Saturated Acyclic Carbonyl Compounds

The photochemical transformations of saturated acyclic carbonyl compounds are dominated by three reaction process, termed as  

The excited (n, n ) saturated acyclic carbonyl compounds undergo an initial cleavage of carbon-carbonyl bond to give an alkyl and an acyl radical. This process is known by a-cleavage or Norrish type I cleavage. 

The mentioned reaction is initiated by low-lying (n,7C ) state because triplet state cleaves more rapidly than the corresponding singlet state, instead of fact that cleavage from singlet is more exothermic. 

The radicals formed in Norrish type I cleavage follow one of the following routes  

If there is a-Hydrogen present in acyl radical then an a-Hydrogen abstraction is takes place, which yields a ketene and alkane. The presence of ketene can be demonstrated by spectroscopic methods or in presence of a nucleophilic species as water or methanol, it is converted into carboxylic acid r ester derivative which gets trapped. 

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This reaction constitutes a method for the transformation of saturated cyclic or acyclic carbonyl compounds in three steps (silyl enol ether formation, halocarbene addition, rearrangement) to a,) -unsaturated carbonyl compounds with one-carbon ring enlargement or chain elongation, respectively. The rearrangement can be induced under acidic, basic, or thermolytic conditions, or with silver(I) salts. 

PhMe2Si)2CuLi undergoes l,4-(conjugate) addition to a variety of cyclic and acyclic a, 8-unsaturated enones to afford saturated 8-silyl carbonyl compounds (eq In acyclic... 

With Fe2(CO)9 as catalyst, the CDC reaction of saturated heterocycles with 1,3-diketones was accomplished using TBP as an eflhcient oxidant (Scheme 2.12) [48]. This protocol shows good compatibility to cyclic and acyclic ethers, thioe-thers, and tertiary amines. Gratifyingly, besides C(sp )-C(sp ) coupling, the oxidative C-N coupling of ethers with azoles also works well (Scheme 2.12) [49]. As a update, with 2-chloranil (tetrachloro-l,2-benzoquinone) as oxidant, benzyl thioethers can be employed as substrates under metal-free conditions [50]. Notably, 2,2,6,6-tetramethylpiperidine-l-oxoammonium tetrafluoro borate is also an effective oxidant for metal-free CDC reaction of isochromanes and carbonyl compounds [51, 52]. 

Remarkable chemoselectivities in acetalization of carbonyl groups promoted by microwave irradiation have been reported. For example, cyclic ketones can be selectively converted to their corresponding acetals, while acyclic ones remain unchanged under the same experimental conditions. Moreover, a,(3-unsaturated aldehydes and ketones react faster then the corresponding saturated compounds. 

Acyclic compounds form a small group of liquid crystals, mainly of the smectic type. Basically, these are unsaturated fatty acids and their derivatives. Several esters of carbonyl-bis(amino acids) can be classified in this same group. Saturated fatty acids of the CH3(CH2)COOH type can be regarded as molecular prototypes which have hydrophobic and hydrophilic compounds, and which form lyotropic liquid crystals. 

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