Aldehydes, aliphatic reduction

Studies of aliphatic a-haloaldehydes in water-dioxane have shown that the electrochemical behavior of these compounds is affected by the equilibrium between the hydrated and unhydrated forms of the aldehyde [92]. Each carbon-halogen bond is broken in a two-electron, one-proton process, and the resulting nonhalogenated aldehyde undergoes reduction at a more negative potential [93]. 

In contrast to aromatic aldehydes, aliphatic aldehydes invariably afforded complex mixtures of products when submitted to the above conditions. Based on their molecular weights, a few products could tentatively be identified as dialkylated, as well as dehydrodialkylated, compounds, such as 24. These perhaps were derived from tautomerization of the initially formed imine 19 to the thermodynamically more stable enamine 20 followed by reaction with a second aldehyde molecule and subsequent reduction (Scheme 3). 

Lysine, 4-amino Aldehydes (aliphatic, aromatic, Reductive alkylation... 

Nickel-catalyzed transformations of SCBs have been studied by Oshima and co-workers <20060L483>. Nickel-catalyzed ring opening of SCBs with aldehydes affords the corresponding alkoxyallylsilanes (Scheme 55). This transformation represents a hydrosilane-free reductive silylation of aldehydes. A wide range of aldehydes (aliphatic, aromatic, electron-rich, and electron-deficient) can be converted to akoxyallylsilanes. 

Bis(N-inethylpiperazinyl)aluniinum hydride (li. This hydride was originally prepared from aluminum hydride and N-methylpiperazine, and was used to reduce carboxylic acids directly to aldehydes. It can be prepared more conveniently from lithium aluminum hydride and the amine. It is useful for reduction of aliphatic and aromatic acids to aldehydes (80-95% yield). Significantly, it reduces a,p-unsaturated acids to aldehydes without reduction of the double bond (70-80% yield). ... 

Pinanyl-9-BBN (Alpine borane 4) is a chiral borane that is readily oxidized by aldehydes. Aliphatic deuterioaldehydes undergo chiral reduction to give alcohols widi 84-98% The chiral alkene is regenerated in the process, only the hydrogen at the 2-position having been utilized, and can be reused. Equation (52) serves as an illustration of the stereochemistry of the process. 

A variety of aldehydes—aliphatic, aromatic, and heterocyclic—have been condensed with hydantoin. Sodium acetate in a mixture of acetic acid and acetic anhydride as well as pyridine containing traces of piperidine serves as condensing agent. Reduction of the double bond is accomplished with phosphorus and hydriodic acid, ammonium sulfide, or stannous chloride, In a more recent modification, the hydantoins are synthesized from aldehyde or ketone cyanohydrins and ammonium... 

Amidines are apparently rather good substrates for selective one-electron reduction to aldehydes. As early as 1908, Merling obtained good yields of some fairly complex aliphatic aldehydes by reduction of the corresponding A/,N-diphenylamidines using sodium in ethanol. As an example, amidine (60) was converted into aldehyde (61) in 60-70% yield. 

Alkylations, A stepwise synthesis of amines via I-aminomethylbenzotriazoles involves reductive alkylation with Zn in DME. Stereoselective synthesis of 1,2-amino alcohols is attained by Zn-mediated allylation of a-amino aldehydes. Change of regioselectivity is observed in the reactions of fluorinated prenylzinc species with aldehydes (aliphatic vs. aromatic)."... 

Reductive couplings. Aliphatic and aromatic aldehydes undergo reductive dimerization on exposure to TiBr2-Cu. 

The electroreduction of aliphatic ketones yields secondary alcohols similarly to aldehydes. The reduction of acetone at Hg and Pt cathodes results in propan-2-ol as the major product, although pinacol and propene are produced as by-products. Reduction of acetone on catalytic cathodes of activated Pt also results in the alcohol... 

Reduction of Carboxylic Acids to Aldehydes. Aliphatic carboxylic acids are reduced rapidly (15 min) and aromatic carboxylic acids are reduced slowly (24 h) by chloro(thexyl)borane-... 

This procedure and others mentioned are particularly valuable for the prepn. of aliphatic aldehydes whereas reduction of acid chlorides with Li-hydridotri-ferf-butoxoaluminate (s. Synth. Meth. 11, 135) is especially valuable for the prepn. of ar. aldehydes. F. e. s. H. G. Brown and A. Tsukamoto, Am. Soc. 83, 2016, 4549 (1961). 

Sulfonimidophosphonates Phosphides s. Alkali phosphides Phosphinaminoboranes 16, 374 Phosphine, reactions with aldehydes, aliphatic 16, 696 —, reductions with — 17, 346 Phosphine boranes... 

In nonaqueous solutions, however, reduction of hydrazones may take place in a different way. As shown by Kitaev and Skrebkova [144], phenylhydrazones, nitrophenylhydrazones, semi- and thio-semicarbazones, nicotinyl- and isonicotinylhydrazones, benzoyl-hydrazines, and also the products from the coupling of diazonium salts with diphenyldiketopyrazolidine and barbituric acid, which are derivatives of aromatic aldehydes, aliphatic aromatic ketones, or... 

