Carbon nucleophiles types

Among several propargylic derivatives, the propargylic carbonates 3 were found to be the most reactive and they have been used most extensively because of their high reactivity[2,2a]. The allenylpalladium methoxide 4, formed as an intermediate in catalytic reactions of the methyl propargylic carbonate 3, undergoes two types of transformations. One is substitution of cr-bonded Pd. which proceeds by either insertion or transmetallation. The insertion of an alkene, for example, into the Pd—C cr-bond and elimination of/i-hydrogen affords the allenyl compound 5 (1.2,4-triene). Alkene and CO insertions are typical. The substitution of Pd methoxide with hard carbon nucleophiles or terminal alkynes in the presence of Cul takes place via transmetallation to yield the allenyl compound 6. By these reactions, various allenyl derivatives can be prepared. 

Alkynyl)oxiranes also react with carbon nucleophiles to afford furan derivatives. Furanes of different types are obtained depending on the structure of the substrates. 7-Methyl-2-ethynyloxirane (95) reacts with acetoacetate to give the furan 97 by the elimination of formaldehyde from the cyclized product 96. The hydroxy ester of the alkylidenefuran 98 and the corresponding lactone 99 are obtained by the reaction of i-methyl-2-(2-propynyI)oxirane[40, 42]. 

The use of carbon nucleophiles in Michael-type addition reactions with pteridine and its derivatives leads to a quite complicated and divergent pattern. These reactions are strongly dependent on the nature of the carbon nucleophile and can be divided into various categories. 

Addition of carbon nucleophiles to vinylepoxides is of particular importance, since a new carbon-carbon bond is formed. It is of considerable tactical value that conditions allowing for regiocontrolled opening of vinyloxiranes with this type of nucleophiles have been developed. Reactions that proceed through fonnation of a rr-allyl metal intermediate with subsequent external delivery of the nucleophile, or that make use of a soft carbon nucleophile, generally deliver the SN2 product. In contrast, the Sn2 variant is often the major reaction pathway when hard nucleophiles are employed. In some methods a nucleophile can be delivered selectively at either the Sn2 or SN2 positions by changing the reaction conditions. 

Stereoselective reactions of this type known at present only deal with four- or five-membered cyclic iV-acyliminium ions. The reactions with carbon nucleophiles usually lead to rra/u-substi-tuted compounds with very high stereoselectivity due to steric control by the substituent already present in the ring. 

Several types of compounds can act as the carbon nucleophile in diazo transfer, including the oxymethylene139 or dialkylaminomethylene140 derivatives of the ketone. These activating substituents are lost during these reactions. 

Instead of alkynes, allenes can also be used as substrates in this type of approach. Finally, one can also apply carbon-nucleophiles such as butadienes in this domino process. Thus, Lu and Xie [145] have treated the alkyne 6/1-303 with an aryl halide 6/1-304 and an amine 6/1-305 to give the substituted pyrrolidinone 6/1-308 via the proposed intermediates 6/1-306 and 6/1-307. As a side product, 6/1-309 is found to have been formed by a cycloaddition of 6/1-303 (Scheme 6/1.81). 

The preparation of 5-chloro-l,2,4-thiadiazol-2-ium chlorides 95 by treatment of formimidoyl isothiocyanates 94 with a twofold excess of methanesulfenyl chloride has been reported in an unusual variation of a type C synthesis. These salts show interesting chemical behavior toward several nitrogen and carbon nucleophiles. The nature of the N-substituent determines the stability of the salt 95. When the substitutent on nitrogen is /-butyl, the salt 95 decomposes readily in solution to give the 5-chloro-l,2,4-thiadiazole 96 (Scheme 10) <2003HAC95>. 

