Skeletal transformations

Fluonnation and skeletal transformation of fluorinated cycloalkanes occurs in the reaction with antimony pentafluoride at high temperature [777] In the case of perfluorinated benzocyclobutanes, an unexpected alicyclic ring cleavage has been observed Perfluorinated alkyl benzocyclobutanes, when treated with antimony pentafluoride, ean be converted to perfluorinated styrenes and then transformed to perfluorinated indans [77S, 779]... 

In the following section, we give a survey of the known types of heteroboranes in question and discuss the structures in terms of the Wade-Williams rules. Readers, who are interested in the synthesis of heteroatom clusters, in their skeletal transformations, or in reactions at the ligand sphere, are referred to the cited literature. conjuncto- Boranes with heteroatoms in the skeleton are not considered in this brief discussion. 

The addition of arylsulfenyl chlorides under doping conditions has also been investigated127 with other compounds of this series where structural features did not permit lactone ring closure and therefore allowed other skeletal transformations. 

Sometimes transformation of aromatic componnds into ion-radicals leads to stereochemically unusual forms. Octamethylnaphthalene is a nonplanar molecule twisted around the bond that is common for the two six-membered rings. The nitrosonium oxidation results in the formation of the cation-radical with the centrosymmetric flatten chairlike geometry (Rosokha and Kochi 2006). According to the authors, such a skeletal transformation improves the overall planarity of octamethylnaphthalene. For example, the mean deviation of the carbon atoms in the naphthalene core for the flatten chairlike cation-radical (0.007 nm) is less than half of the corresponding value for the neutral twisted parent (0.016 nm). Within this flatten carcass of the anion-radical, the spin density can be delocalized more effectively. 

Scheme 76. Skeletal transformations of the Diels-Alder adducts 62 e, 264 of methyl 2-chloro-2-cyclopropylideneacetate (1-Me) [7m, 19b]...

There are thus two classes of acids on surfaces of metal oxides Lewis acids and Brdnsted acids (which are also termed proton acids). The weight of evidence (1-8) shows that strong Brpnsted acids are the primary seat of catalytic activity for skeletal transformations of hydrocarbons. In the solids under review, they consist of protons associated with surface anions. 

On the other hand, Lewis acids are catalytically inactive for skeletal transformations unless proton donors are available at the same locality.1 Lewis acids consist of incompletely coordinated surface ions aluminum ion is the most frequently cited example. Since the relevance of acidity measurements for the prediction of catalytic activity is what we are trying to emphasize, we have concentrated on the determination of Brpnsted acidity in this critique. The problem of finding the most relevant method for acidity measurement has therefore been treated as an evolutionary process in which successive methods have been used more and more successfully for the characterization of a relatively small number of strong Br0nsted acids that are frequently accompanied by a multitude of other surface acids. 

The work of Misono et al. (55) illustrates how acid strength distributions for silica-alumina catalyst can be deduced from catalytic titration measurements by use of an appropriate series of reactants. Surface concentration of amine, pyridine in this case, was adjusted by proper choice of amine partial pressure and desorption temperature while carrier gas flowed over the catalyst sample. At each level of chemisorbed pyridine, pulses of the reactants were passed over silica-alumina at 200°C and the products analyzed. The reactants were t-butylbenzene, diisobutylene, butenes, and f-butanol. It was concluded that skeletal transformations require the presence of very strong acid sites, that double-bond isomerization occurs over moderately strong acid sites, and that alcohol dehydration can occur on weak acid sites. 

Despite the presence of sites that strongly chemisorb a variety of molecules, pure silica gel is catalytically inactive for skeletal transformations of hydrocarbons. However, as has recently been emphasized by West et al. (79), only trace amounts of acid-producing impurities such as aluminum need be present in pure silica gel to provide catalytic activity— especially when a facile reaction such as olefin isomerization is used as a test reaction. They found that addition of 0.012% Al to silica gel resulted in a 10,000-fold increase in the rate of hexene-1 isomerization at 100°C over the pure gel. An earlier study by Tamele et al. (22) showed that introduction of 0.01% wt Al in silica gel produces a 40-fold increase in cumene conversion when this hydrocarbon is cracked at 500°C. The more highly acidic solids that are formed when substantial concentrations of metal oxides are incorporated with silica are discussed in following sections. 

