Tetrahedron
Volume 61, Issue 22,
30 May 2005
, Pages 5147-5208
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Graphical Abstract
Introduction
As mentioned in introductory organic chemistry textbooks, the ideal geometry for a double bond has the olefinic carbon atoms and the four atoms connected to them in the same plane. Thus, in the ground state of ethylene, all six atoms lie in one plane, the bond angles are near 120° and the carbon–carbon distance is 1.34Å. However, as stressed by Mislow as early as 1965, ‘planarity is not expected if the molecules does not have a plane of symmetry passing through the sp2 carbon atoms and all four corresponding ligands’.1 Cyclopentene is a good example, ab initio calculations showing that, when the molecule is allowed to adopt its equilibrium envelope conformation, the sp2 carbon atoms are pyramidalized.2 Pyramidalized alkenes are molecules containing carbon–carbon double bonds in which one or both of the doubly bonded carbon atoms do not lie in the plane defined by the three atoms attached to it. Deviations of the planarity for the vast majority of olefinic carbon atoms are small. However, when a carbon–carbon double bond is located at the bridgehead positions of a polycyclic structure, severe deviations of the usual planar geometry occur.3
In bridgehead alkenes, there are two different types of distortions, the out-of-plane bending and the in-plane bending. The latter is the major distortion observed in small cyclic alkenes (e.g., cyclopropene).4 In bridgehead alkenes, the out-of-plane bending is much more important and two major modes of distortion can be distinguished: twisting and pyramidalization.5 One extreme case is the pure twisting: the two olefinic carbon atoms stay fully sp2 hybridized and thus planar (Fig. 1a). As a consequence, the two p orbitals are misaligned, which weakens the π-component of the double bond. This is visualized by the twisting angle, τ, which is defined as the dihedral angle between the two p orbitals. In the other extreme situation, the syn pyramidalization, the carbon atoms are rehybridized by admixture of additional p character into the original sp2 σ-bonds; this makes the geometry around the carbon non-planar. The π bond is now formed from two p-orbitals with some added s-character; the alignment between the two orbitals is optimal (τ=0), but their orientation in the p-plane is no longer parallel and, for this reason, the distance between them increases and the net overlap is smaller. As we will discuss below, there is now an out-of-plane or flap angle, Ψ, defined as the dihedral angle between a plane containing two cis-substituents and the two olefinic carbon atoms and the plane containing the two other cis-substituents and the two olefinic carbon atoms and this is usually referred to as its supplementary ζ=180°−Ψ, (Fig. 1b). Usually, the situation encountered in anti-Bredt alkenes is intermediate: twisting and pyramidalization occur simultaneously (Fig. 1c).
Another form of pyramidalization, in which the π-type spn orbitals are oriented towards opposite sides of the double bond, is called anti pyramidalization. Although calculations have shown that anti pyramidalization of the carbon atoms of the double bond is energetically more favorable than syn pyramidalization,6 it is much less common and will only be mentioned briefly here.
Bridgehead olefins with torsionally strained double bonds (‘twisted or anti-Bredt alkenes’) have been studied extensively,7 but olefins in which the carbon atoms forming the double bonds are pyramidalized (‘pyramidalized alkenes’) have received much less attention. In this report, we deal with the synthesis and reactivity of untwisted, but pyramidalized, bridgehead olefins. Where appropriate, a mention of the physical and chemical properties will be made. Twisted alkenes, distorted aromatic compounds, such as cyclophanes, fullerenes and fullerene-related compounds are out of the scope of this report.
Some reviews on pyramidalized alkenes have appeared,8 the more comprehensive being the excellent survey by Borden in 1989,8b although more concise accounts have appeared later.9 The present report will cover the material that has appeared since 1989 up to mid 2004, although some previous aspects covered by Borden will be mentioned here.
