Polar functional group coordination

These observations may be rationalized by assuming that the polar functional group coordinates to the metal center in one or more intermediates along the RCM pathway (Scheme 17) [30b]. Such a Lewis-acid/Lewis-base interaction may assemble the reacting sites within the coordination sphere of the ruthenium and hence provide internal bias for cyclization (e.g. structure I). However, if such an... 

The well balanced electronic and coordinative unsaturation of their Ru(II) center accounts for the high performance and the excellent tolerance of these complexes toward an array of polar functional groups. This discovery has triggered extensive follow up work and carbenes 1 now belong to the most popular metathesis catalysts which set the standards in this field [3]. Many elegant applications to the synthesis of complex target molecules and structurally diverse natural products highlight their truely remarkable scope. 

Delivery of hydrogen occurs syn to the polar functional group. Presumably, the stereoselectivity is the result of coordination of iridium by the functional group. The crucial property required for a catalyst to be stereodirective is that it be able to coordinate with both the directive group and the double bond and still accommodate the metal hydride bond necessary for hydrogenation. In the iridium catalyst illustrated above, the cycloocta-diene (COD) ligand in the catalyst is released upon coordination of the reactant. 

In particular, Schrock-type catalysts suffered from extreme moisture and air sensitivity because of the high oxidation state of the metal center, molybdenum. Due to the oxophilicity of the central atom, polar or protic functional groups coordinate to the metal center, poisoning the catalyst and rendering it inactive for metathesis. Since late transition metal complexes are typically more stable in the presence of a wide range of functionalities, research was focused on the creation of late transition metal carbene complexes for use as metathesis catalysts. 

Historically, high-pressure free radical copolymerization has been used to produce highly branched, ill-defined copolymers of ethylene and various polar monomers. Although these materials are in production and extensively used throughout the world, the controlled incorporation of polar functionality coupled with linear polymer structure is still desired to improve material properties. Recent focus in this area has led to the development of new transition metal catalysts for ethylene copolymerization however, due to the electro-philicity of the metal centers in these catalysts, polar functional groups often coordinate with the metal center, effectively poisoning the catalyst. There has b een some success, but comonomer incorporation is hard to control, leading to end-functionalized, branched polyethylenes [44, 46]. These results are undesirable due to low incorporation of polar monomer into the polymer as well... 

What is fhe implication of our work wifh respect to the metal-catalyzed polymerization of polar vinyl monomers FirsL for fhe late metal compounds, fhe polar vinyl monomers can clearly outcompete efhene and simple 1-alkenes wifh respect to insertion. However, fhe ground-state destabilization of the alkene complex that favors the migratory insertion of fhe polar vinyl monomers is a two-edged sword because it biases the alkene coordination towards ethene and l-alkenes. Indeed, we have observed fhe near quantitative displacement of vinyl bromide by propene to form 7 from 3 (Scheme 9.1). Thus, the extent of incorporation of fhe polar vinyl monomer in fhe polymer will depend on the opposing trends in alkene coordination and migratory insertion. The above discussion does not take into account the problem of functional group coordination for acrylates or halide abstraction for vinyl hahdes. 

More polar functional groups such as ketones are hydrogenated effectively with homogeneous cationic rhodium and ruthenium complexes. Polymer catalysts active for both olefin and ketone hydrogenation activity were synthesized from ionic precursors such as Rh(norbornadiene)(acac) + HCIO4 or Rh(norbornadiene)(PE 3)2 + C104. Two types of coordination were observed, depending on the method of attachment " ... 

Amines and amides are usually poor partners in the cross-metathesis reaction because of the possible coordination of the emerging carbene to the polar functional group. To prevent this problem, Elkaim and Grimaud have found that boron-based Lewis acids are effective additives. Introduction of 0.1 equivalent of chlorodicyclo-hexyl borane as an additive seemingly prevents the coordination of the polar groups such as amides to the carbene intermediates, and provides a dramatic increase in the yields of the cross-metathesis products (131) (Equation 79) [82]. 

Polar and coordinatively active functional groups are structural elements frequently found in the constitution of crystal inclusion hosts, mainly including conventional host molecules7 . Typical examples are urea (2), thiourea (5), hydroquinone (4), Dianin s compound (5), deoxycholic add (6) or simply water (Fig. 1). This was the reason to assume that functional groups play an important part in the construction of crystal inclusion compounds. 

Radical polymerization is the most useful method for a large-scale preparation of various kinds of vinyl polymers. More than 70 % of vinyl polymers (i. e. more than 50 % of all plastics) are produced by the radical polymerization process industrially, because this method has a large number of advantages arising from the characteristics of intermediate free-radicals for vinyl polymer synthesis beyond ionic and coordination polymerizations, e.g., high polymerization and copolymerization reactivities of many varieties of vinyl monomers, especially of the monomers with polar and unprotected functional groups, a simple procedure for polymerizations, excellent reproducibility of the polymerization reaction due to tolerance to impurities, facile prediction of the polymerization reactions from the accumulated data of the elementary reaction mechanisms and of the monomer structure-reactivity relationships, utilization of water as a reaction medium, and so on. 

Fig. 4.16 provides an illustration of the adsorption of a neutral polymer, polyvinyl alcohol, on a polar surface, and the resulting effects on the double layer properties. Adsorption of anionic polymers on negative surfaces - especially in the presence of Ca2+ or Mg2+ which may act as coordinating links between the surface and functional groups of the polymer - is not uncommon (Tipping and Cooke, 1982). 

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