高等有机课件Chapter 10 Addition to Carbon–Carbon Multiple Bonds.pptVIP

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高等有机课件Chapter 10 Addition to Carbon–Carbon Multiple Bonds.ppt

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高等有机课件Chapter 10 Addition to Carbon–Carbon Multiple Bonds

Chapter 10 Addition to Carbon–Carbon Multiple Bonds There are four fundamental ways in which addition to a double or triple bond can take place. Three of these are two-step processes, with initial attack by a nucleophile, or attack upon an electrophile or a free radical. The second step consists of combination of the resulting intermediate with, respectively, a positive species, a negative species, or a neutral entity. In the fourth type of mechanism, attack at the two carbon atoms of the double or triple bond is simultaneous (concerted). Which of the four mechanisms is operating in any given case is determined by the nature of the substrate, the reagent, and the reaction conditions. Some of the reactions in this chapter can take place by all four mechanistic types. 10.1 Electrophilic Addition 10.2 Nucleophilic Addition With any substrate, when Y is an ion of the type ZC R2 (Z is as defined above; R may be alkyl, aryl, hydrogen, or another Z), the reaction is called the Michael reaction. In this book we will call all other reactions that follow this mechanism Michael-type additions. 10.3 Free-Radical Addition 10.4 Cyclic Mechanisms There are some addition reactions where the initial attack is not at one carbon of the double bond, but both carbons are attacked simultaneously. Some of these are four-center mechanisms, which follow this pattern: In others, there is a five- or a six-membered transition state. In these cases the addition to the double or triple bond must be syn. The most important reaction of this type is the Diels–Alder reaction. Addition to Conjugated Systems In the case of electrophiles like Br+, which can form cyclic intermediates, both 1,2-and 1,4-addition products can be rationalized as stemming from an intermediate like 18. Direct nucleophilic attack by W- would give the 1,2-product, while the 1,4-product could be formed by attack at the 4 position, by an SN2’-type mechanism. In most cases, more 1,4- than 1,2-addition product is obtain