Metathesis nucleophilic attack

Osborn and Green s elegant results are instructive, but their relevance to metathesis must be qualified. Until actual catalytic activity with the respective complexes is demonstrated, it remains uncertain whether this chemistry indeed relates to olefin metathesis. With this qualification in mind, their work in concert is pioneering as it provides the initial experimental backing for a basic reaction wherein an olefin and a metal exclusively may produce the initiating carbene-metal complex by a simple sequence of 7r-complexation followed by a hydride shift, thus forming a 77-allyl-metal hydride entity which then rearranges into a metallocyclobutane via a nucleophilic attack of the hydride on the central atom of the 7r-allyl species ... 

Besides nucleophilic attack, cyclative cleavage can be eaffected for example by Stille reactions [120], Wittig olefmation reactions [177], Wittig-Horner [178, 179] or metathesis reactions [180-183]. For more details of C-C-bond formation, see Section 3.3.2. 

In these cases also, initial nucleophilic attack on the carbonyl group seems probable. The least stable of these derivatives is [Rh6(CO)i5Cl]-from which the chloride ion can be readily displaced by metathesis with other anions such as SCN- (45) ... 

A similar, but shorter sequence was employed in a synthesis of dihydroxanthatin, with the addition of a copper(II) salt to make the process catalytic in palladium (Scheme 6.33). ° The (3-hydride elimination terminates a sequence that includes a nucleophilic attack and an intramolecular insertion. The alkene insertion from the first t -complex 6.98 to the second t -complex 6.99 proceeds with very high diastereoselectivity. Further aspects of this synthesis, employing metathesis chemistry, can be found in Scheme 8.113. 

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. 

AT-heterocyclic carbenes show a pure donor nature. Comparing them to other monodentate ligands such as phosphines and amines on several metal-carbonyl complexes showed the significantly increased donor capacity relative to phosphines, even to trialkylphosphines, while the 7r-acceptor capability of the NHCs is in the order of those of nitriles and pyridine [29]. This was used to synthesize the metathesis catalysts discussed in the next section. Experimental evidence comes from the fact that it has been shown for several metals that an exchange of phosphines versus NHCs proceeds rapidly and without the need of an excess quantity of the NHC. X-ray structures of the NHC complexes show exceptionally long metal-carbon bonds indicating a different type of bond compared to the Schrock-type carbene double bond. As a result, the reactivity of these NHC complexes is also unique. They are relatively resistant towards an attack by nucleophiles and electrophiles at the divalent carbon atom. 

Attacking the problem of cross-metathesis selectivity from a different angle, Crowe and co-workers explored the reactivity of a more nucleophilic partner for the Ji-substituted alkenes. They chose to use allyltrimethylsilane since they proposed that the CH2SiMe3 substituent should have a negligible effect on alkyli-dene stability, but enhance the nucleophilicity of the alkene via the silicon P-ef-fect (Fig. 1). 

Dihydropyrroles have recently become readily available by ring-closing metathesis. For this purpose, N-acylated or N-sulfonylated bis(allyl)amines are treated with catalytic amounts of a ruthenium carbene complex, whereupon cyclization to the dihydropyrrole occurs (Entries 6 and 7, Table 15.3 [30,31]). Catalysis by carbene complexes is most efficient in aprotic, non-nucleophilic solvents, and can also be conducted on hydrophobic supports such as cross-linked polystyrene. Free amines or other soft nucleophiles might, however, compete with the alkene for electrophilic attack by the catalyst, and should therefore be avoided. 

Several reactions in organometaUic chemistry also appear to contravene the rule, but which can be explained in a somewhat similar way. Hydrometallation [5.45, see (Section 5.1.3.4) page 162], carbometallation, metallo-metallation, and olefin metathesis reactions are all stereospecifically suprafacial [2 + 2] additions to an alkene or alkyne, for which the all-suprafacial pathway is forbidden. Hydroboration, for example, begins with electrophilic attack by the boron atom, but it is not fully stepwise, because electron-donating substituents on the alkene do not speed up the reaction as much as they do when alkenes are attacked by electrophiles. Nevertheless, the reaction is stereospecifically syn—there must be some hydride delivery more or less concerted with the electrophilic attack. The empty p orbital on the boron is the electrophilic site and the s orbital of the hydrogen atom is the nucleophilic site. These orbitals are orthogonal, and so the addition 6.126 is not pericyclic. 

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