Aromatic ketone initiators

The addition followed a radical chain mechanism initiated by photoinitiated electron transfer from the tertiary amine to the excited aromatic ketone and occurred with complete facial selectivity on the furanone ring (99TL3169). The yields increased and best results were obtained with sensitizers (4-methoxyacetophenone,... 

The first step of the reaction involves the (n, it ) excited state of the carbonyl compound reacting with the ground-state alkene. For aromatic ketones, rapid intersystem crossing from the excited singlet state to the excited triplet state occurs, forming initially a 1,4-biradical and then the oxetane ... 

The reaction of aliphatic and aromatic ketone oximes 97 with a dialkyl carbonate 98 in the presence of K2CO3 at 180-190 °C yields 3-alkyl-4,5-disubstituted-2(3//)-oxazolones 104 in 22-48% yields. Mechanistically, it is proposed that N-alkylation of the initially formed oxime O-carbonate 99 yields 100, which affords the enamine 101 in the presence of base. A [3,3] sigmatropic rearrangement ensues to produce 102, which then cyclizes to 104. In cases where 97 contains two methylene groups in proximity to the C=N bond, one of which is benzylic, the above reaction sequence is regioselective for the benzylic methylene group (Fig. 5.25 Table 5.6, Fig. 5.26). ... 

From the decomposition mechanism and the products formed it can be deduced that DCP primarily generates cumyloxy radicals, which further decompose into highly reactive methyl radicals and acetophenone, having a typical sweet smell. Similarly, tert-butyl cumyl peroxide (TBCP) forms large quantities of acetophenone, as this compound still half-resembles DCP. From the decomposition products of l-(2-6 rt-butylperoxyisopropyl)-3-isopropenyl benzene ( ), it can be deduced that the amount of aromatic alcohol and aromatic ketone are below the detection limit (<0.01 mol/mol decomposed peroxide) furthermore no traces of other decomposition products could be identified. This implies that most likely the initially formed aromatic decomposition products reacted with the substrate by the formation of adducts. In addition, unlike DCP, there is no possibility of TBIB (because of its chemical structure) forming acetophenone. As DTBT contains the same basic tert-butyl peroxide unit as TBIB, it may be anticipated that their primary decomposition products will be similar. This also explains why the decomposition products obtained from the multifunctional peroxides do not provide an unpleasant smell, unlike DCP [37, 38]. 

Initiators that operate on the basis of the first mechanisms are commonly different acetophenone derivatives (6) and are mainly used in clear lacquers. The second mechanism employs derivatives of aromatic ketones and quinones... 

Type 2 initiators are bimolecular systems and consist of a photosensitizer and a hydrogen donor. The most well known system is benzophenone/tertiary amine. Because of the relatively weak absorption of benzophenone at 360 nm the efficiency of this system is rather low. A more efficient aromatic ketone is thioxanthone (TX) and its derivatives simply because of the increased at the exposure wave length. 

No substitution product is formed with the enolate ion of cyclohex-2-en-l-one. However, this anion is quite reactive at the initiation step and is often used as an entrainment reagent [17]. Under photo stimulation, the best yields of heteroarylation and phenylation of the enolate ions from aromatic ketones are obtained in DMSO [18]. 

Treatment of the monomer with an acidic catalyst leads initially to polymers of low molecular weight and ultimately to crosslinked, black, insoluble, heat-resistant resin (17). Despite their reportedly excellent properties, virtually no commercial use of such resins exists outside the Soviet Union. The structure and polymerization mechanism of these furfural-ketone polymers are described in a recent study (18). An excellent combustion-resistant resin has been reported (19) from the addition of dialkylphosphites to bis(2-furfurylidene) ketone (6). Furfural condensates with other aliphatic and aromatic ketones have been reported (20,21) to provide photo-crosslinkable resins and hypergol components. 

Even molecules with heteroatoms can be used as electron acceptors. The initiating anion radicals are formed from aromatic ketones [192] (e. g. benzophenone) or nitriles [193] (e. g. benzonitrile, naphthonitrile). It appears that both ketones and nitriles can react in several ways, and the evidence concerning the correctness of the proposed mechanisms does not at present seem to be conclusive [194]. 

In the photopolymerization of methacrylamide by benzoin methyl ether, chain-transfer to monomer has been found to be important, and benzalde-hyde is reported to be an inefficient photoinitiator of methyl methacrylate polymerization unless benzophenone and triethylamine are present. Acetophenone has been found to sensitize the cycloaddition of maleic anhydride to 7-oxabicyclo[2.2.1]heptan-5-one-2,3-dicarboxylic anhydride, , a-hydroxy-acetophenone derivatives have been found to be non-yellowing initiators, and h.p.l.c. has been used to determine residual carbonyl photoinitiators in u.v.-cured resins. In the emulsion-polymerization of methyl methacrylate using an aromatic ketone and a continuous or intermittent laser, the former conditions were found to be similar to those under continuous u.v. irradiation. The dependence of the polymerization rate and average chain-length on the absorbance of the initiator used in the photoinitiated polymerization of vinyl monomers has been studied. Interestingly, irrespective of all conditions, maximum conversion is observed when initiator absorbance is 2.51. "... 

Charge transfer (CT) complexes, different from aromatic ketone/amine systems, such as quinoline-bromine, pyridine-bromine, tetrahydrofuran-bromine etc. have also been reported to behave as initiators of vinyl polymerization, particularly under photoactivation [65-68]. 

The photoreduction of aromatic ketones by polymeric systems having tertiary amine end groups provides an ele nt way for the preparation of block copolymers with high efficiency [138]. The method consists of the synthesis of the bifimctional azo-derivative 4,4 -azobis (iV,i -dimethylaminoethyl-4-cyano pentanoate) (ADCP), successively used as fiee radical thermal initiator for the preparation of tertiary amine-terminated poly(styrene). 

When o-phcnylenediamine reacts with aliphatic or aliphatic-aromatic ketones the initially formed benzimidazolines can be thermally decomposed, losing a hydrocarbon fragment to yield 2-substituted benzimidazoles [81, 131]. While this method does not appear to have major synthetic importance, the analogous reaction with a, 6-diketonc has some application [132]. Presumably it involves an acid-catalysed retro-Claisen condensation of the S-dicarbonyl compound. When acetylacetonc is used, acetone is formed. The process, then, offers an unambiguous approach to compounds such as 2-methyl-4-nitrobenzimidazoles (not available by direct nitration). Neither the numbers of substituents on the arylenediamine nor their natures appear to affect yields or reaction times significantly. 

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