The Alkenes and Alkynes

The hydrocarbons discussed so far in this chapter are referred to as saturated, because all the carbon-carbon bonds are single bonds. Hydrocarbons that have double and triple carbon-carbon bonds are referred to as unsaturated (Fig. 7.10). Ethylene (C2H4) has a double bond between its carbon atoms and is called an alkene. The simplest alkyne is acetylene (C2H2), which has a triple bond between its carbon atoms. In naming these compounds, the -ane ending of the corresponding alkane is replaced by -ene when a double bond is present and by -yne when a triple bond is present. Ethene is thus the systematic name for ethylene, and ethyne for acetylene, although we will continue to use their more common names. For any compound with a carbon backbone of four or more carbon atoms, it is necessary to specify the location of the double or triple bond. This is 

FIGURE 7.11 Sketches of sp hybridized orbitals on carbon, (a) The three sp hybridized orbitals are oriented in a plane with their axes at angles of 120 degrees, (b) The nonhybridized 2p orbital is perpendicular to the plane containing the three sp hybrid orbitals. 

FIGURE 7.12 Bond formation in ethylene, (a) Overlap of sp hybrid orbitals forms a cr bond between the carbon atoms, (b) Overlap of parallel 2p orbitals forms a tt bond. 

These compounds differ in melting and boiling points, density, and other physical and chemical properties. 

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We conclude this introduction to hydrocarbons by describing the orbital hybridization model of bonding m ethylene and acetylene parents of the alkene and alkyne families respectively... 

The catalyst used throughout this study was a 1% w/w palladium on alumina supplied by Johnson Matthey. The support consisted of 0-alumina trilobes (S.A. 100 m g" ) and the catalyst was sized to < 250 p for all catalytic studies. The alkenes and alkynes (all Aldrich >99%) were used without further purification. No significant impurities were detected by GC. Gases (BOG, >99.99%) were used as received. 

A mechanistic pathway is proposed based upon the observed regioselectivities and other results that were obtained during the exploration of the scope and limitations of the Alder-ene reaction.38 Initially, coordination of the alkene and alkyne to the ruthenium catalyst takes place (Scheme 5). Next, oxidative addition affords the metallocycles 42 and 43. It is postulated that /3-hydride elimination is slow and that the oxidative addition step is reversible. Thus, the product ratio is determined by the rate at which 42 and 43 undergo /3-hydride elimination. 

In our study of the simple hydrocarbons, there are only two functional groups. One is a carbon-to-carbon double bond. Hydrocarbons that contain a carbon-to-carbon double bond are alkenes. The other hydrocarbon functional group is a carbon-to-carbon triple bond. Hydrocarbons that contain a triple bond are alkynes. These functional groups are the reactive sites in the alkenes and alkynes. The result is that alkenes and alkynes are more reactive than the alkanes. 

The use of crotylsilanes instead of allylsilanes in both the alkene and alkyne tandem reactions allows the creation of a third and a second stereocenter respectively (Scheme 5.25) [39]. These reactions allow extremely rapid and efficient access to stereochemi-cally complex polyketide chains, as demonstrated by the five-step conversion of alcohol 65 to triol 66 in 31% overall yield. The conversion of 67 to both 68 and 69 establishes the scope of this chemistry to allow access to different structural and stereochemical motifs. 

In Organic I you probably started with the hydroccirbons, compounds of carbon and hydrogen, including the alkenes and alkynes that contained double and single bonds, respectively. Then you probably touched on some of the more common functional groups, such as alcohols and maybe even some aromatic compounds. 

The orthogonal reactivities exhibited by IBr (2.5equiv at —78°C) and Au(PPh3)BF4 (5mol% at 0°C) in the activation of the alkene and alkyne groups of trichloroacetimidate 440 could be utilized in regioselective cyclizations toward 5,6-dihydro-4//-l,3-oxazine derivatives 441 or 442, respectively (Scheme 84) <2006OL3537>. 

In the case of the alkenes and alkynes the A, B, C and A B were not calculated by the above method, since the data for these compounds are much less reliable than in the case of the alkanes. 

The carbolithiation of alkenes and alkynes is a useful transformation for the generation of a new carbon—carbon bond, specially when the alkenes and alkynes are activated by conjugation to carbonyl and related electron-withdrawing groups. Similarly to the intramolecular carbolithiation, it is possible to carry out this reaction with high diastere-o selectivity. 

The nnsaturated hydrocarbons include the alkenes, which contain one double bond per molecule, and the alkynes, which contain one triple bond per molecnle. Their systematic names begin as the names of the corresponding alkanes do, bnt they end with -ene or -yne, respectively. The location of the multiple bond may have to be specified in the name by including the address of the mnltiple-bonded carbon atom that is closer to the end of the chain. The alkenes and alkynes are more active than the alkanes for example, they react with halogens to form halogenated hydrocarbons under mnch less severe... 

