Laser flash photolysis studies of the formation and reactivities of phenyl(naphthyl)methyl carbocations generated from phosphonium salt precursors

Abstract

The photolysis of several substituted phenyl(naphthy1)methyl triphenylphosphonium chlorides has been examined using a combination of laser flash photolysis experiments and product studies. Both carbocation and radical intermediates have been characterized in the transient experiments, with the relative yields depending strongly on the solvent. For example, in alcohols, acetonitrile, or aqueous solvents cation formation predominates while acetonitrile/dioxane mixtures (5-10%) are required for the observation of radicals. Quantum yields for cation formation vary from 0.79 in methanol to 0.093 in 1 : 4 acetonitrile/dioxane, as measured by product studies and transient experiments, respectively. The addition of perchlorate salts leads to dramatic enhancements in the cation lifetimes; the effects are particularly pronounced for acetonitrile/dioxane mixtures where the cation yields also increase by factors of 3-4. In this case the effects are attributed primarily to replacement of chloride by perchlorate in the initial ion pairs. The combined data from both solvent and perchlorate salt effects on the cation lifetimes and yields suggest that the excited state of the phosphonium salt cleaves homolytically, followed by electron transfer within the initial radical/triphenylphosphine radical cation pair to generate carbocation, as opposed to direct heterolytic cleavage. The cation yields also indicate that back reaction to regenerate starting material, as well as product formation within the initial geminate cage, occur in some solvents. The effects of solvent and added perchlorate salts on the rate constants for reaction with nucleophiles have been examined. For example, rate constants that vary by an order of magnitude have been measured for quenching by azide ion in various aqueous acetonitrile and trifluoroethanol mixtures.Faisant appel a une combinaison d'expkriences de photolyse kclair au laser et d'ktudes des produits, on a examink la photolyse de plusieurs chlorures de phCnyl(naphty1)mkthyl triphknyl phosphonium. On a caractkrisk des intermkdiaires tant cationiques que radicalaires dans les expkriences impliquant les espkces transitoires; les rendements relatifs dependent beaucoup du solvant. Par exemple, dans les alcools, l'acktonitrile ou des solvants aqueux, il y a formation prkdominante de cations alors qu'il faut un mklange d'acktonitrile/dioxane (5-10%) pour observer des radicaux. Les rendements quantiques pour la formation des cations, tels que mesurks respectivement par des ktudes de produits et des expkriences sur les espbces transitoires, varient de 0,79 dans le mkthanol a 0,093 dans un mklange 1 : 4 d'acktonitrile/dioxane. L'addition de sels de perchlorate conduit une augmentation dramatique des temps de vie des cations; les effets sont particulibrement prononcks pour les mklanges acktonitrile/dioxane dans lesquels les rendements en cations augmentent aussi par des facteurs de 3-4. Dans ce cas, on attribue les effets principalement a une substitution du chlorure par le perchlorate dans les paires d'ions ...

Nucleophilic reactivities of tertiary alkylamines

“…The choice of photo‐leaving group becomes more critical for less stabilized benzhydryl cations because photoheterolysis gets less favorable with decreasing cation stability. For the generation of benzhydryl cations with E > −2, which cannot readily be isolated as stable salts, we usually use precursors which are known to have a high efficiency of photoheterolysis, such as chlorides ( 5 ),51–53 acetates ( 6 ),54–58 and phosphonium salts ( 4 ) 59–63. The concentrations of Cl − , AcO − , and R 3 P generated by photolysis, which can be calculated from the absorbances and known extinction coefficients51 of the benzhydrylium ions, are so small that the rate of external return with the photo‐leaving group is usually negligible.…”

Photogeneration of carbocations: applications in physical organic chemistry and the design of suitable precursors

“…Since the late 1980s, a large number of studies have employed nanosecond laser flash photolysis to study the generation and the decay kinetics of carbocations . Some of the substrates that were successfully employed for the photogeneration of carbocations are shown in Chart (b) . Valuable information about the formation of carbocationic intermediates in photoreactions has also been derived by the analysis of the products …”

“…Triarylphosphonium salts generally combine high stability, even in strongly ionizing solvents, with a high tendency to produce carbocations upon irradiation . Reactions of triarylphosphines with highly stabilized carbocations, however, do not yield stable triarylphosphonium salts (Fig.…”

“…An efficient method is the laser flash photolysis of suitable neutral (R–X) or charged (R–X + ) precursors (Scheme ), which allows the photogeneration and subsequent real‐time ultraviolet–visible (UV–vis) detection of the carbocations R + in the presence of a nucleophile . This method has widely been employed to determine the rates of fast reactions of carbocations with nucleophiles . Common precursors include alkyl halides R–Hal, acetates R–OAc, aryl ethers R–OAr, ammonium salts R–NR′ 3 + , and phosphonium salts R–PR′ 3 + .…”

Fotólise de brometos de orto-xilila: um estudo em solventes apolares

“…ii, 1977;CRISTOL & GREENWALD, 1976;CRISTOL & BINDEL, 1980;ARNOLD et a/ii. 1985 e referências de 7 a 19,alicitadas; ALONSOet a/ii, 1990;1992 e referências18 e 20 a 22, ali citadas).…”