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Cyclobutadienes—

Ni(r -C4Ph4)2 is notable as the first metal sandwich based on two C4 rings [completing the series Cr(r -C6R6)2, Fe(rj-C5R5)2 and Ni(rj-C4R4)2] and also because its isolation (m.p. 404 °C ) defied theoretical calculations suggesting that d10 complexes of cyclobutadienes would not be [Pg.160]


Colourless liquid with alkenic properties. Many substituted derivatives are known, the preferred method of preparation being the addition of an alkyne to a cyclobutadiene. [Pg.130]

Stabilizing resonances also occur in other systems. Some well-known ones are the allyl radical and square cyclobutadiene. It has been shown that in these cases, the ground-state wave function is constructed from the out-of-phase combination of the two components [24,30]. In Section HI, it is shown that this is also a necessary result of Pauli s principle and the permutational symmetry of the polyelectronic wave function When the number of electron pairs exchanged in a two-state system is even, the ground state is the out-of-phase combination [28]. Three electrons may be considered as two electron pairs, one of which is half-populated. When both electron pahs are fully populated, an antiaromatic system arises ("Section HI). [Pg.330]

A more general classification considers the phase of the total electronic wave function [13]. We have treated the case of cyclic polyenes in detail [28,48,49] and showed that for Hiickel systems the ground state may be considered as the combination of two Kekule structures. If the number of electron pairs in the system is odd, the ground state is the in-phase combination, and the system is aromatic. If the number of electron pairs is even (as in cyclobutadiene, pentalene, etc.), the ground state is the out-of-phase combination, and the system is antiaromatic. These ideas are in line with previous work on specific systems [40,50]. [Pg.342]

The Phase Change Upon CycUzadou of Different s-cis Cyclobutadiene Isomers ... [Pg.369]

THE cvcLOBUTADENE-TETRAHEDRANE SYSTEM. A related reaction is the photoisomerization of cyclobutadiene (CBD). It was found that unsubstituted CBD does not react in an argon matrix upon irradiation, while the tri-butyl substituted derivative forms the corresponding tetrahedrane [86,87]. These results may be understood on the basis of a conical intersection enclosed by the loop shown in Figure 37. The analogy with the butadiene loop (Fig. 13) is obvious. The two CBDs and the biradical shown in the figure are the three anchors in this system. With small substituents, the two lobes containing the lone electrons can be far... [Pg.370]

Within the cubane synthesis the initially produced cyclobutadiene moiety (see p. 329) is only stable as an iron(O) complex (M. Avram, 1964 G.F. Emerson, 1965 M.P. Cava, 1967). When this complex is destroyed by oxidation with cerium(lV) in the presence of a dienophilic quinone derivative, the cycloaddition takes place immediately. Irradiation leads to a further cyclobutane ring closure. The cubane synthesis also exemplifies another general approach to cyclobutane derivatives. This starts with cyclopentanone or cyclohexane-dione derivatives which are brominated and treated with strong base. A Favorskii rearrangement then leads to ring contraction (J.C. Barborak, 1966). [Pg.78]

Cyclobutadiene itself is not stable at room temperature. Several derivatives with stabilizing groups have been prepared by the acid-catalyzed dimerization of alkjmes (R. Gompper, 1975). Less substituted cyclobutadienes could be obtained by photolytic reactions in solid matrix at low temperatures (G. Maier, 1973, 1974). [Pg.329]

Cava, M. P. Mitchell, M. J. 1967, Cyclobutadiene and Related Compounds, Academic Press New York London... [Pg.364]

In addition to benzene and naphthalene derivatives, heteroaromatic compounds such as ferrocene[232, furan, thiophene, selenophene[233,234], and cyclobutadiene iron carbonyl complexpSS] react with alkenes to give vinyl heterocydes. The ease of the reaction of styrene with sub.stituted benzenes to give stilbene derivatives 260 increases in the order benzene < naphthalene < ferrocene < furan. The effect of substituents in this reaction is similar to that in the electrophilic aromatic substitution reactions[236]. [Pg.56]

During our discussion of benzene and its derivatives it may have occurred to you that cyclobutadiene and cyclooctatetraene might be stabilized by cyclic rr electron delocal ization m a manner analogous to that of benzene... [Pg.449]

The same thought occurred to early chemists However the complete absence of natu rally occurring compounds based on cyclobutadiene and cyclooctatetraene contrasted starkly with the abundance of compounds containing a benzene unit Attempts to syn thesize cyclobutadiene and cyclooctatetraene met with failure and reinforced the grow mg conviction that these compounds would prove to be quite unlike benzene if m fact they could be isolated at all... [Pg.449]

