Alicyclic Chemistry Volume 2

A Review of the Literature Published during 1972

By W. Parker

The Royal Society of Chemistry

Copyright © 1974 The Chemical Society
All rights reserved.
ISBN: 978-0-85186-522-5

Contents

Chapter 1 Three- and Four-membered Rings By H. Maskill, 1,
Chapter 2 Five- and Six-membered Rings and Related Fused Systems By D. G. Morris, 174,
Chapter 3 Medium – and Large-ring Compounds By M. S. Baird, 249,
Chapter 4 Bridged Systems By J. M. Mellor, 319,
Author Index, 453,

CHAPTER 1

Three- and Four-membered Rings

BY H. MASKILL

1 Introduction, Theory, and Structure

The first two paragraphs of the corresponding chapter in Volume 1 of this Report also relate to this chapter. Several relevant reviews and books have been published on particular topics and compound types. The report of the International Symposium on the Chemistry of Small Rings held at Louvain (Belgium) in 1971 has also appeared.

Randic and Maksic have reviewed the maximum-overlap method of describing the hybridization of bonded atoms, and a number of cyclopropane and cyclobutane derivatives are included. The method has also been applied to benzocyclopropene and benzocyclobutene. One measure of the hybridization of the bonding orbitals of carbon is the magnitude of the J(13CH) and J(13C13C) n.m.r. coupling constants. For di- and tri-methylcyclopropenes, values of J(13CH) have been calculated using hybrid orbitals, which in turn were calculated by the maximum-overlap method. The agreement with experiment is less satisfactory for hydrogens bonded to a small-ring carbon atom than for those bonded to an acyclic carbon. It is worse for three- than for four-membered rings. Experimental values of J(13C13C) for spiropentane, 1,1,3,3-tetramethylcyclobutane, bicyclobutane, bicyclopentane, and trans-tricyclo[3,1,0,0]hexane have been reported and used to calculate the s-character of the carbons’ bonding orbitals.

Recent generalized valence-bond calculations for cyclopropane, in agreement with the work of Coulson and Moffit of more than 20 years ago, suggest high p-character C — C bonds, the electron density of which is, in part, outside the interatomic vector. The effect of including d-functions in the basis set for ab initio MO energy calculations of small hydrocarbons has been reported. This refinement lowers the calculated energies of cyclopropane and cyclopropene significantly more than it lowers the energies of open-chain isomers. Consequently, the calculated energy differences between cyclic and open-chain isomers are reduced to within ca. 3 kcal mol-1 of the experimental values.

Cyclopropane and tetracyanocyclopropane are among compounds which have been investigated by X-ray induced electron spectroscopy. It was demonstrated that the cyano-groups increase the atomic binding energies, particularly at C-l but also at C-3, of the three-membered rings. Experimental charge distributions were also obtained and compared with calculated values and with values of heterocyclic analogues. MO calculations provide a very satisfactory interpretation of the recently reported photoelectron spectrum of cyclopropene.

Both ab initio and CNDO/2 MO descriptions of cyclobutane have appeared. The Slater-type orbital ab initio method and the floating spherical Gaussian orbital (FSGO) method of Nelson and Frost predict D2d structures with dihedral angles of 15° and 32°, respectively (experimental values are in the range 20–37°). In contrast to the semi-empirical method, the ab initio methods agree that methylene rocking is important and causes the puckered structure to be of lower energy than the planar one. Assuming a D2d structure and C — C and C — H bond lengths based on earlier CNDO/2 calculations, the CNDO/2 method predicts a dihedral angle of 18 [+ or -] 5° and an inversion barrier of 1.29 kcal mol-1 in comparison with 0.31, 3.5, and 1.48 kcal mol-1 (earlier CNDO/2, FSGO ab initio, and experimental values, respectively). The dihedral angle of octafluorocyclobutane has been calculated to be 10° by the MINDO/2 method compared with the experimental value of 17.4°.

