Oxidative coupling of aldehydes

Oxidative coupling of aldehydes,3 Aliphatic aldehydes bearing an a-hydrogen atom on treatment with activated manganese dioxide undergo a-hydrogen abstraction and C-C and C-O dimerization. For example, isobutyraldehyde (1) is converted in about... 

Figure 4.29 A cross-section of results for the Rh(lll) dual catalysis involving oxidative coupling of aldehydes by directed C-H functionalization as reported by Seayad and covirorkers [61].

NHC s catalyse oxidative coupling of aldehydes (R -CHO) with A,A -disubstituted carbodiimides (R -N=C=N-R ) to give Al-acylureas (RCcONR -CO-NHR ) in ambient acetonitrile in the presence of air yield is up to 93%, but is severely lowered under inert gas. " This and other control observations lead to a mechanism with car-bene attacking aldehyde, giving a zwitterion (acyl anion) which attacks carbodiimide. However, some reaction flux may proceed via carboxylic acid or benzoin/acyloin routes. 

Scheme 6.4 Synthesis of phthalides by oxidative coupling of aldehydes.

Alternatively, the oxidation may occur by coupling to a suitable reactant. In 1997, Miyashita explored the scope of oxidative aroylations with nitrobenzene (Scheme ll.lO)." With the same oxidative reagent, nitrobenzene, benzoin reactions had already been developed back in 1982 using cyanide or thiamine. The combination of an aldehyde and nitrosobenzene, in the presence of an NHC, led to the formation of 7V-phenyl hydroxamic acids in good yields. These can be further converted into hydroxamic esters via a one pot reaction in the presence of a second (o,p-unsaturated) aldehyde. Furthermore, the oxidative coupling of aldehydes with carbodiimides in air allowed access to Af-acyl urea derivatives with IMes HCl 19 as the NHC precursor. Related thioesterifications reactions have also been reported using... 

It is postulated that the mechanism of the silane-mediated reaction involves silane oxidative addition to nickel(O) followed by diene hydrometallation to afford the nickel -jr-allyl complex A-16. Insertion of the appendant aldehyde provides the nickel alkoxide B-12, which upon oxygen-silicon reductive elimination affords the silyl protected product 71c along with nickel(O). Silane oxidative addition to nickel(O) closes the catalytic cycle. In contrast, the Bu 2Al(acac)-mediated reaction is believed to involve a pathway initiated by oxidative coupling of the diene and... 

Oxidative nickel-catalyzed coupling of aldehydes and alkynes to generate allylic alcohols. Intermolecular and intramolecular examples are both effective, and the transmetalating agent (MR" ) may be an organosilane, organoborane, organozinc, or alkenylzirconium. ... 

Oxidative coupling of benzenes with , p- unsaturated aldehydes by... 

OXIDATIVE COUPLING OF BENZENES WITH a, fi- UNSATURATED ALDEHYDES BY Pd(OAc)2/HPMoV /O2 SYSTEM... 

Catalysis in reaction systems with undissolved substrates and products is not restricted to biocatalysis. High yields in sobd-state synthesis, sohd-to-sohd reactions, and solvent-free systems have also been reported for aldol condensation, Baeyer-Villiger oxidation, oxidative coupling of naphthols, and condensation of amines and aldehydes [1, 2]. 

The reactions of C02 with aliphatic aldehydes in the presence of Ni(bipy)(cod) have been studied. As the result of oxidative coupling of C02 and RCHO on the Ni(bipy) moiety, five-membered metallacycles are formed (equation 92).280,281... 

Porphycenes with fused aromatic rings between 3,6 and 13,16 positions of 61 (Scheme 29) have been prepared through McMurry coupling of aldehyde 60, followed by DDQ oxidation of the nonaromatic N,N -dihydroporphycene intermediate (05ACI4053,1993PAC143). 

Nickel-catalyzed cyclizations, couplings, and cycloadditions involving three reactive components have been an active area of research for the past decade [39,40]. Central to these reactions is the involvement of a low-valent nickel capable of facilitating oxidative coupling of an unsaturated hydrocarbon (such as an alkyne, allene, or alkene) and a carbonyl substrate (such as an aldehyde or ketone). The use of NHCs as ligands has been evaluated for couplings of aldehydes. Such reactions typically afford O-protected allylic alcohols in good yields. 

Isomerization of alkenes is catalyzed by bis(j7 -allyl)(jj -cyclopentadienyl)vanadium(IV). Cyclohexene nndergoes catalytic stereoselective aerobic oxidation with CpV(CO)4. Reductive coupling (see Reductive Coupling) of aldehydes and aldimines is catalyzed by a Cp-substituted vanadium compound in the presence of a chlorosilane and zinc or aluminum. Vanadocene or bis( ] -cyclopentadienyl)dimethylvanadium(IV) catalyzes the synthesis of 1,2-diphenyldisilane and 1,2,3-triphenyltrisilane from phenylsilane. ... 

The bisdehydro[13]annulenone 191 and bisdehydro[17]annulenone 193 have been prepared by the oxidative coupling of the a,co-diethynyl compounds obtained by the condensation of the aldehydes, 190 and 192 with acetone, and subsequent oxidation of the resulting cyclic alcohols . Only the 17-membered cyclic ketone 193 was found to be paratropic. The atropicity of 191 seems to be attributable to the poor planarity of the highly strained 13-membered ring. 

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