Reaction with probe inserts

The quenching of a triplet carbene reaction with methanol is frequently used as the standard means of probing the singlet-triplet gap. It is widely believed that singlet carbenes insert readily into the O—H bonds of methanol, while the triplet states undergo hydrogen abstraction from the C—H bonds." The behavior of diarylcar-... 

Carbene generation from photolysis of diazirine compounds leads to efficient insertion into C—H or N—H bonds and also causes addition reactions with points of unsaturation within target molecules. Diazirine-containing photoaffinity probes have... 

A potential O-atom donor molecule, N2O, has been allowed to react with SiH+ to probe the formation of HSiO+, a higher energy isomer of SiOH+. The expectations were fulfilled (equation 13)43, as tested by reaction with a base capable of deprotonating HSiO+ but not SiOH+. The formal O-atom insertion product, SiOH+, was ascribed to a proton shuttle mechanism which is dominant when OX is SO2 and CO2 (equation 14). In fact, with these two neutrals the formation of HSiO+ and X as bare species is somewhat endothermic and the proton transfer process within the ion-neutral complex, driven by the stability of SiOH+, is allowed by the proton affinity (PA) of X which is intermediate between those of the Si and O sites of SiO. On the other hand, when O2 is used as the neutral reagent this pathway is not accessible and HSi02+ is the only observed product ion. 

More information can be gained on the mechanism of the reaction if two separate experiments can be carried out with the mechanistic probe inserted at two different sites on the reagents. If we are studying a reaction between a nucleophile and an electrophile, it may be possible to make Hammett plots from the variation of substituents on both reagents. The acylation of amines with acid chlorides is an example. 

Oxidative aminations of olefins with sulfonamides also appear to occur by insertions of alkenes into Pd-N bonds in some cases. An example of the use of stereochemistry to probe for syn or anti addition of palladium and nitrogen aaoss an alkene during oxidative amination is shown in Equation 9.89b. The stereochemistry of the alkene unit indicates that the reactions occur by insertions of olefins into Pd-N bonds. As shown in Equation 9.89b, frie E-olefin containing a pendant sulfonamide would form the E-product if syn addition of the palladium and sulfonamido group occurs during the catalytic process, but it would form the Z-product if anti addition occurred. The product from syn addition is formed, and such syn stereochemistry is consistent with migratory insertion of the olefin into a palladium-sulfonamido intermediate. ... 

The stereochemistry of iron-carbon bond cleavage by electrophilic attack by mercury(n) chloride or bromine, or by carbon monoxide or sulphur dioxide insertion, has been probed by n.m.r. studies of products derived from /Areo-[Mc3C CHD-CHD-Fe(CO)2(A -C5H5)]. Predominant retention occurs in the reaction with mercury(n) chloride, predominant inversion in the reactions with sulphur dioxide or bromine. It is this last result which is of the most interest, for it affects discussions of the stereochemical course of oxidative additions... 

The conversion of LLA was monitored by following the disappearance of the 1240 cm peak associated with the asymmetric C—O—C stretch of LLA. This was accomplished by using an ATR-FTIR probe inserted into the reaction flask. The reaction was stopped by removing from the heat and dissolving the resulting polymer product in chloroform. Reactions 2 and 4 (Table 1) displayed relatively rapid polymerization kinetics and were reacted to high conversion (80%). Reactions 1 and 3 were slower and arbitrarily terminated after 600 min ( 22% conversion). GPC and H-NMR spectroscopy were used to elucidate the structure of PLLA-b-PEOETA-6-PLLA triblock copolymers. Peak integration indicated a terephthalate/adipate ratio of 1.05. 

Instead of immobilizing the antibody onto the transducer, it is possible to use a bare (amperometric or potentiometric) electrode for probing enzyme immunoassay reactions (42). In this case, the content of the immunoassay reaction vessel is injected to an appropriate flow system containing an electrochemical detector, or the electrode can be inserted into the reaction vessel. Remarkably low (femtomolar) detection limits have been reported in connection with the use of the alkaline phosphatase label (43,44). This enzyme catalyzes the hydrolysis of phosphate esters to liberate easily oxidizable phenolic products. 

P 60] The dehydration of 1-hexanol to hexane and of ethanol to ethane were conducted at 155 °C. Heating was accomplished by a heating wire inserted in the micro reactor s top plate. This wire was connected to a potentiostat (0-270 V) temperature was monitored by a digital thermometer with the probe close to the reaction channel. A syringe pump was applied for liquid transport [19]. A flow rate of 3 pi min was applied. The alcohols were purged with nitrogen directly prior to reaction to minimize coke formation. 

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