Carbenes Alkylidenes

The generalizations in Box 5.1 represent two extreme situations and are referred to in the older literature as Fischer-type and Schrock-type carbenes, in deference to the research groups from which the apparently distinct classes of carbene complex originally arose. This dichotomy 

The following tentative generalizations have some utility  

The use of stable free carbenes now allows their coordination to be 

In a similar manner, formaldehyde (0=CH2) and parent imines RN=CH2 are prone to oligomerization. 

The search for halocarbenes led to Fischer s carbyne synthesis (see below) however, such halocarbene complexes are now widely available for elements of Group 6-9, albeit via alternative synthetic strategies. The first stable example arose from a study of the biochemically relevant reactions 

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Carbene (alkylidene) Carbyne (alkylidyne) Cyclopropenyl (cycto-CgHj)... 

Metathesis, which is reversible and can be catalyzed by a variety of organometallic complexes, has been the subject of considerable investigation, and many reviews on this topic have been published.In 1970, Herisson and Chauvin proposed that these reactions are catalyzed by carbene (alkylidene) complexes that react with alkenes via the formation of metallacyclobutane intermediates, as shown in Figure 14-20. This mechanism, now known as the Chauvin mechanism, has received considerable support and is believed to be the pathway of the majority of transition metal-catalyzed olefin metathesis reactions. 

It is known from previous work that diazoalkanes can form carbene(alkylidene)-metal complexes [17, 18], cf. eq. (5). It is thus reasonable to assume that a metal-carbene 9 is formed from the (phosphine oxide-stabilized) species CH3Re 02) (eq. (5)). High oxidation-state metal carbene complexes have ample precedent, especially through the work of Schrock et al. [19], Isolation of type-8 species may be facilitated by sterically more-demanding auxiliary groups (e. g., C5H5 in place of CH3) or, by using heterocyclic carbenes of pronounced Lewis basicity (e. g., 1.3-imidazolin-2-ylidene [20]). 

The currently accepted mechanism-- -- - involves the initial formation of a metal-carbene (alkylidene) complex that reacts with an alkene to form a three-center bound n complex. The three-center bound n complex subsequently rearranges to a metallacyclobutane intermediate (Scheme 6.41). 

The final product is a carbene (alkylidene) complex. Since the carbene C atom has an oxygen-containing substituent, it is an example of a Fischer carbene (see Section 22.19). The structures of two reaction products are shown above. 

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