Allyl hapticity

A change in the allyl hapticity (q3 to p1 slippage) leading to the less substituted titanium-carbon o bond accounts for the observed y-regioselectivity. The anti diastereoselectivity stems from a pseudo-equatorial orientation of the aldehyde group. The diastereoselectivity of the reaction can be reversed through the use of a more coordinating cosolvent such as HMPA (Scheme 13.7) [14]. This reversal of anti to syn diastereoselectivity can be rationalized in terms of an open transition state. 

The geometries in Figs. 4.86 and 4.87 suggest an important distinction in the multicenter hapticity character of ligand attachment to the metal atom. Hapticity refers to the number of atoms in a ligand that are coordinated to the metal. In the Ir+ diammine complex (Fig. 4.86(a)), the metal attaches to each of two nN donor lone pairs in simple monohapto (one-center, q1) fashion. However, in the Ir+ complexes with HCCH or CML the metal attaches to the face of the pi bond or three-center allylic pi system in dihapto (two-center, r 2) or trihapto (three-center, q3) fashion, respectively. The hapticity label q" therefore conveniently denotes the delocalized n -center character of the donated electron pair(s) and the geometry of the resulting coordination complex. 

The effects of altered hapticity can be seen by comparing the q3, four-electron allyl complex of Ir+ (Fig. 4.86(c)) with the corresponding complex of Au+ (Fig. 4.87(c)). Because Au+ requires only two electrons to complete its formal duodectet, the metal-allyl complexation now involves only the anionic nc center of a localized allylic H2C—CH=CH2 moiety, and the hapticity slips from q3 to q1 ... 

In other words, the gold complex is of formal MX type. Thus, the NBO description of the bonding goes considerably beyond the hapticity labels in (4.110) and suggests that only a severely distorted and reorganized form of the allyl anion exists in either of these complexes. 

It is very well known that jr-allyl palladium complex 1, which is a key intermediate for the Tsuji-Trost type allylation, has an electrophilic character and reacts with nucleophiles to afford the corresponding allylation products. We discovered that bis 7r-allyl palladium complex 2 is nucleophilic and reacts with electophiles such as aldehydes [27] and imines [28-32] (Scheme 2, Structure 2). We have also shown that bis 7r-allyl palladium complex 2 can act as an amphiphilic catalytic allylating agent it reacts with both nucleophilic and electrophilic carbons at once to produce double allylation products [33]. These complexes incorporate two allyl moieties that can bind with different hapticity to palladium (Scheme 3). The different complexes may interconvert by ligand coordination. The complexes 2a, 2b and 2c are called as r]3,r]3-bisallypalladium complex (also called bis-jr-allylpalladium complex), r)l,r)3-bis(allyl)palladium complex, -bis(allyl)palladium complex, respectively. Bis zr-allyl palladium complex 2 can easily be generated by reaction of mono-allylpalladium complex 1 and allylmetal species 3 (Scheme 4) [33-36]. Because of the unique catalytic activities of the bis zr-allyl palladium complex 2, a number of interesting cascade reactions appeared in the literature. The subject of the present chapter is to review some recent synthetic and mechanistic aspects of the interesting palladium catalyzed cascade reactions which in-... 

A recent report has shown that tris(pyrazolyl)borate (Tp) (see Tris(pyrazolyl)borates) complexes of Ni tend to form 5-coordinate, formally 18-electron complexes. The hapticity of the tris(pyrazolyl)borate ligand in these complexes appears to be more sensitive to the steric environment around the Ni center as opposed to the electronic factors (i.e. formal electron count of the complex). Thus, the -allyl derivative of tris (3,5-i-Pr2-pyrazolyl)borate NiR (3) displays a -coordination, whereas the -prenyl derivative (4) adopts a /c -coordination... 

Hydride abstraction by Ph3C+ also occurs on the rf-allyl-Mo complex103 shown in equation 8.74, again causing an increase in hapticity of one unit. 

Thus, the amount of phosphane added can be used to markedly influence the product distribution when 2-substituted methylenecyclopropanes are used as substrates. A possible explanation of this observation might lie in the ability of a strong donor ligand, such as an alkylphosphane, to prevent a hapticity change of the ring-cleaved methylenecyclopropane from 1/ -coordination (allyl-type) to 1/ -coordination (trimethylenemethane-type), which is indispensable for induction of the scrambling reaction. 

Reports on arene complexes and interactions of Pd with aryl tr-electron or, in general, conjugated 7r-electron density appear only occasionally. The reports in the period 1993-2004, although scarce, have representation of the formal oxidation states 0, I, and ii for palladium, and hapticities from r] - to rf-zcene. It should be noted, however, that the assignment of oxidation state or hapticity is not clear-cut in some systems. Many of the interactions reported are with pendant aryl groups of ligands already coordinated to Pd. The interactions in the form 77 -benzyl are dealt with in the section on allyl complexes. 

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