Olefin activation reactions

Like NR, SBR is an unsaturated hydrocarbon polymer. Hence unvulcanised compounds will dissolve in most hydrocarbon solvents and other liquids of similar solubility parameter, whilst vulcanised stocks will swell extensively. Both materials will also undergo many olefinic-type reactions such as oxidation, ozone attack, halogenation, hydrohalogenation and so on, although the activity and detailed reactions differ because of the presence of the adjacent methyl group to the double bond in the natural rubber molecule. Both rubbers may be reinforced by carbon black and neither can be classed as heat-resisting rubbers. 

In treating cracked stocks such as steam cracked naphtha or visbreaker naphtha, which are highly olefinic in nature, nickel molybdate or nickel tungstate catalysts are generally employed. These catalysts have much higher activity for olefin samration reactions than does cobalt molybdate. 

Although olefin metathesis had soon after its discovery attracted considerable interest in industrial chemistry, polymer chemistry and, due to the fact that transition metal carbene species are involved, organometallic chemistry, the reaction was hardly used in organic synthesis for many years. This situation changed when the first structurally defined and stable carbene complexes with high activity in olefin metathesis reactions were described in the late 1980s and early 1990s. A selection of precatalysts discovered in this period and representative applications are summarized in Table 1. 

Hexacarbonyldicobalt complexes of alkynes have served as substrates in a variety of olefin metathesis reactions. There are several reasons for complex-ing an alkyne functionality prior to the metathesis step [ 125] (a) the alkyne may chelate the ruthenium center, leading to inhibition of the catalytically active species [125d] (b) the alkyne may participate in the metathesis reaction, giving undesired enyne metathesis products [125f] (c) the linear structure of the alkyne may prevent cyclization reactions due to steric reasons [125a-d] and (d) the hexacarbonylcobalt moiety can be used for further transformations [125c,f]. 

Oxalamidinate anions represent the most simple type of bis(amidinate) ligands in which two amidinate units are directly connected via a central C-C bond. Oxalamidinate complexes of d-transition metals have recently received increasing attention for their efficient catalytic activity in olefin polymerization reactions. Almost all the oxalamidinate ligands have been synthesized by deprotonation of the corresponding oxalic amidines [pathway (a) in Scheme 190]. More recently, it was found that carbodiimides, RN = C=NR, can be reductively coupled with metallic lithium into the oxalamidinate dianions [(RN)2C-C(NR)2] [route (c)J which are clearly useful for the preparation of dinuclear oxalamidinate complexes. The lithium complex obtained this way from N,N -di(p-tolyl)carbodiimide was crystallized from pyridine/pentane and... 

We have recently reported that the addition of Ni results in a promotion of the isomerization activity of sulfated zirconia [10] comparable to that obtained by the addition of Fe and Mn. It has been previously observed that the presence of H2 causes a decrease in isomerization activity, a result consistent with the mechanism that involves olefins as reaction intermediates Here, we... 

Although N-(2-phenylethyl)morpholine is formed in only 14% yield (TOE = 0.3 h ), this is the first example of a transition metal-catalyzed anti-Markovnikov hydroamination of a non-activated olefin. Concerning the reaction mechanism, labeling experiments led the authors to favor activation of the N-H bond over olefin activation [166]. 

Samuel Danishefsky s group at the Sloan Kettering Institute for Cancer Research in New York has also been active in the synthesis of the natural epothilones and biologically active analogs. One of their syntheses also used the olefin metathesis reaction (not shown). The synthesis in Scheme 13.62 used an alternative approach to create the macrocycle, as indicated in the retrosynthetic scheme. The stereochemistry at C(6), C(7), and C(8) was established by a TiCl4-mediated cyclocondensation (Step A). The thiazole-containing side chain was created by reaction sequences F and G. The... 

