Triisopropylphenyl group

In 2006, Akiyama and coworkers established an asymmetric Brpnsted acid-catalyzed aza-Diels-Alder reaction (Scheme 36) [59]. Chiral BINOL phosphate (R)-3o (5 mol%, R = 2,4,6- Pr3-CgH2) bearing 2,4,6-triisopropylphenyl groups mediated the cycloaddition of aldimines 94 derived from 2-amino-4-methylphenol with Danishefsky s diene 95 in the presence of 1.2 equivalents of acetic acid. Piperidinones 96 were obtained in good yields (72 to >99%) and enantioselectivi-ties (76-91% ee). While the addition of acetic acid (pK= 4.8) improved both the reactivity and the selectivity, the use of benzenesulfonic acid (pK= -6.5) as an additive increased the yield, but decreased the enantioselectivity. A strong achiral Brpnsted acid apparently competes with chiral phosphoric acid 3o for the activation of imine 94 and catalyzes a nonasymmetric hetero-Diels-Alder reaction. The role of acetic acid remains unclear. 

The P-addition of alkyl radicals to 4-methyl-2-(arylsulfinyl)-2-cyclopentenone 117 has been shown to occur in a completely stereocontrolled manner. Of a mixture of (4/ )- and (45)-117, only (4R)-117 reacts with t-Bu and i-Pr radicals to give the trans adducts 119a and 119b in 99% yield, while (45)-117 remained entirely unreacted. The stereochemical outcome of the reaction shows that the alkyl radical approaches from the side opposite to the aryl moiety in an antiperiplanar orientation to the carbonyl and sulfoxide bond. The 2,4,6-triisopropylphenyl group on sulfur plays a critical role, as it effectively shields the olefin face at the P-position by one of the isopropyl groups. This was confirmed by the 1 1 diastereomeric mixture obtained in the reaction of 4-methyl-2-(p-tolylsulfmyl)-2-cyclopentanone with the tert-butyl radical. 

A series of l-(pyridin-2-yl)benzimidazolium salts were synthesized by Chianese et al. and used as precursors to rigid, five-membered chelated biden-tate NHC-pyridine ligands [57]. The ligands were constructed with either very small (H, Me) or very bulky (2,6-diisopropylphenyl, 2,4,6-triisopropylphenyl) groups adjacent to the nitrogen and carbon donor atoms. Cationic palladium-allyl complexes were synthesized under mild conditions and used as catalysts for the allylic substitution reaction between allylic carbonates and either sodium dieth)f-2-methylmalonate or Af-methylbenzylamine. 

Using the very bulky rhodium porphyrins Rh(TTEPP)- and Rh(TTiPP)- (which contain triethylphenyl and triisopropylphenyl groups), neither of which can dimerize. direct evidence for an alkene adduct and its subsequent dimerization to the four-carbon bridged product has been obtained. Reaction of Rh(TTEPP)- with ethene... 

Experiments were also conducted to see whether that the alkynyl groups are perfect substitutes for alkoxyl groups towards hydrolysis. Indeed, when 4 is hydrolyzed, it leads to the same butyltin oxide-hydroxide cluster with 12 tin atoms as when butyltin triisopropoxide is used. Moreover, with the more bulky triisopropylphenyl group linked to the tin, two unusual oxide-hydroxide clusters, one with 10 tin atoms and the other with six, are produced, depending on the experimental conditions. ... 

The same authors proposed the origin of this high selectivity by a computational method, namely, ONIOM (B3LYP/6-31G HF/3-21G) calculations. In the TS, chiral phosphoric acid simultaneously activated both carbonyl and enol moieties with Bronsted acidic and Lewis basic sites, respectively. Owing to the steric repulsion between the aryl moiety of the substrate and the triisopropylphenyl group at 3,3 -positions, the attack of the TS from the Re-face was 1.3 kcalmoT more unstable than attack from the Si-face, giving rise to the preferential formation of the (S)-enantiomer. 

