Dediazoniations of Arenediazonium Ions. Part XXI. Dediazoniation of Arenediazonium Ions Complexed with Crown Ethers

The evaluation of the dediazoniation kinetics of various m- and p-substituted benzenediazonium tetrafluoroborates in 1,2-dichloroethane at 50° in the presence of 18-crown-6, 21-crown-7 and dicyclohexano-24-crown-8 demonstrates that the rate constant for the dediazoniation within the complex (k₂) is smallest, and the equilibrium constant for complex formation (K) is largest for the complexes with 21-crown-7 (cf. Scheme 1). The logarithms of the equilibrium constants (K) for complex formation with each of the crown ethers studied correlate well with Hammett's substituent constants, σ, to give reaction constants ρ = 1.18–1.38. A linear correlation between the logarithms of the rate constants for the dediazoniation within the complex with those of the dediazoniation rate constants of uncomplexed diazonium ions (log k₂ vs. log.k₁), found for most substituted diazonium salts, indicates that the dediazoniation mechanism of the complexed diazonium ions is not significantly different from that of the free ions. For very electrophilic diazonium ions (p-Cl, m-CN), k₂ was much larger than expected on the basis of the linear log k₂ vs. log k₁ relationship. Analysis of the dediazoniation products showed that this was due to a change in mechanism from heterolytic to homolytic dediazoniation. The complexation rate of diazonium salts by crown ethers (k_c) is practically diffusion controlled and does not change much with the size of the crown ether. The decomplexation rate (k_d), however, is significantly lower for complexes with 21-crown-7, than for those with 18-crown-6 and dicyclohexano-24-crown-8, and is therefore the reason for the variations in the equilibrium constant (K) and thus for the fact that complexes of arenediazonium salts with 21-crown-7 are the most stable. The amounts of the N_α-N_β rearrangement, as well as those of the exchange of the ¹⁵N-labelled diazonio group with external nitrogen during dediazoniation of p-toluenediazonium salt were independent of the addition of crown ethers. A dediazoniation mechanism involving a charge transfer, as well as an insertion-type diazonium ion-crown ether complex is proposed. In this mechanism, dediazoniation of the insertion complex does not take place directly, but through the charge-transfer complex.     

Genaration of a η6- arene/Cr(co)3-complexed diazonium ion; homolytic and heterolytic dediazoniation via metallic π-bonded aryl radicals and cations

The reaction of NO⁺ with o-toluidinechromium tricarbonyl has been studied. Diazotization (attack on N) competes with NO⁺ attack on the metal and decarbonylation. The Cr(CO)₃-complexed diazonium ion is unstable and dediazoniates even at low temperature. The dediazoniation mechanism is predominantly homolytic. Competing heterolytic dediazoniation is observed in highly ionizing, low nucleophilicity solvents such as CF₃SO₃H (TfOH), FSO₃H and CF₃CH₂OH (TFE).     

Dediazoniation of Arenediazonium Ions. Part XXII. Reactions of 2, 6-Dialkyl-Substituted Benzenediazonium Ions in Super Acids,Acetonitrile and Acetone

Reactions of 2,4,6-trimethylbenzenediazonium ( 1 ), 2,6-diethylbenzenediazonium ( 2 ) and 2,6-diisopropylbenzenediazonium ( 3 ) tetrafluoroborates were studied in magic acid, SbF₅/SO₂ClF, acetonitrile and acetone by ¹H-NMR and by analysis of the dediazoniation products. The N_α-N_β rearrangement of β-N¹⁵-labelled tetrafluoroborates 1–3 was followed by ¹⁵N-NMR of the corresponding arylazonaphthols, as well as by MS analysis of the anilines obtained by reduction of the azo compounds. Diazonium salts 2 and 3 were synthesized for the first time and the steric effect of substituents at C(2) and C(6) on the reactions under study is discussed. All the results obtained can be rationalized by heterolytic dediazoniation of diazonium salts 1 – 3 and product formation from the corresponding aryl cations.     

