Bond formation, transannular

The unusually facile formation of a disulfonium dication from sulfide 10 is the result of stereochemical features of the eight-membered ring, which favor the formation of a transannular bond.31 According to X-ray data (see in Chapter 7.1 Table 1), the distance between the two sulfur atoms in 1,5-dithiacyclooctane 10 is smaller than the sum of their van der Waals radii (3.75 A), which results in a strong non-bonded interaction between the atoms confirmed by photoelectron spectroscopy and mass spectrometry.32 33 This interaction and the sulfur-sulfur distance can be decreased as a result of bond formation with an electronegative substituent as in sulfoxide 13 or sulfoximine 14.34,35... 

The larger (Z,Z)-l,5-cyclononadiene (169) reacts141 stereoselectively with PhSeCl in AcOH to give the substituted hydrindan 170 (equation 138). In consideration of the anti addition mode of selenenyl reagents to double bonds, the transannular reactions of 169 have been rationalized on the basis of the two reaction intermediates, 171 or 172, which are liable to place the PhSe- and AcO- groups in a cis- 1,4-relationship and trans to the bridgehead hydrogen (equation 139). The preferential formation of 170 has thus been attributed to the fact that the pathway via 172 should involve a boat transition state. 

The transannular bond formation between Te-Te and Te-S bonds giving the dications E and E is afforded by treatment of compounds A and B with the oxidizing agents nitrosyl tetrafluoroborate and hexafluorophosphate or with D2SO4. 

The main aspects of the chemical reactivity of helicenes (e.g. electrophilic substitution) equally not deviate from those of planar aromatic compounds, and remarkable reactions of helicenes, which are incidentally found (e.g. the transannular bond formation between a C(l)-substituent and a part of the inner helix) can ultimately be reduced to known principles of aromatic reactivity. 

Transannular interactions have been examined in the hexacyclo-[6.6.0.02 6.03 13.04,11.05 9]-tetradecane system.122 For example, the 14-iodo-10-one derivative (82) reacts with triphenylphosphine to give the salt (83), via C(10)—C(14) bond formation. In contrast, the 10,14-dione does not react. 

The irradiation of palladium complex 124 in acetic acid has been found to promote depalladation and transannular carbon-carbon bond formation.188 ... 

An indole-terminated tris(macrocycle) designed to be a channel-former and which is an effective carrier in bulk membranes but fails to function in a lipid bilayer owing to hydrogen-bond formation has been described <1996CC2147>. Tryptophan- and tryptamine-derived tetraazacyclooctadecane macrocycles have been synthesized <1996J(P1)2427>. Transannular hydrogen bonding stabilizes a left-handed double-helical conformation of the... 

These transannular reactions contrast with those observed on acid-catalyzed cyclization of l, hich results in C5-C10 bond formation (equation I). ... 

Many ring systems have been prepared by cycloaddition of acetylenic compounds, following concerted or multi-step intra- or intermolecular reaction mechanisms. In-particular, the placing in close proximity of triple bonds or a triple bond and another unsaturated system, such that intramolecular cycloaddition might lead to four-seven-membered rings, would seem of interest. This section deals with transannular carbon-carbon bond formation of triple bonds in acyclic and cyclic systems. 

Since the appearance of the theoretical consideration that the formation of tetrahedrane from two acetylenes would be a photochemically allowed process, a number of papers dealing with attempted syntheses of tetrahedranes have been published. It was reported that 2,2 -bis(phenylethynyl)biphenyl (26) was transformed thermally and photochemically into an isomeric hydrocarbon, for which a tetrahedrane (27) or a cyclobutadiene (28) structure would be possible . Actually, the structure of the hydrocarbon was confirmed to be 9-phenyl-l,2 3,4-dibenzo-anthracene (29) derived from transannular bond formation . ... 

On treatment with electrophilic reagents such as bromine and hydrogen bromide, 38 gave diphenylbenzofulvenes (45) through transannular bond formation between proximate triple bonds (44) , whereas addition of bromine to 41 yielded the... 