It is generally admitted that skeletal transformations of hydrocarbons are catalyzed by protonic sites only. Indeed good correlations were obtained between the concentration of Bronsted acid sites and the rate of various reactions, e g. cumene dealkylation, xylene isomerization, toluene and ethylbenzene disproportionation and n-hexane cracking10 12 On the other hand, it was never demonstrated that isolated Lewis acid sites could be active for these reactions. However, it is well known that Lewis acid sites located in the vicinity of protonic sites can increase the strength (hence the activity) of these latter sites, this effect being comparable to the one observed in the formation of superacid solutions. Protonic sites are also active for non skeletal transformations of hydrocarbons e g. cis trans and double bond shift isomerization of alkenes and for many transformations of functional compounds e.g. rearrangement of functionalized saturated systems, of arenes, electrophilic substitution of arenes and heteroarenes (alkylation, acylation, nitration, etc ), hydration and dehydration etc. However, many of these transformations are more complex with simultaneously reactions on the acid and on the base sites of the solid... 

N. Calderon, H. Y. Chen, and K. W. Scott, Olefin Metathesis - A Novel Reaction for Skeletal Transformations of Unsaturated Hydrocarbons, Tetrahedron Lett., 1967, 3327-3329. 

An ene reaction is a concerted intramolecular rearrangement in which a hydrogen atom transfer to a Ti-bond center is accompanied simultaneously by a 7i-bond development, a Ji-bond migration, and a decomposition of the reactant into two unsaturated molecular fragments. The general skeletal transformation is illustrated below... 

Calderon, N., Chen, H. Y., Scott, K. W. Olefin metathesis, a novel reaction for skeletal transformations of unsaturated hydrocarbons. Tetrahedron Lett. 1967, 3327-3329. 

Mention has already been made of the publication of details of the extensive work by Scott and co-workers on skeletal transformations and interconversions in the Aspidosperma-Iboga series. Other transformations include a rearrangement of... 

Enyne metathesis. A profound skeletal transformation results in the formation of unusual ring systems. 

Cluster skeleton rearrangements which can be studied by variable temperature NMR spectroscopy are of comparatively low energy and are often characterized by the presence of weak metal-metal interactions within the cluster core or a low-energy pathway between two polyhedra with similar relative energies. Although these low energy skeletal transformations are not the main subject of this article, some key examples will be briefly described. 

Ru4(CO)i3( -PR2)2 novel small molecule, ligand and skeletal transformations... 

An analogue is normally accepted as being that modification which brings about a carbon-skeletal transformation or substituent synthesis. Examples oxytetracycline, demclocycline, chlortetracycline, trans-diethylstilbesterol with regard to oestradiol. 

Suzuki, H. Sakai, N. Iwahara, R. Fujiwaka, T. Satoh, M. Kakehi, A. Konakahara, T. Novel synthesis of 7-fluoro-8-(trifluoromethyl)-l/f-l,6-naphthyridin-4-one derivatives intermolecular cyclization of an A-silyl-l-azaallyl anion with perfluoroalkene and subsequent intramolecular skeletal transformation of the resulting pentasubstituted pyridines. J. Org. Chem. 2007, 72(15),5878-5881. 

The rate of reactions occurring through heterogeneous acid catalysis is obviously determined by the characteristics of the acid sites (i.e. their number, nature, strength and density). For skeletal transformations of hydrocarbons (and cokipg is obviously of this type) the rate depends essentially on the protonic acidity of the catalysts [10]. The Lewis acid sites alone do not seem to be active. However, when located in the vicinity of protonic sites they can increase their strength and consequently their activity [11]. Therefore for skeletal transformations of hydrocarbons the C/P ratio must depend only on the strength and on the density of the protonic sites. 

The chemical conversion of enmein (62) into enmelol (58) (Scheme 29) constitutes a retrobiogenetic-type conversion 114). In this conversion, the acyloin reaction was used for the skeletal transformation, just as in the conversion of enmein (62) to Az/-kaurane (il2). Thus enmein (62) was converted into compound (145) by acyloin reaction of lactone ester (144). Compound (145) was further transformed into enmelol (58) by the route shown in Scheme 29. 

Oehlschlager, a. C. and G. Ourisson, A comparison of in vivo and in vitro skeletal transformations of diterpenes, in Terpenoids... 

---