Crotyl enol carbonate

Scheme 5. Formation of intenuediates 32 and 33 (a) and mechanistic rationale for the outcome of the Pd-catalyzed crotylation of enol carbonate 10 (b).

The six-member ed transition state for the reaction of an allylic borane or boron ate is very reminiscent of the cyclic transition state for the aldol reaction you met in Chapter 34. In this case the only change is to replace the oxygen of the enolate with a carbon to make the allyl nucleophile. The transition state for the aldol reaction was a chair and the reaction was stereospecific so that the geometry of the enolate determined the stereochemistry of the product aldol. The same is true in these reactions. -Crotyl boranes (or boronates) give anti homoallylic alcohols and Z-crotyl boranes (or boronates)... 

It must be stressed that in the sulfur series, the reaction of crotyl halides takes place regioselectively, leading to S-crotyl derivatives (examples [42, 47]). The oxygen series stand in contrast, with reactions of enolates proceeding on the carbon atom, and not on oxygen. Furthermore, it is not regioselective with crotyl-type halides. 

Addition of an T -allyl-Fp complex to this compound affords an T -aIlyl-Fp-substituted cycloheptatriene system. Two double bonds are involved in an (T -diene)iron complex. The remaining free double bond of the silyl enol ether attacks as a nucleophile at the cationic r -alkene-Fp moiety to form an (Tj -diene)iron complexed cyclopentane annulated cycloheptadienone. Treatment with CAN in methanol under carbon monoxide atmosphere releases the methoxycarbonyl-substituted free ligand (Scheme 4-25). Reaction of the Ti -dienyliumiron intermediate of Scheme 4-25 with an ( , Z)-isomeric mixture of ri -crotyl-Fp proceeds with high diastereoselectivity. Four new stereogenic centers are formed in the course of this formal [3+2] cycloaddition. A hetero [3+2] cycloaddition is also feasible between T -ailyl-Fp complexes and aromatic aldehydes in the presence of zinc chloride or titanium(IV) chloride to provide tetrahydrofuran derivatives (Scheme 4-26). A 1,2-shift of the iron complex fragment occurs in the course of this reaction. Employment of imines affords the corresponding pyrrolidines. ... 

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