Über Pterinchemie. 62. Mitteilung. Protonenkatalysierte [1,2]-H-Verschiebung bei der Umlagerung von 6,7-Diphenyl-5,6-dihydropterin zu 6,7-Diphenyl-7,8-dihydropterin

Proton catalysed [1,2]-H-shift in the rearrangement of 6,7-diphenyl-5,6-dihydropterine (I) to 6,7-diphenyl-7,8-dihydropterine (III) The arrangement from I to the thermodynamically more stable III undergoes through a acid catalysed [1,2]-H-shift (intramolecular 6,7-hydride rearrangement) (see Scheme 1).     

Mechanism of 1,2-shift through π-complex transition state

A systematic calculation of the potential curves or surfaces for 1,2-shift has been realized by using MNDO or other models in MOPAC programs. By referring to the previous authors' viewpoints, the 1,2-shift can be divided into two categories. 1,2-electron-deficient shift is that the electronic configuration of the atom which accepts the migrating group is a cation or an electron-deficient atom, and 1,2-anion shift is the one that the accepted atom of the migration group is a negative ion. In terms of the experimental facts and the calculation of the potential surfaces, in electron-deficient shift such as Beckmann or Baeyer-Villiger rearrangement, the migration occurs through a transition complex formed between the 7i-bond and the cation or electron-deficient migrating group, but in anion shift such as Wittig or Stevens rearrangement, the electron pair in it-orbit excites at first to π* orbit, and then the migration occurs through the new formed complex between the anion migration group and the vacant rc     

Intramolecular thermal transformations of N-phthalimidoaziridines: 1,3-dipolar cyclo-addition and rearrangements*

The intramolecular thermal cycloaddition of N-phthalimidoaziridines at multiple bonds of substituents with the intermediate formation of azomethine ylides leads to condensed pyrrole derivatives, in which the five-membered ring is adjacent to a five-, six-, or seven-membered ring. Rearrangements, which sometimes become the predominant reactions, compete with cycloaddition. Thus, aziridines with aryl substituents readily isomerize to give imines with a 1,2-shift of the phthalimide group to one of the carbon atoms. Aziridines with one electron-withdrawing substituent probably do not open to give 1,3-dipoles but rather undergo a Cope-type rearrangement involving the three-membered ring and C = O bond of the second substituent. Even in intramolecular reactions, very low activity is found for the cyano group triple bond and aromatic ring bonds as dipolarophiles.     

Oxidation of benzyl alcohols with difluoro(aryl)-λ-bromane: formation of benzyl fluoromethyl ethers via oxidative rearrangement

Oxidative rearrangement of benzyl alcohols with difluoro(p-trifluoromethylphenyl)-λ³-bromane (5 × 10⁻² M) in chloroform at room temperature afforded aryl fluoromethyl ethers selectively in good yields, probably via 1,2-shift of aryl groups from benzylic carbon to oxygen atoms.     

Generalized structural and kinetic relationships for 1,2-shifts of a hydrogen atom in carbocations and neutral compounds

Dynamic PMR has been used in a kinetic study of degenerate rearrangement of 5,6-X₂-1-H-1, 2-dimethylacenaphthylenonium ions (X=CH₃, Br), accomplished by means of a 1,2-shift of the hydrogen atom. It has been found that the free energy of activation of the 1,2-shift of the H atom in the 5,6-X₂-1-H-1,2-dimethylacenaphthylenonium cation (X=H, CH₃, Br), in the 1-H-1,2-dimethylcyclopentyl cation, and in 5-H-1,2, 3,4,5-pentamethyl-1,3-cyclopentadiene exhibits a linear correlation with parameters characterizing the electron density on the atom to which the migrant is shifted.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1352–1358, June, 1991.     

