Hydride isomerization

The nature of the counter ion and the solvent medium is also significant as these additions are reversible. The terminal adduct is thermodynamically the more stable, and the initial product mixture can in some cases be converted to it. Thus the zinc and lithium derivatives rearrange on prolonged heating in thf or thf-HMPA (91). The more ionic potassium compounds, however, which can be obtained by addition of potassium hydride, isomerize rapidly, especially in the presence of crown ethers and in polar solvents (92,93) [Eq. (4)]. 

The double bond positional isomerization of alkenes is catalyzed by many transition metals and often occurs as an undesired side reaction. Equations 4 show a simplified scheme of the alkyl-hydride isomerization mechanism in which all the catalytic intermediates have the metal atom in the same oxidation state. 

Isomerization of double bonds in vitamin D analogs such as calciferol by oxidation and reduction has been carried out via the formation of the tt-allylpalladium complex 334 with PdCl2(PhCN)2 in 70% yield, followed by hydride reduction to afford 335[295],... 

Pd hydride. Subsequent enolate formation, double bond isomerization, and carbonylation give the butenolide 582. 

Which of the isomeric C5H12O alcohols can be prepared by lithium aluminum hydride reduction of... 

KTB and KTA are superior to alkaU metal hydrides for deprotonation reactions because of the good solubiUties, and because no hydrogen is produced or oil residue left upon reaction. Furthermore, reactions of KTA and KTB can be performed in hydrocarbon solvents as sometimes requited for mild and nonpolar reaction conditions. Potassium alkoxides are used in large quantities for addition, esterification, transesterification, isomerization, and alkoxylation reactions. 

Several side reactions or post-cuting reactions are possible. Disproportionation reactions involving terminal hydride groups have been reported (169). Excess SiH may undergo hydrolysis and further reaction between silanols can occur (170—172). Isomerization of a terminal olefin to a less reactive internal olefin has been noted (169). Viaylsilane/hydride interchange reactions have been observed (165). 

Positionalisomeri tion occurs most often duting partial hydrogenation of unsaturated fatty acids it also occurs ia strongly basic or acidic solution and by catalysis with metal hydrides or organometaUic carbonyl complexes. Concentrated sulfuric or 70% perchloric acid treatment of oleic acid at 85°C produces y-stearolactone from a series of double-bond isomerizations, hydration, and dehydration steps (57). 

Metal oxides, sulfides, and hydrides form a transition between acid/base and metal catalysts. They catalyze hydrogenation/dehydro-genation as well as many of the reactions catalyzed by acids, such as cracking and isomerization. Their oxidation activity is related to the possibility of two valence states which allow oxygen to be released and reabsorbed alternately. Common examples are oxides of cobalt, iron, zinc, and chromium and hydrides of precious metals that can release hydrogen readily. Sulfide catalysts are more resistant than metals to the formation of coke deposits and to poisoning by sulfur compounds their main application is in hydrodesulfurization. 

N,N-Dimethy1aniline from Nakarai Chemicals was dried over calcium hydride and freshly distilled. Three molar equivalents of N,N-dimethylaniline are used to achieve complete conversion of the n-butyllithium, because In the present particular case free n-butyllithium, if present, causes the isomerization of the (Z)-alkene to the (E)-isomer. 

Reduction with lithium aluminum hydride allows a differentiation from the isomeric nitrones. Whereas 2-tert-butyl-3-phenyloxazirane (9) gives benzylidene-tert-butylamine [Eq. (10)], reduction of the isomeric nitrone leads to iV-benzyl-xV-fert-butylbydroxylaminc [Eq. 

Tetrahydro derivatives are formed when either quinoxaline or 6-chloroquinoxaline is reduced with lithium aluminum hydride in ethereal solution. Similar reduction of 2,3-dimethylquinoxaline gives the meso-(cts)-1,2,3,4-tetrahydro derivative. This is shown to be a stereospecific reduction since lithium aluminum hydride does not isomerize the dl-(trans)-compound. Low temperature, platinum catalyzed, hydrogenation of 2,3-dimethylquinoxaline in benzene also gives meso (cis) -l,2,3,4-tetrahydro-2,3-dimethylquinoxaline. ... 

The vinyl ehloride derivative [(> -Tp )Rh(CNBu-t)(CH2=CH)Cl] when rea-eted with Cp2ZrH2 gives the vinyl hydride [(j -Tp )Rh(CNBu-t)(CH2=CH)H] (990M495). In CgDg at room temperature, the produet transforms into the ethylene eomplex [(>j -Tp )Rh(CNBu-t)(j7 -CH2=CH2)] by intramoleeular isomerization. A similar eomplex, [(>j -Tp )Rh(CN-2,6-xylyl)(>j -CH2=CH2)], is also known (980M4784,980M5148). Photolysis of [(j -Tp )Rh(CNBu-t)(PhN=C=NBu-t)]... 

Abstraction of a hydride ion from a tertiary carbon is easier than from a secondary, which is easier than from a primary position. The formed car-bocation can rearrange through a methide-hydride shift similar to what has been explained in catalytic reforming. This isomerization reaction is responsible for a high ratio of branched isomers in the products. 

Evidence in support of a carbocation mechanism for electrophilic additions comes from the observation that structural rearrangements often take place during reaction. Rearrangements occur by shift of either a hydride ion, H (a hydride shift), or an alkyl anion, R-, from a carbon atom to the adjacent positively charged carbon. The result is isomerization of a less stable carbocation to a more stable one. 

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-... 

The isomerization of an allylic amine to an enamine by means of a formal 1,3-hydrogen shift constitutes a relatively small structural change. However, this transformation could be extremely valuable if it could be rendered stereoselective. In important early studies, Otsuka and Tani showed that a chiral cobalt catalyst, prepared in situ from a Co(ii) salt, a chiral phosphine, and diisobutylaluminum hydride (Dibal-H), can bring about the conversion of certain pro-chiral olefins to chiral, isomeric olefins by double bond migra-... 

Detty published the first example of the titled approach in his pioneering work on teluropyrans (88MI1). The hexafluorophosphate 76 was reduced with diisobutyl aluminium hydride (DIBAL-H) to a 93 7 mixture of isomeric teluropyrans 77 and 78 accompanied by traces (ca. 1%) of the dimeric product 80. The latter was also obtained after the electrochemical reduction of 76 via radicals 79 or by a modification of the reduction with DIBAL-H (Scheme 5). 

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