Allyl and related ligands

TT-Allyl and related complexes can be prepared by reactions such as 24.93-24.97 the last two reactions illustrate formation of allyl ligands by deprotonation of coordinated propene, and protonation of coordinated buta-1,3-diene respectively. Reactions 24.94 and 24.95 are examples of pathways that go via cr-bonded intermediates (e.g. 24.58) which eliminate CO. 

James and J.M. Tour (2005) Topics in Current Chemistry, vol. 257, p. 33 - Molecular wires . 

Packheiser and B. Walfort (2006) Organometal-lics, vol. 25, p. 1836 - Organometallic 7r-tweezers, NCN pincers, and ferrocenes as molecular Tinkertoys in the S5mthesis of multiheterometallic transition-metal complexes . 

Long and C.K. Williams (2003) Angewandte Chemie International Edition, vol. 42, p. 2586 - Metal alkynyl t complexes synthesis and materials . 

Paul and C. Lapinte (1998) Coordination Chemistry Reviews, vol. 178-180, p. 431 - Organometallic molecular wires and other nanoscale-sized devices. An approach using the organoiron (dppe)Cp Fe building block (dppe = PhjPCHjCHjPPhj). 

Photolysis of Fe(CO)5 with buta-1,3-diene gives complex 

Iron tricarbonyl complexes of 1,3-dienes (e.g. cyclohexa-1,3-diene) play an important role in organic synthesis the complexes are stable under a variety of reaction conditions, and iron carbonyls are inexpensive. The Fe(CO)3 group acts as a protecting group for the diene functionality (e.g. against additions to the C=C bonds), allowing reactions to be carried out on other parts of the organic molecule as illustrated by reaction 23.87. 

The presence of the Fe(CO)3 group also permits reactions with nucleophiles to be carried out at the diene functionality with control of the stereochemistry, the nucleophile being able to attack only on the side of the coordinated diene remote from the metal centre. The organic ligand can be removed in the final step of the reaction.  

In Fig. 24.8, we showed the three tt-MOs that the tr-allyl ligand uses in IxMiding to a metal centre. In 

Although commercial applications are stiU future goals, much progress has been made in the design of potential molecular wires. Molecules so far studied have included organic molecules with conjugated alkyne functionalities, porphyrins connected by alkyne units, chains of connected thiophenes, and organometalhc complexes. Examples of the latter are shown below  

Robertson and C.A. McGowan (2003) Chem. Soc. Rev., vol. 32, p. 96 - A comparison of potential molecular wires as components for molecular electronics . 

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Actinide n-Complexes with Allyl, Cyclopolyene, Arene, and Related Ligands... 

The cyclization of ort/zo-allyl phenols was reported by Murahashi in the late 1970s. The reaction of the 2-(2-cyclohexenyl)phenol (Equation 16.110) was one of the early examples of Wacker-type reactions with alcohol nucleophiles and has been re-investigated in more recent years with chiral catalysts. Intramolecular reactions of alkene-ols and alkenoic acids form cyclic ethers and lactones. These reactions were reported by Larock and by Annby, Andersson, and co-workers, and examples are shown in Equations 16.111 and 16.112. °° ° The use of DMSO as solvent was important to form the lactone products. More recently, reactions with alcohols were reported by Stoltz to form cyclic ethers by the use of pyridine and related ligands in toluene solvent. - The type of ligand, whether an additive or the solvent, is crucial to the development of these oxidative processes. However, the features of these ligands that lead to catalysis are not well understood at this time. 

Fig. 24.8). Allyl can also be considered as [C3H5] (see later). Similar schemes can be developed for cyclobutadiene (ri -C4H4), cyclopentadienyl (t -C5H5, see Box 23.1), benzene (t -CgH5) and related ligands as we discuss later in the chapter. 

While the notion that the alkoxides derived from aliphatic alcohols are poor nucleophiles toward 7r-allylmetal complexes has prevailed over the years, much progress made in the recent past has rendered the transition metal-catalyzed allylic alkylation a powerful method for the O-allylation of aliphatic alcohols. In particular, owing to the facility of five- and six-membered ring formation, this process has found extensive utility in the synthesis of tetrahydrofurans (THFs) (Equation (29))150-156 and tetrahydropyrans (THPs).157-159 Of note was the simultaneous formation of two THP rings with high diastereoselectivity via a Pd-catalyzed double allylic etherification using 35 in a bidirectional synthetic approach to halichondrin B (Equation (30)).157 The related ligand 36 was used in the enantioselective cyclization of a Baylis-Hillman adduct with a primary alcohol (Equation (31)).159... 

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