Hydride transfer reactions, multiple reaction

ROP of p-lactones is highly prone to numerous side reactions, such as transester-fication, chain-transfer or multiple hydrogen transfer reactions (proton or hydride). Specifically, the latter often causes unwanted functionalities such as crotonate and results in loss over molecular weight control. Above all, backbiting decreases chain length, yielding macrocyclic structures. All these undesired influences are dependent on the reaction conditions such as applied initiator or catalyst, temperature, solvent, or concentration. The easiest way to suppress these side reactions is the coordination of the reactive group to a Lewis acid in conjunction with mild conditions [71]. p-BL can be polymerized cationically and enzymatically but, due to the mentioned facts, the coordinative insertion mechanism is the most favorable. Whereas cationic and enzymatic mechanisms share common mechanistic characteristics, the latter method offers not only the possibility to influence... 

Supercritical isobutane can provide complete activity recovery from partially deactivated USY catalysts. It can also be applied multiple times to maintain high levels of catalyst activity when the alkylation reaction is performed utilizing commercially available refinery blends. Pressure, temperature, and regeneration time played important roles in the SC isobutane regeneration process because of their effect on solute solubility, diffiisivity, surface desorption, hydride transfer rates, and coke aging. As a consequence, regeneration effectiveness may be maximized by manipulating those variables. 

But before 1980, the foundations for essentially all modern synthetic radical reactions had been laid, sometimes by synthetic organic chemists but more often by physical organic chemists. Kharasch reactions (now often called atom transfer reactions) were known since the 1930s and 1940s, and tributyltin hydride was introduced in the 1960s. In the 1970s, SnAc reactions and redox chain aromatic substitutions (Minisci reactions) were already topical, and allylations with allyl-tributylstannane were first described. In short, there were a number of ways to generate and trap radicals on the one hand, and a number of fundamental transformations of radicals such as addition and cyclization to multiple bonds on the... 

Each of the syntheses of seychellene summarized in Scheme 20 illustrates one of the two important methods for generating vinyl radicals. In the more common method, the cyclization of vinyl bromide (34) provides tricycle (35).93 Because of the strength of sjp- bonds to carbon, the only generally useful precursors of vinyl radicals in this standard tin hydride approach are bromides and iodides. Most vinyl radicals invert rapidly, and therefore the stereochemistry of the radical precursor is not important. The second method, illustrated by the conversion of (36) to (37),94 generates vinyl radicals by the addition of the tin radical to an alkyne.95-98 The overall transformation is a hydrostannylation, but a radical cyclization occurs between the addition of the stannyl radical and the hydrogen transfer. Concentration may be important in these reactions because direct hydrostannylation of die alkyne can compete with cyclization. Stork has demonstrated that the reversibility of the stannyl radical addition step confers great power on this method.93 For example, in the conversion of (38) to (39), the stannyl radical probably adds reversibly to all of the multiple bond sites. However, the radicals that are produced by additions to the alkene, or to the internal carbon of the alkyne, have no favorable cyclization pathways. Thus, all the product (39) derives from addition to the terminal alkyne carbon. Even when cyclic products might be derived from addition to the alkene, followed by cyclization to the alkyne, they often are not found because 0-stannyl alkyl radicals revert to alkenes so rapidly that they do not close. 

Early examples of intramolecular aryl radical addition reactions to heteroatom containing multiple bonds included cyclizations on N=N and C=S moieties [52, 53]. Recently, cyclizations to imines have been used as part of a new enantio-selective approach to indolines (Scheme 8). In the first step of the sequence, the required ketimines 19 were obtained by phase-transfer catalyzed alkylation of 2-bromobenzyl bromides 20 with glycinyl imines 21 in the presence of a cincho-nidinium salt [54], Due to the favorable substitution pattern on the imine moiety of 19, the tributyltin hydride mediated radical cyclization to 22 occurred exclusively in the 5-exo mode. The indoline synthesis can therefore also be classified as a radical amination. 

A transition metal catalyst has also been used to effect the reductive alkylation of amino groups on proteins [41], This reaction uses [Cp Ir(4-4 -dimethoxybipy)(H20)]S04 31 as a mild transfer hydrogenation catalyst and formate ion as the stoichiometric hydride source, in Fig. 10.3-11 (a). Presumably, this reaction occurs via the reversible formation of imine 33 with free amino groups on the protein surface, followed by reduction of iridium hydride 32. For most proteins, multiple modifications are observed (Fig. 10.3-ll(b)), although the overall level of conversion can be altered through variation of either the reaction temperature or the concentrations of the aldehyde and catalyst. In general, the reaction has shown excellent reliability for protein alkylation between pH 5 and 7.4. 

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