Like the aldol reaction, the Diels-Alder reaction of Fischer carbene complexes occurs with the retention of the chromium-carbon double-bond in the product. We had rationalized that -unsaturated Fischer carbene complexes should be very active dienophiles based on two known facts. The first is shown in Scheme VIII, which is that the methyl complex 76 is very acidic (pKa = 12) suggesting that the chromium-carbon double-bonded unit is a very strong electron-withdrawing group.

The second is shown in Scheme IX and is the comparison of the chemical shift of the carbene carbon of the vinyl complex 90 versus that of corresponding ester 94.  The carbene carbon is at d = 323 ppm which is much father downfield than the ester 94 (d = 167).  This data suggests that the carbene complex should have a much greater electron withdrawing effect on the double-bond than the ester group and thus that complex 90 should be a much stronger dienophile than the ester 94.  Our expectations were realized when we found that the complex 90 would react with isoprene 21,000 times faster than the ester 94 [1].[In addition to the increased rate, the reaction of the complex 90 is also more regioselective than the ester 94 (92 : 8 vs. 70 : 30).  The increased rate and increased regioselectivity observed for the reaction of the complex 90 is comparable to the increase in rate (74,000) and regioselectivity (95 : 5) observed when the reaction of methyl acrylate and isoprene is catalyzed by aluminum chloride. Since the cycloadduct carbene complexes 92 and 93 

can be efficiently converted to the esters 95 and 96 by a variety  of mild oxidizing agents, -unsaturated carbene complexes can serve as surrogates for esters in the Diels-Alder reaction when the use of Lewis acid is best avoided. The Diels-Alder reaction of Fischer carbene complexes can also be extended to acetylenic complexes. The reaction of complex 97 with 2,3-dimethylbutadiene gives the cycloadduct 99 in 89 % yield at 50 ^oC in 4 hours [2]. This is an -unsaturated complex, which should be useful in the benzannulation reaction.  In fact, the reaction of 99 with 1-pentyne gives the benzannulation product 98 in 97 % yield.  This overall transformation can be carried out in a one-pot process with all three reagents together since the carbene complex will selectively react with the diene over the alkyne. The one-pot reaction gives the benzannulated product 98 in 80 % yield.  This benzannulation gives a benzene ring rather than a cyclohexadienone since a silyl group at either R¹ or R² in intermediate 16 (Scheme V) will tautomerize via 1,3-migration to oxygen which conveniently gives a protected phenol chromium tricarbonyl complexes which, as discussed above (Scheme IV), are stable to air and can be isolated.

 One of the limitations of the Diels-Adler reaction is, of course, that it is endo selective for most dienophiles and dienes.  We have found that imidazolidinone carbene complexes of the type 103 are exo selective [3].[The reason for this is thought be a consequence of the preference the s-cis conformation over the s-trans conformation of 103. The Diels-Alder reaction from the s-cis conformation results in close contacts between the substituent R³ on the diene and a cis carbon monoxide ligand on the metal that are present in the endo transition states but not in the exo transition states. In addition, the chiral complex 103 results in complete facial selectivity in the addition to the diene leading to a single diastereomer of 105 and, after removal of the chiral auxiliary, a single enantiomer of the exo cycloadduct 106. Since the Diels-Alder reactions of imidazolidinone complexes can be highly exo selective it is anticipated that these complexes would have unique applications in organic synthesis that are not possible by traditional Diels-Alder strategies.