Selectivity of alkenes

On the basis of similar arguments, previously proposed, transition state TSi is expected to have lower energy than TS2 (Scheme 18) and thus account for the geminal selectivity of alkenes with the large L group in the vinyl position. 

The high selectivity of alkene formation is not the result of a large rate difference in the hydrogenation of the triple and double bonds. Rather, it is ensured by the strong adsorption of alkynes compared with that of alkenes. The alkyne displaces the alkene from the surface or blocks its readsorption. 

Data for a selection of alkenes and alkynes are collected in Table 10.2. 

First reported in 1909, the photochemical [2+2] cyclization between alkenes and carbonyl compounds, known as the Patemo-Biichi reaction, is one of the most commonly used methods of synthesizing oxetanes. The scope of the reaction is however limited and only occurs readily between electron-rich alkenes and electron-poor carbonyls. The importance of the reaction is that, with careful selection of alkenes and carbonyl compounds, high regio- and stereoselectivities can be achieved (see CHEC(1984) and CHEC-II(1996) for previous examples) < 1984CHEC(7)363, 1996CHEC-II(1)721>. 

Fig. 7. Panel (a) primary selectivity of small radicals from the decomposition of l-methyl-4-alkyl-cyc/o-hexanes vs. carbon number panel (b) primary selectivity of alkenes from the decomposition of l-methyl-4-alkyl-cyc/o-hexanes vs. carbon number.

Table 5 Stereo-, regio- and chemo-selectivity of alkene bromination in methanol (0.2 m NaBr) at 25°C.a...

FIGURE 9.17 Selectivity of alkene products from tbe cyclization of 11 by the coordination cage. (Reprinted from Ref [29] with permission of American Chemical Society. Copyright 2012.)... 

This chapter will focus almost exclusively on alkene metathesis catalyzed by well-defined homogeneous ruthenium-catalyst systems and is divided into three sections (1) a discussion of how precatalyst structure affects the rate and mechanism of initiation in two key series of metathesis precatalysts (2) a discussion of how substrate structure, both close to and remote from the alkene termini, affects the rate and selectivity of alkene metathesis in synthetic chemistry and (3) the tools that have been used by experimental and theoretical chemists to study alkene metathesis reactions. In each case, the discussion will be focused on the specific topics interested readers are referred to a recent article which covers a wider range of the mechanistic aspects of alkene metathesis with ruthenium complexes, albeit in less depth. 

Rate constants for the reactions of [Ru(NH3)5(OH2)] with ethyl glycinate, methyl sarcosinate, and methylamine have been determined at 298.2 K in aqueous solution. These have been compared with the rate constant for reaction of [Ru(NH3)s(OH2)] with ammonia, to show that alkyl substitution in ammonia reduces the affinity of ruthenium(II) for such ligands.A link between classical formation reactions of this type and organometallic systems is provided by a study of reactions of [Ru(NH3)5(OH2)] with a selection of alkenes and alkynes. ... 

The site selectivity of alkene addition to benzonitrile 20 has been studied with a series of cyclic and acyclic olehns. Again, two reaction modes could be observed [2-1-2]-cycloaddition to the nitrile group leading to 2-azabutadienes, deriving from the primarily formed azetines, (e.g., 21 from 2,3-dimethyl-2-butene), and ortfeo-cycloaddition to the benzene ring (e.g., 22 from 2,3-dihydrofuran). ... 

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