Terminal self-coupling

Termination steps in SrnI reactions compete with the propagation steps and, although these processes have aroused considerable mechanistic and theoretical speculation (see Chapter 9 in ref. 18), their effects, with several important exceptions, are not significant. For example the self-coupling of aryl radicals (equation 11) does not appear to occur under the conditions used for the SrnI reaction. One potentially disruptive termination step is reduction of the intermediate aryl radical (equation 6). The source of the reducing electron can be a dissolving metal (or solvated electron), one of the radical anion intermediates in the reaction (ArNu- or ArX- ), an electrode, or the nucleophile itself. These termination... 

The reaction is carried out by slowly adding the 1-bromoalkyne to a solution containing the terminal alkyne, amine, copper(i) chloride and hydroxylamine hydrochloride. The amine, usually ethylamine, is used in excess, e.g. 1-8 moles/mole of alkyne, and catalytic quantities (1-5 mol %) of copper(i) chloride are used. One of the side-reactions is the self-coupling of the bromoalkyne induced by Cu(i) which in turn is oxidized to Cu(ii) (equation 11). The hydroxylamine salt serves to reduce the copper back to the cuprous state. 

It is also well established [2,6-8] that in the photoinduced vinyl polymerization promoted by benzophenone, the free radical deriving from the hydrogen donor is active in the initiation step, whereas the semipinacol radical mainly undergoes self-coupling and combination with the growing polymer chains. It is pointed out, however, that this termination reaction may involve both hydrogen transfer and direct combination (Scheme 2). 

Not being aware of the earlier work, the present author first noticed the phenomenon in 1981. Geiger and Huber10 had photolyzed dimethylnitrosamine in the gas phase at 1 Torr and under 100 Torr N2 buffer. This compound fragments from the first excited singlet state into dimethylaminyl radicals and nitrous oxide NO with unity quantum yield, but neither photoproducts nor a decrease of the initial compound pressure were observed. Even after 20 h photolysis the back-reaction was complete to more than 99.9% (Scheme 6). This seemed quite puzzling because sterically unhindered aminyl radicals are transient and readily self-terminate by coupling and disproportionation. 

Recently, Brzozowska et al. used NR and ex situ electrochemical techniques to characterize an innovative type of monolayer system intended to serve as a support for a bUayer lipid membrane on a gold electrode surface [51]. Zr ions were used to noncovalendy couple a phosphate-terminated self-assembled monolayer (SAM) formed on a gold surface to the carboxylate groups of negatively charged phos-phatidylserrne (PS). This tethered surface was then used for the formation of a PS hpid bilayer structure formed by vesicle fusion and spreading. NR studies revealed the presence of an aqueous environment associated with the tether layer which arises from nonstoichiometric water associated with the zirconium phosphate moieties [52]. 

The self-coupling of terminal acetylenes is more severe under dilute conditions and is catalyzed by adventitious molecular oxygen. It was found that the strict exclusion of oxygen, low reaction temperatures, and extended reaction times were optimal. Extensive experimentation established that a 50% molar ratio of catalyst to sequence and 0.002 M sequence concentration in dry triethylamine solution gave the highest yield of the 18-unit tetrafunctional sequence 24. Interestingly, the best mode of addition was to add both sequences 22 and 23 together at the start, rather than attempt a controlled slow addition of one sequence to the other. 

With suitably placed hydroxy and ester funcdonal groups, tandem lactonization can ensue (Scheme 11.91). The intermolecular coupling of two alkynes with this system is also possible (Scheme 11.92), provided that only one alkyne is terminal, and the acceptor alkyne is electron poor, otherwise extensive self-coupling of the alkynes may result. " ... 

A non-ferrous dust cap with chain should be provided to close the end of the filling pipe and protect the thread when not in use. When the filling pipe is not self-draining into the tank a gate valve should be fitted as close as possible to the hose coupling. Where there is any possibility of damage or misuse of the terminal equipment a lockable fill cap should be fitted and, if considered necessary, the terminal enclosed in a lockable protective compartment. 

Even though the rate of radical-radical reaction is determined by diffusion, this docs not mean there is no selectivity in the termination step. As with small radicals (Section 2.5), self-reaction may occur by combination or disproportionation. In some cases, there are multiple pathways for combination and disproportionation. Combination involves the coupling of two radicals (Scheme 5.1). The resulting polymer chain has a molecular weight equal to the sum of the molecular weights of the reactant species. If all chains are formed from initiator-derived radicals, then the combination product will have two initiator-derived ends. Disproportionation involves the transfer of a P-hydrogen from one propagating radical to the other. This results in the formation of two polymer molecules. Both chains have one initiator-derived end. One chain has an unsaturated end, the other has a saturated end (Scheme 5.1). 

A characteristic reaction of free radicals is the bimolecular self-reaction which, in many cases, proceeds at the diffusion-controlled limit or close to it, although the reversible coupling of free radicals in solution to yield diamagnetic dimers has been found to be a common feature of several classes of relatively stable organic radicals. Unfortunatly, only the rate constants for self-termination of (CH3)jCSO (6 x 10 M s at 173 K) and (CH3CH2)2NS0 (1.1 X 10 M s at 163K) have been measured up to date by kinetic ESR spectroscopy and consequently not many mechanistic conclusions can be reached. 

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