Photochemical conditions for

Exo-alkylidene oxacycloalkylidene complexes such as chromium 2-oxacy-clopentylidene 30 are reluctant to react thermally with alkynes. Nevertheless, benzannulation can be achieved under photochemical conditions (for a detailed... 

Spiroaziridinium compounds have also been synthesized under photochemical conditions. For example, the photolysis of piperidine 83 in acetonitrile resulted in 3-azoniaspirooctane 85 (Equation 19) <1997JOC6903>. Its presence was detected by H NMR spectroscopy and mass spectrometry. This azoniaspiro species was thought to be transient and went on to form N-substituted piperidines which were qualitatively identified by gas liquid chromatography (GLC). 

In 1987 we reported26 that the three possible C3H2 isomers S-2, T-36, and S-37 can be transformed into each other under photochemical conditions. For several reasons propargylene (36) attracted our attention On the one hand the first C3H2 parent species, identified by direct spectroscopic methods, was triplet propargylene (T-36). Its ESR spectrum was published in 1965,63 and, based on the zero-field-splitting parameters, a linear or nearly linear structure was derived. On the other hand, the structural elucidation of 36 by comparison of the calculated and experimental IR spectra turned out to be rather difficult.64... 

The occurrence of competing side reactions is generally reduced when metal catalysts are employed instead of photochemical conditions. For example, insertions into C—H bonds are of diminished importance, although insertions into O—H and other X—H bonds are still commonplace. The extent to which any of these other reactions compete with cyclopropanation is, however, again dependent upon the choice of catalyst. 

Stannyl- (and -silyl-) carboxylic acids undergo oxidative decarboxylation with LTA under mild conditions to provide the corresponding alkenes. This represents an improvement on the well-known alkene-forming decarboxylation of acids with LTA, which requires thermtd or photochemical conditions, for example. The directing metal effect leads to improved yields and regioselectivity. However, stereo-specific alkene formation did not occur and this could imply free radical involvement or transmetallation (Pb for Sn) (stereochemistry ) followed by cation formation, see for example Scheme 27. 

Dimerization of ethene to cyclobutane, described as a [24-2]-cycloaddition reaction, does not proceed in either the forward or reverse directions under thermal conditions. But the [24-2]-cycloaddition reaction does occur under photochemical conditions. For example, ethene and maleic anhydride when exposed to UV light at — 65°C gave cyclobutane diacid anhydride (8.11) in 77% yield. 

The collimated optics of the set-up demonstrated can be substituted by fibre cities. This type of equipment is shown in Fig. 4.11 using a bifurcated fibre and a diode array. Under these conditions white light has to be used to produce a reflection spectrum monitored by the diode array. However, white light causes undefined photochemical conditions. For this reason a third fibre arm is used, which guides monochromatic radiation to the sample. If the white radiation is blocked by a shutter, it may not disturb the monochromatic photoirradiation. Since modem diode arrays take spectra within a few milliseconds the time of measurement stays short compared to the total photochemical process time. This type of equipment can be used to monitor the photoprocess during irradiation of photoresists [101]. 

According to the Woodward—Hoffman rules, a [tu" s - - tu" s] cycloaddition reaction is symmetry allowed under photochemical conditions. For example, due to the proximity of the two j-cjj-1,3-dienes in the same molecule, cjj-9,10-dihydronaphthalene on photolysis undergoes an intramolecular [4 - - 4] cycloaddition (Scheme 4.25). 

The precise reaction conditions for optimal yields depend upon the specific reagents, and both thermal and photochemical conditions for obtaining good yields have been developed. 

Addition Chlorination. Chlorination of olefins such as ethylene, by the addition of chlorine, is a commercially important process and can be carried out either as a catalytic vapor- or Hquid-phase process (16). The reaction is influenced by light, the walls of the reactor vessel, and inhibitors such as oxygen, and proceeds by a radical-chain mechanism. Ionic addition mechanisms can be maximized and accelerated by the use of a Lewis acid such as ferric chloride, aluminum chloride, antimony pentachloride, or cupric chloride. A typical commercial process for the preparation of 1,2-dichloroethane is the chlorination of ethylene at 40—50°C in the presence of ferric chloride (17). The introduction of 5% air to the chlorine feed prevents unwanted substitution chlorination of the 1,2-dichloroethane to generate by-product l,l,2-trichloroethane. The addition of chlorine to tetrachloroethylene using photochemical conditions has been investigated (18). This chlorination, which is strongly inhibited by oxygen, probably proceeds by a radical-chain mechanism as shown in equations 9—13. 