Dediazoniation of Arenediazonium Ions in Homogeneous Solutions. Part XVII. Kinetics and mechanisms of dediazoniation of p-chlorobenzenediazonium tetrafluoroborate in weakly alkaline aqueous solutions in the presence of oxygen

The kinetics of dediazoniation of p-chlorobenzenediazonium tetrafluoroborate have been studied in buffer solutions in the pH-range 9.0–10.0, ionic strength I = 0.10, at 20.0° in glass and polytetrafluoroethylene vessels. The presence of oxygen (<5 ppb of O₂, 60 to 100 ppb of O₂, air, > 99% of O₂) has a decisive influence on the rate and kinetic order of the dediazoniation. Iodoacetic acid inhibits the reaction, whereas p-chlorophenol has a catalytic effect, and in air and >99% of O₂ it acts as an autocatalyst. The reaction is subject to general-base catalysis by water, hydroxyl ions, hydrogen carbonate and carbonate ions. The kinetic results are interpreted in conjunction with data concerning the reaction products [2] and a ¹⁵N-CIDNP. investigation of a related system [3]. Specific radical chain mechanisms are consistent with the results.     

Dediazoniations of Arene Diazonium Ions in Homogeneous Solution. Part VI: Solvolyses in 2,2,2-trifluoroethanol/water mixtures and exclusion of a potential aryne mechanism

The rates and products of dediazoniation of benzenediazonium tetrafluoroborate in 2,2,2-trifluoroethanol (TFE)/water mixtures has been determined. The results are not consistent with a mechanism in which TFE and water enter the rate-determining part of the reaction as nucleophiles. The influence of the solvent composition on product ratios and rates is explained as a solvent effect, the formation of a (solvated) aryl cation being the rate-determining part of the reaction. The magnitude of the preexponential factor of the Arrhenius equation is consistent with this interpretation. Since the solvolysis of p -chlorobenzenediazonium tetrafluoroborate in TFE yields no detectable m-products, an aryne-like mechanism is excluded.     

Dediazoniation of arenediazonium ions in homogeneous solution. Part VIII. Reaction kinetics and products in dimethyl sulfoxide

p-Nitrobenzenediazonium tetrafluoroborate dissolved in dimethylsulfoxide (DMSO) at 50° forms p-nitrophenol in 88–90% yield. The phenolic oxygen atom originates exclusively from the oxygen atom of DMSO as demonstrated by the use of ¹⁸O-labelled DMSO. The first-order rate of dediazoniation is the same under N₂ as it is in the presence of air. The rate is little influenced by the addition of benzene or iodobenzene. However, the products formed in the presence of these additives are significantly different. UV. spectra and the reactivity of diazonium salt solutions in DMSO when mixed with reagents in aqueous solution demonstrate that a relatively stable charge-transfer complex is formed between the diazonium ion and DMSO. The product analyses and the kinetic and spectral results of dediazoniation in DMSO with and without additives are consistent with a mechanism in which the rate-limiting step is the formation of a p-nitrophenyl radical from the charge-transfer complex. p-Nitrophenol and the products with benzene and iodobenzene are formed in subsequent fast competition steps. In the presence of small amounts of pyridine the dediazoniation is much faster and follows a different kinetic law. Pyridine effectively competes with DMSO in the reaction with diazonium ions.     

Properties of 2,2,2‐Trifluoroethanol/Water Mixtures: Acidity,Basicity, and Dipolarity

In this report, we focus our attention on the characterization of 2,2,2‐trifluoroethanol(TFE)/H₂O mixtures and describe their intrinsic parameters; i.e., solvent acidity (SA), solvent basicity (SB), and solvent dipolarity/polarizability (SPP), by the probe/homomorph‐couple method for a range of mixtures from 0–100% (v/v) TFE. Variation of these parameters is not linear and has a singular and unpredictable behavior depending on the precise composition of the mixture. Based on these parameters, we describe the TFE‐induced changes in some physical properties; i.e., viscosity (η), partial molar volume (V?), density (ρ), dielectric constant (ε), vapor pressure (p_v), and spectroscopic properties; i.e., NMR chemical shifts (δ(¹H)) of TFE Me group for all molar fractions studied. In addition, by means of CD studies, we report that formation of the secondary structure, as percentage of helical content, θ, of a polypeptide, poly(L ‐lysine), in several TFE/H₂O mixtures is adequately described by these mixture parameters. SA, SB, and SPP of TFE/H₂O mixtures provide an excellent tool for the interpretation of formation and stability of intramolecular H‐bonds, and, thus, of secondary structures in polypeptides.     

