Dimerization head-tail

The ligand group can be introduced either on the meso or on the /5-pyrrole position of the porphyrin ring, but the synthesis of the meso-functionalized derivatives is easier and has been more widely exploited. Balch (50-53) reported that the insertion of trivalent ions such as Fe(III) (32) and Mn(III) (33) into octaethyl porphyrins functionalized at one meso position with a hydroxy group (oxophlorins) leads to the formation of a dimeric head-to-tail complex in solution (Fig. 11a) (50,51). An X-ray crystal structure was obtained for the analogous In(III) complex (34), and this confirmed the head-to-tail geometry that the authors inferred for the other dimers in solution (53) (Fig. lib). The dimers are stable in chloroform but open on addition of protic acids or pyridine (52). The Fe(III) octaethyloxophlorin dimer (52) is easily oxidized by silver salts. The one-electron oxidation is more favorable than for the corresponding monomer or p-oxo dimer, presumably because of the close interaction of the 7r-systems in the self-assembled dimer. 

On one-electron rednction, aldehydes and ketones give anion-radicals. It is the carbonyl group that serves as a reservoir for the unpaired electron Ketones yield pinacols exclusively. Thus, acetophenone forms 2,3-diphenylbutan-2,3-diol as a result of electrolysis at the potential of the first one-electron transfer wave (nonaqueous acetonitrile as a solvent with tetraalkylammonium perchlorate as a supporting electrolyte) (van Tilborg and Smit 1977). In contrast, calculations have shown that the spin densities on the carbonyl group and in the para position of the benzene ring are equal (Mendkovich et al. 1991). This means that one should wait for the formation of three types of dimer products head-to-head, tail-to-tail, and head-to-tail (cf. Section 3.2.1). For the anion-radical of acetophenone, all of the three possible dimers are depicted in Scheme 5.21. 

In most cases, none ofthe following were observed ( )-head-to-head dimers, head-to-tail dimers, or trimers. High regio- and stereo-selectivities set this method apart from Rh(I)-catalyzed dimerization methods. 

Irradiation of powdered complexes of benzylidene acetophenone 43 included in the nonchiral 2 yields a single photoproduct, syn head-tail dimer 44 (Scheme 27) [167,168]. Irradiation of 43 in solution gives a mixture of cis... 

There are several reports wherein head-head dimers were not formed preferentially in micellar solutions. For example, the syn head-tail simers, 61, the same ones obtained in solution formed when a number of 7-al-koxycoumarins (60) (alkoxy group chain length was varied between methoxy to octadecyloxy) were irradiated in SDS and HDTC1 micelles [194,195], The same head-tail dimers, obtained in solution, were formed when 9-methyl-anthracene and 9-anthracenecarboxylic acid were irradiated in CTAB and... 

Figure 32. Head-head and head-to-tail orientation of photodimerizable molecules in hexatic BS phase. Head-head dimer results from reactions between molecules present within the same layer whereas head-tail dimer results from molecules present in different layers. [Reproduced with permission from V. Ramesh and R. G. Weiss, J. Org. Chem. 51, 2535 (1986).]...

For instance, head-head photodimers are predicted from the crystal structures of 9-cyanoanthracene and 9-anthraldehyde, but the head-tail isomer is produced. Craig and Sarti-Fantoni and later others found that photoreactions of 9-cyanoanthracene and 9-anthraldehyde take place at defect sites [96,215], Systematic photochemical and crystallographic studies by Schmidt and co-workers uncovered many cases of substituted anthracenes which behave in an unexpected fashion (Scheme 40) [216,217]. Examples shown in Scheme 40 clearly illustrate that, unlike cinnamic acid derivatives, the stereochemistry of the product dimer from anthracenes cannot be predicted on the basis of crystal packing. An example from the laboratories of Venkatesan is noteworthy in this context [218], Irradiation of crystals of 7-... 

Keywords 1,1-diarylethylene, HC1, catalysis, linear dimerization, head-to-tail, waste free, gas-solid reaction, 1,1,3,3-tetraaryl-l-butene... 

