2,4,6-Triisopropylphenyl

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. 

ESR spectrum of the RjSn radical (R = 2,4,6-triisopropylphenyl), spontaneously generated upon dissolving distannane RBSn-SnRj in deoxygenated toluene, revealed at -140°C in the solid state the Sn hfcc of 163 Such a radical is more planar than the PhjSn radical ( Sn hfcc = 186.6mT) but less planar than the MejSn radical C Snhfcc = 161.1 mT)3° ... 

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... 

Disilene 2 having a bulkier substituent, 2,4,6-triisopropylphenyl, was obtained similarly.7 When trisilanes bearing two different substituents on the central silicon atoms are used, a mixture of E- and Z-isomers of disilenes is obtained (Eq. 3).8-10 Attempts to synthesize disilenes with other substituents using trisilane Tip(R)Si(SiMe3)2 (R = H, F, Cl, or 1-pyrroyl) were unsuccessful.10... 

With the availability of stable geometric isomers of doubly bonded germanium compounds, experimental determinations of the 7r-bond strength can be made. The enthalpy of activation for double bond isomerization in Mes(Tip)Ge=Ge(Tip)Mes (Tip = 2,4,6-triisopropylphenyl) has been determined for the Z-E conversion, 22.2 . 3 kcal/mol and for the E-Z conversion, 20.0 0.3 kcal/mol.15 These values agree well with recent theoretical estimations.7 The isomerization barrier in germaphos-... 

Also related is the synthesis of 2,2-dimesityl-l-(2,4,6-triisopropylphenyl)g-ermaphosphene by the reaction between difluorodimesitylgermane and the appropriate dilithiophosphide.35... 

The low-temperature photolysis of hexakis(2,4,6-triisopropylphenyl) cyclotristannane leads quantitatively to the corresponding distannene (Eq. 30).90 The distannene was found to be in equilibrium with the cyclotristannane over a wide range of temperatures. The simplest explanation for this is that the stannylene is formed as an intermediate from either the cyclotristannane or the distannene, but quickly adds to the distannene or dimerizes. This equilibrium can be followed by ll9Sn NMR spectroscopy at elevated temperatures.96... 

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... 

Scheme 7 Friedl-Crafts reaction of l-(2,4,6-triisopropylphenyl)phosphole...

The chloro- or fluorostannanes used were of the type RjXSn-CHRa, where R = bis(trimethylsilyl)methyl or 2,4,6-triisopropylphenyl, R2C is the fluorenylidene moiety and X = Cl or F. Compounds of this type are prepared from R 2SnX2 and LiCHR2. 

A Grignard reaction enables the preparation of hexakis(2,4,6-triisopropylphenyl)tris-tannacyclopropane, 76, obtained from 2,4,6-triisopropylphenylmagnesium bromide and... 

Similar reactions were performed earlier photochemically from 76, which can be converted into tetrakis(2,4,6-triisopropylphenyl)distannene. 

Using alkyllithium compounds, a distannane with a very long Sn—Sn bond, 3.03A, and a syw-peripheral conformation122 was prepared. Thus, reaction of di-t-butyl(chloro)2,4,6-triisopropylphenyl)tin, 78, with f-butyllithium gives 1,1,2,2-tetra-f-butyl-l,2-(2,4,6-triisopropylphenyl)ditin, 79, which displays a restricted rotation about the Sn—C bond as well as the Sn—Sn bond, even at high temperatures. 

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