Hideo Tomioka

Photolysis of regioisomeric diazides of 1,2-diphenylacetylenes studied by matrix-isolation spectroscopy and DFT calculations

A series of diazides of 1,2-diphenylacetylenes was photolyzed in matrices at low temperature and transient photoproducts were characterized by using IR, UV/vis methods combined with ESR studies. Theoretical calculations were also used to understand the experimental findings. The introduction of phenylethynyl groups on phenyl azides has little effect on the photochemical pathway. Thus, upon photoexcitation, (phenylethynyl)phenyl azides afforded the corresponding triplet nitrene, which is in photoequilibrium with the corresponding azacycloheptatetraene. In marked contrast, azidophenylethynyl groups exhibited a dramatic effect not only on the photochemical pathway of phenyl azides but also on the electronic and molecular structure of the photoproducts. The patterns of the effect depended upon the relative position of azide groups in the diphenylacetylene unit. Whenever two azide groups were situated in a conjugating position with respect to each other, as in p,p-, o,o-, and p,o-bis(azides), the azides always resulted in the formation of a quinoidal diimine diradical in which unpaired electrons were extensively delocalizedin the pi-conjugation. The situation changed rather dramatically when azide groups were introduced in the meta position. Thus, the formation of azacycloheptatetraene was noted in the photolysis of the m.m-isomer. ESR studies indicated the generation of a quintet state that was shown to be a thermally populated state with a very small energy gap of ca. 100 cal mol(-1). The m,p-isomer was shown to be an excellent precursor for the high-spin quintet dinitrene. The IR spectra of the photoproduct showed no bands ascribable to azacycloheptatetraene. The observed spectra were in good agreement with that calculated for the quintet state. Strong EPR signals assignable to the quintet state were observed, along with rather weak signals due to mononitrenes. Moreover, the quintet bis(nitrene) was rather photostable under these conditions.

Reactions of carbenes in solidified organic molecules at low temperature

Studies on reactions of carbenes in reactive organic glasses at low temperatures clearly reveal that solution results and liquid phase mechanistic rules cannot be readily extrapolated to matrix conditions. Thus, the usual course of reaction of a carbene with an alkene in solution results in the formation of a cyclopropane for both the singlet and triplet states although a one-step addition possible for singlet carbene produces the cyclopropane stereospecifically and a stepwise pathway with the triplet state affords two possible stereoisomers of the cyclopropane. In a sharp contrast, the formal insertion products into the allylic C-H bonds of alkenes are produced at the expense of the cyclopropane when carbene is generated in alkene matrix at low temperature. Similar results are obtained in the reaction with alcohols, where the C-H insertion products are formed in low temperature alcoholic matrices at the expense of the O-H insertion products which are predominant products in the reaction with alcoholic solution at ambient temperature. The 13C labelling experiments as well as deuterium kinetic isotope effects suggest that these C-H insertion products are most probably produced from the triplet carbene, not from the singlet, by abstraction of H atom from the matrix followed by the recombination of the resulting radical pairs.Kinetic studies using ESR and laser flash photolysis techniques demonstrate that the mechanism of a H-atom transfer reaction changes from a completely classical process in a soft warm glass to a completely quantum mechanical tunneling process in a cold hard glass. Thus, as the reaction temperature is lowered, the classical reaction rate decreases, and eventually becomes much slower than decay by hydrogen atom tunneling. The members of the radical pairs which usually diffuse apart in a fluid solution are not able to diffuse apart owing to the limited diffusibility within a rigid matrix and therefore recombine with high efficiency to give the CH “insertion” products.A rather surprising and intriguing difference between the C-H insertion undergone by singlet carbenes in fluid solution at ambient temperatures and one by triplet carbenes in matrix at low temperature is noted. Thus, a marked increase in the primary and secondary C-H insertion over the tertiary is observed in the matrix reaction indicating that triplet carbenes tend to abstract H from less crowded C-H bonds. This is interpreted to indicate that the distance between carbenic center and tunneling H becomes important in H atom tunneling process. More surprisingly, the C-H insertion by triplet carbene by the abstraction-recombination mechanism in a rigid matrix proceeds with retention of the configuration, suggesting that the solid state prevents motion of the radicals to the extent that does not allow racemization to occur. Reactions with heteroatom substrates such as ethers, amines, alkyl halides and ketones are also subject to the matrix effects and the C-H insertion products increase at the expense of singlet carbene reaction products resulting from the interaction with the heteroatoms. Stereoselectivities of cyclopropanation to styrenes are also shown to be affected by the matrix effects. t-Butyl alcohol matrix is shown to be unreactive toward carbenes and thus can be used as a “solvent” in matrix carbene reactions presumably due to a large inert guest cavity provided by bulky tertiary alcohol which binds a molecular aggregate inside it. H atom tunneling in the matrix is also shown to compete with very efficient intramolecular migration of hydrogen to the carbenic center. Migration aptitude as well as stereochemistry are also found to be subject to the matrix effects.

