Y a-alkylation

Reaction of silylallyl anions 16 with alkyl halides gives a mixture of a- and /-alkylated products 17 and 18, respectively (equation ll)19. The ratio of y/a alkylation increases along with the bulk of the silyl groups by following the order shown in entry 8 of Table 1. 

The photostimulation of Sn2 systems sueh as X + A — Y (A = alkyl) is a niee example for the utility of the VBSCD. This photoreaetion was analyzed before, using the VBSCD model, for predieting the loeation of eonieal interseetions [30]. Here, the transition state for the thermal reaetion is the eollinear [X-A-Y] strueture, whieh is the P (A ) resonating eombination of the two Lewis struetures, while the twin-exeited state, P (A"), is their antiresonating eombination the symmetry labels refer to Cj symmetry. This latter exeited state is readily written as an A symmetry-adapted eombination of Lewis struetures, Eq. (21) ... 

Thiazole acetic acids and their hotnologs can also be prepared by cyclization procedures 4-thiazole alkanoic acids and their salts were prepared by treating a thioamide with a -y-chloro- or 7-bromoacetoacetic or their a-alkyl derivatives (4, 10, 16, 22, 273, 275, 281, 640, 647, 695). 

A nitrogen atom at X results in a variable downfield shift of the a carbons, depending in its extent on what else is attached to the nitrogen. In piperidine (45 X = NH) the a carbon signal is shifted by about 20 p.p.m., to ca. S 47.7, while in A-methylpiperidine (45 X = Me) it appears at S 56.7. Quaternization at nitrogen produces further effects similar to replacement of NH by A-alkyl, but simple protonation has only a small effect. A-Acylpiperidines show two distinct a carbon atoms, because of restricted rotation about the amide bond. The chemical shift separation is about 6 p.p.m., and the mean shift is close to that of the unsubstituted amine (45 X=NH). The nitroso compound (45 X = N—NO) is similar, but the shift separation of the two a carbons is somewhat greater (ca. 12 p.p.m.). The (3 and y carbon atoms of piperidines. A- acylpiperidines and piperidinium salts are all upfield of the cyclohexane resonance, by 0-7 p.p.m. 

Assign a basis name according to the number of carbons in the corresponding unbranched alkane. Drop the ending -ane and replace it by -y/. The alkyl group shown in step 1 is named as a substituted hexyl group. 

The bathochromic shift which results when a quaternary ind-N-unsubstituted p /r-A-alkyl-a-, - 3. 27,28o, 391 y se -8-earbolinium salt is eonverted into the eorresponding anhydro-base is referred to in Seetion VI, A. 

Few other reactions of series of substituted pyridines have been investigated extensively. Dondoni, Modena, and Todesco have measured the rate of N-oxidation of a limited series of pyridines and found a good correlation with normal u-values with a p-value of — 2.23. The A-alkylation of pyridines with alkyl iodides in nitrobenzene has been studied by Brown and Cahn and by Clarke and Rothwell. Unfortunately, the only data available are for the parent compound and for alkyl derivatives, and, since the a-values for the various alkyl groups in a given position are substantially constant, this leaves a correlation of only three independent points. However, the rates of A-alkylation of the j8- and y-alkyl derivatives are so nearly equal that it appears as if no correlation existed. Clarke and Rothwell have also studied the alkylation with allyl bromide in nitromethane at various temperatures, and in this case a more extensive series is available. The authors state that no overall Hammett correlation is obtained however, the j8-substituted derivatives fall on one straight line and the y-derivatives on another one with a different slope. The data are shown in Fig. 2. The line for the j8-compounds, p = — 2.53 0.31, r = 0.95, is seen not to be very good the line for the y-derivatives, p = — 1.42 0.06, r = 0.99, is much more satisfactory. It does not seem likely that the discrepancy is due to the intervention of resonance effects, since in this case one would expect the correlation for the y-derivatives to be poorer than that for the j8-analogs. More extensive studies with a wider variety of substituents would seem very desirable. 

In contrast to the results obtained with the jS-alkoxy-a-alkyl-y-lactol 16 (vide supra), a chelation-directed, anti-Cram selective nucleophilic addition to the a-methyl-y-lactol 1 was not only observed with methyllithium and methylmagnesium bromide but also with (triisopropoxy)methyl-titanium72. In fact, the highest diastereoselectivity (> 98 % de) was observed with the titanium reagent in dichloromethane as reaction solvent. A seven-membered chelate 3 with the a-methyl substituent in a pscudoequatorial position has been postulated in order to explain the stereochemical outcome. 

