Hirao couplings

Added acetate ion promoted the Hirao coupling of PH(0)(0R)2 with aryl bromides and iodides. Formation of a bidentate acetate ligand was suggested to result in PPh3 dissociation, yielding a more reactive Pd electrophile for coordination (followed by deprotonation) of the H-phosphonate. P-C reductive elimination would then yield the product (Scheme 59) [99, 100]. 

Scheme 59 Proposed role of acetate in promoting Pd-catalyzed Hirao couplings...

Catalytic turn-over [59,60] in McMurry couplings [61], Nozaki-Hiyama reactions [62,63], and pinacol couplings [64,65] has been reported by Fiirst-ner and by Hirao by in situ silylation of titanium, chromium and vanadium oxo species with McaSiCl. In the epoxide-opening reactions, protonation can be employed for mediating catalytic turn-over instead of silylation because the intermediate radicals are stable toward protic conditions. The amount of Cp2TiCl needed for achieving isolated yields similar to the stoichiometric process can be reduced to 1-10 mol% by using 2,4,6-collidine hydrochloride or 2,6-lutidine hydrochloride as the acid and Zn or Mn dust as the reduc-tant (Scheme 9) [66,67]. 

Hirao T (2007) Catalytic Reductive Coupling of Carbonyl Compounds - The Pinacol Coupling Reaction and Beyond. 279 53-75 Hirayama N, see Sakai K (2007) 269 233-271 Hirst AR, Smith DK (2005) Dendritic Gelators. 256 237-273 Holzwarth AR, see Balaban TS (2005) 258 1-38... 

Solutions to similar problems of achieving catalytic turnover [22] in McMurry couplings [23], Nozaki—Hiyama reactions [24], and pinacol couplings [25] have been reported by Fiirstner and by Hirao. The key step in these reactions is the in situ silylation of titanium and vanadium oxo species with Me3SiCl and reduction of the metal halides by suitable metal powders, e. g. zinc and manganese dust, as shown in Scheme 12.13. 

The formation of an s/Z-hybridized C—P bond is readily achievable using the Michaelis-Arbuzov reaction. Such an approach is not applicable to form heteroaryl C—P bonds in which the carbon atoms are sp2 hybridized, whereas palladium catalysis does provide a useful method for Csp2—P bond formation. The first report on Pd-catalyzed C—P bond formation was revealed by Hirao et al. [134-136]. Xu s group further expanded the scope of these reactions [137, 138], They coupled 2-bromothiophene with n-butyl benzenephosphite to form n-butyl arylphosphinate 161 [137]. In addition, the coupling of 2-bromothiophene and an alkylarylphosphinate was also successful [138], For the mechanism, see page 19-21. 

Star and miktostar polymers have been synthesized by living cationic polymerization using dienes and trienes as coupling agents and/or multifunctional initiators [Faust and Shaffer, 1997 Hadjichristidis, et al., 1999 Hadjikyriacou and Faust, 2000 Kennedy and Jacob, 1998 Puskas et al., 2001 Sawamoto, 1991]. Multifunctional halides such as hexaiodo-methylmelamines have also been used to obtain star and comb polymers [Ryu and Hirao, 2001 Zhang and Goethals, 2001], Hyperbranched and dendritic polymers have also been studied. 

Shiozaki T, Hirao K, Hirata S (2007) Second- and third-order triples and quadruples corrections to coupled-cluster singles and doubles in the ground and excited states. J Chem Phys 126 244106 (11 pages). 

The palladium-catalysed cross-coupling of aryl halides or vinyl halides with dialkyl phosphonates (31) to yield dialkyl arylphosphonates and dialkyl vinylphosphonates, respectively, was first reported by Hirao and co-workers 69 the halides used most frequently are bromides and the reaction is stereospecific with haloalkenes. Subsequently, analogous reactions of alkyl alkylphosphinates (32), alkyl arylphosphinates (32), alkyl phosphinates (33), and secondary phosphine oxides (34), replacing [P—H] bonds with [P—C] bonds to yield various phosphinates and tertiary phosphine oxides, have been developed (Figure 7.1). Alkyl phosphinates (33) may be mono- or diarylated as desired by the selection of appropriate conditions. Aiyl and vinyl triflates have also found limited... 

