Contributors PENELOPE J. BROTHERS (223), Department of Chemistry. The University of Auckland, Auckland, New Zealand BARRETT EICHLER ( I ), Department ofChemistry, University ofcalifornia-Davis. Davis, California 956 I5 MARK G. HUMPHREY (47) Department of Chemistry, Australian National University. Canberra, ACT 0200 Australia LALEH JAFARPOUR (ISI), Department of Chemistry. University of New Orleans, New Orleans, Louisiana 70 148-2920 IL NAM JUNG ( 145). Organosilicon Chemistry Laboratory. Korea Institute of Science and Technology, Seoul 130-650, Korea NIGEL T. LUCAS (47), Department of Chemistry. Australian National University. Canberra, ACT 0200 Australia STEVEN P. NOLAN (181) Department of Chemistry, University of New Orleans. New Orleans. Louisiana 70148-2920 SUSAN M. WATERMAN (47). Department of Chemistry, Australian National University. Canberra ACT 0200 Australia ROBERT WEST (I), Department of Chemistry, University of Wisconsin. Madison, Wisconsin 53706-1396 BOK RYUL YOO ( l45), Organosilicon Chemistry Laboratory. Korea Institute of Science and Technology. Seoul 130-650, Korea vii ADVANCES IN ORGANOMETALLIC CHEMISTRY, VOL. 46 Chemistry of G roup 14 Heteroallenes BARRETT EICHLER Department of Chemfsfry University of California-Davis Davis, California 95616 ROBERT WEST Department of Chemistry University of Wisconsin Madison, Wisconsin 53706 I. II. III. IV. V. Introduction Bonding Theory Synthesis and Reactions A. Transient I-Silaallenea B. Transient I-Silaketcnes. C. Stable Heteroallenes Physical Properties A. X-ray Structure Determination. B. NMR Spectroscopy Future Prospects References. ,........ .............. .............. .............. .............. .............. I 2 5 5 14 I/ 34 34 40 43 43 I INTRODUCTION An allene (or 1,2-propadiene) is a moiety with two cumulated double bonds between three atoms (R2C=C=CR2). The central atom of the allene is therefore sp-hybridized. The r-bond of one double bond is orthogonal to the other and this unusual n-bonding arrangement can lead to unique electronic effects (Fig. I). This also results in steric properties at the ends of the allene by forcing the substituents to also be orthogonal to each other. This review will focus on allenes which have at least one carbon atom replaced by a heavier group I4 atom, commonly referred to as a heteroallene. Group I4 heteroallenes have appeared in the literature over the last 20 years, and stable examples of this moiety have been synthesized since 1992. Heteroallenes that do not have a group 14 heteroatom will not be discussed, although it is useful to consider phosphaallenes, which have been reviewed by Regitz in 1990.’ To date, heteroallenes with the heteroatom at the end of the allene, the one position, have been easier to synthesize because of their thermodynamic stability compared to those with the heteroatom as the middle atom, the two position. 1 All rights Copyright c’ 2001 by Academic Press. of reproduction in any form reserved. 006%3055/01 $35.00 2 BARRETT EICHLER AND ROBERT WEST II BONDING THEORY Heteroallene structures can be regarded as depending on the extent ofcontribution from each of two bonding arrangements (Fig. 2). Bonding Model A depicts both the heteroatom and the carbon atom in the triplet state, forming one a-bond and one r-bond to make a formal double bond. Lappert et 01.‘~’first postulated bonding Model B, and illustrated bonding between the heteroatom and the carbon atom in singlet states that “may be described as a ‘double’ n-donor-acceptor interaction.” according to Griitzmacher et ~l.‘~ Differences between these two models Model \. / A M. 1 ./ .(‘=c‘