Three Type f arbene omplexes arbene complexes have formal metal-to-carbon double bonds. Several types are known. The reactivity of the carbene and how it contributes to the overall electron counting is dependent on the subtituents and metal involved. ucleophilic Fischer carbenes X 1 L n M 2 Electrophilic X =,, S M = low-valent, middle or late transition metals L-type ligand donating 2 electrons Electrophilic Schrock carbenes L n M 1 2 ucleophilic 1, 2 = alkyl or M = high-valent carbyl or middle transition metals 2X-type ligand ( 2 charge) donating 4 electrons arbenoids L n M 1 EWG L n M = h 2 ( 2 ) 4, 4 u, ( 2 2 )u, or (,)u L-type ligand donating 2 electrons Vinylidenes L n M Electrophilic artwig, rganotransition tal hemistry, 2010, pp 481 504. Semmelhack, rganometallics in Synthesis, Schlosser, Ed., 2002, pp 1024 1041. 1 2 1, 2 = alkyl, aryl, or L-type ligand donating 2 electrons -eterocyclic carbenes L n M = alkyl or aryl generally a spectator ligand L-type ligand donating 2 electrons
ucleophilic L n M X 1 2 Electrophilic X =,, S M = low-valent, middle or late transition metals Fischer arbenes Most developed carbenes are those of r, Mo, and W. Usually synthesized from commerically available and stable M() 6. Usually crystalline solids, easily purified by recrystallizaton or silica gel chromatography. Air stable as solids, slight sensitive in solution. The stability is due to the heteroatom. Dialkyl complexes decompose at low temps. The metal is d 6 and zero valent, coordinateively saturated. To react at the metal, one of the carbonyls must be removed with high temps or photolysis. () 5 M heat or hν ( ) () 4 M L () 4 M M(0), d 6, 18e, sat. M(0), d 6, 16e, unsat. L M The " Wall" can destablize some complexes with α-branching on the group.
Preparation Preparation of the Fischer carbene usually proceeds via an anionic acyl "ate" complex. Alkyl lithium addition: probably most commonly used method () 5 r Li () 5 r 4 Br () 5 r 4 3 BF 4 () 5 r ("ate" complex) charge can be delocalized into all remaining carbonyl groups (ammonium salt) stable solid stable solid eductive routes: r() 6 K- 8 or aaph a 2 r() 5 l () 5 r 3 BF 4 () 5 r 2 () 5 r 3 Sil () 5 r 2 2
Preparation Vinylidene and allenylidiene intermediates: eutral conditions () 6 r hν TF () 5 r () 5 r(thf) () 5 r (an allenylidene) (a vinylidene) () 5 r () 5 r
Addition f ucleophiles The carbonyl groups are strongly electron withdrawing. This makes the M bond electrophilic. eaction mechanisms are similar to the reactions of esters. δ uc () 5 M δ + δ uc () 5 M δ + + Li () 5 W () 5 W S () 5 W () 5 W S 2 Bn () 5 W Bu
Insertions and arbometallations With carbon-based nucleophiles or hydrides, the alkoxide can be slow to eliminate. The metal-based anion can then undergo other reactions. arbonyl migrations and carbometallation of alkenes/alkynes are common. ow facile these reactions are is quite dependent on the group. () 5 M 1 () 5 M 1 L () 4 M L L =, P 3 1 L () 5 M 1 + 1 Low pressures of or the presence of phosphines promotes the insertion. Eur. J. rg. hem. 2004, 2471 2502.
Fischer arbene "Enolates" The α-protons of Fischer carbenes are quite acidic (pk a ~12). Anionic bases needed for irreversible deprotonation. Weak bases (Pyr, DMAP, Et 3 ) can be used, but lead to formation of enol ether. () 5 M 3 () 5 M 3 () 5 M Anionic bases required for irreversible deprotonation. The "enolates" of alkoxycarbenes are only weakly nucleophilic, but can react with electrophiles in the presence of a Lewis acid. () 5 r Et 1. BuLi, Et 2 78 º 2. BF 3 Et 2 () 5 r J. Am. hem. Soc. 1985, 107, 503. () 5 r 1. BuLi, TF 78 º 2. Til 4 () 5 r rganometallics 1991, 10, 807.