The Hofmann-Loffler-Freytag reaction represents formation of pyrrolidines or piperidines by thermal or photochemical decomposition of protonated A -haloamines in the presence of strong acid such as sulfuric acid or trifluoroacetic acid. " The Hofmann-Loffler-Freytag reaction may also be carried out in milder conditions, for example, PhI(OAc)2,12, hv as shown in section 2.3.4. 

An explanation for the finding that concerted [4 -I- 2] cycloadditions take place thermally, while concerted [2 + 2] cycloadditions occur under photochemical conditions, is given through the principle of conservation of orbital symmetry. According to the Woodw ard-Hofmann rules derived thereof, a concerted, pericyclic [4 -I- 2] cycloaddition reaction from the ground state is symmetry-allowed. 

A similar result is obtained for the thermal electrocyclic ring-opening of 3,4-dimethylcyclobutene. The trans isomer yields only (2 ,4 )-2,4-hexadiene when heated, and the cis isomer yields only (2 ,4Z)-2,4-hexadiene. On UV irradiation, however, the results are opposite. Cyclization of the 2E,4 isomer under photochemical conditions yields cis product (Figure 30.4). 

A direct method for preparing a carboxylic acid treats an alkyl halide with NaN02 in acetic acid and DMSO. Reaction of an alkyl halide with ClCOCOaMe and (Bu3Sn)2 under photochemical conditions leads to the corresponding methyl... 

Allylic silanes react with aldehydes, in the presence of Lewis acids, to give an allyl-substituted alcohol. In the case of benzylic silanes, this addition reaction has been induced with Mg(C104)2 under photochemical conditions. The addition of chiral additives leads to the alcohol with good asymmetric induction. In a related reaction, allylic silanes react with acyl halides to produce the corresponding carbonyl derivative. The reaction of phenyl chloroformate, trimethylallylsilane, and AICI3, for example, gave phenyl but-3-enoate. ... 

Alkanes are formed when the radical intermediate abstracts hydrogen from solvent faster than it is oxidized to the carbocation. This reductive step is promoted by good hydrogen donor solvents. It is also more prevalent for primary alkyl radicals because of the higher activation energy associated with formation of primary carbocations. The most favorable conditions for alkane formation involve photochemical decomposition of the carboxylic acid in chloroform, which is a relatively good hydrogen donor. 

Measurement of the light intensity under conditions identical to those used in the photolysis of the compound of interest is essential for the determination of a quantum yield. Although a number of instrumental methods for measuring light intensities are available, unless these are carefully calibrated, the most accurate means is to use a chemical actinometer. This can be any photochemical reaction for which the quantum yield at the wavelength of interest is accurately known. The following photochemical systems are most commonly used for solution actinometry. 

Conditions for reaction thermal photochemical thermal photochemical... 

Many such reactions may indeed be carried out preparatively under photochemical conditions, though, for reasons that cannot be gone into here (the detailed mechanism of photochemical changes), they are often not concerted but proceed via biradical intermediates. One photochemical (2n + 2n) cycloaddition that does, however, proceed via a concerted process is the example we have already referred to ... 

Continuous generation simply means that the intermediate is continuously replenished by some method and examined under pseudoequilibrium conditions. For instance, Whyman (4) was able, using a special IR cell working at high pressure and temperature, to monitor the behavior of several species of importance in the thermal hydro formyl at ion catalytic cycle. Similarly, Koemer von Gustorf and colleagues (5) have monitored the photochemical... 

The enone system itself is usually part of a five- or six membered ring, although acyclic a,(3-unsaturated ketones and enols of P-diketones are also found to undergo cycloadditions under certain conditions. For seven- and higher membered rings the primary photochemical event is Z—E isomerization around the C—C double bond, the E-isomer then eventually undergoing further thermal reactions. 

Me3SnPPh2 will add to allenes under photochemical conditions, giving two regioiso-meric products32. The predominant species is that in which the phosphine residue attaches to the central carbon atom (reaction 23). The overall yield, and relative proportions of 16 and 17 produced, depends on the nature of the substituent R. For R = H, yield = 78%, ratio 16 17 = 89 11 for R = Me, yield = 67%, ratio = 73 27 for R = Bu, yield = 58%, ratio = 88 12. 

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