Reactions of a,(3-unsaturated acylzirconocene chlorides with stable carbon nucleophiles (sodium salts of dimethyl malonate and malononitrile) at 0°C in THF afford the Michael addition products in good yields (Scheme 5.38). Direct treatment of the reaction mixture with allyl bromide in the presence of a catalytic amount of Cul -2LiCl (10 mol%) in THF at 0 °C gives the allylic ketone in a one-pot reaction. This sequential transformation implies the electronic nature of a,P-unsaturated acylzirconocene chloride to be of type E as shown in Scheme 5.37. 

Alkoxycarbenium ions are important reactive intermediates in modem organic synthesis.28 It should be noted that other names such as oxonium ions, oxocarbenium ions, and carboxonium ions have also been used for carbocations stabilized by an adjacent oxygen atom and that we often draw structures having a carbon-oxygen double bond for this type of cations.2 Alkoxycarbenium ions are often generated from the corresponding acetals by treatment with Lewis acids in the presence of carbon nucleophiles. This type of reaction serves as efficient methods for carbon-carbon bond formation. 

Transition metal carbene complexes have broadly been classified into Fischer-type and Schrock-type carbene complexes. The former, typically low-valent, 18-electron complexes with strong 7t-acceptors at the metal, are electrophilic at the carbene carbon atom (C ). On the other hand, Schrock-type carbene complexes are usually high-valent complexes with fewer than 18 valence electrons, and without n-accepting ligands. Schrock-type carbene complexes generally behave as carbon nucleophiles (Figure 1.4). 

Electrophilic vinylidene complexes, capable of reacting with non-carbon nucleophiles to yield Fischer-type carbene complexes, can be obtained by addition of electrophiles to alkynyl complexes (Figure 2.11, Table 2.7, Entries 11, 12) [134,144]. 

As in carboxylic esters it is possible to substitute alkoxy groups of Fischer-type carbene complexes by non-carbon nucleophiles, such as other alcohols [73,214,218], enols [219], aliphatic amines [43,64,66,220-224], aniline [79], imines [225], or pyrroles [226]. Strong nucleophiles can also lead to a dealkylation of methoxy-substituted carbene complexes (5 2 at the methyl group, [227]), in the same way as methyl esters can be cleaved by nucleophiles such as iodide. Carbon... 

The HC1 generated in this reaction destroys one equivalent of diazomethane. This can be avoided by including a base, such as triethylamine, to neutralize the acid.74 Cyclic z-diazoketones, which are not available from acyl chlorides, can be prepared by reaction of an enolate equivalent with a sulfonyl azide. This reaction is called diazo transfer 5 Various arenesulfonyl azides76 and methanesulfonyl azide77 are used most frequently. Several types of compounds can act as the carbon nucleophile. These include the anion of the hydroxymethylene derivative of the ketone78 or the dialkylaminomethylene derivative of... 

The affinity of Cgo towards carbon nucleophiles has been used to synthesize polymer-bound Cgo [120] as well as surface-bound Cjq [121]. Polymers involving G q [54, 68, 69] are of considerable interest as (1) the fullerene properties can be combined with those of specific polymers, (2) suitable fullerene polymers should be spin-coatable, solvent-castable or melt-extrudable and (3) fullerene-containing polymers as well as surface-bound Cgo layers are expected to have remarkable electronic, magnetic, mechanical, optical or catalytic properties [54]. Some prototypes of polymers or solids containing the covalently bound Cjq moiety are possible (Figure 3.11) [68,122] fullerene pendant systems la with Cjq on the side chain of a polymer (on-chain type or charm bracelet ) [123] or on the surface of a solid Ib [121], in-chain polymers II with the fullerene as a part of the main chain ( pearl necklace ) [123], dendritic systems III, starburst or cross-link type IV or end-chain type polymers V that are terminated by a fullerene unit For III and IV, one-, two-and three-dimensional variants can be considered. In addition, combinations of all of these types are possible. 

The reaction of oxiranes with carbon nucleophiles provides a general method for the synthesis of B-type branched sugars. Thus, the diaxial ring-opening of methyl 2,3-anhydro-4,6-0-benzylidene-a-L-mannopy-... 

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