Carbometallation alkenes

Cross-coupling with alkenes (carbometallation of alkenes)... 

Cross-coupling with Alkenes (Carbometallation of Alkenes)... 

An interesting sequential reaction, consisting of an intermolecular alkene carbometallation and subsequent intermolecular alkyne cross-coupling, has been reported (Scheme 8.87) [601]. Starting from an immobihzed tropane framework 459, stoichiometric carbopalladation yields a stable organopalladium intermediate, which in the presence of copper(I) iodide undergoes coupling with an added terminal acetylene. 

An interesting sequential reaction consisting of an intermolecular alkene carbometallation and subsequent intermolecular alkyne cross-coupling has been reported by... 

Hydroalumination of terminal alkenes using EtjAl as the hydride source must be carried out with titanium catalysts [24], since zirconium compounds lead to the formation of alumacyclopentanes [60, 61] (Scheme 2-11) and carbometallated products [62]. Suitable substrates for hydroalumination include styrene, allylnaphthalene and vinylsilanes. Only one of the ethyl groups in EtjAl takes part in these reactions, allowing the synthesis of diethylalkylalanes, which are difficult to obtain by other methods. 

A palladium-catalysed carbometallation-alkyne cross coupling cascade process has been reported for the stereo- and regio-controlled synthesis of dibenzoxepines with substituted exocyclic alkene functionality <06OL1685>. 

The term carbometallation was most probably coined only about a quarter of a century ago.1 However, the history of those reactions that can be classified as carbometallation reactions is much older. If one includes not only the Ziegler-Natta-type organometallic alkene polymerization reactions2 but also various types of organometallic conjugate addition reactions,3 carbometallation collectively is easily more than a century old. In its broadest definition, carbometallation may be defined as a process of addition of a carbon-metal bond to a carbon-carbon multiple bond. As such, it may represent either a starting material-product relationship irrespective of mechanistic details or an actual mechanistic microstep of carbon-metal bond addition to a carbon-carbon metal multiple bond irrespective of the structure of the product eventually formed. 

In principle, carbometallation of an alkene (RCH=CH2) with a coordinatively unsaturated organotransition metal compound (R1 M I. ) can produce a monomeric carbometallation product 1 (Scheme 6). This reaction may not, however, stop at this stage. It can be accompanied by other processes of which (i) hydrogen-transfer hydrometallation to produce a potentially thermodynamically more favorable mixture of a 1,1-disubstituted alkene and a hydrometallation product 2 and (ii) polymerization to produce polyalkenes 3 are representative. The extents to which these side-reactions occur are functions of relative rates of various competing processes. For example, accumulation of the monomeric carbometallation product 1 can be favored in cases where the starting R1 MTL is more reactive toward alkenes than 1. The organometal/alkene ratio is also an important parameter, since neither of the two side-reactions can proceed after all of the starting alkene has reacted. 

Scheme 6 Unified scheme for carbometallation of alkenes with transition metal complexes.

Aside from the Ziegler-Natta polymerization, alkene and alkyne metathesis, and other reactions of Ti-methylene complexes, carbometallation reactions induced by alkyltitanium compounds have been dominated by those involving... 

As already indicated, the carbometallation reactions of zirconacyclopropanes and zirconacyclopropenes with alkenes and alkynes are in many ways similar to the corresponding reactions of titanacycles developed more recently. At the same time, however, there are a number of significant differences, as detailed in Section 10.06.2.2. At the present time, synthetically useful carbotitanation reactions are predominantly cyclic and stoichiometric in Ti and more so than the corresponding chemistry of Zr. It seems reasonable to state that Ti and Zr are complementary to each other more often than not. The cyclic carbozirconation may be either stoichiometric or catalytic. Frequently, the difference between the two is that the stoichiometric reactions lack one or more microsteps for completing catalytic cycles. Otherwise, they often share same stoichiometric microsteps. With this general notion in mind, many stoichiometric carbozirconation reactions have indeed been developed into Zr-catalyzed reactions, as discussed later. 

The currently known carbometallation chemistry of the group 6 metals is dominated by the reactions of metal-carbene and metal-carbyne complexes with alkenes and alkynes leading to the formation of four-membered metallacycles, shown in Scheme 1. Many different fates of such species have been reported, and the readers are referred to reviews discussing these reactions.253 An especially noteworthy reaction of this class is the Dotz reaction,254 which is stoichiometric in Cr in essentially all cases. Beyond the formation of the four-membered metallacycles via carbometallation, metathesis and other processes that may not involve carbometallation appear to dominate. It is, however, of interest to note that metallacyclobutadienes containing group 6 metals can undergo the second carbometallation with alkynes to produce metallabenzenes, as shown in Scheme 53.255 As the observed conversion of metallacyclobutadienes to metallabenzenes can also proceed via a Diels-Alder-like... 

The stoichiometric carbometallation with well-defined organomanganese compounds has thus far been limited to those of organomanganesecarbonyl compounds. One of the apparently common characteristics among them is their strong tendency to exist as coordinatively saturated 18-electron species. As such, they must be rather inert toward alkynes and alkenes (cf. Scheme 3). Dissociation of one of the CO s or some other ligands would facilitate their... 

Controlled single-stage carbometallation reactions of alkenes and alkynes with group 4—7 metals are discussed with emphasis on regio-, stereo-, and chemoselectivity including clarification and understanding of factors governing these synthetically important aspects. 

In this context, alkynes have been investigated under carbometallation conditions in order to generate tri- or tetrasubstituted alkenes with very good stereochemical control.46 For the unactivated triple bonds, the major issue is the regioselectivity of the reaction. 

Trost reported the synthesis of 1,4-dienes with ruthenium catalysis through regioselective carbometallation of alkynes with alkenes.51 Di- and trisubstituted olefins can also be obtained with arylboronic acids through an intermolecular process under rhodium,30 52 55 nickel,56 and palladium catalysis.57 Recently, Larock has reported an efficient palladium-catalyzed route for the preparation of tetrasubstituted olefins.58,59... 

Another approach is based on the palladium-catalyzed intramolecular carbocyclization of the allylic acetate moiety with the alkene moiety (Scheme 96). After the formation of a 7t-allylpalladium complex, with the first double bond the intramolecular carbometallation of the second double bond occurs to form a new C-C bond. The fate of the resulting alkylpalladium complex 393 depends on the possiblity of /3-elimination. If /3-elimination is possible, it generates a metallated hydride and furnishes the cycloadduct 394. This cyclization could be viewed as a pallada-ene reaction, in which palladium replaces the hydrogen atom of the allylic moiety.231... 

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