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%% This is a (brief) example using the beilstein class.
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%%\listfiles

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%% The document class does not have many options yet. It accepts an
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%% manuscript=review,
%% manuscript=bookreport.
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%% For the input encoding:
%% latin1 (default),
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\documentclass[utf8]{beilstein}

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%% Place any additional packages needed here. Only include packages
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%% loads some useful packages, so please have a look at the
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\usepackage{xspace}

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\newcommand*{\CHCL}{\chem{CH_2Cl_2}}
\newcommand*{\HNMR}{\chem{^{1}H~NMR}\xspace}
\newcommand*{\CNMR}{\chem{^{13}C~NMR}\xspace}

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%% Beginning of the article
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\begin{document}

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%% The title of the article is given with the usual \title command.
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%% Each author should be given as a separate \author command.
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%% For corresponding authors please use \author* and give the email
%% address as a second mandatory argument.
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%% The affiliation of authors is given after the authors; the
%% affiliations are numbered consecutively.
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\title{Synthesis of highly substituted allenylsilanes by alkylidenation of silylketenes}
\author*{Stephen P. Marsden}{s.p.marsden@leeds.ac.uk}
\affiliation{School of Chemistry, University of Leeds, Leeds LS2 9JT, United Kingdom}
\author{Pascal C. Ducept}
\affiliation{Department of Chemistry, Imperial College London, London SW7 2AY, United Kingdom}
\maketitle

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\begin{abstract}
\background Allenylsilanes are useful intermediates in organic synthesis. An attractive,
convergent but little used approach for their synthesis is the alkylidenation of stable silylketenes. Reactions thus far have been limited to the use of unsubstituted silylketenes (or equivalents) with stabilised or semi-stabilised ylides only. The current study explores the reactions of substituted ketenes prepared through rhodium(II)-mediated rearrangement of silylated diazoketones.
\results A range of novel 1,3-disubstituted and 1,3,3-trisubstituted allenylsilanes were prepared using stabilised and semi-stabilised ylides. Alkylidenation with non-stabilised phosphorus ylides was not viable, but the use of titanium-based methylenating reagents was successful, allowing access to 1-substituted allenylsilanes.
\conclusion Many novel allenylsilanes may be accessed by alkylidenation of substituted silylketenes. Importantly, for the first time, simple methylenation of silylketenes has been achieved using titanium carbenoid-based reagents.
\end{abstract}

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\keywords{allenylsilanes; rhodium(II) octanoate-mediated rearrangement; silylketenes; titanium carbenoids; ylide}

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%% It is strongly recommended to use BibTeX for managing references.
%% Citations and citation lists can be given with the \cite command.
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\section{Introduction}
Allenylsilanes are versatile intermediates for organic synthesis \cite{Pornet2002,Masse1995}. They have two main modes of reactivity: firstly, as propargyl anion equivalents in thermal \cite{Jian1995,Weinreb1998} or Lewis acid-mediated \cite{Danheiser1980,Danheiser1986} addition to carbonyls, acetals and imines, and secondly as three-carbon partners in [3+2] annulation reactions. Thus, reaction with aldehydes \cite{Danheiser1985}, imines/iminiums \cite{Danheiser1985,Daidouji2005}, enones \cite{Danheiser1981,Danheiser1983,Danheiser1985b} and nitrosyl cations \cite{Danheiser1987} leads to dihydrofurans, dihydropyrroles, cyclopentenes and isoxazoles respectively \cite{Yadav2004}. In most cases the silicon is retained in the final product and can be used as a handle for further synthetic elaboration.

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Amongst the myriad methods to prepare allenylsilanes \cite{Pornet2002,Danheiser1987b}, an attractive disconnection is to consider a Wittig-type alkylidenation of a silylketene (\cref{fig:AlkylidenationApproach}).
\begin{figure}
\caption{Alkylidenation approach to the synthesis of allenylsilanes.}
\label{fig:AlkylidenationApproach}
\includegraphics[width=8.2cm,keepaspectratio]{figure1}
%% \sglcolfigure{figure1}
\end{figure}

[\ldots]

\section{Results and Discussion}
Our investigations began with the preparation of substituted silylketenes \CN{1} as substrates for the alkylidenation chemistry. This was carried out under our previously reported conditions for rhodium(II) octanoate-mediated rearrangement of silyl diazoketones \CN{2}, which in turn were prepared by \textit{C}-silylation of the parent diazoketones \CN{3} with triethylsilyl triflate (\cref{scheme:1}). It should be noted that while the alkyl-substituted silylketenes are relatively stable and show little decomposition at room temperature over several days, the (hetero)aromatic-substituted silylketenes are much less robust and should be used quickly or stored in a freezer.

