Trimerization Reactions

Trimer VI and phenol are the products predicted from the first step of the redistribution reaction trimer VII would be expected from coupling of some of the phenol with the starting dimer (Reactions 18 and 19). 

The desired reaction in catalytic hydroformylation is the addition of carbon monoxide and dihydrogen to the olefin substrate usually to obtain aldehyde. To some extent, however, concurrent reactions of the olefin (substrate) such as hydrogenation, isomerization, and special carbonylations, and consecutive reactions of the aldehyde product such as hydrogenation to alcohol, aldol reaction, trimer-ization, and formate formation take place under the reaction conditions of hydroformylation, which affect both yield and selectivity of the aldehyde products. For an example of product composition obtained using an unmodified cobalt catalyst in the BASF process, see Table 3. 

Dimerization is the main path. However, trimerization to form 1.3,6,10-dodecatetraene (15) takes place with certain Pd complexes in the absence of a phosphine ligand. The reaction in benzene at 50 C using 7r-allylpalladium acetate as a catalyst yielded 1,3,6,10-dodecatetraene (15) with a selectivity of 79% at a conversion of 30% based on butadiene in 22 h[ 19,20]. 1,3,7-Octatriene (7) is dimerized to 1,5,7,10.15-hexadecapentaene (16) with 70% selectivity by using bis-rr-allylpalladium. On the other hand. 9-allyl-l,4,6.12-tridecatetraene (17) is formed as the main product when PI13P is added in a 1 1. ratio[21]. 

Although acetonitrile is one of the more stable nitriles, it undergoes typical nitrile reactions and is used to produce many types of nitrogen-containing compounds, eg, amides (15), amines (16,17) higher molecular weight mono- and dinitriles (18,19) halogenated nitriles (20) ketones (21) isocyanates (22) heterocycles, eg, pyridines (23), and imidazolines (24). It can be trimerized to. f-trimethyltriazine (25) and has been telomerized with ethylene (26) and copolymerized with a-epoxides (27). 

Miscellaneous Reactions. Sodium bisulfite adds to acetaldehyde to form a white crystalline addition compound, insoluble in ethyl alcohol and ether. This bisulfite addition compound is frequendy used to isolate and purify acetaldehyde, which may be regenerated with dilute acid. Hydrocyanic acid adds to acetaldehyde in the presence of an alkaU catalyst to form cyanohydrin the cyanohydrin may also be prepared from sodium cyanide and the bisulfite addition compound. Acrylonittile [107-13-1] (qv) can be made from acetaldehyde and hydrocyanic acid by heating the cyanohydrin that is formed to 600—700°C (77). Alanine [302-72-7] can be prepared by the reaction of an ammonium salt and an alkaU metal cyanide with acetaldehyde this is a general method for the preparation of a-amino acids called the Strecker amino acids synthesis. Grignard reagents add readily to acetaldehyde, the final product being a secondary alcohol. Thioacetaldehyde [2765-04-0] is formed by reaction of acetaldehyde with hydrogen sulfide thioacetaldehyde polymerizes readily to the trimer. 

Acryhc esters dimerize to give the 2-methylene glutaric acid esters catalyzed by tertiary organic phosphines (37) or organic phosphorous triamides, phosphonous diamides, or phosphinous amides (38). Yields of 75—80% dimer, together with 15—20% trimer, are obtained. Reaction conditions can be varied to obtain high yields of trimer, tetramer, and other polymers. 

Chemical Properties. In addition to the reactions Hsted in Table 3, boron trifluoride reacts with alkali or alkaline-earth metal oxides, as well as other inorganic alkaline materials, at 450°C to yield the trimer trifluoroboroxine [13703-95-2] (BOF), MBF, and MF (29) where M is a univalent metal ion. The trimer is stable below — 135°C but disproportionates to B2O2 and BF at higher temperatures (30). 

Oligomerization and Polymerization Reactions. One special feature of isocyanates is their propensity to dimerize and trimerize. Aromatic isocyanates, especially, are known to undergo these reactions in the absence of a catalyst. The dimerization product bears a strong dependency on both the reactivity and stmcture of the starting isocyanate. For example, aryl isocyanates dimerize, in the presence of phosphoms-based catalysts, by a crosswise addition to the C=N bond of the NCO group to yield a symmetrical dimer (15). 

Reportedly, simple alkyl isocyanates do not dimerize upon standing. They trimerize to isocyanurates under comparable reaction conditions (57). Aliphatic isocyanate dimers can, however, be synthesized via the phosgenation of A[,A[-disubstituted ureas to yield /V-(ch1orocarhony1)ch1oroformamidine iatermediates which are subsequendy converted by partial hydrolysis and base catalyzed cycUzation. This is also the method of choice for the synthesis of l-alkyl-3-aryl-l,3-diazetidiones (mixed dimers of aromatic and aUphatic isocyanates) (58). 

