Cycloadditions and Retrocycloadditions

The mechanism again consists of a series of [2 + 2] cycloadditions and retrocycloadditions. 

The mechanism of olefin metathesis does not involve the classic reactions we have covered—namely, oxidative addition, reductive elimination, (3-hydride elimination, etc. Instead, it simply involves a [2+2] cycloaddition and a [2+2] retrocycloaddition. The [2+2] terminology derives from pericyclic reaction theory, and we will analyze this theory and the orbitals involved in this reaction in Chapter 15. In an organometallic [2+2] cycloaddition, a metal alkylidene (M=CR2) and an olefin react to create a metal lacyclobutane. The metalla-cyclobutane then splits apart in a reverse of the first step, but in a manner that places the alkylidene carbon into the newly formed olefin (Eq. 12.83). Depending upon the organometallic system used, either the alkylidene or the metallacycle can be the resting state of the... 

A large fraction of the chemical reactions known are used to form heterocyclic compounds. Displacement reactions and cycloadditions are particularly important, and their rates are therefore of great practical interest. The same is true for the rates of reverse reactions — ring opening by displacements or retrocycloadditions. It was realized over the last 40 years that... 

The mechanism begins with a-hydride elimination to give a benzylidenetitanium complex. A [2 + 2] cycloaddition gives the titanaoxetane, and [2 + 2] retrocycloaddition affords the product and the byproduct. 

Whether cis or trans fusion is observed in nitrone cycloadditions can depend on reaction conditions as first determined by LeBel et al.9 At lower temperature where cycloaddition is irreversible, kinetic control prevails and this usually favors cis fusion. However, at higher temperature equilibration can occur through retrocycloaddition and the more stable product will predominate (i.e. thermodynamic control). The nitrone may also undergo ( )/(Z) isomerization, particularly at elevated temperature, and this complicates the analysis a different kinetically favored ratio might prevail. A recent example of temperature dependence involves formation of isoxazolidines (18) and (19) from aldehyde (17a Scheme 4). At 90 C ds-fused (18) and rrans-fused (19) were formed in 74% and 9% yield, respectively. At 140 C, however, (18) and (19) were formed in 31% and 34% yield. 

Metathesis of alkene 6 to give the new alkenes 11 and 15 is explanined by the following mechanism. The first step is [2+2] cycloaddition between metal carbene 5 and alkene 6 to generate the metallacyclobutane 7 as an intermediate. The real catalyst 8 is generated by retrocycloaddition of the metallacyclobutane 7. Reaction of 8 with alkene 6 generates the metallacyclobutanes 9 and 10 as intermediates. The intermediate 10 is a nonproductive intermediate, which reproduces 6 and 8, while 9 is a productive intermediate and yields the new alkene 11 and the real catalyst 12. Cycloaddition of 12 to alkene 6 produces the productive intermediate 14, from which the new alkene 15 and the active catalytic species 8 are formed. The intermediate 13 is a nonproductive one. 

Retrocycloaddition Many cycloaddition reactions require moderate heating to overcome the activation energy, but if it is heated too much the equilibrium will favour cycloreversion or retrocycloaddition. For example, cyclopentadiene slowly undergoes cycloaddition with itself one molecule of cyclopentadiene acts as a [4ir]-electrons diene and the other as a [2tt]-electrons dienophile. The product is an endo-tricyclo[5.2.1.0]deca-3,8-diene (8.4), often called dicyclopentadiene. The product 8.4 gives back cyclopentadiene on heating at 150° C for an hour. 

The cycloaddition-retrocycloaddition chemistry used to prepare (37) can be employed to prepare (76a-c) in quite respectable yields of 79%, 60%, and 78%, respectively.90... 

The reaction proceeds via metallacyclobutanes as shown in Scheme 13. A [2+2] cycloaddition occurs between the olefin substrate and the metal alkylidene catalyst to produce a metallacyclobutane. Retrocycloaddition then occurs to afford an olefin metathesis product and a new metal alkylidene 88, which works as a further catalyst. 

TpRe(CO)(MeIm)(f/ -anisole) reacts with DMAD to form an f/ -barrelene-complex (Scheme 54). Demetalation gives the barrelene and a trisubstituted benzene that is the product of retrocycloaddition involving elimination of ethyne. TpRe(CO)(MeIm)(5,6-f/ -anisole) also reacts with NMM to give a cycloadduct with the methoxy group at a bridgehead position. In addition, TpRe(CO) (MeIm)(f/ -l-methylpyrrole) undergoes a 1,3-dipolar cycloaddition reaction with dimethylfumarate to give 7-azabicyclo-[2.2.1]heptene (Scheme 54). 

A cycloaddition reaction [18] is the joining together of two independent jt-bonding systems to form a ring with two new a bonds. The reverse is called a retrocycloaddition reaction, and the selection rules apply in both directions of a given reaction. 

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