Zirconacycle substituted

A general method for the synthesis of highly substituted styrenes as 6/4-91, vinyl-cyclohexadienes and related compounds was developed by Xi, Takahashi and coworkers [301] by reacting an intermediately formed five-membered zirconacycle 6/4-89 with propargyl derivatives 6/4-90 or allyl bishalides in the presence of CuCl (Scheme 6/4.21). 

Formation and characterization of zirconacyclopropanes and zirconacyclopropenes In addition to / -H abstraction of dialkylzirconocenes discussed earlier (Schemes 5 and 6), several other methods are also available for the preparation of three-membered zirconacycles as summarized in Scheme 35. From the viewpoint of cyclic carbozirconation reactions, especially those under Zr-catalyzed conditions, /3-H abstraction, 7t-ligand substitution, and decarbometallative ring contraction are particularly important. As such, these three-membered zirconacycles are generally unstable, but they can be stabilized with phosphines, for example, PMe3, and other bases, and are fully identified. Some of the well-identified examples are shown in Scheme 36.206-209... 

It is observed that insertion into a zirconacyclopentene 163, which is not a-substituted on either the alkyl and alkenyl side of the zirconium, shows only a 2.2 1 selectivity in favor of the alkyl side. Further steric hindrance of approach to the alkyl side by the use of a terminally substituted trans-alkene in the co-cyclization to form 164 leads to complete selectivity in favor of insertion into the alkenyl side. However, insertion into the zirconacycle 165 derived from a cyclic alkene surprisingly gives complete selectivity in favor of insertion into the alkyl side. In the proposed mechanism of insertion, attack of a carbenoid on the zirconium atom to form an ate complex must occur in the same plane as the C—Zr—C atoms (lateral attack 171 Fig. 3.3) [87,88]. It is not surprising that an a-alkenyl substituent, which lies precisely in that plane, has such a pronounced effect. The difference between 164 and 165 may also have a steric basis (Fig. 3.3). The alkyl substituent in 164 lies in the lateral attack plane (as illustrated by 172), whereas in 165 it lies well out of the plane (as illustrated by 173). However, the difference between 165 and 163 cannot be attributed to steric factors 165 is more hindered on the alkyl side. A similar pattern is observed for insertion into zirconacyclopentanes 167 and 168, where insertion into the more hindered side is observed for the former. In the zirconacycles 169 and 170, where the extra substituent is (3 to the zirconium, insertion is remarkably selective in favor of the somewhat more hindered side. 

Synthesis of an unsymmetrically substituted bis-zirconacycle was achieved by the sequence shown in Eq. (28).76... 

Further transformations of such zirconacycles are possible before the organic product is removed from the metal center. Buchwald followed the insertion of an alkyne into a zirconaaziridine with the insertion of CO to obtain substituted pyrroles (in a single pot at ambient temperature) in moderate to good yields. A diverse array of substituents can be introduced by varying the amine and alkyne examples are shown in Table 4. The reaction is tolerant not only of various alkyls and aryls, but also of thiophenyl, furyl, and pyrrole substituents, and can create 2,2 -bipyrroles (see Table 4). It does, however, require high CO pressure (as much as 1,500 psi can be necessary) [56]. 

The synthesis of cyclopropanes from carbonyl compounds represents a new approach for converting zirconacycles into carbocycles via a deoxygenative ring contraction under Lewis acid activation. It differs in that from the spontaneous Kulinkovich process. Further progress would involve extension to the synthesis of 1,2-substituted cyclopropanes and the development of catalytic and enantioselective variants. 

The protonolysis of the equally substituted zirconacycle 27 with weak acids such as ethanol leads to the monoprotonated dienyl zirconocene compound 28, which is subsequently converted into substituted diene 29 by a palladium crosscoupling reaction with aryl iodide (Scheme 14) [44]. Selective halogenation of such zirconacycle leads, as well, to the formation of zirconated diene 30 and then to the 1,4-dihalogenodiene 31 (Scheme 15) [45]. 

The reactions of complex (25) with carbonyl compounds show a strong dependence on the substituents leading to the substitution or elimination of the silyl-substituted acetylene or the insertion into the Zr-C bond. Moreover, the reaction of methacrolein with (25) depends strongly of the solvent used in hexane elimination of bis(trimethylsilyl)acetylene leads to the formation of dimer (33) while in THF insertion with the formation of zirconacycle (34) is observed. 

