S Alkyl Groups

One example of nucleophilic attack by a rr-electron system on a sulfur atom of a thiirane 1-oxide is shown in Scheme 51. S-Alkylthiirenium ions react with tetramethylethylene to transfer the S-alkyl group yielding the alkyne and an S-alkyl-2,2,3,3-tetramethylthiiranium ion (79MI50600). 

Migration of 4-Aryl Group with Loss of S-Alkyl Group... 

Ketones form more stable Lewis acid-base complexes with electrophilic metal salts than do aldehydes as a result of the increased basiciLy of the carbonyl oxygen atom thanks to the +1 effect of the ketone s alkyl groups. 

The alcohol s alkyl group is called hydrophobic ( water hating ) because it acts like an alkane It disrupts the network of hydrogen bonds and dipole-dipole attractions of a polar solvent such as water. The alkyl group makes the alcohol less hydrophilic, yet it lends solubility in nonpolar organic solvents. As a result, most alcohols are miscible with a wide range of nonpolar organic solvents. 

Dialkylation of malonic ester proceeds in most cases almost as readily as monoalkylation. Diethyl ethylmalonate is alkylated equally well by s-octyl and n-butyl halides. Di-n-propylmalonic ester is prepared in one step from malonic ester and n-propyl bromide. Methylmalonic ester is alkylated by /3-phenylethyl bromide, and even a-naphthyl-malonic ester may be further alkylated by n-alkyl iodides. Difficulty is encountered, however, in introducing two s alkyl groups into malonic ester. A 23% yield of diisopropylmalonic ester is obtained from iso-ptopylmalonic ester, sodium triphenylmethide, and isopropyl iodide. ... 

Transfer of s-alkyl groups occurs more readily when Hg(I) and Hg(II) alkoxides are used, the former being the superior reagents. For Hg(OMe)2 only one cyclohexyl or other secondary group is transferred, e.g. ... 

That s-alkyl groups do not react under such mild conditions as primary alkyl groups do enables dicyclohexylborane and borabicyclo[3.3.1]nonane to be used in hydro-boration-mercurideboronation sequences, e.g. ... 

Alkenyl groups are transferred in preference to s-alkyl groups. The yields of trans-alkenylmercurials from RC=CH are greater than 85% yields from internal alkynes are lower. Use of (I) rather than tris(alkenyl)boranes leads to higher yields of alkenyl-mercurials. 

Both vernolate and EPTC had half-lives of 3 days in EPTC-history soil compared to 17 and 13 days, respectively, in non-history soil (Figure 4). A single carbon increase in the S-alkyl group (vernolate) did not appear to significantly influence microbial utilization. 

P2i2i2i Z = 4 Dx = 1.34 R = 0.110 for 552 intensities (film measurements). The two S-alkyl groups are trans, with the manno configuration. The conformation lies between < and... 

A problem with this methodology is that only one of the three alkyl groups is transferred to the unsaturated carbonyl compound. A solution to this uses the radical generated from the boronic ester, itself derived from hydroboration with catecholb-orane 8. Treatment of the boronic ester with oxygen and 1,3-dimethyl-hexahydro-2-pyrimidinone (DMPU) in the presence of the a,3-unsaturated aldehyde or ketone gives the desired radical addition product, with transfer of the S-alkyl group. Thus, cyclohexene was converted to l-cyclohexyl-3-pentanone 24 using this chemistry (5.38). 

Also very interesting are the results obtained mainly by Julia in which a substituent Rj is on the terminal double bond 6-position, but this time cis to the aliphatic chain. This effect is well illustrated by entries 7 and 8. As already discussed, the trans isomeric radical (R4 = CH3, R3, = CH3, entry 7) gives the low Cy5/Cy6 ratio of 0.58 because of the R3 substituent. But now, if in place of a trans R4 = CH3, there is a cis one (entry 8, R5 = CH3, R3 = CH3) the Cy5/Cy6 ratio is completely reversed, becoming so high that no Cy6 products are detected. These results have been nicely accommodated by Julia on the basis of Scheme 7 (see the discussion in Section III). It is quite clear that replacement of a c/s-hydrogen atom by a c/s-alkyl group R5 will enhance the... 

The different reaction course followed by the silyl- and alkyl-substituted allenes is fully consistent with the hyperconjugation model, which predicts that a S-silyl group stabilizes carbenium ions better than a S-alkyl group, and with previous conclusions that a-silyl substitution in carbocations is destabilizing relative to a-methyl substitution. The larger space requirements of the trimethylsilyl groups compared with methyl groups may also contribute to the kinetic stability of 413. 

Branched alkyl groups s. Alkyl groups, branched Bridge amidlnes 26,297... 

The use of B-alkyl-9-BBN compounds, especially (15), as mild and very chemoselective reducing agents for aldehydes (Scheme 8), even in the presence of ketones, has been explored and the variation in reducing ability with different S-alkyl groups examined. The cyclic transition state (16) is consistent with an increase in rate both with increase in substitution /3- to boron and when the B—C C—H unit can adopt a planar arrangement. This system has also been... 

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