Carbometallation

2- Addition reactions of three kinds of main group metal compounds, namely R—M X (carbometallation, when R are alkyl, alkenyl, aryl or allyl groups), H—M X (hydrometallation with metal hydrides) and R—M —M —R (dimetallation with dimetal compounds) to alkenes and alkynes, are important synthetic routes to useful organometallic compounds. Some reactions proceed without a catalyst, but many are catalysed by transition metal complexes. 

2- Addition of organometallic compounds of main group metals R—M —X (M = B, Al, Zn, Mg, Sn) to alkenes and alkynes is called carbometallation. Some reactions proceed without a catalyst, but they are promoted or accelerated by transition metal 

Products of the carbometallation 442 and 443 contain very reactive metal carbon bonds, and can not be isolated in many cases. They undergo further reactions such as protonation, transmetallation and nucleophlic attack. The preparation of tri- or tetrasubstituted alkenes 444 and 445 from alkynes via the alkenylmetals 443 is particularly useful. Only catalysed carbometallations are treated here. 

The regioselective carbomagnesation of alkenes is catalysed by Cp2ZrCl2 (449) [173], In this reaction, the zirconocenc ethylene complex (450) is generated and reacts 

Carboalumination of alkynes and alkenes proceeds slowly without a catalyst. The reaction is accelerated by Cp2ZrCl2 [177], Carboalumination of alkenes is catalysed by 

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Carbometalation, an important reaction of RTi(TV) compounds ia which RTi adds to a C=C or CM2 multiple bond and results ia a net R—H addition, is iavolved ia Ziegler-Natta polymerisation as follows ... 

In the foUowiag cases, only those reactions ia which there is no chain growth, or at most dimerisation, are considered (see Olefin polymers). Alkyl titanium haUdes can be prepared from alkyl aluminum derivatives. The ring stmcture imparts regiospecificity to the ensuing carbometalation (216) ... 

Carbometalation of olefins and acetylenes is a useful reaction. For example, in... 

Other authors disagree with the results of this carbometalation sequence. They find that the initial product results stricdy from cis-carbometalation but with a trace of base, H is abstracted from the medium homolyticaHy with loss of stereochemistry (288). They show that M in the above stmcture must be Ti, not Al. If the reaction mixture is quenched with D2O containing NaOD, 95 mol % D is incorporated in the olefin. 

The 1,2-dichloroethane available from Aldrich Chemical Company was distilled from phosphorus pentoxide before use. Although less effective, dichloromethane may also be used in the carbometallation step. 

The retrosynthetic elimination of olefinic stereocenters (E or Z) was illustrated above by the conversion 147 => 148 under substrate spatial control. It is also possible to remove olefinic stereocenters under transform mechanism control. Examples of such processes are the retrosynthetie generation of acetylenes from olefins by transforms such as trans-hydroalumination (LiAlH4), ci5-hydroboration (R2BH), or ci -carbometallation... 

IRC) analysis. It is found that the methylene transfer (path b) is significantly favored over the carbometallation (path a) by about 13 kcal mol . The good agreement between theory and experiment indicates that such studies are valid for this complex system. 

Hydrometallation and carbometallation of alkynylsilanes proceeds regio-and stereospecifically, the metal becoming attached to the silicon-bearing carbon atom in what is normally a co-addition process (hydrostannylation, however, shows the opposite regioselectivity). Electrophilic cleavage, with retention, of the carbon-metal bond then leads to vinylsilanes of various types. 

Cadmium carbonate, 82 Carbodemetallation, 11 Carbometallation, 10 Carboxylic acids, 60,86 Ccrium(ni) chloride heptahydrate, 64 a-Chiral aldehyde, 112 Chiral lanthanide, 112 ff Chloro-a-lithio-a-trimethylsilanes, 21 2 Chloro-2-methylbutane, 135 4-Chloro-2-trimethylsilylamsole. 40... 

Three possible mechanisms may be envisioned for this reaction. The first two i.e. 1) Michael addition of R M to the acetylenic sulfone followed by a-elimination of LiOjSPh to yield a vinyl carbene which undergoes a 1,2 aryl shift and 2) carbometallation of the acetylenic sulfone by R M followed by a straightforward -elimination, where discarded by the authors. The third mechanism in which the organometallic reagent acts as an electron donor and the central intermediates is the radical anion ... 

An early example of the use of a subcatalytic amoimt of sparteine for the activation of an organolithium nucleophile was reported by Lautens et al. in the carbometallation of a meso-unsaturated oxabicycle 25, with ring opening leading to the substituted cycloheptene derivative 26 (Scheme 4) [4]. Both yield and enantiomeric excess remained virtually unchanged when the ratio n-BuLi sparteine was lowered to 1 0.15. However, when a 3 mol% amount of the ligand 1 was used, a 20% decrease in enantioselectivity was observed. 

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. 

Simmons-Smith reagent that contradicts path B, and path A has therefore been widely believed to represent the experimental reality. For lithium carbenoids, on the other hand, the alternative carbometalation/cyclisation pathway has received experimental support. Actually, the factors that determine the... 

The mechanism of carbometallation has been explored computationally.77 The reaction consists of an oxidative addition to the triple bond forming a cyclic Cu(m) intermediate. The rate-determining step is reductive elimination to form a vinyl magnesium (or zinc) reagent, which then undergoes transmetallation to the alkenyl-copper product. 

HfCl4-catalyzed hydro- and carbometalation have been investigated.1 3 /nr/i.v-Addition of tri-butyltin hydride or allyltrimethylsilane to alkynes occurs with high selectivities (Scheme 43).162 In a-olefin polymerization, the Hf mcthylaluminoxanc (MAO) system also works well.184... 

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>. 

Another route to the synthesis of the furanone-containing compounds (e.g., 84, Scheme 18) is via magnesium-mediated carbometallation of propar-gyl alcohols, as described by Forgione et al. [67]. Scheme 20 demonstrates this procedure as a feasible means of producing the Merck anti-inflammatory drug Vioxx, 85. 

Merry-go-round processes involving multiple C-H functionalizations and carbometallations have been described. In the example depicted, a rhodium hydride continues to be formed until steric factors prevail. The 2-norbornene moiety acts as a directing group (Equation (148)).125... 

The addition of 1-alkynes to a,/3-unsaturated ketones in water is catalyzed by a palladium(n)/phosphine combination. Deuteration studies suggest that this reaction proceeds via carbometallation of the olefinic moiety by an alkynylpalladium intermediate (Equation (193)).1... 

C-C Bond Formation (Part 1) by Addition Reactions through Carbometallation Mediated by Group 4-7 Metals ... 

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