Tungsten metallacycles

In another study the kinetics and mechanism of an unprecedented T/2-vinyl isomerization of a highly fluorinated tungsten(II) metalla-cyclopropene complex was studied (92). Photolysis of a tungsten(II) tetrafluoroaryl metallacycle 1 and perfluoro-2-butyne results in the formation of the kinetic rf -vinyl complex 2 in which the fluoride is trans to the inserted acetylene and cis to both carbonyl ligands. Upon heating 2 is converted to the thermodynamic rf -vinyl complex 3 in which the fluoride ligand is now cis to the inserted alkyne and trans to one CO and cis to the second CO ligand as shown in Scheme 1. 

As part of a study of the reactions of metallacyclic y-ketovinyl complexes of molybdenum and tungsten with acetylenes, directed toward the synthesis of complexed -/-lactones, Stone has reported92 the isolation of several vinyl-ketene complexes. When complex 72 was heated with 2-butyne, one molecule of the alkyne was incorporated into the complex with concomitant carbonylation. X-ray analysis of the product (73) has shown unequivocally that the C-l to C-4 vinylketene fragment is bonded in a planar, rj4-configu-ration. In contrast to the thermal reaction, ultraviolet irradiation of 72 or 74 in the presence of 2-butyne affords the complexes 75 and 76, respectively, where the lone carbonyl remaining after alkyne insertion had been replaced by a third molecule of the alkyne. 

It is also shown that it is possible to perform deprotonation reactions on the tungstacyclopentane and form an anionic species 51, in which the unsaturated metallacyclic fragment forms a double bond with the tungsten centre.49... 

The design of transition metal complexes capable of C—F bond activation for the functionalization of fluorocarbons has attracted attention recently. It has been known for several years that oxidative addition of an aromatic C—F bond takes place at tungsten(O) to yield stable tungsten(II) metallacycles, the cleaved carbon and fluorine atoms both finishing up bound to the metal centre (Eqn. (2)) [34-36]. 

Tungsten alkylidenes have also been shown to accomplish carbonyl alkylidenation. Aldehydes, ketones, esters and amides have been used as the carbonyl moiety. These reactions involve metallacycle intermediates and appear to parallel much of the metathesis chemistry (equation 32). 

The heterometallacyclic alkyne-containing complex 85 (M = Mo or W, R = Pr or /7-MeQH4, R = Me or Ph) isomerizes into 86 above 20°C (90). At 55°C the tungsten compound with R being Pr and R Ph rearranges, with incorporation of the alkyne ligand into a metallacycle and extrusion of the thiolate from the chelate ring, to form 87. 

The Schiff base ligand prepared from pentafluorobenzaldehyde and 1,2-diaminobenzene reacted with [W(CO)3(EtCN)] to give [W°(CO)3(NI I2C6H4NCI IC6F5-A%V )(EtCN)] immediately and then the metallacycle [WlT (CO)3(NH2C6H4NCIIC6F4-A, , C )] by cleavage of a C—F bond. The cleaved fluoride atom coordinated to tungsten ion. 01... 

As in the chemistry of the C-H bond, an important problem in organometallic fluorocarbon chemistry is to discover if further reaction chemistry is possible once a C-F bond is successfully attacked. Kiplinger and coworkers demonstrated [48] that the tungsten(II) metallacycles obtained from C-F activation can be... 

---