Metal-alkyne complexes manganese

Metal cluster complexes containing vinylidene ligands have been considered as models of species present when olefins or alkynes are chemisorbed on metal surfaces (114). Vinylidene has been detected in reactions of ethylene or acetylene with Fe(100), Ni( 111), and Pt(l 11) surfaces (115), and was shown to be an intermediate by theoretical studies on a manganese surface (116). The facile cleavage of C-H bonds which occurs in these systems, together with hydrogen addition or abstraction, also occurs on metal clusters. Typical of the reactions considered is the hydrogen transfer reaction... 

Transition metal carbenes constitute a very important class of molecules that have found a multitude of applications. With reference to manganese, a number of new carbene complexes have been reported recently. The ) -alkyne complex (18) was found to undergo oxidation by dimethyldioxirane to afford the a-keto carbene (19). The fascinating aspect of this reaction is the likely existence of an oxirene intermediate (20). Stable oxirene complexes have never been reported, imdoubtedly because of the extreme instability of the antiaromatic oxirene ring. The possibility of trapping an oxirene by epoxidation of a coordinated alkyne is intriguing. Scheme 10 summarizes the chemistry involved. 

Various insertion reactions occur when metal thiolate complexes are treated with fluorinated alkynes under different reaction conditions. The manganese thiolate complexes of type 381 (R = Cp3, QFj) react with hexafluorobut-2-yne at 20°C in pentane to give an insertion product acting... 

Indirect electrochemical oxidative carbonylation with a palladium catalyst converts alkynes, carbon monoxide and methanol to substituted dimethyl maleate esters (81). Indirect electrochemical oxidation of dienes can be accomplished with the palladium-hydroquinone system (82). Olefins, ketones and alkylaromatics have been oxidized electrochemically using a Ru(IV) oxidant (83, 84). Indirect electrooxidation of alkylbenzenes can be carried out with cobalt, iron, cerium or manganese ions as the mediator (85). Metalloporphyrins and metal salen complexes have been used as mediators for the oxidation of alkanes and alkenes by oxygen (86-90). Reduction of oxygen and the metalloporphyrin generates an oxoporphyrin that converts an alkene into an epoxide. 

Rapid development of this area followed the discovery of routes to these complexes, either by ready conversion of terminal alkynes to vinylidene complexes in reactions with manganese, rhenium, and the iron-group metal complexes (11-14) or by protonation or alkylation of some metal Recent work has demonstrated the importance of vinylidene complexes in the metabolism of some chlorinated hydrocarbons (DDT) using iron porphyrin-based enzymes (15). Interconversions of alkyne and vinylidene ligands occur readily on multimetal centers. Several reactions involving organometallic reagents may proceed via intermediate vinylidene complexes. 

The final step in the system of CO-RM 25 is prepared by the same complexation as with CO-RM 24. However the ligand is pre-functiona-lised before the complexation step in this case. Alkyne functionality was introduced with a spacer, which was used to keep the alkyne distanced from the metal complex. This would have been done so that any groups that are subsequently attached on to the alkyne are less likely to interfere with the CO-releasing properties of the manganese unit... 

The Dotz benzannulation reaction (DBR) is the reaction of an a,P unsaturated Fischer carbene with an alkyne to produce a highly substituted phenol. Alternatively, the DBR can be considered a metal templated 3 + 1 + 2 cycloaddition of an allylic carbine (3 carbon unit), carbon monoxide (1 carbon unit), and an alkyne (2 carbon unit). The initial product of the reaction is the arene chromium tricarbonyl complex of the phenol as in 4. These complexes are typically unstable in air such that workup and purification of the product lead to the complete loss of the metal. Chromium is the most often used metal for the benzannulation. Molybdinum, tungsten, and manganese have been used but usually give mixtures of products and require harsh reaction conditions. 

Progress in rhenium chemistry reported in 1991 is reviewed.549 There is much important material in a review of methyltrioxorhenium catalysed alkene metathesis, oxidation and aldehyde alkenation, a review of the insertion of activated alkynes into the metal-metal bond of dinuclear manganese and rhenium carbonyl complexes. i a review of the reactions of [Cp(CO)2Mn=CPh]+ towards electron rich alkynes and nitriles552 and a review of the discovery of tRe(CBu )(CHBu ) OCMe(CF3)2 2l and recent results covering the metathesis of alkenes. s... 

Figure 3.1 clearly illustrates the effect of the two different metal coordination environments shown in Scheme 3.11 on the tautomer energy balance ( (vinylidene) - (alkyne) = E J. While the d manganese complex stabilizes the vinylidene form, the alkyne remains favored for the d palladium complex. In addition, substituent effects can also be observed, with more electron-withdrawing groups (21e, 21g) making the vinylidene more favorable, whUe disubstituted substrates 22a-f increase the preference for the alkyne. In addition, the / ara-substituents seem to have less of an impact in the disubstituted complexes, with data more tightly clustered than that for their monosubstituted counterparts. 

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