Cyclobutadiene escaped chemical charactenzation for more than 100 years Despite numerous attempts all synthetic efforts met with failure It became apparent not only that cyclobutadiene was not aromatic but that it was exceedingly unstable Beginning m the 1950s a variety of novel techniques succeeded m generating cyclobutadiene as a transient reactive intermediate... [Pg.451]

High level molecular orbital calculations of cyclobutadiene itself and experimen tally measured bond distances of a stable highly substituted derivative both reveal a pat tern of alternating short and long bonds characteristic of a rectangular rather than square geometry... [Pg.451]

Thus cyclobutadiene like cyclooctatetraene is not aromatic More than this cyclo butadiene is even less stable than its Lewis structure would suggest It belongs to a class of compounds called antiaromatic An antiaromatic compound is one that is destabi lized by cyclic conjugation... [Pg.451]

One of molecular orbital theories early successes came m 1931 when Erich Huckel dis covered an interesting pattern m the tt orbital energy levels of benzene cyclobutadiene and cyclooctatetraene By limiting his analysis to monocyclic conjugated polyenes and restricting the structures to planar geometries Huckel found that whether a hydrocarbon of this type was aromatic depended on its number of tt electrons He set forth what we now call Huckel s rule... [Pg.451]

Benzene cyclobutadiene and cyclooctatetraene provide clear examples of Huckel s rule Benzene with six tt electrons is a An + 2) system and is predicted to be aromatic by the rule Square cyclobutadiene and planar cyclooctatetraene are An systems with four and eight tt electrons respectively and are antiaromatic... [Pg.452]

FIGURE 11 13 Frost s circle and the TT molecular orbitals of (a) square cyclobutadiene (b) ben zene and (c) planar cyclooctatetraene... [Pg.452]

In the next section we 11 explore Huckel s rule for values of n greater than 1 to see how it can be extended beyond cyclobutadiene benzene and cyclooctatetraene... [Pg.454]

The general term annulene has been coined to apply to completely conjugated mono cyclic hydrocarbons with more than six carbons Cyclobutadiene and benzene retain then-names but higher members of the group are named [jcjannulene where x is the number of carbons m the ring Thus cyclooctatetraene becomes [8]annulene cyclodecapentaene becomes [10] annulene and so on... [Pg.454]

Section 11 18 Although cychc conjugation is a necessary requirement for aromaticity this alone is not sufficient If it were cyclobutadiene and cycloocta tetraene would be aromatic They are not... [Pg.465]

Section 11 19 An additional requirement for aromaticity is that the number of rr elec Irons m conjugated planar monocyclic species must be equal to An + 2 where n is an integer This is called Huckel s rule Benzene with six TT electrons satisfies Huckel s rule for n = 1 Square cyclobutadiene (four TT electrons) and planar cyclooctatetraene (eight rr electrons) do not Both are examples of systems with An rr electrons and are antiaromatic... [Pg.467]

For some systems a single determinant (SCFcalculation) is insufficient to describe the electronic wave function. For example, square cyclobutadiene and twisted ethylene require at least two configurations to describe their ground states. To allow several configurations to be used, a multi-electron configuration interaction technique has been implemented in HyperChem. [Pg.235]


See other pages where Cyclobutadienes— is mentioned: [Pg.121]    [Pg.121]    [Pg.223]    [Pg.351]    [Pg.332]    [Pg.372]    [Pg.199]    [Pg.77]    [Pg.486]    [Pg.449]    [Pg.449]    [Pg.449]    [Pg.451]    [Pg.451]    [Pg.451]    [Pg.451]    [Pg.452]    [Pg.452]    [Pg.453]    [Pg.453]    [Pg.453]    [Pg.465]    [Pg.270]   
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See also in sourсe #XX -- [ Pg.116 , Pg.117 , Pg.277 , Pg.277 ]

See also in sourсe #XX -- [ Pg.199 ]

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1,3-Cyclobutadiene, 1,2,3,4-tetraphenyl-, complexes with

1,3-cyclobutadiene destabilization

Applications cyclobutadiene

Aromatic systems cyclobutadiene

Aryl-substituted cyclobutadienes

Benzene cyclobutadiene

Benzene vs. Cyclobutadiene

Benzo cyclobutadiene

Biradical cyclobutadiene like

Bond distances cyclobutadiene

Bond distances cyclobutadiene derivative

Butadiene and Cyclobutadiene Complexes (4 7r-Systems)

