Zirconium 2-bonds

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... 

Scheme 3.8. Insertion of a carbenoid into a carbon—zirconium bond.

A key feature of the elaboration of organozirconocenes by insertion of a carbenoid is that the product retains the carbon—zirconium bond functionality of the substrate. Several useful elaborations are shown in Scheme 3.16 for the case of the organozirconium product of the insertion of alkenyl carbenoids 52 or 60 [38],... 

Lithiated chloromethyltrimethylsilane is a remarkably stable carbenoid [69] and shows exceptional reactivity in insertions into the alkenyl—zirconium bonds of unsaturated zirconacycles. It is the only known carbenoid that will insert into zirconacyclopentadienes... 

Acylzirconocene chloride derivatives are easily accessible in a one-pot procedure through the hydrozirconation of alkene or alkyne derivatives with zirconocene chloride hydride (Schwartz reagent) [Cp2Zr(H)Cl, Cp = cyclopentadienyl] and subsequent insertion of carbon monoxide (CO) into the alkyl— or alkenyl—zirconium bond under atmospheric pressure (Scheme 5.1) [2],... 

Although this migratory insertion of CO into a carbon—zirconium bond accounts for the majority of acylzirconocene complexes that have been reported, the CO insertion... 

The different reactivities of the carbon—boron and carbon—zirconium bonds toward electrophiles are a consequence of the different bond polarities and the different electronegativities of boron and zirconium. Moreover, zirconium is a transition metal, while boron exhibits intriguing transition metal-like chemistry [55], It is thus reasonable to presume that the combined use of boron and zirconium in organic chemistry should be synergistic, affording products and chemistry not attainable with the individual organo-metallics alone. 

Vinylboronates are generally less reactive than vinylzirconocenes towards various electrophiles and hence selective reactions of the latter should be possible. It was found that selective cleavage of the carbon—zirconium bond in 45 by N-halosuccinimides provides (a-haloalkenyl)boronic esters 53 in excellent chemical yields and with complete re-gioselectivity (Scheme 7.17) [54], An X-ray crystal structure determination of 45 confirmed the configuration of the four-coordinate Zr complex, with two cyclopentadienyl rings, Cl, and C(sp2) as the four ligands (Fig. 7.5) [54,126]. 

As an example of the selective reactivity of borazirconocene alkenes, their hydrolysis was examined [1]. The carbon—zirconium bond is more reactive than the carbon—boron bond towards various electrophiles, and so hydrolysis can be expected to occur with preferential cleavage of the former bond. Since hydrolysis of alkenylzirconocenes is known to proceed with retention of configuration [4,127—129], a direct utility of 45 is the preparation of (Z)-1-alkenylboronates 57 (Scheme 7.17) [12]. Though the gem-dimetalloalkenes can be isolated, in the present case it is not necessary. The desired (Z)-l-alkenylboronates can be obtained in a one-pot procedure by hydrozirconation followed by hydrolysis with excess H20. The reaction sequence is operationally simple and is compatible with various functional groups such as halides, acetals, silanes, and silyloxy protecting groups [12]. 

Short branches, specifically ethyl branches up to about 2 mol%, are formed in the polymerization of ethylene by meso-ansa zirconocenes containing unsubstituted cyclo-pentadienyl and indenyl ligands [Melillo et al., 2002]. Ethyl branches form by an isomerization process in which the usual P-hydride transfer to monomer is immediately followed by reinsertion of the vinyl-terminated polymer into the formed ethyl-zirconium bond. 

Mechanistic studies suggested that a full equivalent of Cp2ZrI2 was not required but the reaction proceeded rather sluggishly in the presence of a lesser amount. Moreover, addition of Et2Zn to 1-octyne could be catalyzed by Cp2ZrMeI and led to an exclusive ethylzincation process, suggesting that the addition of a carbon—zinc bond rather than a carbon—zirconium bond was involved. 

Halogenated hydrocarbons often bring about substitution at silicon (see Section III,C), but in the case shown in entry 4, Si-Ti (but not Si-Si) bonds are cleaved, and rearrangement to a cyclic Si5 derivative occurs. A radical process is postulated, similar to that observed in Ti-Ge (422) and H-Mn-CO (72) systems. Silicon-zirconium bonds are cleaved in an analogous way (entry 5). 

