Head to tail product

The toxicity of 3-methylindole has been attributed to methyleneindolenine trapping of nitrogen and sulfur nucleophiles.57 60-62 Likewise, the ene-imine shown in Scheme 7.9 readily reacted with hydroquinone nucleophiles, resulting in head-to-tail products. Shown in Fig. 7.6 is the 13C-NMR spectrum of a 13C-labeled ene-imine generated by reductive activation. The presence of the methylene center of the ene-imine is apparent at 98 ppm, along with starting material at 58 ppm and an internal redox reaction product at 18 ppm. Thus, the reactive ene-imine actually builds up in solution and can be used as a synthetic reagent. 

If the first diradical were to close to form dimer, the head-to-tail product would be obtained. This indeed is the product that is experimentally observed. If, on the other hand, the second diradical were to close, the head-to-head dimer would be produced. Since no head-to-head product is observed, it is logical to postulate that if this diradical were formed, it must decompose to ground state monomers/83 ... 

Cyclic a,P-enones photodimerize via the Tt-state, as in (4.23)426) and (4.24)427). By chosing the appropriate substituents the reactions proceed regio- and stereoselectively. In contrast, pyrimidine bases dimerize from the first excited singlet state to afford head to tail products (4.25) 428). 

In principle, any cycloaddition involving two dissymmetric compounds can give head-to-head or head-to-tail products. These two compounds are usually obtained in nonequal proportions whose ratio cannot be adequately explained by steric or coulombic factors.1 For example, Reaction (5.1) favors the more hindered product. In Reaction (5.2), the product is formed by creating bonds between atoms having the same charge in the starting material. 

The key [2 + 2]-photocycloaddition reaction between 7 and cyclopente-none afforded a mixture of three photoadducts 14, 20, and 21, in which the head-to-tail products 14 and 20 predominated (Scheme 6.4). The... 

In the same manner that various alkenylboron species can be coupled with alkenyl halides, so too can aryl halides be homologated to styrenes (Scheme 26). - In general, such reactions require a Pd catalyst and the presence of a base, such as sodium ethoxide, in order to achieve this type of head to head coupling. In sharp contrast, mainly head to tail products are formed when a weak base, such as triethyl-... 

Photodimerisation of (61) affords the cw,a t/,cw-head-to-tail product (62). The position of the fluoro substituent appears to play some part in the outcome of the reaction since irradiation of the isomer (63) affords the cis,syn,cis head-to-head dimer. Further studies of the influence of fluoro substituents on the dimerisation of the styrylcoumarins (64) have also been reported. ... 

Irradiation of some 1,4-dihydropyridine derivatives leads in some cases to the formation of cw-dimers and in others to oxidation products, and an enhanced regioselectivity has been observed for the [4 + 4] photodimerisation of 9-aminoacridizinium perchlorate (171 R = NH2, X = C104 ) as compared with (171 R = H, X = Br ). The head-to-tail products syn (172) and anti (173) are produced exclusively. Styryldicyanopyrazines undergo a selective topo-chemical photodimerisation in the solid state by a process whose reactivity and stereochemistry are controlled by differences in their molecular stacking. 

Two striking contrasts with the earlier results stand out the best stabilised of the four diradicals 8.68 is not formed to any significant extent (<1%), and the major products 8.74 and 8.75 (95%) are derived from diradicals that would lead to the head-to-tail product 8.72 if they were to close efficiently. The large amount of head-to-head product 8.71 in the cycloadditions appears to be the result of relatively efficient ring closure of the diradical 8.67. The bonds that are formed most easily are between the a position of one carbonyl compound and the f3 position of the other, the opposite of the frontier orbital prediction in Fig. 8.5 for the combination of two alkenes with a Z-substituent. 

The best studied reaction in this class is that between cyclopentenone 8.65 and ethyl vinyl ether 8.76, which again gives four c/.v-fuscd 1 1 adducts, two stereoisomeric head-to-head products 8.81 and two stereoiso-meric head-to-tail products 8.82.1127 The regioselectivity is slightly less than 3 1 in favour of the head-to-tail isomers, and there must have been intermediate diradicals, some or all of 8.77-8.80. 

Unsymmetrical reactants, such as substituted vinyl monomers, react almost exclusively to give what are called head-to-tail products where the substituents occur on alternative carbon atoms ... 

In the additions of acetylenedicarboxylic esters to the (penta-l,2,4-trienyl)phosphonic esters 484 (R = Me), both head-to-head and head-to-tail products are formed. The initial reaction is then followed by 1,5-sigmatropic rearrangements to give isomers of benzylic phosphonic diesters, 485 (R = Me) and 486 (R = Me). The trienyl phosphonic esters 487 simi-larly give rise to aromatic phosphonic diesters 488 . The ester 484 (R = Et, R = H) is reactive towards A-phenylphthalimide at room temperature, but the product is not the expected 489 but rather 490, which aromatized on chromatography over alumina . The same trienyl phosphonic diesters undergo Diels-Alder reactions with sulphur dioxide and yield sulpholene adducts 491 and... 

In the case of cycloalkenes, it is found that the size of the ring is an important factor in product distribution. Photo [2 -I- 2] cycloaddition of cyclohexenone derivatives (4) to carbomethoxy cyclobutene [47], cyclopentene [48], and cyclohexene [49] (see Scheme 5) demonstrates a gradual reversal of regioselectivity from head-to-head to head-to-tail adducts as indicated in Table 2 [50]. This result of head-to-tail products is not consistent with the dipole-dipole interaction theory. Stability of biradical intermediates is suggested to explain the reversed regioselectivity. 

Cycloadditions of oxa-enone 20, 21, and 22 to polarized alkenes proceed regiospecifically, whereas the nonpolarized isobutene yields only moderate regioselectivity with head-to-tail products dominating (Scheme 9) [52], It has been suggested that the enhanced selectivity is due to larger charge polariza-... 

Figure 10. Two head-to-tail products 3 and 28 are normally formed. The head-to-head isomers 29 and 30 have been observed only in water. ...

The classic de Mayo reaction involves the [2 + 2] photocycloaddition of an alkene to the hydrogen-bonded enol tautomer of a P-dicarbonyl compound as exemplified by the formation of 1,5-diketone 9 from pentane-2,4-dione and cyclohexene (vide supra). In addition to alkenes, allenes are also used as the olefinic component. For example, irradiation of a mixture of dimedone and allene results in the formation of 3,3-dimethyl-7-methylenecycloocta-l,5-dione 12 via the cyclobutane intermediate 11, together with the corresponding head-to-tail product 13, which spontaneously dimerizes to the hetero Diels-Alder adduct 14. Diketone 12 is a versatile building block for the preparation of substituted cyclooctadienones and 8-valerolactones. 

Figure 4 General reaction scheme for the [2-1-2] photodimerization reactions of chalcone and dibenzylidene acetone. In each case, the dimerization reaction can potentially give rise to four possible dimers syn head-to-head (product 1), anti head-to-head (product 2), syn head-to-tail (product 4) and anti head-to-tail (product 3).

Another problem, specffically appearing in the field of pericyclic reactivity of substituted systems is the question of the so-called regioselectivity [43-48]. Under this name the remarkable tendency of pericyclic reactions to prefer the formation of one specific product from the set of possible isomers differing in the mutual position of substituents is imderstood. An example of such a situation is the competition between the formation of the so-called head to head or head to tail products in the dimerization of substituted ethenes. 

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