\begin{scheme}
\caption{Synthesis of substituted silylketenes \CN{1}.}
\label{scheme:1}
\includegraphics[width=8.2cm,keepaspectratio]{scheme1}
\end{scheme}

[\ldots]

With the requisite silylketenes in hand, attention turned to their reaction with the carboethoxy-stabilised phosphoranes \CN{4} and \CN{5}. At the outset, it was by no means certain that these would react efficiently with substituted silylketenes \CN{1} since it is well documented that nucleophiles attack silylketenes \textit{anti} to the silicon, i.e.\ the phosphoranes would be approaching from the same side as the \chem{R^1}-substituent. Since in all previous examples this substituent has been a hydrogen atom, the extension to bulkier substituents could not be taken for granted. In the event, however, we were pleased to find that in nearly all cases the desired allenylsilanes were formed in moderate to excellent yield (\cref{scheme:2}, \cref{tab:1}, see \cref{si:1} for full experimental data).
\begin{scheme}
\includegraphics[width=8.2cm,keepaspectratio]{scheme2}
\caption{Reaction of substituted silylketenes with ester-stabilised phosphoranes.}
\label{scheme:2}
\end{scheme}
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\begin{table}
\caption{Reaction of substituted silylketenes with ester-stabilised phosphoranes.}
\label{tab:1}
\begin{dblcoltabularx}{|l|l|l|l|l|X|X|}\hline
\bfseries Entry & \bfseries Ketene & \bfseries Ylide & \bfseries Temp [\celsius] & \bfseries \textit{t} [h] & \bfseries Solvent & \bfseries Yield 6/7 (8)\\\hline
1 & \CN{1a} & \CN{4} & 80 & 24 & PhH & 54\,\%\\\hline
2 & \CN{1a} & \CN{5} & rt & 3  & \CHCL & 60\,\%\\\hline
3 & \CN{1b} & \CN{4} & 110 & 24 & toluene & 45\,\%\\\hline
4 & \CN{1b} & \CN{5} & reflux & 24 & \CHCL & 77\,\%\\\hline
5 & \CN{1c} & \CN{4} & 80 & 24 & PhH & 60\,\%\\\hline
6 & \CN{1c} & \CN{5} & rt & 6  & \CHCL & 81\,\%\\\hline
7 & \CN{1d} & \CN{4} & 110 & 48 & toluene & 22\,\%%
   \footnote{60\,\% of starting material recovered}\\\hline
8 & \CN{1d} & \CN{5} & 80 & 48 & toluene & 78\,\%\\\hline
9 & \CN{1e} & \CN{4} & 80 & 24 & PhH & 55\,\% (7\,\%)\\\hline
10 & \CN{1f} & \CN{4} & 60 & 5 & \CHCL & 44\,\% (3\,\%)\\\hline
11 & \CN{1h} & \CN{4} & rt & 6 & \CHCL & 0\,\% (57\,\%)\\\hline
12 & \CN{1h} & \CN{4} & 50 & 1 & \CHCL & 7\,\% (23\,\%)\\\hline
13 & \CN{1i} & \CN{4} & rt & 10 & \CHCL & 0\,\% (67\,\%)\\\hline
14 & \CN{1i} & \CN{5} & rt & 2 & \CHCL & 98\,\%\\\hline
15 & \CN{1j} & \CN{4} & 80 & 12 & PhH & 74\,\% (19\,\%)\\\hline
\end{dblcoltabularx}
\end{table}

As expected, reactions with the more substituted ylide \CN{4} were significantly slower than those with the parent ylide \CN{5} (compare reaction temperatures and times, entries 1, 3 and 5 versus entries 2, 4 and 6). [...]

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%% The Supporting Information is an essential part of many articles.
%% They are given inside the ``suppinfo'' environment with the
%% \sifile command which gets the following mandatory arguments:
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\begin{suppinfo}
Supporting information features copies of \HNMR spectra of silylated diazoketones \CN{2} and silylketenes \CN{1}, plus \chem{{}^1H} and \CNMR spectra of allenylsilanes \CN{6}, \CN{7}, and \CN{14}--\CN{19}.
\sifile{S1.pdf}{PDF}{Experimental part}\label{si:1}
\sifile{S2.pdf}{PDF}{NMR spectra of compounds \CN{1}, \CN{2}, \CN{6} and \CN{7}}
\sifile{S3.pdf}{PDF}{NMR spectra of compounds \CN{14--19}}
\end{suppinfo}

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\begin{acknowledgements}
We acknowledge EPSRC grant GR/L60135/01 (PCD) and generous unrestricted research funding from Pfizer for financial support.
\end{acknowledgements}

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\vspace{3cm}
This article is published in full length in \textit{Beilstein J. Org. Chem.}
\textbf{2005}, \textit{1}, No. 5.

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