As discussed in more detail later, pyramidalization changes the typical chemistry of the double bond dramatically. Dimerization of highly pyramidalized alkenes occurs rapidly, and pyramidalized alkenes that do not dimerize at room temperature react with atmospheric oxygen10 and with nucleophilic reagents.11
Pyramidalization allows the 2s atomic orbitals of the olefinic carbon atoms to be mixed into the π bond. The increase in 2s character stabilizes both the π and the π* molecular orbitals. However, this rehybridization also decreases the overlap of the two hybrid orbitals, as compared to the overlap of two parallel p atomic orbitals, by pointing the large lobes of the hybrid orbitals away from each other. The loss of the bonding overlap in the π molecular orbital destabilizes it. The effects of increased 2s character and reduced overlap tend to cancel, so that the energy of the π molecular orbital remains relatively constant. In contrast, a reduction of antibonding overlap in π* stabilizes this molecular orbital further. Hence, the π* molecular orbital drops rapidly in energy upon pyramidalization. Therefore, excitation of an electron from π to π* is made energetically less costly on increasing pyramidalization.12 The unusually low energy of π* is responsible for the ease of reduction of pyramidalized alkenes, for the ability of pyramidalized alkenes to form stable complexes with transition metals, such as Pt(0), that can donate electron density into this molecular orbital,13 and for the deshielding observed for the olefinic carbon atoms in 13C NMR.14
Section snippets
Measurements of pyramidalization
In studying pyramidalized alkenes, it is convenient to use a geometrical parameter to measure pyramidalization.15 More than 20 years ago, Borden et al. introduced the pyramidalization angle, Φ, as a measure of pyramidalization.12a Strictly speaking, the Φ angle is applicable only to those cases having C2v symmetry, with a mirror plane bisecting and perpendicular to the double bond and a mirror plane containing the double bond. As shown in Figure 2a, Φ is the angle between the plane containing
9,9′-Didehydrodianthracene
The title compound 3 has historic interest, because it was the first substantially pyramidalized alkene to be synthesized and it offered the first experimental evidence that pyramidalized alkenes are suceptible to nucleophilic addition reactions, as predicted by frontier orbital theory. Although Applequist et al. had suggested the formation of 3 by the reaction of 9-bromodianthracene 4 with strong bases,18 it was not until 1968 that Weinshenker and Greene described the successful isolation of 3.
Sesquinorbornenes and related alkenes
During the 1980s, the groups of Bartlett, Paquette and Watson synthesized and fully characterized several derivatives of syn- and anti-sesquinorbornene, 84 and 85, respectively, and syn- and anti-sesquinorbornadiene, 86 and 87, respectively (Fig. 3). Most of these derivatives are fairly stable compounds and several X-ray studies were carried out.
Although there are a few exceptions, near-planar alkene geometries have been the rule in the derivatives of 85 and 87. For the syn-sesquinorbornenes,
Bicyclo[3.3.0]oct-1(5)-ene derivatives
In the 1980s, Houk12, 17, 86 and Burkert,87 independently, proposed that pyramidalization occurs to relieve unfavorable torsional interactions, favoring staggering of the bonds at adjacent carbon atoms.9b–c Consistent with this proposal, a pyramidalized, C2v syn geometry of bicyclo[3.3.0]oct-1(5)-ene 201 was calculated to be lower in energy than a planar, C2h anti geometry (Fig. 9).9, 12
Ab initio calculations (HF/3-21G) carried out by Hrovat and Borden predicted a pyramidalization angle of 3.6°
Cubene
Cubane, pentacyclo[4.2.0.02,5.03,8.04,7]octane, is one of the most exciting and extensively studied cage compounds.138 Eaton and co-workers have worked for years on the synthesis of a myriad of cubane derivatives, including the highly pyramidalized 1,2-dehydrocubane (‘cubene’) 433.11, 117
According to ab initio calculations, cubene, with a pyramidalization angle of Φ=84.1° (HF/3-21G), is the most highly pyramidalized alkene yet known.12c Calculations also indicate that sufficient overlap exists
Unsaturated quadricyclanes
The facile access to tetracyclo[3.2.0.02,7.04,6]heptane (quadricyclane) through a photochemically induced [2+2] cycloaddition of the commercially available 2,5-norbornadiene149 made this framework very attractive for the installation of unusual double bonds (Fig. 18).150 As a result of some impressive work, Szeimies and co-workers have found evidence for three unsaturated quadricyclanes, 1(7)-quadricyclene 494,151 1(5)-quadricyclene 495,152 and 1(2)-quadricyclene, 496.153 The elusive anti-Bredt
Anti-pyramidalized alkenes
Anti-pyramidalization is calculated to be energetically less costly than syn-pyramidalization. However, few studies have been carried out in the area of anti-pyramidalized alkenes and, most importantly, there is still a lack of highly anti-pyramidalized alkenes.
