You may examine models of partial diamond and graphite structures by clicking on the appropriate structure below. Diamond Graphite.
A comparison of the coronene and corannulene models discloses an interesting difference in their shapes. Coronene is absolutely flat and, aside from the peripheral hydrogens, resembles a layer of graphite. Its very high melting point reflects this resemblance. Corannulene, on the other hand, is slightly curved, resulting in a bowl-like shape.
If we extend the structure of corannulene by adding similar cycles of five benzene rings, the curvature of the resulting molecule should increase, and eventually close into a sphere of carbon atoms. The archetypical compound of this kind C 60 has been named buckminsterfullerene because of its resemblance to the geodesic structures created by Buckminster Fuller. It is a member of a family of similar carbon structures that are called fullerenes.
These materials represent a third class of carbon allotropes. Alternating views of the C 60 fullerene structure are shown on the right, together with a soccer ball-like representation of the 12 five and 20 six-membered rings composing its surface. Precise measurement by Atomic Force Microscopy AFM has shown that the C-C bond lengths of the six-membered rings are not all equal, and depend on whether the ring is fused to a five or six-membered beighbor.
By clicking on this graphic, a model of C 60 will be displayed. Although C 60 is composed of fused benzene rings its chemical reactivity resembles that of the cycloalkenes more than benzene. Indeed, exposure to light and oxygen slowly degrade fullerenes to cage opened products.
Most of the reactions thus far reported for C 60 involve addition to, rather than substitution of, the core structure. These reactions include hydrogenation, bromination and hydroxylation. Strain introduced by the curvature of the surface may be responsible for the enhanced reactivity of C A model of the C 70 fullerene may be examined by clicking here. A fascinating aspect of these structures is that the space within the carbon cage may hold atoms, ions or small molecules.
Such species are called endohedral fullerenes. The cavity of C 60 is relatively small, but encapsulated helium, lithium and atomic nitrogen compounds have been observed.
Larger fullerenes are found to encapsulate lanthanide metal atoms. Interest in the fullerenes has led to the discovery of a related group of carbon structures referred to as nanotubes. As shown in the following illustration, nanotubes may be viewed as rolled up segments of graphite. The chief structural components are six-membered rings, but changes in tube diameter, branching into side tubes and the capping of tube ends is accomplished by fusion with five and seven-membered rings.
Many interesting applications of these unusual structures have been proposed. A model of a nanotube will be displayed by clicking on the diagram. Many unsaturated cyclic compounds have exceptional properties that we now consider characteristic of "aromatic" systems.
The following cases are illustrative:. The first three compounds cyclic polyenes have properties associated with alkenes in general. Each reacts readily with bromine to give addition products, as do most alkenes. The thermodynamic change on introducing double bonds into the carbon atom ring is also typical of alkenes a destabilization of ca. The remaining four compounds exhibit very different properties, and are considered aromatic.
Furan and pyrrole react more rapidly with bromine, but they also give substitution products. This tendency to favor substitution rather than addition suggests that the parent unsaturated ring system has exceptional stability.
Thermodynamic measurements support this conclusion. A planar or near planar cycle of sp 2 hybridized atoms, the p-orbitals of which are oriented parallel to each other.
Cyclooctatetraene fails both requirements, although it has a ring of sp 2 hybridized atoms. Angle strain is relieved by adopting a tub-shaped conformation; consequently, the p-orbitals can only overlap as isolated pairs, not over the entire ring. Benzene is the archetypical aromatic compound.
The aromatic heterocycle pyridine is similar to benzene, and is often used as a weak base for scavenging protons. Furan and pyrrole have heterocyclic five-membered rings, in which the heteroatom has at least one pair of non-bonding valence shell electrons. By hybridizing this heteroatom to a sp 2 state, a p-orbital occupied by a pair of electrons and oriented parallel to the carbon p-orbitals is created. Four illustrative examples of aromatic compounds are shown above. The second and third compounds are heterocycles having aromatic properties.
Pyridine has a benzene-like six-membered ring incorporating one nitrogen atom. The last compound is imidazole, a heterocycle having two nitrogen atoms. The other electron pair colored black behaves similarly to the electron pair in pyridine. Monocyclic compounds made up of alternating conjugated double bonds are called annulenes. Benzene and 1,3,5,7-cyclooctatetraene are examples of annulenes; they are named [6]annulene and [8]annulene respectively, according to a general nomenclature system in which the number of pi-electrons in an annulene is designated by a number in brackets.
Some annulenes are aromatic e. As shown in the following diagram, 1,3,5,7,9-cyclodecapentaene fails to adopt a planar conformation, either in the all cis-configuration or in its 1,5-trans-isomeric form. The transannular hydrogen crowding that destabilizes the latter may be eliminated by replacing the interior hydrogens with a bond or a short bridge colored magenta in the diagram.
