El encuentro del grupo

developed in the Ritter research group, including the addition of α-olefins, hydroboration, hydrosilylation, and polymerization. All of these transformations are selective for 1,4-addition and a defining feature is catalyst-controlled regioselectivity that can be modified by variation of the iminopyridine ligand of the catalyst. Recent work on the polymerization of isoprene led to interest in the 1,2-addition mode, which has been pursued in the work described in Chapter 2.

Addition of α-olefins to conjugated dienes was the first reaction of 1,3-dienes reported by the Ritter group. In this transformation, an iron (II) precatalyst was reduced in situ to form a low-valent iron catalyst in the presence of substrates (Eq. 35). Olefination of dienes proceeded with >99:1 regioselectivity for olefin E/Z configuration and selectivity for 1,4-addition was as high as >98:2 for some substrates.172

(Eq. 35)

1,4-Selective hydroboration of 1,3-dienes was subsequently developed using a similar catalyst system. Variations of the substituents on the iminopyridine ligand gave either 1,4-addition (Eq. 36) or 4,1-addition (Eq. 37) with >99:1 selectivity in both examples. Selectivity may be controlled by steric hindrance between substituents on the diene substrate and the ligand. Although basic functional groups arrested the reaction, ester, acetal, and ether groups were tolerated.173

172 _{Moreau, B. T.; Wu, J. Y.; Ritter, T. Org. Lett.2008, 11, 337.}

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(Eq. 36)

(Eq. 37)

Hydrosilylation of 1,3-dienes was developed soon after, as discussed above on page 58. In this report, Wu et al. demonstrate not only catalyst-controlled selectivity in the 1,4-hydrosilylation of 1,3- dienes, but also utilize a well-defined low-valent iron catalyst. With this catalyst, Wu et al. were able to investigate the mechanism of 1,4-hydrosilylation.2m

Most recently, a 1,4-selective polymerization of isoprene was developed by the Ritter research group using a similar iminopyridine iron catalyst.174 _{Selectivity for 1,4-insertion to form either the cis-} (Eq. 38) or trans-olefin (Eq. 39) in the growing polymer chain was controlled by variation of the ligand on the iron catalyst. Selectivity was also observed for 3,4-insertion with some ligands (Eq. 40). Because 3,4-insertion of isoprene into a growing polymer chain represents the addition of C–C bonds in a 1,2- selective fashion, we were inspired to investigate similar catalysts and, ultimately, new strategies for achieving 1,2-addition to conjugated dienes.

(Eq. 38)

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(Eq. 39)

(Eq. 40)

1.3.5.

1,2-Selective Additions to Conjugated Dienes

Of the many transition metal-catalyzed addition reactions of 1,3-dienes, very few demonstrate selectivity for 1,2-addition. Most yield primarily 1,4-addition products, likely due to the thermodynamic favorability of π-allyl metal complexes over σ-alkyl isomers.153 _{Examples of 1,2-selective addition} reactions include transition metal-catalyzed hydroboration, diboration, hydrogenation, and hydrosilylation, but only three examples show generality for 1,2-selective addition; in all other reported examples, selectivity for 1,2-addition is isolated to a single substrate.

Transition metal catalysts may be more likely to generate 1,4-addition products of 1,3-dienes because π-allyl metal complexes are thermodynamically preferred over σ-alkyl complexes for many transition metals.153 _{Because reductive elimination is turnover limiting, intermediates D and D’ (Scheme} 14) should reach an equilibrium that reflects the relative stability of the two metal complexes. The concentration of intermediates D and D’, which immediately precede reductive elimination, are likely to influence the product ratio unless the reaction operates under Curtin-Hammet conditions.175 _{Because most} transition metal catalysts thermodynamically prefer a π-allyl (intermediate D’) over a σ-alkyl ligand

175 _{For seminal work on Curtin-Hammet kinetics, see: (a) Curtin, D. Y. Rec. Chem. Prog.1954, 15, 111.} For a review, see: (b) Seeman, J. I. Chem. Rev.1983, 83, 83.

