Zirconocene hydrides, preparation

Scheme 8-5 Zirconocene hydrides prepared in situ alternative to the Schwartz s reagent, ...

Binuclear cationic zirconocene hydrides can be prepared directly from the zirconocene dihydride precursor. Thus, the reaction of the zirconocene dihydride 626 with a trityl borate salt produces the binuclear hydrido complexes 627457 (Scheme 141). The synthesis of 627 may be accompanied by the formation of cationic zirconocene hydrides of higher nuclearity. For example, cationic trinuclear hydrido complex 628 was also isolated during the recrystallization... 

Schwartz s Reagent3 is available commercially (from the Aldrich Chemical Company, Inc.) although it is quite expensive. Two literature preparations of this important reagent are available. The first utilizes LiAI(OtBu)3H to reduce zirconocene dichloride.4 The second method utilizes sodium bis(2-methoxyethoxy) aluminum hydride (RED-AL) as the reducing agent.2a The disadvantages of these procedures have been discussed.3... 

Recently, Oshima et al. developed the conversion of acid chlorides into the corresponding homoallylic alcohols catalyzed by in r(/ -prepared hydridozirconium allyl reagents (Scheme 41),147 147a The proposed mechanism suggests an initial hydride transfer from the zirconocene crotyl hydride species, in equlibrium with its Cp2Zr(l-alkene),147a to the acid chloride with subsequent allylation to afford the corresponding homoallylic alcohols. 

The addition of Cp2Zr(H)Cl, known as the Schwartz reagent [30], to different alkenes and alkynes is known to be a facile process [31]. Therefore, the hydrozirconation of a variety of readily available enynes 12 is among the first methods developed for the stereoselective preparation of dienyl zirconium reagents 13. This process is both completely chemo- and regioselective with a syn addition of the zirconium hydride across the alkyne [32] (Scheme 5). From the same intermediate, the Zr atom can be isomerized in its internal position such as in 15 via a zirconacyclopropene intermediate 14. Moreover, the addition of trimethylstannyl chloride to 14 led to the stannylated dienyl zirconocene 16 [33] (Scheme 5). 

Analogous dienyl zirconium compounds can be prepared by addition of unconjugated dienes that contain enol ethers to 105 (Scheme 19).56-58 These reactions are believed to proceed by initial coordination of the terminal olefin followed by isomerization via zirconocene allyl hydride intermediates to eventually yield zirconacyclopentanes that... 

Hydrocarboxylation can also be used in the synthesis of heterocycles if the heterofunction is introduced at an appropriate position within the substrate. Thus, preparation of 2,4-disub-stituted 2-buten-4-olides can be achieved via stoichiometric transformation of a protected optically active propargyl alcohol without loss of optical activity30. Sequential treatment with zirconocene chloride hydride (Schwartz s reagent), carbon monoxide, and iodine gives a 55% yield of (S)-2-ethyl-4-isobutyl-2-buten-4-olide30. This reaction resembles the intramolecular hydrocarboxylation of allylic alcohols. 

Preparation. This hydride is prepared by reduction of dicyclopentadienyl-zirconium dichloride (zirconocene dichloride) with either LiAlH(0-r-Bu)3 or NaAlHj (OC2 H4 OCH3 >2... 

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. ... 

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