Transmetallation acetylenes

Homoallylic alcohols are provided by Pd-catalyzed reaction of iodobenzene, allene and aldehydes (Scheme 16.15) [19, 20]. A nucleophilic allylindium intermediate is generated through transmetallation of a Jt-allylpalladium species with indium. Such a Jt-allylpalladium complex can alternatively be provided through carbopalladation of ArPdl to a proximate acetylene followed by insertion of allene. 

It is worth mentioning that the sequence hydrotelluration of acetylenes/transmetaUation complements the classical sequence hydrostannillation of acetylenes/transmetallation, which lead to (f )-vinylic organostannates. 

Transmetallations of vinylic tellurides deserve particular attention. These tellurides (prepared by anti-addition of tellurols to acetylenes, see Section 3.16.1.2) exhibit the Z configuration and therefore generate (Z)-vinyUithiums. These results are in sharp contrast to the earlier tin-lithium exchange performed with vinylstannanes (characterized by the E configuration), giving (ii)-vinyllithiums. ... 

Normant and Poisson prepared allenylzinc bromide reagents from TMS acetylenes along the lines of Epsztein and coworkers5, by sequential lithiation with s-BuLi to yield a lithiated species, and subsequent transmetallation with ZnBr2 (equation 35)27,28. Additions to racemic /J-silyloxy aldehydes proceed with low diastereoselectivity to afford mixtures of the anti,anti and anti,syn adducts (Table 17). The latter adducts are formed via an anti Felkin-Anh transition state. Additions to the racemic IV-benzylimine analogs, on the other hand, proceed with nearly complete Felkin-Anh diastereoselectivity to yield the anti,anti amino alcohol adducts (Table 18). 

Another alternate to the Sonogashira coupling was reported by Blum and Molander, where sodium tetraalkynylaluminatcs were coupled with bromoazines and bromoazoles in the presence of a palladium-triphenylphosphine catalyst system. 5-Bromopyrimidine coupled with the TMS-acetylide, for example, to give the cthynylpyrimidinc in excellent yield (7.41.),59 The transmetalating reagents were prepared in situ by the reaction of the appropriate acetylene derivative with sodium aluminiumhydride. 

Dioxa-, oxathia-, and dithiaboroles undergo efficient transmetallation with alkynyl stannanes, furnishing the 5-alkynyl boroles. Thus the reaction of bis(trimethylstannyl) acetylene 273 with 2 equiv of bromoboranes 272 in toluene at room temperature furnishes air-sensitive boroles 274 as colorless solids in good yield (Equation 10) <2004EJI4223>. 

The addition of amines and ethers to alkyllithium compounds profoundly affects polymerization of such species. Amines and ethers alter the association of RLi compounds and change the course of the polymerization and its kinetics. Also, the presence of small amounts of such impurities as water, alcohols, or a-acetylenes, influences the kinetic chain length. The chain-termination reaction with such acidic protons is almost instantaneous. However, there are certain types of protons, such as a-aromatic, secondary amine, and /3-acetylenic, that are not acidic enough to react immediately but will undergo transmetalation during the course of a polymerization reaction. This results in termination or chain transfer of the polymer chain, and limits the realization of polymers of... 

The authors elaborated an innovative synthesis of the Taxane core based on an intramolecular Diels-Alder reaction as the key step. The TBDMS-protected cylohexenone-derived alcohol was converted into the corresponding nitrile intermediate in five steps by known synthetic manipulations (Scheme 43), mainly based on transmetallation protocols. The diene handle for the Diels-Alder reaction was then introduced following a simple but highly efficient four-step procedure. The dienophile for the cycloaddition, the terminal acetylene moiety, was incorporated via lithiation chemistry to furnish the substrate for the intramolecular Diels-Alder cyclization (Scheme 43). 

Acetylenic zinc reagents are readily accessible from 1-alkynes by a deprotonation-transmetallation sequence, and consequently have been successfully used to couple with vinyl halides in many cases (see equation 81), including butadiynyl ones161. Among these, coupling reactions with vinyl bromides have been used in the stereoselective synthesis of enediynes and applied to the synthesis of trienic natural products162. 

Besides simple enones and enals, less reactive Michael acceptors like /3,/3-disubstituted enones, as well as a,/3-unsaturated esters, thioesters, and nitriles, can also be transformed into the 1,4-addition products by this procedure.44,44a,46,46a The conjugate addition of a-aminoalkylcuprates to allenic or acetylenic Michael acceptors has been utilized extensively in the synthesis of heterocyclic products.46-49 For instance, addition of the cuprate, formed from cyclic carbamate 53 by deprotonation and transmetallation, to alkyl-substituted allenic esters proceeded with high stereoselectivity to afford the adducts 54 with good yield (Scheme 12).46,46a 47 Treatment with phenol and chlorotrimethylsilane effected a smooth Boc deprotection and lactam formation. In contrast, the corresponding reaction with acetylenic esters46,46a or ketones48 invariably produced an E Z-mixture of addition products 56. This poor stereoselectivity could be circumvented by the use of (E)- or (Z)-3-iodo-2-enoates instead of acetylenic esters,49 but turned out to be irrelevant for the subsequent deprotection/cyclization to the pyrroles 57 since this step took place with concomitant E/Z-isomerization. 

Symmetrical and unsymmetrical 2,5-fc/s-acetylenic furan derivatives were produced in high yield using Pd-catalyzed cross-coupling reaction of 2,5-/7M(butyltelluro)furan, which was prepared from furan through lithiation-transmetallation and alkylation <03TL1387>. 

Schwartz and Carr found that hydrozirconation of olefins or acetylenes using Cp2Zr(H)Cl and subsequent transmetalation by AICI3 resulted in clean formation of the corresponding alkylaluminum dichlorides these were readily acylated to give ketones in excellent yields as depicted in Sch. 3 [15]. It should be emphasized that because hydrozirconation of internal (or terminal) olefins gives the terminal zirco-... 

Scheme 5-3 Pd-catalyzed cross-coupling reaction of terminal acetylenes with sp- halides i, oxidative addition ii, transmetallation iii, reductive elimination.

The yield of the reaction depends on the degree to which the product can be washed from the sticky trlphenylphosphlne oxide/zinc bromide mass. Elimination and transmetallation followed by protonation gave the acetylene. 

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