Addition reactions Lewis bases

For example, opening of the cluster skeleton from closo to nido to arachno will accompany the addition of electron pairs, whether by reduction reactions or by addition of Lewis base molecules that do not dislodge other Lewis bases. Cluster closing is expected to accompany the removal of electron pairs, whether by oxidation or by loss of a substituent together with the electron pair by which it was formally bound to the cluster ... 

We have considered the addition of three radicals, Cl, H- and CH3, as models for bases. In the Ziegler-Natta reaction, Lewis bases involved are esters of aromatic acids and hindered amines. Bases are supposed either to interact with the support (MgCl2) or to coordinate with Ti [40, 41]. 

The reactions presented in scheme (10) also account for effects exerted by the addition of Lewis bases or acids (as well as other electron donors and acceptors) to the polymerisation system on the microstructure of the polymers formed. As shown in Tables 5.4 and 5.5, some catalysts that are highly stereospecific for the formation of cis- 1,4-polybutadiene yield trans- 1,4-poly butadiene (or eb-m-1,At trans-1,4-polybutadiene) after the addition of a Lewis base or other electron donor to the catalyst system. A plausible explanation of the observed phenomena is that the added component occupies a coordination site at the transition metal, thus forcing the incoming monomer molecule to coordinate as an s-trans-rf ligand. When the additional catalyst component has a basicity comparable with that of the monomer, a competitive monomer/ Lewis base (electron donor) coordination takes place, as shown below [7] ... 

The loss of CO from the hexacarbonyl dicobalt fragment is a key step in the mechanism of the reaction (see section 2.2) the majority of additives are employed to affect this stage of the pathway. Other chemical additives include Lewis bases such as //-butyl methyl sulfide and cyclohexylamine, while molecular sieves have also been employed in order to trap CO out of the reaction mixture.13-16... 

For both cycles, a concerted mechanism is suggested in which the electron-rich double bond of the alkene attacks a peroxidic oxygen of 2. It has been inferred, from experimental data, that the system may involve a spiro arrangement [3, 5 a]. The selectivity toward epoxides can be enhanced by the addition of Lewis O- or Wbases such as quinuclidine, pyridine, pyrazole or 2,2 -bipyridine to the system [3, 6d, lOg-k]. Lewis acids catalyze ring-opening reactions and diol formation. These reactions are suppressed after the addition of Lewis bases. An... 

In many cases, the addition of Lewis bases capable of coordinating to the metal center during epoxidation catalysis has been found to have a beneficial effect on catalyst turnover rate and number as well as epoxide yield. Commonly used additives include pyridine, imidazole, and pyridine N-oxide derivatives. The proposed roles of N-oxide derivatives in [Cr(salen)] -catalyzed and [Mn(salen)] -catalyzed epoxidation reactions include activation of the intermediate metal-0X0 complex [15,50], dissociation of umeactive p-oxo dimer complexes to reactive monomeric species [19,25], and/or solubilization of the active oxidant in bi-phasic reaction media [51]. 

The reaction between SbPh3 and TiCU in dry benzene initially gives a deep violet solution and ultimately a gummy black solid. It is Ukely that reduction of TiCU has occurred. Although there is a paucity of well-characterized aisine and stibine complexes, a surprisingly large number of patents have dealt with catalyst systems employing these compounds (Table 6) which often contain Lewis acids in addition to Lewis bases. The function of the Lewis acid is often not clearly delineated, but its presence has a pronounced effect on the catalytic activity as does the Lewis base. 

In the olefin epoxidation, the mechanistic scheme commonly proposed for the oxygen-transfer reaction consists of a two-step catalytic cycle (Pig. 21) 123). In the first step, an oxygen atom is transferred from the primary oxidant to the Mn -salen catalyst, which in the second step carries the activated oxygen to the olefinic double bond. The main problem in Mn-salen catalyzed epoxidation with H2O2 was the formation of HO radicals by the homolytic cleavage of the weak 0-0 bond, leading to indiscriminate oxidation 124). Addition of Lewis bases, such as imidazole, pyridine, or... 

The quantity of such units in the above polybutadienes is approximately 48-58% transAA, 33-45% cis-XA, and 7-10% 1,2 units. There is little effect of the reaction temperatures upon this composition. As described in Chapter 3, however, addition of Lewis bases has a profound effect. Reactions in tetrahydrofiiran solvent result in 1,2 placement that can be as high as 87%. 

Stille coupling of alkyl halides with alkenyl stannanes can be achieved using electron-rich alkyl phosphines ligands, combined with the addition of a fluoride source as a nucleophilic promoter (Scheme 2.69). ° This probably works by generating a stannate complex 2.201 that participates in transmetallation more readily than the original stannane, in a similar manner to the addition of Lewis bases to Suzuki reactions (Section 2.6). 

Linear and crosslinked products can be formed though addition and condensation reactions. Reactions with unsaturated sites (addition) and Lewis bases (condensation) can occur that, depending on the particular situation, may or may not increase the amoimt of crosslinking. 

If the Lewis base ( Y ) had acted as a nucleophile and bonded to carbon the prod uct would have been a nonaromatic cyclohexadiene derivative Addition and substitution products arise by alternative reaction paths of a cyclohexadienyl cation Substitution occurs preferentially because there is a substantial driving force favoring rearomatization Figure 12 1 is a potential energy diagram describing the general mechanism of electrophilic aromatic substitution For electrophilic aromatic substitution reactions to... 

Dicyclopentadiene is also polymerized with tungsten-based catalysts. Because the polymerization reaction produces heavily cross-Unked resins, the polymers are manufactured in a reaction injection mol ding (RIM) process, in which all catalyst components and resin modifiers are slurried in two batches of the monomer. The first batch contains the catalyst (a mixture of WCl and WOCl, nonylphenol, acetylacetone, additives, and fillers the second batch contains the co-catalyst (a combination of an alkyl aluminum compound and a Lewis base such as ether), antioxidants, and elastomeric fillers (qv) for better moldabihty (50). Mixing two Uquids in a mold results in a rapid polymerization reaction. Its rate is controlled by the ratio between the co-catalyst and the Lewis base. Depending on the catalyst composition, solidification time of the reaction mixture can vary from two seconds to an hour. Similar catalyst systems are used for polymerization of norbomene and for norbomene copolymerization with ethyhdenenorbomene. 

Electrophile Addition Reactions. The addition of electrophilic (acidic) reagents HZ to propylene involves two steps. The first is the slow transfer of the hydrogen ion (proton) from one base to another, ie, from Z to the propylene double bond, to form a carbocation. The second is a rapid combination of the carbocation with the base, Z . The electrophile is not necessarily limited to a Lowry-Briiinsted acid, which has a proton to transfer, but can be any electron-deficient molecule (Lewis acid). 

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