Carcerands cyclobutadiene

Chromium complexes cyclobutadiene

Cobalt cyclobutadiene complexes

Comparison with cyclobutadiene

Complexes cyclobutadiene complex

Complexes cyclobutadiene-nickel

Complexes iron carbonyl-cyclobutadiene

Correspondence cyclobutadiene

Correspondence diagram cyclobutadiene

Cyclic polyenes Cyclobutadiene

Cyclobutadien

Cyclobutadien

Cyclobutadien-Komplexe

Cyclobutadiene

Cyclobutadiene

Cyclobutadiene Hiickel molecular orbitals

Cyclobutadiene Jahn-Teller distortion

Cyclobutadiene Metal Complexes

Cyclobutadiene analogues

Cyclobutadiene and Cyclooctatetraene

Cyclobutadiene anti-aromaticity

Cyclobutadiene antiaromatic compound

Cyclobutadiene antiaromatic molecule

Cyclobutadiene antiaromaticity

Cyclobutadiene aromaticity

Cyclobutadiene attempted synthesis

Cyclobutadiene barrier

Cyclobutadiene bonding

Cyclobutadiene capped

Cyclobutadiene cobalt polymers

Cyclobutadiene cobalt units as building blocks for the elaboration of supramolecular architectures

Cyclobutadiene complex

Cyclobutadiene complexes Subject

Cyclobutadiene complexes bonding

Cyclobutadiene complexes from acetylenes

Cyclobutadiene complexes halogenation

Cyclobutadiene complexes ligand transfer

Cyclobutadiene complexes molybdenum

Cyclobutadiene complexes oxidation

Cyclobutadiene complexes palladium

Cyclobutadiene complexes preparation

Cyclobutadiene complexes properties

Cyclobutadiene complexes reactions

Cyclobutadiene complexes reduction

Cyclobutadiene complexes spectra

Cyclobutadiene complexes stability

Cyclobutadiene complexes structure

Cyclobutadiene complexes substitution

Cyclobutadiene complexes thermal decomposition

Cyclobutadiene complexes with donor ligands

Cyclobutadiene complexes with metals

Cyclobutadiene complexes with nucleophiles

Cyclobutadiene complexes, -coordinated ring

Cyclobutadiene complexes, -coordinated ring structures

Cyclobutadiene compounds, antiaromatic destabilization

Cyclobutadiene derivatives

Cyclobutadiene dianion

Cyclobutadiene dication

Cyclobutadiene dimerisation

Cyclobutadiene dimerization

Cyclobutadiene dimers

Cyclobutadiene distortion

Cyclobutadiene electronic configuration

Cyclobutadiene energy levels

Cyclobutadiene formation

Cyclobutadiene geometry

Cyclobutadiene ground state

Cyclobutadiene intermediates

Cyclobutadiene iron tricarbonyl

Cyclobutadiene iron tricarbonyl complex

Cyclobutadiene iron tricarbonyl electrophilic substitution

Cyclobutadiene iron tricarbonyl oxidation

Cyclobutadiene iron tricarbonyl synthesis

Cyclobutadiene irontricarbonyl

Cyclobutadiene isolation

Cyclobutadiene kekule forms

Cyclobutadiene ligand, aromaticity

Cyclobutadiene matrix isolation

Cyclobutadiene matrix isolation study

Cyclobutadiene molecular orbital

Cyclobutadiene molecular orbital energy

Cyclobutadiene photolysis

Cyclobutadiene point group

Cyclobutadiene properties

Cyclobutadiene reactivity

Cyclobutadiene rectangular

Cyclobutadiene resonance energy

Cyclobutadiene rhodium

Cyclobutadiene rhodium triarylphosphine complexes

Cyclobutadiene ring expansion

Cyclobutadiene rings, coordinated

Cyclobutadiene self-reactivity

Cyclobutadiene square

Cyclobutadiene states

Cyclobutadiene structure

Cyclobutadiene synthesis

Cyclobutadiene tricyclic

Cyclobutadiene tunneling

Cyclobutadiene with Equal Bond Lengths

Cyclobutadiene) cobalt compounds formed by -cycloaddition of alkynes

Cyclobutadiene) tricarbonyliron

Cyclobutadiene, Diels-Alder reaction

Cyclobutadiene, angle strain

Cyclobutadiene, angle strain stability

Cyclobutadiene, antiaromatic

Cyclobutadiene, antiaromaticity electrostatic potential map

