Acyl radicals from aldehydes

In the absence of oxygen, these radicals can be trapped by addition to olefins. Co , on the other hand, generates primarily acyl radicals from aldehydes i.e., the cobalt catalyzed reaction does not proceed through the enol mechanism. 

With polycyclic heteroaromatic bases, such as acridine, the reaction can lead to dihydroderivatives when the metal salt used in the redox system for generating acyl radicals from aldehydes, has a marked reducing character ) (Scheme 14). 

A1BN, iw azobisisnbutyronilrile aldehydes, acyl radicals from 1 18 alkanelhiyl radicals Irom ally] sulfides 300 from disunities 291-2 from thiols 290, 291 polarity 290... 

Another way to construct alkenes is by the addition of carbon radicals to nitrostyrenes such as 5. Ching-Fa Yao of National Taiwan Normal University in Taipei has reported (J. Org. Chem. 2004,69, 3961) an extension of this work, generating the acyl radical from the aldehyde 6, cyclizing it to generate a new radical, then trapping that radical with 5 to give 7. This article includes an overview of the several ways of adding radicals to 5. 

The acyl-alkv biradical obtained by ring-opening of a cyclic ketone is able lo undergo intramolecular disproportionation in one of two ways. A hydrogen atom may be transferred to the acyl radical from the position adjacent to the alkyl group, and this produces an unsaturated aldehyde (4.21). Alternatively, a hydrogen may be transferred to the alkyl radical from the position adjacent to the acyl group, and this results in the formation of a ketene (4.22). Many ketenes are labile, and the use of a nucleophilic solvent or addend. 

Other functional groups provide sufficient stabilization of radicals to permit successful chain additions to alkenes. Acyl radicals are formed by abstraction of the formyl hydrogen from aldehydes. As indicated in Table 12.7, the resulting acyl radicals are... 

Mass Spectrometry Aldehydes and ketones typically give a prominent molecular- ion peak in their mass spectra. Aldehydes also exhibit an M— 1 peak. A major fragmentation pathway for both aldehydes and ketones leads to formation of acyl cations (acyliurn ions) by cleavage of an alkyl group from the carbonyl. The most intense peak in the mass spectrum of diethyl ketone, for exanple, is m/z 57, conesponding to loss of ethyl radical from the molecular- ion. 

The acyl radical 2 can abstract a /3-hydrogen from the radical 3, to give an aldehyde 10 and an alkene 11 ... 

Mechanisms of aldehyde oxidation are not firmly established, but there seem to be at least two main types—a free-radical mechanism and an ionic one. In the free-radical process, the aldehydic hydrogen is abstracted to leave an acyl radical, which obtains OH from the oxidizing agent. In the ionic process, the first step is addition of a species OZ to the carbonyl bond to give 16 in alkaline solution and 17 in acid or neutral solution. The aldehydic hydrogen of 16 or 17 is then lost as a proton to a base, while Z leaves with its electron pair. 

Carbonyl group of the aldehyde decreases the BDE of the adjacent C—H bond. This is due to the stabilization of the formed acyl radical, resulting from the interaction of the formed free valence with Tr-electrons of the carbonyl group. For example, DC—H = 422kJmol 1 in ethane and D( n 373.8 kJ mol 1 in acetaldehyde. The values of Dc H in aldehydes of different structures are presented in Table 8.1. In addition, the values of the enthalpies of acylperoxyl radical reactions with aldehydes were calculated (D0 H= 387.1 kJ mol-1 in RC(0)00 H). 

Aldehydes are oxidized by dioxygen by the chain mechanism in reactions brought about in different ways initiated, thermal, photochemical, and induced by radiation as well as in the presence of transition metal compounds [4-8]. Oxidation chains are usually very long from 200 to 50,000 units [4], Acyl radicals add dioxygen very rapidly with a rate constant of 10s—109 Lmol V1 [4], Therefore, the initiated chain oxidation of aldehyde includes the following elementary steps at high dioxygen pressures [4-7] ... 

The acyl radicals obtained by hydrogen abstraction from aldehydes easily attack protonated heteroaromatic bases. With secondary and tertiary acyl radicals decarbonylation competes with the aromatic acylation [Eq. (12)]. 

Minisci-type substitution is one of the most useful reactions for the synthesis of alkyl- and acyl-substituted heteroaromatics. The acyl radicals are formed by the redox decomposition from aldehyde and /-butyl hydroperoxide or by silver-catalyzed decarboxylation of a a-keto acid with persulfate. Synthesis of acylpyrazines 70 as ant pheromones are achieved by this methodology using trialkyl-substituted pyrazines 69 with the acyl radicals generated from aldehydes or a-keto acids (Equation 10) <1996J(P1)2345>. The latter radicals are highly effective for the acylation. Homolytic alkylation of 6-chloro-2-cyanopyrazine 71 is performed by silver-catalyzed decarboxylation of alkanoic acids to provide 5-alkyl-substituted pyrazines 72 (Scheme 18) <1996CCC1109>. 

On the other hand, carbon dioxide may be derived from the decomposition of the intermediates formed by the reactions of the acyl radical and oxygen, such as the acid radical, peroxy radical, or peracid-aldehyde complex. Therefore, they may be derived as follows ... 

The acyl radical formed from acrolein, maintaining its coordination with a catalyst, may react preferably with oxygen, rather than decompose to produce carbon monoxide, though it is generally believed that a free acyl radical is formed after the abstraction of aldehyde hydrogen by a metal. In such a case, the catalyst metal is considered as behaving as a mononuclear, not a binuclear complex. The molecular weight of the catalyst recovered from the oxidation solution was measured (Table V). 

Protonated pyridines and derivatives readily undergo acylation at C-2 or C-4 (Table 28) (76MI20503). Acyl radicals are usually generated either by hydrogen abstraction from aldehydes (Scheme 210), or by oxidative decarboxylation of a-keto acids (Scheme 211). In the former case (Scheme 210) with acridine as the substrate, reduction can take place to give a dihydroacridine. 

There is some contribution due to / -scission of the alkyl radical formed by the type I process, particularly in the MIPK and tBVK polymers. Loss of carbonyl occurs from photoreduction or the formation of cyclobutanol rings, and also from vaporization of the aldehyde formed by hydrogen abstraction by acyl radicals formed in the Norrish type I process. As demonstrated previously (2) the quantum yields for chain scission are lower in the solid phase than in solution. Rates of carbonyl loss are substantially different for the copolymers, being fastest for tBVK, slower for MIPK, and least efficient for MVK copolymers (Table I and Figure 1). 

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