Chromium complexation, Diels-Alder reaction

Tungsten alkynyl Fischer carbene complexes are excellent dienophile partners in the classical Diels-Alder reaction with 1-azadienes (see Sect. 2.9.2.1). On the contrary, the chromium-derived complexes exhibit a different behaviour and they react through a [4S+3C] heterocyclisation reaction to furnish azepine derivatives [116] (Scheme 68). The reaction is initiated by a 1,2-addition of the nitrogen lone pair to the carbene carbon followed by a [l,2]-Cr(CO)5 shift-pro-... 

Schaus et al.41 have also reported an asymmetric hetero Diels-Alder reaction of Danishefsky s diene 10042 with aldehyde 101 catalyzed by chromium(III) complex 99 bearing a similar chiral salen ligand. Product 102 is obtained in moderate to good yield and stereoselectivity (Scheme 5-31 and Table 5-5). 

In addition to the reaction of vinylcarbene complexes with alkynes, further synthetic procedures have been developed in which Fischer-type carbene complexes are used for the preparation of benzenes. Most of these transformations are likely to be mechanistically related to the Dbtz benzannulation reaction, and can be rationalized as sequences of alkyne-insertions, CO-insertions, and electrocycli-zations. A selection of examples is given in Table 2.18. Entry 4 in Table 2.18 is an example of the Diels-Alder reaction (with inverse electron demand) of an enamine with a pyran-2-ylidene complex (see also Section 2.2.7 and Figure 2.36). In this example the adduct initially formed eliminates both chromium hexacarbonyl ([4 -I- 2] cycloreversion) and pyrrolidine to yield a substituted benzene. 

Chiral catalysts have recently been developed to allow the enantioselective Diels-Alder reactions of 1,2-dihydropyridines. The weakly Lewis-acidic chromium(m) complex 102 was found to catalyze the reaction between iV-phcnoxycarbonyl-1,2-dihydropyridine 85 and /V-acryloyloxazolidinonc 99 to give 2-azabicyclo[2.2.2]octene product 103 in near-quantitative yield and 85% ee (Scheme 27) <2002T8299>. 

Proline-based palladium complex 106 has been shown to catalyze the Diels-Alder reaction of 1,2-dihydropyridine 85 and oxazolidinone 99 to give 103 in similar enantiomeric excess as the chromium(m) complex 102<2005TL5677>. Alteration of the dienophile to pyrazolidin-3-one 107 improves the enantioselectivity of the reaction and gives product 108 in 97% ee (Scheme 29). 

Chromium complex 53 was also shown to efficiently catalyze the inverse electron-demand hetero Diels-Alder reaction of a,(3-unsaturated aldehydes with alkyl vinyl ethers (Scheme 17.19).26 Although the uncatalyzed process required elevated temperatures and pressures to give dihydropyrans in good yields but poor endo. exo selectivities, the reaction proceeded at room temperature in the presence of 5 mol% of ent-53 and 4A molecular sieves in dichloromethane of tert-butyl methyl ether with excellent diastereoselectivity (endo. exo >96 4) and promising enantioselectivities (72-78% ee). Optimal results were achieved using a solvent-free system and excess vinyl ether. 

Crystal structure of 60 revealed that the complex existed as a dimeric species where the two chromium centers are bridged through the indane oxygen and each chromium metal bears one molecule of water (Figure 17.6). It was therefore proposed, as for the inverse electron-demand hetero Diels-Alder reaction, that the barium oxide desiccant removed one molecule of bound water from the catalyst dimer, which opened one coordination site for binding of the substrate carbonyl.26... 

Alkynyl complexes. These complexes, which are readily available by reaction of an alkynyllithium with chromium carbonyl followed by methylation with CH3OSO2F, undergo facile Diels-Alder reactions with dienes to provide a general route to ,p-un-saturated chromium carbene complexes. The latter undergo benzannelation or cyclo-hexadienone annelation on reaction with an alkyne. A (trimethylsilyl)ethynyl complex is useful because benzannelation results in migration of silicon from carbon to oxygen to provide a protected phenol. 

Chiral tridentate chromium(III) complexes catalyze highly enantio- and diastereo-selective hetero-Diels-Alder reaction between various aldehydes and dienes with one oxygen substituent (Sch. 60) [206]. Complex formation between catalysts and aldehydes could not be detected by IR and the successful use of acetone as a solvent casts doubt on a simple Lewis acid mechanism. 

SCHEME 10.29 The stereochemical outcome of the Lewis acid assisted Diels-Alder reaction can be influenced by chromium complexation. 

SCHEME 10.39 An example of an exo-selective Diels-Alder reaction directed by a sugar derivative and chromium complexation. 

Shing, T K M, Chow, H E, Chung, IH E, Sugarometallic chemistry aglycone-chromium complex as chiral auxiliary in asymmetric Diels-Alder reaction. Tetrahedron Lett., 37, 3713-3716, 1996. Akiyama, T, Horiguchi, N, Ida, T, Ozaki, S, Stereodivergent Diels-Alder reactions employing cyclitols as chiral auxiliaries, Chem. Lett., 975-976, 1995. 

Weyershausen, B, Nieger, M, Doetz, K H, Stereospecific exo-selective Diels-Alder reactions with carbohydrate-functionalized a-exo-methylene-2-oxacyclopentylidene chromium complexes, J. Org. Chem., 64, 4206-4210, 1999. 

Baldoli, C., Maiorana, S., Licandro, E., Zinzalla, G., Lanfranchi, M., Tiripicchio, A. Stereoselective hetero Diels-Alder reactions of chiral tricarbonyl ( ti6-benzaldehyde)chromium complexes. Tetrahedron Asymmetry 200, 12, 2159-2167. 

Schaus, S. E., Brnalt, J., Jacobsen, E. N. Asymmetric Hetero-Diels-Alder Reactions Catalyzed by Chiral (Salen)Chromium(lll) Complexes. 

The synthesis is a catalogue of modern asymmetric catalytic methods. The epoxide 25 was resolved by a hydrolytic kinetic resolution (chapter 28) using a synthetic asymmetric cobalt complex. The asymmetric Diels-Alder reaction (chapter 26) was catalysed by a synthetic chromium... 

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