Vanadium. Hirao has established that pinacol couplings of aliphatic aldehydes and aromatic ketones can be accomplished by CP2VCI2 (cat.)/Me3SiCl/Zn with modest to good stereoselectivity (Eqs. 3.43 and 3.44) [64]. Higher diastereoselec-tivity is obtained if a more bulky silyl chloride (PhMeaSiCl) is employed, and a mechanism analogous to that illustrated in Fig. 3-3 is believed to be operative. No catalysis is observed in the absence of Me3SiCl. 

Although a recent report by Hirao and co-workers presents Pd/C as a reusable catalyst for Suzuki coupling of iodophenols in aqueous media, studies that examine reuse of Pd/C for coupling of aryl bromides have not been reported. In this paper we describe both a batch Suzuki cross-coupling reaction with catalyst recycling and a semi-continuous fixed bed system for Suzuki coupling of aryl bromides. 

Hirao et al. synthesized diethyl p-anisylphosphonate 61 by arylation of diethyl phosphonate (60) with p-bromoanisole (59) [1], The reaction was applied to the synthesis of the phosphonate 63a by coupling of the alkenyl bromide 62 with the phosphonate 63 [18],... 

Hirao et al. have further developed the above functionahzation reactions using DPE derivatives in an excellent procedure referred to as the chain-multi-functionalization of living anionic polymers [176]. For this purpose, a new DPE derivative, l,l-bis(3-terFbutyldimethylsilyloxymethylphenyl)ethylene (12). has been synthesized. This DPE is designed in such a way that the tert-butyldimethylsilyl ether acts as a protected functionality in a reaction with living anionic polymers, and is quantitatively transformed into a benzyl bromide (BnBr) or even chloride and iodide functions [176, 183]. As illustrated in Scheme 5.18,12 was first reacted with PSLi to introduce two silyl ether functionalities at the chain-end, followed by treatment with Me3SiCl/LiBr to transform into two BnBr fimctions as a result, a well-defined chain-end-(BnBr)2-functionalized PS was obtained. The same functionalized DPE-derived anion was then separately synthesized, and reacted with the above chain-end-(BnBr)2-functionalized PS. The four silyl ether functionalities thus introduced were transformed into four BnBr functions by the same treatment with MesSiCl/LiBr, and this resulted in a chain-end-(BnBr)4-functionalized PS. As the coupling and transformation reactions proceeded both cleanly and quantitatively, the same reaction sequence could be repeated four more times to successfiiUy introduce 8,16, 32, and 64 BnBr functions at the chain-ends (Scheme 5.19 Table 5.3) [184]. Furthermore, the same reaction sequence could be carried out with a-(BnBr)2-functionalized PMMA to afford a series of weU-defined chain-end-BnBr-multi-functionalized PMMAs with up to 16 BnBr functions [185]. 

Fedorov DG, Schmidt MW, Koseki S, Gordon MS (2004) Spin-oibit coupling methods and applications to chemistry. In Hirao K, Ishikawa Y (eds) Recent advances in relativistic molecular theory. World Scientific, Singapore... 

Analogous chemistry (i.e., end functionalization of polymeric organolithiums with l,l-bis(4-tert-butyldimethylsi-loxyphenyl)ethylene) has been utilized for preparation of dendrimacs, a new type of dendritic polymer wherein a key step in the iterative process utilizes the deprotected phenol end groups in polymers as nucleophiles for Williamson ether-type coupling reactions. Hirao and co-workers " have utilized di-monosaccharide-substituted DPEs to prepare well-defined polyisoprenes and polystyrenes labeled at the chain end with monosaccharide residues. 

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