Acetylenes carbometallation

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

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... 

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 ... 

Next, the cycloisomerization of l, -enynes involving a vinylmetal species originating from the hydro-, hetero-, or carbometallation of the acetylene moiety in the first step is summarized. 

See also Acetoacetyl-CoA in citric acid cycle, 6 633 Acetyl cyclohexanesulfonyl peroxide (ACSP), 74 282 78 478 Acetylene(s), 7 177-227, 227-228 25 633 addition of hydrogen chloride to, 73 821 from calcium carbide, 4 532, 548 carbometalation of, 25 117 as catalyst poison, 5 257t chemicals derived from, 7 227-265 decomposition of, 70 614 Diels-Alder adduct from cyclopentadiene, 8 222t direct polymerization, 7 514 economic aspects of, 7 216-217 explosive behavior of, 7 181-187 as fuel, 7 221-222 health and safety factors related to, 7 219... 

In view of the extensive documentation outlined above, the usefulness of the polarity alternation concept as a primary guide for evaluation of substituent effects can hardly be denied. The influence of a substituent on the ipso site has not been discussed in this article but an even more direct and important effect is implicit. Among the innumerable examples one may cite the preferential formation of geminal dimetallic species [5] in hydrometalation and carbometalation of vinylmetals and acetylenes. On the other hand, chemical systems are usually very complex, inter- and intramolecular forces including steric and stereoelectronic factors may dominate over polarity alternation. Thus, chelation by a proximal donor often directs metalation and stabilizes certain organometallic entities. In these instances the stability gaining from polarity alternation is overwhelmed. 

Vicinal Difunctionalization of Alkenes and Acetylenes via Intermolecular Carbometallation 225... 

Vicinal Difunctionalization ofAlkenes and Acetylenes via intermoiecuiar Carbometallation 229... 

The fruitfulness of the idea of a stepwise addition with an independent variation of the addends was brilliantly illustrated by Normant s studies, which resulted in the elaboration of a general method of alkene synthesis based on the reaction of alkyne carbometallation. Basically this reaction represents a case of the well-known nucleophilic addition to a carbon-carbon triple bond. In the Normant reaction, however, the initial addition of a nucleophile (an organome-tallic reagent) across the triple bond results in the formation of a stabilized carbanion-like intermediate equivalent to a vinyl carbanion. This intermediate can similarly be further reacted with an external electrophile. Most typically, copper-modified Mg or Li reagents, which are unable to react with acidic acetylenic hydrogens, are used in this sequence. 

The retro-Wittig reaction (i) (Scheme 2.39), with disconnection across the double bond, seems to be an obvious candidate for models containing disubstituted double bonds. In these cases, the steric outcome of the reaction can be easily controlled. An alternative route of disconnection at the vinyl bonds (ii) corresponds to a retro-carbometallation reaction. This disconnection is generally applicable to double bonds with any substitution pattern and is especially useful owing to the high stereoselectivity of the carbometallation step. Route (iii) involves a retrosynthetic dehydrogenation leading to the immediate acetylenic precursor, which can be conventionally disassembled into a parent acetylide and a pair of electrophiles as shown. 

A new aspect of synthon application in the use of acetylene (and, in general, alkynes) emerged with the development of methods for the carbometallation of triple bonds in accordance with the Scheme 2.96. As can be seen from the scheme, the net contribution of acetylene in this sequence of conversions corresponds to the insertion of a bipolar C2 unit (synthon 233 with a Z-coniiguration of the double bond) between the nucleophilic and electrophilic fragments. 

The carbometallation can be performed intramolecularly leading in this case to an unsaturated diorganonickel species of type 47 which undergoes a rapid reductive elimination furnishing polyfunctional cyclopentylidenes of type 48 (Scheme 9-41). The syn-addition is proven by using a phenyl-substituted acetylenic iodide such as 49 [78]. 

Vicinal functionalization of olefins or acetylenes is an efficient manipulation of these functional groups, as they have two consecutive reactive centers. From this standpoint, carbometallation of olefins and acetylenes is an attractive transformation, owing to the concomitant formation of a new carbon-carbon bond at one terminus and a Grignard reagent at another position, the latter of which permits an introduction of a variety of carbon or heteroatom electrophiles [Eq. (10)]. 

The enhanced activity of allylic Grignard reagents enables them to react with acetylenic alcohols by an anti-addition [21]. In oxygenated solvents (Et20 or THF), reactions occur regioselectively by oxygen assistance to give the desired carbometallation reaction product [Eq. (9)]. 

This catalytic system is also effective for formation of a stabilized carbanion which undergoes carbometalation to acetylene (Scheme 2.38) [53]. 

The insertion of unsaturated organic molecules such as olefins, acetylenes, CO and RNC into rnetal-carbon bonds is a characteristic and important reaction in organometallic chemistry. The reaction is also termed carbometallation of the unsaturated molecule (i.e., multiply bonded reagents) ... 

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. 

Another example of the iron-catalyzed carbometallation reaction of unactivated alkynes was reported for the aryknagnesiation of alkyl(aryl)acetylenes [64b]. The addition of arylmagnesium bromide to the disubstituted aryl(alkyl)alkynes 234 was found to proceed efficiently in the presence of Fe(acac)j as a catalyst and a substoichiometric amount of an N-heterocyclic carbene (NHC) ligand (IPr) as an additive, which significantly increased the yield of this carbometallation process (Scheme 10.79). 

Carbometallation of l,4-bls(trlmethylsllyl>-l,3-butadlyne with trlmethylalumlnium In the presence of cpaZrCls catalyst selectively produces an alkenylalumlnlum Intermediate which, on treatment with water and various electrophiles, affords 2-methyl-1,4-bls(trlmethylsllyl>-l-buten-3-yne and Its 1-substltuted derivatives (Scheme 8). The TlClA-EtaAlCl catalyst system Induces endo-C2+21 cycloaddltlon of bl8(trlmethylsllyl>acetylene to norbornadlene . 

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