H NMR spectroscopy studies of iron(IIl) a-alkyl and o-aryl porphyrins have been very important in elucidating spin states. Alkyl and most aryl complexes with simple porphyrin ligands (OEP, TPP, or TTP) are low spin, S — I /2 species. NMR spectra for the tetraarylporphyrin derivatives show upheld resonances for the porphyrin pyrrole protons (ca. — 18 to —35 ppm), and alternating upfield and downfield hyperfine shifts for the axial alkyl or aryl resonances. For -alkyl complexes, the a-protons show dramatic downfield shifts (to ca. 600 ppm), upfield shifts for the /3-protons (—25 to — 160 ppm) and downfield shifts for the y-protons (12 ppm). The cr-protons of alkyliron porphyrins are not usually detected as a result of their large downfield shift and broad resonance. These protons were first detected by deuterium NMR in the dcuterated complexes Fe(TPP)CD3 (532 ppm) and Fe(TPP)CD2CDi (562, -117 ppm). ... 

King RS, Sharma V, Pedersen LC, Kakuta Y, Negishi M, Duffel MW. Structure-function modeling of the interactions of the A-alkyl-A-hydroxyanilines with rat hepatic aryl sulfotransferase IV. Chem Res Toxicol 2000 13 1251-8. 

If both X and Y are alkyl groups, the reference compound is a singly branched hydrocarbon, and it is converted to a doubly branched structural unit in the hypothetical dehydrogenation process hence... 

Sulfur dioxide (see above) as well as S02, SO , and SOj have been used as building blocks in three-component sulfone syntheses. It has long been known that aromatic sulfinic acids are easily available from diazonium salts and sulfur dioxide under copper catalysis . Mechanistically, aryl radicals as reactive intermediates add to sulfur dioxide generating arenesulfonyl radicals, which either take up an electron (or hydrogen) yielding a sulfinic acid or add to an olefinic double bond yielding final y -halogenated alkyl aryl sulfones (equation 78). 

Carbanion 239 derived from vinyl sulfone 238 was shown to be alkylated regiospecifi-cally at the a-carbon atom The carbanions 240 and 241 derived from <5-ketosulfones were shown to be alkylated at both a- and y-carbons . Alkylation of 240 gave a-alkylated products predominantly, while the a/y ratio varied markedly by change of the alkylating agent in the reaction with 241. For example, methylation with methyl iodide gave the products in the a/y ratio of 72 28, while the a/y ratio was 27 73 in the ethylation with ethyl bromide ... 

Deprotonation of allylic aryl sulfoxides leads to allylic carbanions which react with aldehyde electrophiles at the carbon atom a and also y to sulfur . With benzaldehyde at — 10 °C y-alkylation predominates , whereas with aliphatic aldehydes at — 78 °C in the presence of HMPA a-alkylation predominates . When the a-alkylated products, which themselves are allylic sulfoxides, undergo 2,3-sigmatropic rearrangement, the rearranged compounds (i.e., allylic sulfenate esters) can be trapped with thiophiles to produce overall ( )-l,4-dihydroxyalkenes (equation 24). When a-substituted aldehydes are used as electrophiles, formation of syn-diols 27 occurs in 40-67% yields with diastereoselectivities ranging from 2-28 1 (equation 24) . ... 

Scheme 1.8 shows some intramolecular enolate alkylations. The reactions in Section A involve alkylation of ketone enolates. Entry 1 is a case of a-alkylation of a conjugated dienolate. In this case, the a-alkylation is also favored by ring strain effects because y-alkylation would lead to a four-membered ring. The intramolecular alkylation in Entry 2 was used in the synthesis of the terpene seychellene. 

The diastereoselective ultrasonically induced zinc-copper 1,4-addition of alkyl iodides to chiral a, (i-unsaturaied systems in aqueous media was studied by Suares and co-workers the Z-isomer gives good diastereos-electivities while reactions with the E-isomer are nonstereoselective.63 The 1,4-addition to chiral y, a-dioxolanyl-a, (i-unsaturated esters also proceeds with good yields (51-99%) (Eq.10.29).64... 

H NMR and 2H NMR spectra of fractionated coal products from El0 and El9 were recorded and analyzed to determine and 2H composition for each structural position. In our study, y y and 2Hx,v are defined as the fraction of the JH and 2H determined from the integrals of the NMR spectra of a given soluble fraction where y equals HS, BS or BMS and x = y-alkyl, 0-alkyl, a-alkyl or aromatic structural positions. The spectral range of the NMR integrations are given in Table V. 

Table VI liststhe calculated Px,y/Fy vaiues for the three soluble coal products from E10 and E19. The values range from 0.58 to 1.63. The a-alkyl regions in E10 and E19 have Px,y >fy which indicate that in liquefaction experiments conducted with a donor solvent, preferential incorporation occurs in the a-alkyl position.

If the chiral auxiliary in Eq. 4.96 is modified by changing MeO into more bulky groups such as trityl (Tr) or t-butyldimethylsilyl (TBS) group, an improved asymmetric nitro-olefination of a-alkyl-y- and 8-lactones is possible (Eq. 4.97).120... 

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