Chapter 6
Catalysts for Silane and Silanol Activation J. B. Baruah
Downloaded by COLUMBIA UNIV on August 3, 2012 | http://pubs.acs.org Publication Date: August 2, 2007 | doi: 10.1021/bk-2007-0964.ch006
Department of Chemistry, Indian Institute of Technology, Guwahati 781039, India (email:
[email protected])
Dehydrogenative coupling reactions of various silanes with alcohols, reductive coupling reactions of silanes with carbonyl compounds as well as dehydration of silanols are studied for silicon oxygen bond formation reactions under catalytic conditions. The dehydrogenative coupling reactions are catalysed by amines and metal catalysts derived from copper(II), gold(III), silver(I) palladium(II), rhodium(I), platinum(II) complexes, whereas reductive coupling reactions between quinone and different silanes are done by rhodium(I) and palladium(II) catalysts. Under near neutral condition dehydration of silanols can be caused by amines such as triethylenetetramine.
Introduction Silicon-oxygen bond containing inorganic compounds constitutes a major portion of the Earth's crust. Thus, transformations leading to silicon oxygen bonds by reagents under ambient condition are of great value. Such transformations have close relevance towards the goal to convert Si0 to useful organic or inorganic materials. For achieving such a goal, it is a prime necessity to understand the reactivity of the silicon oxygen bond starting from its formation to its cleavage. The process of developing new reagents should also complement green chemical paths. We have been interested in the generation of new Si-0 bond forming reactions through dehydrogenative coupling , reductive coupling of carbonyl compounds with silanes as illustrated in scheme 1. The dehydrogenative reaction would leave hydrogen as the side product, whereas in the case of substitution of Si-OH group by an alcohol would lead to water as the side product. The reductive coupling reaction will have excellent atom economy. We have studied these reactions that are catalysed by various transition metal catalysts and the important observations that provide advantages of using a particular reagent are presented here. 1
2
2
3
4
5
© 2007 American Chemical Society
In Science and Technology of Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.
69
70 catalyst Si^ / *H
+ROH *"
/
/^""Λ ν ν
+
OR
Being a gas it can be trapped and can be used as fuel y 9
o r , a b o r a t o r
^
rea
ent
catalyst
> ς
+
^
x
S
/ 'vi
R
0
H
Ο * /
— -
Κ
catalyst
\
- f à ) χ
*"
=>
Harmless b i p r o d u c t
^
^'"oH^
> Excellent atom economy
ill
Downloaded by COLUMBIA UNIV on August 3, 2012 | http://pubs.acs.org Publication Date: August 2, 2007 | doi: 10.1021/bk-2007-0964.ch006
Scheme 1
Experimental The synthetic aspects of most of the work presented here have been published elsewhere. 5
Platinum catalysed Si-O bond forming reaction In a typical experiment l,3-diphenyl-2-propene-l-one (2.1g, lmmol) was dissolved in 10ml of toluene in a Schlenck tube. To this K PtCl (0.004g, O.Olmmol) was added make a homogeneous solution. The triethylsilane (0.47g, 3mmol) was added by a micro syringe to this solution. The reaction was continued for 6hrs at 80°C by constant stirring. The reaction was monitored by TLC from time to time and after 6hrs the stirring and heating was stopped and filtered. From the filtrate the solvent was removed under reduced pressure. The crude product obtained was further purified by column chromatography [silica gel with hexane/ethylacetate (5%)]. The products were further characterized by comparing their proton nmr, IR with authentic samples. The ratio of the isomers in each case was evaluated from the proton nmr spectra. 2
4
Results and Discussion Palladium catalysed Si-Η bond activation 6
Palladium complex catalysed silicon oxygen bond formation is well known. Palladium complexes having phosphine ligands are generally used for such reactions but are expensive and toxic. Thus, we felt that the use of nitrogen containing ligands might reduce toxicity as well as ease the manipulation of air sensitive reagents. We found that the tetraethylethylenediamine (TEEDA), tetramethylethylenediamine (TMEDA) complexes of PdCl are good catalysts for dehydrogenative coupling reaction between silane and alcohol. Varieties of silanes were reacted with different phenolic and alcoholic compounds to obtain the corresponding silyl ethers in excellent yield . In order to know the structure of the catalyst we have determined the structure of one of the catalyst Pd(TEEDA)Cl and the structure is shown in Figure 1. The structure is a distorted square planar structure. Some of the important bond distances are Nl-Pd, N2-Pd, Cll-Pd, C12Pd, 2.104, 2.078, 2.301 and 2.318A respectively. Similarly the
