Benzoins derivatives

It is worthy of mention at this point that the benzoin ester PPG bears a chiral center, which is a potentially problematic feature with chiral substrates. However, this 

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The benzoin ethers (75, R-alkyl R H) and the ot-alkyl benzoin derivatives (75, R=H, alkyl R =alkyl) undergo a-scission with sufficient facility that it is not quenched by oxygen or conventional triplet quenchers.276 This means that the initiators might be used for UV-curing in air. Unfortunately, it does not mitigate the usual effects of air as an inhibitor (Section 5.3.2). The products of a-scission (Scheme 3.53) are a benzoyl radical (13) and an ( -substituted benzyl radical (76) both of which may, in principle, initiate polymerization, 76 2"... 

It should be pointed out that not all benzoin derivatives (75) are suitable for use as photoinitialors. Benzoin esters (75, R=aeyl) undergo a side reaction leading to furan derivatives. Aryl ethers (75, R=aryl) undergo (3-seission to give a phenoxy radical (an inhibitor) in competition with a-scission (Scheme 3.54). Benzoin derivatives with a-hydrogens (75 R-Il) are readily autoxidized and consequently can have poor shelf lives. 

The acyl phosphonates, acyl phosphine oxides and related compounds (e.g. 81. 82) absorb strongly in the near UV (350-400 nm) and generally decompose by rescission in a manner analogous to the benzoin derivatives.381"285 Quantum yields vary from 0.3 to 1.0 depending on structure. The phosphinyl radicals are highly reactive towards unsaturated substrates and appear to have a high specificity for addition v.v abstraction (see 3.4.3.2). 

Klos et at.2is6 described a range of polymerizable benzoin derivatives as photoinitiators (e.g. 83, 84). These and other polymeric photoinitiators have advantages as initiators over low molecular weight analogs in circumstances where migratory stability is a problem.287"2 ... 

Phenacyl radicals are produced by photodecomposition of initiators containing the phenone moiety (Scheme 3.74). These initiators include benzoin derivatives and acylphosphine oxides (see 3.3.4.1.1). Acyl radicals can be formed by... 

Besides aromatic hydrocarbons such as pyrene (above), benzanthracene (Becker et al., 1977a), benzoin derivatives (Baumann et al., 1982), aromatic and heteroaromatic ketones (Fomin et al., 1980 Baumann et al., 1985), azo dyes (Timpe et al., 1982), methylene blue (Becker et al., 1986 a Becker and Kohrs, 1990), acridine yellow... 

A growth area is UV or electron beam curing. This can be via radical polymerisation, e.g., a benzoin derivative,... 

Regio-control of the formation of benzoic esters of mixed benzoin derivatives results from the initial formation of the cyanhydrin benzoate (see Chapter 3). Subsequent base-catalysed hydrolysis of the esters produces the mixed benzoins (Scheme 6.20) [66]. 

In 2002, Enders and co-workers took advantage of the bicyclic restriction first introduced by keeper and Rawal to develop catalyst 20. Use of this catalyst provides a number of benzoin derivatives 22a-h in up to 95% ee (Table 1) [41]. The stereochemistry of the benzoin reaction catalyzed by thiazolium and triazo-lium pre-catalysts has subsequently been modeled by Honk and Dudding [42]. 

For related reasons, and because their excited-state energies are lower than for dialkyl ketones, diaryl ketones and simple alkyl aryl ketones do not fragment on irradiation in solution, even at higher temperatures. This leads to a photostability that is one factor contributing to the successful employment of ketones such as benzophe-none tPh-CO) or acetophenone (PhCOMe) as triplet sensitizers. a-Cleavage for ketones in solution at room temperature is promoted if structural factors cause the bond adjacent to the carbonyl group to be somewhat weaker than normal. Hence t-alkyl ketones give decar-borylation products readily (4.5), as do benzyl ketones (4.6 and benzoin derivatives (4.7). 

Furo[3,2-c][l]benzopyran-4-ones have been prepared either by acid catalyzed condensation of 4-hydroxycoumarin with a benzoin derivative (equation 11) (81IJC(B)614) or from 3-allyl-4-hydroxycoumarins on oxidation with osmium tetroxide/potassium periodate followed by cyclization of the intermediary aldehyde with PPA (equation 12) (79G109). 

Resolution of the enantiomeric compounds, aromatic amino acids, amino acid derivatives, aromatic sulfoxides, coumarin derivatives, benzoin, and benzoin derivatives have been accomplished on the albumin columns. The a-acid glycoprotein protein column has been used to resolve 50 enantiomeric drugs [288]. The mobile-phase requirements of these sorbents have been reviewed [288]. Although the selectivity of these sorbents are often outstanding, the solute capacity is only 1 nmol per injection. 

Benzoin derivatives and benzophenone derivatives were commercial products of the highest purity grade. They are 2,2 - dime-thoxy-2-phenyl acetophenone (DMPA), benzophenone (BP), 4-benzoyl-benzoic acid (BBA), 3,3, 4,4 -benzophenone and tetracarboxylic dianhydride (BTCD). 

The carbanion derived from diethyl l-(trimethylsiloxy)-l-phenylmethanephosphonate 291 served as an acyl anion equivalent. Its reaction with carbonyl compounds afforded the silylated benzoin derivatives 292 (equation 182)443. This reaction was useful for the synthesis of 2-phenylbenzo[h]furans without laborious isolation of the intermediate benzoin. 

Benzoin derivatives are used as initiators for the photochemical curing of printing inks, lacquers and other surface coatings, since the intermediate radicals in a reaction such as (4.7) can be diverted to initiate the polymerization of vinyl monomers. The use of an unsvm-metrical ketone (4.8) also shows that discrete radicals are produced in the cleavage reaction, since the ratio of hydrocarbon products is close to that expected tor a random combination of separated radicals. Esters that give rise to similar stabilized radicals undergo loss of carbon dioxide (decarboxylation) by a closely related mechanism, and this has proved useful in making quite strained cyclic systems bv irradiation of readily prepared cyclic diesters (4.9). 

Electron donating radicals may be produced thermally or photochemically. Benzoin derivatives and phenylazotriphenylmethane were found to be effective to generate electron donating radicals, directly by a photochemical process (Scheme 9). 

In general, group X in the prepolymer could be a number of possible photolabile groups. Benzoin alkylethers are usually superior to other photosensitizers studied for the photopolymerization of allylamines Therefore, prepolymers are prepared in which group X is derived from an unsaturated benzoin derivative of which several suitable structures are shown ... 

Reduction of tosylhydrazones Tosylhydrazones of benzoin derivatives (1) are reduced by NaBHsCN in the presence of TsOH stereoselectively to eryffcro-dia-stereomers (2) in excellent yield. 

The efficiencies of benzoin derivatives in promoting the light induced hardening of styrene-containing unsaturated polyester resins have been found to vary considerably (7), benzoin alkyl ethers and a-alkylated benzoins being more reactive than benzoin itself. However, as photoinitiators for the polymerization of bulk methyl methacrylate, benzoin and benzoin methyl ether exhibit comparable efficiencies (20), and this is also indicated for polymerisation of methyl acrylate in tert-butanol (21). The relative photoinitiating efficiencies of benzoin derivatives may be influenced by several factors, some of which will depend on the nature of the monomer system and the environment in which the polymerizations are carried out. 

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