S. Sternhell

Estimation of the chemical shifts of olefinic protons using additive increments—II : The compilation of additive increments for 43 functional groups

Abstract Additive shielding parameters for olefinic protons were derived for 36 functional groups by means of a least squares procedure applied to 4298 chemical shifts. Additional shielding parameters for a further 7 functional groups were obtained by assuming simple additivity. A number of conspicuous exceptions to the calculated values are listed.

Estimation of the chemical shifts of olefinic protons using additive increments—III: Examples of utility in NMR studies and the identification of some structural features responsible for deviations from additivity

Abstract Examples of utilization of the additive shielding parameters for olefinic protons in making NMR assignments are given and modifications of some published data are suggested. The role of ground-state mesomeric effects on chemical shifts in numerous classes of alkenes is stressed. Some structural features associated with significant deviations from results calculated on the basis of the additivity principle are: (i) unusual juxtaposition of remote functional groups, (ii) planar conjugated carbonyl derivatives, (iii) bicyclic olefins and dienes, (iv) vinyl ethers in which the lone pairs on the oxygen are constrained into certain conformations, (v) vinyl iodides.

Mechanochemistry of some hydrocarbons

Abstract Aromatic hydrocarbons (biphenyl, naphthalene, anthracene and phenanthrene) were subjected to ball milling (SPEX® 8000) with approximately ten-fold weight of inorganic materials (alumina or silica). After about 24 h all of the hydrocarbons were converted largely to carbon (graphite), but at intermediate stages disproportionation products (tetralin, phenylcyclohexane, bicyclohexyl, 9,10-dihydroanthracene, 1,2,3,4-tetrahydroanthracene, 1,2,3,4,4a,9,9a,10-octahydroanthracene, 1,2,3,4,5,6,7,8-octahydroanthracene, 9,10-dihydrophenanthrene, 1,2,3,4-tetrahydrophenanthrene, 1,2,3,4,4a,9,9a,10-octahydrophenanthrene, 1,2,3,4,5,6,7,8-octahydrophenanthrene) were also obtained in significant yields.

Studies of Australian soft corals. XXIII the co-occurrence of bicyclogermacrene and lemnacarnol derivatives in Parerythropodium fulvum

Abstract The isolation of oxygenated bicyclogermacrenes, likely precursors of several classes of sesquiterpenes, is reported from the soft coral Parerythropodium fulvum .

NMR studies of bond-orders

Allyliclorfho-benzylic interproton coupling constants involving a methyl group, i.e. 4J~e<--<-~ henceforth designated as JOB. correlate well* with mobile bond-orders calculated in a number of ways in a representative series of hydrocarbons. Moreover, JOB is essentially independent of ring size and varies little with substituents in the conjugated systems studied2. Thus JOB appears to be a useful parameter for the study of mobile bond-orders and this work is concerned with applications to simple heterocyclic systems, biphenylene, 1,6-rnethano[l Olannulene, azulene, quinones, the Mills-Nixon effect, fulvenes, porphyrins, sterically distorted benzenes, and electronically distorted benzene derivatives and heteroaromatics. INTRODUCTION We have recently1 established that in a basic series of hydrocarbons including propanic single bonds of fixed geometry, cycloalkenes, benzene and 15 derivatives of simple polynuclear aromatic systems containing only six-membered rings, JOB correlates well with either the Pauling bond-orders or the squares of the SCF MO bond-orders. While it is easy to use JOB as a semiquantitative measure of bond-orders in other systems, we wish to show in this preliminary study that interesting insights into electron distributions in conjugated systems can be obtained by the JOB method without resorting to use of equations obtained from the above correlations. HETEROAROMATIC SYSTEMS JOB values for 20 methyl derivatives of simple heteroaromatic systems (furan, thiophene, pyridine, quinoline, pyridine oxide, pyrazole, pyridazine, pyrimidine, 2-pyridone and indole) correlate reasonably well with the squares of the relevant mobile bond-orders calculated by the SCF MO method, which however appears to overestimate delocalization in furan and underestimate delocalisation in 2-pyridone. SOME POLYNUCLEAR AROMATICS The values for JOB in biphenylene shown in structure (1) clearly confirm the radialene-like electron distribution. Similarly the values for JOB in azulene (2) are in accord with the accepted redistribution of r-electrons from the seven-membered ring to the five-membered ring. However the apparently equal bond-orders of C2<3 and C3-C4 bonds in 1,6-methano[lO]annulene (3) is contrary to expectations from the relevant bond lengths3* QUINONES The JOB values in 11 derivatives of orfhoand para-benzoquinones correlate well with the SCF MO bond-orders, but more significant is the insight

