Keith Prout

CRYSTALS version 12: software for guided crystal structure analysis

The determination of small-molecule structures from single-crystal X-ray data is being carried out by researchers with little or no crys- tallographic training. At the same time, completely automatic crystal structure analysis can still only be achieved under very favourable conditions. Many of the problems that cause automatic systems to fail could be resolved with suitable chemical insight, and until this is built- in, programs continue to need human guidance. CRYSTALS version 12 contains a modern crystallographic human-interface design, and novel strategies incorporating chemical knowledge and sensible crystallographic guidance into crystal structure analysis software.

WIZARD: AI in conformational analysis

SummaryA program which utilizes the techniques of Artificial Intelligence and Expert Systems to solve problems in the area of Conformational Analysis is described. The program searches conformational space in a systematic fashion, based on the technique known as heuristic state-space search. The program proceeds by recognizingconformational units, assigning one or moreconformational templates to each unit, andjoining them to form conformational suggestions. These suggestions arecriticized to discover logical inconsistencies, and any resulting stresses areresolved. The resulting conformational suggestions are sometimes accurate enough for immediate use, or may be further refined by a numerical program. The latter combination is shown to be quite efficient compared to purely numerical conformational search techniques.

The asymmetric synthesis of β-lactams : Stereocontrolled Asymmetric Tandem Michael Additions and Subsequent Alkylations of E-[(η5-C5H5)Fe(CO)(PPH3)COCH = CHME]. X-ray Crystal Structure of (RS)-E-[(η5-C5H5) Fe (CO) (PPH3) COCH = CHME]

Abstract Michael addition of methyllithium to the E-crotonyl complex (RS)-[η5-C5H5)Fe(CO)-(PPh3)COCHCHMe] followed by trapping of the resultant enolate with methyl iodide gives (RS)-[(η5-C5H5)Fe(CO)(PPh3)COCH(Me)CHMe2] (d.e. > 100:1), also generated by treatment of (RS)-[(η5-C5H5)Fe(CO)(PPh3)COCH2CH(OMe)2] with three equivalents of methyllithium and methyl iodide. Addition of n -butyllithium to the (RS)-E-crotonyl complex followed by protonation with methanol occurs with high diastereoselectivity. Quenching with methyl iodide gives (RS)-[(η5-C5H5)Fe(CO)(PPh3)COCH(Me)CH(Me) n -Bu], also generated by treating either diastereoisomer of [(η5C5H5)Fe(CO)(PPh3)COCH2CH(Me)OMe] with two equivalents of n -butyllithium and methyl iodide. Decomplexatlon gives the known erythro -2,3-dimethyl-heptanoic acid. Similarly, Michael addition of lithium benzylamide and electrophilic quenching with methanol or methyl iodide occurs with high diastereoselectivity and gives upon decomplexation, 4-methyl- and cis -3,4-dimethyl-N-benzyl-β-lactams respectively. The stereochemical results are rationalised by addition occurring to the E-crotonyl complex in the anti (CO to CO) and cisoid conformation and subsequent alkylation of the unhindered face of the E-enolate generated. Confirmation is provided by an X-ray crystal structure analysis of (RS)-E-[(η5-C5H5)Fe(CO)(PPh3)COCHCHMe]. When repeated with the optically pure (S)-E-crotonyl complex, decomplexation gives essentially optically pure (2R) ,(3R)-(-)-N-benzyl-2,3-dimethylheptanamide, (4S)-(-)-4-methyl- and (3R),(4S)-(-)- cis -3,4-dimethyl-N-benzyl-β-lactams.

