Ethyl dimerisation

The use of stoichiometric ruthenium-NHC complexes generated in situ from [Ruljd-COCKp-cymene)], an imidazohnm salt [4] or an imidizol(idin)ium-2-carboxylate [4] has been applied in the cyclopropanation of styrene 5 with ethyl diazoacetate (EDA) 6 (Scheme 5.2). No base was necessary when imidazolium-2 carboxylate were employed. The diastereoselectivity was low and the cis/trans ratio was around 50/50 (Table 5.1). Although the diastereoselectivity was moderate, the reaction was highly chemoselectivity as possible side reactions (homologation, dimerisation and metathesis) were totally or partially suppressed. 

The continuous and batch microwave reactors have been particularly useful for heating reactions in which thermally labile products are formed. For example, alkyl 2-(hydroxymethyl)acrylates have considerable potential as functionalised monomers and synthons128. Published syntheses at ambient temperature, however, required several days and were not conducive to scale-up129-133. The microwave procedure involved a modified Baylis-Hillman reaction, in which the parent acrylate derivative was reacted with formalin in the presence of 1,4-diazabicyclo [2.2.2] octane (DABCO). Preparations from starting acrylates, including methyl, ethyl and n-butyl esters, were easily achieved within minutes with multiple passes through the CMR, at ca. 160-180°C (Scheme 9.16). Rapid cooling was required to limit hydrolysis, dimerisation and polymerisation. Yields... 

Bis-[trifluoromethyl]-2-(penta-fiuoro-ethyl)-undecafluoro- E10 (Defluoro-dimerisation)... 

Within the scope of this review we shall only consider those compounds possessing one or more alkenyl functions susceptible to activation by electrc hilic attack. Included in this family is a vast array of monomers varying in basicity from ethylene, which is so resistant to protonation that the ethyl carbenium ion has hitherto eluded observations even under the most drastic conditions (see below), and which in fact is equally resistant to cationic polymerisation, to N-vinylcarbazole, whose susceptibility to this type of activation is so pronounced that it can be polymerised by almost any acidic initiator, however weak. We shall also deal with olefins which, because of steric hindrance, can only dimerise (e.g., 1,1-diphenylethylene) or cannot go beyond the stage of protonated or esterified monomeric species (e.g., 1,1-diphenylpropene). The interest of such model compounds is obvious they allow clean and detailed studies to be conducted on the kinetics and mechanism of the initiation steps and on the properties of the resulting products which simulate the active species in cationic polymerisation. The achievements and shortcomings of the latter studies will be discussed below. 

Head-to-tail dimerisation of methyl acrylate to the dimethyl ester of 2-methylenepentane-dioic acid (126) occurred in 82-85% yield in the presence of catalytic amounts of P(RNCH2CH2)3N with R = PP, Bu or Bz but the less sterically hindered proazaphosphatrane with R = Me, gave oligomer or pol-ymer. The proazaphosphatrane, P(RNCH2CH2)3N with R = Bu also acts as an effective ligand for the palladium-catalyzed direct arylation of ethyl cyano-acetate (127) with aryl bromides (e.g. 128) to form (129) in high yield. ... 

A further evidence on the acceleration enjoyed by a typical Pd-catalysed reaction, the Heck reaction, in an ionic phase ( V-mcthyl-Y.Y. V.-trioctylammonium chloride or Aliquat 336) is found in a triphasic protocol developed by Tundo and coworkers. 7b.The arylation of electron poor olefins is catalysed by palladium supported on charcoal (Pd/C) and is carried out in the heterogeneous isooctane/Aliquat 336/water system (Figure 27). Under this multiphasic condition, Aliquat 336 forms a third liquid phase between the organic and the aqueous phase that traps the catalyst. The use of phosphines is not necessary. As a matter of fact, Aliquat 336 incorporates the solid-supported catalyst and ensures an efficient mass transfer between the bulk phases resulting in an increase of the reaction rate of an order of magnitude compared to the reaction in the absence of the ionic liquid. A determing role is played by the base while I LN drives the reaction towards the formation of ethyl cinnamate, reaction carried out in the presence of KOH lead to formation of Ullmann dimerisation products. 

