Carbonylative metallacycle intermediates

The alkyne-cobalt carbonyl complex 3 formed from the alkyne 1 and dicobalt octacarbonyl 2 should lose at least one of the GOs on the metal to provide the vacancy for the incoming olefins. Subsequently, an olefin-bound complex 5 rearranged oxidatively to yield a metallacyclic intermediate 6. Migratory insertion of GO of 6 would provide the homologated ring intermediate 7, and the following two successive reductive eliminations afford the cyclopentenone... 

Tungsten alkylidenes have also been shown to accomplish carbonyl alkylidenation. Aldehydes, ketones, esters and amides have been used as the carbonyl moiety. These reactions involve metallacycle intermediates and appear to parallel much of the metathesis chemistry (equation 32). 

For the direct carbonylation with group VIII transition metal catalysts two main types of mechanisms have been proposed so far, involving the formation of a metal-imido (e. g.. Structure 4) or a metallacyclic intermediate (e. g.. Structure 5) [3]. 

Scheme 3. Mechanism for the direct carbonylation involving a metallacyclic intermediate [3],...

A Ni -based system related to those already described in connection with syntheses of both cyclobut-enediones and several five-membered heterocycles (Sections 9.4.2.1, 9.4.3.3 and 9.4.3.5) is also capable of catalyzing pyridone synthesis from isocyanates and alkynes. However, the structures found for the isolable metallacyclic intermediates in this system imply a completely different insertion sequence isocyanate first, followed by the two alkynes (Scheme 35). As a consequence, the regiochemistry found in the products of reaction of unsymmetrical alkynes is the reverse of that typical of Co larger substituents end up at positions 4 and 6. Thus, starting with the carbonyl carbon of the isocyanate, each carbon-carbon bond forming event is strongly regioselective for the less-hindered alkyne carbon (equation 46). The... 

Hydroboration of carbonyl compounds by pinacolborane is chemoselectively catalysed by titanocene bis(catecholborane) (A). Aryl aldehydes and ketones produce alkoxypinacolboronate esters (B) in moderate to high yields. The facile hydrolysis of B over silica affords alcohols in good yields. The catalytic hydroboration of electron-poor acetophenones is faster than that for electron-rich acetophenones. Computational studies with benzophenone and benzaldehyde indicated that hydroboration is spontaneous and probably proceeds via Ti metallacycle intermediates whose structures... 

This transformation was proposed to proceed via the pathway shown in Scheme 17.11 [14b]. l-(l,2-Propa-dienyl)cyclopropanols (as exemplified by the reaction of compound 47) reacts with Co2(CO)g to give the carbonyl inserted intermediate 51, which undergoes cyclopropane cleavage to give a metallacyclic intermediate 52. Reductive elimination transforms 52 to cyclohexenedione 53. Then 53 readily tautomerizes to hydroquinone 49, which is readily further oxidized to benzoquinone 50 by air. Hydroquinone 49 can be trapped by acetic anhydride to give 48, or can be... 

The 14-electron species denoted here as Cp2M (M = Ti, Zr), generated from Group 4 metallocenes, mediate useful oxidative cyclizations of dienes 231 [103,104], enynes 233 [105] and diynes 235 [105,106] via the metallacycles 232, 234 and 236. These metallacycles are useful intermediates, which are hydrogenolysed, carbonylated, halogenated, attacked by electrophiles and converted to functionalized cyclic compounds such as 238 and 239 [107],... 

Metallacycles, e.g. 307 (see next page), are intermediates in these processes and several have been isolated and characterized crystallographically. In addition, certain of these species may be carbonylated to give cyclic ketones. 

In any chain reaction, apart from initiation steps, the termination steps are also important. In metathesis there are many possibilities for termination reactions. Besides the reverse of the initiation step, the reaction between two carbene species is also a possibility (eq. (17)). The observation that, when using the Me4SnAVCl6 system, as well as methane traces of ethylene are also observed [26] is in agreement with this reaction. Further reactions which lead to loss of catalytic activity are (1) the destruction of the metallacyclobutane intermediate resulting in the formation of cyclopropanes or alkenes, and (2) the reaction of the metallacycle or metal carbene with impurities in the system or with the functional group in the case of a functionally substituted alkene (e. g., Wittig-type reactions of the metal carbene with carbonyl groups). 

General evidence for that kind of mechanism comes from ab initio theoretical calculations performed on a related platinum complex [3, 44] and from the reactivity of four- and five-membered heterometallacyles [45] structurally close to some of the intermediates in postulated mechanisms (e.g.. Scheme 3). Moreover, related metallacycles have often been isolated from the reaction medium after nitroaromatic carbonylation, indicating that such species can easily be generated under typical carbonylation conditions [46, 47]. 

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