Allyl anions alkylation

Corey has utilized the Brook-generated allyl anion alkylation in several syntheses. A recent example was in the preparation of ZF-cation cyclization precmsor 28. Chemoselective addition of 2-propenyllithium to the acyl silane carbonyl of 27, followed by transmetalation and alkylation provided the (Z)-silyl enol ether 28. ... 

Recent advances include alkyl iodides as substrates that can be activated by metal complexation. Also Jt-allyl "anions", when co-ordinated to palladium, are activated toward attack by nucleophiles. This is very similar to the activation of co-ordinated alkenes and it shows the very high electrophilicity of palladium. The valence state of palladium, and/or the charge on palladium, and therefore also the ligands attached to it are very important ... 

With respect to the above-mentioned unsaturated carbonyl compounds with a double bond and a carbonyl group separated by three carbon atoms (14), it can be stated here that they may be disconnected to an alkyl vinyl ketone and an allylic anion (Scheme 7.5), through an oxy-Cope rearrangement (C/. Scheme 5.22). 

Hard nucleophiles (alkyl anions) give predominant retention of configuration. At the opposite extreme, softer nucleophiles, such as benzyl or allyl anions, lead mainly to inversion. The following examples are quite significant ... 

The synthesis of 4-alkyl thioketones is possible by exploiting the stabilizing effect of a sulfur atom upon an adjacent carbanionic center. Ambident allylic anions react so that conjugate addition proceeds exclusively with the a-carbon of the nucleophile,129 243 244 as illustrated in equation (S3) 245 arylsulfinyl and arylsulfonyl groups normally246 behave similarly.247-249 Sulfur-stabilized vinylic carbanions can be prepared and function as Michael donors in difunctionalization sequences.250... 

A limited number of other anionic species have been employed as Michael donors in tandem vicinal difunctionalizations. In a manner similar to sulfur ylides described above, phosphonium ylides can be used as cyclopropanating reagents by means of a conjugate addition-a-intramolecular alkylation sequence. Phosphonium ylides have been used with greater frequency261-263 than sulfur ylides and display little steric sensitivity.264 Phosphorus-stabilized allylic anions can display regiospecific 7-1,4-addition when used as Michael donors.265... 

Deprotonation of BSMA imines followed by trapping with an electrophile of the intermediate anion has been described (see Section IV.C.4). However, when these imines derive from conjugated carbonyl compounds, attack of the base takes place at the end of the conjugated system to give delocalized bis(allyl) anion that could be alkylated or silylated in the (3-position from the nitrogen.227... 

Pyridyl allyl sulfide, on the other hand, gives only one regioisomer in its alkylation reactions. U lsensible here to show the allyl anion as a compound with a C-Li bond. 

X-Substituted Allyl Anions. Allyl anions with alkyl substituents almost always react with carbonyl electrophiles at the more substituted a position, as in the reaction of the prenyl Grignard reagent with aldehydes to give the product 4.39, presumably because the metal attaches itself preferentially to the less-substituted end of the allyl system and then delivers the electrophile in a six-membered transition structure 4.38. In contrast, alkylation of a similar anion with an alkyl halide gives mainly the product 4.40 of y attack, which is normal for an X-substituted allyl anion when a cyclic transition structure is not involved. 

Phen and the radical anion of the alkene. Secondary electron transfer from allylsilane to Phen produces the radical cation of allylsilane and neutral Phen. The radical cation of allylsilane is cleaved by assistance of acetonitrile to generate an allyl radical. The allyl radical adds to the radical anion of the alkene to give the allylated anion which is converted into the product upon protonation. Alkyl and arylmethyl radicals can be generated in a similar manner from tetraalkyl tin compounds and arylmethylsilanes, respectively [124]. These radicals add regioselectively to the -position to the cyano groups in the radical anions of alkenes. 

If one compares the 1,2,3-triphenylcyclopropyl anion as well as the cyclopropyl anion 320" with the many more cyclopropyl anions not showing the ring-opening reaction such as all cyclopropyl anions with H or alkyl groups at C(l) like the parent 323 or Walborsky s 324" it is immediately clear that substituents which stabilize a negative charge at both carbon atoms which become terminal centers of the allyl anion facilitate the ring scission. If the cyclopropyl anion is prepared by deprotonation of a cyclopropane it is... 

In accordance with this model one finds diastereoselectively anti products on reaction of aldehydes with ( )-allyl compounds, whereas allyl systems with the (Z)-configuration give mainly syn products and it is even possible to effect asymmetric induction. As the double bond of the product can be oxidatively cleaved to a CW3 group, the reaction can be regarded as a stereoselective aldol reaction, an aspect which explains the widespread interest in this type of reaction. With heterosubstituted allylic anions it is sometimes possible to effect predominantly y-attack with different electrophiles by the choice of the heteroatom.2 For instance it is well known that with sulfur substituents like —SR, —SOR or —SOjR the a-attack dominates, but doubly lithiated allenethiol possesses high y-reactivity and can be used as a homoenolate anion equivalent in reaction with electrophiles such as alkyl halides (Scheme 7). ... 

The trimethylsilyloxy group of allylic or benzylic alcohols is readily displaced by nucleophiles (malonate anion, alkyl Grignard reagents, hydride (AlCl3-LiAlHif)).91 This limitation as a protecting group should be noted. 

Reactions alkylations, reactions with epoxides and aldehydes, conjugate additions Heterocyclic synthesis with allyl silanes Reactions with Co-stabilised cations An Allyl Dianion The Role of Tin in Anion Formation Halide Exchange with Chelation Indium Allyls Allyl Anions by Deprotonation The synthesis ofall-trans dienes The synthesis ofall-trans retinol... 

X-substituted AUyl Anions. The allyl-lithium reagents 4.53-4.56 are relatively simple examples of X-substituted allyl anions. With oxygen303,304 or nitrogen305,306 substituents, 4.53 and 4.54, y alkylation is almost always the major pathway, but with sulfur, while y alkylation is known307 4.55, a alkylation is much more common 4.56.308,309... 

From this point on, the regioselectivity of substituted allyl anions is much less regular, and somewhat less explicable. For a start, X-substituted allyl anions react with carbonyl electrophiles with a selectivity. This is explicable, but it is determined by the site of coordination by the metal, not by the frontier orbitals. We can contrast the reaction of the oxygen-substituted lithium anion 4.57 with an alkyl halide, which is y selective, as usual, and the reaction of the zinc anion 4.58 with a ketone, which is a selective.304 The oxygen substituent coordinates to the zinc cr-bound at the y position, and the aldehyde is then delivered to the a position in a six-membered cyclic transition structure 4.59. The same reaction with the lithium reagent 4.57 gives a 50 50 mixture of a and y products, and so lithium is not so obviously coordinated in the way that the zinc is. This type of reaction is often brought under control in the sense 4.59 for synthetic purposes by... 

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