1.3- Cyclobutanediones

Cyclobutanediones, once exotic compounds represented by a few perhalo derivatives, have become readily available as a result of new synthetic developments in recent years. These include the modified acyloin condensation 52) in which the intermediate enediolate is trapped as bis-trimethylsilyl ether (28) which can be converted to cyclobutanedione by reaction with bromine or hydrolyzed to acyloin and oxidized in a separate step. In addition to this efficient and general method, bi- or polycyclic unsaturated cyclobutanediones (30) have become available from photolysis of bridged cyclohexenediones (29) to be discussed in the following section. Photocycloaddition of dichlorovinylene carbonate (DCVC) to olefins53) promises to provide a third route if the problems associated with hydrolysis of the photoadducts (31) can be overcome. 

The most common reaction, with the exceptions to be noted later, is photo-bisdecarbonylation to alkene plus two molecules of carbon monoxide (C202 ). This is observed with tetramethylcyclobutanedione 57) (33), the propellanedione 3 (in a 

As noted earlier, compounds of type 30 are obtained by photolysis of 29. Irradiations of 30 at X 500 nm where 29 have no absorption (cf. Fig. 1 for typical spectra) allow examination of the possible occurrence, even to a small extent, of the reverse photoisomerization 30 -+ 29. No such isomerization has been observed except for the overcrowded tetrasubstituted substance 34 where a trace of such reversal has been detected 6l) together with a small amount of monodecarbonylation in addition to the major product, the diene 35. 

The contrast between thermal and photochemical reactions of vinyl cyclobutane-dione 36 is of interest60 . Photolysis of 36 proceeded quantitatively with bisdecarbon-ylation to tetrachlorodiene 37 while thermolysis (70°) of 36 resulted in quantitative isomerization to isomeric diketone 39 (the photochemical precursor of 36). Similar effects have been observed with related compounds. The most reasonable mechanism for isomerization 36 - 39 is homolysis to biradical 38 which can either revert to 36 or 

Attempts to observe intermediate(s) in photolysis of 3 by irradiation at 77 K were thwarted by the extremely low quantum yield for reaction at this temperature 23). No such marked temperature dependence was observed with compounds of type 30 where quantum yields at 77 K were only slightly lower than room temperature values 23,55,62) Resujts obtained with tricyclic compounds which are of special interest in 

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DimeriZa.tlon. A special case of the [2 + 2] cyclo additions is the dimerization of ketenes. Of the six possible isomeric stmctures, only the 1,3-cyclobutanediones and the 2-oxetanones (P-lactones) are usually formed. Ketene itself gives predominandy (80—90%) the lactone dimer, 4-methylene-2-oxetanone (3), called diketene [674-82-8], approximately 5% is converted to the symmetrical dimer, 1,3-cyclobutanedione [15506-53-3] (4) which undergoes enol-acetylation to so-called triketene [38425-52-4] (5) (44). 

Aldoketenes also form piedorninantly the lactone dimers, although the ratio of isomers can be influenced by base catalysis. Ketoketenes dimerize symmetrically, and at a slower rate, to 1,3-cyclobutanediones, unless acidic or basic catalysts are present. 

Ketene trimer can be recovered from the tarry residue of diketene distillation and converted into valuable building blocks like 1,3-cyclobutanedione and squaric acid [2892-51-5] (140,141), an important intermediate in the synthesis of pharmaceuticals and squaryHum dyes used in photostatic reproduction (142,143). 

The most obvious features of this synthesis are its simplicity and overall yield, which appear to be superior to those of any other published report. An important merit lies in the generality of the reaction, and the fact that it is an example of a reasonably large-scale photochemical preparation. Tetramethylethylene is readily produced from commercially available tetramethyl-1,3-cyclobutanedione by an identical route.7... 

