Iodinations of aldehyde

The first asymmetric direct a-iodination of aldehydes has also been described to provide products in moderate to good enan-tioselectivities using an organocatalyst. 3-Methyl-1-butanal was reacted with NIS in the presence of a chiral pyrrolidine catalyst to provide the halogenated product in 78% yield with 89% ee (eq 27). 

SCHEME 13.38. Enantioselective a-iodination of aldehydes via enamine catalysis. 

Keiji Maruoka of Kyoto University devised J. Am. Chem. Soc. 2008,130, 3728) a chiral amine that mediated the enantioselective iodination of aldehydes such as 12. Direct cyanohydrin formation dehvered 13 in high de and ee. The epoxide 14 is readily prepared in high ee from crotyl alcohol. Barry M. Trost of Stanford University loveoA Organic Lett. 2008,10, 1893) that 14 could be opened with 15, to give 16 with high regio- and diastereocontrol. 

Such bifunctional catalysis by a chiral secondary amino alcohol catalyst is also effective for the direct asymmetric iodination of aldehydes with N-iodosuccinimide, in which a slightly modified catalyst [17b with a bis(pentafluorophenyl) hydroxymethyl group] displayed remarkable catalytic and chiral efficiencies (Scheme 7.30) [53]. 

Kano T, Ueda M, Maruoka K. Direct asymmetric iodination of aldehydes using an axiaUy chiral bifunctional amino alcohol catalyst. J. Am. Chem. Soc. 2008 130(12) 3728-3729. 

However, there is another approach for the rare direct asymmetric a-iodination of aldehydes (Scheme 4.23). In this reaction, catalyst 46b and benzoic acid as cocatalyst were used to promote the a-iodination of several aldehydes 17 using A-iodo succinimide (95c) as a iodinating agent, affording the corresponding chiral a-iodoaldehydes 102 in moderated isolated yields (30-80%) and with excellent enantioselectivities (90-99% ee) [132]. 

As in the acid-catalyzed halogenation of aldehydes and ketones, the reaction rate is independent of the concentration of the halogen chlorination, bromination, and iodination all occur at the same rate. Fomnation of the enolate is rate-detemnining, and, once fomned, the enolate ion reacts rapidly with the halogen. 

A mixture of 1,4-dioxane and water is often used as the solvent for the conversion of aldehydes and ketones by H2Se03 to a-dicarbonyl compounds in one step (Eq. 8.117).331 Dehydrogenation of carbonyl compounds with selenium dioxide generates the a, (i-unsaturated carbonyl compounds in aqueous acetic acid.332 Using water as the reaction medium, ketones can be transformed into a-iodo ketones upon treatment with sodium iodide, hydrogen peroxide, and an acid.333 Interestingly, a-iodo ketones can be also obtained from secondary alcohol through a metal-free tandem oxidation-iodination approach. 

The transformation of aldehydes (44) (R3 = H) and ketones (44) (R3 = Alkyl) to the corresponding a-hydroxylated acetals (45) has been performed in an MeOH-KOH/KI-(Pt/C) system. The first step of the oxidation is considered to proceed by the attack of an electrogenerated active iodine species [I]+ to an... 

Removal of the iodine atom from aryl iodides proceeds smoothly in a DMF-Bu4NCl04-(Hg) system [561, 562]. A Cd-modified electrode-assisted allylation of aldehydes and ketones has been attained in a DMF-Et4NCl04 system, in which the... 

Iodine is known to catalyze the condensation of aldehydes, benzyl carbamate and allyltrimethylsilane to homoallylic amines. However, in this case the involvement of an in sitn prepared [MejSi] species was suggested to be the active catalyst [235], An iodine catalyzed acetalization of carbonyl compounds was reported, where the active catalyst was believed to be hydroiodic acid [236],... 

Living olefin polymerization allows the synthesis of end-functionalized polyolefins if appropriate initiation and/or quenching methods are used. Doi et al. first showed the utility of living olefin-polymerization catalysts for the preparation of end-functionalized polyolefins. They synthesized iodine-, amine-, aldehyde-, hydroxy-, and metha-cryl-terminated PPs using living V-PP species and appropriate reagents. " " ... 

Iodine-catalysed hydroperoxidation of cyclic and acyclic ketones with aqueous hydrogen peroxide in acetonitrile is an efficient and eco-friendly method for the synthesis of gem -dihydroperoxides and the reaction is conducted in a neutral medium with a readily available low-cost oxidant and catalyst.218 Aryl benzyl selenoxides, particularly benzyl 3,5-bis(trifluoromethyl)phenyl selenoxide, are excellent catalysts for the epoxidation of alkenes and Baeyer-Villiger oxidation of aldehydes and ketones with hydrogen peroxide.219 Efficient, eco-friendly, and selective oxidation of secondary alcohols is achieved with hydrogen peroxide using aqueous hydrogen bromide as a catalyst. Other peroxides such as i-butyl hydroperoxide (TBHP), sodium... 

First an iodination of the alcohol is accomplished by treatment with iodine in presence of Ph3P and imidazole.13 Successive iodide displacement with the nucleophilic vinyl cuprate derived from 2-lithiopropene (2-bromopropene, tBuLi) finished the emplacement of the C-l side chain. Cleavage of the TPS ether by tetrabutylammonium fluoride and subsequent oxidation with the Dess-Martin periodinane provides aldehyde 9. 

Data for aliphatic aldehyde enolisation are very scarce, probably because the enolisation process is often complicated by oxidation and hydration. Nevertheless, the rate constants for base- and acid-catalysed iodination of R R2CHCHO were determined in aqueous chloroacetic acid-chloroacetate ion buffers (Talvik and Hiidmaa, 1968). The results in Table 4 show that alkyl groups R1 and R2 increase the acid-catalysed reactivity in agreement with hyperconjugative and/or inductive effects. This contrasts with aliphatic ketones for which steric interactions are important and even sometimes dominant. Data for base-catalysis are more difficult to interpret since a second a methyl group, from propionaldehyde to isobutyraldehyde, increases the chloroacetate-catalysed rate constant. This might result from a decrease of the a(C—H) bond-promoted hyperconjugative stabilisation of the carbonyl compound... 

Catalytic rate coefficients for iodination of aliphatic aldehydes in chloro-acetic acid-chloroacetate ion buffers11... 

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