Alkyl zirconocenes

The synthesis of analogous iminoacyl complexes by isonitrile insertion into linear alkyl-zirconocene chlorides is also known. In an overall regiospecific hydrocyanation of alkenes, iminoacyls 21 derived from tBuNC or Me3SiCN (as the Me3SiNC isomer) may be treated with I2 to rapidly generate an imidoyl iodide and subsequently the nitrile 22 (Scheme 3.6) [22], Less hindered iminoacyl complexes (e. g. R = Bu, Cy) may be hydrolyzed to afford aldehydes 23 [23]. 

Scheme 4.37. Diastereoselective aldol reactions of an alkyl zirconocene.

Dimeric association (71) can be obtained by an acid-base reaction between alkyl zirconocene and a terminal alkynyl ruthenium complex (equation 32). Surprisingly, the same reaction was observed with an iron acetylide opposed to Schwartz s reagent see Schwartz s Reagent) (equation 33). ... 

Some examples of the dioxolenium ion alkenylation reaction are shown in Scheme 8.29 [58]. The use of AgC104 on Celite (easier to handle than pure AgC104) and triphenyl phosphite was stated to improve the reproducibility in these reactions. At present, the method is not applicable to alkyl zirconocene chlorides. 

Carboboration. An alkyl group is delivered from (alkyl)zirconocene chlorides to a triple bond accompanied by the formation of an oxaborolidine unit. Remarkably, Me3P (vs. other phosphine ligands) has a unique stereochemical influence. 

Usefid reviews address alkyl zirconocene catalysts for the pol)mierisation of silanes to poly silanes by a a-bond metathesis mechanism, chiral titanates as promoters in aldol reactions, and MeTiCl3 as a reagent for chelate-controlled carbonyl addition reactions. 5 xhe reactions of terminally functionalized alkenes with zirconocene hydrides are reviewed. Thermochemical studies show that while the bond dissociation enthalpies of Zr—C6H13 and Zr— CgHjj in zirconocene systems are comparable, the insertion of cyclohexene into the Zr—bond is more exothermic than the insertion of hexene. 

In MAO-activated catalyst systems, alkyl zirconocenium cations are likewise thought to be present, presumably in weakly botmd inner-sphere ion pairs with anions of the type MeMAO [26, 29]. These anions - still only vaguely characterized as large agglomerates [32] - are assumed to be formed from MAO by uptake of a methyl anion from the alkyl zirconocene precursor. In equilibrium with these inner-sphere ion pairs A, outer-sphere ion pairs B (Fig. 3) are observed in MAO-activated pre-catalyst systems [29, 32-34] that contain a heterobinuclear cationic AlMes adduct [35], presumably together with MeMAO as counter-anion. 

An 5h2 mechanism is reported for the displacement of alkyl radicals from alkyl zirconocenes by di-r-butyl nitroxide. Substitution in vanadium carbonyls in THF solution according to equation (13) follows a D mechanism and is weakly counterion dependent in the order Na" > Et4N. ... 

Zirconocene alkene complexes 29 are readily prepared by the p-hydride activation route from dialkyl zirconocenes 28 or methyl alkyl zirconocenes 27 (Scheme 5.10). It is also possible to prepare the zirconocene complexes of ethene, styrene, and, to a lesser extent, vinyltrimethylsilane by displacement of the weakly bound 1-butene ligand from 4. ... 

Stable transition-metal complexes may act as homogenous catalysts in alkene polymerization. The mechanism of so-called Ziegler-Natta catalysis involves a cationic metallocene (typically zirconocene) alkyl complex. An alkene coordinates to the complex and then inserts into the metal alkyl bond. This leads to a new metallocei e in which the polymer is extended by two carbons, i.e. 

