Intramolecular dimerization

The linear dimerization of substituted conjugated dienes is difficult, but the Pd-catalyzed intramolecular dimerization reaction of the 1,3,9,11-tetraene 13 gives the 3-propenylidene-4-allylpiperidine derivative 14, which has the 1,3,7-octatriene system. The corresponding 1,3,8,10-tetraene also affords the 3-pro-penylindene-4-allylcyclopentane derivative[18]. 

Johansson MK, Cook RM (2003) Intramolecular dimers a new design strategy for fluorescence-quenched probes. Chemistry 9 3466-3471... 

Packard, B. Z., Toptygin, D. D., Komoriya, A. and Brand, L. (1996). Profluorescent protease substrates intramolecular dimers described by the exciton model. Proc. Natl. Acad. Sci. USA. 93, 11640-11645. 

Intramolecular dimer radical cation and dimer dication formation... 

Just as intermolecular interactions result in dimer radical cation formation from a radical cation and a molecule of starting material, the presence of two chalcogens atoms within a molecule in proximity to one another can result in intramolecular dimer radical cation formation. 

Ditellurole (122) differs from the compounds described in the previous two sections due to its small ring size, combined with the presence of two sp -hybridized carbons (Fig. 45). As a result, formation of the intramolecular dimer radical cation... 

As was mentioned earlier, furoxans are often encountered as unwanted byproducts in nitrile oxide cycloadditions. There are, however, some efforts to exploit this facile C C forming dimerization for synthesis. In one case, an intramolecular hw(nitrile oxide) cycloaddition was used for a synthesis of biotin (322a). More recently, the intramolecular dimerization was employed for the construction of medium- and large-size rings. This was feasible if one of the two nitrile oxide functionahties was relatively hindered and stable (see Section 6.1.4). Unsymmetrical... 

Recently, by using improved nanosecond pulse radiolysis with the monitoring wavelength region from 300 to 1600 nm [44], absorption spectra due to main reactive intermediates such as the intramolecular dimer cation radical in the near-IR wavelength region were clearly observed in the pulse radiolysis of polystyrene in various solutions [47]. For example. Fig. 1 shows the absorption spectra observed in the pulse radiolysis of polystyrene solutions in CH2CI2. 

The transient absorption spectrum assigned to 9 and intramolecular rr-dimer" of 9 was observed immediately after 8-nsec e , and decayed with the formation of new peaks at 480 and 520 nm assigned to Np"2 (Fig. 6). This indicates that intramolecular dimerization in 9 occurs to reversibly form TV-dimeratk = 1.0 x 10 -1.0 x 10 sec In DCE, 9 and rr-dimer disappeared by neutralization with Cl generated by the initial radiolytic processes at = 4.7 x 10 ° sec From the time profile of the transient absorption at 520 nm involving the formation of Np"2 and decay of tt-dimer" of 9" k, = 5.3 x 10 sec is estimated for the reaction. 

Scheme 10 Intramolecular dimer during electron attachment of 14a-e.

Remarkable enhancements of the unimolecular c-t isomerization of c-S with p-MeO and oxidation of S with -MeO are explained by charge-spin separation in such S Unimolecular c-t isomerization of such c-S proceeds with a chain mechanism, while regioselective oxidation occurs in such S because of the spin localization. Cycloreversion of t,c,t-TPCB occurs to give a a-St 2, while the photochemical cycloreversion of TPCB and t,t,t-TPCB gives Tr-St 2 and t-St /t-St pair, respectively. Radical cations of phosphorus compounds (9 and 10 form intramolecular rr-dimer between two Nps from which Np 2 forms. Formation of intermolecular a-dimer of aromatic acetylene (11 - and 12 -) and intramolecular dimer of 13 and diarylmethanoT was observed, and the n = 3 rule is not effective for intramolecular dimer -. 

In the aromatic series, 2,2 -dinitrobiphenyl has been reduced with zinc and potassium hydroxide [88] or with sodium sulfide to give low yields of benzofc]-cinnoline dioxide, an intramolecular, dimeric, nitroso compound [89, 100] (Eq. 51). 

Head-to-tail intramolecular dimerization of nonactivated vinyl groups can also occur in compounds in which the tether is only two atoms long under conditions of flash-vacuum pyrolysis (FVP). Under such conditions strained bridged polycyclic derivatives can be prepared.78 The formation of 18 is the result of intramolecular head-to-tail cycloaddition of the Cope rearrangement intermediate derived from 1,2-divinylacenaphthene.78... 

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. 

Photochemically induced intramolecular dimerizations are known. For example, when irradiated with 254-nm light, 28 gives the housane products 29 and 30 and butadiene gives 31. Both these reactions apparently proceed through biradical intermediates.63... 

The major (almost exclusive) isomer formed is the anti-trans dimer, but two other dimers (anti-cis and syn-trans) are also obtained under a variety of conditions. The analogous intramolecular dimerization of a, >-bis(l -methyl-2,5-diphe-nylsilacyclopentadienyl)alkanes has been also described39. 

Table 8 Quantum Yields for Intramolecular Dimerization and Fluorescence Quantum Yields of 350...

Table 12 Fluorescence Quantum Yield and Quantum Yield for Intramolecular Dimerization...

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. 

Scheme 6.15 Tether length dependence of half-rings (molecular weights relative to PS the intramolecular dimerization of templated standards).

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