Imine zirconocene complexes

CautionI All procedures should be carried out in an efficient fume hood. Disposable vinyl or latex gloves and safety glasses should be worn. 

Place a B14 septum on the stopcock unit and flush the space between it and the stopcock by inserting a vent needle (Fig. 5.1C) and slightly opening stopcock A. Add 5 mL of dry THF by syringe followed by the dropwise addition of 2 mL of the 3.0 M MeMgCI solution. This should give a rapid evolution of methane and the solution should become warm. Allow to stir for 20 min at ambient temperature after addition is complete. 

Carefully allow argon to enter the apparatus from the manifold avoiding blowing the zirconocene dichloride around. Repeat the evacuate/refill cycle twice. The apparatus should be left with stopcock A open so that the manifold maintains a positive pressure of argon in the flask. Stopcock B should be kept closed except when adding reagents. 

Add 5 mL of dry THF to the flask by syringe through stopcock B and after the zirconocene dichloride has dissolved cool the flask to -30X in an acetone bath cooled with liquid nitrogen or solid carbon dioxide. 

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However, none of the previous reports described isolation and characterization of z]1-imine zirconocene complexes. Addition of isocyanides on organozir-conocenes is known to give the corresponding iminoacyl complexes after insertion into a zirconium-carbon bond [5,39-41]. These compounds have been conveniently converted for example to aldehyde [5,39] or nitriles [42]. 

Isocyanides can also insert into metallacyclopentadienes [52]. Majoral and coworkers reported insertion of isocyanides into indenylzirconacyclopentadi-enes and proved the formation of rj -imine zirconocene complex, which is in-tramolecularly stabilized by a phosphino group. Elimination of the metal fragment Cp2Zr give the corresponding jS-phosphino imine derivatives or the unexpected jS-iminophosphine by hydrolysis with HCl (Eq. 49) [53]. 

The C = C bond of several alkynes inserted into the Zr-C bond of the rj2-thioaldehyde zirconocene complexes 81 (see Scheme 21) to form the five-membered thiazirconacycles 144. The imine metallacycle obtained from 81 and butyronitrile tautomerized under the reaction conditions to cleanly give the enamine metallacycle 145 (Scheme 38).70... 

Insertion of isonitriles into zirconacycles affords )] -iminoacyl complexes that smoothly rearrange to stable f-irame zirconocenes. In the case of 2-phosphanozirconaindenes, rearrangement proceeds leading to -imine zirconocenes (Scheme 35). Subsequent formation of a /3-phosphano indenoimine is obtained by elimination of Cp2Zr fragment for the 2,6-dimethylphenyl isocyanide." ... 

A chiral zw a-metallocene triflate complex catalyzes the Diels-Alder cycloaddition reaction between an oxazolidinone-based dienophile and cyclopentadiene [206]. Triflate in titanocene and zirconocene complexes is labile [207,208] and thus the polarity of solvent influences the reactivity and enantioselectivity. Asymmetric hydrogenation of imines and enamines catalyzed by chiral aw a-titanocene catalyst provides amines with high enantioselectivity [209,210]. 

Cyclopropane ring cleavage is also observed in the case of zirconocene 2-alkene and j 2-imine complexes with adjacent cyclopropane rings to give t/3-allyl, /3-azaallyl, and t/ -enamine complexes [29]. 

A wide variety of five-membered zirconacydes 8 may be formed by the formal co-cycliza-tion of two 7i-components (3 and 6 alkene, alkyne, allene, imine, carbonyl, nitrile) on zir-conocene ( Cp2Zr ) (Scheme 3.2) [2,3,8]. The co-cydization takes place via the r 2-complex 5 of one of the components, which is usually formed by complexation of 3 with a zircono-cene equivalent (path a) ( Cp2Zr itself is probably too unstable to be a true intermediate) or by oxidation on the metal (cyclometallation/p-hydrogen elimination) (path b). Two additional routes to zirconocene r 2-complexes are by the reverse of the co-cyclization reaction (i. e. 8 reverting to 5 or 9 via 7), and by rearrangement of iminoacyl complexes (see Section... 