Self-association of thiols. The baggy fit problem in weak complexes

The PMR technique has been used to obtain thermodynamic data for hydrogen bonding of alkanethiols (RSH) in 1:1 dimers in carbon tetrachloride. At ca. 303°K these are (R, 10⁴K(M^?1), ?ΔH°(kcal/mole), ?ΔS°(eu)): n-C₃H₇, 51 ± 5, 0.9 ± 0.15, 13 ± 1; i-C₃H₇, 50 ± 10, 0.8 ± 0.3, 13 ± 1; n-C₄H₉, 35 ± 2, 0.8 ± 0.15, 14 ± 1; t-C₄H₉, 14 ± 4, 1.1 ± 0.7, 16 ± 2; C₆H₁₁, 1.3 ± 2, 0.7 ± 0.3, 15 ± 1. Alkanethiol self-association is weak, and although an exact expression [Eqn. (5)] reproduces spectral data precisely, the fit is sufficiently ‘loose’ or ‘baggy’ so that values of K, ΔH° and ΔS° are uncertain. The methodology of the treatment of self-association data and their errors is examined and Deranleau's useful approach is extended. The impossibility of obtaining reliable data for very weak (< 10 %) or very strong (> 90 %) associations by techniques equivalent to ours is emphasized. The possibility of cyclic thiol dimers is discussed. It is suggested that the PMR method cannot give trustworthy self-association data for aryl or arylalkylthiols because of the relatively large anisotropy effects introduced into the dilution shift.     

Dediazoniations of Arene Diazonium Ions in Homogenous Solution. Part IV: Change-over from a heterolytic to a homolytic mechanism in 2,2,2-trifluoroethanol pyridine mixtures

There are only two dediazoniation products of benzenediazonium tetrafluoroborate in 2,2,2-trifluoroethanol (TFE), namely phenyl 2,2,2-trifluoroethyl ether ( 1 ) and fluorobenzene ( 2 ). The reaction kinetics are strictly first-order with respect to the diazonium salt. The addition of increasing amounts of pyridine to the system results in a gradual decrease in the yields of 1 and 2 and an increase in the yields of the homolytically formed products, benzene ( 3 ), biphenyl ( 4 ), isomeric phenylpyridines ( 5 ) and diazo tar ( 6 ). The reaction kinetics show that the rate of dediazoniation of the benzene diazonium salt increases with increasing amounts of pyridine. The reaction with added pyridine is no longer first-order with respect to the diazonium ion. The product analyses and the kinetic data are consistent with the view that in pure TFE this diazonium salt decomposes completely by a heterolytic mechanism. The addition of pyridine brings about a competitive homolytic mechanism which becomes increasingly dominant as the concentration of pyridine increases.     

Solvent and Substituent Effects in the Solvolysis of 9‐Aryl‐9‐bromofluorenes

The solvolysis of eight 9‐aryl‐9‐bromofluorenes ( 6b～6i ) in a variety of solvents were studied. Correlation analysis using single‐parameter Grunwald‐Winstein equation (Eqn. 1) with different Y scales showed good linearity (R ≥ 0.98) for most cases if Y_xBnBr was employed. Linear relationships were observed from Hammett‐type analysis of logarithm of rate constants using Brown‐Okamoto σ⁺ constants (Eqn. 3), although inverse order of k(p‐CF₃)/k(m‐ CF₃) was realized in a number of cases. The ρ values were found to vary slightly with different solvent systems. Calculated atomic charge reveals the similarity between 9‐phenyl‐9‐fluorenyl cation ( 7 ) and triphenylmethyl cation ( 8 ). An extended charge delocalization throughout the fluorenyl ring led to the conclusion of the insignificance of antiaromaticity, which was in harmony with that obtained in previous studies. The variation of relative k_Br/k_Cl rate ratios was attributed to the electrophilic pull by solvents in solvolysis.     

Construction of Highly Substituted Nitroaromatic Systems by Cyclocondensation. Part II. Conversion of 4-nitro-3-oxobutyrate to 3-nitrosalicylates

The base-catalyzed reaction of 4-nitro-3-oxobutyrate (6) with acetylacetone ( 8 Scheme 3), formylacetone ( 13 , Scheme 4), formylcyclohexanone ( 31 , Scheme 5), 2,4-dioxopentanoates 39 and 40 (Scheme 6), and 2,4,6-heptanetrione ( 2 , Scheme 7) affords substituted 3-nitrosalicylates, products of a double aldol condensation. With unsymmetrical dicarbonyl compounds both regioisomers are formed. High selectivity was found in the case of β-keto-aldehydes 13 and 31 with preferred addition of the NO₂-substituted carbon to the aldehyde carbonyl. The major products of these cyclocon-densations, which are isolated in yields ranging from 20% to 80%, are all new compounds. Less successful are the conversions with β-alkoxy- and β-chloro-vinyl ketones ( 23, 25 , and 26 ), and with alkinone 24 , where the condensation products are formed in very low yield (Scheme 4).     

