Iridium alkylidene complexes

The ruthenium and iridium methylene complexes and the osmium alkylidenes 48-50 also react with S02 to afford sulfene complexes (104). 

Cyclometallation in an iridium complex was used to form an unusual alkylidene complex in the conformationally restricted backbone of certain diphos-phines. Such a transformation occurs via a-hydride elimination [Eq. (6.76)] ... 

Co(I) or Co(I), Co(III) couples/ Rhodium phosphine complexes cyclize a,a>-alkynoic acids to alkylidene lactone. The suggested mechanism (Scheme 32) is supported by the isolation of two iridium trialkylphosphine complexes (47) and (48), which serve as models for the proposed intermediates A and C and whose crystal structure has been determined. ... 

Finally, the iridium carbene complex 580, obtained from the double C-H activation of 2-ethylphenol by Tp Ir(C6H5)2(N2) (211), illustrates the rare incidence of an equilibrium between alkylidene hydride and alkene hydride complexes. The alkylidene forms in admixture with ca. 5% of the alkene hydride isomer 581, illustrating a preference for the a- over hydrogen in the second activation step. However, in isolation 581 is observed to re-establish the same equilibrium mixture (i.e. 20 1 580 581, Scheme 61) a rare example of a metal-alkene converting to a metal-alkylidene, the reverse reaction being more typical. 

As mentioned above in Section 1.25.5.2, rhodium and iridium pincer complexes have been used to catalytically dehydrogenate alkanes, giving terminal olefins as the kinetic products. In a recent report by Goldman and Brookhart, the iridium Pincer complexes were combined with Schrock s alkylidene metathesis catalyst... 

A series of benzylidene and low-valent alkylidene complexes have also been prepared from sulfur ylides. Deprotonation of benzyl or alkyl diphenylsulfonium salts with a strong base, such as KN(SiMe3)2, KNPr j or LiNPr generates the corresponding sulfur ylide. Reaction of this ylide with low-valent ruthenium, osmium, rhodium, or iridium complexes containing a labile ligand leads to transfer of the carbene unit from sulfur to... 

Recently, metathesis of cyclooctane was reported using the tandem system having the pincer-ligated iridium complexes for hydrogenation/dehydrogenation and Schrock-type Mo-alkylidene complexes for olefin metathesis [78]. However,... 

Building off of seminal work on the development of iridium complexes for dehydrogenation - hydrogenation catalysts, an improved, homogeneous, dual-catalytic system for alkane metathesis has recently been discovered [101, 102]. Goldman and Brookhart found that a combination of an Ir-pincer-based catalyst with Schrock-type, Mo- or W-alkylidene complexes (as the olefin metathesis catalyst) transformed alkanes into lower and higher new alkanes (Figure 2.12). 

The reduction behaviour of the alkylidene adduct of a cobalt-dithiolene complex (423) has been examined548 and the study has shown that, when the alkylidene-bridged structure (423) is reduced by one electron, it isomerizes rapidly and quantitatively to the ylide form (424). This represents the first example of reversible isomerization of the metal-carbon bond in a cobaltadithiolene complex. A surprising cis- to tra .s-dihydride isomerization which is unprecedented for 18-electron six-coordinate complexes has been observed549 in an octahedral iridium-c7.y-di hydride complex. 

Alkylidene carbonyl iridium complexes, reactions, 7, 275 Alkylidene compounds, NLO properties, 12, 121 Alkylidene-containing complexes, in molybdenum complexes, Schrock-type complexes, 5, 524 a-Alkylidene cyclic carbonyl compounds, isomerization,... 

More recentiy, Ruiz et al. have developed the synthesis of the ruthenium(II) complex 28 [154]. This makes use of an appropriate combination of previous strategies. The ruthenium atom is stereogenic and this leads to a mixture of dia-stereoisomers, which are eight times more active than cisplatin on T47D breast cancer cell line. This approach can be extended to include complexes of rhodium and iridium [155], and other variants of the method have been used with ruthenium [156]. Alkylidyne and alkylidene derivatives of osmium, exemplified by molecule 29, have been reported [157], but as yet they have not been evaluated biologically. 

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