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Bridging rhodacycles related to 200 have been proposed as intermediates in the rearrangement of
strained cycloalkanes by Halpern 135 and Gassman (Scheme 3.46). 136 For example, treatment of
bicyclo[2.2.0]hexane 204a with catalytic amount of [Rh2(nbd)2Cl2] leads to exclusive formation
of cyclohexene (207a). This observation is readily accommodated by a mechanism involving
oxidative insertion of Rh(I) into the strained C-C bond, to form a bridging metallocycle 205
which undergoes a β-hydride elimination and reductive elimination to give cyclohexene. The
same reaction of tetradeuteriated bicyclo[2.2.0]hexane 204b results in 3,4,5,6-
tetradeuteriocyclohexene 207b in support of this mechanism. Moreover, when 204a is treated
with 25 mol% [Rh(CO)2Cl]2, it affords an isolable acyl-rhodacycle 208 in 88% yield.
Interestingly, treatment of this compound with PPh3 initiates a sequence of β-hydride elimination
and reductive elimination steps to give aldehyde 210. These results clearly indicate that bridging
rhodacycle 200 is a viable intermediate in the cycloisomerization of ene-allenes as outlined in
Path A (Scheme 3.45).
Scheme 3.46 Bridged Rh(III)-metallocycles via oxidative addition of Rh(I) into strained bonds.
X
X
X
X
204a, X = H
204b, X = D
5 mol% [Rh 2(nbd) 2Cl 2]
benzene, 50 o C, 4h
65%
nbd = norbornadiene
25 mol% [Rh(CO) 2Cl] 2
CCl 4, 40 o C, 10h
88% (based on Rh)
X
X
Cl CO
Rh
O
H H
RhLn X
X
205 206
PPh 3
208 209
RhLn X
X
RhLn H O -[RhCl(CO)(PPh3) 2]
H
X
X
X
X
207a, X = H
207b, X = D
The alternative mechanistic path B involves an oxidative addition of Rh(I) into the allyl
C-H bond of intermediate 199 leading to a π-allyl rhodium hydride species 202 (Scheme 3.45).
Insertions of this type are common for many transition metals (e.g., Rh, Ru, Ir) and often result
in isomerization of the allylic amide to an enamide. 137 Next, endo-carbocyclization of this
76
X
X
H
O
210