Cyclization temperature

The patent literature on the synthesis of bismaleimides is quite extensive. It has always been the target to optimize the synthetic conditions in order to maximize the yield of pure bismaleimide by minimizing the side reactions. Low cyclization temperatures were found to be advantageous and could be achieved with catalysts such as triethyl amine (20), cobalt acetate (21) and sodium carbonate (22) or metal salts in combination with tertiary amines (23). 

It has since been shown that the cycloaddition is catalyzed by transition metal salts (78JOC667). Although the yields are generally lower than irr the uncatalyzed reaction, this is outweighed by the advantages of shorter reaction times and a lower cyclization temperature. Illustrative examples of the formation of 2-alkoxy-3,4-dihydro-2//-pyrans are presented in Table 3, which includes a comparison of the two methods of synthesis. 

Another 6-exo closure from our enantioselective total synthe.ses of (—)-morphine (40) and its enantiomer was published in 1993 (Scheme 6-7) [12]. The pivotal cyclization of 38 to 39 was accomplished in 60% yield. This is a demanding intramolecular Heck reaction, since the palladium-bond arylmethyl side chain of 38 must rotate over the octahydroisoquinoline ring to coordinate with the trisubstituted alkene. Reflecting this difficulty, Pd(OCOCF3)2(PPh3)7 [a precatalyst that generates the reactive Pd(0) catalyst Pd(PPh3)2] cyclization temperature of 120 °C were required. 

Rawat, D.S. Zaleski, J.M. Syntheses and thermal reactivities of symmetrically and asymmetrically substituted acyclic enediynes steric control of Bergman cyclization temperatures. Chem. Commun. 2000, 2493-2494. 

In the context of enediyne reactivity, the electronic structure properties and their relationship to the cyclization chemistry of only the porphyrins have been probed in detail to date. Electronic absorption measurements and Raman analysis of the octa(phenylacetylene) derivatives (Scheme 99) show a correlated redshift in the absorption profile of 13nm/alkyne (Fig. 39), as well as a concomitant decrease in the frequency of the porphyrin marker bands between 1300 and 1600 cm as a function of increasing number of alkynes and increasing planarity of the macrocycle (Fig. 40) (584). These compounds also exhibit structure-dependent Bergman cyclization temperatures, but due to the steric crowding and large substituents at the alkyne termini positions,... 

Typically, cyclization temperatures evaluated by DSC are 60-90 C higher in temperature than their corrsponding values in solution. 

The reaction between tri-O-acetyl-D-glucal 7 and chlorosulfonyl isocyanate has been studied in the past, but neither formation of a cycloadduct nor of a rearranged product has been observed. Isocyanate acted only as acid catalyst causing decomposition of sugar material. On the other hand, [2+2]cycloaddition of active isocyanates to dihydro-2H-pyran and to its derivatives has been widely investigated, under a variety of conditions. The reaction of tosyl isocyanate with dihydro-2H-pyran 1 at low temperature (0 ) led to the formation of bicyclic p-lactam 2. Elevation of the cyclization temperature resulted in the rearrangement of the four-membered ring to the open-chain amide 3 (Scheme 4). 

Some representative monomers have been shown in Tables 3 and 4. As many of these copolyimides are soluble in organic solvents, the polycondensation reaction can be carried out as a two-step, one pot process, because once the intermediate poly(amic acid)s are formed at low temperature, the imidization step can proceed to quantitative conversion by merely raising the temperature beyond the cyclization temperature, usually 160-200 °C, for a few hours. Water produced by cyclodehydration can be more easily removed if a cosolvent is present that can help for an azeotropic water separation [81,200]. In this way, the polyimide is attained in solution, and it can be isolated by precipitation or by direct transformation from the polymer solution. 

The Bergman cyclization temperature of 18-membered macrocyclic enediyne 3.885 is 130°C, while cyclization of its complex with Ag(I) 3.889 occurs at the lower temperature of 110°C. In the case of complex Ag+-l,8-diazabicyclotriyne 3.891, there was a decrease in the distance between the nitrogen atoms to 4.614 A compared with 5.049 A for the free diamine 3.890 (Scheme 3.123) [24a, 404]. 

The Bergman cyclization temperature of the rerm-N-substituted enediynes 3.898 decreases from 160°C (for the base 3.898) to 90°Cupon the complexation with Cu(II) to give complex 3.899 (Scheme 3.126). A similar drop in temperature was observed for the Bergman cyclization of corresponding Ni(Il) complex (from 160°C to 124°C). The reaction products, however, were not identified. 

While aromatic enediynes are fairly stable compounds, their cyclization temperature can be reduced sometimes even to the physiological level by... 

Table 2.1 Bergmann cyclization temperatures (7b) for 6>-dppeb and complexes thereof ...

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