Allene from thermolysis

The positional selectivity on formation of the cydoadducts from 221 is less pronounced than that of the isobenzene 162, but it is the conjugated double of the allene moiety as well that predominantly undergoes the reaction. As demonstrated by the thermolysis of several products, these are formed from 221 under kinetic control. For example, on heating, the styrene adduct 240 and the furan adduct 231 rearranged virtually completely to 241 and 232, which are formally the cycloadducts to the non-conjugated double bond of the allene subunit of 221 [92, 137]. The cause of the selectivity may be the spin-density distribution in the phenylallyl radical entity of the diradical intermediates. 

The diketone 64 was also readily prepared from 59 as outlined in Scheme 20.15. Condensation between 64 and 2 equiv. of 51b gave 65 in excellent yield. Thermolysis of 65 in 1,4-CHD at 75 °C also promoted the Myers-Saito cyclization reaction to generate the biradical 66. The aryl radical center in 66 was then captured by the allenic moiety to form 67, having two stabilized triarylmethyl radical centers. Subsequent hydrogen-atom abstractions from 1,4-CHD then furnished 68. 

The benzannulated analog 115 was likewise synthesized from 114 (Scheme 20.24) [56, 63], However, unlike 109, thermolysis of 115 resulted in its slow decomposition without the formation of the cycloaromatized adduct 116. The lack of propensity for 115 to undergo the Myers-Saito cyclization reaction was attributed to unfavorable steric interactions between the diphenylphosphinyl group and the aryl ring of the benzannulated enyne-allene system, causing the allenic moiety to be rotated out of the plane defined by the aryl ring and preventing the cyclization reaction. 

Allenes can also be synthesized from diazocyclopropanes by chemical or photochemical [l,2,l]-elimination of nitrogen. In the thermolysis of 28 to give 30 the carbene-intermediate 29 could be trapped 18), and in the low-temperature photolysis of 31 the triplet carbene 32 could be detected by EPR-spectroscopy 17). 32 is longlived in a polycrystalline matrix and rearranges to 33 (28 %) at a temperature of —154 °C 19>. Numerous applications are included in Ref. 20). Especially noteworthy are the syntheses of stable cyclobutadienes by Masamune (90%)21) and Regitz (67%)22). 

The higher members of the [l,n,l]-eliminations are also of preparative importance. Thus, the Doering allene synthesis 14) leads to bicyclobutanes (e.g. 66, 28 %) 45), if bulky substitution as in 65 favours the [l,3,l]-elimination. The related cyclopropene syntheses from 6746) and 6946, respectively (40 and 6% 68 in derivatized form lithiation and carboxylation), are to be classified as [l,3,(2)l]-eliminations of bromine (reductive) as well as of hydrogen chloride. The thermolysis or the photolysis of diazo... 

Numerous unsuccessful attempts to synthesize cyclopropanethione have been reported. Thermal or photochemical generation of the C3H4S species from different sources always leads to allene episulfide. Some representative experiments include (a) in vacuo pyrolysis of the sodium salt of 2,2,4,4-tetramethylthietanone tosylhydrazone (4) into the stable tetramethylallene episulfide (S), (b) pyrolytic extrusion of nitrogen from perfluorinated thiadiazoline 6, (c) in vacuo pyrolysis of spiro compound 8 into methylenethiirane (3), (d) the flash vacuum pyrolysis-microwave spectroscopic approach applied to spiro compounds 9 and 10, (e) pyrolysis of anthracene adduct 11 and tosylhydrazide salt 12, (f) thermolytic nitrogen extrusion from pyrazoline-4-thione 13, thermolysis of tetramethylallene episulfide (5) or pyrazoline 13 in dig-lyme solution, and photolytic nitrogen extrusion from pyrazoline 13, ° (g) thionation of methylenecyclopropanone 15, and (h) reaction of donor-acceptor substituted allenes 18 with elemental sulfur. ... 

Allenes A convenient synthesis of allenes is accomplished by thermolysis of a-alkylidene-P-lactones in DMF. The substrates are avaUable from P-lactones by benzeneselenylation followed by oxidative elimination. 