The catalytic reduction of alkenes and alkynes are important methods for the synthesis of alkanes. The hydroboration and hydrosilylation of alkenes are alternatives to catalytic methods. Again, both the alkene and alkyne may have played an important role in the construction of the hydrocarbon chain. 

The IR spectra of hexane, 1-hexene, and 1-hexyne illustrate the important differences that characterize the IR spectra of hydrocarbons above 1500 cm. Although all three compounds contain C-C bonds and sp hybridized C-H bonds, the absorption peaks due to C=C and C=C readily distinguish the alkene and alkyne. 

The most likely scenario for enyne metathesis is an intramolecular combination of the alkene and alkyne moieties to form a ring, which is really a variation of RCM. Examples of intermolecular enyne metathesis (CM) have been successful if they are run in an atmosphere of ethene, using it as the alkene component. Even for some intramolecular enyne metatheses, preequilibration of the catalyst with ethene caused vastly improved yields.74 Equations 11.2475 and 11.2576 show... 

The tethering of the alkene and alkyne expands the scope of alkene partners to include more hindered disubstituted alkenes and especially trisubstituted alkenes. Equations 1.47 and 1.48 show two differently arranged trisubstituted alkene partners as well as being able to form five- and six-membered rings [45, 46]. 

In the case of the cyclic substrate 51, the intervention of a C-H insertion pathway reveals itself in terms of the diastereoselectivity, not regioselectivity. Thus, exposure of enyne 51 to the standard Ru catalyst at ambient temperature produced the transfused bicyclo[5.4.0]undecene 52 (Equation 1.60, path a) [55]. If a metallacycle mechanism was operative, coordination of the metal with both the alkene and alkyne must occur to form the cis-fused product. On the other hand, coordination of the Ru with the Lewis basic bridgehead substituent directs it to abstract an allylic C-H on the same face as the substituent, which then leads to the trans-fused product as observed. On the other hand, cycloisomerization using a Pd(0) precatalyst does indeed lead to the Z-fused bicycle (Equation 1.60, path b). 

Alkanes are saturated hydrocarbons because they contain only carbon and hydrogen and have only carbon-to-hydrogen and carbon-to-carbon single bonds. The alkenes and alkynes are unsaturated hydrocarbons because they contain at least one carbon-to-carbon double or triple bond, respectively. 

Alkenes and alkynes are unsaturated hydrocarbons. Alkenes are characterized by the presence of at least one carbon-carbon double bond and have the general molecular formula C H2 . Alkynes are characterized by the presence of at least one carbon-carbon triple bond and have the general molecular formula C H2 2- The physical properties of the alkenes and alkynes are similar to those of alkanes, but their chemical properties are quite different. 

Our experience with cyclopropanes has been limited. There is not very much data, but, in this case (in contrast to the alkenes and alkynes), it is difficult to parameterize to that which is available. Because of the very fragmentary nature of the results, they will not be discussed further here. 

Production of the tetrasubstituted cyclopentenone (15a, 15b)) involves condensing the protected vinyl ether (16) with (12) via a Pauson-Khand reaction. Studies on the Pauson-Khand reaction , 1 have shown that upon condensing an unsymmetrical alkene with an unsymmetrical alkyne in the presence of a cohalt carbonyl compound give rise to mixtures of diastereomers (Fig 1) with the bulkier substituents of the alkene and alkyne being a to the carbonyl of the cyclopentenone. However, the stereochemistry a to the... 

There are four subfamilies of hydrocarbons, known as alkanes, alkenes, alkynes, and aromatics. (These families will be discussed in detail in Chapters 4 and 5.) The alkane and aromatic families of hydrocarbons occur naturally the alkenes and alkynes are manmade. Both types of hydrocarbons are used to make other families of chemicals, known as hydrocarbon derivatives. Radicals of the hydrocarbon families are made by removing at least one hydrogen from the hydrocarbon and replacing it with a nonmetal other than carbon or hydrogen. Ten of these hydrocarbon derivatives will be discussed in detail in the appropriate chapters associated with then-major hazards alkyl halides, nitros, amines, ethers, peroxides, alcohols, ketones,... 

Branching can also occur in the alkene and alkyne famihes. In order for branching to occur in hydrocarbons, there must be at least a four-carbon compound. Propane cannot be branched until the hydrocarbon derivatives, that is, elements other than carbon and hydrogen, are added to the structure of the compound. Other types of branching will be discussed in the Hydrocarbon Derivatives section of Chapter 5. 

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