Two-step elimination

When thermolyzing azido compounds, the question arises as to whether an observed rearrangement is achieved in one step, concerted to the elimination of Nj, or whether there are two steps, elimination and rearrangement, with nitrenes as intermediates. Generally, thermolysis of azidoboranes is more probably a concerted process (22). The decomposition of (tPr2N)2BN3, however, carried out at 450°C, gives a product, besides the main product of Eq. (19), that has to be interpreted in terms of a nitrene stabilized by an intramolecular CH insertion [Eq. (19b)]. 

Results of an ab initio study of H2 elimination from protonated fonnaldehyde and formaldhnme suggest that the process is concerted, whereas a two-step elimination mechanism applies in the case of protonated thiofonnaldehyde.102... 

The classical Bamford-Stevens reaction6 is a two-step elimination process involving a hydrazone dianion intermediate. The latter sequentially... 

Let us now turn to /3-eliminations that take place via acyclic transition states. There three elimination mechanisms (Figure 4.16) depending on the order in which the C—H and the C—Het bonds of the substrate are broken. If both bonds are broken at the same time, it is a one-step E2 elimination. When first one and then the other bond is broken, we have two-step eliminations. These can take place via the El or the Elcb mechanism. In the El mechanism, the C— Het bond is broken first and the C—H bond is broken second. Conversely, in the Elcb mechanism the C—H bond is broken first, by deprotonation with a base. In this way the conjugate base (cb) of the substrate is produced. Subsequently, the C—Het bond breaks. 

In summary, the benzyne mechanism operates when the halobenzene is unactivated toward nucleophilic aromatic substitution, and forcing conditions are used with a strong base. A two-step elimination forms a reactive benzyne intermediate. Nucleophilic attack, followed by protonation, gives the substituted product. 

ElcB mechanism (Section 24.IB) A two-step elimination mechanism that goes by a carbanion intermediate. EleB stands for Elimination Unimoleeular, Conjugate Base. ... 

The other is a two-step, elimination-addition mechanism. In the elimination step, / elimination occurs by an E2 mechanism to give a ketene, a very reactive compound that is not usually isolable. In the addition step, the alkoxide adds to the electrophilic carbonyl C of the ketene to give the enolate of an ester. Acyl chlorides lacking a-hydrogens (t-BuCOCl, ArCOCl), of course, can react only by the addition-elimination mechanism. 

The generally accepted mechanism for this reaction is a two-step elimination-addition mechanism. In the / elimination step, NIk promotes the / elimination of the elements of HI to give benzyne. In the addition step, Nik adds to the very strained triple bond to give an aryl anion, which undergoes proton transfer. Workup gives the neutral product. 

A number of promoieties have been designed to undergo the two-step elimination of a drug (216-218). Coumarinic acid is one such... 

By analogy with the SnI mechanism, can you think of a two-step elimination mechanism ... 

PROBLEM 10.20 The first step in eq. 10.26 really involves two reactions, addition of ammonia to the carbonyl carbon to form an ammonium alkox-ide followed by a proton transfer from the nitrogen to the alkoxide oxygen. Illustrate this process with equations using the arrow-pushing formalism. The second step in eq. 10.26 also involves two steps, elimination of an alkoxide (R O ) followed by deprotonation of the hydroxyl group. Write a detailed mechanism for these steps. 

In Chapter 10 we looked at El reactions (two-step elimination reactions that form a car-bocation intermediate) and E2 reactions (concerted elimination reactions). The preceding base-catalyzed dehydration represents the third kind of elimination reaction— namely, an ElcB (elimination unimolecular conjugate base) reaction, a two-step elimination reaction that forms a carbanion intermediate. ElcB reactions occur only if the carbanion can be stabilized by electron delocalization. 

The first total synthesis of archazohd A 111, disclosed by Menche et al. [89], was achieved from the assembly of three main building blocks - ketone 104, aldehyde 105, and alkene 107 (Scheme 6.20). The synthesis was accomplished first by employing a boron-mediated aldol reaction of 105 and 104 followed by a two-step elimination to give iodide 106. Heck reaction of 106 with 107 using dichloropaUadium catalyst and tetra-n-butylammonium chloride (TBACl) additive followed by reaction with phosphonate 108 furnished alkene 109 with E/Z selectivity (6 1). Oxidative removal of the PMB group in 109 and Swern oxidation afforded ketophosphonate 110. 

This elimination of a LG and H to form a double bond is called a two-step elimination (or El). 

Consider the following rate data for the two-step elimination in Model 3. 

Table 14.1 Rate dependence on concentration for Two-Step Elimination...

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