Allyl complexes fluxionality

The structures of jj3-allyl complexes of platinum have been fully summarized by Hartley in Chapter 39 of Comprehensive Organometallic Chemistry . This article also details the NMR methods used to investigate fluxionality of the r/3-allyl ligand. 

Scheme 7.15] or S -type mechanism [Equation (7.9)]. Depending on the nature of the nucleophile and catalyst employed, the subsequent nucleophilic substitution of the metal can follow either via a-elimination [path A, Equations (7.8) and (7.9), Scheme 7.15], via an SN2 reaction (path B) or via an SN2 -type reaction (path C). For reasons of clarity, only strictly concerted and stereospecific SN2- or SN2 -anti-type mechanistic scenarios are shown in Scheme 7.15. The situation might, however, be complicated if, e.g., the initial S l -anti ionization event is competing with an Sn2 -syn reaction. Erosion in stereo- and regioselectivity can be the result of these competing reactions. Furthermore, fluxional intermediates such as 7t-allyl Fe complexes are not shown in Scheme 7.15 for reasons of clarity. These intermediates are known for a variety of late transition metal allyl complexes and will be referred to later. Moreover, apart from these ionic mechanisms, radicals might also be involved in the reaction. So far no distinct mechanistic study on allylic substitutions has been published.

Allyl complexes are characteristically fluxional, the principal pathway being the rf-rf-rf process shown below. 

Variable temperature H NMR spectroscopy of a series of Sc and Y metallocene allyl complexes indicates flux-ional (see Fluxional Molecule and Structure Property Maps for Inorganic Solids) behavior involving C-C... 

K. Vrieze, Fluxional allyl complexes, in Dynamic Nuclear Magnetic Resonance Spectroscopy (Eds. L. M. Jackman, F. A. Cotton), Academic Press, New York, 1975. 

Many ij -allyl complexes are fluxional (p. 914) at room temperature or slightly above. 

The complex (328) has been obtained from /rans-cyanohalogeno-bis-(triphenylphosphine) Pt(II) by halide displacement with azide ion followed by cycloaddition of acetonitrile.619 Above 150°C fluxional exchange of the coordinated N atom around the tetrazole ring was observed.619 New dimeric zr-allyl complexes of Pd and Pt with tetrazolate groups as bridging ligands bonded at the 2-N and 3-N atoms (329) have... 

The dominant mode of fluxional behavior in 7) -allyl complexes involves a 17 -> r/ V interconversion (47, 49). These features have been reviewed by Vrieze (706) and are discussed in this volume by Tsutsui and Courtney. The formation of the rj -intermediate provides a pathway for interchanging syn and anti substituents and inverting stereochemistry in chiral allyl complexes (49). 

Temperature-dependent NMR studies have shown that the uranium allyl complex is fluxional. This fluxionality has been proposed as a o- ... 

Fluxionality in allyl complexes is a widespread phenomenon. The 77 -allyl complexes [Ni(i7 -allyl)(aryl)(diphos)] (10) are nonrigid 18-electron species which exhibit syn-anti proton exchange and, in the case of the complex (R = Me, R = H), rapid site exchange of the two allylic termini. The syn-anti exchange is thought to occur via a short-lived Ty -allyl intermediate (11). ... 

A bis(Pentamethylcyclopentadienyl) lutetium allyl and (C5Mc5)2LuCH2-CMe=CH2 are found to be among the final products of the decomposition of (C5Me5)2LuCH2CHMe2 as shown by Watson and Roe (1982). The NMR spectrum of the allyl complex shows 5CH2 at 68 and 5CH at 163 ppm, proving a fluxional a-allyl structure. 

At higher temperatures, allyl complexes are no longer rigid they become dynamic, fluxional compounds. Therefore, AM2X2 spectra become AX4 spectra. Temperatures of such transitions for various compounds range from 200 to 450 K. The hafnium allyl compound [Hf( / -C3H5)4] exhibits an AX4-type spectrum even at 199 K. For dynamic... 

In the n -C3H5 complexes, fluxional behavior involves movement of the metal from one end of the allyl ligand to the other. In t -C3H3 systems, the syn protons (Hj, H ) and anti protons (Hj, H3) may interchange and the C and C3 may be observed to interchange if other ligands make the two sides of the complex different. The bonding and fluxional aspects of these systems have been reviewed. 

Nucleophilic attack to palladium is one of the important reactions that Pd 77 -allylic complexes undergo. Ligand-substitution reactions and metathesis of the auxiliary ligands lead to new TT-allyl complexes, and they are a valuable and common synthetic route for a large number of compounds. These reactions have been collected in Section 8.06.6.2.1. The fluxional behaviour of Pd 77 -allyls often involves nucleophilic attack to palladium, and some examples have been discussed in Section 8.06.6.2.3. 

With Tj -allyl complexes the two major types of fluxionality operating are allyl rotation about the M-allyl axis and interconversion. 

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