Zirconium alkenyl

Erker and co-workers (41, 41a) have studied the photochemistry of several zirconium alkenyl complexes and observed both isomerization of the alkenyl ligand and reductive coupling of the alkenyl ligand with X. 

The extent to which each reaction occurred was found to relate closely to the nature of the ligand X. For example, irradiation of ( )-ZrCp2(Cl) (CH=CHPh) gave, after 2.5 hours, a photostationary mixture of E and Z isomers of the zirconium alkenyl complex [Eq. (41)], in a 30 70 ratio. 

Photostationary equilibria between E and Z isomers were also observed with the three zirconium alkenyl complexes shown in Eq. (43), but these... 

The Roussel-Uclaf group has published full details of the synthesis of dihydro-10,11 -PGA2, and Trost and his co-workers have described an interesting chiral synthesis of the PG analogue (92). The application of conjugate addition of zirconium alkenyls to cyclopentenones leading to prostaglandin intermediates has been reported, and Kondo et a/. have extended their approach to... 

Hydrozirconation of the tert-butyl ester of 5-hex-ynoic acid 1 proceeds in 81% yield with little discernible competitive reduction of the ester functional group, whereas comparative hydrozirconation of the methyl ester gave only 41% of the desired zirconium alkenyl, with a large amount of reduced carboxylate product [3] (Scheme 12.2). 

The hydrides can also be used to form primary alcohols from either terminal or internal olefins. The olefin and hydride form an alkenyl zirconium, Cp2ZrRCl, which is oxidized to the alcohol. Protonic oxidizing agents such as peroxides and peracids form the alcohol direcdy, but dry oxygen may also be used to form the alkoxide which can be hydrolyzed (234). 

Mixed-Metal Systems. Mixed-metal systems, where a zirconium alkyl is formed and the alkyl group transferred to another metal, are a new apphcation of the hydrozirconation reaction. These systems offer the advantages of the easy formation of the Zr—alkyl as well as the versatiUty of alkyl—metal reagents. For example, Cp2ZrRCl (R = alkyl or alkenyl) reacts with AICI3 to give an Al—alkyl species which may then be acylated with... 

Alkenyl zirconium complexes derived from alkynes form C—C bonds when added to aHyUc palladium complexes. The stereochemistry differs from that found in reactions of corresponding carbanions with aHyl—Pd in a way that suggests the Cp2ZrRCl alkylates first at Pd, rather than by direct attack on the aUyl group (259). 

Carbon-carbon bond formation from alkyl and alkenyl zirconium reagents usually involves transmetallation reactions and are discussed in Chapter 8. 

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... 

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... 

For application in organic synthesis, the regiochemistry of insertion of carbenoids into un-symmetrical zirconacydes needs to be predictable. In the case of insertion into mono- and bicydic zirconacydopentenes where there is an wide variety of metal carbenoids insert selectively into the zirconium—alkyl bond [48,59,86], For more complex systems, the regiocon-trol has only been studied for the insertion of lithium chloroallylides (as in Section 3.3.2) [60]. Representative examples of regiocontrol relating to the insertion of lithium chloroal-lylide are shown in Fig. 3.2. 

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. 

General structure 24 is used throughout to indicate a wide variety of zirconacyclopentanes and zirconacyclopentenes. Generally, these are unsubstituted on alkyl carbons a to zirconium, whereas alkenyl carbons generally have an alkyl, aryl, or trimefhylsilyl substituent a to the zirconium. 

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],... 

The first example of a stable 1,1-bidentate Lewis acid based on boron and zirconium has been reported [35]. The synthesis of 22 is outlined in Scheme 7.12. Treatment of hex-l-yne with HBBr2 Me2S followed by conversion of the dibromoboronic ester to the corresponding alkenyl boronic acid and esterification with propane-1,3-diol provided the alkenyl boronic ester. Hydrozirconation of this compound with 3 equivalents of the Schwartz reagent, Cp2Zr(H)Cl [57], afforded the desired product 22 in 86% yield. 

For the asymmetric 1,4-addition of alkenyl groups in aprotic media, alkenyl zirconium reagents can be used, which are generated by hydrozirconation of terminal alkynes (Figure 3.30). Under these conditions, alkenyl groups derived from alkylacetylenes are efficiently installed, but those from arylacetylenes are not as effective (entry 3). 

Casey was able to prepare related zirconocene alkenyl complexes according to Scheme 8.18. Alkene coordination was established by a number of NMR techniques. While zwitterionic compounds 38 allowed the determination of the alkene dissociation energy, AG = 10.5 kcal mol , very similar to that of 35. Thermally more stable complexes were obtained by protonation of 37 with [HNMePh2][B(C5F5)4[. Dynamic NMR spectroscopy and line shape analysis allowed the measurement of the barriers of alkene dissociation (AG = 10.7 and 11.1 kcal mol ), as well as for the site epimerisation ( chain skipping ) at the zirconium center (AG = 14.4 kcal mol" ) (Scheme 8.19) [77]. 

More recently, a novel metal-substituted methylenecyclopropene (triafulvene) derivative was obtained when bis(propyne)zirconocene was treated with one equivalent of tris(pentafluorophenyl)borane, followed by excess of benzonitrile (equation 367)430. The first step involves alkynyl ligand coupling to give the isolable Cp2Zr(//-2,4-hexadiyne)B(C6F5)3 betaine. This undergoes a formal intramolecular nitrile insertion into the Zr—C(sp2) c-bond of the adjacent alkenyl zirconocene unit, leading to the zirconium-boron triafulvene-betaine. X-ray analysis of the triafulvene confirmed the planar... 

Alkenyl-aluminum and -zirconium derivatives have been found to couple with allyl halides in the presence of Pd° catalysts (equation 38), although simple alkyl-aluminum and -zirconium reagents fail in the reaction.154 The 1,4-dienes thus generated are important intermediates in organic synthesis. 

C-M bond addition, for C-C bond formation, 10, 403-491 iridium additions, 10, 456 nickel additions, 10, 463 niobium additions, 10, 427 osmium additions, 10, 445 palladium additions, 10, 468 rhodium additions, 10, 455 ruthenium additions, 10, 444 Sc and Y additions, 10, 405 tantalum additions, 10, 429 titanium additions, 10, 421 vanadium additions, 10, 426 zirconium additions, 10, 424 Carbon-oxygen bond formation via alkyne hydration, 10, 678 for aryl and alkenyl ethers, 10, 650 via cobalt-mediated propargylic etherification, 10, 665 Cu-mediated, with borons, 9, 219 cycloetherification, 10, 673 etherification, 10, 669, 10, 685 via hydro- and alkylative alkoxylation, 10, 683 via inter- andd intramolecular hydroalkoxylation, 10, 672 via metal vinylidenes, 10, 676 via SnI and S Z processes, 10, 684 via transition metal rc-arene complexes, 10, 685 via transition metal-mediated etherification, overview,... 

Before adding aldehyde 14 a transmetalation from zirconium to zinc is necessary because of low reactivity of the sterically hindered organozirconocene compounds like 18 toward most organic electrophiles.9 Resulting alkenylethylzinc 19 reacts in a 1,2-addition with the cr,y3-unsaturated aldehyde 14 transferring exclusively the alkenyl moiety. The formation of Z -allylic alcohol 20 reveals stereochemical retention of the double bond configuration in the transmetalation and addition steps. 

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