Base-catalysed prototropic rearrangements of cyclic polyacetylenes have often been used to synthesize annulene and dehydroannulene systems. In such reactions, transannular carbon-carbon bond formation between proximate triple bonds occasionally takes place to give polycyclic compounds. Treatment with a base of cyclic polyacetylenes 57 and 59 gave benzenoid aromatic compounds 58 and 61, with the desired dehydro[16] and [I8]annulenes, respectively. ... 

Similarly, biphenylene (63) was obtained in ca. 7% yield, from treatment of the thermally unstable tetrayne 8a with potassium /-butoxide, probably by transannular bond formation between the triple bonds closely placed in the intermediate 62 . ... 

It is well known that the transannular carbon-carbon bond formation is often induced in catalytic hydrogenation of severely strained cyclic acetylenes where triple bonds are closely placed to each other. Hydrogenation of the highly strained tetrayne (69) with 10% Pd-C gave the polycyclic compounds, 70 and 71, in addition to the expected hexadecahydro compound (50% yield). On the other hand, Birch reduction of 69 gave another perhydro product (72) in good yield. From a similarly constructed tetrayne (73) a polycyclic compound (74) was obtained by catalytic hydrogenation over platinum . 

Transannular reactions are defined as those reactions which lead to the formation of a covalent bond between atoms on opposite sides of a ring compound. They occur frequently in medium, i.e. 8-, 9-, 10-and 11-membered, rings where favored conformations allow opposite sides of the rings to come close to each other for bond formation. Probably the most commonly encountered transannulation processes are... 

This chapter summarizes those transannular reactions in medium ring carbocycles which result in carbon-carbon bond formation via alkylation of carbon, in the presence of electrophilic reagents. It is therefore a logical extension of two other chapters in this volume, covering Friedel-Crafts Alkylations (Chapter 1.8), and Polyene Cyclizations (Chapter 1.9). The discussion of these processes is organized according to the size of the cycloalkene from which transannular cyclization is effected. 

The biosynthesis of tabersonine is shown as occurring with 17,20-bond formation before 7,21 largely on the mechanistic grounds evident in Scheme 10 experiments considered to point against such a transannular ring-closure can be interpreted in ways not in conflict with Scheme 10. 

Addition of carbenium ions to C —C double bonds and C—C triple bonds, as well as to allylic systems, is of great interest in organic chemistry. Extensive investigations due to the importance of terpenes in biosynthesis (e.g., generation of steroids, transannular reactions of humulene. etc.) led to the development of a great number of reactions employing this method of C —C bond formation. 

The initial reaction is a Prins reaction, catalysed by the boron trifluoride complex. In order to achieve maximum overlap of the -orbitals of the olefin and aldehyde groups, the aldehyde must approach the olefin from below, as will easily be seen using molecular models. This means that the resultant alcohol function is located on the downward side of the molecule as shown in Figure S21. A 1,2-carbon shift followed by a transannular bond formation with concomitant loss of a proton, provides the skeletal rearrangement to the product. It may not be too obvious in the figure, but an experiment with molecular models will soon... 

Another transannular C-C bond formation reaction which starts with the attack of an electrophile at the triple bond is the formation of bityclic systems from tyclic diynes shown in Scheme 8-13, where H2SO4 induces the transannular addition of H2O to a cyclic diyne... 

Schiess examined the 1,5-shift of carbonyl groups in 1,3-cyclohexadiene and found that the rate of rearrangement for the formyl material is roughly 125 times faster than that for the hydrogen shift while that for the acetyl derivative is only 4 times faster. Further, the carbomethoxy derivative is substantially slower (by a factor of 70). Here it is likely that there is substantial bond formation to the carbonyl group recognizing that it is carbocation-like so the interaction is not unlike that in transannular tt participation however, it is unclear as to the extent of ionic character in this transition state, so (Scheme 7.42) is merely meant to be provocative. 

This reaction is formally a 1,5-shift of a butadiene bridge However, it was suggested that transannular bond formation to a highly stabilized biradical followed by, or less likely coupled with, C1-C2 bond fission could account for the product. ... 

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