Substituent effects fluoro

Substituent Effect F-nmr Shifts for BCI3 Adduct Formation with p-Fluoro-p -Substituted... 

To provide further experimental clues to the nature of substituent effects, we decided to test the additivity of orf/io-substituent effects. Specifically, we set out to compare methyl torsional potentials in S0, Sp and D0 for the sequence of molecules o-fluorotoluene (studied by Ito and coworkers), o-chlorotoluene, and 2-fluoro-6-chlorotoluene. 

Comparing the potentials across each row, we can test the idea of additivity of ortAo-substituent effects for 2-fluoro, 6-chloro, and then 2-fluoro-6-chloro substitution. The definition of a = 0 changes across each row to permit easy visual addition of the potentials in the physically appropriate manner to assess the degree of additivity of ort/io-chlorine and ort/io-fluorine substituent effects. For o-fluorotoluene and 2-fluoro-6-chlorotoluene, a = 0 denotes the (minimum-energy) pseudo-trans conformation for o-chlorotoluene, a=0 denotes the pseudo-cis conformation. The notion of additivity has considerable merit in all three electronic states. 

Alkyl- and aryl-hydrazones of aldehydes and ketones readily peroxidise in solution and rearrange to azo hydroperoxides [1], some of which are explosively unstable [2], Dry samples of the p-bromo- and p-fluoro-hydroperoxybenzylazobenzenes, prepared by oxygenation of benzene solutions of the phenylhydrazones, exploded while on filter paper in the dark, initiated by vibration of the table or tapping the paper. Samples were later stored moist with benzene at —60°C to prevent explosion [3], A series of a-phenylazo hydroperoxides derived from the phenyl-or p-bromophcnyl-hydrazones of acetone, acetophenone or cyclohexanone, and useful for epoxidation of alkenes, are all explosive [4], The stability of several substituted phenylazo hydroperoxides was found to be strongly controlled by novel substituent effects [5],... 

The inversion barriers for compounds 59 (ca. 26—27 kcal/mole) and 71 (ca. 30— 32 kcal/mole) have been estimated from the values obtained for diaziridines and oxaziridines (Table 3) and from a study of substituent effects in pyrrolidines 68>. From Fig. 4 barriers of ca. 40 and 18—20 kcal/mole may be estimated for N-fluoro aziridines and N-fluoro pyrrolidines respectively. 

A fluorine substituent shortens the adjacent C—C bonds and lengthens the opposite bond in the cyclopropane ring. The lengthening of the C—C bond opposite to a CF2 group is remarkable (cf 98 and 104 in Table 9), but the mean C—C distance is smaller even in these molecules than in cyclopropane (1). In the other fluoro derivatives, all C—C bonds are shorter than in 1. The C—C bonds appear to be longer, the C—F bonds shorter in a cis isomer than in the trans isomer , and the same applies to C—C and C—F bonds in cis compared to trans CHF—CHF moieties within the same molecule. Ab initio calculations with a 4-2IG split valence basis set reflect the above trends quite well, except for the distinction of effects in the cis and trans forms ". The additivity of substituent effects, which... 

The cross-coupling reaction of alkenyl(fluoro)silanes with aryl halides sometimes produces, in addition to the desired ipso-cowpled products, small amounts of cmc-coupled products [14]. The czne-coupling is often striking in the reaction with organotin compounds. The isomer ratio of products produced by the reaction of l-fluoro(dimethyl)silyl-l-phenylethene with aryl iodides is found to depend on the electronic nature of a substituent on aryl iodides (Eq. 11) an electron-withdrawing group like trifluoromethyl and acetyl favors the formation of the ipso-coupled product. To explain the substituent effect, the mechanism depicted in Scheme 3 is proposed for the transmetalation of alkenylsilanes with palladium(ll) complexes. It is considered that an electron-donating substituent on Ar enhances... 

The related reactions of y-fluoro-y-methyl-i5-aminocarbonyloxy-a,/ -unsaturated esters 22 with high 1,3-syn diastereoselectivity, irrespective of the stereochemistry at the y-position72, are quite interesting in terms of the y-substituent effect (vide supra). 

Do j5-substituents effect the stereochemistry of the cyclopropyl radical In order to evaluate the j -substituent effect let us examine a cyclopropyl radical that can maintain its configuration, such as one with an a-fluoro as a substituent, and determine whether a P-substituent will alter its configurational stability. Inspecting the data in Table 11 one would have to conclude that j -substituents such as methyl, phenyl and ether groups have no effect on the stereochemistry of the cyclopropyl radical Also, chlorine as a )S-substituent does not have any effect on the stereochemistry. Jefford and coworkers have shown that LiAlH4 reduction of the tricyclic compound 80 (R=H, Cl) gave the same ratio (2 1) of anti-... 

Further studies by Stanislawski and West were conducted on the fluoro- and chloro-terminated linear polydimethylsilanes117. Chlorine and fluorine substituent parameters were determined for the a, ft and y positions and found to be + 53.0, — 2.6 and 0.7 ppm for F, and +42.2, +2.5 and —0.3 ppm for Cl. These parameters gave good values for all chemical shifts with the exception of the dihalodisilanes. From the substituent effects it is apparent that the effect of the halogen is rapidly attenuated down the chain. 

The photoelectron spectra of the compounds RCS2 CF3 (R = F, Cl, or CFs-S) have been analysed with reference to the spectra of the thioketones R2CS (R = F, Cl, or alkylthio) and through CNDO calculations the CFs-S group has a substituent effect comparable to that of chlorine. i A study of the reactions of the acid derivatives XCO SCl (X = F or Cl) has shown that nucleophilic substitution at carbonyl carbon is more rapid with the dichloro-compound (X = Cl), although the S—Q bonds in both compounds are of comparable reactivity some chemical conversions of the fluoro-compound are shown in Sicheme 32. 

The cr-substituent effects and solvent effects (extended Grunwald-Winstein equation) for solvolyses of / -X-substituted benzoyl chlorides (X = OMe, Me, H, and Cl) in 97% wt/wt hexafluoroisopropanol/water was explained by two competing pathways. Plots of log k versus a were consistent with a cationic path and explained by an 5 n2-5n1 spectrum of mechanism. Electron-donating groups favoured the reaction and values of p increased in the order AcOH < HCO2H < TEE < HFIP. A benzoyl cation intermediate was trapped in hexafluoroisopropanol. Ab initio calculations of heterolytic bond dissociation energies of various chloro- and fluoro-substituted and other benzoyl chlorides were correlated with log k for solvolyses... 

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