Thermal Ring Opening

The thermal ring opening of l,2-bis(trimethylsiIoxy) cyclobutenes (from acyloin condensation of 1,2-dicarboxylic esters) was used in ring expansion prodecures (see p. 53f.). 

Aziridines, e.g. (91), undergo thermal ring opening in a conrotatory manner to generate azomethine ylides. These azomethine ylides are 47r-components and can participate in [4 + 2] cycloadditions with 1-azirines acting as the 27r-component 73HCA1351). 

There are several general classes of pericyclic reactions for which orbital symmetry factors determine both the stereochemistry and relative reactivity. The first class that we will consider are electrocyclic reactions. An electrocyclic reaction is defined as the formation of a single bond between the ends of a linear conjugated system of n electrons and the reverse process. An example is the thermal ring opening of cyclobutenes to butadienes ... 

Figure 30.6 Thermal ring-openings of cis- and trans-dimethylcyclobutene occur by conrotatory paths.

FIGURE 18.2 Thermal ring opening of 1,2-dimethylcyclobutene. The two hydrogens and two methyls are forced into conrotatory motion so that the resulting p orbitals have the symmetry of the HOMO of the diene. 

As early as 1895, the synthesis of polydichlorophosphazene was attempted by H.N. Stokes by thermal ring-opening polymerization of hexachloro-triphosphazene [(NPCl2)3]. The product obtained by H.N. Stokes was a high-molecular weight cross-linked rubbery material called inorganic rubber which is insoluble in all solvents and hydrolytically decomposes into phosphates, ammonia, and hydrochloric acid in the presence of moisture. Because of its insolubility and hydrolytic instability, the polymer found no technological application and remained as a laboratory curiosity. 

In 1964, H.R. Allcock filed a patent claiming the preparation of a soluble polydichlorophosphazene. The polymer obtained can be soluble in organic solvents such as benzene and toluene. This thermal ring-opening polymerization of hexachloro-triphosphazene in the melt at 250°C is the most fully developed and commonly used method for polydichlorophosphazene synthesis to date. 

Also of interest are the thermal ring opening polymerizations of 1,1-dimethyl-l-silabenzocyclobutene (eq. 10) [50] and of 1,1,2,2-tetramethyl-1,2-disilabenzocyclobutene (eq. 11) [51]. The polymer produced in the latter reaction is a polycarbodisilane. 

A thermal ring opening reaction of an imine-substituted cyclopropene led to a mixture of 2,3,4-trisubstituted and 3,4,5-trisubstituted pyrroles <06TL5793>. 

The hybrid polymer 140 is generated by a thermal ring-opening polymerization (ROP) process (Scheme 10).300,301 Polymers containing three-coordinate sul-fur(IV) are generally hydrolytically sensitive even when the chloro substituents are replaced by phenoxy groups.302... 

The thermal ring opening of the cyclobutene in the heterocycle (339) led also to cyclohexene derivatives (340) through intermolecular Diels-Alder reaction 119). 

Photocycloaddition of allene to cyclohexenone (341) gave the (3,y-enone (342), which reacted with vinyl magnesium bromide to produce the tertiary alcohol (343) in 79% yield. When the compound (343) was treated with KH and 18-crown-6 in THF at room temperature for two hours and quenched with aq. NH4C1, the cyclobutene (344) was obtained. The thermal ring opening of the cyclobutene (344) proceeded in toluene in a sealed-tube at 180 °C for twelve hours to give a readily separable 5 1 mixture of the civ-olefin (345), and the trans-olefin (346) respectively in 95 % yield. Moreover, (345) could be converted to a mixture of (346) and (345) in the ratio of 10 1 by irradiation. The compounds (345) and (346) possess the skeleton of the germacranes (347), (348) and (349) 122). 

Little information has appeared on derivatives of [l,2,3,5]thiatriazolopyridines as mentioned also in CHEC-11(1996) <1996CHEC-II(8)405>. In a recent study, the thermal decomposition of the sulfoxide derivative 44 in methanol in the presence of sodium triflate was investigated <1996JFC161> (Scheme 13). After a prolonged reflux, two products 2-pyridyl triflate 46a and 2-methoxypyridine 46b, was isolated in 34% and 16% yields, respectively. The authors concluded that the first step of this transformation is a thermal ring opening of 45 to a carbene intermediate. 

II. RING-OPENING REACTIONS A. Thermal Ring Opening... 

Thermal ring opening to form the merocyanine form is less dependent upon the type of substituent keeping a AG of near to 101 kJmol for BIPS, 6-nitro-BIPS and 6,8-dinitro-BIPS. [37] The ring opening reaction s rate-determining step is probably a rotation cis to trans about the central p-methine bond and this is also consistent with the polar CCC or CCT transition state which have a rather double p-methine bond. 

Scheme 1. Preparation of l-chloro-l-(trichloroethenyl)cyclopropanes 7 by the addition of thermally ring-opened tetrachlorocyclopropene 5 onto alkenes 6 [7-10,25]...

Pyrrolidine derivatives such as 206 have been synthesized by the addition of the corresponding azomethine ylide generated by the thermal ring opening of the aziridine 205 (or similar aziridines) as shown in Scheme 4.33 [204, 210-212],... 

Temperature range of thermal ring opening measurements. = Activation energy, Kcal/mole, for thermal ring opening. 

Half-life time for thermal ring opening. 

The activation energies and entropies for the thermal ring opening show an isokinetic dependence indicating a common structure of the transition state in all members of the DHP series. 

In the examples presented in CHEC-II(1996) in which a pyridazin-3(2//)-one is the 1,3-dipolarophile, two types of 1,3-dipoles are used nitrile oxides and diazoalkanes. Two other 1,3-dipoles have to be mentioned now. The 1,3-dipolar cycloaddition of the azomethine ylide 95 generated in situ by thermal ring opening of dimethyl trans- -(A-methoxyphenyl)aziridine-2,3-dicarboxylate 94 to some 4- or 5-substituted 2-methylpyridazin-3(2//)-ones has been... 

Substituted quinazolines 970 have been prepared by treatment of dihydrotriazolines 968 with ammonia, in a process which involves initial thermal ring opening to an amidine 969, and cyclocondensation of the ammonia with... 

Whereas the thermal ring-opening reaction of oxrranes and aziridines is frequently used for generation of carbonyl ylides and azomethine ylides, the analogous procedure starting with thiiranes does not produce the expected thiocarbonyl ylides (8). However, in the case of tetraaryl-substituted thiiranes, the photolytically mediated reaction with tetracyanoethylene (TCNE) is believed to occur via a single electron transfer (SET) mechanism, also involving a thiocarbonyl ylide as a likely intermediate (75,76) (Scheme 5.14). 

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