Dimerization is reportedly catalyzed by pyridine [110-86-1] and phosphines. Trialkylphosphines have been shown to catalyze the conversion of dimer iato trimer upon prolonged standing (2,57). Pyridines and other basic catalysts are less selective because the required iacrease ia temperature causes trimerization to compete with dimerization. The gradual conversion of dimer to trimer ia the catalyzed dimerization reaction can be explained by the assumption of equiUbria between dimer and polar catalyst—dimer iatermediates. The polar iatermediates react with excess isocyanate to yield trimer. Factors, such as charge stabilization ia the polar iatermediate and its lifetime or steric requirement, are reported to be important. For these reasons, it is not currently feasible to predict the efficiency of dimer formation given a particular catalyst. 

Commercially, polymeric MDI is trimerized duting the manufacture of rigid foam to provide improved thermal stabiUty and flammabiUty performance. Numerous catalysts are known to promote the reaction. Tertiary amines and alkaU salts of carboxyUc acids are among the most effective. The common step ia all catalyzed trimerizations is the activatioa of the C=N double boad of the isocyanate group. The example (18) highlights the alkoxide assisted formation of the cycHc dimer and the importance of the subsequent iatermediates. Similar oligomerization steps have beea described previously for other catalysts (61). 

Also, the presence of strong bases, even in trace amounts, can promote the formation of isocyanurates or carbodiimides. In the event of gross contamination, the exothermic reaction can sharply increase the temperature of the material. Normally, the trimerization reaction occurs first and furnishes heat for the carbodiimide reaction. The carbodiimide reaction Hberates carbon dioxide and forms a hard soHd. The Hberation of carbon dioxide in a sealed vessel could result in overpressurization and mpture. 

The stringency of the conditions employed in the unmodified cobalt 0x0 process leads to formation of heavy trimer esters and acetals (2). Although largely supplanted by low pressure ligand-modified rhodium-catalyzed processes, the unmodified cobalt 0x0 process is stiU employed in some instances for propylene to give a low, eg, - 3.3-3.5 1 isomer ratio product mix, and for low reactivity mixed and/or branched-olefin feedstocks, eg, propylene trimers from the polygas reaction, to produce isodecanol plasticizer alcohol. 

The molecular size of the product is limited insofar as the reaction is terrninated at the dimer or trimer stage. Thus the process is more properly termed oligomerization. The four- to twelve-carbon compounds required as the constituents of Hquid fuels are the prime products. 

The cychc haUdes can be converted to discrete substitution products by reaction with amines, alcohol, or alkylating agents. For example, (NPCl ) reacts with ammonia to form (NP(NH2)2)3 [13597-92-7] withy -NaOCgH CH to form (NP(OCgH4CH2)2)3 [27122-73-2] and with CH MgCl to form (NP(CH3)2)3 [6607-30-3]. Among the cychc members, the trimeric haUdes are the most inert toward substitution and tetrameric haUdes are the most active. 

The backbone of poly(phenylene oxide)s is cleaved under certain extreme reaction conditions. Lithium biphenyl reduces DMPPO to low molecular weight products in the dimer and trimer molecular weight range (20) and converts poly(2,6-diphenyl-l,4-phenylene oxide) to 3,5-diphenylphenol in 85% yield (21) (eq. 4). 

Primary aromatic amines react with aldehydes to form Schiff bases. Schiff bases formed from the reaction of lower aUphatic aldehydes, such as formaldehyde and acetaldehyde, with primary aromatic amines are often unstable and polymerize readily. Aniline reacts with formaldehyde in aqueous acid solutions to yield mixtures of a crystalline trimer of the Schiff base, methylenedianilines, and polymers. Reaction of aniline hydrochloride and formaldehyde also yields polymeric products and under certain conditions, the predominant product is 4,4 -methylenedianiline [101 -77-9] (26), an important intermediate for 4,4 -methylenebis(phenyhsocyanate) [101-68-8], or MDI (see Amines, aromatic amines, l thylenedianiline). 

The formation of isocyanurates in the presence of polyols occurs via intermediate aHophanate formation, ie, the urethane group acts as a cocatalyst in the trimerization reaction. By combining cyclotrimerization with polyurethane formation, processibiUty is improved, and the friabiUty of the derived... 

Reaction between butadiene and CO2 has been extensively studied (171) since the reaction was first demonstrated (167—170). This reaction has been shown to be catalyzed by Pd (172,173), Ni (174), Ru (175), Pt (178), and Rh (172,173) catalysts. Products include gamma (5) and delta lactones (6), acids (7,8), and esters (9). Mechanistic studies have shown that butadiene initially forms a dimer (Pd, Ru, Ni) or trimer (Rh) intermediate followed by CO2 insertion (171). The fate of these intermediates depends on the metal, the ligands, and the reaction conditions. 

Hydrogen chloride or a few drops of hydrochloric acid cataly2e the conversion of //-butyraldehyde iato the trimer, parabutyraldehyde, C22H24O2, (2,4,6-tripropyl-l,3,5-trioxane [56769-26-7] (1). The reaction is reversed by heating the parabutyraldehyde ia the presence of acid. Anhydrous hydrogen chloride at —40°C converts //-butyraldehyde iato l,l -dichlorodibutyl ether, (2) ia 70—75% yield (10). 

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