Fischer arbene "Enolates" The "enolates" of aminocarbenes are only more nucleophilic (compare ester enolates to amide enolates), and do not require Lewis acids to react. () 5 r 1. BuLi, TF 78 º 2. () 5 r (S) (S) Tf 51% yield 95% ee () 5 r 2 1. BuLi, TF 78 º 2. () 5 r 2 DMS, 60 º, 71% or DMD, 78% 2 2 1. LDA, TF 78 º 2. 2 + 2 > 95:5, 88% J. Am. hem. Soc. 1993, 115, 4602.
ycloadditions α,β-unsaturated Fischer carbenes undergo cycloaddition reactions but are much more reactive than the corresponding ester. () 5 M Et 3, TMSl () 5 M Li M() 6 () 5 M Et () 5 M [4+2] [2+2] TMS [3+2] Et () 5 M TMS
onjugate Additions α,β-unsaturated Fischer carbenes can also serve as Michael acceptors. Et Li Et () 5 r () 5 r anion "protects" carbene from further reactions with nucleophiles Li pyr () 5 r () 5 r Et 98% yield 93% yield J. Am. hem. Soc. 1992, 114, 2985.
emoving the tal There are sevral methods available for removing the metal and converting the carbene into a different functional group. 2 Pyridine 2 2 X A, DMD 3, or DMS () 5 M X Tf, or TFA Bu 3 Sn pyr/hexane, 70º Bu 3 Sn
yclopropanation Fischer carbenes will react with electrophilic a olefins to form cyclopropanes. The yields can vary, but generally work well. () 5 M 1 2 Δ 1 2 2 = 2, 2, P() 2, S 2, olefin eaction is suppressed by pressure. This points to formation of a metallocyclobutane intermediate. () 5 M 1 2 () 4 M 1 [2+2].E. 2 (a metallocyclobutane) 1 2 In order to cyclopropanate electron-rich olefins, acyloxycarbenes must be used. Likely involves a different (polar) mechanism. 1 + () 4 M 3 3 2 = () 5 M 2 1 3 2 = Ac 1 Ac 3
yclopropanation eutral alkenes are usually poor substrates, but can react in an intramolecular sense. More complex alcohols can be introduced using acyloxycarbenes (triflates would be difficult to handle/prepare). () 5 r 4 1. l 2. () 5 M Δ 88% yield () 5 r 4 l 2 l 2, 10 º to rt 53% yield eactions with alkynes are more facile than with alkenes, but gives an α,β-unsaturated carbene. This can go on and do other chemistry. () 5 M 1 2 M() 4 2 () 4 M 1 2 (a metallocyclobutene) 1 (a vinylogous Fischer carbene)
Dötz eaction eaction of alkenyl and aryl alkoxycarbenes with alkynes produces highly substituted benzene rings or quinones, depending on work-up conditions. () 5 r L Δ, S () 4 r () 4 r L L S S insertion S insertion () 4 r L air hν r() 4 r() 4 L L L S get quinone with A S S r-vinyl ketene The intermediates in this process can be intercepted by other functional groups before the final ring closure, leading to cascade processes and complex products.
r-bound Ketenes The formation of r-bound ketenes can be used to explain a number of synthetic transformations. The ease of formation appears to be dependent on the substitution around the carbene and can be promoted by photochemical means (in the visible). The process of inserting is quite reversible and, unless trapped, will deinsert and return to the carbene. () 4 r X hν () 4 r X X () 4 r The ketene is generated in low concentrations and is metal-bound. This prevents many of the side reactions commonly encountered with ketenes. The reactivity pattern still mimics that of normal ketenes. 1 X X 1 2 2 2 Znl 2 2 X 1 2 () 4 r X 2 X 1 X 1 2 1