This dimerization is so rapid that ketene does not form P-lactones with aldehydes or ketones, except at low temperatures. Other ketenes dimerize more slowly. In these cases the major dimerization product is not the P-lactone, but a cyclobutanedione (see 15-61). However, the proportion of ketene that dimerizes to p-lactone can be increased by the addition of catalysts such as triethylamine or triethyl phosphite. Ketene acetals R2C=C(OR )2 add to aldehydes and ketones in the presence of ZnCl2 to give the corresponding oxetanes. ... 

Tetramethyl-2,4-cyclobutanedione See Sodium 1,3-dihydroxy-1,3-bis(ac/-nitromcthvl)-2,2,4.4-tctramcthvlcyclobutan-diide... 

During a very early study on tetramethylcyclobutadiene16 it was anticipated by one of us that 2,2,4,4-tetramethyl-l,3-dicarbenacyclobutane formed in the pyrolysis of dry di-sodium salt of 2,2,4,4-tetramethyl-l,3-cyclobutanedione-bisto-sylhydrazone might be a reasonable precursor for the cyclobutadiene derivative. But the only product — isolated in a cooling trap — was tetramethylbutatriene. 

In a 500-ml. three-necked flask equipped with a thermometer, a mechanical stirrer, and a reflux condenser are placed 200 g. (1.43 moles) of tetramethyl-l,3-cyclobutanedione (Note 1) and 50 g. of chlorobenzene (Note 2). The mixture is heated to 135° with stirring while a total of 1.8 g. of reagent grade anhydrous aluminum chloride is added in 0.3-g. portions over a 3-hour... 

Tetramethyl-l,3-cyclobutanedione is available from Eastman Organic Chemicals. It melts at 115-116° and is typically 99% pure by vapor-phase chromatography. 

The isomerization is usually complete in 5 hours and can easily be followed by vapor-phase chromatography. Heating periods up to 20 hours are not detrimental. The only failure among numerous preparations occurred when tetramethyl-1,3-cyclobutanedione contaminated with 4% of isobutyric acid was used. In case of partial conversion after 5 hours, additional increments (0.5 g.) of aluminum chloride should be added to complete the reaction. 

Tetramethyl-1,3-cyclobutanedione, reaction with aluminum chloride, 48,72... 

Electrochemical reduction of carbon monoxide in dry nonaqueous media at moderate to low pressures leads to the formation of the 1,3-cyclobutanedione dianion (squarate) at current efficiencies, up to about 45% depending on the cathode material [1,2]. In aqueous solution, electroreduction can lead to the formation of methane and other hydrocarbon products. The role of the metal/adatom in determining the extent of CO and hence hydrocarbon formation during the reduction of carbon dioxide is related to the ability of the electrode material to favor CO formation (Cu, Au, Ag, Zn, Pd, Ga, Ni, and Pt) and stabilize HCCO [3, 4]. 

Even in solution the lowest excited singlets of certain compounds may react at rates competitive with intersystem crossing. An example is tetramethyloxetanone, 27, which dissociates with a quantum yield close to unity no matter how much triplet quencher is present in solution.303 The 1,3-cyclobutanediones behave similarly.304... 

Electron impact mass spectrometry of the cyclobutanedione (24) gives rise to dimethylcarbene radical cation.35 Appearance energy measurements and ah initio calculations indicated that the radical cation lies 84 kJ mol 1 above the propene radical cation and is separated from it by a barrier of 35 kJ mol-1. Diarylcarbene radical cations have been generated by double flash photolysis of diaryldiazomethanes in the presence of a quinolinium salt (by photo-induced electron transfer followed by photo-initiated loss of N2).36 Absolute rate constants for reactions with alkenes showed the radicals to be highly electrophilic. In contrast to many other cation radicals, they also showed significant radicophihc properties. 

Cyclobutanedione, 169 Cyclodeamination, 450 1,6-Cyclodecadiene, 539 Cyclodehydration, 321, 427, 428, 430 (3-Cyclodextrin, 150-151 Cycloheptatriene, 144 Cyclohcxanc-1,2-diol, 145 Cyclohexanol annelation, 10... 

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