The MINDO/2 procedure has also been used to give satisfactory agreement with experiment for the bond lengths of cyclobutene and hexafluorocyclobutene. In both molecules the C-3 — C-4 bond is correctly predicted to be particularly long. The ground and first electronically excited states of 3,4-dimethylenecyclobutene have been the subject of CNDO, INDO, and modified CNDO–CI calculations. Using experimental bond lengths, good agreement was obtained between calculated and experimental dipole moments, and the known single-bond–double-bond structure was justified. Such a structure also allows a ready interpretation of the results of a recent i.r. investigation. The magnitude of the dipole moment, which is not easily accounted for by an alternant structure, and the negative resonance energy have led to an anti-aromatic description of 3,4-dimethylenecyclobutene.

Two ab initio MO investigations of bicyclobutane have been described, and in both considerable attention is paid to the C-l — C-3 bond. Both show that there is considerable electron density between and above the bridgehead carbons and both describe the bond as essentially a σ-bond which is formed by overlap of atomic hybrid orbitals of high p-character. The electronic charge distribution within bicyclobutane has been estimated experimentally by measuring the molecular Zeeman effect in a high magnetic field. There is no unusual ring current or delocalization effect and the dipole moment has its positive end towards the bridgeheads, as calculated earlier. MO calculations of the structure and reactivity of propellanes which contain three- and four-membered rings have also been published.

Besides electronic structure and bonding, other molecular features of cyclopropane derivatives, such as conformation, have been theoretically scrutinized. The barrier to rotation about the bond joining methyl to cyclopropyl in methylcyclopropane has been calculated to be 1.82 kcal mol-1 using the INDO approximation. When methyl is replaced by amino- or phosphino-groups, there is more than the single minimum in the rotation energy profile but, for both compounds, (1) is the most stable conformation. It is calculated (INDO and CNDO/2) to be 3.73 kcal mol-1 more stable than the other conformation which corresponds to a minimum for cyclopropylamine, and the highest maximum in the energy profile is 4.82 kcal mol-1 above the ground-state minimum. The corresponding values for cyclopropylphosphine (CNDO/2) are 2.8 and 5.93 kcal mol-1. For each of these three compounds, the conformation which is calculated to be most stable is in accordance with experimental observation and the quantitative aspects are in reasonable agreement. The energy barriers to rotation about the double bonds in methylenecyclopropane, methylenecyclopropene, and dicyclopropylidene (inter alia) have been calculated using MINDO/2. The cyclopropane ring reduces the calculated energy barrier to rotation about the ethylenic bond by ca. 5 kcal mol-1 per cyclopropane. A single cyclopropene produces a dramatic reduction of 23 kcal mol-1, though a second cyclopropene causes a small increase (5 kcal mol-1) rather than a further decrease (see Table 1).

Internal rotation within vinylcyclopropane and vinylcyclobutane and their simple derivatives has been studied by an ab initio method. The most stable conformations are (2), in agreement with experiment and (3), for which there is no experimental comparison, though other minima were detected. The barriers to complete rotation are calculated to be 3 and 2.7 kcal mol-1 for vinylcyclopropane and vinylcyclobutane, respectively. The effects of fluoro- and cyano-groups upon the calculated rotation energy profile are discussed. A conclusion from this work is that cyclopropyl and cyclobutyl interact with the olefinic bonds to extents between those characteristic for formally saturated and unsaturated substituents. This is in agreement with structural work, but a microwave study of cyclopropylacetylene and a deuterio-derivative supports an earlier view that conjugation between a triple bond and cyclopropyl either does not exist or does not lead to a detectable modification of bond lengths.

The different conformers of vinylcyclopropane will have different long-range n.m.r. coupling constants between the vinyl and the cyclopropyl hydrogens. This has been the subject of a semi-empirical theoretical study. There have also been reports of a theoretical study of the conformations of cyclopropene monohydrate and of MO calculations on carbo-cations which contain three- and four-membered rings.

The absolute configurations of two sorts of naturally occurring compounds have been determined by X-ray crystallography, and structures of synthetic compounds, all of which contain cyclopropane groups. Compound (4) has been reported as a bisnorcaradiene with a particularly long C-1 — C-6 9bond (1.80 Å). This non-aromatic structure for (4) corroborates earlier n.m.r. work. The cyclopropene moiety of the benzocyclopropene (5) has been shown to have the same interatomic distances as cyclopropene itself. The benzene to which the cyclopropene is fused, however, is deformed from regularity as anticipated by earlier calculations and n.m.r. work on the parent benzocyclopropene.

