C-H insertions complexes

Carmona also saw ethylene activation with the tris-(3,5-dimethylpyra-zolyl)borate-bis-ethyleneiridium(I) complex, but there are two main differences from Crabtree s results. First, reaction occurs thermally in solution at 60°C. Second, the C-H insertion complex undergoes further C-C coupling to give an allyl hydride product (Eq. 38) [128]. 

A coordinatively unsaturated rhodium(I) complex generated from 10 reacts with biphenylene to give C-H inserted complex 11 as the kinetic product. Complex 11 is then thermally converted to the C-C inserted complex 12 [27]. This re-... 

Tetraphenylmolybdenocene dihydride Mo(r 5-C5HPh4)CpH2 (45) was formed by addition of diphenylacetylene to MoCpL(PhC CPh)CH3 (L = P(OMe)3) (Eq. 15), presumably via an ot-hydrogen abstraction to an intermediate methylidene hydrido complex, followed by addition of two equivalents of diphenylacetylene and C — H insertion with concomitant elimination of L [57 b],... 

The only other reaction with an aromatic substance is the C-H insertion into ferrocene [85], giving 41,which illustrates the highly electrophilic character of the phosphinidene complex. Other aromatic C-H insertions have been observed, but these likely occur by means of intermediate P,0- and P,N-ylids,such as the reaction of (0C)5W=PR withbenzophenone and azobenzene that give 42 and 43,respectively [56a, 86]. 

Interestingly, [Ee(F20-TPP)C(Ph)CO2Et] and [Fe(p2o-TPP)CPh2] can react with cyclohexene, THF, and cumene, leading to C-H insertion products (Table 3) [22]. The carbenoid insertion reactions were found to occur at allylic C-H bond of cyclohexene, benzylic C-H bond of cumene, and ot C-H bond of THF. This is the first example of isolated iron carbene complex to undergo intermolecular carbenoid insertion to saturated C-H bonds. 

With the iron complex [Fe(Cl3terpy)2]( 104)2 (Clsterpy = 4,4, 4"-trichloro-2,2 6, 2"-terpyridine) as catalyst, sulfamate esters react with Phl(OAc)2 to generate iminoiodanes in situ which subsequently undergo intramolecular nitrenoid C-H insertion to give amidation products in good yields (Scheme 30) [48]. 

Fig. 29. Possible mechanism for C—C activation involving formation of a 7r-complex followed by direct sp2-sp3 C-C insertion. This mechanism is ruled out based on the much larger potential energy barrier for C—C insertion relative to C—H insertion from the 7T-complex.

Allylic C/H insertion accompanied by an allylic rearrangement has been observed for carbenoid reactions of ethyl diazoacetate with allylamines (Scheme 23)1S1). Apparently, metal-catalyzed isomerization 117 118 proceeds the C/H insertion process. Although mechanistic details have not yet been unraveled, T)3-allyl complexes... 

Cyclometallation of thiophene imine 110 with platinum complex 111 proceeded to give metallacycle 112 via an intramolecular C-H insertion on the thiophene ring . 

Although concerted C=C and C-H insertion into benzene by methylene is possible, several experiments on the effect of various solvents on the product ratio suggest the intermediacy of a complex between methylene and benzene.67-69 Based on the obtained kinetic and thermodynamic data, the transient is believed to be a complex formed between singlet methylene and benzene. 

In recent years, the related C-H insertion chemistry of nitrenes has gained considerable momentum.36 Effective chiral catalysts have been developed as well as new methods for generation of the nitrene precursors. Even more impressive has been the application of this chemistry to the synthesis of complex natural products. The scope of this chemistry is described in Section 10.04.4. 

The reactivity between a tertiary C-H site and a sterically accessible, secondary C-H site is relatively even in the reactions catalyzed by TpBf3Cu. This can be seen in the reaction with 2-methylpentane (Equation (7)),38,49,56 which gave rise to a mixture of only two products. No insertion into the methyl or the sterically crowded methylene C-H bonds was seen. The C-H insertion has the possibility of selectively functionalizing relatively complex alkanes. An impressive example is the C-H insertion to 1 (Equation (8)).56 A mixture of two alkylation products derived from insertion at the tertiary C-H bonds was obtained. This transformation has been extended to the selective functionalization of hydrocarbon polymers.75... 

