Functional group intolerance

Olefin-metathesis is a useful tool for the formation of unsaturated C-C bonds in organic synthesis.186 The most widely used catalysts for olefin metathesis include alkoxyl imido molybdenum complex (Schrock catalyst)187 and benzylidene ruthenium complex (Grubbs catalyst).188 The former is air- and moisture-sensitive and has some other drawbacks such as intolerance to many functional groups and impurities the latter has increased tolerance to water and many reactions have been used in aqueous solution without any loss of catalytic efficiency. 

If the metal cation is too electrophilic, CO coordination will be too strong, possibly by coordination via its oxygen atom, and CO will act as a poison rather than participating in the polymerization [40], The moderate electrophilicity of Pd" catalysts makes them tolerant also to a variety of heteroatom functionalities in the olefin substrate. In this respect, polyketone catalysis can have a wider applicability than early transition metal catalysis of polyolefins, which is highly intolerant of functional groups. 

Many heterogeneous metathesis catalysts cannot catalyze the metathesis of functionalized olefins because of their intolerance to functional groups. Interference of the functional groups also reduces the activity of the catalysts that are active in this case. This increases the costs of the catalyst because much higher catalyst/substrate ratios must be used than are required for normal olefins. 

The absence of functionality in hydrocarbon polymers limits applications where good adhesive properties, affinities for dyes, permeability, and compatibility with polar polymers are necessary. One of the limitations of conventional Ziegler-Natta catalysts is their intolerance of functional groups due to the high Lewis acidity of the transition metal component and the presence of Lewis acidic cocatalysts based on alkyl aluminums or aluminoxanes, as was anticipated in Section 2. 

The potential for introduction of chiral centres using a Diels-Alder approach makes this a particularly attractive route to benzo-fused cyclohexanes. However, the chemistry of orthoxylylenes is relatively unexplored as previous routes to this elusive intermediate involved the use of reagents such as disodium tetracarbonylferrate. This reagent is extremely intolerant of other functional groups, mainly as a result of its high basicity and the yields obtained are only a fraction of those available using the zinc/ultrasound route. 

All common methods of synthesis are either unable to yield high molecular weights while guaranteeing narrow or monomodal mass distributions, or they are intolerant towards some functional groups. This limits the spectrum of available polysilanes. 

Meanwhile, conjugated unsaturated ketones and nitroal-kenes can inhibit the reaction altogether under relatively mild conditions. This is an issue with this specific class of substrates rather than an intolerance of ketone functional groups. 

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