Carbenoids Carbocations

In this chapter, die term carbene includes the species called carbenoid which is often considered as an a-metal-substituted carbocation or its equivalent. In some cases the term carbenoid also includes encumbered carbenes complexed with metal halides. 

The reaction also takes place with other bases (e.g., LiH,213 Na in ethylene glycol, NaH, NaNH2) or with smaller amounts of RLi, but in these cases side reactions are common and the orientation of the double bond is in the other direction (to give the more highly substituted olefin). The reaction with Na in ethylene glycol is called the Bamford-Stevens reaction,214 For these reactions two mechanisms are possible—a carbenoid and a carbocation mechanism.215 The side reactions found are those expected of carbenes and carbocations. In general, the carbocation mechanism is chiefly found in protic solvents and the carbenoid mechanism in aprotic solvents. Both routes involve formation of a diazo compound (34) which in some cases can be isolated. 

Carbenes and carbenoids, 565—566, 571—572 Carbenium ions, 140. See also Carbocations Carbinolamine intermediates, 672—673, 674 Carbobenzoxy. See Benzyloxycarbonyl Carbocations... 

The understanding of this catalysis started in 1952, shortly after the concept of carbenes was introduced (see Sect. 8.1). Yates postulated that transition-metal catalysts react with diazo compounds by formation of transient electrophilic metal carbenes, because that complex can be depicted as a metal-stabilized carbocation (8.104). Doyle (1986 a) proposed the catalytic cycle (8-46) for the formation of the carbenoid 8.104 and its reaction with an electron-rich substrate S . The reagent S is, first of all, an alkene in cyclopropanation, but can also belong to other groups of compounds, to be discussed later in this section. 

Carbenoid A reactive substance that, like a carbene, converts alkenes to cyclopropanes stereospecifically. Carbocation Positively charged carbon. 

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