Formaldehyde biradical

Qualitatively, the interaction diagram would closely resemble that in Fig. 3, since electron-donating substituents in both addends would raise the molecular levels of both the carbonyl compound and the olefin. Only the energy gap, E(n)-> F(n), would increase, the net result being that the calculated ratio of concerted to biradical reaction, Eqs. 40 and 41, should be even closer to unity than in the formaldehyde-ethylene case. Detailed calculations 38> support this conclusion, so PMO theory predicts that the overall stereochemical results are due to a combination of concerted (singlet) and biradical (triplet) mechanisms. This explanation agrees with the experimental facts, and it bypasses the necessity to postulate differential rates of rotation and closure for different kinds of biradical intermediates. 

Mono- and di-alkylated furans were synthesized in a one-pot preparation from 2-propynyl-2-tetrahydropyranyl ether (106), butyllithium and formaldehyde. The intermediate allenyl ether (107) presumably cyclizes via a 2-(2-tetrapyranyloxy)-2,5-dihydrofuran (108) to afford the heterocycle (109) (79AG(E)875). In a similar manner, singly and doubly branched tetrahydropyranyloxybutynolates afforded the substituted furans (110) (Scheme 20). The thermocatalytic isomerization of ethyl l-methyl-2-phenylcyclopropene-3-carboxylate yielded the furan, possibly by a 1,3-sigmatropic displacement step or by a non-concerted biradical intermediate (75T2495). 

A Planar Molecule Becomes Bent. In the ground state formaldehyde is planar, but in the n-7r state an electron is promoted from the n to the 7r orbital. The excited molecule can be represented approximately as a biradical (Figure 3.47). The carbon atom now has four nearly equivalent bonds ,... 

Here, pathway 1 (reaction 1) is the coordinated addition of ozone (1) to ethylene (2), which proceeds through the formation of a weakly-boimd complex that transforms into primary ethylene ozonide (PO) or 1.2.3-trioxolene upon passing through the symmetrical transient state (TSl). Pathway 2 (reaction 2, the DeMore mechanism [15]) involves the collision during spontaneous orientation of the reagents (3) and the rotational transition to the biradical transient state (TS2) (4) followed by the formation of the same PO. Proceeding from the above-said, we supplement this pathway with the reaction of detachment of molecular oxygen and the formation of intermediate biradical (5) the latter may either decompose with the formation of formaldehyde (6) and carbene (7) or transform into acetaldehyde (8) or epoxide (9). Finally, pathway 3 involves the transition of ozone into the triplet state (10). This pathway is similar to reaction 2. Here, the same biradical (5) is formed it transforms into the... 

POZ). The reaction enthalpy is retained as the internal energy of the products, resulting in formation of the vibrationally excited ozonide, which subsequently undergoes unimolecular decomposition to yield a chemically activated biradical, known as the carbonyl oxide or Criegee intermediate (Cl), and an aldehyde (e.g., MVK, MACR or formaldehyde). A total of nine carbonyl oxides (methyl vinyl carbonyl oxide, derived from 1,2-ozonide isopropyl carbonyloxide, derived from... 

Thietanes are photochemically unstable and should be protected from light if they are to be stored for any length of time. Short-lived (hot) biradical intermediates, for example, 98a, appear to be formed and can undergo a variety of reactions, as shown for thietane. " Mercury-sensitized photolysis gives triplet biradicals that are longer lived than the singlet biradicals formed on direct excita-tion. " Some cyclopropane product is produced in the sensitized photolysis. The second excited singlet state of thietane decomposes either to ethylene and thio-formaldehyde via a 1,4-biradical or to elemental sulfur and cyclopropane. ... 

Tetrafluoroethylene oxide decomposes through the biradical to give perfluoro-formaldehyde and difluorocarbene . The latter intermediate could be trapped as perfluorocyclopropane by tetrafluoroethylene. The mechanism proposed is... 

