Allylsilanes transmetallation

Although the palladium catalysed conversion of allylsilanes bearing a pendant hydroxyl group to fiirans (3.64.), reported by Szabo, bears remarkable similarity to the reaction depicted in 3.63., it has also some distinctive features. The formation of the allylpalladium complex is achieved in a transmetalation step and therefore requires the presence of a... 

Allylmetal compounds can be prepared by the Pd-catalysed allylation of dimetal compounds. The phenylallylboronate 195 is prepared by the reaction of 2-phenylallyl acetate with bis(pinacolate)diboron (194) catalysed by ligandless Pd in DMSO [95], A good synthetic route to the allylsilane 197 is the reaction of Me3SiSiMe3 with geranyl trifluoroacetate (196) catalysed by ligandless Pd via transmetallation of the n-allylpalladium intermediate at room temperature in DMF [96],... 

As before, a silver ion and its counterion could act together in the transmetallation step, such as depicted in Scheme 10.44. Such a hypothesis seemed supported by the discovery in 2006 that 2-pyridinyl, but not 3-pyridinyl, allylsilanes reacted under similar conditions and transferred the pyridine moiety to various aryl iodides (Scheme 10.45).78 In this coupling, the nitrogen atom of the pyridinyl moiety probably played a role similar to that of the hydroxyl group in the silanol-based Hiyama couplings. 

Generally, the Lewis acid-promoted allylation of aldehydes with allyltrimethylsilanes proceeds via carbonyl activation. However, the SnCU-promoted allylation with allylsilanes bearing a coordinate site is rationalized by a transmetallation mechanism, in which allyltrichlorostannanes work as the actual allylating agents (Equation... 

It is a palladium-catalyzed cross-coupling reaction between organosilanes (vinyl, ethynyl and allylsilanes) and organic halides (aryl, vinyl and allyl halides). Allylpal-ladium chloride dimmer [( ri -C3H5PdCl)2] and either tris(diethylamino)sulfonium difluorotrimethylsilicate (TASF) or tetra-n-butylammonium fluoride (TBAF) are used as catalysts. Fluoride ion acts as an activator for the coupling, forming an intermediate hypervalent anionic silicon species, which can then transmetallate with palladium as a preliminary reaction to coupling. 

The preparation of allylsilanes from allyl ethers and chlorosilanes is catalyzed by Cp2TiCl2.1247 The process involves reaction of Cp2TiCl2 with BunMgCl to generate Cp2TiBun2 which readily decomposes to Ti(ll) complexes with subsequent reaction of allyl ether to afford bis-Cp allyl derivatives and further transmetallation with BunMgCl.24 1248... 

In the Lewis acid-promoted allylation with allylsilanes, the promoter is usually regarded as activating electrophiles. Thomas [374] and Dias [375] have, however, described SnCLj-promoted allylation reactions proceeding via a transmetalation mechanism (Scheme 10.130). In the latter equation of Scheme 10.130, the formation of the allylstannane species has been observed by NMR spectroscopy [375 b]. 

Yamamoto et al. have systematically studied the Lewis acid-catalyzed carbosilylation of alkynes with allylsilanes and related compounds [512]. They have disclosed that the EtAlCl2 (cat.J-MeaSiCl (excess) catalyst system or a sub-stoichiometric amount of HfC effectively induces the trans-aUylsilylation of the alkynes (Scheme 10.192) [514]. The HfCLj-promoted method, in particular, is quite useful for stereo- and regioselective aUylsilylation of a variety of aUylsilanes under mild conditions. In sharp contrast to the mechanism proposed by Jung et al., Yamamoto et al. have proposed that EtAlCh or HfCLi adds to an alkyne, the activated alkyne is allylated by an allylsilane, and the resulting vinylmetal becomes an aUylsilylation product by transmetalation. 

The corresponding allylsilane can be used for access to allyltrichlorostannane. Consecutive transmetalation with SnCLj. and exposure of a dipeptide aldehyde to the resulting allyltrichlorostannane led to a 1,2-syn adduct with high stereoselectivity (Scheme 12.22) [60]. 

In polar solvents or at higher temperatures, the fluorine-silicon bridge would be cleaved to switch the transition-state model to the Sg2(open) (72), thus resulting in inversion. On the other hand, open and cyclic Sg2 mechanisms have been proposed to justify the observed stereochemistry in the y-selective cross-coupling of allylsilanes [298]. Allylic silanolates undergo the palladium-catalyzed Hiyama cross-coupling with aromatic bromides with excellent stereoselectivity through a syn SE transmetallation [299]. 

In a similar fashion, the sila-aza precursor 34 resulted in the corresponding allylsilane 37. In both examples, the transmetalation reactions were found to be complete by trapping the silyllithiums 32 and 36 with MesSiCl to give the corresponding disilanes in good yields (68% and 86% respectively). 

The most popular promoters are BF3 OEt2, TiCU, and SnCU. They are followed, arguably, by AICI3, AlC Et, and TMSOTf. Protic acids are used more rarely since they tend to cause an unwanted protodesilylation. BF3, TiCU, and SnCL have somewhat different properties. BF3 does not react with allylsilanes directly, and has only one coordination site. TiCU is also inert towards allylsilanes, even at room temperature, and has two coordination sites. TiCU forms 1 1 or 1 2 adducts with aldehydes, depending on the stoichiometry of the reagents. It can also form chelates with a- and P-alkoxyaldehydes, which influences the stereochemistry of the reaction (chelation control). TiCU is best used at low temperatures (-78 °C) to prevent the formation of unwanted chlorination products that appear at room temperature. The major characteristic of SnCU is that, in the presence of an allylsilane, it undergoes a transmetallation within minutes at -80 °C. Hence,... 

---