Preparation of bis(diazo) compounds incorporated into butadiyne and thiophene units and generation and characterization of their photoproducts

(2,6-dimethyl-4-tert-butylphenyl)(2,4,6-tribromophenyl)diazomethane(-N(2)) was found to be stable enough to survive under Sonogashira coupling reaction conditions, and aryldiazomethyl substituents were introduced at the 1,4-positions of butadiyne (4-2N(2)) and the 2,5-positions of thiophene(5-2N(2)). Irradiation of those bis(diazo) compounds generated bis(carbenes), which were characterized by using ESR and UV/vis spectroscopic techniques in a matrix at low temperature as well as time-resolved UV/vis spectroscopy in solution at room temperature. These studies revealed that both of the bis(carbenes), 4 and 5, have singlet quinoidal diradical ground states with a very small singlet-triplet energy gap of less than 1 kcal mol(-1). A remarkable increase in the lifetime of bis(carbenes), as opposed to that of the monocarbene (2), was noted and was interpreted to indicate that bis(carbenes) are thermodynamically stabilized as a result of delocalization of unpaired electrons throughout the pi net framework. In spite of the stability, both bis(carbenes) are readily trapped by molecular oxygen to afford bis(ketones). Presumably, the reaction of the upper-lying localized quintet states with oxygen is much faster than that for lower-lying states.

Triplet carbenes: From fleeting existence to attractive molecular units

Attempts to stabilize the triplet state of carbenes to the extent that they can survive under normal conditions were made, in which triplet carbenes having a half-life of 19 min are realized. A way to use the resulting persistent carbene as a unit to construct photosensitive magnetic materials is also discussed.

Structures/properties relationships in photoinitiators of polymerization: XI. Behavior of new benzylbenzoin benzyl ethers

Abstract The photochemistry of five new substituted benzoin ethers has been investigated and their polymerization activities evaluated; the cleavage mechanisms have been explored using steady-state photolysis and mass spectroscopy and the excited state processes studied with time-resolved laser spectroscopy. The five compounds can be separated into two groups: three of them present a very interesting initiation efficiency for the polymerization of acrylates; photolysis leads to a cleavage reaction resulting from a very short triplet state and is followed by subsequent cleavage. The other two, which exhibit a better near UV–Vis light absorption, have very poor efficiencies as photoinitiators since they are very stable under irradiation. Full diagrams of excited state processes are discussed.

Benzylbenzoin benzyl ethers as new photoinitiators

The title benzoins are prepared and their photochemistry has been investigated. Product analysis studies indicate that the benzoin undergoes Norrish Type I photocleavage to generate benzoyl and benzyloxybenzyl radical, followed by thermal fragmentation to produce deoxybenzoin thereby generating benzyl radical. All three transient species are actually detected and characterized by laser flash photolysis of the benzoin in solution at room temperature. The efficiency of the benzoins as photoinitiator has been also investigated. Effects of ring substituents and polycyclic aromatic rings on the photoreactivity have been revealed.

Tunneling Insertion Reaction of Carbenes

Many organic reactions proceed by multi-step processes in which intermediate molecular species are involved in the conversion from one molecule to another. Since organic chemistry is basically the chemistry of carbon compounds, it is natural that the greatest attention has been paid to reactive intermediates that contain carbon. The normal state of carbon is tetravalent (1).

Special Articles on Organic and Inorganic Optical Materials. Preparation of Polyion Complexes Consisting of Spyropyrans and Photochromism of Their Spin-coated Films.

1´位側鎖にカルボキシル基を持つスピロピラン(SP)を合成し,そのカルボン酸ナトリウム塩とカチオン性ポリマー,ポリ(塩化ジアリルジメチルアソモニウム)(PDDA+Cl-)を水中混合することによって標記ポリイオンコンプレックスを合成した。このポリイオンコンプレックスは1H-NMR,IRによってSPがカルボキシレートの形でPDDA+ に取り込まれることを確認した。ポリイオンコンプレックスはそのクロロホルム溶液をスピンコートすることによって無色透明な薄膜を形成し,これは光照射によって着色し,加熱によって消色する正ホトクロミズムを示した。着色体メロシアニソ(MC)の吸収極大から,ポリイオンコンプレックスではSPは比較的非極性な部分に取り込まれていることを示し,またMCの熱消色速度は速い過程と遅い過程の二つの一次速度の和として解析できることから,SPはPDDA+主鎖にそって取り込まれたものと主鎖からはずれて取り込まれたものとの二成分からなることも示した。ポリィオソコソプレックスの挙動をSPの溶液中およびポリ(メタクリル酸メチル)フィルム中でのそれと比較し,ポリィオンコンプレックス中のSPはその着色体がかなり安定化されていることを明らかにした。