Direct metathesis

Et2NCS)3Mo(NS)], but this reaction s not general. An alternative to direct metathesis with [SN]+ is dissociative oxidative addition (e.g. [MCl2(PPh3)2] + 1/3(S3N3C13) -->... 

The two-electron reduction of a-diimines to prepare the required starting materials for a subsequent condensation is usually achieved by reaction with lithium but other alkaline (Na) or alkaline earth (Mg) metals should be useful as well. The synthesis of the heterocycles 10 is either accomplished by direct metathesis of the formed metal enediamide with PC132 [19] or, alternatively, by quenching the diamide with a suitable acid to produce an enediamine or a-aminoimine, respectively, and subsequent base-induced condensation with PC13 or RPC12 [18, 20] (Scheme 3). 1,3-l )i-/e/t-butyl-2-chloro-1,3,2-diazaphospholene was also prepared from the reaction... 

Bimolecular reactions may be classified into two major groups direct metathesis and association reactions. The latter also are related to the reverse of the unimolecular reactions discussed above. However, as we shall see, significant differences would exist when there are energy transfer limitations. It is also convenient further to classify bimolecular reactions as radical-molecule, radical-radical, and molecule-molecule reactions. The application of TST to bimol ular reactions, described symbolically by... 

Direct metathesis reactions are characterized by tight transition states and involve the transfer of atoms or radicals as a consequence of the close proximity of the two reactants, and they are the only type of reactions that are not subject to becoming energy-transfer-limited at high temperatures and low pressures. A typical energy diagram for metathesis reactions is shown in Fig. 8c. Examples of such reactions include... 

Another process mechanistically related to imine exchange is the dynamic production of pyrazolotriazinones reported in 2005 by Wipf and coworkers [29]. After first verifying that reaction of either 16 or 17 with equimolar quantifies of isobutyraldehyde and hydrocinnamaldehyde at 40°C in water (pH 4.0) resulted in the same 3 7 mixture of 16 and 17 at equilibrium (Fig. 1.6, Eq. 1), the authors demonstrated that a library could be generated by reaction of pyrazolotriazinone 16 with a series of aldehydes (Fig. 1.6, Eq. 2). Direct metathesis of pyrazolotriazinones was also demonstrated, as was reaction with ketones. Importantly, equilibration was halted by raising the pH to 7. 

Two types of side reaction have been noted. The catalyst system WCVMe4Sn generates a certain amount of HCl, which adds to terminal double bonds to the extent of about 10% of the metathesis reaction (Baker 1977). The catalyst system WCl6/Et3B causes some double-bond shift in the substrate prior to metathesis with methyl undec-lO-enoate (10) a series of products of the type MeOCO (CH2)gCH=CH(CH2) COOMe are formed, the yield decreasing as n decreases from 8 (the direct metathesis product) to 7, 6, and 5 (Nakamura, R. 1976a). 

The direct metathesis polymerization of acetylenes is not the only route to polyacetylenes using olefin metathesis chemistry. Below are summarized some of the other methods that have been developed in recent years. 

Synthesis. Diphenyliodonium hexafluorophosphate, Ph2lPF6, and triphenylsulfonium hexafluoroantimonate, PhaSSbFe, were synthesized using literature procedures (25). Tetra-n-butylammonium hexafluorophosphate, TBAPFe, was obtained by direct metathesis between tetra-n-butylammonium bromide (99%) and potassium hexafluorophosphate (tech.), KPFe. 

A number of synthetic routes have been reported to generate these species and their tetrahydrofuran (THF) adducts, including direct metathesis with alkali metal salts [50,52-55], or transmetallation with Be(T) -C5H5)2 or Mg(ri -C5Hs)2 [56-62]. In addition, the trivalent compounds may be obtained from chemical [53] or photochemical [63,64] reduction of suitable tetravalent actinide precursors [51,65-67]. Examples of these preparations are given in Eqs. 2-5 ... 

Moreover, binding through a methylene tether the arene ligand with the dihydroimidazolm-2-ylidene unit, in a half-sandwieh mode, led to the r[ Ti -arene-carbene Ru-allenylidene eomplex 55, whieh was highly aetive in RCM of 1,6-dienes. This eomplex allowed to seleetively direct metathesis of dienes either towards RCM or cycloisomerization products. 

As in the case of other sulfonium salts, the above simple halide-containing salts are not directly suitable for use as photoinitiators because of the tendency of these anions to act as terminating agents n in cationic polymerizations. Direct metathesis of the sulfonium halides with the appropriate acid or alkali metal salt of such weakly nucleophilic anions as PF, and SbFg, and AsFg" suffices to convert them to the active photoinitiators. Tables 6 and 7 give the structures and UV absorption characteristics of some representative dialkylphenacylsulfonium and dialkyl-4-hydroxyphenyl-... 

---