Tandem isomerization

Scheme 8 Tandem isomerization/dehydration of glucose over Zr02 and Ti02...

The mechanism of the catalytic cycle is outlined in Scheme 1.37 [11]. It involves the formation of a reactive 16-electron tricarbonyliron species by coordination of allyl alcohol to pentacarbonyliron and sequential loss of two carbon monoxide ligands. Oxidative addition to a Jt-allyl hydride complex with iron in the oxidation state +2, followed by reductive elimination, affords an alkene-tricarbonyliron complex. As a result of the [1, 3]-hydride shift the allyl alcohol has been converted to an enol, which is released and the catalytically active tricarbonyliron species is regenerated. This example demonstrates that oxidation and reduction steps can be merged to a one-pot procedure by transferring them into oxidative addition and reductive elimination using the transition metal as a reversible switch. Recently, this reaction has been integrated into a tandem isomerization-aldolization reaction which was applied to the synthesis of indanones and indenones [81] and for the transformation of vinylic furanoses into cydopentenones [82]. 

A nickel hydride complex, NiHCl(diphenylphosphinoethane), catalyses the tandem isomerization-aldolization reaction of allylic alcohols with aldehydes.156 The atom- (g) efficient process proceeds at or below ambient temperature with low catalyst loading, and works well even for bulky aldehydes. Magnesium bromide acts as a co-catalyst, and mechanistic investigations suggest that a free enol is formed, which then adds to the aldehyde in a hydroxyl-carbonyl-ene -type reaction. 

Indeed, a ruthenium-catalyzed tandem olefin-migration/aldol-type reaction has been realized when an aldehyde is present in aqueous media [18,19]. For 3-butene-2-ol (9), the tandem isomerization/aldol-type reaction was examined. The mixture of 9, aldehyde (10), and a catalytic amount of RuCl2(PPh3)3 in H20/toluene(4/l) (Eq. 6) or H20 alone (Eq. 7) was stirred for 5 h at 110 °C (oil bath temperature) and afforded the aldol adduct 11. 

Catalytic tandem isomerization/Claisen reaction of bis allyl ether was reported by Dixneuf et al. [23]. A cationic bis-oxazoline-ruthenium-arene complex S3 in the presence of both l,3-bis(2,6-diisopylphenyl)imidazolinium chloride and CS2CO3 catalyzes the selective transformation of bis-allyl ether 51 into ) c5-unsaturated aldehyde 52 via successive alkene isomerization and Claisen rearrangement (Eq. 12.21). 

Branchadell, V., Crevisy, C., Gree, R., From Allylic Alcohols to Aldols by Using Iron Carbonyls as Catalysts Computational Study on a Novel Tandem Isomerization Aldolization Reaction, Chem. Eur. J. 2004, 10, 5795 5803. 

The RhI-catalysed rearrangement of A-allylaziridines to (Z)-A-alkenylaziridines has been investigated by computational methods and the reaction occurs via a hydrometalation//3-hydride elimination. Alkylidene cycloheptadienes have been prepared via a rhodium(I)-catalysed tandem isomerization of cyclopropylenynes through a [1,5] carbon-carbon migration via an allene intermediate with high Z E selectivities (Scheme 152). ... 

In a tandem isomerization/Prins strategy utilizing cooperative catalysis between an iridium(III) catalyst and a Bronsted acid, indole 182 underwent an isomerization/protonation sequence via a Prins-type oxocarbenium intermediate, with subsequent C—C bond formation to give oxepane-fused indole 183 (13AGE12910). Various anthranilic acids were coupled with chiral a-haloacids to afford N-acylated anthranilic acid intermediates which underwent cyclization to (3R)-3-alkyl-4,l-benzoxazepin-2,5-diones... 

Scheme 5.4 Simplified mechanisms for two cycles of tandem isomerization-hydroformylation reaction (HF = hydroformylation Iso = isomerization only the most important relationships are depicted equilibria are not indicated).

