Substituent equation

A closely related procedure for preparing thietane dioxides is the one-step conversion of cyclic a-amino ketoximes (222) to 2-( -cyanoalkyl)-3-dialkylaminothietane dioxides (226), with trans-orientation of the substituents " (equation 86). 

An alternative route to allenyl stannanes involves organocuprate displacements on propargylic chlorides bearing an alkynyl PhsSn substituent (equation 39)79. Interestingly, transmetallation by attack of the cuprate on the tin substituent is not observed in these systems. A parallel strategy can be employed for allenylgermanes (equation 39)79. The... 

Ruthenium-catalyzed carbonylations of a variety of heterocycles have been disclosed. With benzimidazole derivatives, the regioselectivity changes upon introduction of a C(2) substituent (Equations (107) and (108)).100... 

The selectivity of hydrogenation of dimethyl tetramethylbicyclo[2.2.0]hexa-2,5-diene-5,6-dicarboxylate depends on the catalytically active metal and on the bulk of the ester substituent (equations 59 and 60)147. 

Saczewski and colleagues reported a similar pathway when the A-aryl-A-(4,5-dihydro-l//-imidazol-2-yl)hydroxylamine hydrochlorides 37 were refluxed in acetone in the presence of acetylenic derivatives 38 carrying electron-withdrawing substituents (equation 10). However, when 37 free bases were used, the adducts rearranged at room temperature, producing only the ketene aminals 39 in poor yields (equation 11). 

Aryl-4,5-dihydrooxazoles 556 underwent cobalt-catalyzed carbonylation to give 4,5-dihydro-l,3-oxazin-6-ones 557, usually in good yields, but an exception was the ring enlargement of 556 containing a 5-methyl (R = Me) or a sterically bulky 2-(o-tolyl) substituent (Equation 67) <20030L1575>. 

The moderate diastereoselectivity common with PKR is substantially increased by introduction of bulkier substituents (Equation (41)). 

The model involving the minimization of the A-1,3 ally lie strain followed by an alkoxy-directed cyclopropanation is quite reliable to predict the sense of induction in the cyclopropanation of chiral acyclic allylic alcohols. However, the application of this model is not as straightforward in the case of allylic ethers. Indeed, it was shown that the cyclopropanation of benzyl ethers led to a major anti or syn isomer depending upon the nature of the substituents (equation 67). ... 

An example of a sugar-derived chiral -haloketone is offered by 24. When ulosyl bromide 24 is coupled to acetaldehyde in a Grignard-type process, a mixture of isomeric adducts is formed, where the 50% is represented by 25, possessing the (R) configuration at the hydroxy ethyl substituent (equation 21). The same protocol is applied in an efficient nucleophilic C-glycosidation reaction of 24 with galactose-derived aldehyde 26 to give 27 (equation 22)20. 

Stereochemical investigation on the vinylogous 1,5-elimination reactions of 16 to give cyclopropylalkenes has revealed highswi stereoselectively of the intramolecular SN process (equation 18)30. The stereoselectivity of the cyclization reaction of 17 to give the ( )-isomer is improved by the increase in steric bulkiness of the sulfonyl substituent (equation 19)31. 

This method, sometimes referred to as transmetallation, is by far the most versatile synthetic route to transition metal amide complexes. The amide, typically lithium or sodium, is reacted with the corresponding transition metal halide either in a hydrocarbon or, more typically, an ether solvent. The method has been applied to virtually all of the transition elements and normally results in complete substitution except for the most bulky amido substituents (equations 12,37 13,38 14,39 1540... 

Template reactions between malonaldehydes and diamines in the presence of copper(II), nickel(II) or cobalt(II) salts yield neutral macrocyclic complexes (equation 15).99-102 Both aliphatic102 and aromatic101 diamines can be used. In certain cases, non-macrocyclic intermediates can be isolated and subsequently converted into unsymmetrical macrocyclic complexes by reaction with a different diamine (Scheme ll).101 These methods are more versatile and more convenient than an earlier template reaction in which propynal replaces the malonaldehyde (equation 16).103 This latter method can also be used for the non-template synthesis of the macrocyclic ligand in relatively poor yield. A further variation on this reaction type allows the use of an enol ether (vinylogous ester), which provides more flexibility with respect to substituents (equation 17).104 The approach illustrated in equation (15), and Scheme 11 can be extended to include reactions of (3-diketones. The benzodiazepines, which result from reaction between 1,2-diaminobenzenes and (3-diketones, can also serve as precursors in the metal template reaction (Scheme 12).101 105 106 The macrocyclic complex product (46) in this sequence, being unsubstituted on the meso carbon atom, has been shown to undergo an electrochemical oxidative dimerization (equation 18).107... 

Table 9 Control of Stereochemistry by Allylic Oxygen Substituents (Equation 38)...

Table 11 Relative Asymmetric Induction by Homoallylic Substituent (Equation 44)129...

High stereoselectivity has been observed in the cyclizations to y-lactams of thioimidate systems with a homoallylic substituent (equation lll).217W,7c The selectivity is considered to arise from A1 2 strain favoring a quasiaxial orientation of the homoallylic substituent in (17 see above). In comparison, the cyclizations to y-lactams shown in Table 29 were not stereoselective.233... 

Since enolates also add via a ligand attack process, the regioselectivity that they exhibit is quite comparable to soft caibon nucleophiles. Alkyl or aiyl substituents at the allyl termini direct attack to the less substituted terminus (equations 228-232) functional groups such as COaMe and halogen at one allyl terminus direct attack to the remote terminus (equations 233 and 234). Remote functionalities such as —OR also direct addition to the allyl terminus more removed from the substituent (equations 23S and 236). 

Functionality remote to the allyl ligand, whether it be oxygen,181 183 190 193 195 201,410 nitrogen,185 or phosphorus,209 directs selective attack of an amine remote to the electronegative substituent (equations... 

Hwu and Gilbert have shown the Nef conversion of nitro compounds to ketones to be promoted by a y-silyl substituent (equation 112)180. 

The protonation of silyl-substituted styrenes is regioselective and leads to secondary benzyl cations with a /1-silyl substituent (equation 49). The isomeric carbocation 332 is not formed. 

Lithiated fluorosilanes such as 801 and 802 may be considered as fluoride donor adducts to silanimines and behave as the free silanimines (e.g. 803) in cycloaddition reactions with C=0 compounds. The stability of the four-membered cycloadducts obviously depends on the substituents (equation 274)355. 

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