SchifT s bases A -Arylimides, Ar-N = CR2, prepared by reaction of aromatic amines with aliphatic or aromatic aldehydes and ketones. They are crystalline, weakly basic compounds which give hydrochlorides in non-aqueous solvents. With dilute aqueous acids the parent amine and carbonyl compounds are regenerated. Reduction with sodium and alcohol gives... 

Aldehydes and ketones are similar in their response to hydrogenation catalysis, and an ordering of catalyst activities usually applies to both functions. But the difference between aliphatic and aromatic carbonyls is marked, and preferred catalysts differ. In hydrogenation of aliphatic carbonyls, hydrogenolysis seldom occurs, unless special structural features are present, but with aryl carbonyls either reduction to the alcohol or loss of the hydroxy group can be achieved at will. 

Notable examples of general synthetic procedures in Volume 47 include the synthesis of aromatic aldehydes (from dichloro-methyl methyl ether), aliphatic aldehydes (from alkyl halides and trimethylamine oxide and by oxidation of alcohols using dimethyl sulfoxide, dicyclohexylcarbodiimide, and pyridinum trifluoro-acetate the latter method is particularly useful since the conditions are so mild), carbethoxycycloalkanones (from sodium hydride, diethyl carbonate, and the cycloalkanone), m-dialkylbenzenes (from the />-isomer by isomerization with hydrogen fluoride and boron trifluoride), and the deamination of amines (by conversion to the nitrosoamide and thermolysis to the ester). Other general methods are represented by the synthesis of 1 J-difluoroolefins (from sodium chlorodifluoroacetate, triphenyl phosphine, and an aldehyde or ketone), the nitration of aromatic rings (with ni-tronium tetrafluoroborate), the reductive methylation of aromatic nitro compounds (with formaldehyde and hydrogen), the synthesis of dialkyl ketones (from carboxylic acids and iron powder), and the preparation of 1-substituted cyclopropanols (from the condensation of a 1,3-dichloro-2-propanol derivative and ethyl-... 

A reiterative application of a two-carbon elongation reaction of a chiral carbonyl compound (Homer-Emmonds reaction), reduction (DIBAL) of the obtained trans unsaturated ester, asymmetric epoxidation (SAE or MCPBA) of the resulting allylic alcohol, and then C-2 regioselective addition of a cuprate (Me2CuLi) to the corresponding chiral epoxy alcohol has been utilized for the construction of the polypropionate-derived chain ]R-CH(Me)CH(OH)CH(Me)-R ], present as a partial structure in important natural products such as polyether, ansamycin, or macro-lide antibiotics [52]. A seminal application of this procedure is offered by Kishi s synthesis of the C19-C26 polyketide-type aliphatic segment of rifamycin S, starting from aldehyde 105 (Scheme 8.29) [53]. 

The lithium cnolate generated by deprotonation of 2-/m-butyl-6-methyl-l,3-dioxan-4-onc, readily available from polyhydroxybutyric acid (PHB), predominantly affords the diastereo-mers 7 when reacted with aldehydes. The diastereomeric ratios of aldol adducts 7/8, produced by reactions with aliphatic aldehydes, range from 87.5 12.5 to >99 1. Pure diastereoiners7are obtained by recrystallization in 25-74% yield116-118. Only marginal diastereoselectivities with respect to the carbinol center are obtained with aromatic aldehydes111-119. Benzoylation of the dioxanones 7, followed by reduction with lithium aluminum hydride, affords enan-tiomerically and diastereomerically pure triols 9 in >85% yield 11. ... 

Metal-induced reductive dimerization of carbonyl compounds is a useful synthetic method for the formation of vicinally functionalized carbon-carbon bonds. For stoichiometric reductive dimerizations, low-valent metals such as aluminum amalgam, titanium, vanadium, zinc, and samarium have been employed. Alternatively, ternary systems consisting of catalytic amounts of a metal salt or metal complex, a chlorosilane, and a stoichiometric co-reductant provide a catalytic method for the formation of pinacols based on reversible redox couples.2 The homocoupling of aldehydes is effected by vanadium or titanium catalysts in the presence of Me3SiCl and Zn or A1 to give the 1,2-diol derivatives high selectivity for the /-isomer is observed in the case of secondary aliphatic or aromatic aldehydes. 

Acyl sulfonylhydrazides (117) are cleaved with base to give aldehydes. This is known as the McFadyen-Stevens reduction and is applicable only to aromatic aldehydes or aliphatic aldehydes with no a hydrogen. " RCON=NH (see 10-87) has been proposed as an intermediate in this reaction. " °... 

This is redueed with anhydrous SnCl2 to RCH=NH, which precipitates as a complex with SnC and is then hydrolyzed (16-2) to the aldehyde. The Stephen reduction is most successful when R is aromatic, but it can be done for aliphatic R up to about six carbons. It is also possible to prepare 21 in a different way, by treating ArCONHPh with PCI5, which can then be converted to the aldehyde. This is known as the Sonn-Muller method. 

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