The important points that emerge from the studies on the five chlorocoumarins in the solid state are the following, (a) All chloro substituted coumarins (7-chloro, 6-chloro, 4-chloro, 4-methyl 7-chloro and 4-methyl 6-chloro coumarins) undergo dimerization in the solid state. While four of these give syn head-head dimer as the photoproduct, 4-chlorocoumarin gives a mixture of anti-head-tail and syn head-tail in poor yields ( 25%). (b) Two of them namely 4-... 

NMR spectrum is suggestive of a head-to-head, tail-to-tail linked dimer, revealing two NMe (8 2.19, 2.62), three OMe, and 10 aromatic H, including two at higher field [5.84, 6.01 (H-8 and H-8 )]. The mass spectrum shows mile 198 [base, cleavage at a (doubly benzylic)], indicating that the OH lies in the lower portion of the molecule. 

If sterol content and conformation are so important for membrane stability, we should study the biosynthesis of sterols (Figure 3). The first enzyme in terpenoid biosynthesis is the 3-Hydroxy-3-Methyl-Glutary1-Coenzyme A-reductase (HMG-CoA-reductase) that catalyzes the synthesis of mevalonate. Two phosphorylations and decarboxylation of mevalonate lead to isopentenylpyrophosphate, the basic C -unit in sterol synthesis. Isopentenylpyrophosphate reacts with its isomer, the dimethylally1-pyrophosphate, in a head/tail-reaction to geranyl-pyrophosphate reaction with another C -unit leads to farnesyl-pyro-phosphate, that dimerizes in a tail/tail-reaction to squalene. After expoxidation of its A -double bond, squalene cyclizes to lano-... 

Cathodic coupling of a, jS unsaturated carbonyl compounds yields three types of dimers head-to-head (112, Eq. (189) ), tail-to-tail (113) and head-to-tail hydrodimers (114). 

Scheme 3. Dipolar species tend to dimerize head to tail.

Rapid formation of polyethylene Catalytic dimerization (head-to-tail) at 80 °C [29]... 

Unexpectedly, the vicinyl dihalides react differently (18). For example, an equimolar mixture of styrene and 1,2-dibromoethane reacts on lithium metal yielding a head-to-head, tail-to-tail polystyrene, ethylene and lithium bromide. Apparently, the adsorbed styrene is reduced and dimerized to the dianions. 

Condensation Products. In a condensation reaction, the aquo complex dimerizes to give the respective head-tail Pt dimers with N3,04 bridged uracil (thymine) among other products. 

A similar sequence of reactions takes place when sodium metal replaces the alloy, but then tetrameric dianions, instead of dimeric ones, are the main products of that process. Apparently, the slowness of reduction by sodium metal is responsible for this result. The tetramer was described in earlier papers as a tail-to-tail, head-to-head, tail-to-tail bonded oligomer84). Subsequent NMR studies85 have conclusively shown that they result from the addition of two monomeric molecules to the dimeric dianions, i.e. 

General Considerations. In the well-known photodimerization of anthracenes in liquid solution 9, lO /lO, 9 -dimers (head-to-tail [4 + 4]) are formed in most cases. However, there have also been instances where head-to-head-photodimers (9,9 /10,10 ) are produced [19], and these were overseen previously. The solid phase photoreactions of anthracenes charged the topochemical postulate [7] for decades with hitherto unsolvable difficulties. All examples that contradict this assumption were eliminated without hesitation from the scope of topochemistry and termed to be crystal defect reactions, because the topochemically allowed processes were taken as support for topochemistry without further proof. The later provision, that the dimerizations occur within reaction cavities in the bulk of the crystal [20], did not help in this respect. A summary of the various arguments is given in Ref. 8. From examples 7 to 8 only b and perhaps c formally meet... 

Figure 2. Schematic models of gramicidin A dimers (A) head-to-head helical dimer (B) tail-to-tail helical dimer (C) antiparallel double-stranded helix (D) parallel double-stranded helix. (Reproduced with permission from reference 301. Copyright 1986 Biophysical Society.)...

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