Multibond Processes

Many reactions result in a nearly simultaneous formation of a pair of o bonds. In some cases, a carbene is a transient intermediate. Carbene, iCH, is electron-deficient it lacks two electrons for a complete octet. Although there is no net charge and little or no dipole, it is highly electrophilic and will attack both jt and o electrons to form pairs of new bonds. The lack of specificity in this high reactivity renders iCH of little synthetic value, but selective cyclopropanation of alkenes may be accomplished using Simmons-Smith conditions (Eq. 7.36) [60], with a variety of methods available [61]. 

Cyclopropanation of alkenes can also be accompUshed via other transient electrophilic intermediates that are possibly metal complexes of carbenes. Copper, rhodium, ruthenium, cobalt, or palladium catalyze the decomposition of diazocarbonyl compounds [62] which, in the presence of alkenes, gives cyclopropyl derivatives. The intramolecular example shown in Equation 7.37 uses rhodium bis(l-adamantate) dimer as the catalyst [63]. 

Concerted reactions are commonly used to join carbons. For example, the Diels-Alder reaction is the formation of a cyclohexene from a diene and an alkene. Usually the alkene is rendered electrophilic by conjugation with a carbonyl group, and the diene may be rendered nucleophilic by electron-donating substituents. In the case shown in Equation 7.38 the alkene is further electron depleted by association with a Lewis acid [64], a common technique for accelerating Diels-Alder reactions. In some cases, the alkene is nucleophilic and the diene is electrophilic as in Equation 7.39 [65]. Examples of this sort are called reverse-electron-demand Diels-Alder reactions. It is important to point out here that the concerted reactions differ from the foregoing in that no carbanion or cation intermediate is involved, and in many cases, electrophilic and nucleophilic factors are not present, as in the very favorable dimerization of cyclopentadiene. These reactions are covered in more detail in Chapter 5. 

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Approach to Chemical Reactivity. Specificity of Multibond Processes. 

For multibond processes we take the effective bond length as the arithmetical mean of the bond lengths of the corresponding reactive bonds of reactants and products. 

Wender, P. Miller, B.L. (1993) Toward the Ideal Synthesis Connectivity Analysis and Multibond-Forming Processes. In Organic Synthesis Theory and Applications, Vol. 2 (ed. T. Hudlicky), pp. 27-65. JAl Press, Greenwich, CT. 

The continuing accumulation of experimental material made it, however, still more apparent that the above criteria, both kinetic and stereochemical [61-64], cannot be regarded as absolute [65], since there are probably both concerted reactions which are forbidden as well as nonconcerted ones which are allowed. An example of the first type of reaction is some 2 + 2 retro-cycloadditions on sterically strained systems [62,66] the second type of processes is then represented by the so-called multibond reactions [67]. This, of course, stimulates to formulate a new and more universal criterion of concertedness as well as a deeper understanding of the very concept of concertedness. 

It should be noted in conclusion that the above-considered (Sect. 9.2.5) double proton migrations may also be regarded as typical multibond reactions. The rule cited above is actually quite applicable to these reactions. At the same time, given a favorable stereochemistry, a concerted synchronous mechanism may be realized for some systems of this type. Analysis of the factors that may provide for the possibility of such exceptions in the case of this and some other multibond reactions would be of particular interest for understanding the nature of cooperative processes. 

The Ma group has reported some very elegant multibond forming processes to synthesize dendralenes, including both palladium(II)- and rhodium(I)-catalyzed cycloisomerizations of di-allenes 106 to form cyclic dendralenes 107 (Scheme 1.15) [68, 69, 71], and also the remarkable palladium(0)-catalyzed three-component reaction of di-allenes 108, propargylic carbonates 109, and boronic acids 110 to form bicyclic [3]dendralenes 111 (Scheme 1.15) [70]. Similar, intermolecular transformations to those used to form dendralenes 107 had been reported earlier by Alcaide, to synthesize dihydrofuran-containing dendralenes [72, 73]. 

The multibond character of the related processes implies that although the traditional closed-shell HF approach is capable of providing acceptable descriptions of both the reagents and the final product, it cannot account for all electronic rearrangements along the reaction pathway, such as, e.g., the... 

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