Acid-Catalyzed Rearrangements of 5,6-exo-Epoxy-7-oxabicyclo[2.2.1]hept-2-yl Derivatives. Migratory Aptitudes of Acyl vs. Alkyl Groups in Wagner-Meerwein Transpositions

In the presence of HSO₃F/Ac₂O in CH₂CL₂, 2-exo- and 2-endo-cyano-5,6-exo-epoxy-7-oxabicyclo[2.2.1]hept-2-yl acetates ( 6a , b ) gave products derived from the epoxide-ring opening and a 1,2-shift of the unsubstituted alkyl group (σ bond C(3)–C(4)). In contrast, under similar conditions, the 5,6-exo-epoxy-7-oxabicyclo[2.2.1]heptan-2-one ( 6c ) gave 5-oxo-2-oxabicyclo[2.2.1]heptane-3,7-diyl diacetates 20 and 21 arising from the 1,2-shift of the acyl group. Acid treatment of 5,6-exo-epoxy-2,2-dimethoxy-7-oxabicyclo[2.2.1]heptane ( 6d ) and of 5,6-exo-epoxy-2,2-bis(benzyloxy)-7-oxabicyclo[2.2.1]heptane ( 6e ) gave minor products arising from epoxide-ring opening and the 1,2-shift of σ bond C(3)–C(4) and major products ( 25 , 29 ) arising from the 1,3-shift of a methoxy and benzyloxy group, respectively. Under similar conditions, 5,6-exo-epoxy-2,2-ethylenedioxy-7-oxabicyclo[2.2.1]heptane ( 6f ) gave 1,1-(ethylenedioxy)-2-(2-furyl)ethyl acetate ( 32 , major) and a minor product 33 , arising from the 1,2-shift of σ bond C(3)–C(4). The following order of migratory aptitudes for 1,2-shifts toward electron-deficient centers has been established: acyl > alkyl > alkyl α-substituted with inductive electron-withdrawing groups. This order is valid for competitive Wagner-Meerwein rearrangements involving equilibria between carbocation intermediates with similar exothermicities.     

Hydrazulene ring systems via heteroatom-assisted [1,2]-shift of oxonium and sulfonium ylides

[reaction: see text] Cyclic mixed acetals with pendant diazoketone side chains undergo rearrangement to ether-bridged cycloheptane ring systems on treatment with Cu(hfacac)(2). Stevens [1,2]-shift of an oxonium ylide furnishes the major product (7), in some cases accompanied by minor amounts of a product (8) resulting from [1,2]-shift of a sulfonium ylide. In the subsequent sulfur-triggered cleavage of the bridging ether, the desired bicyclo[5.3.0]heptene was obtained, along with the product of novel S(N)2' attack on the resulting allylic ketal.     

Rearrangements of 1-(1-methylprop-1-en-1-yl)-1,2-dimethylacenaphtyylenonium ions

It was discovered by means of dynamic NMR that the 1-(cis-1-methylprop-1-en-1-yl)-1,2-dimethylacenaphthylenonium ion generated under conditions of “long life” for carbocations underwent fast (ΔG^#35.8 kJ mol^?1 at ?103°C) degenerate 1,2-shift of the cis-dimethylvinyl group. Quantum-chemical calculations by DFT method predict lower rate of 1,2-shift for the trans-dimethylvinyl group compared to cis-dimethylvinyl group and dependence on the cations conformation of the rates of these processes and of the rearrangement mechanism into the ions of phenalenyl type.     

Reaction of tetracyanoethylene with aldehydes. Synthesis of 6-imino-2,7-dioxabicyclo[3.2.1]octane-4,4,5-tricarbonitriles

It was discovered by means of dynamic NMR that the 1-(cis-1-methylprop-1-en-1-yl)-1,2-dimethyl-acenaphthylenonium ion generated under conditions of “long life” for carbocations underwent fast (ΔG^#35.8 kJ mol^?1 at ?103°C) degenerate 1,2-shift of the cis-dimethylvinyl group. Quantum-chemical calculations by DFT method predict lower rate of 1,2-shift for the trans-dimethylvinyl group compared to cis-dimethylvinyl group and dependence on the cations conformation of the rates of these processes and of the rearrangement mechanism into the ions of phenalenyl type.     