Negishi E, Tan Z (2005) Diastereoselective, Enantioselective, and Regioselective Carbo-alumination Reactions Catalyzed by Zirconocene Derivatives. 8 139-176 Netherton M, Fu GC (2005)Pa]ladium-catalyzed Cross-Coupling Reactions of Unactivated Alkyl Electrophiles with Organometallic Compounds. 14 85-108 Nicolaou KC, King NP, He Y (1998) Ring-Closing Metathesis in the Synthesis of EpothUones and Polyether Natmal Products. 1 73-104 Nishiyama H (2004) Cyclopropanation with Ruthenium Catalysts. 11 81-92 Noels A, Demonceau A, Delaude L (2004) Ruthenium Promoted Catalysed Radical Processes toward Fine Chemistry. 11 155-171... 

The regioselective hydrozirconahon of internal unsymmetrical alkenes remains a challenge, as it could considerably expand the use of zirconocene complexes. Little is known about the mechanism of zirconium migration along an alkyl chain. 

Then, contrary to what was reported previously, the olefin dissociates from the zirconium metal complex. This conclusion was further supported by other experimental observations. However, it cannot be completely excluded that competition between dissociative and direct rearrangement pathways could occur with the different isomerization processes studied up to now. Note that with cationic zirconocene complexes [Cp2Zr-alkyl], DFT studies suggest that Zr-alkyl isomerizations occur by the classical reaction route, i.e. 3-H transfer, olefin rotation, and reinsertion into the Zr-H bond the olefin ligand appears to remain coordinated to the Zr metal center [89]. 

Styrene hydrozirconation led to a ratio for terminal and benzylic zirconocene products of around 85 15 [97]. Experimental evidence on alkyl-substituted styrene suggested that both electronic [98,99] and steric effects [41,86] are important for the formation of the benzyhc and terminal zirconium isomers. Migration of the metal fragment during the reaction of the zirconocene moiety might occur, perturbing the terminal/benzyl regioisomer ratio of the isolated products (Scheme 8-11) [67, 83, 98-102]. 

Some hydrometalation reactions have been shown to be catalyzed by zirconocene. For instance, CpiZrCf-catalyzed hydroaluminations of alkenes [238] and alkynes [239] with BU3AI have been observed (Scheme 8-34). With alkyl-substituted internal alkynes the process is complicated by double bond migration, and with terminal alkynes double hydrometalation is observed. The reaction with "PrjAl and Cp2ZrCl2 gives simultaneously hydrometalation and C-H activation. Cp2ZrCl2/ BuIi-cat-alyzed hydrosilation of acyclic alkenes [64, 240] was also reported to involve hydrogen transfer via hydrozirconation. 

The facile isomerization of alkyl and vinyl zirconocenes complicates the stereocontrol in the transformations of internal acyclic aikenes and allenes, and so further studies are certainly needed to circumvent this problem. 

Replacement of one of the Cp groups in the titanocene or zirconocene-based catalysts by an alkyl group destroys the... 

In 1978, Schwartz and Gell found that CO would induce reductive elimination of alkane in various zirconocene alkyl hydride complexes with concurrent formation of Cp2Zr(CO)2 (2) (52,53). It was postulated that CO initially coordinates to the 6-e complex 23 forming the coordina-tively saturated species 24 which can then reductively eliminate alkane and/or rearrange to a zirconocene acyl hydride intermediate. When R = cyclohexylmethyl, methylcyclohexane reductively eliminated and Cp2Zr(CO)2 was isolated in 25% yield. 

It has since been shown that if less coordinating anions are used, then cationic zirconocene alkyls may serve as highly active single-component catalysts. Hence, treatment of Cp2ZrMe2 with... 

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... 

Access to non-terminal ( ,2)-dienes and ( ,Z, )-trienes 61 is provided analogously through deprotonation of ( , )-4-alkyl-l-chloro-l,3-butadienes followed by insertion of the resultant carbenoid 60 into alkyl- and alkenyl-zirconocene chlorides (Scheme 3.14) [38], The corresponding internal (Z,Z)-dienes and (Z,Z, )-trienes are also readily obtained by insertion of (3-alkynyl carbenoids 62 [44] into alkyl- and alkenylzirconocene chlorides, respectively (Scheme 3.14). Reduction of the triple-bond moiety in the products 63 to afford the cis-alkenes is well known [45—47]. 

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