Allenes 4,4,5,5-tetramethyl-2-

Methylene thiirane is obtained by thermolysis of several spirothiirane derivatives which are formally Diels-Alder adducts of methylenethiirane and cyclopentadiene or anthracene <78JA7436). They were prepared via lithio-2-(methylthio)-l,3-oxazolines (c/. Scheme 121). A novel synthesis of the allene episulfide derivatives, 2-isopropylidene-3,3- dimethylthiirane (good yield) or its 5-oxide (poor yield), involves irradiation of 2,2,3,3-tetramethyl-cyclopropanethione or its 5-oxide (81AG293). Substituents on the thiirane ring may be modified to give new thiiranes (Section 5.06.3.9). The synthesis of thiirane 1-oxides and thiirane 1,1-dioxides by oxidation is discussed in Section 5.06.3.3.8 and the synthesis of 5-alkylthiiranium salts by alkylation of thiiranes is discussed in Section 5.06.3.3.4. Thiirene 1-oxides and 1,1-dioxides may be obtained by dehydrohalogenation of 2-halothiirane 1-oxides and 1,1-dioxides (Section 5.06.4.1.2). 

Spirocyclic 2-alkylidenecyclobutanones can be prepared from vinylidenecycloalkane additions with dimethylketene (see Tabic 1). Alternatively, cycloaddition of cycloalkylidenemethanone to allenes permits synthesis of spirocyclic cyclobutanones. This latter route is more attractive because of the greater availability of simple allcnes as compared with the alkylidenecycloalka-nes. The in situ generation of cyclohexylidenemethanone in the presence of excess tetramethyl-allcnc produced the spirocyclic ketone 9 in reasonable yield.5... 

Photochemical reactions of quinones with allenes have also been studied and in some cases cyclobutane formation occurs, although in competition with products derived from attack of the allene on the carbonyl oxygen. Thus, photocycloaddition of tetramethyl-l,4-benzoquinone with 1,1-dimethylallene affords the four-membered carbocycle 6 in good yield.12... 

Because only a small number of stable selenoketones exist, studies of their decomposition are scarce. Irradiation of dwerf-alkylselones with UV light in hydrogen-donating solvents affords diselenides (80CJC6 87MI1), and the selone 37 behaves likewise. A second path of photolysis involves extrusion of molecular nitrogen from 37 to yield eventually tetramethyl-allene and elemental selenium, probably via the elusive... 

This contrasts with the protonation occurring at the central allene carbon of 1,3-dimethyl-, 1,1,3,3-tetramethyl- and 1,3,3-trimethyl-l-phenyl allenes 415 (R = H, Me,... 

On the other hand, treatment of the mono-2-butylallene oxide 67 with one equivalent of peracid yields the spiro-Ws-epoxide 68 and the acetoxy ketone 69.55b> Similarly, the peracid oxidations of tetramethyl-allene, 1,1-dimethylallene, and 1,2-cyclononadiene do not give the... 

The behavior of allene (1,2-propadiene) and its derivatives as ligands in transition metal complexes has been investigated for more than 20 years (197,198). Allene may be coordinated by only one C=C double bond to a metal (197,199), or it may link to two metal centers as a 2 2 or 3 1 electron donor. Dinuclear /i-f/2l2-allene complexes are known for Mo (200,201), W (201), Mn (202-203), and Rh (199,204). In [Fe2(CO)7(/ -f/3 1-C3H4)] a bonding mode (205,206). In the course of the reaction, often two or more allene molecules become connected with transition metal complexes. Two allenes linked by C-2 yield tetramethyl-eneethane (207,208), which is stabilized by coordination on Fe (205,208), Ni (270), or Pd (210,211). 

Many dimerizations have been reported for allenes bearing four identical substituents allene itself and tetramethyl 127>, tetrafluoro 13>n4), tetrachloro 49,80,94,99-101,103,108,128) and tetrabromo 103,106) analogs all give 1,2-dimethylenecyclobutanes completely devoid of information regarding reaction stereochemistry. Tetrachloroallene dimerizes with activation parameters riH = ll kcal/mole, [Pg.8]

Treatment of (E)-2-alkenyl sulfoxides (2) with lithium 2,2,6,6-tetramethyl-piperidide induces elimination to allenes. An example is the conversion of 3 to 4. This transformation permits conversion of aldehydes and ketones into terminal allenes. 

