Treasures from the Free Radical Renaissance Period - Miscellaneous hexenyl radical kinetic data

Rate constant data and Arrhenius parameters have been determined for a series of substituted hexenyl radicals of differing electronic and steric demand. Electron-withdrawing groups (CF3, CO2Et) directly attached to the radical centre slighly accelerate 5-exo ring-closure (kcis + ktrans ∼ 2.1 × 105 s-1 at 25°) relative to donating groups (OMe; 1.6 × 105 s-1 at 25°). Sterically demanding groups (tert-Bu), as expected, slow the cyclization process (1 × 105 s-1). These observations are consistent with subtle changes in activation energy for 5-exo ring-closure. Interestingly, the nature of the solvent would appear to have a significant influence on this chemistry with the cis/trans stereoselectivity sometimes improved as the solvent polarity is increased. Except for the system containing the CF3 (electron-withdrawing) group which displays an increase in the cyclization/capture rate constant (kc/kH), a general decrease in the kc/kH ratio as solvent polarity is increased is noted; these changes have been speculated to arise mainly from changes in kH in the various solvents employed

Factors affecting the rates of addition of free radicals to alkenes?Determination of absolute rate coefficients using the persistent aminoxyl method.

The rate of coupling of alkyl radicals with the persistent aminoxyl radical 1,1,3,3-tetramethyl-isoindolin-N-oxyl (1) has been used as a kinetic probe to determine absolute rate coefficients for the addition of alkyl radicals to methyl acrylate. The results are discussed in terms of the role of the structure and functionalization of the attacking radical on the rates of addition, particularly as they affect steric, polar, and enthalpic factors. The aminoxyl method is assessed against other methods for determining free radical addition rate coefficients

Diastereoselective radical cyclization reactions; the synthesis of O -methylcorytenchirine

Highly diastereoselective cyclization of radicals such as 4 provides a model for the synthesis of 8-substituted berbines. Thus the reaction of 6,7-dimethoxyisoquinoline 21 with the acid chloride 19 affords the key intermediate 22, which undergoes free radical cyclization on treatment with tributylstannane to give (±)-23 as the sole product. Reduction of 23 affords (±)-O-methylcorytenchirine 14. The carbamate 24 does not undergo radical cyclization when treated with tributylstannane, but the acetyl pyridine 33 affords the cyclized products 37 and 38 in reasonable yield and with good diastereoselectivity

Hanns Fischer : radical pioneer

For four decades, Hanns Fischer played a major and highly innovative role in the development of free radical chemistry. The present article describes the science in several of Fischer's papers. These papers were chosen for analysis because they nicely illustrate Fischer's originality, passion for exactitude, and impact on chemistry, and because they are well-read favorites of the present authors

The frequently overlooked importance of solvent in free radical syntheses

This tutorial review is designed to dispel the myth, still believed by many synthetic organic chemists, that radical-based syntheses are free from significant solvent effects. However, many synthetically valuable radical reactions do exhibit large kinetic solvent effects. It is therefore important to select the solvent for any proposed radical synthesis with considerable care if good product yields are to be achieved

Highly diastereoselective radical reactions of substituted methylideneimidazolidinones and related systems

Stannane-mediated radical addition to tnethylideneimidazolidinones occurs with good to excellent diastereo-selectivity. The stereochemical outcome of addition is highly dependent on the nature of the NI substituent on the imidazolidinone ring

A comparison of methods for measuring relative radical stabilities of carbon-centred radicals

This article discusses and compares various methods for defining and measuring radical stability, including the familiar radical stabilization energy (RSE), along with some lesser-known alternatives based on corrected carbon-carbon bond energies, and more direct measures of the extent of radical delocalisation. As part of this work, a large set of R-H, R-CH3, R-Cl and R-R BDEs (R = CH2X, CH(CH3)X, C(CH3) 2X and X = H, BH2, CH3, NH2, OH, F, SiH3, PH2, SH, Cl, Br, N(CH3)2, NHCH3, NHCHO, NHCOCH3, NO2, OCF3, OCH2CH3, OCH3, OCHO, OCOCH3, Si(CH3)3, P(CH3)2, SC(CH 3)2CN, SCH2COOCH3, SCH 2COOCH3, SCH2Ph, SCH3, SO 2CH3, S(O)CH3, Ph, C6H 4-pCN, C6H4-pNO2, C 6H4-pOCH3, C6H4-pOH, CF2CF3, CF2H, CF3, CCl2H, CCl3, CH2Cl, CH2F, CH2OH, CH 2Ph, cyclo-CH(CH2)2, CH2CHCH 2, CH2CH3, CH(CH3)2, C(CH3)3, CCH, CHCH2, CHCHCH3, CHO, CN, COCH3, CON(CH2CH3)2, CONH 2, CONHCH3, COOC(CH3)3, COOCH 2CH3, COOCH3, COOH, COPh), and associated radical stability values are calculated using the high-level ab initio molecular orbital theory method G3(MP2)-RAD. These are used to compare the alternative radical stability schemes and illustrate principal structure-reactivity trends

The mechanism of Bu 3 SnH-mediated homolytic aromatic substitution

The fate of intermediate π radicals is crucial in Bu 3SnH-mediated cyclization by homolytic aromatic substitution, for example, of bromo compound 1 via radical 2 to give oxindole 3 (AIBN = azobisisobutyronitrile). The results indicate that the mechanis