Butyl decomposition pathway

The substituted hydroxylamine C NOPP from reaction 7) can take part in various dark reactions, even at ambient temperature. From a study of the low molecular weight model I in the liquid phase, two decomposition pathways are possible (reaction 8) (12). The products from the disproportionation reaction 8a were only observed in the absence of a radical trap such as O2. In a given solvent ks kaa-Uo (solvent air saturated and degassed respectively). Both k8a and ke were found to increase by an order of magnitude on going from a non-polar solvent (iso-octane) to a polar solvent (methanol or tert.-butyl hydro peroxide, BuOOH). 

Fig. 10.7 Decomposition pathways of 1 -butyl-3-methylimidazolium via a biradical transition state.

The rates of all the various decomposition pathways of the molecular ion of n-butyl acetate have been investigated using a double-focussing mass spectrometer [743]. There appears, however, to have been either an error in the calculation of times or the calculations are simply imprecise. The times reported appear to be too short, perhaps by something like a factor of 3. This being so, the agreement between these incorrect times and decomposition times for related ions [520] would not be significant. 

No investigations on growth kinetics of di-/i-butyltin(iv) diacetate have been published, but pryrolysis experiments were carried out to obtain some information on the decomposition pathway by Yagi and coworkers [180]. The reaction under vacuum consists of two steps first the n-butyl-groups are eliminated betwen 280-310 °C and then above 320°C, the acetoxy groups are cleaved from the tin atom. Both reactions take place at lower temperatures in the presence of oxygen. 

Scheme 3.1 Decomposition of (a) AiBN and (b) fert-butyl peroxyester initiators. In (b), the decomposition pathway is influenced by the nature of the R substituent of the carbonyl group.

The most important factor affecting the selectivity of the epoxidation reaction (226) is the choice of metal complex used as the catalyst [374-377]. Table 11 summarizes the results of several studies which indicate that in general, molybdenum complexes are superior catalysts for this reaction. The lower selectivity for several of the catalysts listed in Table 11 is due to competing metal catalyzed hydroperoxide decomposition via homolytic bond cleavage under reaction conditions. Sheldon and Van Doom have shown that half times for decomposition of tert-hnXyl hydroperoxide in benzene at 90 were in the order [Co(Oct)2] >[Cr(acac)3] >[VO(acac)2] > [Mo(CO)6] > [W(CO)6] > [Ti(OBu)4]. On the other hand, the relative rates of epoxide formation in reactions of ferf-butyl hydroperoxides with cyclohexene in benzene at 90°C were in the order [Mo(CO)6] > [VO(acac)2] > [Ti(OBu)4] > [W(CO)6j. Thus, the relative rates of homolytic decomposition pathways and heterolytic epoxidation for any given complex determine the epoxide selectivity. 

When 35 was treated with AgBp4 and butyl acrylate under stoichiometric conditions, decomposition products resulting from reductive elimination from all of the key catalytic intermediates were observed. The stability of the catalyst under the actual reaction conditions was much higher, and was attributed to faster rates of p-hydride elimination from the arylated acrylate at the elevated reaction temperatures, which removed at least two of the decomposition pathways. However, it should be noted that these decomposition pathways could be responsible for the generation of catalytically active Pd complexes containing one or zero carbene ligands in this or other systems. 

The tendency for N-nitrosamides to undergo hydrolysis by a nucleophilic catalysed pathway has been confirmed by studies of N-alkylnitroso acetamides (22) Results summarised in Table I for N -n-butyl-JJ -nitroso acetamide show that its decomposition is also subject to steric constraints (2,6-lutidinestrong nucleophiles (eg. imidazole, thiols) irrespective of their base strength (pK ). Further, the second order dependence on [Imidazole] is more clearly defined for the decomposit-... 

For further examples of dichotomous solvent-influenced radical/ionic perester decompositions, see the base-catalyzed perester fragmentation shown in Eq. (5-39) in Section 5.3.2 [110], as well as the decomposition of t-butyl heptafluoroperoxybutyrate, C3p7-C0-0-0-C(CH3)3 [691]. The relative extent of monomolecular and induced thermal decomposition of disubstituted dibenzyl peroxydicarbonate, ArCH2-0-C0-0-0-C0-0-CH2Ar, is also substantially influenced by the reaction medium [692]. The thermolysis of suitable dialkyl peroxides can also proceed by two solvent-dependent competitive reaction pathways (homolytic and electrocyclic reaction), as already shown by Eq. (5-59) in Section 5.3.4 [564]. 

