Aldehydes chemoselectivity

The oxidation of terminal alkenes with an EWG in alcohols or ethylene glycol affords acetals of aldehydes chemoselectively. Acrylonitrile is converted into l,3-dioxolan-2-ylacetonitrile (69) in ethylene glycol and to 3,3-dimetho.xy-propionitrile (70) in methanol[28j. 3,3-Dimethoxypropionitrile (70) is produced commercially in MeOH from acrylonitrile by use of methyl nitrite (71) as a unique leoxidant of Pd(0). Methyl nitrite (71) is regenerated by the oxidation of NO with oxygen in MeOH. Methyl nitrite is a gas, which can be separated easily from water formed in the oxidation[3]. 

Ketone vs. aldehyde chemoselective addition of allyltitanium derivatives. 

The synthesis of the non-racemic cyclopentanone (+)-93 is outlined in Scheme 15. Starting with 2-methyl-cyclopent-2-enone (90), sequential cuprate addition and enolate alkylation afforded the racemic cyclopentanone rac-92 as a single diastereomer. The double bond was cleaved by ozonolysis, the resulting aldehyde chemoselectively reduced in the presence of the keto function and the primary hydroxyl function was subsequently protected as a silyl ether to provide racemic rac-93. This sequence has been applied fre-... 

How would you convert this bromo-aldehyde chemoselectively into the two products shown ... 

Oxidation of terminal alkenes bearing an EWG in alcohols or ethylene glycol affords acetals of aldehydes chemoselectively. 3,3-Dimethoxypropioiutrile (53) is produced commercially in MeOH from acrylonitrile by use of methyl lutrite (52) as... 

PdCl2(PPh3)2 is an excellent catalyst for the carbonylative addition of (PhS)2 to alkynes at lower CO pressures. The obtained carbonylative addition products (/3-thio-a,j8-unsaturated thioesters or /3-seleno-a,/3-unsaturated selenoesters) can be converted to the corresponding aldehydes chemoselectively, upon treatment with tributyltin hydride in the... 

At a lower temperature (120 C), also the Co catalyst was able to generate aldehydes chemoselectively (Scheme 6.91) [41]. Subsequent replacement of syngas by hydrogen allowed the reduction of the aldehydes with the same catalyst. Alternatively, the reaction was conducted in methanol as a solvent to trap the aldehyde as dimethyl acetal. 

The water-soluble pyrazolato complex (Rh(p-Pz)(CO)(TPPTS)]2 was used as precursor for olefin hydroformylation in an aqueous heptane solvent system 9]. Without additional ligand, olefin isomerization dominated hydroformylation at 7 bar CO/H2 (1 1) giving large amounts of 2-hexene. Isomerization was suppressed most effectively at 49.8 bar, leading to aldehyde chemoselectivities >90% and a 2.4-3.S njiso ratio. It was demonstrated that aerobic recycling of the aqueous catalyst phase by... 

Sodium cyanoborohydride is remarkably chemoselective. Reduction of aldehydes and ketones are, unlike those with NaBH pH-dependent, and practical reduction rates are achieved at pH 3 to 4. At pH 5—7, imines (>C=N—) are reduced more rapidly than carbonyls. This reactivity permits reductive amination of aldehydes and ketones under very mild conditions (42). 

The imidazolidine was prepared from an aldehyde with A/,N -dimethyl-1,2-eth-ylenediamine (benzene, heat, 78% yield) and cleaved with Mel (Et20 HjO, 92% yield). Derivatization is chemoselective for aldehydes. The imidazolidine is stable to BuLi and LDA. The diphenylimidazolidine has been prepared analogously and can be cleaved with aqueous HCl. ... 

Allyl bromide, Sb(OEt)3, 80°, 2-6 h, 85-98% yield. This method is chemoselective for aldehydes in the presence of ketones. 

Nal, CeCl3 7H20, CH3CN, rt, 0.5-21 h, 84-96% yield. Chemoselective cleavage of ketone derivatives is observed in the presence of aldehyde derivatives, and enone ketals are cleaved in the presence of simple ketone ketals. 

HSCH2CH2SH, FeCb-Si02, CH2CI2, < 1 min-7 h."" Montmorillonite Clay can also be used as a support medium for the ferric ion (75-98% yield ). In this case, the reaction is chemoselective for aldehydes."" ... 

This chemoselectivity stands in contrast to that of 2,6-disubstituted pyridines. For example, 2,6-dimethylpyridine 35 was reacted with hydrogen peroxide and acetic anhydride to produce the expected acetoxy derivative 36. A second iteration of the previous reaction conditions did not afford an aldehyde, as in the previous example, but 2,6-bis-acetoxy derivative 37. 

The combination of thionation by Lawesson s reagent [98] of oxoenamino-ketones 96 with normal electron-demand Diels-Alder reaction of conjugated aldehydes allows a variety of thiopyrans 97 to be synthesized by a regio-selective and chemoselective one-pot methodology [99] (Equation 2.28). Thionation occurred at the more electrophilic ketonic carbonyl group. O O... 

This looks at first sight like a formidable problem of chemoselectivity, but changing the oxidation level to an aldehyde (25) gives a simple 1,3-diX disconnection to available aldehyde (26). The ester would have done as well. 

---