or reaction with another alkene moiety, depending on the electronic requirements of the metal and the particular alkene considered. 

Pd° has also been found to catalyze thiono-thiolo allylic rearrangement of O-ally 1 phosphoro- and phosphono-thionates via a ir-allyl intermediate (equation 62).220... 

The insertion of 1,3-dienes into a ir-allylpalladium complex is believed to proceed via an intermediate in which the metal is complexed to the less hindered double bond of an unsymmetrical diene, followed by an electrocyclic rearrangement which links the more substituted allyl terminus with the more substituted alkene (equation 77).246-251 Electron-withdrawing substituents on the ir-allyl fragment generally increase the rate of insertion,248 whereas substituents on the diene generally slow the rate.268... 

The driving force for a cr-7r rearrangement can be understood therefore in terms of ligand field stabilization energy AE, the concept of hard and soft acids and bases, and preferred coordination number of the metal atom. Keeping these concepts in mind, specific types of cr-ir rearrangements may be discussed. 

Elimination of a hydride ion may also cause a cr-ir rearrangement (14), for example, the treatment of the o-ethyl complex (21) with triphenylmethylperchlorate produces the ir-ethylene complex (22) which... 

Recently, it has been discovered that catalysis by rhodium compounds is more effective than by the older cobalt catalyst when tris(triphenylphosphine)rhodium chloride is treated with carbon monoxide, the catalyst bis(triphenylphosphine)rhodium carbonyl chloride is formed. This catalyst is very effective under very mild conditions (49-51). It is believed that the tr-ir rearrangement is also important with this catalyst and operates in a manner analogous to that in the cobalt-catalyzed process, since stablization of the cr complex has been shown to lead to olefin isomerization and lower linear selectivity (52). 

A ir,o domino-Heck reaction under concomitant rearrangement of the tetracyclic allylcyclopropane endo,exo-bishomobarrelene has been reported 138 the stereoselective reaction proceeds via an intramolecular insertion of a primarily formed carbopallada-tion intermediate into a strained cyclopropane C-C cr-bond (Scheme 96). 

Alternatively, Pd(0) adds oxidatively to the double bond of a glycal derivative resulting in the formation of a ir-allyl complex, which may react with carbon nucleophiles to give C-glycosides with a double bond between C(2) and C(3).26 A rt-allyl complex may also be formed starting from a Ferrier rearrangement product (2,3-unsaturated sugar derivative).22... 

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