Trimeric side products

With aliphatic aldehydes the yields are poor at best. In a relatively recent study, Zil berman and Py-ryalova used a modified Stephen procedure to reduce a series of aliphatic nitriles RCN. As the size of R increased, the yield decreased from a maximum of 51% (R = Et). A major cause of the low yields was found to be the formation of trimeric side products (46). 

Using controlled reaction conditions such as the temperature profile and rate and time of the addition of polyols, more uniform materials can be produced. The correct spacing of the hard segments required to produce the physical properties can be obtained. The controlled conditions will also help prevent the formation of undesirable side products such as allophanate, biuret, and trimers. These reactions will give branching of the polymer chains. 

The synthesis of a fused pyridothietone 79 by the FVP of 2-mercapto nicotinic acid was described (Scheme 13) <2002TL5285>. However, the selectivity of this reaction was shown to be poor, since the formation of trimer 80 was observed as a side product. 

The high regioselectivity to phenylacetaldehydes may be related to the stabilization of a developing alpha-cation (5). A possible side product is trimeric phenylacetaldehyde (triphenylbenzene), which is formed especially at lower temperatures (10% at 200°C vs. 2% at 300°C). 

G. M. Whitesides and W. J. Ehmann, 7. Am. Chem. Soc., 1969, 91, 3800 see also G. A. Ville, K. P. C. Vollhardt and M. J. Winter, Organometallics, 1984, 3, 1177 for further studies along these lines, and J. R. Strickler, P. A. Wexler and D. E. Wigley, Organometallics, 1988, 7, 2067 for studies of relevant model systems associated with alkyne trimerizations catalyzed by early transition metals. Note that although cyclobutadiene-like structures are not involved in most alkyne trimerization mechanisms, metal-complexed cyclobutadienes are fairly common side products in alkyne trimerization reactions. As their yields are usually low, their formation does not present practical problems in arene synthesis. 

Wakefield and Cook have indicated that the high yield and relative absence of side-products by using organolithium reagents make this reaction advantageous in comparison to trimerization of benzonitrile in the presence of organolithium reagents (Section VIII,A,1). 

The main side product was a cyclic trimeric aldol product isophorone (see Fig. 3), which was formed with a selectivity 12% throughout the test. 

It has also been reported that the reaction of N(Si(CH3)3)3 with PCI5 can be manipulated to maximize yields of either the cyclic trimer (76%) or the pure phosphoranimine monomer (40%) through variations in reaction conditions (25). These reactions use milder conditions than previously reported methods, with higher yields. In addition, the major side product, (CHslsSiCl, can be recycled to form one of the starting materials, NlSilCHslsls. 

Whereas 7 isolated as a side product, in 1989 Tashiro etal. deliberately prepared trimeric and tetrameric metacyclophanes by the conventional sulfone method. Cyclisation of monomeric or dimeric thiol- and chloride-functionalised precursors linked by (CH2) bridges in various combinations, yielded, after ring contraction via sulfone pyrolysis, a multitude of [2 ]- and [m.n]-metacyclophanes as shown in Figure 3 [19-24],... 

The minor products are generally 1-3% of the total yield and arose from (a) side-chain fragmentation producing hydrogen and low-molecular-weight hydrocarbons (b) addition of these fragments to the free olefin (c) dimerization and trimerization of the free olefin (d) fragmentation of the alkyl radical and cation intermediates. 

Numerous modifications of this procedure have been used. The synthesis of ben-zimidazolone monomers (12), dimers (13) and trimers demonstrate this versatile, simple approach to bridged heterocycles (76TL79). N-Alkylated products (e.g. 14) are in many cases the result of side reactions caused by a competitive transmetallation process (78MI52202) polymerization is also a common reaction course. 

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