Isobutyl groups

Those derived from isobutane are the 2 methylpropyl (isobutyl) group and the 1 1 dimethylethyl (tert butyl) group Isobutyl is a primary alkyl group because its poten tial point of attachment is to a primary carbon tert Butyl is a tertiary alkyl group because Its potential point of attachment is to a tertiary carbon... 

Isobutyl group (Section 2 13) The group (CH3)2CHCH2— Isoelectric point (Section 27 3) pH at which the concentration of the zwittenonic form of an amino acid is a maximum At a pH below the isoelectric point the dominant species is a cation At higher pH an anion predominates At the isoelec tnc point the ammo acid has no net charge Isolated diene (Section 10 5) Diene of the type... 

Construct such a model for isotactic polypropylene. Estimate the volume of an isobutyl group on the scale of your model and examine whether interference between successive substituents would occur if this were the R group present. 

A loss of 55 is possibly the loss of C4H7 from esters (double hydrogen rearrangement). The loss suggests a butyl or isobutyl group, especially when m/z 56 is also present. 

In Step C a dithiane anion was used as a nucleophilic acyl anion equivalent to introduce the C(10)-C(13) isobutyl group. 

Attempts to use the isobutyl group in the carbometalation of alkynes only give rise to hy-drometalated products, but ethyl and n-propyl groups can be successfully transferred from the corresponding dialkylaluminum chlorides. The regioselectivity in these reactions is lower than that for the methyl transfer. Indeed, the reaction mechanism may be different from that of methylalumination [62]. 

Triisobutylaluminum (TIBA) is an effective reducing agent for ketones. However, in most cases only one isobutyl group is available for reduction. Enolization occurs after a rapid reduction involving the first isobutyl group (143,147). For example, an enolate is formed in the reaction of TIBA with cyclohexanone (143) (eq. [31]). 

Ketone signature Methyl group. Isobutyl group J i... 

Figure 10.4 Structure of the self-dimerized V complex on Si02 modeled by DFT. (a) Top view (b) side view. Red vanadium green nitrogen blue oxygen gray Ph ring yellow isobutyl group derived from L-leucine.

The cationic polymerization of vinyl isobutyl ether at —40°C produces stereoregular polymers (structure 5.21). The carbocations of vinyl alkyl ethers are stabilized by the delocalization of p valence electrons in the oxygen atom, and thus these monomers are readily polymerized by cationic initiators. Poly(vinyl isobutyl ether) has a low Tg because of the steric hindrance offered by the isobutyl group. It is used as an adhesive and an impregnating resin. 

Whereas the van der Waals radius of the fluorine atom is the smallest one after that of hydrogen, its volume is actually closer to that of oxygen (Table 1.16). Note that if the volume is an intrinsic property, steric effects are dependent on the observed phenomena. They frequently appear in dynamic processes. This allows comparison of steric parameters of various groups, fluorinated or not. These parameters show that the CF3 group is at least as bulky as an isopropyl or isobutyl group (Table 1.17). These data are confirmed by the values of the rotation, or of inversion barriers, of fluorinated diphenyl-type compounds (Figure 1.6). 

Yet a further increase in potency is observed when the para-isobutyl group is replaced by a benzene ring. One published synthesis for that compound is quite analogous to the malonate route to the parent drug. The acetyl biphenyl (50-1) is thus converted to the corresponding arylacetic acid by reaction with sulfur and morpholine, followed by hydrolysis of the first-obtained thiomorpholide. This is then esterified and converted to malonate anion (50-2) with sodium ethoxide and ethyl formate. The anion is quenched with methyl iodide hydrolysis of the esters followed by decarboxylation yields the NSAID flubiprofen (50-3) [51]. 

It of interest to note that the isobutyl group may also be replaced by a heterocyclic ring. The route to this compound, pirprofen (51-6), starts with the direct methylation of unesterihed 4-nitrophenylacetic acid (51-1). The observed selectivity for monoalkylation in this case may reside in the structure of the dianion, whose most stable form is presumably that depicted in (51-2). Catalytic reduction of the product (51-3) gives the corresponding aniline this is then converted to its acetanilide (51-4) with acetic anhydride. Treatment with chlorine followed by hydrolysis gives the chloroaniline (51-5). Double alkylation of this last intermediate with 1,4-dichloro-but-2-ene (depicted as the cis isomer for aesthetic reasons) forms the dihydropyrrole ring. There is thus obtained the NSAID pirprofen (51-6) [52]. 

Sterically bulky reducing agents, e.g. diisohutylaluminium hydride (DIBAH), can selectively reduce esters to aldehydes. The reaction is carried out at low temperatures (—78 °C) in toluene. Diisohutylaluminium hydride has two bulky isobutyl groups, which make this reagent less reactive than LiAlH4. 

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