Chiral acetate enolate equivalent for the synthesis of β-hydroxy acids and esters: X-ray crystal structure of RR,SS-[(η5-C5H5)Fe(CO)(PPh3)(COCH2CH(OH)CH2CH3)]

Abstract The aluminium enolate derived from the iron acetyl complex [(η5-C5H5Fe(CO)(PPh3)COCH3], in contrast to the lithium enolate, undergoes highly stereoselective aldol reactions with aldehydes to generate RR,SS-β-hydroxyacyl complexes which on decomplexation liberate β-hydroxy acids or esters. Determination of the molecular structure of RR,SS-[η5-C5H5)Fe(CO)(PPh3)(COCH2CH(OH)CH2CH3] allowed assignment of the relative configuration of the new chiral centre.

Evidence for a direct bonding interaction between titanium and a β-C–H moiety in a titanium–ethyl compound; X-ray crystal structure of [Ti(Me2PCH2CH2PMe2)EtCl3]

The X-ray crystal structure of [Ti(Me2PCH2CH2PMe2)EtCl3](1) shows that the Ti–C–C angle and the Ti–C(methyl) distance of the Ti–Et moiety are 85.9(6)° and 2.516(10)A, respectively and that the Ti–H–C (methyl) distances are 2.29(Ti–H) and 1.02(H–C)A a direct bonding interaction between the titanium atom and the β-C–H system is proposed.

γ-Lactam analogues of penicillanic and carbapenicillanic acids

Abstract Synthesis and biological activity of γ-lactam analogues of penicillanic and carboapenicillanic acids, and the sodium periodate mediated rearrangement of pyrrolidine-2,3-diones are described. 1,3- Dipolar addition of cyclic nitrone (6) and methyl acrylate afforded the bicyclic adducts (7a) and (7b). Reductive cleavage of the N-O bond and subsequent cyclisation of a regioisomer (11a) gave the γ-lactams (12a) and (12b) in a ratio of 85 : 15. They are transformed to the carbapenam analogues (1)-(4). Their stereochemistry was assigned according to the X-ray structure of the γ-lactam (12b). Benzyl 6-oxopenicillanate (20) was directly transformed to the γ-lactam analogue (5) via a novel ring expansion. These synthetic analogues did not show antibiotic activity or β-lactamase inhibition. Treatment of pyrro1idine-2,3-diones (25a) and (25b) with sodium periodate gave ring contracted β-lactams (26a) and (26b) respectively. Similar treatment of (27) followed by diazomethane afforded an unexpected spiro epoxide.

Mono-η-cycloheptatrienyltitanium chemistry: synthesis, molecular and electronic structures, and reactivity of the complexes [Ti(η-C7H7)L2X](L = tertiary phosphine, O- or N-donor ligand; X = Cl or alkyl)

Bis(η-toluene)titanium reacts with cycloheptatriene in the presence of (AlEtCl2)2, at > 60 °C to form [Ti(η-C7H7)(η-C7H9)] and at room temperature [{Ti(η-C7H7)(thf)(µ-Cl)}2](thf = tetrahydrofuran) is also formed. The crystal structure of the latter has been determined. The dimer reacts with the ligands L2= R2PCH2CH2PR2(R = Me or Ph), trans-1,2-bis(dimethylphosphino)cyclopentane, MeOCH2CH2OMe, 2PMe3 or Me2NCH2CH2NMe2, forming the compounds [Ti(η-C7H7)L2Cl]. Reaction of alkyl Grignard reagents with the appropriate chloroderivatives gives the titanium–alkyls [Ti(η-C7H7)L2R][L2= Me2PCH2CH2PMe2, R = Me or Et; L2=trans-1,2-C5H8(PMe2)2, R = Me]. The crystal structure of [Ti(η-C7H7)(Me2PCH2CH2PMe2)Et] has been determined: there is no evidence for Ti–H–C interactions between the Ti and the hydrogens of the ethyl group. The photoelectron spectra of several Ti(η-C7H7) compounds are discussed in terms of the nature of the Ti(η-C7H7) bonding. It is proposed that the chemistry of the Ti(η-C7H7) system corresponds most closely to a formal description of the η-C7H7 group as having a –3 charge rather than the more conventional description as +1. Homogeneous mixtures of [{Ti(η-C7H7)(thf)(µ-Cl)}2] and aluminium alkyls are shown to catalyse ethylene polymerisation.

