Allyl anion equivalent

Nickel(O) complexes are extremely effective for the dimerization and oligomerization of conjugated dienes [8,9]. Two molecules of 1,3-butadiene readily undergo oxidative cyclization with a Ni(0) metal to form bis-allylnickel species. Palladium(O) complexes also form bis-allylpalladium species of structural similarity (Scheme 2). The bis-allylpalladium complexes show amphiphilic reactivity and serve as an allyl cation equivalent in the presence of appropriate nucleophiles, and also serve as an allyl anion equivalent in the presence of appropriate electrophiles. Characteristically, the bis-allylnickel species is known to date only as a nucleophile toward carbonyl compounds (Eq. 1) [10,11],... 

Allylsilanes and allylboranes are allyl anion equivalents, which are stable enough to be included in subsequent allylation reactions of aldehydes under... 

During the preparation of allylic zinc reagents, the formation of Wurtz-coupling products may be observed, especially if the intermediate allylic radical is well stabilized. However, the direct insertion of zinc foil to allyl bromide in THF at 5 C is one of the best methods for preparing an allylic anion equivalent. Allylic zinc reagents are more convenient to prepare and to handle than their magnesium and lithium counterparts... 

In organic syntheses allylsilanes and allylstannanes have been used extensively as allyl anion equivalents during the last two decades [187-190]. The regioselective attack of electrophiles, which finally yields products with allylic inversion (Scheme 43), has been explained by the hyperconju-gative stabilization of carbenium centers by the carbon-silicon or carbon-tin bond in the j3-position [191-196], which has initially been derived from solvolytic experiments [197-199]. 

Electrophilic selenenylation and seleninylation (RSe+) of allyl anion equivalents (allyl silanes, stannanes). 

An intramolecular variation of the Heck reaction was used in the construction of the monoterpene alkaloid ( )-oxerime 395 [138]. Nucleophilic addition to 392 of an allyl anion equivalent produced 393, a precursor to an intramolecular Heck cyclization. Palladium-catalyzed ring closure afforded 394, an intermediate in the synthesis of the natural product 395. 

The most useful of all allyl anion equivalents are the allyl silanes.20 This is because it is easy to make them regioselectively, because they do not undergo allylic rearrangement (silicon does not do a [1,3] shift) and because their reactions with electrophiles are very well controlled addition always occurring at the opposite end to the silicon atom. Symmetrical allyl silanes can be made from allyl-lithiums or Grignards by displacement of chloride from silicon. A useful variant is to mix the halide with a metal, e.g. sodium, and Me3SiCl in the same reaction, rather after the style of the silicon acyloin reaction,21 as in the synthesis of the acetal 80. 

Methylene-l,4-alkanediols. The PdCl2-(PhCN)2 - SnCl couple transforms an allylic alcohol into an allyl anion equivalent. 2-Methylene-1,3-propanediol can be used to prepare 2-methylene-1,4-alkanediols on reaction with carbonyl compounds. 

The 5yn-dianions (39) of tosylhydrazones have also been reported to act as allyl anion equivalents. They add to carbonyl compounds to yield /3-o ty-adducts that, on treatment with excess alkyl-lithium, give jise to homoallylic alcohols... 

As help, we will remember how an allyltributylstannane reacts with an electrophile. Allylstannanes (like allylsilanes) have been extensively used as allyl anion equivalents. Considering the ability of die metal to stabilize a positive charge in the P-position by hypereonjugation, it seems reasonable to propose a mechanism involving the formation of an intermediate P-carbocation. Then, the first step would be the eleetrophilie addition to the allylic system leading to the formation of cation 20, stabilized by the metal in the P-position. Displacement of the metallic fragment subsequently produees the final product (Scheme 3.9). 

Allylalion of the alkoxymalonitrile 231 followed by hydrolysis affords acyl cyanide, which is converted into the amide 232. Hence the reagent 231 can be used as an acyl anion equivalent[144]. Methoxy(phenylthio)acetonitrile is allylated with allylic carbonates or vinyloxiranes. After allylation. they are converted into esters or lactones. The intramolecular version using 233 has been applied to the synthesis of the macrolide 234[37]. The /i,7-unsaturated nitrile 235 is prepared by the reaction of allylic carbonate with trimethylsilyl cyanide[145]. 

