Carbon rr-bonding

Insertion Reactions of Transition Metal-Carbon rr-Bonded Compounds. II. Sulfur Dioxide and Other Molecules, 12,31... 

Enamines that would formally be derived from ammonium cannot be formed by any process that would expose the product to either acid or base, since the conversion to the imine is exothermic as both the carbon-nitrogen ir-bond (74 kcal mol" 1 cal = 4.18 J) and carbon-hydrogen bond (99 kcal mol" ) are stronger than the carbon-carbon rr-bond (63 kcal mol" ) and nitrogen-hydrogen bond (93 kcal mol" ). However, ethenamine and several substituted analogs have been prepared for spectral analysis (at low temperature) by retro-Diels-Alder reactions (equation 10). ... 

These systems were extensively studied with the goal of finding even better, more efficient catalysts. Based on these studies it appears that the key attributes for high activity appear to be three-fold (1) the transition metal center is complexed only by metal olefin Tt-bonds and a metal-carbon rr-bond (which can be part of an allylic system), (2) the metal center is cationic, and (3) the counter-ion is a weakly coordinating anion. 

In Chapter 15, we will examine the structural features that cause a compound to be aromatic. We will also look at the features that cause a compound to be antiaromatic. Then we will take a look at the reactions that benzene undergoes. You will see that although benzene, alkenes, and dienes are all nucleophiles (because they all have carbon-carbon rr bonds), benzene s aromaticity causes it to undergo reactions that are quite different from the reactions that alkenes and dienes undergo. 

Discuss the differences observed in the IR and NMR spectra of maleic anhydride and of bicyclo[2.2.1]hept-2-ene-c do-5,6-dicarboxylic anhydride that are consistent with the involvement of the carbon-carbon rr-bond of maleic anhydride in this experiment. 

The carbon-carbon rr bond is relatively weak, and the chemistry of the alkenes is governed largely by its reactions. The most common transformation is addition of a reagent A-B to give a saturated compound. In the process, the A-B bond is broken, and A and B form... 

As portrayed m Figure 2 20 the two carbons of acetylene are connected to each other by a 2sp-2sp cr bond and each is attached to a hydrogen substituent by a 2sp-ls CT bond The unhybndized 2p orbitals on one carbon overlap with their counterparts on the other to form two rr bonds The carbon-carbon triple bond m acetylene is viewed as a multiple bond of the ct + rr + rr type... 

Section 2 20 Carbon is sp hybridized in ethylene and the double bond has a ct com ponent and a rr component The sp hybridization state is derived by mix mg the 2s and two of the three 2p orbitals Three equivalent sp orbitals result and their axes are coplanar Overlap of an sp orbital of one car bon with an sp orbital of another produces a ct bond between them Each carbon still has one unhybridized p orbital available for bonding and side by side overlap of the p orbitals of adjacent carbons gives a rr bond between them... 

Bonding m alkenes is described according to an sp orbital hybridization model The double bond unites two sp hybridized carbon atoms and is made of a ct component and a rr component The ct bond arises by over lap of an sp hybrid orbital on each carbon The rr bond is weaker than the CT bond and results from a side by side overlap of p orbitals... 

Isomeric alkenes may be either constitutional isomers or stereoisomers There is a sizable barrier to rotation about a carbon-carbon double bond which corresponds to the energy required to break the rr component of the double bond Stereoisomeric alkenes are configurationally stable under normal conditions The configurations of stereoisomeric alkenes are described according to two notational systems One system adds the prefix CIS to the name of the alkene when similar substituents are on the same side of the double bond and the prefix trans when they are on opposite sides The other ranks substituents according to a system of rules based on atomic number The prefix Z is used for alkenes that have higher ranked substituents on the same side of the double bond the prefix E is used when higher ranked substituents are on opposite sides... 

We can consider the hydroboration step as though it involved borane (BH3) It sim phfies our mechanistic analysis and is at variance with reality only m matters of detail Borane is electrophilic it has a vacant 2p orbital and can accept a pair of electrons into that orbital The source of this electron pair is the rr bond of an alkene It is believed as shown m Figure 6 10 for the example of the hydroboration of 1 methylcyclopentene that the first step produces an unstable intermediate called a tt complex In this rr com plex boron and the two carbon atoms of the double bond are joined by a three center two electron bond by which we mean that three atoms share two electrons Three center two electron bonds are frequently encountered m boron chemistry The tt complex is formed by a transfer of electron density from the tt orbital of the alkene to the 2p orbital... 

An orbital overlap description of electron delocalization mil dimethylallyl cation H2C=CH—C(CH3)2 is given m Figure 10 2 Figure 10 2a shows the rr bond and the vacant p orbital as independent units Figure 10 2b shows how the units can overlap to give an extended rr orbital that encompasses all three carbons This permits the two rr electrons to be delocalized over three carbons and disperses the positive charge... 

Aldol condensation offers an effective route to a p unsaturated aldehydes and ketones These compounds have some interesting properties that result from conjugation of the carbon-carbon double bond with the carbonyl group As shown m Figure 18 6 the rr systems of the carbon-carbon and carbon-oxygen double bonds overlap to form an extended rr system that permits increased electron delocalization... 

The diminished rr electron density m the double bond makes a p unsaturated aide hydes and ketones less reactive than alkenes toward electrophilic addition Electrophilic reagents—bromine and peroxy acids for example—react more slowly with the carbon-carbon double bond of a p unsaturated carbonyl compounds than with simple alkenes... 

Figure 1.16 The structure of acetylene. The two sp-hybridized carbon atoms are joined by one sp-sp rr bond and two p-p 7t bonds.

You might recall from Section 1.9 that a carbon-carbon triple bond results from the interaction of two sp-hybridized carbon atoms. The two sp hybrid orbitals of carbon lie at an angle of 180° to each other along an axis perpendicular to the axes of the two unhybridized 2py and 2pz orbitals. When two sp-hybridized carbons approach each other, one sp-sp a bond and two p-p -rr bonds are... 

A carbon-carbon double bond is stronger than one carbon-carbon single bond but weaker than the sum of two single bonds (Section 2.15). A carbon-carbon triple bond is weaker than the sum of three carbon-carbon single bonds. Recall that a single C—C bond is a o-bond, but the additional bonds in a multiple bond are TT-bonds. One reason for the difference in strength is that the side-by-side overlap of p-orbitals that results in a rr-bond is not as great as the end-to-end overlap that results in a o-bond. 

FIGURE 14.30 Structure of diamond. Each carbon atom is sp hybridized and forms tetrahedral rr-bonds to four neighbors. This pattern is repeated throughout the crystal and accounts for diamond s great hardness. 

The heterobenzenes of the group 15 elements (1-5) comprise a series in which elements of an entire column of the periodic table have been incorporated into aromatic rings. The comparative study of this series has been extremely valuable for evaluating p-p rr-bonding between carbon and the heavier elements.1 However, the heterobenzene series has two important limitations. Only arsabenzene has a well developed aromatic chemistry. Moreover, stibabenzene and particularly bismabenzene are so labile that it has been difficult to obtain derivatives stable enough for study. 

In all previous RRS measurements of dermal carotenoids we measured the total concentration of all long-chain carotenoid species since the method only detects the chain s carbon double-bond vibration, which is identical in all species. Lycopene has an increased conjugation length compared... 

The presence of Ga—C rr-bonds in many dimeric precursor complexes was found to affect the quality of the resultant films and the film growth rate in many cases films obtained from dimers were found to contain significant levels of carbon. Monomer species have also been investigated. 

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