Naphthalene and azulene are [10]annulene analogs stabilized by a transannular bond. The bridged [14]annulene compound on the far right, also has aromatic properties. A modified [10]annulene, aromatic by nmr criteria, was prepared recently by chemists at California Institute of Technology. Remarkably, this hydrocarbon is chemically unstable, in contrast to most other aromatic hydrocarbons.
To learn more Click Here. A synthesis of barrelene bicyclo[2. Zimmerman Wisconsin , using a double Hofmann elimination. As shown in the following diagram, the chemical behavior of this triene confirmed it was not aromatic in the accepted sense of this term.
Bromine addition took place rapidly with transannular bond formation, in the same fashion as with norbornadiene bicyclo[2. Pyrolysis of barrelene gave the expected cycloreversion products benzene and acetylene. The heat of hydrogenation of barrelene reflects its thermodynamic stability. Furthermore, the first double bond of barrelene is reduced with the release of An explanation for the lack of aromatic behavior in the case of barrelene may be found by comparing the orbital symmetry of the six component p-orbitals with those of benzene.
Benzene is an annulene in which all six p-orbitals may be oriented with congruent overlapping phases. The cylindrical array of p-orbitals in barrelene cannot be so arranged, as shown in the diagram on the right. Arylglyoxals in Synthesis of Heterocyclic Compounds. Chemical Reviews , 5 , The Journal of Organic Chemistry , 78 6 , Wilson , Jonathan M. Percy , Joanna M. Redmond , and Adam W. The Journal of Organic Chemistry , 77 15 , Journal of Physical Organic Chemistry , 32 6 , e Journal of Physical Organic Chemistry , 32 5 , e Computational and Theoretical Chemistry , , Journal of Physical Organic Chemistry , 30 6 , e Ring Rearrangement Metathesis in 7-Oxabicyclo[2.
Some Applications in Natural Product Chemistry. Natural Product Communications , 12 5 , X Journal of Molecular Graphics and Modelling , 70 , Mojtahedi , M. Journal of Chemical Research , 40 4 , Theoretical study of the regio- and stereoselectivity of the intramolecular Povarov reactions yielding 5H-chromeno[2,3-c] acridine derivatives. RSC Advances , 6 19 , What is the origin of its regio- and endo stereospecificity?
RSC Advances , 6 79 , Ormachea , Pedro M. Kneeteman , Luis R. Understanding the participation of 3-nitropyridine in polar Diels—Alder reactions.
A DFT study. Helvetica Chimica Acta , 98 9 , Prakash Reddy. General Aspects of Organofluorine Compounds. Decostanzi , J. Campagne , E. Fluorinated enol ethers: their synthesis and reactivity. RSC Advances , 5 79 , RSC Advances , 5 72 , Vallejos , Silvina C. Competing mechanisms for the reaction of dichloropropynylborane with 2-tert-butylbutadiene.
Devising a systematic nomenclature system for heterocyclic compounds presented a formidable challenge, which has not been uniformly concluded. Many heterocycles, especially amines, were identified early on, and received trivial names which are still preferred.
Some monocyclic compounds of this kind are shown in the following chart, with the common trivial name in bold and a systematic name based on the Hantzsch-Widman system given beneath it in blue. The rules for using this system will be given later. For most students, learning these common names will provide an adequate nomenclature background. An easy to remember, but limited, nomenclature system makes use of an elemental prefix for the heteroatom followed by the appropriate carbocyclic name.
A short list of some common prefixes is given in the following table, priority order increasing from right to left. The Hantzsch-Widman system provides a more systematic method of naming heterocyclic compounds that is not dependent on prior carbocyclic names. It makes use of the same hetero atom prefix defined above dropping the final "a" , followed by a suffix designating ring size and saturation. As outlined in the following table, each suffix consists of a ring size root blue and an ending intended to designate the degree of unsaturation in the ring.
In this respect, it is important to recognize that the saturated suffix applies only to completely saturated ring systems , and the unsaturated suffix applies to rings incorporating the maximum number of non-cumulated double bonds. Systems having a lesser degree of unsaturation require an appropriate prefix, such as "dihydro"or "tetrahydro". Despite the general systematic structure of the Hantzsch-Widman system, several exceptions and modifications have been incorporated to accommodate conflicts with prior usage.
Examples of these nomenclature rules are written in blue, both in the previous diagram and that shown below. Note that when a maximally unsaturated ring includes a saturated atom, its location may be designated by a " H " prefix to avoid ambiguity, as in pyran and pyrrole above and several examples below. When numbering a ring with more than one heteroatom, the highest priority atom is 1 and continues in the direction that gives the next priority atom the lowest number.
All the previous examples have been monocyclic compounds. Polycyclic compounds incorporating one or more heterocyclic rings are well known. A few of these are shown in the following diagram. As before, common names are in black and systematic names in blue. The two quinolines illustrate another nuance of heterocyclic nomenclature. Thus, the location of a fused ring may be indicated by a lowercase letter which designates the edge of the heterocyclic ring involved in the fusion, as shown by the pyridine ring in the green shaded box.
