CHROMIUM AND TUNGSTEN PENTACARBONYL GROUPS AS REACTIVITY AUXILIARIES IN THE DIELS-ALDER REACTIONS OF ALKENYL CARBENE COMPLEXES WITH 1,3-DIENES

The Diels-Alder reactions of Fischer carbene complexes of the group 6 metals bearing alkenyl substituents on the carbene ligand are surveyed for 10 chromium, molybdenum, and tungsten complexes with 11 1,3-dienes. The reactions of complexes of the type R1CH=CHC(OMe)=M(CO)5 (R1 = H, CH3 (trans), M = Cr, Mo, W) have been examined with isoprene, and it was found that the rates can be as much as 2 ⨯ 104 faster than their organic ester analogues. The regioselectivity of the cycloadditions of alkenyl carbene complexes with isoprene were also found to be significantly higher (⩾91:9) than their organic esters analogues and comparable to that observed with Lewis acid mediated cycloadditions of acrylate esters with isoprene. The cycloadditions of the seven complexes of the type R1R2C=CR3C(OMe)=M(CO)5 (R1, R2, R3 = H; R1 = CH3, R2, R3 = H; R1, R3 = H, R2 = CH3 (cis and trans); R1 = H, R2, R3 = CH3: M = Cr, W) were investigated with 2,3-dimethyl-1,3-butadiene and cyclopentadiene. The reactions with cyclopentadiene were found to be stereoselective in favor of the Alder endo adduct at a level much higher than is observed for the corresponding α,β-unsaturated esters. The reaction with cyclopentadiene was also found to be stereospecific with cis- and trans-propenyl tungsten complexes where the stereochemistry about the olefin in each carbene complex was retained in the cycloadducts. The cycloadditions of six complexes of the type R1R2C=CR3C(OMe)=M(CO)5 were also investigated with three monooxygenated dienes: 1-methoxy-, 2-methoxy-, and 1-acetoxy-1,3-butadiene. It was found that these carbene complexes display greatly enhanced reactivity compared to α,β-unsaturated esters with these dienes under either thermal or high-pressure conditions. All of the reactions of these three acyclic dienes with alkenyl carbene complexes were highly regioselective; however, like their organic ester analogues, they occurred with relatively low stereoselectivity. The endo adduct (but not the exo) from the reaction of the trans-propenyl chromium (but not tungsten) complex 12 with 1-methoxy-1,3-butadiene forms a methoxyl-chelated tetracarbonyl carbene complex. Several cycloadditions were examined for four complexes of the type R1R2C=CR3C(OMe)=W(CO)5 with Danishefsky's diene and trans, trans-1-methoxy-2-[(trimethylsilyl)oxy]-4-ethoxy-1,3-butadiene (27). The reaction of the Zrens-propenyl tungsten complex with diene 27 occurs with retention of the stereochemistry in both the diene and the carbene complex. The reaction of the ris-propenyl tungsten complex with 27 occurs with competing isomerization to the Zra/v-propenyl complex prior to cycloaddition. The chromium and tungsten cyclohexenyl complexes C6H9C(OMe)=M(CO)5 were found to display different chemoselectivities toward derivatives of Danishefsky's diene; the chromium complex produced divinylcyclopropanes, whereas the tungsten complex gave rise to Diels-Alder adducts. The reaction of the vinyl tungsten complex CH2=CHC(OMe)=W(CO)5 with 6,6-dimethyl-6-sila-α-pyran (28) illustrates the advantages of rate and tolerance of sensitive organic functionality that are possible when alkenyl carbene complexes are employed as synthons in the Diels-Alder reaction. Finally, the versatility with which the metal unit can be removed from cycloadducts 43 and 44 demonstrates that alkenyl carbene complexes can serve as synthons in the Diels-Alder reaction for esters, aldehydes, ketones, methoxyallenes, 2-methoxybutadiene, and simple alkenes. © 1990, American Chemical Society. All rights reserved.