Diels-Alder reactions of methacrolein

Chiral Lewis acids supported on silica gel and alumina, and their use as catalysts in Diels-Alder reactions of methacrolein and bromoacrolein [103]... 

Oxazoborolidinone 8 is an example of catalyst supported on silica gel. It is prepared by immobilizing the N-tosyl-0-allyl-(S)-tyrosine with mercaptopropyl silica and treatment with BF3 and has been used to catalyze the Diels-Alder reaction of methacrolein with cyclopentadiene [17] (Equation 4.2). The cycloaddition occurs with good diastereoselectivity but with low enantioselec-tivity. 

Figure 39 The enantioselective polymer-supported catalysts (61) of chiral oxazaborolidinone with cross-linking structures for use in the Diels-Alder reaction of methacrolein with cyclopentadiene. (Adapted from ref. 85.)...

Chiral rhodium284 and ruthenium catalysts285,286 have been reported to catalyze the Diels-Alder reaction of methacrolein with cyclopentadiene. Several bis(oxazolidine) and 2-pyridyl-l,3-oxazolidine ligands were used as chiral ligands. The adducts were obtained with only moderate enantioselectivities. 

The chiral dialuminum Lewis acid 14, which is effective as an asymmetric Diels-Alder catalyst, has been prepared from DIBAH and BINOL derivatives (Scheme 12.12). " The catalytic activity of 14 is significantly greater than that of monoaluminum reagents. The catalyst achieves high reactivity and selectivity by an intramolecular interaction of two aluminum Lewis acids. Similarly, the chiral trialuminum Lewis acid 15 is quantitatively formed from optically pure 3-(2,4,6-triisopropylphenyl)binaphthol (2 equiv) and MeaAl (3 equiv) in CH2CI2 at room temperature (Scheme 12.12). " The novel structure of 15 has been ascertained by NMR spectroscopic analysis and measurement of the methane gas evolved. Trinuclear aluminum catalyst 15 is effective for the Diels-Alder reaction of methacrolein with cyclopentadiene. Diels-Alder adducts have been obtained in 99% yield with 92% exo selectivity. Under optimum reaction conditions, the... 

A chiral dinuclear Ti(IV) Lewis acid catalyst 18 can be prepared in situ from a 1 2 molar mixture of (7 )-3,3 -di(2-mesitylethynyl)binaphthol and Ti(Oi-Pr)4 at ambient temperature. The 3- and 3 -substituents on the chiral ligand are effective for preventing undesired aggregation between Ti(IV) complexes and increasing the enantioselectivity (up to 82% ee) in the Diels-Alder reaction of methacrolein with cyclopentadiene (Scheme 12.16). ... 

Cationic chiral Rh and Ru complexes were prepared by reaction of [(T -C5H5)RhCl2]2 and [RuCl2(T 6-mes)]2 with chiral bidentate or monodentate oxazoline ligands, respectively. Treatment of these monocationic metal complexes, with AgSbF produced dicationic complexes, which were also found to be highly effective for the enantioselective Diels-Alder reaction of methacrolein [12,13] (Eq. 8A.6). On the basis of spectroscopic and structural studies, a full catalytic cycle of a chiral Ru complex was proposed for the Diels-Alder reaction of cyclopen-tadiene with methacrolein [14]. 

The moderate Lewis acidity of ruthenium complexes was used to promote catalytic Diels-Alder reaction of dienes and acrolein derivatives [21-23]. The enantioselective Diels-Alder reaction of methacrolein with dienes was catalyzed with cationic ruthenium complexes containing an arene or cyclo-pentadienyl (Cp) ligand and a chiral ligand such as phosphinooxazoline, pyridyl-oxazoline, monoxidized 2,2 -bis(diphenylphosphino)-1, T-binaphthyl (BINPO)or l,2-bis[bis(pentafluorophenyl)phosphanyloxy]-l,2-diphenylethane (BIPHOP-F). The reaction gave the cycloadduct in high yields with excellent... 

