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Conjugated dienes photochemical

Evidence for the formation of oxabicyclobutane (6) in the irradiation (S) was negative (72JA1193). The striking difference in photochemical behavior of enones such as (5) and the analogous conjugated dienes, which readily form bicyclobutanes, is attributed to the substantially greater endothermicity of the isomerization in the oxa series. [Pg.186]

A steroid very closely related structurally to cholesterol is its 7-dehydro derivative. 7-Dehydrocholesterol is fonned by enzymatic oxidation of cholesterol and has a conjugated diene unit in its B ring. 7-Dehydrocholesterol is present in the tissues of the skin, where it is transfonned to vitamin D3 by a sunlight-induced photochemical reaction. [Pg.1096]

Figure 30.7 Photochemical cyclizations of conjugated dienes and trienes. The two processes occur with different stereochemistry because of their different orbital symmetries. Figure 30.7 Photochemical cyclizations of conjugated dienes and trienes. The two processes occur with different stereochemistry because of their different orbital symmetries.
The conjugated diene dienoestrol (65) was irradiated at 254 nm in 90% aqueous methanol. Rotation and cis-trans photoisomerization gave (66) which underwent a photochemical [1, 5]sigmatropic rearrangement to give (67). Photocyclization followed by enol-keto tautomerism then gave the isolated dihydrophenanthrene dione (68) [56]. [Pg.68]

Conjugated dienes can undergo photochemical conversion into cyclobutenes ... [Pg.152]

There are a variety of photochemical reactions that non-conjugated dienes can undergo. One of these that is currently of considerable interest is the reactivity brought about by electron-accepting sensitizers such as the cyanoarenes. The photoreactivity of these systems involves the photochemical excitation of the sensitizer to an excited state7. Thereafter, the reactivity is dependent on the ease of oxidation of the alkene or diene. With the transfer of an electron from the diene to the photoexcited sensitizer a radical cation is formed. It is this intermediate that brings about the various processes which occur within the diene systems under investigation. [Pg.258]

One of the exciting areas that has gained importance over the recent decade is the photochemical cyclization of non-conjugated dienes in the presence of species that can act as templates. One such species that has been used is copper(I) salts. The earliest example of the use of copper salts in the intramolecular photocycloaddition of non-conjugated dienes is that described for cycloocta-1,5-diene. When this is irradiated in the presence... [Pg.270]

Perhaps the most convincing evidence for internal conversion is provided by the observations of Hammond et al.sl who find that the quenching of molecular fluorescence by conjugated dienes is not accompanied by either an increase in yield of molecular triplet states (kfa = 0), by any detectable photochemical change (kg = 0), or by the appearance of a characteristic exciplex band (k% = 0). Since the observed quenching constants, KQ, vary by several orders of magnitude it may be assumed that reversible photoassociation is operative in these systems (Section III.C) in which case with (cf. Eq. 33)... [Pg.206]

Such reactions may occur thermally or photochemically, and the differences between the two normally show up in two ways. First, in a thermal reaction the direction of change will be towards the equilibrium position, favouring the more thermodynamically stable compound. whereas in a photochemical reaction the direction of change will be towards a photostationary state that favours the compound with the lower absorption coefficient at the wavelength of irradiation. It is therefore normal for conjugated dienes to be converted efficiently into cyclobutenes using wavelengths that are absorbed bv the diene but not by the cydoalkene 12.12). [Pg.47]

An important group of conjugated diene/triene systems are those in the vitamin D series. The key reactions in the commercial manufacture of vitamin D (and probably also in its formation in skin exposed to daylight) are a photochemical, conrotatory electrocydic ring-opening in the provitamin, and a thermal 1.7-shift of hydrogen in the previtamin so formed (2.23). High conversions to the vitamin are not normally possible because all three species absorb appreciably at the... [Pg.50]

The outcome of all this for photochemical sigmatropic shifts is that those most commonly observed are of order (1.3) or (1.7) these involve 4 or 8 electrons, respectively, and occur in a suprafacial manner. Examples of photochemical 1.3-shifts of hydrogen are found for monoalkenes (2.25) and for conjugated dienes 2.26). In the case of dienes a 1,3-shift is favoured over a 1,5-shift, because for the latter to occur photochemically it would have to take place in an antarafacial manner. Note that in both examples the direction of... [Pg.52]