Codimers with alkynes can also be obtained from 1,2-diphenylcyclopropene when the trimethylphosphane-stabilized metallocene complexes 20 (M = Ti, Zr) of this cyclopropene derivative are used as substrates. The titanocene complex is prepared from bis(>/ -cyclopen-tadienyl)bis(trimethylphosphane)titanium (18), while the zirconocene complex is most conveniently formed from (f7 -but-l-ene)bis(f7 -cyclopentadienyl)(trimethylphosphane)zirconium (19) via a substitution reaction of the butene ligand. When 20 (M = Ti) is reacted with but-2-yne, the titanacycle 21 can be isolated as blue crystals in 71% yield. The zirconacycle 21 (M = Zr) is obtained in 78% yield as an orange powder. ... 

Reductive coupling of enynes or dienes perhydroindoles. Reaction of ZrCp2 with the cyclohexenylaminc 1 in THF at 25° results in a zirconacycle (a) that is hydrolyzed by HCI (10%) to the perhydroindole 2 (75% yield). The Zr—C bonds can be cleaved by various other electrophiles to give substituted perhydroindoles. Treatment of a with CO gives the tricyclic ketone 3. 

Lithiation of commercially available bromoarenes (281) followed by treatment with zirconocene (methyl) chloride affords zirconocene complexes of substituted benzynes (283), which react with symmetric alkynes and l-(trimethylsilyl)propyne to give single regioisomeric zirconacycles, (284) and (285), respectively (Scheme 66). Both (284) and (285) react with disulfur dichloride to produce benzothiophenes (286) and (287), respectively, in 60-80% isolated yields in a one-pot procedure <89JOC2793>. Protodesilylation of (287) can be accomplished in >90% yield to give the corresponding 2-unsubstituted benzothiophenes by treatment with tetrabutylammonium fluoride in tetrahydrofuran. Application of the procedure to the preparation of 2,3-dihydrobenzo[6]thiophenes was also reported <9iOM537>. 

The co-cyclisation of substituted enynes is often highly diastereoselective. Substituents next to the alkene component exert complete control over the adjacent ring junction stereochemistry for example, 21 and 22 are formed as single isomers. In some cases the zirconacycles must be given time to equilibrate thermally to the more stable isomer in order to achieve high levels of diastereocontrol (the formation of zirconacyclopentenes is a reversible process). 

The X-ray structures of M(diene)Cp2 (M = Zr, Hf) suggest they are best regarded as a rir complexes but the u/ir ratio is less for M = Zr and acyclic dienes. These structural differences affect the reactivity.The bis-phosphine complex Ti(PMe3)2Cp2 reacts with HCsCH to give the titanacycle Ti(CH=CHCH=CH)Cp2 and polyacetylene via Ti(HCCH)(PMe3)Cp2. Substituted alkynes lead to various isomeric metallacycles. Zirconacycles Zr(CPh=CRCR=CPh)Cp 2 arise in the reaction of [ZrH(y-H)Cp 2 2 PhCsCR (R = H, Ph)." The... 

In organic chemistry, A -heterocyclic compounds are cyclic compounds containing one or more nitrogen atoms. A -Heterocyclic compounds include aromatic A -heterocycles such as pyrrole, pyridine, and imidazole, as well as saturated A -heterocycles such as aziridine, piperidine [1]. A -heterocyclic compounds are very important motifs in biochemical compounds such as nitrogenous bases, as well as pharmaceuticals and materials (Fig. 1.1). Significant synthetic efforts had been made toward A -heterocycles with different structures and substitutents [1] however, it is still demanding to develop new synthetic methods toward A -heterocyclic compounds, especially via metallacycles such as zirconacycles. 

Synthesis of Substituted Heterocycles Cu-mediated intermolecular coupling reaction of zirconacycles with dihalogenated heteroaromatic compounds is applicable for the synthesis of fused aromatic heterocycles. Zirconacyclopentadi-ene reacted with 2-iodo-3-bromothiophene in the presence of 2 equiv of CuCl and DMPU at 50 °C to afford the corresponding benzothiophenes 71. When 2-chloro-3-iodopyridine and 4-chloro-3-iodopyridine were used, the corresponding substituted quinolines 72 and isoquinolines 73 were obtained in high yields, respectively (Scheme 11.28) [28],... 

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