Formation and Decomposition of Diazo Ethers under Acidic Conditions. Effects of Ethanol Concentration,Acidity, and Temperature on the Ethanolysis of 4‐Methylbenzenediazonium Ions

We investigated the effects of solvent composition, acidity, and temperature on the dediazoniation of 4‐methylbenzenediazonium (4MBD) ions in EtOH/H₂O mixtures by employing a combination of spectrometric and chromatographic techniques. First‐order behavior is found in all solvent composition ranges. HPLC Analyses of the reaction mixtures indicate that three main dediazoniation products are formed depending on the particular experimental conditions. These are 4‐cresol (ArOH), 4‐phenetole (ArOEt), and toluene (ArH). At acidities (defined as ?log [HCl])<2, the main dediazoniation products are the substitution products ArOH and ArOEt but upon decreasing the acidity, the reduction product ArH becomes predominant at the expense of ArOH and ArOEt, indicating that a turnover in the reaction mechanism takes place under acidic conditions. At any given EtOH content, the plot of k_obs values against acidity is S‐shaped, the inflexion point depending upon the EtOH concentration and the temperature. Similar S‐shaped variations are found when plotting the dediazoniation–product distribution against the acidity. The acid dependence of the switch between the homolytic and heterolytic mechanisms suggests that the homolytic dediazoniation proceeds via transient diazo ethers, and this complex kinetic behavior can be rationalized by assuming two competitive mechanisms: i) the spontaneous heterolytic dediazoniation of 4MBD, and ii) an O‐coupling mechanism in which the EtOH molecules capture ArN$\rm{{_{2}^{+}}}$ to yield a highly unstable (Z)‐adduct which undergoes homolytic fragmentation initiating a radical process (Scheme). Analyses of the effects of temperature on the equilibrium constant for the formation of the diazo ether and on the rate of splitting of the diazo ether allowed the estimation of relevant thermodynamic parameters for the formation of diazo ethers derived from methylbenzenediazonium ions under acidic conditions.     

Aminierende,reduktive Kupplung aromatischer Aldehyde mit Tris(dialkylamino)methylvanandium (IV) zu N,N,N′,N′-Tetraaalkyl-1,2-diaryläthylendiaminen

Aminative Reductive Coupling of Aromatic Aldehydes to N,N,N′,N′-Tetraalkyl-1,2-diarylethylenediamines, Induced by Tris(dialkylamino)methylvanadium (IV) In a novel type of reaction, certain aromatic aldehydes (benzaldehyde, p-methoxybenzaldehyde, 1-naphthaldehyde, furan-2-carbaldehyde) and secondary amines are coupled to give N,N,N′,N′-tetraalkyl-1,2-diarylethylenediamines 1–6 . The reagents are tris(dialkylamino)methylvanadium(IV) compounds (cf. Eqn. 2). These are generated in situ either from isolable chlorotris(dialkylamino) vanadium(IV) (Eqn. 3), or preferably, from an Et₂O/pentane solution of VCl₄ which is treated sequentially with 3 equiv. of lithium dialkylamide, 1 equiv. of MeLi, and 0.8 equiv. of an aromatic aldehyde, to give the products 1–6 in a one-pot preparation (Scheme 2). The yields range from 14 to 54%. The diastereoisomeric mixtures (meso- and (±)-forms) obtained are separated by chromatography (Al₂O₃, petroleum ether/Et₂O/Et₃N), and the pure stereoisomers fully characterized. A mechanism of the reductive coupling induced by CH₃V (NR₂)₃ is proposed (Scheme 1).     

