Rhenium alkylidene complex

Rhenacyclobutane complexes 110 can be synthesized by the [2+2] cycloaddition of ethylene with rhenium alkylidene complexes at low temperatures (Scheme 19) <1993JA2980>. However, even at low temperature, the alkylidene and rhenacyclobutane complexes combine to yield the rhenacyclic alkylidene 111 by an indeterminant mechanism. At room temperature, the most electron-deficient rhenacyclobutane 110c extmdes ethylene and reverts to the corresponding alkylidene complex. 

Recently, a well-defined heterogeneous rhenium-alkylidene complex supported on silica was reported, which brought about the metathesis of methyl oleate with a TON of 900 [51] ... 

Co-condensation of rhenium atoms with benzene alone gave no isolable products at ambient temperature. It was not expected, however, when rhenium atoms were co-condensed with a benzene alkane mixture that l-alkylidene complexes, analogous to the l-arylidene complexes were formed. Thus, co-condensation of rhenium atoms with a 1 1 mixture of benzene... 

The rhenium perhydrocarbyl complex Re(=CBu )(= CHBu )(CH2Bu )2 reacts with the surface hydroxyl groups of a silica(7oo) (Scheme 2.29) to form a well-defined surface species, monografted on silica and containing one alkyl, one alkylidene and one alkylidyne ligands according to mass balance analysis IR, NMR, EXAFS [79-82] and calculations [83, 84]. 

There are no reports of such a reaction with alkylidene complexes of rhenium such as [Re(=CCMe3)(=CHCMe3)(CH2CMe3)2] [21-23], tungsten (]W(=NAr) (=CHCMe3)(CH2CMe3)2]) [24] and other complexes of molybdenum... 

The reaction of the rhenium alkylidyne complex 277 with diisopropyl-acetylene and with diethylacetylene [Eq. (196)] demonstrates the sensitivity of metathesis reactions toward steric factors (57). With diisopropylace-tylene an alkylidyne complex is obtained whereas the reaction with diethylacetylene gives a metallacyclobutadiene. In the metathesis reactions the alkyne with the bulkiest groups cleaves most easily from intermediate metallacyclobutadiene complexes. The rhenacyclobutadienes with the smallest substituents thus become sinks and slow down the effective rate of metathesis. The alkylidyne alkylidene rhenium complex 278 is an active olefin metathesis catalyst (52). Reaction with hexene transforms the neo-pentylidene group into a propylidene group as shown in Eq. (197). 

In the 1970 s an intense interest in the olefin metathesis reaction [35] served as a driving force for research concerning metal-carbon double bonds. In its simplest form the olefin metathesis reaction consists of a redistribution of alkylidene components of olefins (Eq. 6). It was known to be promoted by tungsten, molybdenum, and rhenium, although at the time the catalytic reactions were black boxes, with nothing known about the mechanism or the detailed nature of the catalyst. Among the proposals was one that consisted of a reaction between an alkylidene complex (or carbene complex at that time) and an olefin to give a metallacyclobutane intermediate, from which an olefin could be lost and a new alkylidene complex formed, [36] as shown in... 

It is clear that electron-withdrawing alkoxides in M(NAr)(CHR)(OR )2 complexes dramatically increase the rate of reaction of an olefin with the M=CHR bond. In fact, no other X ligands in M(NAr)(CHR)X2 complexes are as successful as alkoxides for sustained metathesis activity, [66] either because they are not bulky enough to stabilize an electron deficient metal center and prevent bimolecular decomposition (e.g., halides) or because they donate too much electron density to the metal in a a and/or Ji fashion (e.g., amides or thiolates). A recent example is Mo(NAr)(CHR)(diamide) where the diamide is a N,N -disubstituted-2,2 -bisamido-l,r-binaphthyl ligand no ready reaction was observed between this complex and ethylene or even benzaldehyde. [108] Alkylidene complexes are now known for all metals in groups 4, 5, and 6, plus rhenium. For example, it was shown that Cp2Ti(CH2) could be trapped... 

There will be circumstances other than those I have described here in which "high oxidation state" organometallic chemistry of rhenium in a catalytic reaction will be viable, although it is becoming clear that the balance necessary to achieve this feat is more difficult to maintain as one moves to the right in the transition metal series, and that some of the d rhenium chemistry in fact may look like chemistry of dP osmium species. On this basis it would seem unlikely that the principles that have been used to prepare Re(VII) alkylidyne and alkylidene complexes (a hydrogen migration reactions) can be extended further (to technetium, or especially osmium or ruthenium), at least in a routine fashion. 

In the last decade, impressive advances have been made in the synthesis of alkylidene and alkylidyne complexes that contain metals from groups 4 [5] and 5 [5i,j,m, 6], especially Ti and V, but - except for the ROMP of norbornene by vanadium complexes - group 4 and 5 metals have not shown wide-ranging activity for olefin metathesis. Well-characterized rhenium(VII) complexes are known to be active for metathesis, and many rhenium(Vll) alkylidene and alkylidyne complexes have been isolated [2a], but little attention has been paid to the syntheses of rhenium alkylidene or alkylidyne complexes in the last decade. Theoretical calculations have been carried out on M(X)(CHR )(Y)(Z) complexes... 

When alkenes are treated with certain catalysts (most often tungsten, molybdenum, or rhenium complexes), they are converted to other alkenes in a reaction in which the alkylidene groups (R RX=) have become interchanged by a process schematically illustrated by the equation ... 

An interesting C-metalation of a coordinated pyrrol ligand in a rhenium complex has been described [19a]. Methylated aromatic hydrocarbons have been activated by (ZrCU) [19b], A novel mode of electrophilic activation of an aliphatic C-H bond, assisted by porphyrin complexes ofZr(IV) and achieved by the use of hydrides of lithium, sodium or potassium, has been reported [19c]. It has been shown that the ligands 2,3,5,6-tetraphenylphenoxide and 3,5-dimethyl-2,6-diphenylphenoxide undergo intramolecular activation by tantalum alkylidene groups at rates 20 and 100 times slower than that of the simple 2,6-diphenylphenoxide ligand [19d]. 

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