Ketone—zirconocene complexes

Conversion to Ketone— and Ketene Zirconocene Complexes and Reactions Thereof 5.3.2.1 Ketone zirconocene complexes... 

Scheme 5.9. (CH3)3AI-mediated formation of a ketone-zirconocene complex and reactions.

Reaction of saturated acylzirconocene chlorides with (CH3)2Cu(CN)Li2 gives the secondary alcohol (73%), and D20 work-up of the reaction mixture gives the a-deuterio alcohol. This observation suggests the formation of a ketone—zirconocene complex (Scheme 5.40 see also Section 5.3.2.1). Thus, for the reaction of a,p-unsaturated acylzirconocene chlorides with R2Cu(CN)Li2, initial formation of an unsaturated ketone—zirconocene complex followed by 1,3-rearrangement of the zirconocene moiety to an oxazirconacyclopentene, which is a ketone carbanion equivalent, has been proposed (Scheme 5.41). 

The sequential double migratory insertion of CO into acydic and cydic diorganozircono-cene complexes through acylzirconocene and ketone—zirconocene species provides a convenient procedure for preparing acyclic and cyclic ketones (Scheme 5.6) [8], Thus, the bi-cydic enones from enynes can be obtained through CO insertion into zirconacyclopen-tenes followed by a subsequent rearrangement (Scheme 5.7). The scope and limitations of this procedure have been described in detail elsewhere [8d]. This procedure provides a complementary version of the well-known Pauson Khand reaction [9]. 

The assumed structures of the ketone—zirconocene and oxazirconacyclopentene complexes have not been established spectroscopically due to the excess of cuprate reagent present. However, as discussed in Section 5.3.2.2, (i) direct nucleophilic addition to the... 

Cyclohexylisonitrile inserts selectively into the endocyclic vinyltitanium bond of a-methylenetitanacyclobutene complexes 37, although the reaction fails for many other isonitriles (Equation 12) <19960M1176>. No insertion occurs upon treatment with nitriles or ketones, in contrast to the established reactivity of typical titanacyclobutenes <1996CHEC-II(lb)887>. The reaction of fert-butylisonitrile with 1,9-anthracenediyl-zirconocene complex 38 proceeds exclusively by insertion into the terminal ring zirconium-carbon bond, presumably due to steric considerations (Equation 13) <2000JA9880>. 

Similar to the benzynezirconocene, cyclohexyne, cyclopen-tyne, alkyne, alkene, cycloaUcene zirconocenes, and related species insert various substrates such as alkynes, alkenes, aldehydes, ketones, nitriles or phosphaalkynes. They lead in general five-membered zirconacycles, which can be converted by transmetalation or exchange reactions into fused-ring aromatic or heterocyclic compounds. The extension of this chemistry to heterobenzyne complexes can be realized, for instance, in phosphinine compounds. Consequently, under mild conditions, ) -phosphabenzyne-zirconocene complexes are formed and can be isolated either as PMes adducts or as dimers when the elimination reaction is carried out without added phosphane (Scheme 28). 

As a case in point, 3,3-dimethylbutan-2-one (139 Scheme 16) upon reaction with the zirconocene complex of isoprene in benzene at 60 C for 2 h affords (140a), whereas irradiation of the reaction mixture at low temperature in toluene leads to (140b). Mixtures of (140a) and (140b) are obtained by irradiation of isolated (138) and ketone (139) at higher temperatures. As the reaction temperature is lowered, an increasing amount of (140b) is formed. Therefore, it is reasoned that the zirconocene complex reacts in the cis form (138), under thermal conditions, and in the trans form under photochemical eolations. 

The zirconocene complex [2ZrH2] catalyses an Oppenauer-type oxidation of alcohols in the presence of an appropriate hydrogen acceptor. On oxidation of diols containing two primary alcohols, and of diols containing two secondary alcohol groups, one of the alcohol groups is selectively oxidized to form hydroxy-aldehydes and hydroxy-ketones respectively. This system... 

Several alcohol-, ether-, acid-, ester- and ketone-functional alkenes (Fig. 12) were tested as comonomers in polymerization experiments [19,21] A bridged zirconocene complex rac-Et(Ind)2ZrCl2 was selected as catalyst for the studies because it is a relatively good copolymerization catalyst and capable of both ethylene and propylene polymerizaticais. MAO was used as cocatalyst. MAO and the comonomers were pre-contacted for 15 min in the reactor just before the start of the polymerization. 

A related intramolecular reaction of this type is the so-called Evans-Tishchenko reaction [166]. Here, a P-hydroxy ketone is reduced in the presence of an aldehyde yielding 1,3-diol monoesters. Several metal catalysts such as samarium iodide [166, 167] and zirconocene complexes [168] are effective. The reaction is highly diastereo-... 

Carbon monoxide rapidly inserts into the carbon—zirconium bond of alkyl- and alkenyl-zirconocene chlorides at low temperature with retention of configuration at carbon to give acylzirconocene chlorides 17 (Scheme 3.5). Acylzirconocene chlorides have found utility in synthesis, as described elsewhere in this volume [17]. Lewis acid catalyzed additions to enones, aldehydes, and imines, yielding a-keto allylic alcohols, a-hydroxy ketones, and a-amino ketones, respectively [18], and palladium-catalyzed addition to alkyl/aryl halides and a,[5-ynones [19] are examples. The acyl complex 18 formed by the insertion of carbon monoxide into dialkyl, alkylaryl, or diaryl zirconocenes may rearrange to a r 2-ketone complex 19 either thermally (particularly when R1 = R2 = Ph) or on addition of a Lewis acid [5,20,21]. The rearrangement proceeds through the less stable... 

Instead of titanocene dichloride, zirconocene dichloride can be used for the preparation of the carbene complex 219. The Lewis acidity of Zr is lower than that of Ti, and the Zr reagent 219 reacts smoothly with aldehydes and ketones, but not with esters [69]. 

Butadiene)zirconocene reacts with metal carbonyls in a similar way as with ketones. Carbon-carbon coupling between a butadiene terminus and the carbonyl carbon atom occurs readily to yield a metallacyclic product, only that generally a metallacyclic n-allyl metallocene system is obtained from the reactions with metal carbonyls.8 Sometimes, O-zirconated Fischer-type carbene complexes are readily formed in this way that cannot be easily obtained by other routes. A typical example is the... 

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