Alkenes, viii cycloadditions

Cycloaddition reactions between alkenes and noncarbohydrate, carbonyl compounds have been described in discussing the reactions of alkenes (see Table I and Scheme 1). The depiction of the excited carbonyl given in Scheme 6 is useful in understanding the regiochem-istry of the cycloaddition process, as it suggests that the electron-deficient oxygen atom in the excited carbonyl will react with the alkene to produce the (more-stable) 1,4-diradical. Table VIII lists cycloaddition reactions in which the excited carbonyl is part of a carbohydrate. 

Turning from iminophosphanes to alkylidenophosphanes (phospha-alkenes), the orientation of the [2 + 2]-cycloaddition is inverted, as far as phosphorus is concerned only one example has been worked out (product VIII) 19). The phosphaalkyne iBuC=P does not react with the iminoborane BuB=NtBu, which instead trimerizes (IS). An exotic [2 + 2]-cycloaddition is observed when the very reactive titanaethene... 

In Fig. 1.6 a simplified mechanism for as -dihyroxylation of alkenes and ketohydroxylation of R CH=CHR by RuCl3/Oxone /aq. Na(HC03)/Et0Ac-CH3CN is shown. The cA-dihydroxylation route involves (3 + 2) cycloaddition of RuO to the alkene giving a Ru(VI) ester (1) which is oxidised by (HSOj) to the Ru(VIII) ester (2). Reversible nucleophilic addition of water to (2) gives the diol R CH(OH) CH(OH)R (3). Ketohydroxylation ensues when the activated Ru(VIII) ester... 

Many examples of the intramolecular [2 + 2] photocydoaddition of alkenes to benzene derivatives have been reported. The acetophenone derivative 41 undergoes an efficient [2 + 2] photocydoaddition, leading to the cyclobutane derivative 42 (Scheme 5.9, reaction 18) [46, 47]. It was shown that, in this case, a nn triplet state is involved. The presence of a nitrile group in compound 43 induces a [2 + 2] cycloaddition at position 1,2 of the aromatic moiety, leading to intermediate VIII (reaction 19) [48]. Following tautomerization, the final product 44 is formed. 

Alkenes can be dihydroxylated cis-selectively by reaction with a stoichiometric amount of yV-mcthylmorpholinc-yV-oxidc (NMO for a preparation, see Figure 14.30), a catalytic amount of a suitable Os(VIII) reagent, and in the presence of water. This reaction, the cis-vic dihydroxylation, was not discussed in the section on cis-selective additions to C=C double bonds in alkenes (Section 3.3), and it also was not discussed in the context of cycloadditions (Chapter 12). From a preparative point of view, this reaction is closely related to oxidative cleavages, and for this reason it is introduced now (Figure 14.16). 

Stable five-membered dioxametallacyclic adducts are obtained from the reaction of Group VIII metal-peroxo complexes (Rh, Ir, Pd, R) with nucleophilic ketones and electrophilic alkenes or ketones. The adducts of R02(PPh3)2 with acetone and l,l-dicyano-2-methylpropene have been characterized by X-ray crystallography. The reaction of (Ph3P)2R02 with nucleophilic ketones involves, as the major pathway, the precoordination of the carbonyl compound to the vacant axial site of platinum prior to intramolecular 1,3-dipolar cycloaddition (equation 42). ... 

Reviews.—Recent reviews involving olefin chemistry include olefin reactions catalysed by transition-metal compounds, transition-metal complexes of olefins and acetylenes, transition-metal-catalysed homogeneous olefin disproportionation, rhodium(i)-catalysed isomerization of linear butenes, catalytic olefin disproportionation, the syn and anti steric course in bi-molecular olefin-forming eliminations, isotope-elfect studies of elimination reactions, chloro-olefinannelation, Friedel-Crafts acylation of alkenes, diene synthesis by boronate fragmentation, reaction of electron-rich olefins with proton-active compounds, stereoselectivity of carbene intermediates in cycloaddition to olefins, hydrocarbon separations using silver(i) systems, oxidation of olefins with mercuric salts, olefin oxidation and related reactions with Group VIII noble-metal compounds, epoxidation of olefins... 

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