Trimethylphosphine as a reactive solvent: synthesis, crystal structures, and reactions of [Ta(PMe3)3(η2-CH2PMe2)(η2-CHPMe2)] and [W(PMe3)4(η2-CH2PMe2)H] and related studies

Reduction of WCl6, MoCl5, TaCl5, ReCl5, and RuCl3 using sodium sand in pure trimethylphosphine as a reactive solvent gives the compounds [W(PMe3)4(η2-CH2PMe2)H], [Mo(PMe3)5H2], [Ta(PMe3)3(η2-CH2PMe2)(η2-CHPMe2)], [Re(PMe3)5H], [Ru(PMe3)3(η2-CH2PMe2)H], and [(PMe3)3 HRu(µ-CH2PMe2)2RuH(PMe3)3], respectively. The crystal structures of the tungsten and tantalum compounds have been determined. The previously unknown ligand η2-CHPMe2 is shown to be present in the tantalum compound. The reduction of WCl6 in PMe3 by magnesium is shown to proceed in the sequence [W(PMe3)3Cl4], [{W(PMe3)3Cl2}2], [W(PMe3)4Cl2]. Reduction of [W(PMe3)3Cl4] with sodium sand under hydrogen gives [W(PMe3)4Cl2H2]. The compound [W(PMe3)4(η2-CH2PMe2)H] reacts with butadiene giving cis-[W(PMe3)2(η-C4H6)2] and with cyclopentadiene forming [W(η-C5H5)(PMe3)3H] and [W(η-C5H5)2H2]. Variable-temperature n.m.r. studies on [W(PMe3)4(η2-CH2PMe2)H] show it to be fluxional. Reduction of RUCl3 in trimethylphosphine–cyclopentene gives [Ru(PMe3)4(σ-[graphic omitted]H2)H]. The compound [W(PMe3)3H6] with spiro[2.4]hepta-4,6-diene gives [W(η-C5H4Et)(PMe3)3H].

A titanium–methyl group containing a covalent bridging hydrogen system: X-ray crystal structure of Ti(Me2PCH2CH2PMe2)MeCl3

The X-ray crystal structure of Ti(Me2PCH2CH2PMe2)MeCl3 shows that a Ti–C–H angle and the corresponding Ti–H distance of the Ti–Me moiety are 70(2)° and 2.03(4)A, respectively; the bonding of the Ti-H-C(methyl) system is described in terms of a two-electron three-centred covalent bond and it is proposed that such distorted transition metal–α-hydrogen interactions will occur quite widely.

The crystal and molecular structures of hexaaquacopper(II) benzenesulphonate, toluene-4-sulphonate and d-camphor-10-sulphonate

The structures of hexaaquacopper(II) benzenesulphonate (triclinic, a = 22.51, b = 6-26, c = 6.96/k, <t = 92.5, fl = 93.0, y -89.5 °, space group P1, Z = 2, 2970 reflexions, R w = 0.041), hexaaquacopper(II) toluene-4-sulphonate (monoclinic, a = 5.85, b = 25.71, c = 7.35 A, fl = 105.4 °, space group P2,/c, Z = 2, 3645 reflexions, R w = 0.048) and hexaaquacopper(II) o-camphor-10-sulphonate (monoclinic, a = 17.17, b = 7.05, c = 11.64 A, fl = 94.1 °, space group P 2 , Z = 2, 1920 reflexions, R w = 0.089) have been determined from diffractometer intensity measurements. The overall packing arrangements in the three structures are broadly similar, but whilst the Cu n ions in the arylsulphonates are in elongated, near tetragonally distorted, octahedral environments, that in the o-camphor-10-sulphonate is better described as a rhombically distorted octahedron. Details of these, and related structures, may be rationalized in terms of Jahn-Teller distortions and hydrogen bonding.