The cyclization step of Equation 28-8 is a photochemical counterpart of the electrocyclic reactions discussed in Section 21-10D. Many similar photochemical reactions of conjugated dienes and trienes are known, and they are of great interest because, like their thermal relatives, they often are stereospecific but tend to exhibit stereochemistry opposite to what is observed for formally similar thermal reactions. For example,... [Pg.1387]

Conjugated dienes also undergo photochemical cycloaddition reactions. Related thermal cycloadditions of alkadienes have been discussed in Sections 13-3A, 21-10A, and 21-10D, but the thermal and photochemical reactions frequently give different cyclic products. Butadiene provides an excellent example of the differences ... [Pg.1388]

Photochemical cycloaddition reactions of t-1 and several of its analogues with conjugated dienes have been reported. The reaction of 1-t with 2,5-dimethyl-2,4-hexadiene yields the isomeric [2+2] cycloadducts 49 and 50 in a 1.2 1 ratio with a... [Pg.197]

N. J. Turro, Jr., and G. S. Hammond Mechanisms of photochemical reactions in solution. XXXI. Activation and deactivation of conjugated dienes by energy transfer. J. Amer. chem. Soc. 87, 3406 (1965). [Pg.76]

Although complexes with C—H—metal three-center, two-electron bonds were first observed several years ago (40-42), they have received increasing attention recently as model systems for C—H activation by transition metal complexes (43). A general route to such compounds involves the protonation of diene (35,44-51) or olefin complexes (52-56). The resulting 16-electron species are stabilized by the formation of C—H—metal bridges. Irradiation of the complexes [Cr(CO)s L] [L = CO, P(CH3)3, P(OCH 3)3 jin presence of conjugated dienes having certain substituents provides a photochemical route to electron-deficient >/4 CH-diene complexes. [Pg.315]

Ultraviolet irradiation of [Cr(CO)3(ty6-l,3,5-cycloheptatriene)] (41) in the presence of conjugated dienes causes no CO evolution. A photochemical [6 + 4] cycloaddition takes place with the dienes la-Id, li, and 2,3-dimethyl-1,3-butadiene (It) (69-71) [Eq. (21)]. Within the coordination sphere of chromium, two C—C bonds are formed between C-l and C-6... [Pg.325]

As already mentioned, no [6 + 4] cycloaddition is observed when tri-carbonyl-i 6-l,3,5-cycloheptatrienemolybdenum(0) is irradiated in presence of conjugated dienes. So an explanation of these experimental results has to be provided. One reason might be an unfavorable fit of the CuH14 n(CH3 ) and C, 4H18 - (CH3) hydrocarbons into the coordination sphere of molybdenum, arising from the increased Mo—C bond lengths. The other reason concerns the possible different stabilities of the intermediates of the photochemical [6 + 4] cycloaddition with molybdenum as the central metal. [Pg.331]

The preparative potential of the photochemical reactions of Mn2(CO)10 and Re2(CO)10 with conjugated dienes has hitherto been used only rarely (51,147,166-168). The reason for this might be that the reactions do not proceed in a simple fashion. Normally, mixtures of several products are obtained. Powerful separation techniques such as low-temperature column chromatography or high-performance liquid (HPL) chromatography are necessary to isolate the products, and this has thrown some light on the nature of the reactions. [Pg.353]

The photochemical reactivity of alkenes is also of great interest [1,2]. Studies in this area have led to an expansion of the synthetic utility of these substances. Typical photochemical reactions include cis-trans isomeriza-tions, inter- and intramolecular cycloadditions, photooxidations, and electrocyclic ring opening and closing of conjugated dienes and polyenes. Many of these photoreactions have thermal counterparts. In contrast,... [Pg.161]


See other pages where Conjugated dienes photochemical is mentioned: [Pg.128]    [Pg.38]    [Pg.1081]    [Pg.361]    [Pg.67]    [Pg.143]    [Pg.224]    [Pg.258]    [Pg.260]    [Pg.298]    [Pg.906]    [Pg.917]    [Pg.929]    [Pg.41]    [Pg.168]    [Pg.24]    [Pg.28]    [Pg.30]    [Pg.40]    [Pg.44]    [Pg.62]    [Pg.278]    [Pg.376]    [Pg.274]    [Pg.74]    [Pg.260]   
See also in sourсe #XX -- [ Pg.268 , Pg.269 ]

See also in sourсe #XX -- [ Pg.268 , Pg.269 ]




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Conjugation Dienes, conjugated)

Dienes conjugated

Photochemical cycloaddition conjugated dienes

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