Similar observations have been reported by Saveant et al. in their studies on the electrochemical reduction of simple aliphatic halides [132g] (/ -.. s-, and t-butyl halides) at a glassy carbon electrode. Cyclic voltammetry of the butyl halides showed one or two irre crsible waves, depending on the relative reducibility of the alkyl halide RX and of the radical R. All transfer coefficients reported were smaller than 0.5 (between 0.2 and 0.32). The fact that the transfer coefficient was small was taken as further evidence that the reduction pathways do not involve the RX anion radical as an intermediate. Our observations from the electrochemical reduction of 18, 158, 19, and 131 at a glassy carbon electrode are in agreement with this. The cyclic voltammetric shape, the peak width tp 2 — p = (180-150) mV, and the value of a =0.25-0.32 at different scan rates [137] showed, without question, that electron transfer and decomposition of the anion radical... 

However, somewhat to our surprise, mononuclear Tp -Pr- Co 02 proved unstable in solution at ambient temperature. Its decomposition produced [(Tp P M Co)2(ll-0)2] as a transient intermediate and ultimately proceed to the hydroxide. Significantly, no additional metal complex was needed to initiate Ae decomposition, and the reaction took place even in the presence of dioxygen. It is highly unlikely that the substitution of an isopropyl group for a tert-butyl substituent would change the electronic nature of the mononuclear dioxygen complex sufficiently to shift the O2 dissociation equilibrium. Thus there must be another pathway, which is open to Tp P > Co-02, but closed to the sterically more encumbered Tp Co-O2. 

As in the uncatalyzed reactions with enamines (vide supra), there is potentially more than one point where stereochemical differentiation can occur (Scheme 59). Selectivity can occur if the initial addition of the enol ether to the Lewis acid complex of the a,/J-unsaturated acceptor (step A) is the product-determining step. Reversion of the initial adduct 59.1 to the neutral starting acceptor and the silyl enol ether is possible, at least in some cases. If the Michael-retro-Michael manifold is rapid, then selectivity in the product generation would be determined by the relative rates of the decomposition of the diastereomers of the dipolar intermediate (59.1). For example, preferential loss of the silyl cation (or rm-butyl cation for tert-butyl esters step B) from one of the isomers could lead to selectivity in product construction. Alternatively, intramolecular transfer of the silyl cation from the donor to the acceptor (step D) could be preferred for one of the diastereomeric intermediates. If the Michael-retro-Michael addition pathway is rapid and an alternative mechanism (silyl transfer) is product-determining, then the stereochemistry of the adducts formed should show little dependence on the configuration of the starting materials employed, as is observed. 

It was speculated that the second step of over-oxidation to acid might take place via a free radical pathway, arising from the catalytic decomposition of the t-BuOOH. It was further thought that the use of a free radical inhibitor might reduce the extent of the acid formation and inqjrove the overall aldehyde selectivity. The use of free radical scavengers such as 2,6-di-ier/-butyl-4-methylphenol (Table 4, Run 24), the stable free-radical, TEMPO, (Run 25) or the amine type inhibitor, N-Phenyl -2-Naphthylamine (Run 26), did not show any improvement in the reaction selectivity towards the formation of the aldehyde. The lack of any significant reduction in the amoimts of ester formed when using these modifiers showed that both steps of aldehyde and acid formation most likely do not include the involvement of free radical intermediates. 

Thermodynamic parameters have been obtained from kinetic studies of the thermal decomposition of ethyl 2,7-di-t-butyl-5-methyl-4-thiepinecarboxylate (19) <90AG(E)424> and 1-benzothiepine (61a) <7472431 >. The activation parameters for the sulfur extrusion reaction of (19) and (61a) are A// = 93.7 14 kJ mol- for (19), A/f = 75 13 kJ mor for (61a), and AS = -112.5 15 JK" mol" for (19), AS = —100 42 JK" mol" for (61a), respectively. The large negative AS values are well in accord with a decrease in entropy in the transition state which would be anticipated to be close to a thianorcaradiene structure. In contrast, AS value for the selenium extrusion reaction from selenepine (62) is 9.3 JK mol-, which implies that the reaction pathway is different to that of thiepines. A pathway via incipient fission of a C—Se bond has been suggest <90AG(E)424>. 

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