Non-metathesis reactions

Besides alkene or alkyne metathesis, a broad range of other non-metathetical reactions promoted by NHC Ru complexes was reported in the literature. Some of them were discovered serendipitously, as they constituted side reactions in metathesis catalysis. Other were deliberately investigated and optimised. Despite the usefiilness of several of these processes, their significance has remained undervalued due to the huge appeal of olefin metathesis and related reactions. 

Non-metathetical reactions catalysed by ruthenium-carbene complexes are multifaceted and cover a broad range of transformations, thanks in part to the large number of oxidation states and coordination geometries available for the metal centre, in sharp contrast with other elements such as rhodium, palladium and platinum, which reluctantly form compounds with high oxidation states and have a strong preference for the square planar geometry. Several 

Building on these results, Hanessian and co-workers devised an efficient protocol for the isomerisation of terminal olefins with minimal self-dimerisation or cross-metathesis by employing methanol to generate hydride complex 33 in situ from its parent 10. The procedure was successfully applied to a variety of allylic compounds, including O- and A-allyl ethers, and cleanly afforded the corresponding propenyl species as EjZ isomeric mixtures, without any further isomerisation or conjugation in the cases of ketones, esters and lactams. 

Another promising isomerisation process, the atom-economical conversion of allylic alcohols into aldehydes or ketones, was reported in 2006 by Fekete and Joo. Reactions were carried out in aqueous/organic biphasic systems 

Following this seminal work, Arisawa and Nishida investigated the behaviour of A-tosyl diallyl amine in the presence of the second-generation Grubbs initiator (10) and different catalyst modifiers. Trimethyl(vinyloxy)silane afforded the best yields of cycloisomerised versus RCM product, allowing for the formation of diverse exo-methylene benzofurans and indolins via diene cycloisomerisation (Equation (7.7)). The method was also successfully applied to the synthesis of a novel indole alkaloid that displayed antifungal activity. As mentioned previously (see Section 7.3.2.2), it is known that in the presence of the vinyl trimethylsilyl ether, compound 10 decomposed into hydrido-car-bonyl complex 33, which was believed to be the actual catalyst for the cycloisomerisation process. 

---

The thiolate-figated catalysts (e.g., 3) developed by Jensen and coworkers also displayed good Z selectivity in homodimerization reactions. However, the addition of proton sponge (l,8-bis(dimethylamino)naphthalene) was frequently necessary to discourage non-metathesis reactions, such as olefin isomerization or walking. Overall, Z selectivity also depended heavily on the substrate and the reaction conditions used [21]. 

From the foregoing, however, it should not be concluded that the approach of Mango and Schachtschneider is appropriate for the understanding of the metathesis reaction. The main difficulty is the supposition that the metathesis is a concerted reaction. If the reaction is not concerted, it makes no sense, of course, to correlate directly the orbitals of the reactants with those of the products. Recently, non-concertedness has been proved probable for several similar reactions, which were formerly believed to be concerted. For instance, Cassar et al. (84) demonstrated that the Rh catalyzed valence isomerization of cubane to sj/w-tricyclooctadiene proceeds stepwise. They concluded that a metallocyclic intermediate is formed via an oxidative addition mechanism ... 

An interesting way to control the stereoselectivity of metathesis-reactions is by intramolecular H-bonding between the chlorine ligands at the Ru-centre and an OH-moiety in the substrate [167]. With this concept and enantiomerically enriched allylic alcohols as substrates, the use of an achiral Ru-NHC complex can result in high diastereoselectivities like in the ROCM of 111-112 (Scheme 3.18). If non-H-bonding substrates are used, the selectivity not only decreases but proceeds in the opposite sense (product 113 and 114). 

Like styrene, acrylonitrile is a non-nucleophilic alkene which can stabilise the electron-rich molybdenum-carbon bond and therefore the cross-/self-metathe-sis selectivity was similarly dependent on the nucleophilicity of the second alkene [metallacycle 10 versus 12, see Scheme 2 (replace Ar with CN)]. A notable difference between the styrene and acrylonitrile cross-metathesis reactions is the reversal in stereochemistry observed, with the cis isomer dominating (3 1— 9 1) in the nitrile products. In general, the greater the steric bulk of the alkyl-substituted alkene, the higher the trans cis ratio in the product (Eq. 11). 

See Sodium azide See METAL NON-METALLIDES METAL HALIDES METATHESIS REACTIONS... 

As will be discussed more thoroughly in Section 3.2.5, transition metal carbene complexes can mediate olefin metathesis. Because heteroatom-substituted carbene complexes are usually less reactive towards olefins than the corresponding nonheteroatom-substituted complexes, it is, e.g., possible to use enol ethers to terminate living polymerization or other types of metathesis reaction catalyzed by a non-heteroatom-substituted carbene complex. Olefin metathesis can also be used to prepare new heteroatom-substituted carbene complexes (Figure 2.15, Table 2.11). 

Since radical and atom metathesis reactions generally have low activation energies, their rate coefficients are expected to exhibit non-Arrhenius behavior because of the increased importance of the term noted previously. In Fig. 9, rate coefficient data for H-atom abstractions from CH4... 

Olefins can be divided into four categories on the basis of their propensity to homodimerize (Figure 2). Type I olefins are able to undergo rapid homodimerization and whose homodimers can equally participate in CM. A CM reaction between two olefins of this type will generally result in a statistical product mixture. Type II olefins homodimerize slowly, and, unlike type I olefins, their homodimers can only be consumed with difficulty in subsequent metathesis reactions. Type III olefins are unable to undergo homodimerization, but have the capacity to undergo CM with either type I or II olefins. As with type I olefins, the reaction between either two type II or type III olefins should result in non-selective CM. Type IV olefins are inert to olefin CM, but do not inhibit the reaction therefore, they can be regarded as spectators to CM. 

