Diels–Alder Reactions at Carbene Ligands: A Route to Olefin‐Carbene Complexes

Abstract

The stereocontrol exerted by the (C0)5Cr fragment is revealed in the endo/exo ratio of (2b + 3)/2a =60/40. A similar ratio is observed for the isolobal methyl methacrylate in AICl,-catalyzed reactions; ['] however, the uncatalyzed reaction is known to satisfy poorly the Alder endo rule161 and results therefore in a reversal of the product distribution."] The olefin-carbene complex 3 belongs to a class of compounds that is important for the discussion of the mechanism of the olefin metathesis reaction.[*] To date only a few compounds containing other metals have been isolated;191 3 is the first example of a chromium compound. Complex 3 is preferentially formed in good yield by heating the mixture of isomers 2a,b and can be separated from the unchanged ex0 isomer 2a by column chromatography.The structure of 31"'1 confirms the alkene coordination of the olefin-carbene ligand to the Cr(CO), fragment, which is possible owing to the endo position of the carbene carbon atom C1 on the norbornene skeleton (Fig. 1). The alkene function is thereby bonded unsymmetrically to the metal ; the Cr-Calkene distances (2.286(3) and 2.366(3) A'' 'I) are relatively longLiZ1 Both findings are a consequence of the rigidity of the norbornene skeleton and the short separation of the chromium-coordinated carbene and alkene functions by only two carbon atoms (C31, C36). The deviation from ideal octahedral symmetry at the central metal atom is due to the chelate structure. The Cr-C1 distance of 2.017(3)A places 3 in the class of complexes with short Cr-Cc;lrbrnc

Metal-Assisted Cycloaddition Reactions in Organotransition Metal Chemistry

“…Among the most widely used organic dienophiles are alkenyl and alkynyl esters such as acrylate, maleate, fumarate, and propynoate or acetylenedicarboxylate. Applying the isolobal relation , between a d 6 -metal pentacarbonyl fragment and an oxygen atom, vinyl and alkynyl Fischer-type carbene complexes have been used as dienophiles in inter- and intramolecular Diels−Alder reactions by the groups of Wulff ,− and Dötz − (with respect to ref , see also Scheme in the following section). It has been shown, that the methoxy(vinyl)carbene complexes 256a − c (Scheme ) react with a variety of dienes with rate accelerations of greater than 10 4 over that of the isolobally related methyl acrylate 256d ; concurrently the regioselectivity toward isoprene is increased ( 257a : 258a = 92:8 vs 257d : 258d = 70:30), and a high endo/exo stereoselectivity with cyclopentadiene is found ( 259a : 260a = 94:6 vs 259d : 260d = 78:22).…”

High Stereoselectivity and Facial Selectivity in Diels−Alder Cycloadditions of Novel Captodative Olefins: α-Alkoxyvinyl(ethoxy)carbene Chromium Complexes with Cyclopentadienes

“…In accord with Wulff,5b the high reactivity of alkenylcarbene complexes as dienophiles correlates with the electrophilicity of the carbene carbon, expressed by the resonance structures 13a − c (Scheme ), which control most of the reactivity pattern of these complexes. It was uncertain, however, whether the introduction of the α-alkoxy group would make complexes 3 - 6 reactive enough as dienophiles in Diels−Alder cycloadditions.…”

“…If the contribution of the resonance structure 13d is important in these complexes, it would be anticipated that the presence of the α-alkoxy group leads to a decrease in reactivity toward dienes such as cyclopentadiene ( 1 ) and pentamethylcyclopentadiene ( 7 ). The former hypothesis would be supported by the chemical shifts of the vinyl carbons shown in Table , in which a shielding effect is exhibited by carbon C-2 in comparison with complexes 6 Z and 12 5b…”

“…In contrast, the torsion angle W−C(1)−C(2)−C(3) in 4 is 172.7°, making the pentacarbonyl chromium moiety a stronger electron-withdrawing group with respect to the double bond. As further proof of the above, Table includes 13 C NMR data of the isopropenyl carbene of tungsten 12 ,5b where the α-substituent is a methyl group. In this molecule, there is no electron donation to carbon C-2, and the corresponding chemical shift is similar to that of 6 Z .…”

“…The report of Wulff in 1983, that the vinylcarbene complexes of Fischer can be involved in Diels−Alder reaction as dienophiles, broadened even further the scope of their applications. Thus, it has been established that the Diels−Alder reactions of α,β-unsaturated complexes with cyclopentadiene ( 1 ) are stereoselective in favor of the endo adduct, although there are some cases of exo selectivity depending on the nature of the dienophile (conformationally s-cis restricted, α-substituted 6c,8 ) or of the dienes (furan or cyclopentadiene derivatives 9 ).…”