Synthesis of new calix[4]arenes bearing silylether groups

The Si-H groups of 25,26,27,28-tetrakis[4-(tris(dimethylsilyl)methyl)butoxy]calix[4]arene (II) were treated with methanol, ethanol, propanol, butanol, pentanol, hexanol, isopropanol, 1-methyl propanol, 2-methylpropanol and 2-chloroethanol in the presence of Karstedt catalyst (platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex, solution in xylene) to give the corresponding 25,26,27,28-tetrakis[4-(tris(alkoxydimethylsilyl)methyl)butoxy]calix[4]arene. It is found that alcoholysis of calix[4]arene 1 in the presence of Speier catalyst (H 2 PtCl 6 .6H 2 O) was unsuccessful using reflux conditions over seven days. In addition, the rate of alcoholysis is dependent on the amounts of the catalyst and reaction temperature.


Introduction
Calixarenes 1 have been widely studied as hosts and potential hosts for molecular recognition.Calixarenes containing organosilicon groups are of potential interest for molecular recognition of anions and nucleophilic substances, [2][3][4] as many silicon compounds can interact with nucleophiles to form hypervalent silicon adducts. 5[8][9][10][11] Among the diverse reactions at silicon centers, those involving siliconoxygen bond formation are particularly important and silyl ethers are among the most widely used protecting groups for hydroxyl functions in organic synthesis, 12 and also for the preparation of "prodrugs" for drug delivery systems. 13They also play an important role in inorganic synthesis as precursors in the preparation of sol-gels and other condensed siloxane materials. 14The dehydrocoupling reaction between hydrosilane and an alcohol is a typical well-known method for the preparation of Si-O bonds using transition-metal catalysts. 15We have recently developed a convenient procedure for the preparation of a series of tris(alkoxydimethylsilyl)methanes by the reaction of (HMe2Si)3CH with monofunctional alcohols in the presence of chloroplatinic acid (H2PtCl6.6H2O)as catalyst. 16Since alkoxysilane-calixarenes represent a new class of calixarenes, our interest in the preparation of calix [4]arenes bearing the tris(alkoxydimethylsilyl)methyl group was greatly enhanced.Herein we report the use of the dehydrocoupling reaction for the preparation of calixarenes containing silyl ether groups via reaction between a calixarene bearing Si-H group and various alcohols in the presence of Karstedt catalyst.
We have recently reported the preparation of tris(alkoxydimethylsilyl)methanes via the reaction of tris(dimethylsilyl)methane and various alcohols in the presence of the Speier catalyst (H2PtCl6.6H2O)under mild and aerobic conditions 16 (Scheme 2).Since we were interested in extending this methodology to the coupling of the calixarene bearing Si-H groups such as 2 with various alcohols, it was decided to study the dehydrocoupling between compound 2 and some alcohols in different conditions (Scheme 3).Scheme 2. Preparation of tris(alkoxydimethylsilyl)methanes.Scheme 3. Preparation of calix [4]arene bearing tris(alkoxydimethylsilyl)methane groups.
Alcoholysis of calixarene 2 with primary alcohols (methanol, ethanol, propanol and butanol) failed to go to completion using the Speier catalyst under reflux conditions over seven days.These reactions were therefore repeated using various amounts of the catalyst ([Pt]/[Si-H]=0.055to [Pt]/[Si-H]=0.39).However these conditions also failed to achieve a complete reaction.
The Karstedt catalyst (platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex, solution in xylene) which is more active than the Speier catalyst (H2PtCl6.6H2O)was then examined.Reaction conditions were optimized by using propanol as a typical alcohol and the Karstedt catalyst at 80 °C.As shown in Figure 1, increasing the loading of the Karstedt catalyst to a certain extent can affect the reaction time.By increasing the amount of Pt catalyst from -3 , we were able to reduce the reaction time from 36h to 8h.The colorless reaction mixture gradually turned to homogeneous black-colored liquid, indicating the generation of colloidal Pt(0) particles.
The method was found to be applicable also to secondary alcohols and leads to selective formation of the corresponding calixarenes bearing alkoxysiloxanes in good yields (Table 1).As shown in Table 1 reaction of calixarene 2 with primary alcohols gave higher yields than the analogous reactions with secondary alcohols, probably due to the increase in steric hinderance.The reaction progress was monitored by FTIR spectroscopy on the basis of absorption measurements at the Si-H stretching bond frequency (2107 cm -1 ) with reference to the standard curve.The FTIR spectrum of the propoxysilane-bearing calixarene 5 does not show a sharp peak at 2107 cm -1 indicating the absence of a Si-H bond, and shows the concomitant appearance of the Si-O peak at 1084 cm -1 (Figure 2).In addition, the 1 H NMR spectrum of (5) for example, shows the presence of 36 protons (CH3CH2CH2O) at 1.1 ppm and 32 protons (12×CH3CH2CH2O, 4×CCH2CH2CH2CH2O) at 3.5-3.8ppm (Figure 3).Similar results were observed for other alcohols.

Conclusions
A series of new calixarenes bearing tris(alkoxydimethylsilyl)methyl groups were obtained in good yields from the catalytic reaction of calixarene 2 with several alcohols.Using the Speier catalyst, alcoholysis was unsuccessful under reflux conditions over seven days.However, in the presence of the Karstedt catalyst, the desired reaction went to completion in only 8h at 80°C to afford the desired products in high yields.Primary alcohols gave higher yields than the secondary alcohols, which might be the result of the increase in steric hindrance.Furthermore, the newly-obtained calixarenes containing alkoxysilanes at the lower rim are novel calixarenes which open up a new field in the chemistry of calixarenes.

Experimental Section
General.The reactions involving organolithium reagents were carried out under dry argon.Solvents were dried by standard methods.Substrates for preparation of (HSiMe2)3CLi, viz.HSiMe2Cl, Mg, CHBr3, n-BuLi, lithium diisopropyl amide (LDA) and the substrate for preparation of 25,26,27,28-tetrakis[4-bromobutoxy]calix [4]arene, viz.p-tert-butylphenol, formaldehyde 35-40%, NaH, DMF, 1,4-dibromobutane, and all alcohols used in the alcoholysis reactions were purchased from Merck and all alcohols were purified by standard methods.Karstedt catalyst was purchased from Aldrich.The 1 H NMR and 13 C NMR were recorded with a Bruker FT-400MHz spectrometer at room temperature and using CDCl3 as the solvent.The FTIR spectra were recorded on a Bruker-Tensor 270 spectrometer.Elemental analyses were carried out with a Heareus CHN-ORAPID instrument.

Figure 1 .
Figure 1.The effect of the amount of the Karstedt catalyst on the reaction time.

Figure 2 .
Figure 2. The FTIR spectra of the compounds 2 and 5.