NEWS & VIEWS RESEARCH INORGANIC CHEMISTRY (109.7°), perhaps implying that their silylene is more reactive than Rekken and colleagues’ compound. Both silylenes exhibit striking thermal Two-armed silicon stability for divalent silicon compounds: Rekken and colleagues’ compound is stable up Compounds of transition metals are often used to activate small molecules for to 146 oC, whereas Protchenko and colleagues’ chemical reactions. The discovery of unusual silicon-containing compounds compound is stable up to 130 oC. The stability raises the prospect of metal-free activators. of the new silylenes must be due largely to steric protection of the reactive silicon atoms by the attached groups (in other words, the bulky ROBERT WEST from a starting material that was structurally groups shield the silicon atoms from attack identical to the silylene product, but in which by other molecules). The bulky groups might ny chemist will tell you that silicon two bromine atoms were also attached to the also stabilize the compounds by donating atoms are tetravalent — that is, they silicon atom. They removed the two bromine electrons to the silicon atom. have four electrons available for atoms using a magnesium-containing reduc- Further insight into the chemical bonding Achemical bonding, and therefore usually form ing agent. in the two silylenes was obtained by looking at four single bonds to other atoms. But two In the second paper2, Protchenko and col- the signals (the chemical shifts) of silicon-29 papers1,2 published in the Journal of the Am­­ leagues’ silylene (Fig. 1b) has a more complex nuclei in the nuclear magnetic resonance eri can Chemical Society report some remark- structure than Rekken and colleagues’ com- spectra for the molecules. In both cases, the able silicon-containing compounds known as pound (for example, it is not symmetrical, chemical shifts have strongly positive val- silylenes, in which the silicon atom forms only and the silicon atom sits between a nitrogen ues (285.5 parts per million for the silicon in two single bonds and so is said to be di valent. atom and a boron atom), and it required a Rekken and colleagues’ silylene, and They are the first stable silylenes that are more complicated synthesis. Protchenko 439.7 p.p.m. for that in Protchenko and col- acyclic (the silicon does not form part of a ring et al. also started from a brominated material, leagues’ compound). These values fall between of atoms), and therefore open up a new chapter but they reacted it with a boron-containing the corresponding chemical shift (78 p.p.m.) in the chemistry of divalent silicon. reagent12 that not only reduces the silicon– measured in a classic silylene4 that is stabilized Among the elements in group 14 of the bromine bonds in the starting material, but by electron donation from nitrogen atoms on periodic table (from carbon to lead, all of also attaches itself to the silicon atom to yield either side of the silicon, and that of another which have four electrons available for bind- the final product. previously reported silylene13 that is stabilized ing), carbon and silicon are the most reluc- Both Rekken et al. and Protchenko and col- only by steric hindrance (567 p.p.m.). The tant to form divalent compounds. The first leagues obtained X-ray crystal structures of chemical shifts for the new silylenes provide stable divalent carbon compounds, known their silylenes to provide conclusive proof of further clear evidence that the compounds are as carbenes, were discovered3 in 1991, and what they had made, and also to learn more indeed divalent silicon compounds. Moreover, the synthesis of stable divalent silylenes fol- about the atomic bonding in the compounds the shifts suggest that Rekken and colleagues’ lowed a few years later4–6. These first, highly — which in turn can help to explain the com- silylene is partially stabilized by electron dona- reactive silylenes are now regarded as classic pounds’ reactivities and stabilities. Rekken tion from the sulphur atoms adjacent to the compounds, and have become the starting et al. found that the sulphur–silicon–sulphur silicon atom, and that Protchenko and col- materials for an elaborate branch of silicon bond angle in their silylene is 90.52°. The leagues’ silylene may also be partially stabilized chemistry7–9. angle at the silicon atom in Protchenko and by electron donation from the atoms on either The classic silylenes are cyclic molecules colleagues’ compound is somewhat larger side of the silicon, but to a lesser extent. in which two nitrogen atoms are The interest in silylenes lies in attached to the silicon. These their reactivity — particularly the a nitrogens are believed to stabi- Me Me Me Me possibility