Benzylic C-H borylation

Benzylic C-H borylation giving benzylboronates was first reported by Marder [58]. The reaction of HBpin in toluene in the presence of RhCl P(i-Pr)j 2(N2) (1 mol%) at 140 °C gives (borylmethyl)benzene and bis(boryl)methyl benzene in 69% and 7% yields respectively, along with several products arising from aromatic C-H borylation (ca. 15%). 

Aromatic C-H borylation catalyzed by Ir-bpy complexes was carried out as a joint project in collaboration with Professor Hartwig s group. We are greatly indebted to Professor John F. Hartwig and his co-workers for their contribution to the elucidation of the mechanism and for helpful discussions with him. 

Borane Reagents, Academic Press, London, 1988. (b) D. S. Matteson, Stereodirected Synthesis with Organoboranes, Springer Berlin, 1995. 

3 Rh-catalyzed 1,4-addition to enones (a) N. Miyaura, in ACS Symposium Series 783 Organoboranes for Syntheses, P. V. Ra-machandran, H. C. Brown (Eds), American Chemical Society, Washington DC, 2001, Chapter 7. (b) K. Fagnou, M. Lautens, 

5 Petasis reaction (a) N. A. Petasis, I. A. Zavialov, in Advances in Boron Chemistry, The 

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The reaction of HBpin in toluene in the presence of RhCl P(/-Pr)3 2(N2) (1 mol%) at 140 °C resulted in a mixture of (borylmethyl)benzene (69%) and bis(boryl)methyl benzene (7%), along with several products arising from aromatic C-H borylation (ca. 15%).345 Rhodium-bpy complexes catalyzed the borylation at the benzylic C-H bond.351 Pd/C was found to be a unique catalyst for selective benzylic C-H borylation of alkylbenzenes by B2pin2 or HBpin (Equation (70)).360 Toluene, xylenes, and mesitylene were all viable substrates however, the reaction can be strongly retarded by the presence of heteroatom functionalities such as MeO and F. Ethylbenzene resulted in a 3 1 mixture of pinacol 1-phenylethylboron and 2-phenylethylboron derivatives. 

Marder s group also reported the first example of benzylic C-H borylation, using a Rh catalyst. Later, Pd/C was reported to catalyze borylation of toluene, xylenes, and mesitylene at the... 

Functional group tolerance of the borylation is quite high. The reaction occurs selectively at the C-H bond for 1 bearing MeO, Me, I, Br, Cl, F3C, Me02C, and NC groups (Scheme 2) [4—10], In particular, it is interesting to note that aromatic C-H bonds are selectively borylated even in the presence of weaker benzylic C-H bonds or C-halogen bonds. 

Direct borylation of hydrocarbons catalyzed by a transition metal complex has been extensively studied by several groups and has become an economical, efficient, elegant, and environmentally benign protocol for the synthesis of a variety of organoboron compounds. The Rh-, Ir-, Re-, and Pd-catalyzed C-/H borylation of alkanes, arenes, and benzylic positions... 

Smith and Marder reported the dehydrogenative borylation of arenes, yielding arylboronates, with pinacolborane in the presence of rhodium and iridium catalysts such as Cp Rh(7/ -C6Me6), CpIrPMes, and [RhClP( Pr)3]2N2 (eq 22). Toluene and other methyl substituted arenes react with pinacolborane in the presence of [RhClP(Tr)3]2N2 and furnish benzylboronates via benzylic C-H activation and dehydrogenative borylation (eq 23). 

The functionalization of benzylic or arene C-H bonds with boranes leads to synthetically useful boranates. Shimada and co-workers showed that [RhCl(P/Pr3)2(N2)] is an effective catalyst precursor for the borylation of aromatic and benzylic C-H bonds with the use of pinacolborane, resulting in high selectivity for benzylic C-H functionalization. ... 

The ready availability of arylboronates by an aromatic C-H borylation provides a synthetic link to the well-established palladium-catalyzed cross-coupling reactions, rhodium-catalyzed 1,4-addition to a,p-unsaturated carbonyl compounds, and other bond forming reactions using arylboronic esters (Scheme 2.12). Borylation of 1,3-dichlorobenzene with pinacolborane is followed directly by a cross-coupling reaction with methyl p-bromobenzoate for the synthesis of a biaryl product in 91% yield [60]. Pinacol esters of arylboronic acids react much slower than the free acids [62], but both derivatives achieve high isolated yields and comparable enantioselectivities (91% ee) in asymmetric 1,4-addition to N-benzyl crotonamides [63]. Borylation of arenes followed by oxidation of the C-B bond is synthetically equivalent to an aromatic C-H oxidation to phenols [64]. Oxidation of the resulting arylboronates with Oxone in a 1 1 acetone-water solution is completed within 10 min at room temperature. 

Benzyl esters, cyclization-hydrosilylation, 11, 386-387 Benzyl ethers, cyclization-hydrosilylation, 11, 386-387 Benzyl groups, C-H bond silylation, 10, 240 Benzylic alcohols, catalytic alkylation, 11, 146 Benzylic carbon-hydrogen bonds borylation, 9, 174... 

Marder reported that the alkyl C-H bonds in benzylic compounds are selectively borylated with a rhodium catalyst. Aromatic borylation prodncts were only observed as minor by-products. Similar results were obtained by Miyaura with Pd/C (equation 28). ... 

Independently, in 2001 Marder and his coworkers reported a high degree of benzyl selectivity in the borylation reactions of C-H bonds in toluene, p-xylene, and mesitylene with HBpin (pin = 0CMe2CMe20) using the catalyst precursor [(P Pr3)2Rh(Cl)(N2)] (Scheme 11) [78]. Oxidative addition of HBpin to [(P Pr3)2Rh(Cl)(N2)] yields frans-[(P/Pr3)2Rh(Cl)(H)(Bpin)], the structure of which was determined by low temperature single-crystal X-ray... 

Borylation of an ortho-methyl group of IMes with lithium borohydride has been achieved through an iridium-mediated pathway involving oxidative addition of the C—H to iridium followed by migration of the benzyl carbon to boron (Scheme 15.2). This illustrates the possibility of selective borylation of an NHC at one of the less-reactive carbons, while using an inexpensive boron source [71]. 

Borylation of the benzylic position of arenes via C—H functionalization has been achieved. Yu has also reported the ability construct C-B bonds via directed C—H functionalization of arylbenzamides catalyzed by Pd(II). Formation of carbon-carbon bonds between arenes and alkyltrifluoroborates, alkylstannanes, and alkyl halides has been reported. The incorporation of amino acid derived... 

C R5)(CO)2W=CTo1] (R = H, Me) react with H2B2Et4 to afford the boryl substituted Tj -benzyl complexes [(ri-C5R5)(CO)2W a,l,2-Ti -a-BEt2CH2Tol ]. Column chromatography on silica converts this complex to the T 3-benzyl complex [CpW(CO)2(a,l,2-T 3-CH2Tol)J.548... 

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