Crossover reaction

Therefore, most of the nonoxidative generation methods that have evolved can be viewed as a crossover reaction of sorts whereby one o-QM product is exchanged for another by application of heat. The stereochemistry accruing in the products of these procedures is expectedly subject to thermodynamic control. For example, while exploring a synthetic approach for nomofungin (Fig. 4.2), Funk recently showed that... 

Triblock terpolymers PS-b-PBd-b-P2VP and PBd-b-PS-b-P2VP, where PBd is polybutadiene (mostly 1,2-PBd), were prepared in order to study the microphase separation by transmission electron microscopy, TEM and SAXS. In the first case the triblocks were synthesized by the sequential addition of monomers in THF using s-BuLi as the initiator [26]. For the second type of copolymers, living PBd-b-PS diblocks were prepared in benzene at 40 °C in the presence of a small quantity of THF in order to obtain the desired 1,2-content and to accelerate the crossover reaction as well. DPE was then added to decrease the nucleophilicity of the active centers in order to avoid side reactions with the THF, which in combination with benzene was the solvent of the final step. 

Anionic polymerization and suitable Unking chemistry were employed for the synthesis of 3-arm PCHD-fc-PS star-block copolymers with PCHD either as the inner or the outer block (Scheme 77) [153]. The block copolymers were prepared by sequential addition of monomers. It was shown that the crossover reaction of either PSIi or PCHDLi was efficient and led to well-defined block copolymers. However, in the case of the PCHD-fc-PSLi copolymers, longer polymerization times were needed for long PCHD... 

Scheme 8.6 The radical crossover reaction between alkoxyamine derivatives 38 and 39 [36],...

Figure 8.11 Schematic diagram of the polymer scrambling by the radical crossover reaction of polyester 43 and polyurethane 44 having alkoxyamine units [37].

The exchange in the alkoxyamine-based polymer occurs in a radical process that is tolerant of many functional groups. The exchange process is therefore applicable to polymers with various functional groups. TEMPO-based polyester 43 and polyurethane 44 were synthesized for studies of the scrambling of disparate polymers imder thermodynamic control (Fig. 8.11) [37], Two kinds of TEMPO-based polymers were mixed and heated in a closed system. After 24 hours when the crossover reaction achieved equilibrium, GPC and NMR analyses revealed that they were totally scrambled through bond recombination on the TEMPO units. 

Table 8.2 Changes in molecular weights and molecular weight distributions in radical crossover reactions of 43...

Dynamic formation of graft polymers was synthesized by means of the radical crossover reaction of alkoxyamines by using the complementarity between nitroxide radical and styryl radical (Fig. 8.13) [40]. Copolymer 48 having alkoxyamine units on its side chain was synthesized via atom transfer radical polymerization (ATRP) of TEMPO-based alkoxyamine monomer 47 and MMA at 50°C (Scheme 8.9). The TEMPO-based alkoxyamine-terminated polystyrene 49 was prepared through the conventional nitroxide-mediated free radical polymerization (NMP) procedure [5,41], The mixture of copolymers 48 and 49 was heated in anisole... 

Figure 8.13 Schematic representation for the dynamic formation of graft polymer through radical crossover reactions of alkoxyamine units [32],...

Scheme 8.11 Thermodynamic formation of crosslinked polymer 54 via radical crossover reaction of alkoxyamines in copolymers 52 and 53 [42],...

Figure 8.14 Schematic diagram of the interconversion between diblock copolymer and star-like nanogel throngh the radical crossover reaction of alkoxyamine units [44],...

Otsuka, H. Aotani, K. Higaki, Y Takahara, A. Polymer scrambling Macromolecular radical crossover reaction between the main chains of alkoxyamine-based dynamic covalent polymers. J. Am. Chem. Soc. 2003, 125, 4064 065. 

Higaki, Y Otsuka, H. Takahara, A. Dynamic formation of grafted polymers via radical crossover reaction of alkoxyamines. Macromolecules 2004, 37, 1696-1701. 

Amamoto, Y Higaki, Y Matsuda, Y Otuska, H. Takahara, A. Programmed formation of nanogels via a radical crossover reaction of complementarily reactive diblock copolymers. Chem. Lett. 2007, 36, 1098-1099. 

The initiator used is important for copolymerizations between monomers containing different polymerizing functional groups. Basic differences in the propagating centers (oxonium ion, amide anion, carbocation, etc.) for different types of monomer preclude some copolymerizations. Even when two different monomer types undergo polymerization with similar propagating centers, there may not be complete compatibility in the two crossover reactions. For example, oxonium ions initiate cyclic amine polymerization, but ammonium ions do not initiate cyclic ether polymerization [Kubisa, 1996]. 

In sharp contrast to the result shown in Scheme 4, complex 11 selectively forms 14 in the presence of an equal amount of phenylacetylene and Me2PhSiH under GO pressure regardless of a stoichiometric or catalytic reaction. The fact that almost identical results are obtained in a pair of crossover reactions between different triorganosilyl groups suggests the presence of a pre-equilibrium between the hydrosilane and Rh-Si species (Scheme 6). ... 

Qualitatively, we have observed that the initial crossover reaction from the polybutadienyllithium anion to that of the vinylbenzyl anion (II) (Reaction 1) was much slower than the corresponding reaction involving polyisoprenyllithium anions. This observation was in agreement with the Young and Fetters U.V. - visible analysis (15). 

When an excess of styrene (S) was added to the CH2C(C6H5)2Li active center, DLi, the ensuing crossover reaction followed pseudo first order kinetics 271) ... 

During the crossover reaction of styrene to the DLi active centers, the following equilibria are maintained for the dimeric aggregates ... 

Scheme 16. Application of radical-polar crossover reaction in the total synthesis of ( )-aspidospermidine...

The synthetic procedure for the synthesis of the inverse starblock copolymers is given in Scheme 25. Diblock arms (I) having the living end at the PS chain end were prepared by anionic polymerization with sequential addition of monomers. In order to accelerate the crossover reaction from the PILi to the PSLi chain end a small quantity of THF was added prior the addition of styrene. The living diblock (I) solution was added dropwise to a stoichiometric amount of SiCl4 until two arms are linked to the silane. This step was monitored by SEC and is similar to a titration process. The end point of the titration was determined by the appearance of a small quantity ( 1%) of trimer in the SEC trace. The diblock (I) was selected over the diblock (II) due to the increased steric hindrance of the styryl anion over the isoprenyl anion, which makes easier the control of the incorporation of only two arms into the silane. 

The product mixture is then analyzed. There are two possible outcomes. It can contain nothing other than the two products that were already obtained in the individual experiments. In this case, each substrate would have reacted only with itself. This is possible only for an intramolecular reaction. The product mixture of a crossover experiment could alternatively consist of four compounds. Two of them would not have arisen from the individual experiments. They could have been produced only by crossover reactions between the two components of the mixture. A crossover reaction of this type can only be intermolecular. 

In all the following examples, the targeted double bonds were activated by suitable substituents to increase the efficacy of the desired cyclization mode. For the total synthesis of acutumine (26), an activated a,p-unsaturated ketone 27 was chosen as precursor (Scheme 10) [74, 75], Aryl radical additions to this type of alkenes are known to proceed about ten times faster than to comparable allylic alcohols. In a radical-polar crossover reaction, the spirocyclic product 28 was obtained in the presence of triethylaluminum as promoter and an oxaziridine as hydroxylating agent. The fact that even the efficient hydrogen donor tetrahydrofuran could be used as solvent nicely demonstrates the high efficacy of the cyclization step. 

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