Nucleophilic 1,2-Shifts of Alkoxycarbonyl and Carboxylate Groups in the Benzilic-Acid Type Rearrangement of α,β-Dioxobutyric Esters

tert-Butyl α,β-dioxobutyrate (hydrate; 1d ) undergoes, at medium or high pH, the benzilic-acid rearrangement with exclusive 1,2-shift of the COO(t-Bu) group; the same is true for the corresponding isopropyl ester 1c and ethyl ester 1b at high pH, whereas at lower pH, the overall picture of these reactions is complicated by concurrent hydrolysis of the ester, followed by a 1,2-shift of the COO ^? group. Consequently, the shift of these electron-attracting groups cannot be considered to be systematically disfavoured (compared, e.g., with alkyl-group shifts). Kinetic measurements of the rearrangement show for both esters (as well as for the analogous ethyl ester 1b , and also for ethyl 3-cyclopropyl-α,β-dioxopropionate ( 4 )) a characteristic rate profile: at relatively low pH, k is proportional to [HO^?], approaching saturation with increasing [HO^?] (interpreted as complete transformation of the substrate into the hydrate monoanion), which is followed at higher pH by another rate increase with k proportional to [HO^?] (probably due to the reaction of the hydrate dianion). The similarity of k values for 1b-d shows that in the shift of COOR steric hindrance caused by R is negligible.     

Über die 1,2-Verschiebung der Säureamidgruppe bei der Benzilsäureumlagerung von Chinisatin

Labelling with ¹⁴C shows that the benzilic acid type rearrangement of quinisatine (I) to isatine (III) proceeds essentially by a 1,2-shift of the intact carbonamid group to the carbonyl group at C-4. The reaction rate was determined by polarography.     

Unprecedented double benzylic rearrangement: regio- and stereospecific tandem 1,4-shift and Curtin rearrangement

An α-benzyloxyketone forming part of a strained cyclopentane carbon framework when treated with 10 equiv of anhydrous NaOH in absolute ethanol, for 2 h, affords in a 65% yield a new 2-benzyl-2-hydroxyketone, resulting from an unprecedented double benzylic rearrangement. This new rearrangement could be interpreted as an initial benzylic 1,4-shift between the O-enolate alkoxide of the ketone group and the oxygen atom of the benzyloxy ether, followed by a Curtin type benzylic 1,2-shift. Apart from the novelty and the synthetic application of this transformation it is worth noting the complete regio- and stereoselectivity observed. The structures of both substrate and product have been confirmed by X-ray diffraction studies. A tentative mechanism is herein proposed.     

A quantum-chemical study of alternative chlorotropic isomerization mechanisms for bis(diethylamino)-chlorophosphonium methylide

The MNDO-PM3 method has been used in the supermolecular approximation to examine alternative mechanisms for the chlorotropic isomerization of unsolvated bis(diethylamino)chlorophosphonium methylide and solvated forms of it containing chloroform of 1∶1 and 1∶2 compositions. It is shown that the most favorable channel for the chlorotropic rearrangement of unsolvated P-chloroylide corresponds to the sigmatropic mechanism. There are effects from specific interactions with the chloroform that make themselves felt in the preferential chlorotropic transformation of the isomers by the dissociation-recombination mechanism, whose activation energy E_1∶2 ^≠=kcal/mol is comparable with the observed activation barrier for the 1,2-shift of the chlorine atom in the related P-chloroylide. Institute for Bioorganic Chemistry and Petroleum Chemistry, National Academy of Sciences of Ukraine, 1 Murmanskaya ul., Kiev 253094, Ukraine. Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 35, No. 4, pp. 215–221, July–August, 1999.     

Synthesis and thermal stability of silicon-containing esters of phosphorus acids

A number of trialkylsilylmethyl diphenyl phosphates MeRR′SiCH₂OP(O)(OPh)₂ (1a-e: R=Et (a), Pr (b), CF₃CH₂CH₂ (c, e), Me₃SiCH₂ (d); R′=Me (a-d), Et (e)) were synthesized and their thermal rearrangement, of the 1,2-shift type, was studied. The rearrangement consists of the migration of an alkyl group from Si atom to the methylene carbon atom and gives the corresponding silyl esters. The rate of the rearrangement was found to increase in the order1d<1b<1a<1 (R=R′=Me)<1c corresponding to the enhancement of the total inductive effect (−I) of the substituents at the Si atom. The relative migration ability of the substituents at the Si atom, determined by GC/MS analysis of the disiloxane fraction resulting from hydrolysis of pyrolyzed phosphates1a-e, increases in the order R=Pr<Et<CF₃CH₂CH₂<Me≪Me₃SiCH₂, which differs substantially from the order in which the rate of the rearrangement of phosphates1a-d changes. The electronegativity of the migrating group affects noticeably the relative ability to migrate. For Part 4, see Ref. 1. Deceased. Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 9, pp. 1767–1772, September, 1998.     