Molecules that contain two or three bulky aryl groups. An example is 2,2-dimesitylethenol (112). In this case, the keto content at equilibrium is only In cases such as this steric hindrance (p. 189) destabilizes the keto form. In 112 the two aryl groups are 120° apart, but in 113 they must move closer together (w 109.5°). Such compounds are often called Fuson-type enols ° There is one example of an amide with a bulky aryl group, A-methyl l w(2,4,6-triisopropylphenyl)acetamide, that has a measurable enol content, in sharp contrast to most amides. 

In order to overcome the difficulties of thermal instability, the phosphasilene derivatives 15, which bear a silyl or germyl group attached to phosphorus, were synthesized. Indeed, they proved to have stronger Si=P bonds (stable up to 100°C), thus allowing studies of their structures and reactivity.l0b 14 Phosphasilenes 15a-15i were synthesized from the corresponding Iithium(fluorosilyl)phosphanides 16a-16i by the thermally induced elimination of LiF (see Scheme 4).10b It has been shown that excellent steric protection of the highly reactive Si=P bond in 15 is provided by the 2,4,6-triisopropylphenyl (Is = isityl) substituent attached to the low-coordinate silicon center. The appropriate precursors 16a-16i were synthesized in a multiple-step procedure, starting from 17 (Scheme 4).10b U... 

Stannene 1 was found to be an extremely air-sensitive compound, which was identified only by its trapping reactions.99 Replacement of the bis(tri-methylsilyl)methyl groups with the bulky aromatic group 2,4,6-triisopropylphenyl led to the isolation of the stable stannene 2 as its diethyl ether... 

Dehydrofluorination has also been utilized for the synthesis of stable stannaphosphenes. The stannaphosphene with bis(trimethylsilyl)methyl groups attached to tin could not be obtained in pure form because of its high reactivity92 replacing these alkyl groups with the bulky aryl substituent 2,4,6-triisopropylphenyl eliminated this problem and resulted in a stannaphosphene that is still air- and moisture-sensitive but stable under an inert atmosphere for prolonged periods of time and in solution at 60°C.93-94... 

In one of their notable examples, the hydroboration polymerization of low molecular weight allyl-telechelic polyisobutylene with tripylborane (trip = 2,4,6-triisopropylphenyl) was found to yield air-stable organoboron segmented block copolymers. These boron main-chain polymers (8) (Fig. 8), unlike the general ones, were stable to air. The stability was due to the steric hindrance of the bulky tripyl groups preventing oxygen attack of the borons.28... 

With the voluminous 2,6-bis(2,4,6-triisopropylphenyl)phenyl group, it is only possible to introduce one unit of this substitutent to lead to obtain an arylbromoplumbylene. In the solid state, this compound exists as the halogen-bridged dimer 74.89 Starting from 74, not only the heteroleptic diorganylplumbylenes 75-77,89 but also the again heteroleptic metalloplumbylenes 78-8090 can be prepared (Equation (27)). 

Simple enols stabilized by bulky aryl groups have been reviewed.131 Amide enols, tip2C=C(OH)NR1R2 (tip = 2,4,6-triisopropylphenyl), can be generated by reaction of amines with ditipyl ketene, are observable by NMR, and slowly tautomerize. Vinyl alcohols with two or three bulky aryls have propeller conformations and are chiral, but are not easily resolved. 

Stereospecific syn addition of methoxysilane to silacyclobutadiene was reported by Fink and coworkers (Scheme 12)72. Ethanol also adds the silacyclobutadiene stereospecifically in a syn fashion to give 42 (Scheme 12), but the ethanol adduct undergoes photoisomerization to a photostationary mixture of 42 and its stereoisomer. A similar system, but with the more bulky 2,4,6-triisopropylphenyl (Tip) groups, also gave the syn ethanol adduct 4373. The rather complicated photochemical-thermal isomerization process was discussed by Fink74. 

The treatment of hexakis(2,4,6-triisopropylphenyl)tetrasilabutadiene with maleic anhydride furnished the 2,9-dioxa-5,6,7,8-tetrasilatetracyclodecan-3-one derivative 29 (Scheme 19). The reaction pathway involves a [2+2] cycloaddition of one of the Si=Si bonds of tetrasilabutadiene to the highly reactive C=0 group, followed by a second cycloaddition of the remaining Si=Si bond across the C=C double bond to complete the formation of the final product 29 <20010M2451>. 

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