According to its X-ray crystal structure and DFT theoretical calculations, heptafulvalene 688 has an anti-pyramidalized C2h structure. This conformation is lower in energy than the syn-pyramidalized C2v conformation.194
Probably, the best
Perspectives
Fifteen years have passed since the publication of the seminal review on pyramidalized alkenes by Borden in Chemical Reviews in 1989.8b In this period, many landmark achievements have been attained. The synthesis of several derivatives of the highly pyramidalized tricyclo[3.3.0.03,7]oct-1(5)-ene, the synthesis of polyunsaturated dodecahedranes, the generation of acepentalene and related compounds, the dimerization of cubene and the synthesis of cyclopropene-fused norbornene derivatives are
Acknowledgements
Financial support from Ministerio de Ciencia y Tecnologia and FEDER (Project No. PPQ2002-01080 and Ramón y Cajal fellowship to S. V.) and Comissionat per a Universitats i Recerca (Project No. 2001SGR00085) is gratefully acknowledged. The authors wish to express their deep gratitude to our co-workers, who have contributed significantly over the past 20 years with hard work, skill and enthusiasm to our contributions to the field of pyramidalized alkenes. We also thank the Centre de
Santiago Vázquez was born in Barcelona in 1968. He studied Pharmacy (1986–1991) at the Universitat de Barcelona. He obtained his PhD in Organic and Medicinal Chemistry at the same university in 1996 under the direction of Professor P. Camps. After spending 2 years (1998–1999) in the Christopher Ingold Laboratories (University College London) with Professor William B. Motherwell as a Marie Curie Research Fellow, he returned to Barcelona. In 2001 he took up his present position as ‘Investigador
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Santiago Vázquez was born in Barcelona in 1968. He studied Pharmacy (1986–1991) at the Universitat de Barcelona. He obtained his PhD in Organic and Medicinal Chemistry at the same university in 1996 under the direction of Professor P. Camps. After spending 2 years (1998–1999) in the Christopher Ingold Laboratories (University College London) with Professor William B. Motherwell as a Marie Curie Research Fellow, he returned to Barcelona. In 2001 he took up his present position as ‘Investigador Ramón y Cajal’ at Universitat de Barcelona. His scientific interests include polycyclic cage compounds, drug synthesis and free radical and computational chemistry.
Pelayo Camps graduated in chemistry in the Universitat de Barcelona (UB). His PhD studies were carried out at the Organic Chemistry Institute (CSIC, Barcelona) under the direction of Prof. Dr. José Pascual, completing his doctoral thesis and obtaining the PhD degree from the same University in 1972. This year started his docent carrier at the Universitat Autònoma de Barcelona (UAB) as Assistant Professor. In 1978, after a postdoctoral stage at the University of Aix-Marseille-III with Prof. Dr. José Elguero, he was promoted to Associate Professor of the Faculty of Pharmacy of the Universidad de Valencia (UV), where he stayed for a year, returning to the UAB in 1979. Two years later obtained a Full Professor position in the Faculty of Chemistry (San Sebastian) of the Universidad del País Vasco where he spent 2 years. After five more years in the Faculty of Pharmacy of the UV, in 1988, he moved to his actual position as a Full Professor in the Faculty of Pharmacy of the UB where he is currently the Head of the Pharmaceutical Chemistry Unit. His research interest has been always related with the organic synthesis of different kind of compounds: polycyclic and cage compounds via highly pyramidalized alkenes, acetylcholinesterase inhibitors for the treatment of Alzheimer's disease, and the use of chiral auxiliaries for the asymmetric synthesis of drugs and related compounds. He has previously held a visiting professorship at the University of Bordeaux.
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