Catalyst and ring size effects on periselectivity of oxonium ylide rearrangements

Diazoketones were subjected to carbene-transfer with Rh(II) or Cu(II) catalysts to probe the selectivity for rearrangement via five- or six-membered oxonium ylides. 4,5-Bis(benzyloxy) and 4-allyloxy-5-benzyloxy substrates 3a,b showed a large preference for rearrangement via the five-membered ylide under all conditions. However, a sharp divergence was seen with 5-allyloxy-4-benzyloxy substrate 3c, which underwent predominantly a [2,3]-shift to pyran 5c via the six-membered ylide with Cu(II) catalysis and a [1,2]-shift to furan 4c via the five-membered ylide with Rh(II) catalysis.     

On the Mechanism of the α-Alkynone Cyclization: A Theoretical Study

The acetylene → carbene rearrangement of three model a-alkynones, namely propynal ( 1a ), 2-butynal ( 1b ) and butynone ( 1c ) is studied by ab initio double-zeta, double-zeta plus polarization and double-zeta plus self-consistent electron pairs calculations. Transition states are located by force-gradient geometry optimization. Calculated minimum energy reaction paths reveal substituent effects on the activation parameters, which indicate that a H-atom or an alkyl group competes favorably with an acyl group in the [1,2]-shift from a-alkynones 1 to the corresponding acylvinylidenes 5 .     

Stereochemical memory in the regioselective and diastereoselective rearrangement of tricyclo[3.3.0.0 2,4]octanes (housanes) by electron transfer (1,3-cyclopentanediyl radical cations) and acid (cyclopentyl carbenium ions) and silver-ion catalysis

The electron-transfer-catalyzed rearrangement of the housanes 5 affords regioselectively only the two cyclopentenes 6 (CH(3) migration) and 7 (R migration) by 1,2-migration of the two groups at the methano bridge to the methyl terminus. The 1,2-shift of the CH(3) group prevails, and the rearrangement ratio is essentially insensitive to the migratory aptitude of the R substituent. This stereochemical memory effect derives from the conformational impositions on the stereoelectronic requirements during the 1,2-migration in the 1,3-radical-cation intermediates. Similar regioselectivities and diastereoselectivities are observed for the TFA-catalyzed and silver(I)-ion-promoted rearrangements, whereas the rearrangement catalyzed by HClO(4) affords a complete reversal in the product selectivity and both the regioselectivity and the diastereoselectivity are much reduced. Migration to the phenyl terminus is favored to afford the 6' and 7' cyclopentenes, of which the former (CH(3) migration) dominates. For the minor regioisomer, only the cyclopentene 6 is formed by an exclusive 1,2-shift of the CH(3) group. This dichotomy in product selectivities is rationalized in terms of two distinct mechanisms for the various activation modes: a common one for the electron-transfer-induced, TFA-catalyzed, and silver(I)-ion-promoted rearrangements and a different one for HClO(4).     

A new reaction of vicinal sulfonyliminocarboxylates with phosphites

The reaction of alkyl trifluoro(organylsulfonylimino)propionates with phosphites occurs with NC transfer of the RSO₂ group and leads to sulfonyl-substituted trifluoroalanine derivatives. The novel rearrangement is interpreted as cheletropic 1,4-cycloaddition of the phosphite and subsequent 1,2-shift of the sulfonyl group in the intermediate cyclic phosphorane.     

Allylation of epoxides with allylic indium reagents

Allylindium sesquihalide reacts with epoxide to give homoallyl alcohol via a 1,2-shift reaction. In contrast, allylindate gives the ring-opening product without rearrangement.     

A convenient catalytic oxidative 1,2-shift of arylalkenes for preparation of a-aryl ketones mediated by NaI

Using a catalytic amount of Nal and a stoichiometric oxidant Oxone^@,a convenient procedure has been developed for the catalytic oxidative 1,2-shift of arylalkenes in CH₃CN/H₂O at room temperature,which provides the corresponding α-aryl ketones in moderate to good yields.In this protocol,sodium iodide is first oxidized into hypoiodous acid,which reacts with arylalkene to afford iodohydrin.Then,the iodohydrin is transformed into the α-aryl ketone via an oxidative 1,2-shift rearrangement.