Tandem Isomerization-Hydroformylation Reactions 393 Table 5.2 Product formation in relation to the syngas pressure. ... 

Tandem Isomerization-Hydroformylation Reactions with Consecutive Steps... 

Behr, A., Obst, D., Schulte, C., and Schosser, T. (2003) Highly selective tandem isomerization-hydroformylation reaction of trans-4-octene to n-nonanal with rhodium-BIPHEPHOS catalysis. J, Mol, Catal, A-Chem, 206, 179-184. 

Ong and coworkers have successfully applied the Ni(cod)2/NHC catalyst combination to the tandem isomerization of allylarenes and subsequent regioselective hydroheteroarylation. Through appropriate choice of NHC it was possible to select for linear or branched products IMes preferentially gave the branched products (Scheme 5.25), the more hindered NHC, IPr, preferentially gave the linear product, and this preference was amplified to near-complete selectivity by the addition of trimethylaluminum (Scheme 5.26). ... 

A tandem isomerization/cyclization of a-methallyloxy carboxylic acids has been catalyzed with Al(OTf)3 to give substituted l,3-dioxolan-4-ones (eq 8). Interestingly, in this case, isomerization occurred before cyclization. ... 

Using a ruthenium complex and tri-tert-butylphosphine as a ligand, a tandem isomerization reaction of allylic alcohols followed by a C-H bond activation directed by a carbonyl group formed in situ was reported. The transformation, which is performed in the presence of triethoxyvinylsUane, leads to the incorporation of a (triethoxysilyl)propyl unit at the C3 position of l-(furan-2-yl)prop-2-en-l-ol (eq 31). ... 

A simple base-induced tandem isomerization/hydroxyalkylation of the easily accessible 3,4-allenoates 96 has been described that furnishes, in a fully regio- and stereo-controlled manner, the expected dienic esters 97 (Scheme 48) [87]. The addition step involves an aldehyde but it can be extended to sulfonimines. 

Ruthenium/Br0nsted Acid System The tandem isomerization/Friedel-Crafts reaction of aUylamides reported by Sorimachi and Terada was another early example of a cascade catalysis procedure using Brpnsted acid and metal salts as catalysts (Scheme 2.93) [127]. With the promotion of the ruthenium catalyst 342, the aUylamides isomerized to enamide X32, which was then tautomerized by a Brpnsted acid (343 or 344) to form imine X33 then the electron-enriched aromatic substrates 341 attacked the Brpnsted acid-activated imine intermediates to afford Friedel-Crafts products 345 in up to 91% yield. 

SCHEME 2.93 Tandem isomerization/Friedel-Crafts (aza-Petasis-Ferrier) reaction via a cascade catalysis procedure. 

In 2008, Sorimachi and Terada reported a relay catalysis using a rhodium hydride complex/Brpnsted acid (129) binary system (Scheme 5.88) [89]. The sequential transformation involves a three-step relayed catalysis, where (1) isomerization of allylamide 130 to enamide A is catalyzed by RhClH(CO)(PPh3)j (2) subsequent isomerization of A to imine B is relayed by 129 and (3) the catalytic sequence is terminated by a carbon-carbon bond forming the reaction of B with a nucleophilic component 131 under 129. This approach enables the generation of reactive imines B from readily available allylamides 130 in a one-pot reaction via tandem isomerization. 

Scheme 4-293. Tandem isomerization-aldolization of allylic alcohols.

Scheme 4-294. One-pot tandem isomerization-aldolization of TBS-protected 3-vinyl-1,3-dihydroisobenzofiiran-1 -ol.

The hydroformylation of internal and substituted alkenes with 35-Rh, such as 2-octene, cyclohexene, isobutene and methallyl alcohol showed better performance compared to the PPh3-Rh catalyst. Most strikingly, selective hydroformylation of a-pinene and tetramethylethylene, which undergoes a tandem isomerization-hydroformylation, can be achieved with 35-Rh, although with low... 

---