A trimethylsilyl group (abbreviated TMS) is a functional group in organic chemistry. This group consists of three methyl groups bonded to a silicon atom [−Si(CH3)3], which is in turn bonded to the rest of a molecule. This structural group is characterized by chemical inertness and a large molecular volume which makes them useful in a number of applications.
A trimethylsilyl group bonded to a methyl group forms tetramethylsilane, which is abbreviated as TMS as well.
Compounds with trimethylsilyl groups are not normally found in nature. Chemists sometimes use a trimethylsilylating reagent to derivatize rather non-volatile compounds such as certain alcohols, phenols, or carboxylic acids by substituting a trimethylsilyl group for a hydrogen in the hydroxyl groups on the compounds. This way trimethylsiloxy groups are formed on the molecule. Trimethylsilyl groups on a molecule have a tendency to make it more volatile, often making the compounds more amenable to analysis by gas chromatography or mass spectrometry. Such derivatizations are often done on a small scale in special vials.
In an NMR spectrum, signals from atoms in trimethylsilyl groups in compounds will commonly have chemical shifts close to the tetramethylsilane reference peak at 0 ppm. Also compounds, such as high temperature silicone "stopcock" grease, which have polysiloxanes (often called silicones) in them will commonly show peaks from their methyl groups (attached to the silicon atoms) having NMR chemical shifts close to the tetramethylsilane standard peak, such as at 0.07 ppm in CDCl3.
Otherwise very reactive molecules can be isolated when enveloped by bulky trimethylsilyl groups. This effect can be observed in tetrahedranes.
Super silyl groups
Related to trimethylsiliyl groups are so-called super silyl groups of which there exist two varieties: A silicon group connected to three trimethylsilyl groups makes a tri(trimethylsilyl)silyl group (TTMSS or TMS3Si) and a silicon group connected to three tert-butyl groups. The TTMSS group was proposed in 1993 by Hans Bock. With a van der Waals volume of up to 7 cubic angstrom it surpasses the related TIPS group (around 2)   and one potential application is its use as a temporary substituent promoting asymmetric induction for example in this diastereoselective one-pot reaction involving two sequential Mukaiyama aldol reactions :
- Gottlieb, H. E.; Kotlyar, V.; Nudelman, A. NMR Chemical Shifts of Common Laboratory Solvents as Trace Impurities. J. Org. Chem. 1997, 62(21), pp 7512-7515. doi:10.1021/jo971176v
- "Super Silyl" Group for Diastereoselective Sequential Reactions: Access to Complex Chiral Architecture in One Pot Matthew B. Boxer and Hisashi Yamamoto J. Am. Chem. Soc.; 2007; 129(10) pp 2762 - 2763; (Communication) doi:10.1021/ja0693542
- Tris(trimethylsilyl)silyl-Governed Aldehyde Cross-Aldol Cascade Reaction Boxer, M. B.; Yamamoto, H. J. Am. Chem. Soc.; (Communication); 2006; 128(1); 48-49. doi:10.1021/ja054725k
- The starting materials are acetaldehyde and benzophenone which are both converted to silyl enol ether by reaction with tris(trimethylsilyl)silane and triflic acid with evolution of hydrogen. The aldol reaction is catalyzed by bis(trifluoromethane)sulfonimide