If the reaction is non-electron-transfer, then treat it as a metathesis reaction. 

Interestingly, the complexes CpCp Hf(SiHPhSiHPhSiH2Ph)Cl and CpCp Hf(SiHPh SiH2Ph)Cl are observed to form concurrently with the small polysilanes. It seems that these complexes are not necessarily the intermediates in the dehydrogenative process as suggested in Scheme 6, but perhaps only side products formed via non-productive cr-bond metathesis reactions (equation ll)22. 

The alkene metathesis reaction was unprecedented - such a non-catalysed concerted four-centred process is forbidden by the Woodward-Hoffmann rules - so new mechanisms were needed to account for the products. Experiments by Pettit showed that free cyclobutane itself was not involved it was not converted to ethylene (<3%) under the reaction condition where ethylene underwent degenerate metathesis (>35%, indicated by experiments involving Di-ethylene) [10]. Consequently, direct interconversion of the alkenes, via an intermediate complex (termed a quasi-cyclobutane , pseudo-cyclobutane or adsorbed cyclobutane ) generated from a bis-alkene complex was proposed, and a detailed molecular orbital description was presented to show how the orbital symmetry issue could be avoided, Scheme 12.14 (upper pathway) [10]. 

Metal cyanides(and cyano complexes), 216 Metal derivatives of organofluorine compounds, 217 IV-Metal derivatives, 218 Metal dusts, 220 Metal fires, 222 Metal fulminates, 222 Metal halides, 222 Metal—halocarbon incidents, 225 Metal halogenates, 226 Metal hydrazides, 226 Metal hydrides, 226 Metal hypochlorites, 228 Metallurgical sample preparation, 228 Metal nitrates, 229 Metal nitrites, 231 Metal nitrophenoxides, 232 Metal non-metallides, 232 Metal oxalates, 233 Metal oxides, 234 Metal oxohalogenates, 236 Metal oxometallates, 236 Metal oxonon-metallates, 237 Metal perchlorates, 238 Metal peroxides, 239 Metal peroxomolybdates, 240 Metal phosphinates, 240 Metal phosphorus trisulfides, 240 Metal picramates, 241 Metal pnictides, 241 Metal polyhalohalogenates, 241 Metal pyruvate nitrophenylhydrazones, 241 Metals, 242 Metal salicylates, 243 Metal salts, 243 Metal sulfates, 244 Metal sulfides, 244 Metal thiocyanates, 246 Metathesis reactions, 246 Microwave oven heating, 246 Mild steel, 247 Milk powder, 248... 

Reetz and coworkers tested catalysts for different reactions such as enantiose-lective acylation of a chiral secondary alcohol by lipases, the enantioselective ring opening of epoxides to non-racemic diols, and metathesis reactions [11, 12]. The two first examples are exothermic reactions and catalyst activity is revealed by hot spots in the IR image. The catalytic performance found by use of time-resolved IR-thermography correlated well with already known activity of the tested catalysts [11]. The metathesis reaction is particularly interesting, because it is the first example of the monitoring of endothermic reactions by means of an IR camera [12]. 

Both the intramolecular and the intermolecular secondary metathesis reactions affect the polymerisation kinetics by decreasing the rate of polymerisation, because a fraction of the active sites that should be available as propagation species are involved in these non-productive metathesis reactions. The kinetics of polymerisation in the presence of metal alkyl-activated and related catalysts shows in some cases a tendency towards retardation, again due to gradual catalyst deactivation [123]. Moreover, several other specific reactions can influence the polymerisation. Among them, the addition of carbene species to an olefinic double bond, resulting in the formation of cyclopropane derivatives [108], and metallacycle decomposition via reductive elimination of cyclopropane [109] deserve attention. 

Molecular models suggest that such a P -connection between remote alkenyl residues in a dimer is possible. To check whether it really occurs under the conditions of the metathesis reaction, we synthesized the monoloop tetra-urea compounds 16, in which one of the non-cyclic urea residues is substituted by a bulky group which cannot penetrate the loop (Scheme 5.18). (The synthesis is analogous to that of 15, introducing in the final two steps the bulky residue first.). 

Cubic boron nitride is an important materia that is widely used in cutting tools and as grinding, abrasive materials. Both Hu et al. [8] and Cui et al. [9] have synthesized cubic BN via this method. By using the solvothermal metathesis reaction of BBrs and LisN, Cui and co-workers obtained better yield of cubic BN, and the TEM image and the XRD pattern are shown in Fig. 3. Some other metastable non-oxides have also been prepared and reported using solvothermal method, e.g. AIN [10-11] and Si3N4 [12]. 

Microwave-assisted reactions have become well established in contemporary synthetic methodology. Non thermal microwave effects, though, have been shown not to be a factor in the observed rate enhancement with the ring-closing metathesis reaction to form the azepine derivative 2 (88% coversion 20 minutes, 100 °C) from the acyclic diene 1 precursor with the ruthenium catalyst 3 <03JOC9136>. 

The non-classical divalent lanthanide complexes have stronger reducing power than divalent samarium complexes because of their higher reduction potentials. Dinitrogen is not an inert atmosphere for these non-classical divalent lanthanide complexes. Therefore, attempts to prepare non-classical divalent organolanthanide complexes by metathesis reactions in dinitrogen atmosphere have been unsuccessful, and the dinitrogen-activated products were isolated. A typical example is shown in Equation 8.36 [112]. 

---