Chromium complexes amides

Stereoselective deprotonation of componnd 581 is possible (Scheme 233), bnt the yields and enantioselectivities obtained are poorer than for the chromium-complexed analogues (see below). With an internal electrophilic quench it was possible to form the axially chiral benzamide 582 in 89% ee using hthium amide 360 . 

Chromium (III) hexaurea Complexes, also called Chromium Hexacarb.amides. of general formula [CrfH. CO.NI ) ] X, where X stands for chlorate, chromate, nitrate, nitrite, perchlorate, etc, are briefly described in Mellor (Ref 1). They can be prepd by crystg from so Ins of urea and Cr salt. Tomlinson et al (Ref 2) detd the following expl props of chromium (III) hexaurea nitrate, [Cr(H2N.CO.NH2)6] (N03)3 ... 

Direct synthesis of atropisomeric benzamides and anilides from prochiral precursors has been reported using chiral-amide-mediated deprotonation of 2,6-dimethyl-substituted ben-zamide and anilide chromium complexes. A screening of amides revealed that (R,R) 3 was the most selective in the deprotonation of the benzylic methyl groups (Scheme 51)92 94. 

With A-pivaloyl 2,6-dimethylaniline chromium complex 69, the chiral lithium amide 70 derived from l-phenyl-2(4/-methylpiperazinyl)ethylamine turned out to be the base of choice for the asymmetric lithiation at the benzylic methyls (Table 3). With almost all electrophiles, chromium complexes bearing different substituents on the nitrogen atom could be obtained in up to 99% ee92,93. In contrast, the chiral bases (R,R)-3, (R)-27 and (S)-4 resulted in modest asymmetric induction ranging from 44% to 78% ee. 

In the case of A-benzoylaniline complex 71, the chiral lithium amide asymmetric induction was found to be dependent upon the substituent group on the IV-acyl part of the chromium complex. Thus for EX = BnCl, (R)-21 resulted in higher enantioselectivity than (R,R)-3 (Scheme 52)92. This was attributed to an equilibrium between the trans- and c -rotamers of the amide. 

The same chiral auxiliary has also been used for the stereoselective synthesis of arene-chromium complexes treatment of an aromatic aminal with chromium hexacarbonyl gives the corresponding complex with high diastereomeric excess. This protocol was recently applied in a total synthesis of (—)-lasubine (eq 4). A successful application of 1,2-diaminocyclohexane (as its IR,2R enantiomer) as a chiral auxiliary is illustrated by the di-astereoselective alkylation of the potassium enolate of bis-amide (3) with electrophiles such as benzyl bromide to give bis-alkylated products with excellent diastereoselectivity (eq 5). Lower levels... 

PC8HM, Phosphine, dimethylphenyl-, 22 133 iridium complex, 21 97 PC 2H27, Phosphine, tributyl-, chromium complexes, 23 38 PC18H1S, Phosphine, triphenyl-, 21 78 23 38 cobalt complexes, 23 24-25 cobalt, iridium, and rhodium complexes, 22 171, 173, 174 iridium complex, 21 104 palladium complex, 22 169 palladium and platinum complexes, 21 10 ruthenium complex, 21 29 PNOC 2Hl2, Phosphinic amide, diphenyl-, lanthanoid complexes, 23 180 PNAH.2, Propionitrilc, 3,3, 3 -phosphinidy-netri-,... 

The synthesis of an enantiomerically enriched chromium complex via asymmetric lithiation of a prochiral tricarbonyl(ri -arene)chromium complex using a chiral lithium amide base was first demonstrated in 1994 by Simpkins [88]. Arene complex 44 was treated with C2-symmetric chiral base ent-39 in the presence of TMSCl as an internal quench and silylated complex 45 was obtained in 84% ee (Scheme 24). 

Amide hydrolysis was described in the case of the chromium complex shown in Eq. 27.84 It is noteworthy that no alteration of the organometallic moiety occurs. In addition to the relative stability of chromium compoimds under sonication, the low-energy irradiation conditions selectively allowed the biphasic hydrolysis to occur, without promoting the more energy-demanding metal group sonolysis. 

Fig. 8 The structure of (a) (5, 5 )-Whelk-01 and the proposed chiral recognition model between (b) the CSP and the most retained enantiomer of amide-type planar tricarbonyl-chromium complexes (adapted from [111])...

Chromium aminocarbenes [39] are readily available from the reaction of K2Cr(CO)5 with iminium chlorides [40] or amides and trimethylsilyl chloride [41]. Those from formamides (H on carbene carbon) readily underwent photoreaction with a variety of imines to produce /J-lactams, while those having R-groups (e.g.,Me) on the carbene carbon produced little or no /J-lactam products [13]. The dibenzylaminocarbene complex underwent reaction with high diastereoselectivity (Table 4). As previously observed, cyclic, optically active imines produced /J-lactams with high enantioselectivity, while acyclic, optically active imines induced little asymmetry. An intramolecular version produced an unusual anti-Bredt lactam rather than the expected /J-lactam (Eq. 8) [44]. 

Photodriven reactions of Fischer carbenes with alcohols produces esters, the expected product from nucleophilic addition to ketenes. Hydroxycarbene complexes, generated in situ by protonation of the corresponding ate complex, produced a-hydroxyesters in modest yield (Table 15) [103]. Ketals,presumably formed by thermal decomposition of the carbenes, were major by-products. The discovery that amides were readily converted to aminocarbene complexes [104] resulted in an efficient approach to a-amino acids by photodriven reaction of these aminocarbenes with alcohols (Table 16) [105,106]. a-Alkylation of the (methyl)(dibenzylamino)carbene complex followed by photolysis produced a range of racemic alanine derivatives (Eq. 26). With chiral oxazolidine carbene complexes optically active amino acid derivatives were available (Eq. 27). Since both enantiomers of the optically active chromium aminocarbene are equally available, both the natural S and unnatural R amino acid derivatives are equally... 

Chromium(III) is a commonly-used crosslinker for preparing profile control gels with polymers having carboxylate and amide functionalities (la,b). Cr(III) is applied in many forms. For example, it can be used in the form of simple chromic salts of chloride and sulfate, or as complexed Cr(III) used in leather tanning (2), or as in situ generated Cr(III) from the redox reaction of dichromate and bisulfite or thiourea. The gelation rate and gel quality depend on which form of Cr(III) is used. 

Mori has reported that in the reaction of chromium carbene 25 with an alkyne containing a tethered 4-amidobutyne unit (26), a postulated vinylketene complex (27) is intercepted by nucleophilic amide attack, yielding a mixture of lactams (28 and 29).15 The expected naphthol 30 was also isolated in low yield. 

---