Tungsten reaction with functional groups

Although the reactions of metal-carbene complexes with various functional groups can lead to catalyst-substrate compatibility problems, these same reactivity profiles can be used in tandem reaction sequences. In the following two examples, a tungsten-carbene complex (Eq. 4.9) and a titanium complex (Eq. 4.10) are... 

Reactions similar to the PKR that provide cyclopentenones from the [2 + 2 + 1] cycloaddition of an alkyne, an alkene, and carbon monoxide can be achieved with metals other than cobalt. Chromium, iron, iridium, molybdenum, nickel, palladium, rhodium, ruthenium, titanium, tungsten and zirconium have all been reported to catalyze the cycloaddition. The mechanism, selectivity, and functional group compatibility varies with each metal, making their discussion beyond the scope of this chapter. 

O-Acyl esters (Barton esters) of A hydrox3q)yridine-2-thione are excellent precursors of carbon-centered radicals through the rapid decarboxylation of the acyloxyl radicals formed, under mild reaction conditions (photolysis with a tungsten lamp at room temperature, or refluxing in benzene or toluene). Carbon-centered radicals formed in this manner can be used for various types of functional group conversions and C-C bond formations (Figure 1). ... 

Vinyl ethers are known to react with ruthenium carbene complexes to give the so-called Fischer carbenes which show greatly reduced olefin metathesis activity [13]. Therefore, ethyl vinyl ether is typically use din the nonfunctional termination of most living ROMP reactions using ruthenium initiators. Figure 3.2. When substituted vinyl ethers are employed, functional groups or even complex molecules can be transferred onto the polymer chain end in one step. This type of functional termination reaction works only for ruthenium carbene complexes because titanium, molybdenum, and tungsten carbenes are tolerant toward vinyl ethers. 

In spite of its versatility, metathesis could not, until lately, be developed to its full synthetic potential because the traditional catalysts were ill-suited for application being relatively short-hved and susceptible to air, moisture, or side reactions. Schrock was the first to develop an entire family of tungsten-alkylidene and, more importantly, molybdenum-alkylidene complexes with very high activity and selectivity in olefin metathesis. Further on, Grubbs discovered ruthenium (Ru) catalysts which up to now are among the most tolerant of functional groups initiators. ... 

More evidence has appeared showing that the olefin metathesis reaction can tolerate the presence of functional groups. The catalytic system Re207-Al203, promoted by a small amount of tetramethyltin, effects metathesis of olefins in fair yield (17—40%) in the presence of unsaturated ethers and ketones, alkenyl esters, and halogeno-alkenes. The reaction is performed in carbon tetrachloride as solvent at room temperature over 6 h. Electro-reduction of tungsten hexa-chloride with an aluminium anode in halogenated solvents appears to form a complex suitable for a clean metathesis, exemplified by the conversion of pent-2-ene into its equilibrium mixture with but-2-ene and hex-3-ene. ... 

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