The reactivity of a silyl-zirconium complex is interesting because an unsaturated bond would be inserted into the silyl-zirconium bond to provide an alternative zirconium complex. It has zirconium-carbon and silyl-carbon... 

Treatment of j]2-iminosilaacyl complex 6c with LiEt3BH gave azazircona-cyclopropane 10, which was hydrolyzed to give (silylmethyl)aniline in 82% yield (Scheme 5). Treatment of 10 with 4-octyne gave alkene 12 in 73% yield after hydrolysis. Presumably, the insertion of alkyne into the carbon-zirconium bond of 10 gives silazirconacyclopentene 11. When CuCl and allyl chloride were added to a THF solution of silazirconacyclopentene 11, tetrasubstituted alkene... 

To examine the reactivity of the silyl-zirconium bond, Mori et al. used Me2PhSiLi instead of Ph2tBuSiLi. A THF solution of Me2PhSiLi 8b (1 equiv.) was added to a THF solution of Cp2ZrCl2 (1 equiv.) and diphenylacetylene 14a (1 equiv.) at -78 °C, and the solution was stirred at room temperature for 3 h. After hydrolysis of the reaction mixture, vinylsilane 15a was obtained in 36% yield along with the starting material 14a in 40% yield. When the reaction mixture was treated with D20, compound 15a-D2 having two deuteriums was obtained. One deuterium was introduced at the vinylic position and the other was incorporated into the methyl proton on the silicon (39% yield, each D content quant.). If insertion of alkyne 14a into the zirconium-carbon bond of lb occurs, vinylsilane 15a should be formed, but in this case only one deu-... 

The reasons why alkynes 14 and 19 having aryl or vinyl groups, respectively, insert into the zirconium-silicon bond of 3, whereas the alkyne 17 having alkyl groups inserts into the carbon-zirconium bond of 3, are still not clear. Presumably, electronic factors are important for the insertion reaction. 

The possible reaction mechanism for the formation of 31 is shown in Scheme 15. Insertion of alkyne 14 into silazirconacyclopropane 3 gives silazirconacyclopentene 22. Then, insertion of carbon monoxide into the carbon-zirconium bond in silazirconacyclopentene 22 gives silazirconacyclohexenone 34, whose carbonyl oxygen would coordinate to zirconium metal. Then the zirconium carbon bond migrates onto silicon to afford oxazirconacyclohexene 36 via 35 [26]. Deuterolysis of 36 would afford 31-D2, which has two deuteriums. 

Once single zirconacyclopentadienes 20 are formed, only the regiospecific reaction of one carbon-zirconium bond with an electrophile could lead to the stereoselective preparation of metalated dienes 26 (Scheme 13). 

This is also the evidence of a substantial fraction of ionicity of zirconium bonds with ligands. However, zirconium forms strong directed bonds. With coordination number 8, it uses hybrid p-orbitals of d sp type, that leads to dodecahedral configuration of bonds. Thus, zirconium compounds are prone to hydrolysis and hydrolytic condensation. In particular, in the solutions of zirconium chloride ZrOCl2 SHjO, a tetramer [Zr4(0H) (H20)sl° is the prevailing form, which is able to participate in further polymerisation processes. According to the data [42], the structure of the tetramer corresponds to the following scheme ... 

Creemers, H. M. J. C. Verbeek, F. and Noltes, J. G., Studies in Group IV organometallic chemistry XXX. Synthesis of compoimds containing tin-titanium and tin-zirconium bonds. J. Organomet. Chem. 15(1968) 125-130. 

In fact, cleavage of the carbon-zirconium bond occurs classically with various electrophiles (see Electrophile), generally under mild conditions. In addition, the other carbon-metal bond in gem metaUozirconocenes undergoes a broad range of transformations. The different reactivities of the carbon-metal and carbon-zirconium bonds are the results of the difference of electronegativity of the two metal centers and the two different bond polarities. 

Acylzirconocene chlorides are easily accessible in a one-pot procedure through the hydrozirconation see Hydrozirconation) of alkene or alkyne derivatives with the Schwartz s reagent and subsequent migratory insertion see Migratory Insertion) of carbon monoxide into the alkyl- or alkenyl zirconium bond. The stability of the acylzirconocene chlorides is remarkable at room temperature, and consequently allows many applications in organic synthesis. 

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