Line widths of nuclear magnetic resonance signals of tertiary methyl groups

The widths at half-height (WH) of the NMR signals due to tertiary methyl groups in a number of compounds of known geometry are correlated with spin-spin interactions across four single bonds and can be used for some stereochemical assignments. The mass spectra of some steroids, including a methyl migration, are discussed.

Properties of 2-arylnorbornene oxides and of the dimer formed by dehydration of 2-P-anisylnorbornane-2,3-cis-exo-diol

Abstract 2-Phenylnorbornene-2,3- exo -oxide and its p -chloro analog have been prepared. A thorough NMR analysis of the former compound in conjunction with its facile epoxide opening reactions prompted reexamination of an earlier structural assignment of the p -methoxy analog to the product formed from acid catalyzed dehydration of 2- p -anisylnorbornane-2,3- cis - exo -diol. The product was found to be a p -dioxane, a dimer of molecular weight 432. The 100 MHz NMR spectrum of the dimer recorded at 29° shows that a considerable barrier exists to rotation of the p -anisyl group about the bond joining it to the norbornane skeleton. Variable temperature NMR studies on a sample substituted with deuterium at the positions ortho to the methoxy group gave ΔG 383 * = 19·6 kcal/mole for this process.

Electrophilic metal-alkyl bond cleavage in tetraorganosilicon and tetraorganotin compounds by lead tetracarboxylates and aryllead tricarboxylates

Abstract Methyl-silicon bond cleavage of tetramethylsilane occurs with aryllead(IV) tricarboxylates in trifluoroacetic acid to produce aryl(methyl)lead(IV) bistrifluoroacetates in high yield. An extension of the study to tetraalkylstannanes has shown that alkyl-tin cleavage by lead(IV) carboxylates proceeds even in chloroform, while a detailed study of the reactions of lead tetraacetate with tetraalkylstannanes indicated the reaction to be an electrophilic cleavage rather than one involving an electron transfer mechanism.

Electrocyclic reactions. Part X. Photochemical cyclisation of trans,trans-dibenzylideneacetone

Photolysis of trans,trans-dibenzylideneacetone (1,5-diphenylpenta-1,4-dien-3-one) in benzene solution at ca. 300 nm gave, as the major product by head-to-head staggered [π2s+π2s] cycloaddition, the truxinic-type dimer (IV), whose structure has been established by both chemical and physical methods. As minor products, four stereoisomeric trimers have been isolated; these appear to be derivatives of tri(cyclobuta)[a,d,g]nonane-3,6,9-trione (XII), and their structures and modes of formation are discussed.

The photochemistry of 2-acetoxynaphthalen-1(2H)-ones

The photochemistry of the 2-acetoxynaphthalen-1(2H)-ones (4), (13), (20), and (26) has been examined. In each case there was rapid formation of a mixture of products, which arose either by homolytic dissociation of the C–OAc bond or by rearrangement involving acetoxy group migration. Two types of rearrangement were observed. The first involved 1,2-acetoxy group migration with C(2)–C(4) bond formation to give the dihydrocyclopropindenones (7), (15), (22), and (28). This reaction, which is stereospecific, yields the endo-6-acetoxy isomer only, and it is thought to proceed by a concerted non-ionic pathway. The second rearrangement, which is believed to have an ionic mechanism, resulted in formation of 3-acetoxynaphthols from naphthalenones (4) and (26), and 4-acetoxynaphthols from naphthalenones (13) and (20), in which the 3-position is substituted.