Cyclobutadiene, antiaromaticity reactivity

Cyclobutadiene, attack

Cyclobutadiene, degenerate orbitals

Cyclobutadiene, formation from alkynes

Cyclobutadiene, generation

Cyclobutadiene, generation in situ

Cyclobutadiene, preparation

Cyclobutadiene, radical cations

Cyclobutadiene, radical cations NMR spectra

Cyclobutadiene, radical cations isomerization

Cyclobutadiene, radical cations mass spectra

Cyclobutadiene, reactions

Cyclobutadiene, stabilization

Cyclobutadiene, stabilization coordination

Cyclobutadiene-Fe

Cyclobutadiene-carbon monoxide complex

Cyclobutadiene-metal

Cyclobutadiene-silver nitrate complex

Cyclobutadienes Cyclobutane

Cyclobutadienes Cyclobutanes

Cyclobutadienes Cyclobutanones

Cyclobutadienes Cyclobutenes

Cyclobutadienes complexes

Cyclobutadienes conformation

Cyclobutadienes cycloreversion

Cyclobutadienes derivatives

Cyclobutadienes ferrocenyl groups

Cyclobutadienes from alkenes

Cyclobutadienes from alkyne dimerization

Cyclobutadienes isolation

Cyclobutadienes mass spectra

Cyclobutadienes matrix trapping

Cyclobutadienes metal complexes

Cyclobutadienes metal-coordinated structures

Cyclobutadienes molecular orbitals

Cyclobutadienes nickel complex

Cyclobutadienes oxidation

Cyclobutadienes polymers

Cyclobutadienes push-pull

Cyclobutadienes radical cations

Cyclobutadienes reactions with

Cyclobutadienes reactions with alkynes

Cyclobutadienes synthesis

Cyclobutadienes thermolysis

Cyclobutadienes, Diels-Alder reaction

Cyclobutadienes, and

Cyclobutadienes, cycloaddition

Cyclobutadienes, dimerization

Cyclobutadienes, dimerization Cyclobutanation

Cyclobutadienes, dimerization intramolecular

Cyclobutadiene—Heavy Atom Tunneling

Cyclopentadienyl derivatives metal-coordinated cyclobutadienes

Cyclopentadienyl rings cyclobutadiene complexes

Dewar benzene from acetylene + cyclobutadiene

Dimerization of Cyclobutadiene

Diradicals cyclobutadiene

Electrocyclic cyclobutadiene

Electrostatic potential map cyclobutadiene

Energy-level diagrams for cyclobutadiene and benzene

Example 2 Square-Planar Cyclobutadiene (D)

Frost circles cyclobutadiene

Furans cyclobutadienes

Hiickel cyclobutadiene

Hiickel theory cyclobutadiene

Inner phase reactions cyclobutadiene

Inner-phase stabilization cyclobutadiene

Intramolecular cyclobutadiene

Iron carbonyl-cyclobutadiene

Iron complexes cyclobutadiene

Iron complexes cyclobutadienes

Iron complexes, with cyclobutadiene

Irradiation cyclobutadiene

Matrix isolation of cyclobutadiene

Metal complexes, of cyclobutadiene

Mobius cyclobutadiene

Molecular orbitals cyclobutadiene

Molecular orbitals of cyclobutadiene

Nucleus Independent Chemical Shift cyclobutadiene

Of cyclobutadiene, -iron tricarbonyl

Orbital cyclobutadiene-like

Organometallic polymers cyclobutadiene complexes

Palladium complexes cyclobutadiene derivatives

Photolysis of Cyclobutadiene

Platinum complexes cyclobutadiene

Polymers Containing Cyclobutadiene Complexes

Preparation of Cyclobutadiene Complexes

Reactions of cyclobutadiene

Reactive intermediate generation cyclobutadiene

Rhodium complexes cyclobutadiene

Ring structures cyclobutadiene complexes

Ruthenium cyclobutadiene complex

SOME STABLE CYCLOBUTADIENES

Subject index Cyclobutadiene)metal complexes,

Supramolecular chemistry cyclobutadiene

Tetra cyclobutadiene

The Cyclobutadiene Molecule

The Molecular Orbital Picture of Cyclobutadiene

Titanium complexes cyclobutadiene

Transition metal complexes cyclobutadienes

Tricyclic cyclobutadiene complexes

Triplet state cyclobutadiene

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