Heterocyclic rings are found in many naturally occurring compounds. Most notably, they compose the core structures of mono and polysaccharides , and the four DNA bases that establish the genetic code. By clicking on the above diagram some other examples of heterocyclic natural products will be displayed.
Oxiranes epoxides are the most commonly encountered three-membered heterocycles. Epoxides are easily prepared by reaction of alkenes with peracids, usually with good stereospecificity. Because of the high angle strain of the three-membered ring, epoxides are more reactive that unstrained ethers. Addition reactions proceeding by electrophilic or nucleophilic opening of the ring constitute the most general reaction class.
Example 1 in the following diagram shows one such transformation, which is interesting due to subsequent conversion of the addition intermediate into the corresponding thiirane. The initial ring opening is stereoelectronically directed in a trans-diaxial fashion, the intermediate relaxing to the diequatorial conformer before cyclizing to a 1,3-oxathiolane intermediate. Other examples show similar addition reactions to thiiranes and aziridines.
The acid-catalyzed additions in examples 2 and 3, illustrate the influence of substituents on the regioselectivity of addition. Example 2 reflects the S N 2 character of nucleophile chloride anion attack on the protonated aziridine the less substituted carbon is the site of addition. The phenyl substituent in example 3 serves to stabilize the developing carbocation to such a degree that S N 1 selectivity is realized.
The reduction of thiiranes to alkenes by reaction with phosphite esters example 6 is highly stereospecific, and is believed to take place by an initial bonding of phosphorous to sulfur. By clicking on the above diagram , four additional example of three-membered heterocycle reactivity or intermediacy will be displayed. Examples 7 and 8 are thermal reactions in which both the heteroatom and the strained ring are important factors. Note that two inversions of configuration at C-2 result in overall retention.
Many examples of intramolecular interactions , such as example 10, have been documented. As illustrated below, acid and base-catalyzed reactions normally proceed by 5-exo-substitution reaction 1 , yielding a tetrahydrofuran product. However, if the oxirane has an unsaturated substituent vinyl or phenyl , the acid-catalyzed opening occurs at the allylic or benzylic carbon reaction 2 in a 6-endo fashion.
Preparation Several methods of preparing four-membered heterocyclic compounds are shown in the following diagram. The simple procedure of treating a 3-halo alcohol, thiol or amine with base is generally effective, but the yields are often mediocre. Dimerization and elimination are common side reactions, and other functions may compete in the reaction.
In the case of example 1, cyclization to an oxirane competes with thietane formation, but the greater nucleophilicity of sulfur dominates, especially if a weak base is used. In example 2 both aziridine and azetidine formation are possible, but only the former is observed. This is a good example of the kinetic advantage of three-membered ring formation. Example 4 demonstrates that this approach to azetidine formation works well in the absence of competition.
Indeed, the exceptional yield of this product is attributed to the gem-dimethyl substitution, the Thorpe-Ingold effect , which is believed to favor coiled chain conformations.
The relatively rigid configuration of the substrate in example 3, favors oxetane formation and prevents an oxirane cyclization from occurring. Finally, the Paterno-Buchi photocyclizations in examples 5 and 6 are particularly suited to oxetane formation. Reactions Reactions of four-membered heterocycles also show the influence of ring strain. Some examples are given in the following diagram. In the thietane reaction 2 , the sulfur undergoes electrophilic chlorination to form a chlorosulfonium intermediate followed by a ring-opening chloride ion substitution.
Strong nucleophiles will also open the strained ether, as shown by reaction 3b. Example 5 is an interesting case of intramolecular rearrangement to an ortho-ester. Such electron pair delocalization is diminished in the penicillins, leaving the nitrogen with a pyramidal configuration and the carbonyl function more reactive toward nucleophiles.
Preparation Commercial preparation of furan proceeds by way of the aldehyde, furfural, which in turn is generated from pentose containing raw materials like corncobs, as shown in the uppermost equation below. Similar preparations of pyrrole and thiophene are depicted in the second row equations. Equation 1 in the third row illustrates a general preparation of substituted furans, pyrroles and thiophenes from 1,4-dicarbonyl compounds, known as the Paal-Knorr synthesis.
Many other procedures leading to substituted heterocycles of this kind have been devised. Two of these are shown in reactions 2 and 3. Furan is reduced to tetrahydrofuran by palladium-catalyzed hydrogenation. This cyclic ether is not only a valuable solvent, but it is readily converted to 1,4-dihalobutanes or 4-haloalkylsulfonates, which may be used to prepare pyrrolidine and thiolane. Dipolar cycloaddition reactions often lead to more complex five-membered heterocycles.
Indole is probably the most important fused ring heterocycle in this class. By clicking on the above diagram three examples of indole synthesis will be displayed.
The first proceeds by an electrophilic substitution of a nitrogen-activated benzene ring.
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