Intermolecular dimerization

The intermolecular dimerization of nitrile oxides has been described as a procedure to prepare Fx with identical substituent both in the 3 and 4 position (Fig. 3). This procedure is a [3 -F 2] cycloaddition where one molecule of nitrile oxide acts as 1,3-dipole and the other as dipolarophile [24-26]. Yu et al. has studied this procedure in terms of theoretical calculus [27,28]. Rearrangement of isocyanates competes with the bimolecular dimerization, with the former becoming dominant at elevated temperatures. 

DFT studies of the intramolecular ene-like (or the so-called 1,3-dipolar ene) reaction between nitrile oxides and alkenes show that this reaction is a three-step process involving a stepwise carbenoid addition of nitrile oxide to form a bicyclic nitroso compound, followed by a retro-ene reaction of the nitrosocyclopropane intermediate. The competitive reactions, either the intramolecular [3 + 2] cycloaddition between nitrile oxides and alkenes or the intermolecular dimerization of nitrile oxides to form furoxans, can overwhelm the intramolecular 1,3-dipolar ene reaction if the tether joining the nitrile oxide and alkene is elongated, or if substituents such as trimethylsilyl are absent (425). 

With the bulky metallo-organic Pd(II) catalyst 98, on the other hand, selective formation of 99 was possible here functional groups are tolerated that would react with an Ag(I) catalyst (for example, terminal alkynes, alkyl chlorides, alkyl bromides and alkyl iodides) [59]. With l,n-diallenyl diketones (100), easily accessible by a bidirectional synthesis, up to 52-membered macrocycles (101) could be prepared in an end-group differentiating intramolecular reaction (Scheme 15.26) [60], For ring sizes lager than 12 only the E-diastereomer is formed overall yields of the macrocydes varied between 17 and 38%. Only with tethers shorter than 11 carbon atoms could the Z-diastereomer of the products be observed, a stereoisomer unknown from the intermolecular dimerization reactions of 96. 

Azetidones (p-lactams) are generally obtained in high yield from (3-halopropion-amides (Table 5.18) and the low yield from the reaction of N-phenyl (3-chloropropi-onamide can be reconciled with the isolation of A-phenyl acrylamide in 58% yield [34]. The unwanted elimination reaction can be obviated by conducting the cyclization in a soliddiquid system under high dilution [35, 36]. Azetidones are also formed by a predominant intramolecular cyclization of intramolecular alkylation to yield aziridones. Aone-pot formation of azetidones in 45-58% yield from the amine and P-bromocarboxylic acid chloride has also been reported [38]. 

The 3,4-dimethylenethiophene (333) has been generated from diazene (332) by either thermolysis or photolysis (310-380 nm). The purple-colored biradical (333) undergoes intramolecular dimerization to (334) at - 78°C (the formation of intermolecular dimers is possible at higher temperatures). However, (333) is quiet stable up to 160 K at frozen glassy solution in 2-methyltetrahydrofuran. 

The intermolecular dimerization of ketone enolates to give 1,4-diketones has been accomplished earlier with cupric6.7 and ferric salts.6 These transition metal salts have also been used to achieve intramolecular carbon-carbon bond formation.7.6.16 However, step C represents the only reported example11 of cyclopropane construction via technology of this type. 

The photoactivated intermolecular dimerization reaction of aziridinyl ketone 18 leading to heterocycle 67 after the initial oxidation of piperazine 66 has also been described [84] (Scheme 1.17). 

Photodimerization of anthracene has frequently been cited as a photochemical switch to create photoresponsive crown ethers. Photoirradiation of 3 in the presence of Li+ gives the photocyclo-isomer 4 [5,6], 4 is fairly stable with Li+ but readily reverts to the open form 3 when Li+ is removed from the ring. In this system, however, intermolecular dimerization may take place competitively... 

In this system, however, intermolecular dimerization may take place competitively with intramolecular dimerization. To rule out this possibility, compound 5, in which two anthracenes are linked by two polyether chains, was synthesized.171 It was found that intramolecular photodimerization proceeds rapidly in the presence of Na+ as the template metal cation. Compound 6 was also synthesized.181 Although this compound has not been applied in a photoswitch system, it displays a remarkable fluorescence change upon binding with RbC104 or H3N+(CH2)7NHj.[81 Yama-shita et al.[9] also synthesized 7, in which intermolecular photodimerization of anthracene is completely suppressed. The photochemically produced cyclic form 8 displayed excellent Na+ selectivity. 

Intermolecular dimerization has also been effected by a comparable protocol.24-26 Treatment of triethylborane with silver nitrate and sodium hydroxide in water at 25°C led to the rapid evolution of M-butane (72%), ethylene (9%), and ethane (9%). Reaction of two different alkylboranes led to statistical mixtures of dimerized and cross-coupled products. Furthermore, this strategy has been used successfully in the synthesis of olefins from dihydroborated internal acetylenes,27 and in polymerizations of bifunctional organoboron compounds.28... 

Many biologically active peptides are cyclic in nature, and the SPS of this class of peptides, exemplified by 2.7, has also received attention with several different strategies for the final cyclization. The phenomenon of pseudodilution on bead in which each resin site is essentially isolated from its neighbors favors the intramolecular cyclization reaction compared to the intermolecular dimerization, which occurs in solution even at high dilutions. The SPS of cyclic peptides has recently been covered in two excellent reviews (31, 32). The technique of cyclative cleavage via the N- or the C-terminus (see Section 1.2.7) has been used, as has anchoring through amino acid side chains with sequential cyclization and peptide release. 

Quantum Studies of Intermolecular Dimer Formation and Intramolecular Dynamics... 

Within the framework of current transport theories, the task of the chemist is to prepare donor molecules in which the molecule has the appropriate oxidation potential, orbital delocalization, and solubility. Further, the effects of dipolar disorder and intermolecular dimer sites must be minimized. Finally, the physical and chemical interactions of the transport and generation materials (whether in a single or dual-layer configuration) must promote efficient charge generation and injection. The systematic integration of all of these characteristics is indeed formidable. 

Certainly, the stability of chemical compounds has many facets. In the case of silenes, germenes, and staimenes it involves among others, the kinetic or thermodynamic stability against intermolecular dimerization or intramolecular migrations. 

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