Transition metal allyl complexes with -

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.

Early transition metal allyl complexes have an enormous practical importance as either catalytic precursors or stoichiometric reagents in organic synthesis [103-108]. In the majority of the Group 4 complexes containing the allyl moiety, the metals exhibit the higher oxidation state (+4). Very few of these compounds are available in the literature with a +3 oxidation state, presumably because of their paramagnetic nature (reactivity) and difficulty in their handling. 

Allylmagnesium halide and lithio reagents are available from allyl halides by lithiation and Mg reaction techniques. These active reagents combine with binary or complex transition-metal halides to form transition-metal-allyl complexes, including both homoleptic and ligand-substituted tj -allyl complexes with substituted and unsubstituted allyl groups. 

A wide range of transition metal-allyl complexes are known to react with many types of nucleophiles. In most cases, these reactions occur between cationic allyl complexes and amines or stabilized, anionic carbon nucleophiles. The reaction typically occurs between the nucleophile and the form of the allyl complex, and attack usually occurs at the face of the allyl ligand opposite the metal. However, there are exceptions to these trends. For example, several experiments suggest that unstabilized carbon nucleophiles react first at the metal center, and C-C bond formation occurs between the alkyl and the allyl group by reductive elimination. In addition, a recent study has shown through deuterium labeling that attack of malonate anion on a molybdenum-allyl complex occurs with retention of configuration. ... 

Cycloaddition reactions of 18-electron transition metal ti -allyl complexes with unsaturated electrophiles to form five-membered rings have been extensively investigated. These transformations constituted a family of metal-assisted cycloaddition reactions in which the metal functions as an electron-donor center. These are typically two-step processes that involve the initial formation of a dipolar metal r) -alkene intermediate (2) and subsequent internal cyclization (equation 2). The most extensively investigated application of this methodology has been with dicarbonyl-ii -cyclopentadienyliron (Fp) complexes from the laboratory of Rosenblum. These (ri -allyl)Fp complexes are available either by metallation of allyl halides or tosylates with a Fp anion, or by deprotonation of (alkene)Fp cations. ... 

This chapter will focus on the nucleophilic addition reactions of transition metal vinylidenes and allyl complexes with Grignard reagents. Reactions with transition metal vinylidenes will be discussed initially, and then a brief review of allyl complexes with Grignard reagents will conclude the chapter. The synthesis and some general reactions of these vinylidene and allyl complexes will be presented. A more detailed description of the chemistry of these metal complexes can be found in the literature [1]. 

Besides the conventional methods, the metallo-carbene route to access cyclic compounds has become a versatile tool in sugar chemistry. Synthesis of stavudine 112, an antiviral nucleoside, from an allyl alcohol [101] is realized by a Mo(CO)5-mediated cyclization reaction (O Scheme 26). Molybdenum hexacarbonyl smoothly reacts with the triple bond of 113 to generate the intermediate Mo-carbene, which undergoes a clean cyclorearrangement to yield the furanoid glycal 114. Alkynol isomerization is effected by group-6 transition metal carbonyl complexes [102]. 

Reactions of transition metal a-allyl complexes with SO2 afford two types of... 

Quite a number of transition-metal complexes are capable of isomerizing allyUc alcohols into transition-metal-enol complexes through an internal redox process. Those can then be trapped in situ with aldehydes in an aldol reaction. Motherwell and coworkers developed a RhClIPPhjIj-catalyzed domino isomerization-aldol reaction of secondary allyl alcohols, which gave rise to a mixture of syn- and owti-aldol products, with only small amounts of the regioisomeric aldol product occasionally being formed (Scheme 8.25) [42]. 

There are also complexes containing polyhapto ligands with a more extended TT-system than that in T -allyl or -benzyl groups. Pentadienyl systems are the next higher anionic homolog of an allyl system. Pentadienyl systems are often formed by the addition of protons to, or removal of hydrides from, transition metal polyene complexes. Alternatively, pentadienyl groups have been generated by the addition of... 

Transition metal-benzyl complexes also react readily with nucleophiles, but the origin of this enhanced reactivity results from effects other than the typical rapid rate of nucleophilic attack at an electrophilic benzyUc carbon. As shown in Equation 11.17, benzyl groups can be bound in an fashion, much like an aUyl group. As presented later in this chapter, cationic iq -allyl complexes react with a variety of nucleophiles. iq -Benzyl groups are common in the chemistry of palladium(II), - and these ti -benzyl and phen-ethyl complexes react with a variety of nucleophiles. For example, these complexes react with malonate anions, and they have been shown to react with amines " during some recently developed hydroamination processes. These reactions occur with predominant inversion of configuration. ... 

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