Organometallic transformations

It is clear that one of the most valuable aspects of the chemistry of vinylketene complexes is that the ligands in question are more often than not extremely reactive in their uncomplexed state. Use of the complexed vinylketene allows easy access to a class of synthon that would be at best difficult to handle, and at worst unobtainable. As we have seen, a wide variety of organic fragments are available from the reactions of such complexed species. The relative stability of many of the vinylketene complexes has also allowed their isolation from reaction mixtures, leading to insights into the mechanisms of various organic and organometallic transformations. 

In the following paragraphs the general strategies used in organometallic transformations of imidazoles to prepared histaminergic ligands will be reviewed. 

These descriptions, while helpful, are by their nature noncomprehensive and there are many exceptions if such definitions are taken too literally. The problem may be linked to the definition of organometallic chemistry as the chemistry of compounds with metal-to-carbon bonds . This immediately rules out Wilkinson s compound, RhCl(PPh3)3, for example, which is one of the most important industrial catalysts for organometallic transformations known in the field. Indeed, it is often the objectives and thought processes of the chemist undertaking the work, as much as the work itself, which determine its field. Work in modern supramolecular chemistry encompasses not just host-guest systems but also molecular devices and machines, molecular recognition, so called self-processes ... 

As can be seen from Scheme III, lanthanide halides are suitable precursors for the synthesis of homoleptic derivatives such as silylamides [114], cyclopen-tadienyls [115] and aryloxides [116]. Such organometallies can be readily obtained in a pure form by sublimating them from the reaction mixture. They themselves are important precursors in organometallic transformations (vide infra). Heteroleptic complexes of the type CpxLn(halide)y (x + y = 2,3) are important synthetic precursors with respect to formation of various Ln-X bonds via simple metathesis reactions [2-29]. Fig. 4 indicates the lanthanide element bonds which are involved in these ubiquitous heteroleptic cyclopentadienyl systems. 

In order to facilitate the presentation, Scheme 12 is included to illustrate how A, B, and C are interrelated as critical intermediates in the conversion of (>/5-hexadienyl )Fe(CO)3+, with the ligand originally bound to iron in a cis configuration, to the all-trans hexadiene ligand present in the final nucleophilic adduct by a series of standard organometallic transformations... 

Terminal alkyne complexes of zirconium, while challenging to isolate, have been implicated in a number of organometallic transformations. Mixing an alkenyl zirconocene 218 with a transient dialkyl zirconocene 111 furnishes a yu-acetylide complex 219 (Scheme 33).112 This reaction is believed to proceed by initial transmetallation to form a zirconocene alkenyl alkyl 220, which undergoes subsequent /3-hydrogen abstraction to generate the terminal alkyne complex 221. This proposed intermediate can be trapped with PMe3 222 from the alkylation of the zirconocene alkenyl bromide with butyllithium. Comparison of the spectroscopic features of this product to the... 

The remainder of this section will focus on true SBMs, which have been the subject of vigorous research. Despite the electron deficiency of early transition metal, lanthanide, and actinide complexes, several groups reported that some of these d f" complexes do react with the H-H bond from dihydrogen and C-H bonds from alkanes, alkenes, arenes, and alkynes in a type of exchange reaction shown in equation 11.32. So many examples of SBM involving early, middle, and late transition metal complexes have appeared in the chemical literature over the past 20 years that chemists now consider this reaction to be another fundamental type of organometallic transformation along with oxidative addition, reductive elimination, and others that we have already discussed. 

The left-hand dicarbonyl complex is a singlet, whereas the monocarbonyl derivative is a triplet, so the process is clearly spin-forbidden. Despite this, the authors observed the equilibrium between the two to be rapid, and they concluded It seems thus implausible that changes in spin state should slow organometallic transformations as a general rule. ... 

Spin-crossing effects can dramatically affect reaction mechanisms, rate constants, branching ratios, and temperature behaviors of organometallic transformations. 

Lanthanide halides, nitrates and triflates are not only common reagents in organic synthesis (Fig. 1) but also represent, in dehydrated form, key precursor compounds for the more reactive organometallics (Scheme 2). As a rule, in compounds of strong monobasic acids or even superacids, cation solvation competes with anion complexation, which is revealed by fully or partially separated anions and solvated cations in their solid state structures. The tendency to form outer sphere complexation in coordinating solvents [47] is used as a criterion of the reactivity of inorganic salt precursors in organometallic transformations. 

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