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(intermediate D), the relative proportion of D’ in solution should be much greater, which can explain the selectivity for 1,4-addition at most transition metal catalysts.

1,4-Hydrosilylation predominates in addition reactions catalyzed by palladium and rhodium catalysts, while the few examples of 1,2-hydrosilylation are catalyzed by platinum complexes. The only example of 1,2-selective hydrosilylation that produces a single product utilizes Pt(PPh3)4 and adds Cl2MeSiH to 1,3-pentadiene to generate 3-pentenylsilane in in 86% yield (Eq. 41).176

(Eq. 41)

Limited 1,2-selective addition of trichlorosilane to butadiene has been described by Rericha and Capka using the same platinum-phosphine catalyst (Eq. 42). This example is notable as higher selectivity for 1,2- addition is observed than in other reported reactions of butadiene, but the low overall yield (5%) after 5 hours at 95 °C renders this method impractical for preparation of 3-butenylsilanes.

(Eq. 42)

Other catalysts can also produce mixtures in which the 1,2-hydrosilylation product is the major component, but isomers are also generated in significant quantities. A similar precatalyst to that from Eq. 41 and Eq. 42, which is likely to generate the same active species in situ, forms both the 1,2-mono- and the dihydrosilylation products of isoprene, but the product of 1,4-hydrosilylation is not observed (Eq. 43).2g

(Eq. 43)

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In the case of Speier’s catalyst, hydrosilylation of myrcene with trichlorosilane is partially selective for the 1,2-addition product, but also produces over-silylated products (Eq. 44).177 _{In contrast, other silanes} and dienes show 1,4-selectivity with Speier’s catalyst, indicating that regioselectivity is substrate- dependent.

(Eq. 44)

The most general 1,2-selective addition reported to date is the diboration of conjugated dienes by an olefin-supported platinum(0) catalyst, originally reported in 1997 by Ishiyama et al. as the 1,2- diboration of pentadiene.178 _{In this report, while Pt(dba)}

2 catalyzed the 1,2-addition of bis- pinacolatodiboron to pentadiene (Eq. 45), the platinum phosphine precatalyst Pt(PPh3)4 generated exclusively 1,4-addition products (Eq. 46).

(Eq. 45)

(Eq. 46)

An enantioselective variation was later developed by Kliman et al. that produced enantioenriched 1,2- addition products of a variety of diene substrates with pinicolatodiboron (Eq. 47), which can be further elaborated through reactions of the C–B bonds to form diols, alcohols, and other synthetically useful building blocks.179

177 _{Nasiak, L. D.; Post, H. W. J. Organomet. Chem.1970, 23, 91.}

178 _{Ishiyama, T.; Yamamoto, M.; Miyaura, N. Chem. Commun.1997, 689.}

179 _{Kliman, L. T.; Mlynarski, S. N.; Ferris, G. E.; Morken, J. P. Angew. Chem. Int. Ed. Engl. 2012, 51,} 521.

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(Eq. 47)

A single example of 1,2-selective hydroboration has been reported using a nickel-phosphine catalyst and catecholborane (Eq. 48). Although no mechanistic proposal is provided, this hydroboration is also generally selective for 1,2-addition across butadiene, isoprene, myrcene, and trans-pentadiene.180

(Eq. 48)

1,2-Selective partial hydrogenation of dienes has also been reported using a platinum catalyst. Bertani et al. synthesized an allylplatinum hydride precatalyst that was stable below −20 °C (Eq. 49). When warmed, the allylplatinum hydride complex reductively eliminates propene, which suggests that LPt0_{(diene) is the active form of the catalyst. Dienes such as butadiene, isoprene, and 2,3-} dimethylbutadiene were partially hydrogenated to the terminal olefin products under these conditions. Under these conditions, no butane was detected and isolation of other butene isomers was not reported.181

(Eq. 49)

180 _{Zaidlewicz, M.; Meller, J. Tetrahedron Lett.1997, 38, 7279.}

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