Transmetalation to aluminum

As yet, there are not many examples of this. As a representative example, the reaction of zirconacyclopentadienes with aldehydes in the presence of A1C13 affords cyclopentadiene derivatives. In this case, the aluminum abstracts the oxygen atom from the aldehyde moiety (Eq. 2.31) [11]. Aluminacyclopentadiene might be the intermediate in this reaction. 

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The aldehyde homoenolate synthons discussed above do not add to aldehydes with a high degree of stereoselectivity. However, transmetalation to aluminum or titanium... 

Transmetalation to give l-methyl-2-propenylaluminum followed by isomerization to 2-butenyl isomers may be involved in reactions between aldehydes and 2-butenyl(tributyl)-stannane induced by aluminum(III) chloride in the presence of one mole equivalent of 2-propanol. Benzaldehyde and reactive, unhindered, aliphatic aldehydes give rise to the formation of linear homoallyl alcohols, whereas branched products are obtained with less reactive, more hindered, aldehydes66,79. 

AICI3 is a moisture-sensitive and strong Lewis acid. It is a first choice for Friedel-Crafts-type reactions, which provide numerous important transformations in laboratory and industry. It can also be applied to the transformation of alkenes to ketones via alkylaluminum halides.303 Hydrozirconation of an olefin and subsequent transmetalation from zirconium to aluminum gives the corresponding alkylaluminum dichloride, and the subsequent acetylation by acetyl chloride affords the corresponding ketone in high yield (Scheme 66). 

Mechanistically, the pre-catalyst Cp2TiCl2 104 is reduced to a [Ti]-H 105 species which is subsequently able to hydrotitanate the triple bond 106. A transmetallation from titanium to aluminum regenerated the [Ti]-H species to generate the (yy )-hydroaluminated compound 107 (Scheme 13). 

Carr, D. B., Schwartz, J. Transmetalation preparation of organometallic reagents for organic synthesis by transfer of organic groups from one metal to another. Transmetalation from zirconium to aluminum. J. Am. Chem. Soc. 1977, 99, 638-640. 

The lithium enolates of thioesters are also amenable to this transformation, involving transmetalation, to Et2AlCl, producing the corresponding aluminum enolates which undergo Mannich-type reaction with aldimine 48 subsequent intramolecular cyclization gave /i-lactams in fairly good yields (Scheme 6.28) [49]. 

Transmetalation. Carr and Schwartz have found that an organic group bound to zirconium can be transferred to aluminum. Thus reaction of 1, obtained by hydrozirconation of alkenes or alkynes, with AlCln results in a color change from bright yellow to orange as an organoaluminum dichloride is formed. On addition of an acid chloride, a ketone (2) is formed rapidly. This new ketone synthesis proceeds in high yield from alkyl- and alkenylaluminum dichlorides. 

Direct, stereospecific introduction of angular allyl groups into fused cyclic a,/ -enones by means of titanium tetradiloride has been reported Reactions, including C-alkylation, have been effected via metalation of arsine oxides Syntheses with organometallic reagents may be facilitated by transmetalation, e.g. from zirconium to aluminum... 

Transmetalation of lithium enolate 1 a (M = Li ) by treatment with tin(II) chloride at — 42 °C generates the tin enolate that reacts with prostereogenic aldehydes at — 78 °C to preferentially produce the opposite aldol diastereomer 3. Diastereoselectivities of this process may be as high as 97 3. This reaction appears to require less exacting conditions since similar results are obtained if one or two equivalents of tin(ll) chloride arc used. The somewhat less reactive tin enolate requires a temperature of —42 C for the reaction to proceed at an acceptable rate. The steric requirements of the tin chloride counterion are probably less than those of the diethyla-luminum ion (vide supra), which has led to the suggestion26 44 that the chair-like transition state I is preferentially adopted26 44. This is consistent with the observed diastereoselective production of aldol product 3, which is of opposite configuration at the / -carbon to the major product obtained from aluminum enolates. 

Transmetalation of 19 by treatment with two equivalents of diethylaluminum chloride generates the aluminum enolate species 23. The latter reacts with acetaldehyde to produce the stable aluminum aldolates 24 which do not undergo the Peterson elimination23. A protic quench then provides the a-silylated aldol adducts of tentative structures (2 R)-25 and (2 V)-25 with little diastereoselectivity. Other diastereomers are not observed. 

When the metallic additive to the intermediate 374 was zinc dihalide (or another Lewis acid, such as aluminum trichloride, iron trichloride or boron trifluoride), a conjugate addition to electrophilic olefins affords 381 . In the case of the lithium-zinc transmetallation, a palladium-catalyzed Negishi cross-coupling reaction with aryl bromides or iodides allowed the preparation of arylated componnds 384 ° in 26-77% yield. In addition, a Sn2 allylation of the mentioned zinc intermediates with reagents of type R CH=CHCH(R )X (X = chlorine, bromine) gave the corresponding compounds 385 in 52-68% yield. ... 

In the following year, this method was also applied to the total synthesis of tjipanazole FI (371) (784). For this synthesis, the required bisindole 1444 was obtained starting from 5-chloroindole (1440) in three steps and 47% overall yield. Acylation of 1440 with oxalyl chloride led to the glyoxylic acid chloride 1441. Transmetalation of indolylmagnesium bromide with zinc chloride, followed by addition of the acid chloride, provided the ot-diketone 1443. Exhaustive reduction of 1443 with lithium aluminum hydride (LiAlFl4) afforded the corresponding bisindolylethane 1444. Executing a similar reaction sequence as shown for the synthesis of tjipanazole F2 (372) (see Scheme 5.243), the chloroindoline (+ )-1445 was transformed to tjipanazole FI (371) in two steps and 50% overall yield (784) (Scheme 5.244). 

The other commercially important routes to alkyltin chloride intermediates utilize an indirect method having a tetraalkyltin intermediate. Tetraalkyltins are made by transmetallation of stannic chloride with a metal alkyl where the metal is typically magnesium or aluminum. Subsequent redistribution reactions with additional stannic chloride yield the desired mixture of monoalkyltin trichloride and dialkyltin dichloride. Both -butyltin and /z-octyltin intermediates are manufactured by one of these schemes. 

Scheme 5 accounts for the observation in Eq. (206). At first, the bulky aluminum reagent occupies a less-hindered space of the substrate so that an incoming nucleophile should approach the ketone from its superficially more-hindered side (i.e., by axial attack). The change in selectivity here has been considered to arise from a steric control, rather than transmetallation of the Grignard reagent to an aluminumate complex before the addition. Other selective reactions based on the same notion have also been developed [451-453]. 

Many other types of organometallics which are not acylated directly acid chlorides and which do not undergo addition to ketones may still transmetallate into the acylpalladiumfll) complex. Simple alkyl organomercurials have been acylated in this fashion to give moderate to good yields of ketones. " Larock has studied the palladium-catalyzed acylation of vinylmercury(II) compounds with acyl halides (equation 104). The reaction was only modestly productive and could not compare to the yield provided by aluminum chloride catalysis. 

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