Oompound 1, 2-(hydroxymethyl )a11y1trimethylsilane, represents a conjunctive reagent which can be considered as the equivalent of zwitterion 2, possessing a nucleophilic allyl anion synthon and an electrophilic allyl... 

Several reviews cover hetero-substituted allyllic anion reagents48-56. For the preparation of allylic anions, stabilized by M-substituents, potassium tm-butoxide57 in THF is recommended, since the liberated alcohol does not interfere with many metal exchange reagents. For the preparation of allylic anions from functionalized olefins of medium acidity (pKa 20-35) lithium diisopropylamide, dicyclohexylamide or bis(trimethylsilyl)amide applied in THF or diethyl ether are the standard bases with which to begin. Butyllithium may be applied advantageously after addition of one mole equivalent of TMEDA or 1,2-dimethoxyethane for activation when the functional groups permit it, and when the presence of secondary amines should be avoided. 

Finally, intermediate cationic allyl complexes of palladium15,16 and ruthenium17, produced from allylic esters by the action of substoichiometric amounts of the metal catalyst, have been electronically inverted by reduction to become nucleophilic anion equivalents, which are capable of carbonyl addition. 

In 1989 we reported on the synthesis and structure of the first l,3-diphospha-2-sila-allylic anion 3a [4], mentioning its value as a precursor for phosphino-silaphosphenes. In analogy to 3a we obtained the anions 3b-f [5] by treatment of 4 equivalents of the lithium phosphide 1 with the adequately substituted RSiC, of which 3b and 3c were investigated by X-ray analyses. The very short P-Si bond lengths (2.11-2.13 A) of 3a-c and the almost planar arrangement of Pl-Sil-P2-Lil indicate the cr-character of the Lithium P-Si-P allyl complex. 

In this transformation, manganese(IV) oxide oxidizes allylic or benzylic alcohols to aldehydes followed by nucleophilic attack of the in situ formed triazolinyli-dene carbene (Scheme 41). The authors suggest the formation of an acyl anion equivalent LX is slow in MeOH compared to oxidation to allow for an activated carboxylate LXII. 

No other synthetic method Is known that achieves the equivalent transformation. Rather elaborate procedures using an allylic anion type of the homoenolate "equivalents" or homoenolate radicals have been reported, but their tolerance to the structure of the enone acceptor is much narrower. 

Alternatively, the ambident oi-hetero substituted allyl anions have been utilized as homoenolate equivalents. For example, in the presence of HMPA, allyl phenyl sulfides (251),192 allyl phenyl sulfones (252)192b c and allyl phenyl selenides (253)192d e add to a,(3-enones in a l,4(0)-mode, while allyl phenyl sulfoxides (254) and allyl phosphine oxides (255) afford 1 A j-addition exclusively, irrespective of solvent used.193 Hua has shown that additions of either chiral sulfoxide (254 R1 = R2 = R3 = R4 = H, R5 = p-tolyl) or allyl oxazaphospholidine oxide (256) occur with excellent enantioselectivity (>95% ee).194 Similarly, Ahlbrecht reports that the a-azaallyl (257) adds exclusively in a 1 A j-mode to acceptor (59) to afford 1,5-diketones (Scheme 86).195... 

Whereas an allylsilane can serve as an allyl anion synthon, the reaction of 1,3-bis(silyl)propene with electrophiles can afford the 1,3-disubstituted propene. Thus, treatment of a mixture of (If)- and (Z)-2-aryl-l,3-bis(trimethylsilyl)propenes 5 with 2 equivalents of NBS at —78 °C stereoselectively yields the corresponding (Z)-2-aryl-l,3-dibromopropene 6. When 1 equivalent of NBS is employed, the monobromo product 7 is obtained (equation 5). The reactions apparently proceed via the pattern of sequential displacement of allylsilane moieties40,41. 

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