Preparation and Characterization of Molybdenum Tris(pyrazolyl)borate Dicarbonyl η³-Dienyl Complexes

Abstract

Treatment of allenic electrophiles with Mo-(CO) 3 (py) 3 followed by potassium hydrotris(1-pyrazolyl)-borate (KTp) or treatment of an enyne with TpMo(CO) 3 H followed by decarbonylation cleanly produced stable, new η 3 -dienyl complexes.We have previously described many examples of S N 2′ reactions of cobaloxime and Co(salen) anions with allenic electrophiles to yield cobalt substituted 1,3-dienes, 1 but in our experience, S N 2 reactions of transition-metal anions with allenic electrophiles have been rare. [2][3][4] In 1997, we began a detailed investigation of reactions of molybdenum cyclopentadienyl carbonyl anions (1; Scheme 1) with allenic electrophiles (2). 2b,c These reactions produced complexed cyclobutenones (3) as the major products, however, in several cases we also isolated small amounts of η 3 -dienyl complexes (4 and 5).In contrast to η 3 -allyl complexes, η 3 -butadienyl complexes are rare and their reaction chemistry is largely unexplored. 5 Platinum η 3 -butadienyl complexes have been prepared via oxidative addition of allenic chlorides 6 and via insertion into methylenecyclopropanes. 7 Bruce and co-workers prepared a number of dicyano-bis-(trifluoromethyl)-substituted η 3 -butadienyl complexes via 2 + 2 cycloaddition of metal acetylides with dicyanobis(trifluoromethyl)ethylene followed by ring opening and alkene complexation. 8 The reaction of metal carbonyl anions with 1,4-dichloro-2-butyne, as well as with diynes, has been used to prepare several carbonylated η 3 -butadienyl complexes. 9 Green and co-workers reported the synthesis of Me 5 CpMo(CO) 2 (η 3 -butadiene) via deprotonation of a cationic η 4 -diene complex. 4 In principle, η 3 -dienyl complexes should participate in many of the same kinds of reaction chemistry which are known for η 3 -allyl complexes. 5 However, very few reactivity studies have been done on any of the η 3 -butadienyl complexes that have been reported to date, and this group can be expected to participate in interesting addition reactions and cycloaddition chemistry. 4,6,10 Here, we report two different routes to, and preliminary reaction chemistry of, TpMo(CO) 2 (η 3 -dienyl) complexes (Tp ) hydrotris(1-pyrazolyl)borate),] including X-ray crystallographic characterization of one of these complexes and one of the precursors to this family of complexes.Liebeskind 11 and Pearson 12 have both recently shown that TpMo(CO) 2 (η 3 -allyl) complexes offer some practical advantages for organic chemists over the analogous CpMo(CO) 2 complexes. They are easy to prepare from inexpensive starting materials (Mo(CO) 6 and potassium hydrotris(1-pyrazolyl)borate (KTp); KTp can be synthesized in one step on a multimole scale from KBH 4 and pyrazole 13 ), they are very stable to air at 25°C, and their preparation does not require the preparation and handling of air-sensitive metal carbonyl anions. We have † Wake Forest University. ‡ Guilford College. § To whom inquiries regarding X-ray crystallography should be addressed.* Corresponding author.(1) For reviews of our work in this are...