Radiation induced terpolymerization of tetrafluoroethylene with propylene and n-butyl vinyl ether in bulk

Terpolymerization of tetrafluoroethylene (TFE) with propylene (P) and n-butyl vinyl ether (NBVE) induced by γ-rays at room temperature at dose rate 5 × 10⁵ rad/h and P/NBVE molar ratio from 49/1 to 10/40 was carried out. An alternating copolymerization between TFE and two α-olefins was found to take place in this system, so that 50 mole % of TFE containing terpolymer is always formed at various monomer compositions. The terpolymer composition can be explained successfully by the treatment by a complex mechanism. The complex reactivity ratios of r_I (TFE–complex) and r_II (TFE-NBVE complex) were calculated to be 0.5 and 0.6, respectively, assuming a complex mechanism. The polymerization rate and molecular weight increase with NBVE concentration in the monomer mixture. Colorless transparent rubber-like polymers were obtained at each monomer composition. The glass transition temperature sharply decreases with NBVE concentration in the terpolymer but the thermal and chemical resistances of the terpolymer slightly decrease. Considering these results together with the mechanical properties it has been concluded that the 45/48/7 terpolymer of TFE/P/NBVE molar ratio is good as a practical elastomer useful at relatively low temperatures.     

Liquid phase oxidation of transition metals. Part 6. iron and copper complexes containing dimethylsulphoxide,dimethylformamide, and acetonitrile. Anomalous states of ligands

Summary Direct oxidation of iron and copper in a donor-acceptor medium, L + CCl₄, where L is dimethylsulphoxide, dimethylformamide or acetonitrile was employed to obtain complex compounds:cis-[FeCl₂(DMSO)₄]Cl] (3), 2 FeCl₃ · 3 DMSO (5), [FeCl(DMSO)₅][FeCl₄]₂] (6), [FeCl(DMSO)₅][Fe₂Cl₆O] (7),cis-[FeCl₂(DMF)₄][FeCl₄] (8), [Fe(MeCN)₆][FeCl₄]₂ (9) andcis-[CuCl₂(DMF)₂]₂ (10), The structures of complexes (9) and (10) have been established by x-ray diffraction analysis and compared with those of (3), (6), (7) and (8) which are reported elsewhere.The [FeCl(DMSO)₅][Fe₂Cl₆O] complex (7) is formed by oxidation of iron fromcis-[Fe^IIICl₂(DMSO)₄]₂[Fe^IICl₄] (4) in ethanol. One of the 5 DMSO molecules of (7) was found to be disordered; the Mössbauer spectroscopy data suggest that it can move within the cation coordination sphere.Mössbauer spectroscopy and x-ray diffraction analysis indicate electron isomerism in one of the complexes.For papers 4 and 5 of these series see refs. 1 and 2.     

Oxidation of ascorbic acid in the presence of phthalocyanine metal complexes. Chemical aspects of catalytic therapy of cancer 2. Catalysis by cobalt octacarboxyphthalocyanine. Reaction products

The products of ascorbic acid oxidation in the presence of cobalt octa-4,5-carboxy-phthalocyanine sodium salt (TPH) were identified. These include the ascorbate radical (A^·), hydroxyl radical (OH^·), and hydrogen peroxide (H₂O₂). The kinetics of accumulation and consumption of the reaction products was studied. For the concentration ranges of ascorbic acid = 0–2.5 ⋅ 10⁻³ mol L⁻¹ and the catalyst C _TPH = 0–3.5 ⋅ 10⁻⁵ mol L⁻¹, the the highest possible concentration of the ascorbate radical is ∼10⁻⁷ mol L⁻¹, the concentration of H₂O₂ is 7 ⋅ 10⁻⁴ (30% of the starting concentration of ascorbic acid) and the concentration of the hydroxyl radical is at most 10⁻⁶ mol L⁻¹.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2224–2228, October, 2004.     

The lead tetraacetate oxidation of bisacetylhydrazones of α-dicarbonyl compounds

The oxidative cyclization of the title compounds results in generally two different kinds of products. The first, 1-(N,N-bisacetylamino)-1,2,3-triazole 7 (R³ = CH₃) is the primary product, while the second, 1-N-acetylamino-1,2,3-triazole 8 (R³ = CH₃), when observed, is obtained via hydrolysis from the former during work-up and separation of the reaction mixture. The primary products are considered as resulting from intramolecular nucleophilic attack on the acetyl group, of the presumed zwitterionic intermediate 5 (R³ = CH₃), by the N of the ambident N-acetylimine site of 5 .     

