Metal stabilized carbocations

The metal-carbene complexes are electrophilic in character. They can, in fact, be represented as metal-stabilized carbocations. 

Stable transition-metal complexes of this type are known and others have been recognized as likely intermediates in a number of reactions. Rightly or wrongly, they are called carbene-metal complexes, although they also can be regarded either as metal-stabilized carbocations or as metal-stabilized ylides (Section 16-4A). 

Pearson, A J 1987, Transition Metal-Stabilized Carbocations in Organic Synthesis, in Hartley, F R (ed), Chemistry of the Metal-Carbon Bond, Vol 4, chapter 10, Wiley Chichester, UK. 

Two resonance-contributing structures (3a and 3b), in the formalism of ylide structures, can be used to describe metal carbene intermediates. The highly electrophilic character of those derived from Cu and Rh catalysts suggests that the contribution from the metal-stabilized carbocation 3b is important in the overall evaluation of the reactivities and selectivities of these metal carbene intermediates. Emphasis on the metal carbene structure 3a has led to the subsequently discounted proposal that cyclopropane formation from reactions with alkenes occurs through the intervention of a metallocyclobutane intermediate [18]. The metal-stabilized carbocation structure 3b is consistent with the cyclopropanation mechanism in which LnM dissociates from the carbene as bond-formation occurs between the carbene and the reacting alkene (Eq. 5.4) [7,15]. 

Diazocarbonyl compounds, especially diazo ketones and diazo esters [19], are the most suitable substrates for metal carbene transformations catalyzed by Cu or Rh compounds. Diazoalkanes are less useful owing to more pronounced carbene dimer formation that competes with, for example, cyclopropanation [7]. This competing reaction occurs by electrophilic addition of the metal-stabilized carbocation to the diazo compound followed by dinitrogen loss and formation of the alkene product that occurs with regeneration of the catalytically active metal complex (Eq. 5.5) [201. 

Transition-metal-stabilized carbocations can be generated from functionalized butadieneiron carbonyl or arenechromium tricarbonyl complexes [92], Reactions of such carbocations formed from chiral complexes have been studied, but low selectivities are usually observed [526, 528, 535]. However, chromium tricarbonyl complexes derived from ephedrine 5.66 suffer cyclization in acidic medium. After decomplexation, c/s-tetrahydroquinolines are formed with a high diastereo-and enantioselectivity [540,542] (Figure 5.44). 

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. 

For a discussion of whether this is a metal carbene or a metal-stabilized carbocation or both or something in between, see Fiirstner, A. Davies, P. W. Angew. Chem., Int. Ed. 2007, 46, 3410. Similarly, an oxonium ion, may also be viewed as an oxygen-stabilized carbocation. 

The remarkable stability of the gold complexes is due to significant metal-metal bonding. However, their isolation and structural study are remarkable and greatly contributed to our knowledge of higher-coordinate carbocations. 

Metal-Complexed Alcohols. It is well known that carbocations are frequently stabilized when organotransition metal centers are present in adjacent portions of... 

The unusual stereospecific elimination via a carbocation was rationalized by suggesting that extensive C-metal a-ir conjugation is present in the carbocation intermediate (Figure 3). The awP -periplanar stereochemistry is found because the a-jt stabilization to the SnMe3 group is strongest when the metal is involved in a vertical stabilization of the carbocation. 

Chiral Rh(II) oxazolidinones Rh2(BNOX)4 and Rh2(IPOX)4 (25a,b) were not as effective as Rh2(MEPY)4 for enantioselective intramolecular cyclopropanation, even though the steric bulk of their chiral ligand attachments (COOMe versus i-Pr or CH2Ph) are similar. Significantly lower yields and lower enantioselectivides resulted from dinitrogen extrusion from prenyl diazoacetate catalyzed by either Rh2(4.S -lPOX)4 or Rh2(4S-BNOX)4. This difference, and those associated with butenolide formation [91], can be attributed to the ability of the carboxylate substituents to stabilize the carbocation form of the intermediate metal carbene (3b), thus limiting the Rh2(MEPY)4-catalyzed reaction to concerted carbene addition onto both carbon atoms of the C-C double bond. 

This concludes the discussion of the stabilities of carbocations with hydrocarbon-based structures and also of different methods for deriving equilibrium constants to express these stabilities. The remainder of the chapter will be concerned mainly with measurements of stabilities for oxygen-substituted and metal ion-coordinated carbocations. Consideration of carbocations as conjugate acids of carbenes and derivations of stabilities based on equilibria for the ionization of alkyl halides and azides will conclude the major part of the chapter and introduce a discussion of recent studies of reactivities. 

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