A Route to Carbasugar Analogues - Jonathan Clayden - The ...

Chapter 1: Introduction
1.4 Nucleophilic Dearomatisation of Metal Complexes
High levels of diastereoselectivity have been achieved for a range of nucleophiles and
substrates in the dearomatising addition to transition metal-complexed arenes. Most
widely applied are the chemistries of Cr(CO) 3 and Mn(CO) + 3 arene complexes, which
are discussed below.
1.4.1 (η 6 -Arene)Cr(CO) 3
The chemistry of arene-tricarbonyl chromium complexes is extensive, allowing a
broad range of functionalisations to be performed prior to dearomatisation (Scheme
1.21). These are possible due to the electron-withdrawing properties of the Cr(CO) 3
R=F, Cl, OR
Nu
R
complex. 49 Cr(CO) 3 Cr(CO) 3
NuLi
R
75 76
Nu
E +
[O]
R
77
E
Nu
Crystal Structure for
Nu = dithiane, R = H
R
Nu
6
Scheme 1.21 – general reactivity of (η -arene)Cr(CO) 3 complexes
A range of carbon nucleophiles may attack the chromium-complexed arene 75 from
the face exo to the bulky metal centre, yielding the η 5 -cyclohexadienyl 76 (Scheme
1.21) which has been characterised by crystal structure for the addition of dithiane to
benzene. 50
Addition to the arene complex is relatively facile and labile. This has the advantage of
high levels of selectivity – the thermodynamic product is formed rapidly – but it also
means that regioselectivity is highly substrate-dependent. Furthermore, the addition of
an electrophile other than a proton will often react with the dissociated nucleophile. In
light of this, many studies have been conducted with dithiane as nucleophile since it
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