Effects of Ascorbic Acid on Arenediazonium Salts Reactivity: Kinetics and Mechanism of the Reaction

We have examined the kinetics and mechanism of dediazoniation of o‐, m‐ and p‐methylbenzenediazonium (ArN) tetrafluoroborate in the presence of ascorbic acid (H₂A) at different pHs by combining spectophotometric (VIS‐UV), high performance liquid chromatography (HPLC), and polarographic measurements. Kinetic data show that, at low pH, observed rate constants increase linearly with increasing ascorbic acid concentration, but the saturation kinetics observed at higher pH suggest the formation of a transient diazo‐ether complex preceding the slow step of the reaction. Experimental evidence for the formation of such a complex was obtained from a competitive coupling reaction with the Na salt of `2‐naphthol‐6‐sulfonic acid' and by titration of ascorbic acid (H<sub>2</sub>A) with the arenediazonium ions (electrochemical measurements). HPLC Analysis of dediazoniation products indicates that, in the absence of H<sub>2</sub>A, only the heterolytic phenol derivative, ArOH, is formed quantitatively, in keeping with the predictions of the D<sub>N</sub>+A<sub>N</sub> mechanism. In the pH 2 – 4 range and in the presence of H<sub>2</sub>A, reduction products (ArH) are obtained in addition to heterolytic products (ArOH), corroborating that certain biological reducing agents like ascorbate (HA<sup>−</sup>) are capable of inducing reductive fragmentation of ArN into aryl radicals. All evidence is consistent with two competitive reaction pathways, the thermal decomposition of ArN, and a rate‐limiting decomposition of the transient diazo ether `complex', formed during the reaction of ArN with HA⁻ in a rapid pre‐equilibrium step.     

Nucleophilic Aromatic Substitutions. Part XIV.. Investigation of the mechanism of hydroxy-denitration of 4,2-and 2,4-chloronitrobenzenediazonium ions as a function of pH

4,2-Chloronitrobenzenediazonium ions in aqueous buffer solutions between pH 2.9 and 7.9 do not hydrolyze by dediazoniation as previous authors have assumed, but by denitration. The isomeric 2,4-compound reacts by denitration (ca. 70%) and dechlorination (ca. 30%). The reactions are general base catalyzed. The products and kinetics are consistent with an S_NAr-mechanism in which the general base-catalyzed addition of a hydroxyl group at the reacting C-atom is rate-limiting. The rate maxima at or near the pH-values corresponding to (pK₁ + pK₂)/2 of the diazonium ? cis-diazotate equilibria can be rationalized on the assumption that the diazonium ion is the only equilibrium form of the diazo compound which enters the substitution proper, and the superposition of the rate term k_B[B] of all nucleophiles involved (H₂O, OH^? and buffer bases).     

Singlet-Oxygen Ene Reactions of (E)-4-Propyl[1,1,-2H3]oct-4-ene. Short Communication

Rose bengal-sensitized photooxygenation of 4-propyl-4-octene ( 1 ) in MeOH/Me₂CHOH 1:1 (v/v) and MeOH/H₂O 95:5 followed by reduction gave (E)-4-propyl-5-octen-4-ol ( 4 ), its (Z)-isomer 5 , (E)-5-propyl-5-octen-4-ol ( 6 ), and its (Z)-isomer 7 . Analogously, (E)-4-propyl[1,1,1-²H₃]oct-4-ene ( 2 ) gave (E)-4-propyl[1,1,1-²H₃]oct-5-en-4-ol ( 14 ), its (Z)-isomer 15 , (E)-5-[3′,3′,3′-²H₃]propyl-5-octen-4-ol ( 16 ), its (Z)-isomer 17 , and the corresponding [8,8,8-²H₃]-isomers 18 and 19 (see Scheme 1). The proportions of 4–7 were carefully determined by GC between 10% and 85% conversion of 1 and were constant within this range. The labeled substrate 2 was photooxygenated in two high-conversion experiments, and after reduction, the ratios 16/18 and 17/19 were determined by NMR. Isotope effects in 2 were neglected and the proportions of corresponding products from 1 and 2 assumed to be similar (% 4 ≈? % 14 ; % 5 ≈? % 15 ; % 6 ≈? % ( 16 + 18 ): % 7 ≈? % ( 17 + 19 )). Combination of these proportions with the ratios 16/18 and 17/19 led to an estimate of the proportions of hydroperoxides formed from 2 . Accordingly, singlet oxygen ene additions at the disubstituted side of 2 are preferred (ca. 90%). The previously studied trisubstituted olefins 20–25 exhibited the same preference, but had both CH₃ and higher alkyl substituents on the double bond. In these substrates, CH₃ groups syn to the lone alkyl or CH₃ group appear to be more reactive than CH₂ groups at that site beyond a statistical bias.