The formation of these compounds has been rationalized according to Scheme 6. The reaction of Os (E )-CH=C 11 Ph C1 (C())( P Pr3)2 with n-BuLi involves replacement of the chloride anion by a butyl group to afford the intermediate Os (/i> CH=CHPh ( -Bu)(CO)(P Pr3)2, which by subsequent hydrogen (3 elimination gives OsH ( >CI I=CHPh (CO)( P Pr3)2. The intramolecular reductive elimination of styrene from this compound followed by the C—H activation of the o-aryl proton leads to the hydride-aryl species via the styrene-osmium(O) intermediate Os r 2-CH2=CHPh (CO)(P Pr3)2. In spite of the fact that the hydride-aryl complex is the only species detected in solution, the formation of OsH ( )-CH=CHPh L(CO)(P Pr3)2 and 0s ( )-CH=CHPh (K2-02CH)(C0)(P,Pr3)2 suggests that in solution the hydride-aryl complex is in equilibrium with undetectable concentrations of OsH ( )-CH=CHPh (CO)(P,Pr3)2. This implies that the olehn-osmium(O) intermediate is easily accessible and can give rise to activation reactions at both the olefinic and the ortho phenyl C—H bonds of the... 

Reactions leading to the formation of the catalytically active nickel hydride species from organonickel precursors (Section III) can be regarded as model reactions for olefin oligomerization reactions. The reactions described by Eq. (8) and Scheme 3 (Section III) show that RNiX compounds (R = methyl orallyl, X = halide or acetylacetonate) activated by Lewis acids add to double bonds under mild reaction conditions (-40° or 0°C). It follows further from these reactions that under conditions leading to olefin dimerization a rapid nickel hydride /3-hydrogen elimination reaction occurs. The fact that products resulting from olefin insertion into the nickel-carbon bond are only observed when /3-hydride... 

Rhodium(n) carboxamidates are clearly superior to all other types of catalysts in effecting highly chemo-, regio-, diastereo-, and enantioselective intramolecular C-H activation reactions of carbenoids derived from diazoacetates. Specifically, Rh2(4Y-MPPIM)4 is the catalyst of choice for C-H activation reactions of simple primary and secondary alkyl diazoacetates. Likewise, Rh2(4Y-MACIM)4 thus far has been the most successful catalyst with tertiary alkyl diazoacetates, whereas for primary acceptor-substituted diazoacetates with a pendant olefin side chain, Rh2(4A-MEOX)4 has proved to be highly selective. 

The ruthenium compounds described above show a distinctly lower metathetic activity than the molybdenum alkenylidene complex 24 developed by Schrock et al. (Fig. 4, see also the chapter by R.R. Schrock, this volume) [18], which is another standard catalyst for any type of olefin metathesis reaction. However, they... 

Dicyclopropylallene reacts with olefins activated by ester groups at 200 °C to yield the adducts 20 predominantly (Table 12.1). In contrast, the reactions with methylenemalononitrile occurred at room temperature with selective production of the adduct 19 [18], A dipolar mechanism is assumed for the latter reaction. 

Sm(III)-catalyzed olefin hydroboration reaction by catecholborane has recently been theoretically investigated [28], The stationary structures on the model reaction path considering ethylene as the olefin, Cp2SmH as the active catalyst, and HB(OH)2 as the model borane were obtained at the RHF and MP2 levels. MP4SDQ energy calculations were also carried out at the MP2 structures. 

An optimum isomerization activity versus Si/Al2 ratio was reported for mordenite [5,13,16,17]. At too high an acid density, the acid strength decreases and olefin oligomerization reaction is favored. On the other side, too few acid sites impair activity. Reported optimum Si/Al2 ratios are around 20, which is close to the value... 

A special application of bimetallic ruthenium complexes was found in the olefin metathesis reaction vide infra) The two metal centers were closely attached to one another through /r-halide anions. The labile assembly was the key feature to the formation of highly active catalysts. 

Ruthenium(n) systems containing imidazol-2-ylidene or imidazolidin-2-ylidene have been used to catalyze the synthesis of 2,3-dimethylfuran starting at (Z)-3-methylpent-2-en-4-yn-l-ol [Eq. (54)]. The activity of the catalyst strongly depends on the nature of the NHC ligand. Benzimidazolin-2-ylidenes give the best results for this transformation. Similar systems have also been used for olefin metathesis reactions. ... 

Section 11.06.4 of this chapter highlights the substrate scope of olefin CM reactions. Based on this survey of the literature, olefins will then be placed into their appropriate category based upon catalyst activity and substrate tolerance, citing specific examples (Section 11.06.4.6). It is important to note that olefin-type characterization can change in response to catalyst reactivity. For example, an olefin may be characterized as a type III olefin in CM... 

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