Allylchromium

The advantages of allylchromium reagents in stereoselective synthesis were soon shown7 8- 9. Due to mild reaction conditions, sensitive functionalities - even epoxide rings10 -... 

Allylchromium reagents of similar reactivity may also be generated from allyl tosylates1, acrolein diethyl acetal13, and vinyloxiranes14. 

Ethyl (Z)-2-bromomethyl-2-heptenoate and aldehydes condense on reaction with chromium(II) chloride to furnish cw-3,4-disubstituted dihydro-3-methylene-2(3 //)-( uranones exclusively16, indicating that a (Z)-allylchromium complex might serve as reactive intermediate in the. mv-selec-tive addition step due to the bulky 2-substitucnt. Alternatively, an acyclic transition state for the reaction of the ( )-diastereomer, mediated by the Lewis acid dichloroaluminum hydride, has been discussed16. 

Oxygen-substituted allylchromium reagents are conveniently generated from vinyloxi-ranes by chromium(II) chloride in the presence of lithium iodide and aldehydes with high anti diastereoselectivity14. qh oh... 

The diastereofacial selectivity of addition of achiral allylchromium to chiral aldehydes is mainly determined by steric approach control even a- and /5-alkoxy-subsliluents apparently do not give rise to a rate-determining chelation9. 

The steady-state polymerization in the presence of Cr (it-CsHb) was first order with respect to the monomer concentration (125) the effective activation energy was 4.7 0.5 kcal/mole. When the concentration of Crfir-CaHs was varied, first a linear rise of the polymerization rate occurred with an increase of tris-ir-allylchromium concentration to the upper limit then the rate does not depend on Crfx-CaHs concentration (126). The value of the upper limit of the polymerization rate increased with an increase in the water content of the solvent used. 

Tris-7r-allylchromium, Cr(ir-C3H5)3. Powdered CrCl3 3THF was added to 3.5 equivalents of C3H5MgCl in (C2H5)20 solution at — 20°C. Evaporation of the ether solution at — 20 °C yields a dark colored solid which on extraction with pentane at — 20 °C yielded a blood red solution identified by IR as Cr(7r-allyl)3 (Figure 2) (38). 

This topological rule readily explained the reaction product 211 (>90% stereoselectivity) of open-chain nitroolefins 209 with open-chain enamines 210. Seebach and Golinski have further pointed out that several condensation reactions can also be rationalized by using this approach (a) cyclopropane formation from olefin and carbene, (b) Wittig reaction with aldehydes yielding cis olefins, (c) trans-dialkyl oxirane from alkylidene triphenylarsane and aldehydes, (d) ketenes and cyclopentadiene 2+2-addition, le) (E)-silyl-nitronate and aldehydes, (f) syn and anti-Li and B-enolates of ketones, esters, amides and aldehydes, (g) Z-allylboranes and aldehydes, (h) E-alkyl-borane or E-allylchromium derivatives and aldehydes, (i) enamine from cyclohexanone and cinnamic aldehyde, (j) E-enamines and E-nitroolefins and finally, (k) enamines from cycloalkanones and styryl sulfone. 

It is interesting to compare the above results with the reactions of other allyl metal reagents. Whereas allyllithium and allylmagnesium are slightly ketone-selective (Tables 3-5), allylboranes911 as well as certain allylchromium derivatives92)... 

The addition to a,p-unsaturated aldehydes proceeds exclusively in a 1,2-fashion (equation 24). This regiochemical preference is general for all organochromium compounds. The addition to 4-r-butylcyclo-hexanone (32) occurs predominantly via equatorial addition (equation 25), as it provides (33) and (34) in an 88 12 ratio. Allylchromium is one of the most efficient reagents for this transformation (Table 1). 

Table 8 Enantioselective Addition of Allylchromium Reagents to Aldehydes (equation 61)...

Allylzinc and allylchromium reagents coordinated by chiral ligands have variable degrees of success in the preparation of optically active homoallylic alcohols. 

A-Protected amino aldehydes have been employed in the ehromium-mediated reaction of allylic halides [146]. A series of amino aldehydes bearing various N-protective groups are treated with allyl bromide and CrCl2 to produce the homo-ally lie alcohols 243 and 244 (Table 10-26). The reactions are very unselective with the simple allylchromium reagents unless a disubstituted amino group is employed. 

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