Unusual Pathways for Metal-Assisted C−C and C−P Coupling Reactions Using Allenylidenerhodium Complexes as Precursors

The rhodium allenylidenes trans-[RhCl{CCC(Ph)R}(PiPr3)2] [R = Ph (1), p-Tol (2)] react
with NaC5H5 to give the half-sandwich type complexes [(η5-C5H5)Rh{CCC(Ph)R}(PiPr3)] (3, 4). The
reaction of 1 with the Grignard reagent CH2CHMgBr affords the η3-pentatrienyl compound [Rh(η3-CH2CHCCCPh2)(PiPr3)2] (6), which in the presence of CO rearranges to the η1-pentatrienyl derivative
trans-[Rh{η1-C(CHCH2)CCPh2}(CO)(PiPr3)2] (7). Treatment of 7 with acetic acid generates the
vinylallene CH2CH−CHCCPh2 (8). Compounds 1 and 2 react with HCl to give the five-coordinate
allenylrhodium(III) complexes [RhCl2{CHCC(Ph)R}(PiPr3)2] (10, 11). An unusual [C3 + C2 + P] coupling
process takes place upon treatment of 1 with terminal alkynes HC⋮CR‘, leading to the formation of the
η3-allylic compounds [RhCl{η3-anti-CH(PiPr3)C(R‘)CCCPh2}(PiPr3)] [R‘ = Ph (12), p-Tol (13), SiMe3
(14)]. From 12 and RMgBr the corresponding phenyl and vinyl rhodium(I) derivatives 15 and 16 have been
obtained. The previously unknown unsaturated ylide iPr3PCHC(Ph)CCCPh2 (17) was generated from
12 and CO. A [C3 + P] coupling process occurs on treatment of the rhodium allenylidenes 1, 2, and trans-[RhCl{CCC(p-Anis)2}(PiPr3)2] (20) with either Cl2 or PhICl2, affording the ylide−rhodium(III) complexes
[RhCl3{C(PiPr3)CC(R)R‘}(PiPr3)] (21−23). The butatrienerhodium(I) compounds trans-[RhCl{η2-H2CCCC(R)R‘}(PiPr3)2] (28−31) were prepared from 1, 20, and trans-[RhCl{CCC(Ph)R}(PiPr3)2] [R
= CF3 (26), tBu (27)] and diazomethane; with the exception of 30 (R = CF3, R‘ = Ph), they thermally
rearrange to the isomers trans-[RhCl{η2-H2CCCC(R)R‘}(PiPr3)2] (32, 33, and syn/anti-34). The new
1,1-disubstituted butatriene H2CCCC(tBu)Ph (35) was generated either from 31 or 34 and CO. The
iodo derivatives trans-[RhI(η2-H2CCCCR2)(PiPr3)2] [R = Ph (38), p-Anis (39)] were obtained by an
unusual route from 1 or 20 and CH3I in the presence of KI. While the hydrogenation of 1 and 26 leads to
the allenerhodium(I) complexes trans-[RhCl{η2-H2CCC(Ph)R}(PiPr3)2] (40, 41), the thermolysis of 1
and 20 produces the rhodium(I) hexapentaenes trans-[RhCl(η2-R2CCCCCCR2)(PiPr3)2] (44, 45)
via C−C coupling. The molecular structures of 3, 7, 12, 21, and 28 have been determined by X-ray
crystallography. (Abbreviations used:  p-Tol = p-tolyl, 4-C6H4CH3; p-Anis = p-anisyl, 4-C6H4OCH3.)