Diphenylcyclopropenethione (6) has been shown by X-ray crystallography to have phenyl–cyclopropenyl bond lengths (1.440 Å) which are very similar to those of triphenylcyclopropenium perchlorate (1.438 Å) and shorter than the more usual C(sp2)–C(sp2) bonds (1.48 Å). The bonds of the three-membered ring are not equal in length and there is a distinct shortening of the two equivalent bonds (1.403 Å) compared with those of cyclopropene. The third bond (1.338 Å) is similar in length to the double bond in ethylene (1.332 Å).

Crystal structures of several three- and four-membered carbo-cations have been reported. The cation of 1,2,3-tri(dimethylamino)cyclopropenium perchlorate (7) is symmetrical, with C — C bond lengths of 1.363 Å as expected for a cyclopropenyl cation. The N — cyclopropenyl bond is considerably shorter (1.333 Å) than a normal C — N bond; this is indubitably a measure of the delocalization of electron density from nitrogen to the three-membered-ring carbons. The hydrogen–hydrogen interactions of the methyl groups on adjacent nitrogen atoms cause the ion to be non-planar. The three-membered-ring part of cation (8) is more like diphenylcyclopropenethione than either (7) or triphenylcyclopropenium ion, as it has two long bonds and one short one. This is a reflection of the electronic structure in the sense that the canonical with the tropylium residue electron-deficient probably contributes to the hybrid to the same extent as the canonical which is shown.

A tetra-arylcyclobutenyl salt has been prepared with 13C enrichment in the cyclobutenyl ring. This allowed determination by 13C n.m.r. spectroscopy of the π-charge density at the carbons of the four-membered ring for comparison with earlier calculations. The same report also confirms that the three aryl groups which are bonded to the unsaturated carbons of the four-membered ring are conjugated with the allylic electron-deficient group.

Although no u.v. maxima were detected in the region 185–200 nm, several derivatives of the tetra-substituted cyclopropanes (9) show strong negative Cotton effects (Δε ca. – 3.5 to – 7.5) in the range 185–195 nm. Vibrational assignments have been made for the i.r. and Raman spectra of deuteriated and undeuteriated 1,1,2,2-tetrafluorocyclopropane, 1,3,3-trifluorocyclopropene,and cyclopropanecarboxylic acid and its sodium salt. Detailed consideration of results from microwave spectroscopy of cyclopropyl derivatives has led Penn and Boggs to conclude that an unsaturated substituent at C-1 causes a slight shortening of the C-2 — C-3 bond compared with the effect of a saturated substituent. Dipole-moment measurements of 1,1-dichloro- and 1,1-dibromo -2-phenylcyclopropane have been reported and suggest that the dihedral angles between the two rings in the two compounds are ca. 30° and 30–40°, respectively, in their most stable conformations.

A correction to an earlier report of the calculations on the vibrations of [1H6]- and [2H6]-methylenecyclopropane has appeared, as also has an account of the use of n.m.r. and i.r. spectroscopy for structural and configurational assignments to fifteen bicyclo[3,1,0]hex-3-en-2-ols.

Structure determinations by X-ray crystallography have also been reported for several cyclobutane derivatives. The four-membered ring of cyclobutane –cis- 1,2-dicarboxylic acid in the crystalline state is puckered with a dihedral angle of 156°. The ENDOR-detected n.m.r. spectrum of (CH2)3C(CO 2D)2 has been recorded for single crystals at 4.2 K. The results are best accommodated by a puckered cyclobutane ring with a dihedral angle of 165°. Comparison with earlier X-ray work implies that the energy barrier between the two equivalent ground-state puckered conformations is approximately only 20 cm-1 (a value much lower than those for other cyclobutanes investigated in gas and liquid phases). A low-temperature X-ray crystallography investigation on d-spiro[3,3]heptane-2,6-dicarboxylic acid at – 160°C supports the earlier conclusion, based upon o.r.d. and c.d. data, that this enantiomer has R absolute configuration (10). Both rings are puckered and the dihedral angles are 152.6° with both carboxylic acid groups in pseudo-equatorial positions. The molecular structure of 1,2-dichlorocyclobutenedione has been determined by X-ray crystallography.