Diels-Alder Reaction. The Diels-Alder reaction of methacrolein with 1,3-dienol derivatives can also be catalyzed by the chiral BINOL-derived titanium complex BINOL-T1CI2. The endo adduct was obtained in high enantioselectivity (eq 15). The sense of asymmetric induction is exactly the same as observed for the asymmetric catalytic reactions shown above. Asymmetric catalytic Diels-Alder reactions with naphthoquinone derivatives as a dienophile provide an efficient entry to the asymmetric synthesis of anthracyclinone aglycones (eq 16). ... 

Itsuno et al. explored the possibility of using polymer-supported chiral Lewis acids in a model Diels-Alder reaction of methacrolein with cyclopentadiene [23a]. By using an insoluble polymer-supported Lewis acid [23], prepared from borane with cross-linked polymers with a chiral moiety such as an A-sulfonylamino acid, the Diels-Alder adduct is obtained in good yield with almost perfect exo selectivity and moderate enantioselectivity (Eq. 23). 

Derivatives of (S)-tyrosine were supported on silica gel through the phenolic oxygen atom and treated with BH3 to give Lewis acids able to accelerate the Diels-Alder reactions of methacrolein and bromoacrolein with cyclopentadiene. (S)-Prolinol has been supported on silica gel and alumina and then treated with EtAICI2 to give a supported catalyst... 

Kiindig et al [14] have reported the synthesis of the new chiral ruthenium complexes, [CpRu(biphop-F)] (45) and [(Indenyl)Ru(biphop-F)] (46), and have shown their efficiency as Lewis acids in Diels-Alder reactions of methacrolein (24a) with cyclopentadiene (Scheme 16.14, Table 16.3). The corresponding cycloadduct (25) was observed in high exo selectivities. They found that the rate of Diels-Alder reaction varied with the catalyst counteranion and it increased in order BF4 < SbF6 < TFPB (tetrakis[3,5-bis(trifluoromethyl)phenylj borate). 

Isolated reports mentioned the use of NHC-Ru species for the asymmetric Diels-Alder reaction of methacrolein and cyclopentadiene, cyclopropanation of styrene with ethyl diazoacetate, or dehydrative condensation of A -(2-pyr-idyl)benzimidazole and allyl alcohol. Whittlesey and co-workers described the stoichiometric activation of C-F bonds in perfluoroaromatic substrates such as hexafluorobenzene, perfluorotoluene and pentafluoropyridine using the dihydrido complex [(ICy)Ru(dppp)(CO)(H)2] (41) (dppp= l,3-bis(diphenylphosphino) propane). Modification of the NHC nitrogen substituents from A -alkyl to... 

Kami996 Kamahori, K., Ito, K. and Itsuno, S., Asymmetric Diels-Alder Reaction of Methacrolein with Cyclopentadiene Using Polymer-Supported Catalysts Design of Highly Enantioselective Polymeric Catalysts, J. Org. Chem., 61 (1996) 8321-8324. 

One of the most important application of MAD is e%o-selective Diels-Alder reaction of methacrolein with cyclopentadiene (Scheme 6.124) [148]. It is known that, in the presence of Me3Al, this reaction proceeds with moderate exo selectivity (endolexo = 1 15). On the other hand, the use of MAD instead of Me3Al resulted in improvement of exo selectivity as high as 48 1. ATPH is also a nice catalyst for e%o-selective Diels-Alder reaction of a-methyl-a,p-unsaturated ketones with cyclopentadiene. 

The reactions shown in Scheme 9.72 require a stoichiometric amount of (105). In contrast, a highly enantioselective reaction using a catalytic amount of a chiral silacycle has also been developed by Leighton and coworkers [33]. Silacycle (106) catalyzes the Diels-Alder reaction of methacrolein with cyclopentadiene to give... 

Recently, the iron and ruthenium complexes 178 and 179 were use for enantioselective Diels-Alder reaction of methacrolein with nitrones [118]. 

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