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... 

Cyanosilanes can be isocyanide sources since a tautomeric equilibrium exists between cyanosilanes and the corresponding isocyanides. The equilibrium largely favors the cyano tautomer. The use of such dilute isocyanide donors realizes efficient Ti(n)- and Ni(0)-catalyzed cyclizations of enynes to iminocyclopentenes via metallacyclopentene intermediates (Scheme 19).266,266 266b Treatment of zirconacyclopentanes and -pentenes with Me3SiCN provides zirconocene-imine complexes, which serve for carbon-carbon bond formation with various unsaturated bonds.267... 

The formal complex of an imine RCH=NR with a zirconocene of C2v symmetry will be formed as a racemic mixture, with equal amounts of two enantiomers. If a zirconaaziridine possesses another stereogenic element, it will have two diastereomers of unequal energy, and they may be generated in a kinetic ratio that is not equal to their thermodynamic ratio. 

Imido zirconocenes see Imide Complexes) are synthesized from thermolysis of Cp2Zr(Me)NHR. They react with a wide variety of organic substrates (alkynes, imines, azides, carbodiimides, ketones) to yield [2 + 2] cycloaddition products (Scheme 4) see Metallacycle) ... 

Zirconocene-imine complexes such as 789 insert isocyanates and cyclic carbonates and in some cases C02.606 For example, isocyanates can insert either exclusively into the Zr-G bond, to form complexes 790 when R = bulky group such as Buc and SiMe3, or preferentially into the Zr-N bond to give complexes 791 when R = Me, Et, GH2Ph, and... 

Scheme 207). Diazametallacycles such as 831 are key intermediates involved in imine metathesis catalyzed by zirconocene imido complexes,628 the authentic samples of which were synthesized independently by the reaction of 825 with 2 equiv. of the corresponding imine.629 The zirconocene imido complex 825 also ring-opens epoxides such as styrene oxide regioselectively to give metallacycle 832 (Scheme 207).630... 

Three classes of catalysts have been studied for the asymmetric hydrogenation of imines. One class of catalyst is generated from late transition metal precursors and bisphosphines. These catalysts have typically been generated from rhodium and iridium precursors. A second class of catalyst is based on the chiral titanocene and zirconocene systems presented in the previous section on the asymmetric hydrogenation of unfunctionalized olefins. The third class of catalyst is used for the transfer hydrogenation of imines and consists of ruthenium or rhodium complexes containing diamine, amino tosylamide, or amino alcohol ligands. " ... 

Table 5.1 Formation and trapping of zirconocene T -imine complexes. ...

Generation and trapping of a zirconocene T) -imine complex (Scheme 5.22)... 

Insertion of isocyanides into zirconacyclopentanes and zirconacyclopentenes affords the corresponding zirconocene q -imine complexes. Cyclopentylamine derivatives could be prepared by trapping of the insertion intermediates using alkenes, alkynes, and carbonyl compounds [50]. For example, insertion of phenylisocyanide into bicyclic zirconacyclopentenes affords iminoacyl complexes that rearrange to give a, )0-unsaturated zirconocene q -imine complexes. These complexes react with alkenes or alkynes to give cyclopententylamines (Eq.48) [51]. 

Zirconocene j -imine complexes formed by a C-H activation route from a variety of amines can be trapped by halo-alkenes or alkynes to afford functionalized adducts (Scheme 12.32) [43]. 

The inteimolecular hydroamination of allenes is readily catalyzed by early transition metal complexes to yield imines. An addition of aromatic and ahphatic amines to aUene requires high reaction temperatures (90-135°C) and long reaction times (1-6 days) when mediated by zirconocene- [41] and tantalum-imido [178] catalysts. The more efficient titanium half-sandwich imido-amide complex 42 operates under significantly milder reaction conditions (27) [179], Because the metal-imido species are prone to dimerization, sterically more hindered aliphatic and aromatic amines are more reactive. Simple, sterically unencumbered aliphatic amines add to aUenes in the presence of the bis(amidate) titanium complex 43 (28), although higher reaction temperatures are required [180]. 

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