The molecular structures of cyclobutane and several of its simply substituted derivatives have been investigated by i.r. and Raman spectroscopy. The energy barriers between the two puckered minima for cyclobutane and [2H8] -cyclobutane have been determined (518 [+ or -] 5 cm-1 and 508 [+ or -] 8 cm-1, respectively). With chloro-, bromo-, and cyano-substituents, asymmetry is introduced into the potential function for ring inversion, and conformational isomers become possible. However, the best fit of the far-i.r. data was for asymmetric potentials with single minima. It was presumed that the minima correspond to puckered conformations with equatorial substituents. The Raman spectra of chloro-, fluoro-, and methyl-cyclobutane lead to similar conclusions. The vibrational spectrum of crystalline cyclobutanecarboxylic acid has been reported and is best interpreted in terms of a hydrogen-bonded dimeric structure with a centre of symmetry.

Recent Raman results for methylenecyclobutane, as well as earlier i.r. data, can be accommodated by a double-minimum potential with a barrier of only 141 [+ or -] 5 cm-1 between two puckered forms. This is the first experimental observation of a ring-puckering vibration for methylenecyclobutane. If tetramethylcyclobutane-l,3-dione has a double-minimum potential, recent i.r. and Raman results indicate that the barrier must be lower than the value for cyclobutanone (7.9 cm-1). The spectral results were interpretable on the basis of the known D2h symmetry. Tetramethylenecyclobutane and [2H8] tetramethyl-enecyclobutane, which have been isolated for the first time recently, have been shown by i.r. and Raman spectroscopy to be planar molecules (D4h symmetry). Assignments were made for 23 of the 24 allowed fundamental vibrations. The n.m.r. spectrum of 2-methylenecyclobutanone, a molecule which is expected to be planar or rapidly inverting, has been reported and interpreted.

The first X-ray structure determination on a bicyclobutane has appeared. The structure for l,3-dicyanobicyclo[l,l,0]butane is in agreement with those obtained previously for analogues by spectroscopic and electron-diffraction methods. The microwave spectra of bicyclopentane and labelled analogues have been recorded and yield molecular parameters significantly different from those of a recent electron-diffraction investigation. The microwave values are shown in (11) with the electron-diffraction values in parentheses.

The dihedral angle is 62.26°, compared with 70.6° by electron diffraction. The n.m.r. spectra of three diphenylbicyclo[2,1,0]pentanes have been reported and discussed in detail. New results for the structure of bicyclo[2,1,0]pent-2 -ene by electron diffraction have also been reported. The molecular parameters are in agreement with the values by microwave spectroscopy which were reported last year.

8,8-Dichlorotricyclo[3,2,1,0]octane (12) is a bridged bicyclopentane, and it has been confirmed by X-ray crystallography at – 40 °C that the bridgehead carbon atoms have ‘inverted’ tetrahedral geometry, i.e. the four bonds from a bridgehead carbon are all to one side of a plane through the bridgehead carbon. The inter-hybrid angles, however, at the bridgehead carbons are 101–116°C; values much less exceptional than the angles between the interatomic vectors. The central C-1 — C-5 bond is seen to be longer than the corresponding bonds of bicyclopentane or bicyclopent-2-ene.

The dissociation constants of eight 6-substituted spiro[3,3] heptane-2-carboxylic acids in 50% aqueous ethanol at 25 °C have been measured and used to correlate non-conjugative substituent effects. Detailed analysis of the results suggests that a field effect rather than a σ-inductive effect is the more reliable model. The first and second dissociation constants of squaric acid (3,4-dihydroxycyclobut-3-ene-l,2-dione) in aqueous 3M-NaClO4 have been measured by e.m.f. methods. Results were in agreement with earlier ones after correction for ionic strength.

(Continues…)Excerpted from Alicyclic Chemistry Volume 2 by W. Parker. Copyright © 1974 The Chemical Society. Excerpted by permission of The Royal Society of Chemistry.
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