Carbon-fluorine bond activation coupled with carbon-carbon bond formation at iridium. Confirmation of complete kinetic diastereoselectivity at the new carbon stereocenter by intramolecular trapping using vinyl as the migrating group.
The iridium(perfluoropropyl)(vinyl) complex CpIr(PMe(3))(n-C(3)F(7))(CH=CH(2)) (5) has been prepared. It has been characterized by X-ray crystallography, and its ground state conformation in solution has been determined by (19)F{(1)H} HOESY NMR studies. It reacts with the weak acid lutidinium iodide to afford the eta(1)-allylic complex CpIr(PMe(3))((Z)-CH(2)CH=CFC(2)F(5))I (6), which has also been characterized crystallographically. The mechanism of C-F bond activation and C-C bond formation leading to 6 has been elucidated in detail by studying the reaction of 5 with lutidinium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate [LutH(+)B(ArF)(4)(-)], containing a weakly coordinating counteranion. The main kinetic product of this reaction, determined by (19)F{(1)H} HOESY studies at -50 degrees C, is the endo-CpIr(PMe(3))(anti-eta(3)-CH(2)CHCFCF(2)CF(3))[B(ArF)(4)] diastereomer 9, along with a small amount of the exo-syn-isomer 8. Isomer 9 rearranges at -20 degrees C to its exo-anti isomer 7, and subsequently to the thermodynamically favored exo-syn-isomer 8, which has been isolated and crystallographically characterized. Complex 8 reacts with iodide to afford complex6. On the basis of the unambiguously defined kinetically controlled stereochemistry of 9 and 8, a detailed mechanism for the C-F activation/C-C coupling reaction is proposed, the principal conclusion of which is that C-F activation is completely diastereoselective.
Hughes
Russell P
RP
Department of Chemistry, 6128 Burke Laboratory, Dartmouth College, Hanover, New Hampshire 03755, USA. rph@dartmouth.edu
Laritchev
Roman B
RB
Zakharov
Lev N
LN
Rheingold
Arnold L
AL