Alkenyl complexes, reaction with electrophiles

The insight that zinc ester enolates can be prepared prior to the addition of the electrophile has largely expanded the scope of the Reformatsky reaction.1-3 Substrates such as azomethines that quaternize in the presence of a-halo-esters do react without incident under these two-step conditions.23 The same holds true for acyl halides which readily decompose on exposure to zinc dust, but react properly with preformed zinc ester enolates in the presence of catalytic amounts of Pd(0) complexes.24 Alkylations of Reformatsky reagents are usually difficult to achieve and proceed only with the most reactive agents such as methyl iodide or benzyl halides.25 However, zinc ester enolates can be cross-coupled with aryl- and alkenyl halides or -triflates, respectively, in the presence of transition metal catalysts in a Negishi-type reaction.26 Table 14.2 compiles a few selected examples of Reformatsky reactions with electrophiles other than aldehydes or ketones.27... 

All around this chapter, we have seen that a,/J-unsaturated Fischer carbene complexes may act as efficient C3-synthons. As has been previously mentioned, these complexes contain two electrophilic positions, the carbene carbon and the /J-carbon (Fig. 3), so they can react via these two positions with molecules which include two nucleophilic positions in their structure. On the other hand, alkenyl- and alkynylcarbene complexes are capable of undergoing [1,2]-migration of the metalpentacarbonyl allowing an electrophilic-to-nucleophilic polarity change of the carbene ligand /J-carbon (Fig. 3). These two modes of reaction along with other processes initiated by [2+2] cycloaddition reactions have been applied to [3+3] cyclisation processes and will be briefly discussed in the next few sections. 

Palladium complexes have been used for the electroreductive cycliza-tion of Ai-alkenyl-2-bromoanilines to the corresponding indoline derivatives (Scheme 69) [101]. The postulated carban-ion intermediate undergoes a reaction with the electrophiles (H+, CO2). 

Protonation of alkenyl complexes has been used [56,534,544,545] for generating cationic, electrophilic carbene complexes similar to those obtained by a-abstraction of alkoxide or other leaving groups from alkyl complexes (Section 3.1.2). Some representative examples are sketched in Figure 3.27. Similarly, electron-rich alkynyl complexes can react with electrophiles at the P-position to yield vinylidene complexes [144,546-551]. This approach is one of the most appropriate for the preparation of vinylidene complexes [128]. Figure 3.27 shows illustrative examples of such reactions. 

Since our present interest is the reaction of r-bonded systems, the carbon-carbon double bond of alkenyl monomers, with electrophiles to give classical carbenium ions (or esters) as end products, we will concentrate on these entities. Whether nonclassical carbonium ions ( TT-complexes ) are involved in these reactions as hi -energy transition states, is still uncertain, and this interesting aspect will be discussed specifically in later sections (see Chaps. Ill and IV). 

The iron-catalysed reaction of heteroarenes, including indoles, pyrroles, thiophene, and furan, with 3-methyl-2-quinonyl boronic acids allows the formation of alkylated products, such as (68), rather than the more usual alkenylated products. The unusual alkylation, at the 5-position, of oxindoles to give products such as (69) has been reported using the acid-catalysed reaction with benzylic alcohols in nitromethane. Silylation of indole, at the 3-position, to give (70) has been achieved using a cationic ruthenium(II) sulfide complex as a catalyst. A sulfur-stabilized silicon electrophile is formed resulting in a Wheland intermediate which is deprotonated by sulfur atom. ... 

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