Rademacher and co-workers studied the gas-phase thermolysis of 4-substituted 3,3,5,5-tetramethyl-3,5-dihydo-477 pyrazoles by photoelectron spectroscopy <1996T1695>. Among the systems investigated was 76, which afforded 77 at 470 °C. The alkylidene thiirane rearranged at 640 °C to thiol 78. Thermolysis of 79 at 425 °C gave alkylidenethi-irane 3 -oxide 80, which lost SO at 700 °C to yield allene 81 (Scheme 13). 

The hydrazone 364 of 3-thietanone reacts with hydrogen sulfide to give the dispiro derivative 384. Thermolysis of the sodium salts of p-toluenesulfonyl-hydrazones of 3-thietanone,2,2,4,4-tetramethyl-3-thietanone and 2-isopropylidene-4,4-dimethyl-3-thietanone yields allene or cumulene epi-sulfides, for example, 385. The reactions of some tris-iminothietanes are discussed in Section XV.3.B. ... 

Fluxional processes involving acyclic unsaturated hydrocarbon ligands, such as allyls and allenes, are also common. For the -n-complex formed between tetramethyl-allene and tetracarbonyliron (Fig. 15.53), the proton magnetic resonance spectrum below -60 C shows three peaks in the ratio 1 1 2. representing the three cis hydrogen atoms, three Irans hydrogen atoms, and six hydrogen atoms in a plane perpendicular to the carbon-iron bond. With an increase in temperature, the spectrum collapses to a... 

A novel method of preparing tetramethylallene is by cycloelimination from 3,3,5,5-tetramethyl-l-pyrazolin-4-one hydrazone, which occurs on treatment of this compound with an excess of nickel peroxide at room temperature, yielding nitrogen and 87-91% of the allene 163... 

The interactions of these hindered-piperidine compounds with other conventional stabilizers has also attracted widespread interest. Allen et have clearly shown that the presence of phenolic anti-oxidants antagonize the stabilizing performance of hindered-piperidine compounds. Furthermore, their performance is drastically reduced by processing and this effect is demonstrated in Table 5 for bis(2,2,6,6-tetramethyl-4-piperidinyl) sebacate (known commercially as Tinuvin 770) and a commercial phenolic anti-oxidant, Irganox 1010. With u.v. absorbers such as the 2-hydroxybenzophenones the hindered piperidines seem to... 

The question of whether the bisallyl biradical is involved in the allene dimerization has been answered in the affirmative. Dimerization of 1,1-dimethylallene at 150°C and thermolysis of 4,5-dimethylene-3,3,6,6-tetramethyl-3,4,5,6-tetrahydropyridazine at the same temperature gave the three isomeric allenes in the same ratio. Further, direct photolysis of the azo compound just above room temperature gave the same product distribution when the thermal data was extrapolated to that temperature. 

Later on, the annulation of allenes with 2-iodophenols and 2-iodoaniline derivatives were further explored. In 2009, Shi and Li developed an efficient method for the coupling reactions of diarylvinylidenecyclopropanes with 2-iodophenols and N-(2-iodophenyl)-4-methylbenzenesulfonamide. A variety of cyclopropane-containing 2,2-diaiyl-3-(tetramethyl(yclopropylidene)-2,3-(dihydro)benzofuran and 2,2-diaryl-3-(tetramethyl(yclopropylidene)-l-(toluene-4-sulfonyl)-2,3-dihydro-l//-indole derivatives were produced in moderate to good yields with palladium as the catalyst (Scheme 2.48a). 

Another interesting class of compounds reported by Ando and coworkers is tetrasilacyclohexynes and polysilabridged allenes. Thus, l,l,2,2,3,3,4,4-octaalkyl-l,2,3,4-tetrasilacyclohexynes (10a and 10b) were prepared and their Si NMR spectral data were reported (10a 5 = -30.8, -19.2 10b -17.7, -8.3). NMR spectral data (—32.2, —7.9) were also reported for l,3,5,5-tetraphenyM,4,6,6-tetramethyl-4,5,6-trisilacyclohexa-1,2-diene (11). ... 

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