Figure 4. Examples of low-temperature limit of rate constant of solid-state chamical reactions obtained in different laboratories of the USSR, United States, Canada, and Japan (1) formaldehyde polymerization chain growth (USSR, 1973 [56]) (2) reduction of coordination Fe-CO bond in heme group of mioglobin broken by laser pulse (United States, 1975 [65]) (3) H-atom transfer between neighboring radical pairs in y-irradiated dimethylglyoxime crystal (Japan, 1977, [72], (4, 5) H-atom abstraction by methyl radicals from neighboring molecules of glassy methanol matrix (4) and ethanol matrix (5) (Canada, United States, 1977 [11, 78]) (6) H-atom transfer under sterically hampered isomerization of aryl radicals (United States, 1978 [73]) (7) C-C bond formation in cyclopentadienyl biradicals (United States, 1979 [111]) (8) chain hydrobromination of ethylene (USSR, 1978 [119]) (9) chain chlorination of ethylene (USSR, 1986 [122]) (10) organic radical chlorination by molecular chlorine (USSR, 1980 [124,125]) (11) photochemical transfer of H atoms in doped monocrystals of fluorene (B. Prass, Y. P. Colpa, and D. Stehlik, J. Chem. Phys., in press.).

After the formaldehyde polymerization described in the beginning of this section, the next, example of a low-temperature kinetic plateau in reactions related to heavy molecular fragment transfer was observed by Buchwalter and Closs [111] in the photoisomerization of a 1,3-cyclopentadiyl (CPDY) biradical isomerized into a bicyclo-(2,10)-pentane (BCP) ... 

The 03-isoprene reaction proceeds by initial addition of 03 to the C=C double bonds to form two primary ozonides, each of which decomposes to two sets of carbonyl plus biradical products (a minor channel is apparently formation of a 1,2-epoxymethyl butene). This mechanism is consistent with the formation of formaldehyde, methacrolein, and methyl vinyl ketone. As in other 03-alkene reactions, OH radicals are observed in significant yield, about 0.27 molecule of OH per 03-isoprene reaction. 

The yields of the three main products increase in the presence of water vapour, as well as in the presence of cyclohexane. The simultaneous presence of both scavengers leads to still higher yields for these carbonyl compounds. This trend, as well as the absolute yields, agrees well with literature data obtained under comparable conditions [8-10]. The explanation for this behaviour is not clear. One idea is that high concentrations of water vapour and cyclohexane suppress reactions of biradicals and OH radicals, respectively, with other products such as MAC, MVK and formaldehyde, so that higher yields are measured for these products. Also the IR spectrum of the non-identified products changes clearly with different concentrations of water vapour, but the nature of these products is still speculative. Nevertheless, it is evident that water vapour participates in the O3 reaction. 

Reactions.—Pyrolysis of thietan at 700 °C is the best source of thio-formaldehyde. Photofragmentation of thietans, sensitized by (86), gives good yields of highly substituted olefins. A careful study of the mercury-sensitized decomposition of thietan suggests the trimethylenethiyl biradical as the first intermediate. Photolysis of (87) gives (88). ... 

In 1946, Klute and Walters provided the first exploration of the pyrolysis of THF, observing the formation of ethene, methane, and CO as major products during pyrolysis over a temperature range of 802— 842 K and a pressure range of 50—300 torr. They proposed that the main breakdown pathway of THF involved formation of ethene and acetaldehyde (which ultimately decomposed to methane and carbon monoxide) a secondary pathway involved formation of propene and formaldehyde (which then formed carbon monoxide and H2). Their work was followed several decades later by Lifshitz et al., in a 1986 study exploring initiation pathways available via a shock tube study of isotopic derivatives of THF over the temperature range 1070—1530 K. These authors noted that two pathways were possible the first yielded ethene and a biradical [(CH2)2—O], while the second yielded propene and formaldehyde. 

The overall mechanisms for reaction of O3 and NO3 with 2-buten-l-ol are similar to those with 2-propen-l-ol described previously. O3 will add to the C=C bond resulting in formation of a trioxide which decomposes to form two carbonyl compounds and two biradicals. Grosjean and Grosjean (1995) reported acetaldehyde and glycolaldehyde as major products from the ozonolysis of 2-buten-l-ol with formation yields of (64 6)% and (79 18)%, respectively. Formaldehyde was also observed as a product (23 7)%, its formation involves the reactions of biradicals (CH3CHOO and HOCH2COO). 

The ozonolysis of 3-buten-2-ol leads mainly to formaldehyde (44 5)%, 2-hydroxypropanal (30 3)% and acetaldehyde (19 2)% as reported by Grosjean and Grosjean (1995). Acetone and glyoxal were observed as minor products in yields of (2 1)% and (3 1)%, respectively. The proposed mechanism is similar to that for the ozonolysis of other olefins with the initial formation of a trioxide which decomposes to stable carbonyls and biradicals. 

---