More recently, substituted transient thioxosilanes have been obtained by a retro-ene reaction according to Scheme 6.3. In this study, the thermolysis of several thiosilanes such as 9a and 9b (Scheme 6.3) was examined using both high-resolution mass spectrometry (HRMS) and photoelectron spectroscopy (PES). While monomeric (/-Pr)2Si=S (10b) was obtained from 9b and identified independently by both methods employed, 10a was not observed in the mass spectrum of the thermolysis products of 9a. Moreover, the detection of monomeric 10a by its PE spectrum was hampered by strong bands due to allene and undecomposed precursor obscuring bands assumed to stem from lOa.PH... 

The allene PrCH=C=CHCH2NH2 isomerizes to the 3-pyrroline (122) under the influence of silver tetrafluoroborate. Photolysis of aroyl azides in the presence of diketen (123) yields the hydroxy-pyrrolinones (124).AT-Ray analysis has shown that the adduct of the imine Pr"CH=NPr to N-phenylmaleimide has structure (125). Treatment of dichloromaleimide with ethoxycarbonyl-methylenetriphenylphosphorane affords the Wittig product (126). The formation of the pyrrolidinone (128) in the thermolysis of the iV-cyclohexenylacryl-amide (127) represents an intramolecular ene-reaction (see arrows). The perfluoropyrrolidinone (130) results from the reaction of the cyclobutane (129) with potassium fluoride. Pyrrolidinols (131) are obtained in moderate yields by photochemical cyclization of the amides ArCOCH2CH2NBzCH2Ph. ... 

The pentacyclic alkaloid ( )-meloscine (134) was prepared by Feldman and Antohne using a clever allenyl azide cycloaddition/cyclization cascade to deliver the core azabicyclo[3.3.0]octadiene substructure (2012OL934). Strain-driven release of nitrogen from the dipolar cycloadduct 129 derived from 128 promotes formation of the azatrimethyl-enemethane diradical 130 en route to the bicyclic product 131 (Scheme 30). For the synthesis ofmelo-scine 134, the thermolysis of a dilute solution of allene 132 in toluene gave the desired bicycle 133 whose structure was estabfrshed by single crystal X-ray analysis. Subsequent manipulation of the peripheral functionality in 133 then delivered ( )-meloscine 134. 

Thermolysis of the oxazolinone (164), which is prepared in six steps from the indole (163), in chlorobenzene leads to decarboxylation and the allene (165)- Tautomerization of (165) to (166) followed by electrocyclic ring closure then leads to the ortho-quinone (16 ) which only requires hydrolysis of the three ester... 

Other methods were also used for the synthesis of trifluoromethylfurans. Thus, flash vacuum thermolysis of silyl enol ether of 1,3-diketone 148 at 800 °C afforded furan 69 (70 %) via intermediate allenic ketone 149 [110], Another synthesis of trifluoromethylated furan from 1,3-diketone comprised reaction of diazomethane with 2-(trifluoroacetyl)dimedone 150. This approach produced dihydrofuran 152 in a mixture with methylated product 151. Compound 151 underwent aromatiza-tion into 153 under heating withp-toluenesulfonic acid [111]. 

Another cycloaddition, in which this time the silyl enol ether functions as a dienophile, is the SnCl4-catalysed addition of butadiene to (180 R = Me) giving (181). If R = H in the starting material a 4 + 3 reaction takes place, producing the seven-membered ring (182). Flash-vacuum thermolysis of P-keto-trimethylsilyl enol ethers has been used in a substituted furan synthesis, and the same process has been used to prepare a-allenic acids (183) from siloxy-dienes. ... 

Radical C -C Myers-Saito cyclization of enyne-allene, as well as the reaction of C -C cyclization do not depend on the donor properties of the solvent [427]. For enyne carbodiimides the situation is different, because the nitrogen atom is a potential donor center and is well known for the high electrophilicity of the central carbon atom of the car-bodiimide group. Indeed, the study of the thermolysis of carbodiimide 3.971a showed a strong dependence of the cyclization rate constant on the solvent properties. At 85°C, the reaction was seven times faster in dioxane k = 5.3 x 10 s ) and was nine times faster in acetonitrile K = 6.93 X 10 s ) than in benzene (k = 7.83 x 10 - s ). The rate constant of the thermal C -C cyclization of enyne-carbodiimides correlates better with the donor properties of the solvent rather than its dielectric constant, which is different from the reactions of enyne-allenes. Therefore, any discussion of the mechanism requires the consideration of alternative routes (Scheme 3.147) [424]. 

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