Halogen shift

Halogen shifts have been found for tungsten, with assumed formation of iodovinylidenes in reactions of 1-iodo-l-alkynes with W(CO)5(thf) en route to cyclization of 2-(iodoethynyl)styrenes to naphthalenes and of iodo-alkynyl silyl enol ethers [147], while more substantial confirmation is found in Mn =C=C(I)CH (OR)2 (CO)2Cp [R = Me, Et (OR)2 = 0(CH2)30], of which the XRD structure of Mn =C=C(I)CH(OMe)2 (CO)2Cp was determined [148]. 

The photolysis of (Me3Si)3SiPh in the presence of functional substituted olefins is of considerable interest (55). Irradiation of 20 with a low-pressure mercury lamp in the presence of vinyl chloride or l-bromo-2-methyl-propene affords the respective 1-alkenyl-1-halo-1-phenyltrimethyldisilanes as the sole volatile product. The fact that the reaction of trimethylsilylphenylsilylene with butyl bromide does not give any volatile products suggests that compound 23 and 24 must come from a 1,2-halogen shift of... 

Haloallenyl ketones cyclization gives halofuranes via two competitive pathways depending on the catalyst (Scheme 34) [194]. Thus, more oxophilic Au(III) activates the carbonyl group leading to 3-halofurans by a 1,2-halogen shifts, whereas Au(I) selectively activates the terminal double bond of the allene to form 2-halofurans. 

The possible presence in the 4-chloro-4-hexenyl trifluoroacetate of small amounts of two cis-trans pairs of products of addition of trifluoroacetic to the triple bond without concomitant halogen shift remains speculative. In any event these compounds would be removed as ketones upon hydrolysis of the trifluoroacetate. Both the 4-chloro-4-hexenyl trifluoroacetate and the alcohol resulting from its hydrolysis have been shown to contain 9% of the (E) isomer. In the present study the hydrogen decoupled magnetic resonance spectra of the ester and alcohol were shown to contain peaks attributable to approximately 9% of E) isomer. 

Scheme S.10 Proposed mechanism of rhodium(l)-catalyzed halogen shift.

I 8 The Rhodium(l)-Catalyzed Alder-Ene Reaction Tab. 8.9 Scope of halogen shift reaction... 

Moreover, both the as- and trons-aUcenes react at the same rate and yield the same product (Tab. 8.9, entries 1 and 2). As demonstrated above, this was not the case with the N-tethered enynes (Tab. 8.2, entry 6) which gave a mixture of products. Nitrogen-and oxygen-tethered enynes were subjected to the rhodium(I) conditions affording high yields of the halogen shift product (Tab. 8.9, entries 1-10). 

The phenomenon of halotropism occurs in the reaction of tribromoacetic, 2-bromopropanoic and fnm -3,4-dibromocyclopentane-l-carboxylic acid with sulfur tetrafluoride. Tribromoacetic acid reacts at 25 C to give l,l,2-tribromo-l,2,2-trifluoroethane (6a) in 95% yield, while the other two acids give mixtures of trifluoromethyl derivatives 7 and rearranged 1,1,2-tri-fluoroalkanes6.110 The halogen shifts confirm the carbocationic mechanism of the fluorination reaction (vide supra). 

Answer Halogens shift the proton signal downfield and the shift is the larger, the larger the number of halogen atoms. 

Electronegative elements (N, O, and halogen) shift absorptions downfield. 

The calculation of magnetic shielding parameters in the alkali metal halides using Hartree-Fock wavefunctions was carried out by Ikenberry and Das, but their approach did not reproduce the observed halogen shift values of the systems studied (chemical shifts were in error by about 200-300 ppm). The pressure dependence of the shifts of several alkali halides was calculated by Hafemeister and Flygare. ... 

Thus, halocarbenes undergo smooth addition to the P/C triple bonds of compounds 9 to furnish 1-phosphirenes (for intramolecular ring closures phosphavinylcarbenes to give the same products, see [16]), which rearrange to the thermodynamically more stable 2-isomers 11 by [l,3]-halogen shifts [17]. 

Several aminochromes have been characterized spectrophotometrically (65AHC205). They all display absorption maxima at around 205-215 nm and near 300 nm in the UV region, and also a less intense, broader absorption in the range 470-490 nm. Halogenation shifts the first absorption to ca. 230 nm and the visible band to ca. 520 nm. 

A weak band in the 275—300 nm range is an indicator of a ketone or aldehyde carbonyl group. Substituents like OH, NH2, NHR or halogen shift the n tt" transition towards shorter wavelengths. Conjugation of a C=0 group with C=C bonds shifts the n n transition towards the VIS range. 

The cyclization of 1,6-enynes by RhCl(PPh3)3 can generate functionalized 1,3- or 1,4-diene cyclic compounds. For example, treatment of l,6-en)mes containing a haloalkenyl group with Wilkinson s catalyst in dichloromethane at reflux produces cyclization product which incorporates an intramolecular halogen shift a wide spectmm of enynes can be applied in this transformation (eq 75). ... 

The reaction of difluoroiodotoluene with a four-, flve-, or six-membered carbocycles 47 affords the ring-expanded ( )-S-fluoro-3-halovinyliodonium tetrafluoroborates 48 stereoselectively in high yields (Scheme 3.17) [42], This reaction proceeds via a sequence of X -iodanation-1,4-halogen shift-ring enlargement-fluorination steps. 

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