The much-used dimethyl l,2,4,5-tetrazine-3,6-dicarboxylate is generated by the dimerisation of ethyl diazoacetate, then oxidation. ... 

In a related context, 2-(p-toluenesulfonyl)ethylamine was used as an ammonia equivalent in an aza-ene reaction by which aldehyde 92.1 was converted to the bicycle 92.2 [Scheme 8.92]. After N-acetylation and Pd(0)-catalysed hydro-stannylation, the alkenylstannane 92.4 dimerised under copper(II) nitrate catalysis. The 2-(p-toluenesulfonyl)ethyl group was then discharged by -elimination with potassium rerf-butoxide. 

Not surprisingly the cyclopentadienide anion reacts readily with electrophilic reagents. With carbon dioxide it gives a dicarboxylic acid which dimerises [1,4]. It is acylated or aroylated by acid chlorides, no catalyst being required, to give diacyl [19,20] or diaroyl [21,22] derivatives which are monomeric. The monoacetyl-cyclopentadienide ion has been obtained by reaction with ethyl acetate in tetrahydrofuran [23]. With methyl chloroformate mono-and di- methoxycarbonyl)cyclopentadienide salts are formed [24]. 

Some of the methylphosphine boranes presented in the previous tables have been used to prepare several diphosphine boranes with an ethyl bridge, i.e. analogues of DiPAMP via Cu(II) promoted dimerisation (Scheme 2.23 and Table 2.14). 

In this reaction the thermodynamically favoured enantiomeric pair Z-67 is preferentially formed. A complication is that ethyl diazoacetate can dimerise to form ethyl fumarate and maleate so the rate of addition of ethyl diazoacetate has to be slow to achieve high conversions. 

Treatment of [Ru(Ti6-cymene)(Cl)2]2 with bis(oxazolinyl)pyridine replaces the arene ligand with the tris-chelate ligand. This product reacts with Me3SiCHN2 between 0°C and 15°C yielding a monomeric trimethylsilylcarbene complex which cyclopropanates styrene. The complex also catalyses the dimerisation of ethyl diazoacetate.205... 

The catalytic reaction of buta-1,3-diene with ethyl methylacetoacetate in the presence of [Pd(T)2-buta-l,3-diene) R2P(CH2)nPR2 l complexes has been investigated . The product was found to be a mixture of 1 1 adducts and the reaction to be fastest when n=2 and R = Pr in dichloromethane as the solvent. The X-ray crystal structure of a side product [ ( H n -CgHi2)Pd 2 Me2P(CH2)2PMe2 ] (42), formed by dimerisation of the butadiene ligand, has been determined. 

HF calculations of the structures and vibrational frequencies of monomers and dimers of lithium alkyl carbonates (methyl, ethyl, and propyl carbonate lithium) and lithium alkoxides (lithium methoxide, lithium ethoxide, lithium propoxide, and lithium butoxide) indicate that they adopt dimeric structures. Dimerisation energies of 214 kJ mol for lithium alkyl carbonates and 266 kJ mol for lithium alkoxides are calculated and are found to be approximately independent of the chain length. 

Asymmetric dimerisation of 3,4-dimethyl-1-phenylphosphole in the presence of an organoplatinum(II) complex derived from (R)-Ar,AT-dimethyl-l-(l-naph-thyl)ethylamine affords an optically pure P-chiral diphosphine. " " Reaction of racemic Sb-chiral (+ )-l-phenyl-2-trimethylsilylstibindole with di-p-chloro-bis (iS)-2-[l-(dimethylamino)ethyl]phenyl-C,Ar dipalladium(II) gives a 1 1 mixture of the diastereomeric complexes 51a and 51b. Separation of the complexes by chromatography followed by treatment with triphenylphosphine affords the optically pure l-phenyl-2-trimethylsilylstibinoles in quantitative yields. " The... 

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