Allylic Zirconium Reagents

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. 

Other acyl organometallic reagents are known. An acyl zirconium reagent, such as RCOZr(Cl)Cp2, reacted with allylic bromide in the presence of Cul to give the corresponding ketone, but with allylic rearrangement. ... 

Whereas ethylmagnesiation of allyl alcohols affords the syn diol with 95 5 diastereoselectivity, the same reaction on the allyl methyl ether affords predominantly the anti diastereomer with 90 10 selectivity [92] (Scheme 7-80). The reversal of stereoselectivity was attributed to the association of the magnesium alkoxide (and not of ethers) with the zirconium reagent (Scheme 7-80). 

The preparation of allylic zirconium reagents from allylic ethers and a low valent zirconium source has been described [187]. The subsequent reaction with aldehydes proceeds in good to excellent yield to afford homoallylic alcohols. The diastereoselectivity observed is highly dependent upon the substitution of the Start-... 

Table 10-31. Addition of allylic zirconium reagents to aldehydes.

The generation and subsequent reaction of oxy-functionalized allylic zirconium reagents to a wide range of aldehydes proceeds with excellent anti selectivity [188]. Allylic zirconium reagents can also be prepared from the hydrozirconation of allenes [189]. Very high levels of diastereoselectivity for both simple aliphatic and aromatic aldehydes are observed in these reactions for the production of the anti homoallylic alcohol (Scheme 10-97). 

Some examples of combined syn-anti and diastereofacial selectivity employing /V-n-propyl- and N-isopropyl-aldimines, derived from a-phenylpropionaldehyde, and crotyl-9-BBN, -magnesium and -zirconium reagents have been reported by Yamamoto et al. Cram selectivity, which is observed in the analogous reactions of these chiral imines with allyl organometallics (see Section 4.3.2.1.2i), is preserved as ratios of Cram anti-Cram products are consistently about 8 1. Anti selectivity is also observed but the ratios do not exceed 7 3. The weak anti selectivity parallels that observed in reactions of crotyl-9-BBN with branched a-alkylaldimines. Since syn-anti selectivity is influenced more by the a-substituent than by the A-substituent of the aldimine, more synthetically useful levels of combined syn-anti and diastereofacial selectivity might be expected in other series of a-substituted aldimines. 

Allyl chromium, titanium and zirconium reagents fall into the Type 3 category. Enantioselective reactions in this class are relatively rare, although the diastereoselectivities can be quite high (reviews [7,8,15]). 

A particularly clear example of syn versus anti addition was carried out in stoichiometric fashion using vinyl-zirconium reagents, in direct comparison with the stereochemical outcome of the catalytic allylic alkylation with soft anions (Scheme 17). The zirconium reagent gives the natural steroid configuration, while the malonate anion gives the opposite epimer. ... 

J.2 Vinylzirconium Reagents Addition to a.P-Unsaturated Ketones 1.53 J Allylic Titanium and Zirconium Reagents... 

Very few Ni-allene complexes have been isolated and characterized, but many such species have been postulated as key intermediates in organic transformations catalyzed by Ni precursors. Notable examples of such applications include the cyclization of two arynes and one allene to give 10-methylene-9,l-dihydrophenanthrenes, " the one-pot coupling of allenes with aldehydes and silanes to give allyl silyl ethers, selective [3+2]-cyclization of allenes and Fischer carbenes, regio- and stereoselective, three-component assembly of allenes with aryl iodides and alkenyl-zirconium reagents, and regio- and chemoselective [2+2+2]-cycloaddition of electron-deficient diynes with allenes to polysubstituted benzene derivatives. ... 

The corresponding zirconium allyl derivatives have also been investigated6,7, but, in general, do not have any particular advantages over the titanium reagents. In addition, the starting materials are often more expensive and more difficult to purify. 

I.3.3.3.8.2.I.I. With Titanium(IV) and Zirconium(IV) Reagents Nonhetero-Substituted Allylic Reagents... 

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