Al reaction pathways

Scheme 4b depicts condensation between a hydroxymethyl group and a phenolic ring where the hydroxybenzyl attacks at a ring position that is already hydroxymethylated. In this case, a methylene linkage is produced between the rings with concurrent loss of one mole each of formaldehyde and water. Both Jones and Grenier-Loustalot et al. demonstrated the occurrence of this reaction pathway beyond doubt under basic conditions. 

Wawzonek et al. first investigated the mechanism of the cyclization of A-haloamines and correctly proposed the free radical chain reaction pathway that was substantiated by experimental data. "" Subsequently, Corey and Hertler examined the stereochemistry, hydrogen isotope effect, initiation, catalysis, intermediates, and selectivity of hydrogen transfer. Their results pointed conclusively to a free radical chain mechanism involving intramolecular hydrogen transfer as one of the propagation steps. Accordingly, the... 

The first reaction pathway for the in situ formation of a metal-carbene complex in an imidazolium ionic liquid is based on the well loiown, relatively high acidity of the H atom in the 2-position of the imidazolium ion [29]. This can be removed (by basic ligands of the metal complex, for example) to form a metal-carbene complex (see Scheme 5.2-2, route a)). Xiao and co-workers demonstrated that a Pd imida-zolylidene complex was formed when Pd(OAc)2 was heated in the presence of [BMIMjBr [30]. The isolated Pd carbene complex was found to be active and stable in Heck coupling reactions (for more details see Section 5.2.4.4). Welton et al. were later able to characterize an isolated Pd-carbene complex obtained in this way by X-ray spectroscopy [31]. The reaction pathway to the complex is displayed in Scheme 5.2-3. 

O Brien et al. provided the first examples of olefin formation by reductive alkylation of aziridines [97]. Treatment of aziridine 267 with s-BuLi gave olefin 270 in 76% yield (Scheme 5.68). For the formation of olefin 270 they suggest a reaction pathway that proceeds in a manner analogous to that proposed for epoxides [36] namely, nucleophilic attack of s-BuLi on lithiated aziridine 268 to form dilithiated species 269, which eliminates Li2NTs (TsNH2 was observed as a product of this reaction) to yield olefin 270. 

One is the concerted decomposition of a dioxetanone structure that is proposed for the chemiluminescence and bioluminescence of both firefly luciferin (Hopkins et al., 1967 McCapra et al., 1968 Shimomura et al., 1977) and Cypridina luciferin (McCapra and Chang, 1967 Shimomura and Johnson, 1971). The other is the linear decomposition mechanism that has been proposed for the bioluminescence reaction of fireflies by DeLuca and Dempsey (1970), but not substantiated. In the case of the Oplopborus bioluminescence, investigation of the reaction pathway by 180-labeling experiments has shown that one O atom of the product CO2 derives from molecular oxygen, indicating that the dioxetanone pathway takes place in this bioluminescence system as well (Shimomura et al., 1978). It appears that the involvement of a dioxetane intermediate is quite widespread in bioluminescence. 

Bradshaw et al. 67) were the first to propose a reaction pathway that is compatible with a transalkylidenation scheme. They suggested that the reaction proceeds via a quasi-cyclobutane intermediate. Applied to linear alkenes, this is pictured as follows ... 

The reaction of alkoxyarylcarbene complexes with alkynes mainly affords Dotz benzannulated [3C+2S+1C0] cycloadducts. However, uncommon reaction pathways of some alkoxyarylcarbene complexes in their reaction with alkynes leading to indene derivatives in a formal [3C+2S] cycloaddition process have been reported. For example, the reaction of methoxy(2,6-dimethylphenyl)chromium carbene complex with 1,2-diphenylacetylene at 100 °C gives rise to an unusual indene derivative where a sigmatropic 1,5-methyl shift is observed [60]. Moreover, a related (4-hydroxy-2,6-dimethylphenyl)carbene complex reacts in benzene at 100 °C with 3-hexyne to produce an indene derivative. However, the expected Dotz cycloadduct is obtained when the solvent is changed to acetonitrile [61] (Scheme 19). Also, Dotz et al. have shown that the introduction of an isocyanide ligand into the coordination sphere of the metal induces the preferential formation of indene derivatives [62]. 

Lim et al. also investigated HMTA-phenolic reactions with somewhat larger model compounds (e.g., two- and four-ring compounds) and established that similar reaction pathways to those described previously occurred.50 For these model compounds (as opposed to one-ring model compounds), which are more representative of typical oligomeric systems, increased molecular weight favored die formation of hydroxybenzylamines but not benzoxazines. This was suggested to be a steric effect. 

Aminopyridinato ligands form a special class of anionic ligands in which an aromatic ring is part of an amidinate system. These ligands have frequently been employed in early transition metal and lanthanide coordination chemistry. Their diverse and interesting chemistry has been described in detail by Kempe et al. ° and will thus be covered here only briefly. Typical reaction pathways leading to titanium aminopyridinato complexes are outlined in Scheme 169. Metathetical as well as salt-free routes have been developed. 

For both methanol oxidation and formic acid oxidation, a dual-pathway mechanism has been proposed (for methanol oxidation, see Lamy et al. [1983] Jarvi and Stuve [1998] Cuesta [2006] Housmans et al. [2006] Iwasita [2003] for formic acid oxidation, see Parsons and VanderNoot [1988] Sun et al. [1988] Willsau and Heitbaum [1986] Miki et al. [2002] Samjeske and Osawa [2005] Chen et al. [2006a, b, c] Samjeske et al. [2005, 2006] Miki et al. [2004], Chang et al. [1989]), in which one reaction pathway proceeds via formation and subsequent oxidation of COad (P, indirect pathway ), while the other leads, via one or more reaction intermediates RI, directly to CO2 ( direct pathway ) (Fig. 13.8a). 

In the original proposal of the dual-pathway mechanism (for formic acid oxidation, see [Capon and Parsons, 1973a, b, c] for methanol oxidation, see [Parsons and VanderNoot, 1988 Jarvi and Stuve, 1998 Leung and Weaver, 1990 Lopes et al., 1991 Herrero et al., 1994, 1995]), both pathways lead to CO2 as the final product, as illustrated in the reaction scheme depicted in Fig. 13.8a [Jarvi and Smve, 1998]. In this mechanism, desorption of incomplete oxidation products was not included. The existence of a direct reaction pathway for methanol oxidation, following the dual-pathway mechanism, was justified by the observation of a methanol oxidation current at potentials where COad oxidation is not yet active [Sriramulu et al., 1998, 1999 Herrero et al., 1994, 1995]. The validity of this interpretation was questioned, however, by Vielstich and Xia (1995), who claimed that CO2 formation is observed only with the onset of COad oxidation and that the faradaic current measured at lower potentials is due to the formation of the incomplete oxidation products formaldehyde and formic acid. The latter findings were later confirmed by Wang et al. [2001], Korzeniewski and Childers [1998], and Jusys et al. [2001, 2003]. In more... 

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