Activation of a C-H bond requires a metallocarbenoid of suitable reactivity and electrophilicity.105-115 Most of the early literature on metal-catalyzed carbenoid reactions used copper complexes as the catalysts.46,116 Several chiral complexes with Ce-symmetric ligands have been explored for selective C-H insertion in the last decade.117-127 However, only a few isolated cases have been reported of impressive asymmetric induction in copper-catalyzed C-H insertion reactions.118,124 The scope of carbenoid-induced C-H insertion expanded greatly with the introduction of dirhodium complexes as catalysts. Building on initial findings from achiral catalysts, four types of chiral rhodium(n) complexes have been developed for enantioselective catalysis in C-H activation reactions. They are rhodium(n) carboxylates, rhodium(n) carboxamidates, rhodium(n) phosphates, and < // < -metallated arylphosphine rhodium(n) complexes. 

A very impressive example of the synthetic utility of this chemistry is the one-pot enantioselective double G-H activation reaction of 86 to generate chiral spiran 87 (Equation (73)).172 In this case, the phthalimide catalyst Rh2(enantiotopically selective aromatic C-H insertions of diazo ketoesters (Equation (74)).216 Moreover, dirhodium(n) tetrakisIA-tetrafluorophthaloyl- )-/ /-leucinate], Rh2(hydrogen atoms of the parent dirhodium(n) complex are substituted by fluorine atoms, dramatically enhances the reactivity and enantioselectivity (up to 97% ee). Catalysis... 

As shown in the previous two sections, rhodium(n) dimers are superior catalysts for metal carbene C-H insertion reactions. For nitrene C-H insertion reactions, many catalysts found to be effective for carbene transfer are also effective for these reactions. Particularly, Rh2(OAc)4 has demonstrated great effectiveness in the inter- and intramolecular nitrene C-H insertions. The exploration of enantioselective C-H amination using chiral rhodium catalysts has been reported by several groups.225,244,253-255 Hashimoto s dirhodium tetrakis[A-tetrachlorophthaloyl-(A)-/ r/-leuci-nate], Rh2(derived rhodium complex, Rh2(i -BNP)4 48,244 afforded moderate enantiomeric excess for amidation of benzylic C-H bonds with NsN=IPh. 

An asymmetric C-H insertion using a chiral 3,3, 5,5 -tetrabromosubstituted (salen)manganese(m) complex 107 with TsN=IPh afforded insertion products with ee up to 89%.258 Che reported the first amidation of steroids such as cholesteryl acetate with (salen)ruthenium(n) complexes 108.259... 

The rhodium acetate complex catalyzed the intramolecular C-H insertion of (/ )-diazo-fR)-(phenylsulfonyl)acet-amides 359 derived from (f )-amino acids to afford in high yield the 6-benzenesulfonyl-3,3-dimethyl-7-phenyl-tetrahydro-pyrrolo[l,2-c]oxazol-5-one 360 (Equation 63) <2002JOC6582, 2005TL143>. 

Although the asymmetric C-H insertion reactions catalyzed by Cu complexes are far less common than their Rh counterparts, a few interesting examples have been published. Each of these cases relies on an intramolecular insertion initiated by a diazocarbonyl functionality. 

In an approach to planar chiral ferrocenes, Siegel and Schmalz (85) report that bis(oxazoline)-copper complexes induce efficient aromatic C-H insertion from a... 

Other five-membered germanium heterocycles were prepared by the Lewis acid-catalyzed C-H insertion of a germylene (Equation (15)),26 thermal dissociation of a germaketenethioacetal and trapping of the resulting germy-lene 11 (Equation (16)),27 and from the zircona-bicyclic complexes 12 and 13 (Scheme 2 and Equation (17)).28... 

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