that they can ‘activate’ lize the divalent silicon atom, in small molecules, such as hydro- Ar Si Ar Ar = part by donating electrons to a S S gen, to allow such molecules to vacant orbital on the silicon. The Me Me take part in potentially useful cyclic structure also seems to give chemical reactions. Rekken et al. the molecules stability. Acyclic b report that their silylene does not Dipp silylenes that have two nitrogen react with hydrogen, although it Me Si atoms bonded to divalent silicon 3 N Me Me does combine with another small have been detected, but these Dipp Si molecule (iodomethane; CH3I) in Dipp = Me Me compounds are not stable at room N B a typical silylene reaction. Protch- temperature10,11. The synthesis of enko et al., however, find that their stable acyclic silylenes is there- N Dipp compound captures hydrogen fore a crucial advance because it readily to yield a dihydride prod- greatly expands the scope of diva- uct that has two silicon–hydrogen lent silicon chemistry. bonds. This is the first example of The compound now reported Figure 1 | Acyclic silylenes. Two teams report the synthesis of the first the reaction of hydrogen with a by Rekken et al.1 has a remarkably thermally stable compounds in which a divalent silicon atom — one that forms silylene. What’s more, the reaction two single bonds — does not form part of a ring of atoms. a, In Rekken and simple structure consisting of a 1 is a remarkable single-site activa- colleagues’ silylene , a silicon atom is flanked by two sulphur atoms. The dots silicon atom flanked by two iden- on the silicon atom represent a lone pair of electrons, and the bond crossed by tion process — one in which both tical, bulky arylsulphur groups a wavy line in the Ar group indicates the point of attachment of the group to hydrogen atoms become attached (aromatic rings connected to the the silylene. Me, methyl group. b, In Protchenko and colleagues’ silylene2, the to the same atom. Most other acti- silicon atom by sulphur atoms; silicon atom is between a nitrogen and a boron atom. Divalent silicon atoms vation processes involve two sites. Fig. 1a). The authors prepared it in a and b are shown in red. Further study of the chemical 3 MAY 2012 | VOL 485 | NATURE | 49 44-53 News and Views.indd 49 30/04/2012 11:18 RESEARCH NEWS & VIEWS reactions of these acyclic silylenes will no 1. Rekken, B. D., Brown, T. M., Fettinger, J. C., Commun. 1931–1932 (1995). doubt lead to the synthesis of a variety of inter- Tuononen, H. M. & Power, P. P. J. Am. Chem. 7. Hill, N. J. & West, R. J. Organomet. Chem. 689, 4165–4183 (2004). esting chemical compounds. And now that Soc. http://dx.doi.org/10.1021/ja301091v (2012). 8. Asay, M., Jones, C. & Driess, M. Chem. Rev. 111, Rekken et al. and Protchenko et al. have shown 2. Protchenko, A. V. et al. J. Am. Chem. Soc. http:// 354–396 (2011). the way, we should see a growing family of dx.doi.org/10.1021/ja301042u (2012). 9. Gehrhus, B. & Lappert, M. F. J. Organomet. Chem. 617–618, 209–223 (2001). 3. Arduengo, A. J. III, Harlow, R. L. & Kline, M. J. Am. acyclic, di valent silylenes with as yet unknown 10. Tsutsui, S., Sakamoto, K. & Kira, M. J. Am. Chem. ■ Chem. Soc. 113, 361–363 (1991). structures and properties. Soc. 120, 9955–9956 (1998). 4. Denk, M. et al. J. Am. Chem. Soc. 116, 2691–2692 11. Lee, G.-H., West, R. & Müller, T. J. Am. Chem. Soc. Robert West is in the Department of (1994). 125, 8114–8115 (2003). 5. West, R. & Denk, M. Pure Appl. Chem. 68, 785–788 12. Segawa, Y., Yamashita, M. & Nozaki, K. Science 314, Chemistry, University of Wisconsin, Madison, (1996). 113–115 (2006). Wisconsin 53706-1396, USA. 6. Gehrhus, B., Lappert, M. F., Heinicke, J., 13. Kira, M., Ishida, S., Iwamoto, T. & Kabuto, C. J. Am. e-mail: [email protected] Boese, R. & Bläser, D. J. Chem. Soc. Chem. Chem. Soc. 121, 9722–9723 (1999). CANCER BIOLOGY is controlled by mTOR. However, these previ- ous studies were performed using rapamycin, which only partially inhibits 4E-BP activity6. Importantly, an earlier study10 found that The director’s cut overexpression of 4E in cells was not sufficient to drive the translation of 5ʹTOP-mRNAs and A genome-wide characterization of active translation of messenger RNA that the level of translation of 5ʹTOP-mRNAs following inhibition of mTOR will transform our view of this signalling protein’s versus other mRNAs showed similar sensitiv- regulatory role in cancer. See Article p.55 & Letter p.109 ity to a 4E-inhibitory molecule. These findings suggested that the selective effects of 5ʹTOP- mRNA translation are independent of 4E and ANTONIO GENTILELLA & GEORGE THOMAS The two new studies2,3 broaden this view of possibly mediated by a regulatory factor that mTOR activity.