[54962-98-0] · C5H7Li · Pentadienyllithium · (MW 74.06)
(pentadienylating agent in substitutions and additions; used to attach pentadienyl groups to other metals and nonmetals)
Physical Data: orange crystals. Solution NMR studies and trapping experiments indicate a predominant planar W shape; pentadienylpotassium prefers a U shape under most conditions and is used when a U shape is desired.2
Solubility: usually prepared and used without isolation, in hydrocarbon/ether mixtures.
Preparative Methods: by metalating 1,4-pentadiene with BuLi/THF3 or Li/amines,4 or the more readily available 1,3-Pentadiene with Li/amines (best for large scale),4 or by cleaving alkyl 2,4-pentadienyl ethers with alkyllithiums.5
Form Supplied in: prepared in situ and used directly.
Handling, Storage, and Precautions: must be prepared and transferred under inert gas to exclude oxygen and moisture.
Alkylations give mixtures of products from reaction at the central and end carbons.1 Borylation,2a,b silylation,2c -e,6 and related reactions6 occur almost exclusively at an end carbon to provide (E) derivatives (eq 1; cf. eq 22c). The TMS derivatives thus produced are more selective in their additions to aldehydes and ketones than are the pentadienylmetals.7 Additions of pentadienyllithium to aldehydes5 and ketones8 were found to be reversible, with the product on the left (eq 3) reverting to the thermodynamic product on the right under more vigorous conditions. Pentadienyllithium has been used to make open versions of the well-known cyclopentadienyl transition metal complexes.1,9
Table 1 lists some useful pentadienyllithium derivatives.1,10-14 See also Allyllithium, Allylmagnesium Bromide, and Crotyllithium.
Robert B. Bates & Sriyani Caldera
University of Arizona, Tucson, AZ, USA