Electrophilic Attack on Allyl Groups

Electrophilic reagents such as and PhaC cause allyl-metal bond splitting, leading to the formation of corresponding complexes and organic derivatives  

The action of acids on allyl palladium compounds also causes olefin evolution. However, the reaction conditions must be very carefully selected because slight changes or a small excess of acid may lead to various side reactions owing to lability of olefin palladium complexes. 

---

Alkylsilanes are not very nucleophilic because there are no high-energy electrons in the sp3-sp3 carbon-silicon bond. Most of the valuable synthetic procedures based on organosilanes involve either alkenyl or allylic silicon substituents. The dominant reactivity pattern involves attack by an electrophilic carbon intermediate at the double bond that is followed by desilylation. Attack on alkenylsilanes takes place at the a-carbon and results in overall replacement of the silicon substituent by the electrophile. Attack on allylic groups is at the y-carbon and results in loss of the silicon substituent and an allylic shift of the double bond. 

Electrophilic attack on allyl silanes represents one of their most commonly used modes of reaction. The relative reactivity of allyl silanes and alkenes towards diarylmethyl cations has been used to calculate the value of the 0- effect of a trimethylsilyl group. 

A second type of organopalladium intermediates are 7r-allyl complexes. These complexes can be obtained from Pd(II) salts and allylic acetates and other compounds with potential leaving groups in an allylic position.79 The same type of 7i-allyl complexes can be prepared from alkenes by reaction with PdCl2 or Pd(02CCF3)2.80 The reaction occurs by electrophilic attack on the n electrons followed by loss of a proton. The proton loss probably proceeds via an unstable species in which the hydrogen is bound to... 

Sesquiterpenes are formed from three C5 units where a further molecule IPP reacts with GPP to form famesyl diphosphate (famesyl pyrophosphate, FPP). FPP is further converted to linear or cyclic products. By cleaving the diphosphate group, an allylic cation is produced that underlies an electrophilic attack on the central or distal double bond leading to cyclization. Due to the increased chain length and an additional double bond, a huge number of structures can be built up. Sesquiterpene cyclase enzymes typically produce a major product accompanied by a range of related structures [44]. 

The first route we saw in Section 5.1 the second and third resemble the synthetic reactions most commonly used for alkyl complexes. In Eq. 5.15 and Eq. 5.16, the metal reacts with the sterically slim terminal CH2 group, and Eq. 5.17 shows an electrophilic attack on a diene complex. Equation 5.18 shows that when a C=C group of a diene undergoes insertion into an M-H bond, the hydrogen tends to add to the terminal carbon (Markovnikov s rule). The resulting methylallyl can become if a vacant site is available. In Eq. 5.19, when an allene inserts into an M-H bond, the hydride adds to the central carbon to give an allyl. 

A possible mechanism of the aldol-type Mukaiyama reaction and the Sakurai allylation was investigated [98-100]. The proposed mechanism involves the catalytic activation of the aldehyde and its interaction with the silyl ketene acetal or allylsilane, resulting in an intermediate. Thereafter two possible pathways can lead either to the release of TMS triflate salt and its electrophilic attack on the trityl group in the intermediate or to the intramolecular transfer of the TMS group to the aldolate position, resulting in the evolution of the trityl catalyst and the formation of the product (Scheme 16.30). To explore both possibilities a series of experimental and spectroscopic studies were performed. 

Allyl silanes react with electrophiles with even greater regioselectivity than that of vinyl silanes. The cation 3 to the silyl group is again formed but there are two important differences. Most obviously, the electrophile attacks at the other end of the allylic system and there is no rotation necessary as the C-Si bond is already in a position to overlap efficiently with the intermediate cation. Electrophilic attack occurs on the face of the alkene anti to the silyl group. The process is terminated by loss of silicon in the usual way to regenerate an alkene. 

The reaction of butadiene iron tricarbonyl, Fe(CO)3(C4H6), in liquid SO2 with BF3 leads to an interesting product containing an O-bonded RSO2 moiety A crystal structure analysis (Fig. 34) of the product showed the presence of allylic and O-sulfinate interactions with Fe, resulting from electrophilic attack of SO2 on the coordinated butadiene group. In addition, a BF3 group was found to be bound reversibly to the... 

In order to predict the stereochemical outcome of a cyclization, some rules have been proposed based on a model for the attack of an electrophile, under kinetic control, to an alkene containing an internal nucleophile. The selectivity is determined by the relative affinity of the diastereotopic face of the double bond towards a proton syn to H in an OH-in-plane-conformer, or syn to OH in a H-in-plane-conformer, and the cyclization involves a probable intramolecular attack on a 7i-compIex. In fact, when a hydroxy or an alkoxy group is present, the electrophile preferentially attacks the OH-in-plane-conformer from the face of the double bond syn to the allylic hydrogen 22. Thus, starting from terminal double bonds, the ci.v-diastereomer is prevalent in the reaction mixture. 

The Lewis acid combines with allyl bromide to give either the allyl cation or, maybe, the complex we jhow here. In either case, electrophilic attack occurs on the 3-position, as is almost always the case ith indole (pp. 1170-1), and the benzyl group migrates to the 2-position where there is a hydrogen itom that can be lost to restore the aromaticity. 

Allyl complexes are susceptible to nucleophilic and electrophilic attack. A typical reaction of an allyl complex with a nucleophile is illustrated in Eq. (5) [17]. Nucleophilic attack at one of the terminal carbons of an allyl ligand most often occurs on the face of the allyl group opposite the metal, as shown in Eq. (5) however, the nucleophile may bind to the metal center initially if there is a vacant site available and, then, transfer to the endo face of the allyl group. 

The observed moderate sitoselectivity of the furan ring oxidation at C-5 was attributed to a coordination of the electrophilic Br" " species with the allylic C-8-OR group, prior to the attack on the aromatic ring (60). 

---