Alkenes rearrangement

Another feature of systems that are subject to B-strain is their reluctance to form strained substitution products. The cationic intermediates usually escape to elimination products in preference to capture by a nucleophile. Rearrangements are also common. 2-Methyl-2-adamantyl p-nitrobenzoate gives 82% methyleneadamantane by elimination and 18% 2-methyl-2-adamantanol by substitution in aqueous acetone. Elimination accounts for 95% of the product from 2-neopentyl-2-adaman l p-nitrobenzoate. The major product (83%) from 2-r-butyl-2-adamantyl p-nitrobenzoate is the rearranged alkene 5. 

The reaction mechanism has been confirmed by trapping of intermediates 13, 14 and 15. Because of the fact that neither a carbene nor a carbenium ion species is involved, generally good yields of non-rearranged alkenes 2 are obtained. Together with the easy preparation and use of tosylhydrazones, this explains well the importance of the Shapiro reaction as a synthetic method. 

Formation of a Rearranged Alkene During Dehydration of a Primary Alcohol... 

The enantioselective synthesis of azabicyclic y-lactams starting from 2-azanorbornenones after treatment of a catalytic amount of RuCl2(PCy3)2 (= CHPh) in the presence of ethylene or allyl acetate proceeds also via ring rearrangement—alkene metathesis (ROM-CM-RCM) [41] (Scheme 19). If n = 0 or 3, no RCM occurs and a cyclic dialkenyl compound is formed by cascade ROM-CM reactions. 

To our knowledge, the analog of the double dehydrochlorination reaction for 1,1-dibromo-cyclopropanes has not been reported. While the reaction of 9,9-dibromobicyclo[6.1.0]non-4-ene with potassium terr-butoxide in dimethyl sulfoxide is known, only rearranged alkenes were obtained." On the other hand, many 1,1-dichloro- and 1,1-dibromocyclopropanes fused to unsaturated six-membered rings undergo double dehydrohalogenation to give cycloproparenes (see Section 5.2.2.I.2.5.). 

In 1972, Lewis and Winstein reported that the reaction of a,a-dimethylallyl phenyl sulfide (1) with thiophenol in the presence of tert-butyl hydroperoxide gave the isomeric compound 7,7-dimethylallyl phenyl sulfide (3) (Scheme 1) [1]. It was proposed that this reaction occurred by addition of thiophenoxy radical to the terminal end of the alkene to produce radical intermediate 2. This radical then underwent y9-scission with loss of the tertiary thiophenoxy group to form the rearranged alkene 3. 

The acid-catalyzed dehydration of an alcohol to a rearranged alkene is known as a Wagner-Meerwein rearrangement. Propose a mechanism for the following Wagner-Meerwein rearrangement ... 

Easic Principles Practical Photochemistry General Considerations Carbonyl Compounds a-Cleavage Carbonyl Compounds Hydrogren Abstraction Steroids Carbonyl Compounds Cycloaddition Enone and Dienone Rearrangements Alkenes Isomerisation and Rearrangement Alkenes Cycloaddition Alkenes Photo-Cxidation Terpenoids Aromatic Compounds Isomerisation and Cycloaddition Practical Photochemistry Scale-up Aromatic Compounds Substitution and Cydisation Alkaloids Photoinitiated Free-radical Chain Reactions. 

The relative amount of base has a significant effect on the product distribution. When the tosyl hydrazone of pinacolone was treated with > 1.8 equivalents of n-butyllithium, 3,3-dimethyl-l-butene was formed. When only 1 equivalent of n-butyllithium was used, however, a 57% yield of 3,3-dimethyl-l-butene, 40% of 1,1,2-trimethylcyclopropane, and 3% of the rearranged alkene (2,3-dimethyl-1-butene) were isolated.264 jhe type of base employed is also important. The product distribution from the hydrazone of cyclopentanone, for example, gave a mixture of cyclopentene/( )-2-pentene/(Z)-2-pentene.265 When sodium methoxide was used, a 2 84 14 mixture of these products was obtained whereas sodium hydride led to a 83 14 3 mixture and sodium amide a 60 35 3 mixture.265... 

Pre-equilibrium protonation (Scheme 4-22) was found to be typical for diazomethane, for diazoacetates (R = C2H5O2C — R = H), a-diazo ketones (R = R" - CO, R = H) in aqueous or partly aqueous systems. These reactions are characterized by specific, not general, acid catalysis, by faster reaction in D2O than in H2O, because D30" is a stronger acid than H30", and by deuterium exchange (R = H replaced by D). Results have been summarized by More O Ferrall (1967). The mechanism of the dediazoniation and subsequent product-forming steps (reaction with nucleophiles Nu, rearrangements, alkene formation) are not within the scope of this section (see, however. Sects. 7.3 and 7.4). 

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