Migration, hydrogen

Step 2 The 7C complex rearranges to an organoborane Hydrogen migrates from boron to carbon carrying with it the two electrons m its bond to boron Development of the transition state for this process is shown m 2(a) and its transformation to the organoborane is shown m 2(b)... 

Diazo compounds react with alkenes to afford A -pyrazolines, which in turn izomerize to A -pyrazolines if there is a hydrogen atom a to the N=N bond (Scheme 54). In those cases where two possible ways of isomerization exist, the more acidic hydrogen migrates preferentially. The alkene configuration is conserved on the A -pyrazoline (stereospecificity) but the regioselectivity depends on the substituents of both the alkene and the diazo compound. 

A fluorine-hydrogen migration is typical for the reactions of aldehydes branched at the carbon atom a to the formyl group. Comparable amounts of 1,1 -difluoroalkanes and 1,2-difluoroalkanes together with bis(l -fluoroalkyl) ethers are obtained [169] (equation 84). 

A peculiar dehydrofluorination occurs when tnfluoromethyl dihydropyndine derivatives are treated with organic bases A double-bond shift and a hydrogen migration convert one tnfluoromethyl group to a difluoromethyl group and aromatize the ring [22] (equation 20)... 

A second hydrogen migration from the methyl group back to the central carbon (IV, which is equivalent to T)... 

A primary allylic hydrogen at the ene 1 is especially reactive a secondary hydrogen migrates less facile, and a tertiary one is even less reactive. The enophile unit should be of an electron-poor nature it can consist of a carbon-carbon double or triple bond, a carbonyl group or an azo group. Mixtures of regioisomeric products may be obtained with substituted enophiles. The acrylic ester 6 reacts with... 

Free radicals, unlike carbocations, do not normally undergo isomerization by methyl or hydrogen migration. However, hydrogen transfer (chain transfer) occurs when a free radical reacts with other hydrocarbons. 

Whereas exo-norbornene oxide rearranges to nortricyclanol on treatment with strong base through transannular C-H insertion (Scheme 5.11), endo-norbornene oxide 64 gives norcamphor 65 as the major product (Scheme 5.14) [15, 22]. This product arises from 1,2-hydrogen migration very little transannular rearrangement is observed. These two reaction pathways are often found to be in competition with one another, and subtle differences in substrate structure, and even in the base employed, can have a profound influence on product distribution. 

The dominant pattern for the thermal fragmentation of thietane dioxides involves extrusion of sulfur dioxide leading to a 1,3-diradical (i.e. 242) which closes to final products, mainly cyclopropanes, accompanied by rearrangement products resulting from hydrogen migration within the diradical191,1930 230,256-258 (equation 92). 

Reactions in which a group (other than hydrogen) migrates from one position in a ring to another. Such migrations can be either inter- or intramolecular ... 

Norbomyl cations, besides displaying the 1,2 shifts of a CH2 group previously illustrated for the isobomeol —> camphene conversion, are also prone to rapid hydride shifts from the 3 to the 2 position (known as 3,2 shifts). These 3,2 shifts usually take place from the exo side, that is, the 3-exo hydrogen migrates to the 2-exo position. This stereoselectivity is analogous to the behavior we have previously... 

The R groups may be alkyl, aryl, or hydrogen, though if hydrogen migrates, the... 

The overall pathway for the conversion of the unsaturated azido ether 281 to 2,5-dihydrooxazoles 282 involves first formation of the dipolar cycloaddition product 287, which thermolyzes to oxazoline 282 or is converted by silica gel to oxazolinoaziridine 288. While thermolysis or acid-catalyzed decomposition of triazolines to a mixture of imine and aziridine is well-documented [71,73], this chemoselective decomposition, depending on whether thermolysis or exposure to silica gel is used, is unprecedented. It is postulated that acidic surface sites on silica catalyze the triazoline decomposition via an intermediate resembling 289, which prefers to close to an aziridine 288. On the other hand, thermolysis of 287 may proceed via 290 (or the corresponding diradical) in which hydrogen migration is favored over ring closure. 

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