Carbene complexes oxygen-stabilized

The increased stability of ruthenium carbene complexes towards oxygen-containing compounds might be because later transition metals, having more d-electrons, are softer and hence react better with soft bases, e.g. olefins. The early transition metals, on the other hand, having few d-electrons, are generally harder and react preferentially with hard bases, such as water or carbonyl compounds. 

X-ray crystallographic data, as mentioned, show double bond character in both the Cr-C and C-O bonds. This supports the statement made early in the discussion of carbene complexes that n bonding in complexes of this type (containing a highly electronegative atom—in this case oxygen) may be considered delocalized over three atoms. Although not absolutely essential for all carbene complexes, the delocalization of n electron density over three or more atoms provides an additional measure of stability to many of these compounds.14... 

This increase in acidity is mainly the result of the weaker 77-donor effect of the MeS (c7r = - 0.15) " compared to that of the MeO group (ctr = - 0.43) " which leads to a weaker stabilization of the neutral thia compared to the oxa carbene complexes. The higher pK values of entries 1-3 compared to that of 5 can be attributed to 77-donation by the respective oxygens in entries 1-3 that is stronger than in 5. In the case of entry 3, the ethyl group is a better stabilizer of the oxyanion resonance structure of the carbene complex (140). For the cyclic carbene complexes (entries 1 and 2), the 77-donor effect is enhanced further because, by virtue of the cyclic structures of entries 1 and 2, the oxygen is locked into a position for better 77-overlap with carbene carbon (142). Cr NMR data are in agreement with this assessment. ... 

A significant development for the selective synthesis of alkenes makes use of alkene metathesis. Metathesis, as applied to two alkenes, refers to the transposition of the alkene carbon atoms, such that two new alkenes are formed (2.110). The reaction is catalysed by various transition-metal alkylidene (carbene) complexes, particularly those based on ruthenium or molybdenum. The ruthenium catalyst 84, developed by Grubbs, is the most popular, being more stable and more tolerant of many functional groups (although less reactive) than the Schrock molybdenum catalyst 85. More recently, ruthenium complexes such as 86, which have similar stability and resistance to oxygen and moisture as complex 84, have been found to be highly active metathesis catalysts. 

The phosphorus and sulfur ylides are more stable than the nitrogen and oxygen analogs (66). The former species are comprised of carbenes complexed to soft donors, whereas in the latter the carbenes are not stabilized by the adjoining hard bases. The high stability of CH2I compared to that of CHjF is at variance with the classical theory of inductive effect, but it is exactly as predicted on the basis of HSAB principle if regarded as [X complexes. 

Fischer Carbenes Fischer recognized the first carbene complexes in 1964. They were formed by the attack of an alkyllithium on a metal carbonyl followed by methylation (Eq. 11.1). Going back to the bonding picture mentioned above, we saw that the methoxy substituent will also help stabilize the empty p orbital on the carbene carbon by ir donation from one of the lone pairs on oxygen. Resonance form 11.3, which is probably the dominant one in the heteroatom stabilized Fischer carbenes, shows the multiple character of this bond. This effect is responsible for the restricted rotation often observed for the heteroatom-carbene carbon bond in NMR studies. For example cis and trans isomers 11.8 and 11.9 of methoxymethyl carbenes are rapidly interconverting at room temperature (Eq. 11.2), but can be frozen out in the proton NMR at -40 C. ... 

Carbene complexes, generated by the reaction between metal salts and diazo compounds can insert into C-H bonds in a form of CH activation (see Chapter 3 for other CH activation reactions). While early reactions involved the use of copper salts as catalysts (Schemes 8.143 and 8.144), rhodium complexes are now more widely used. In molecules such as cyclohexane, there is no issue of regioselectivity, but this issue is critical for the use of the reaction in synthesis. Both steric and electronic factors influence selectivity. Carbon atoms where a build up of some positive charge can be stabilized are favoured. Hence, allylic positions and positions a- to a heteroatom such as oxygen or nitrogen, are favoured. The reaction at tertiary C-H bonds, rather than primary C-H bonds is also favoured for the same reason, but, in this case, are also disfavoured by steric effects. Reactivity and selectivity are also influenced by both the structure of the catalyst, and the... 

CO is very seasitive to nucleophilic attack when coordinated to metal sites of low n basicity. On such a site, the CO carbon is positively charged because L-to-M a donation is not compensated by M-to-L back donation, and the CO rr orbitals are open to attack by the nucieophile. Nucleophilic lithium reagents convert a number of metal carbonyls to the corresponding anionic acyls. The net negative charge now makes the acyl liable to electrophilic addition to the acyl oxygen to give the Fischer (heteroatom-stabilized) carbene complex, 8.1. ... 

The metal-carbene complexes stabilized by the presence of an heteroatom, most often oxygen (but sometimes also N or S) or even two heteroatoms. 

Carbonyl ylides can be viewed as an adduct between a carbonyl group and a carbene and, in fact, some ylides have been prepared this way (see above). The application of carbonyl ylides to the synthesis of complex natural products has been greatly advanced by the finding that stabilized carbenoids can be generated by the decomposition of ot-diazocarbonyl compounds with copper and rhodium complexes. The metallocarbenoids formed by this method are highly electrophilic on carbon and readily add nucleophiles such as the oxygen of many carbonyl derivatives to form carbonyl ylides. This type of reaction is in fact quite old with the first report being the addition of diazomalonate and benzaldehyde (33,34). 

Ito, Kawakami and Sawamura recently described the borylation of al-lylic carbonates by B2pin2, catalyzed by bis(phosphine)copper(I) alkox-ides. It was proposed that bis(phosphine)copper(I) boryl species formed by alkoxide/boryl a-bond metathesis are key intermediates in the catalytic cycle [231]. Making use of related N-heterocyclic carbene stabilized precursors, Sadighi and co-workers have very recently isolated the thermally labile copper boryl complex (IPr)CuBpin (11.1) together with the products of oxygen atom, styrene and aldehyde insertion into the Cu-B bond (11.2-11.5 Scheme 24) [232,233,237]. The structure of 11.1 in the solid state reveals an approximately linear Cu(I) coordination geometry [ZB-Cu-C 168.1(2)°] and a Cu-B distance [2.002(3) A] which is somewhat shorter than the sum of the expected covalent radii [2.05 A] [106]. Yet further evidence for the... 

A very recent example of carbene/palladium chemistry is reported by Stahl in which (NHC)2Pd add molecular oxygen to form a peroxo complex. Oxidative chemistry involving this type of complex as catalyst is expected to follow. This represents a fundamental departure of NHC ligands from phosphines due to their stability to oxidation Konnick MM, Guzei lA, Stahl SS (2004) J Am Chem Soc 126 10212... 

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