2- allyl metal species

A variety of routes are available for the preparation of allylsilanes (/) with the simplest and most direct being the silylation of allyl-metal species. Other routes exemplified in this chapter include Wittig methodology, the use of silyl anions/anionoids in allylic substitution, and hydrometallation of... 

Compared with allylation, homoallylation has received little attention both in stoichiometric organometallic chemistry and in catalytic transition metal chemistry [24-26]. This is apparently owing to the difficult availability and the poor reactivity of a homoallyl metal species as compared with an allyl metal species. 

The isomerisation of allyl and propargylic alcohols involves the 1,3-shift of an oxygen atom rather than a hydrogen atom. Isomerisation of allyl alcohols can be catalysed by a variety of metal oxo complexes and in this instance the reaction does not involve metal carbon bonds as we will see. One could imagine that allylic metal species can participate in isomerisation of allylic compounds, but for the alcohols themselves this is not an easy reaction. In chapter 13 reactions of allyl acetates and the like, which are more prone to... 

From Hard Allylic Organometallics. The most common preparation of allylic boranes and boronates is the addition of a reactive allylic metal species to a borinic or boric ester, respectively (Eqs. 10 and 11). Preparations from allyllithium, " " allylmagnesium, and allylpotassium " ° reagents are all well known. These methods are popular because the required allylic anions are quite easy to prepare, and because they generally lead to high yields of products. 

Abstract Palladium-catalyzed cascade reactions have gained steadily increasing importance over the last decade. The important factor in these reactions is the catalytic generation of jr-allyl palladium intermediates which further undergo a variety of reactions. jr-Allyl palladium complexes can be easily formed by the treatment of allylic substrates with Pd(0). A jr-allyl palladium complexes on treatment with allylic metal species produce bis jr-allyl palladium complex. In this review, the palladium catalyzed cascade reactions involving jr-allyl palladium chemistry is described. The first part deals with catalytic reactions involving jr-allyl palladium complexes as an intermediate, while the second part features catalytic reactions involving bis jr-allyl palladium complex as an intermediate. 

Allylsilanes may be prepared by silylation of allyl-metal species, provided that the carbanions can be trapped regioselectively by RgSiCl. 

An alternative to the direct electrophific allylic amination of an alkene is the reaction of an allylic acetate or carbonate with a transition metal (typically a palladium or rhodium complex) to give a Tr-allyl metal species that reacts with a nitrogen nucleophile to give an allylic amine (see Section 1.2.4). 

The o-allylic metal species experiences various transformations (Scheme 5.2). 

It is necessary to suppress this isomerization for selective reactions. The allylic transition metal complexes generated in situ can undergo protonation, fi-hydride elimination, nucleophilic addition, or reductive elimination. The fate of the initially formed o-allylic metal species depends on the reaction conditions, mainly on the character of the transition metal used. With proper conditions, retro-allylation is a useful tool in organic synthesis. This section categorizes the chemistry of retro-allylation according to the transition metals used. 

Y Lactone Synthesis Using 2-( Alkoxycarbonyl)allyl Metal Species... 

Lactone synthesis through the reaction of 2-(alkoxycarbonyl)allyl metal species to carbonyl substrates holds a main position for the synthesis of the a-methylene derivatives. In 2(X)5, Hall et al. [30] obtained an interesting result in a TfOH-catalyzed reaction of a benzaldehyde derivative bearing electronically rich arene structure with (E)-crotyl boron reagent (E)-39 (Scheme 16). In this case, if the reaction proceeds via normal cyclic six-membered transition... 

This result suggests that diene itself is not involved in the cleavage reaction. A TT-allyl-metal species w ith agostic interaction of H with Co is the key intermediate. The same type of reaction also proceeds in 5-membered ring compounds on Co (Eq. 14) [23]. A simple cobalt rr-allyl complex reacted w ith 2-bu-tyne to give a 7-membered ring cobalt complex where one carbon and two carbons of the allyl moiety were separated in the final product [11]. The mechanism is not yet clear. 

Scheme 4 Two pathways for producing r -coordinated allyl metal species in hydroamination...

For producing ri -coordinated allyl metal species, two pathways are proposed as shown in Scheme 4, and in either case an acid is involved, often added as a cocatalyst or in situ generated path (a) formation of metal hydride species followed by coordination of C-C double bond and subsequent migratory insertion into M-H bond [hydrometallation], and path (b) coordination of C-C double bond followed by protonation of the coordinated alkene [41]. To the terminal carbon of the rj -allyl system, an amine attacks from external side. This type of hydroamination has different characteristics in that the formation of C-H bond precedes by the formation of C-N bond, by contrast to the reactions of other mechanisms which have the opposite bond-forming order, that is, the formation of C-N bond occurs first. 

As shown in Eq. (8), the reaction is able to proceed through either rj -coordination or -coordination pathway, and it depends on the catalyst species used in the reaction. In the reaction directed to left, r] -coordination of alkyne followed by nucleophilic attack to carbon-a occurs [22, 23]. On the other hand, in the reaction directed to right, alkyne isomerization to aUene takes place first, followed by q -allyl coordination, and subsequent nucleophilic attack to carbon-I> (the terminal carbon of q -allyl metal species) takes place [48]. 

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